The Ordovician Earth System

Special Paper 466 THE GEOLOGICAL SOCIETY OF AMERICA® THE ORDOVICIAN EARTH SYSTEM EDITED BY STANLEY C. FINNEY AND WILL...

Author: Stanley Charles Finney | William B. N. Berry (editors)

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Special Paper 466

THE GEOLOGICAL SOCIETY OF AMERICA®

THE ORDOVICIAN EARTH SYSTEM

EDITED BY STANLEY C. FINNEY AND WILLIAM B.N. BERRY

The Ordovician Earth System

edited by

Stanley C. Finney Department of Geological Sciences California State University at Long Beach Long Beach, California 90840 USA William B.N. Berry Department of Earth and Planetary Science University of California, Berkeley Berkeley, California 94720 USA

Special Paper 466 3300 Penrose Place, P.O. Box 9140

Boulder, Colorado 80301-9140, USA

2010

Copyright © 2010, The Geological Society of America (GSA), Inc. All rights reserved. GSA grants permission to individual scientists to make unlimited photocopies of one or more items from this volume for noncommercial purposes advancing science or education, including classroom use. For permission to make photocopies of any item in this volume for other noncommercial, nonprofit purposes, contact The Geological Society of America. Written permission is required from GSA for all other forms of capture or reproduction of any item in the volume including, but not limited to, all types of electronic or digital scanning or other digital or manual transformation of articles or any portion thereof, such as abstracts, into computer-readable and/or transmittable form for personal or corporate use, either noncommercial or commercial, for-profit or otherwise. Send permission requests to GSA Copyright Permissions, 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Copyright is not claimed on any material prepared wholly by government employees within the scope of their employment. Published by The Geological Society of America, Inc. 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA www.geosociety.org Printed in U.S.A. GSA Books Science Editors: Marion E. Bickford and Donald I. Siegel Library of Congress Cataloging-in-Publication Data The Ordovician earth system / edited by Stanley C. Finney, William B.N. Berry. p. cm. — (Special paper ; 466) Includes bibliographical references. ISBN 978-0-8137-2466-9 (pbk. : alk. paper) 1. Paleobiogeography—Ordovician. 2. Paleoecology—Ordovician. 3. Biodiversity. I. Finney, Stanley C. (Stanley Charles), 1947–. II. Berry, William B. N. QE721.2.P24O76 2010 560′.45—dc22 2010000939 Front cover: Indian River Falls. Outcrop of uppermost Ordovician strata in the Indian Falls Conservation area 5 km NNW of Owen Sound, Ontario. The basal red mudstones, which are overlain by about 2.5 m of gray siltstones, represent the uppermost portion of the shallow-water, poorly fossiliferous Queenston Formation that consists of clastics transported from eastern highlands formed during the Late Ordovician Taconic orogeny. The Queenston is unconformably overlain by about 7.5 m of shallow-water fossiliferous limestones of the Manitoulin Formation. Traditionally, this unconformity has been taken to represent the Ordovician/Silurian boundary but recent work on conodonts and brachiopods indicates that the Manitoulin Formation is of Hirnantian age. This is consistent with the fact that the Hirnantian Isotope Carbon Excursion (HICE) is present in the upper Queenston and lower Manitoulin formations. The subManitoulin unconformity is now interpreted as reflecting a eustatic regression during a middle Hirnantian Saharan glaciation and the Manitoulin as deposited during the following post-glacial transgression. Photo by Stig M. Bergström. Back cover: Conodont, Pygodus anserinus. The platform element of this species, illustrated by a specimen from the Sops Arm Formation in Newfoundland, is very distinctive. The evolution of P. anserinus from P. serra occurs below and very close to the boundary between the Middle Ordovician and Upper Ordovician Series, and the lowest occurrence of P. anserinus, stratigraphically above occurrences of P. serra, can be used in carbonate successions to locate the series boundary with considerable accuracy. Photo by Godfrey Nowlin. 10 9 8 7 6 5 4 3 2 1

Contents

Preface Stanley C. Finney and William B.N. Berry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v 1. Global setting of Ordovician orogenesis Cees R. van Staal and Robert D. Hatcher, Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Ordovician explosive volcanism Warren D. Huff, Stig M. Bergström, and Dennis R. Kolata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3. Toward identifying potential causes for stratigraphic change in subtropical to tropical Laurentia during the Mohawkian (early Late Ordovician) Achim D. Herrmann and Bernd J. Haupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4. The Upper Ordovician Guttenberg δ13C excursion (GICE) in North America and Baltoscandia: Occurrence, chronostratigraphic significance, and paleoenvironmental relationships Stig M. Bergström, Birger Schmitz, Matthew R. Saltzman, and Warren D. Huff . . . . . . . . . . . . . . . . 37 5. The Ordovician brachiopod radiation: Roles of alpha, beta, and gamma diversity David A.T. Harper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 6. Ordovician paleogeography and tectonics of the major paleoplates of China Chen Xu, Zhou Zhi-yi, and Fan Jun-xuan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 7. Ordovician of the Siberian Platform Alexander V. Kanygin, Tatiana N. Koren, Anastasia G. Yadrenkina, Alexander V. Timokhin, Oleg V. Sychev, and Tatiana Yu. Tolmacheva . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 8. Early–Middle Ordovician conodont paleobiogeography with special regard to the geographic origin of the Argentine Precordillera: A multivariate data analysis Guillermo L. Albanesi and Stig M. Bergström . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 9. Black shales: An Ordovician perspective William B.N. Berry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 10. Paleogeographic, paleoceanographic, and tectonic controls on early Late Ordovician graptolite diversity patterns Daniel Goldman and Wu Shuang-Ye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

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Contents 11. Origin of Late Ordovician (mid-Mohawkian) temperate-water conditions on southeastern Laurentia: Glacial or tectonic? Frank R. Ettensohn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 12. Correlations across a facies mosaic within the Lexington Limestone of central Kentucky, USA, using whole-rock stable isotope compositions John W. Coates, Frank R. Ettensohn, and Harold D. Rowe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Preface

Stanley C. Finney† Department of Geological Sciences, California State University at Long Beach, Long Beach, California 90840, USA William B.N. Berry† Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, California 94720, USA With a duration of 44 million years (488–444 Ma), the Ordovician Period is a most significant chapter in Earth’s history. It includes the great Middle Ordovician biodiversification event, the Hirnantian glaciation, and long-term greenhouse conditions. During the period, many continents were dispersed widely along the equator and in the southern temperate belt (Laurentia, Baltica, Siberia, South China, and Tarim), and large Gondwana (Africa, South America, India, Antarctica, and Australia) occupied much of the Southern Hemisphere from the pole to the equator. Richly fossiliferous carbonate sediments, along with occasional shale facies and quartz sands, were deposited in shallow seas that flooded tropical continents, while siliciclastic sediments, including Hirnantian glacial and glacio-marine sediments, were deposited over large parts of Gondwana and peri-Gondwana terranes. After nearly complete submergence in the Early Ordovician, Laurentia underwent a dramatic, extensive regression and subsequent transgression that produced the continentwide, post-Sauk, pre-Tippecanoe unconformity, which may be expressed to various degrees in stratigraphic successions on most other paleocontinents that lay at low to middle paleolatitudes. The margins of the Ordovician oceanic realm, now preserved in orogenic belts (Antler, Taconic, Ouachita, and Ellesmerian of North America, Caledonides of northwest Europe, Central Asia, and Lachlan in eastern Australia), were sites of deep-marine sedimentation of thick turbiditic sandstone packages, shale, and chert. Magmatic-volcanic arcs along paleo-plate boundaries produced impressive belts of Ordovician granites in northwestern Argentina, and numerous, often thick, geographically widespread K-bentonites that are found today in eastern North America, northwest Europe, China, and the Argentine Precordillera and which represent some of the largest known fallout ash deposits of the Phanerozoic Era. Subduction along the margins of Laurentia, Gondwana, Baltica, and Siberia formed extensive linear orogenic belts. Accordingly, rocks of the Ordovician System are widespread on all continents except Antarctica (where the present ice cover may hide them), and capping Mount Everest they occupy the highest elevation on Earth (Ross, 1984). A 30 yr project of formally defining the Cambrian-Ordovician and Ordovician-Silurian boundaries, and a single set of global series-epochs and stages-ages for the Ordovician System-Period was recently completed by the International Subcommission on Ordovician System when the GSSP (global stratotype section and point) for the base of the Dapingian Stage and the Middle Ordovician Series was approved by the International Commission on Stratigraphy and ratified by the International Union of Geological Sciences in 2007 (Fig. 1). This scientific process was especially difficult and complex for the Ordovician System, because the high degree of biogeographic provincialism and ecologic differentiation of Ordovician faunas greatly limited global correlations (Finney, 2005). British series-epochs were often used as de facto nomenclature on stratigraphic correlation charts and geologic time scales. However, they were not widely adopted outside of the British Isles because they could not be correlated biostratigraphically with precision and high †

E-mails: [email protected]; [email protected].

v

Preface

GLOBAL SERIES

vi

443.7 Ma 445.6 Ma

GLOBAL STAGES

BRITISH SERIES

NORTH AMERICAN SERIES

HIRNANTIAN ASHGILL

UPPER

CINCINNATIAN

471.8 Ma

478.6 Ma

MIDDLE

468.1 Ma

CARADOC

MOHAWKIAN

SANDBIAN

DARRIWILIAN

LLANVIRN WHITEROCKIAN

DAPINGIAN ARENIG FLOIAN

LOWER

460.9 Ma

ORDOVICIAN

455.8 Ma

KATIAN

IBEXIAN TREMADOCIAN

TREMADOC

488.3 Ma

Figure 1. Chart showing correlation between global series and stages and regional series for Britain and North America. Calibrated ages from Ogg et al. (2008).

resolution. As a result, several independent and very different regional sets of series and constituent stages were established for the Ordovician System, with each generally applicable to a different paleoplate or modern continent, but this greatly confused Ordovician chronostratigraphy-geochronology and often resulted in imprecise correlations of Ordovician stratigraphic successions. The new set of global series and stages, and the precise definitions of their boundaries as well as those of the system, now provide a common language for correlation of Ordovician strata and for expressing geologic time (Fig. 1). During the many years required to establish the complete set of new global units, many Ordovician stratigraphers used the regional classification that they preferred, and some lamented the replacement of the British Series as the standard units of Ordovician stratigraphy (Cope, 2007). Nevertheless, now that the global set of units is complete, they are universally accepted and regularly used in papers on Ordovician stratigraphy. Regional chronostratigraphic classifications can still be used for those regional stratigraphic successions on which they were defined and may offer the most precise local correlation, and the correlation of the varied regional units to the new global series and stages is fully elucidated in Bergström et al. (2009). The process of establishing global chronostratigraphic units for the Ordovician System depended on, and resulted in, extensive, focused, often multidisciplinary studies of stratigraphic successions worldwide by a multitude of specialists. It generated a huge body of stratigraphic information, primarily on biostratigraphy and paleontology, but also on chemostratigraphy, sequence stratigraphy, and magnetostratigraphy as well as sedimentology and geochemistry. In addition it fostered the acquisition of radiometric dates that could be used to calibrate the ages of the chronostratigrapic units, particularly their boundaries, thus refining a significant part of the Geologic Time Scale (Cooper and Sadler, 2004). The primary goal of this long-term project was establishment of a single set of global chronostratigraphic units for precise, high-resolution correlations worldwide. However, it also fostered integrated, multidisciplinary studies that documented the state of, and important changes in, the Earth system during the Ordovician Period (e.g., Huff et al., 1992; Finney et al., 1999). As the chronostratigraphic project neared completion, members of the Ordovician Subcommission redirected their attention toward using the greatly expanded knowledge of Ordovician stratigraphy to address major events of the Ordovician Period. Former subcommission chair Barry Webby organized and led the most

Preface successful International Geological Correlation Programme (IGCP) 410, the Great Ordovician Biodiversification Event, which produced an impressive examination and summary of one of the major biotic events of not only the Ordovician but also the entire Phanerozoic (Webby et al., 2004). Along the same lines, a symposium titled “Global Ordovician Earth System” was organized at the 32nd International Geological Congress in Florence, Italy, in 2004. It was composed largely of contributions that used an Earth systems approach to examine aspects of the Earth’s lithosphere, biosphere, hydrosphere, and atmosphere during the Ordovician and to integrate them where possible. Several of those contributions, further elaborated since the Congress in Florence, form the core of this volume with additional contributions later solicited by the editors. The contributions in this volume range from global to regional in scope. Their focus varies from tectonic to paleogeographic and from sedimentological to paleontological. Yet all provide views of the Ordovician world. They are only a small sample of the diverse current research on the Ordovician System, yet they provide entry into the Ordovician world and in varying degree to interactions among Earth’s systems. Thus, the title of the volume is The Ordovician Earth System. In the lead article, Cees van Staal and Robert Hatcher conclude from their study of early Paleozoic orogenic belts that Ordovician orogenesis was due principally to accretion of arc terranes and ribbon microcontinents. Warren Huff and co-authors review the distribution of Ordovician K-bentonites in eastern North America, northwest Europe, Argentina, and China. Achim Herrmann and Bernd Haupt employ numerical models of the Ordovician ocean-climate system to propose that upwelling cold-water masses that penetrated the epicontinental sea of Laurentia during the early Late Ordovician indicate the onset of cooling conditions that plunged the Earth system toward icehouse conditions that later led to the Hirnantian glaciation as well as produced a regional extinction during the early Late Ordovician (Mohawkian) of eastern Laurentia. Stig Bergström and co-authors demonstrate the powerful correlation potential of the early Katian (early Late Ordovician) Guttenberg carbon isotope excursion, which occurred within the time of massive fallout ash deposits and the Mohawkian extinction, but they consider its cause to be enigmatic. Turning to the biosphere, David Harper’s detailed analysis of brachiopod diversity illustrates well the great Middle Ordovician biodiversification event, the Hirnantian mass extinction, and less dramatic changes and relates them to paleogeographic changes and tectonic events. Contributions by Chen Xu and colleagues on the Ordovician paleoplates of modern China and by Alexander Kanygin and co-authors on the Ordovician of the Siberian platform provide valuable summaries of stratigraphic information that are essential for reconstructing Ordovician paleogeography for significant continents of the Ordovician world. Through statistical analysis of Ordovician conodonts from globally widespread areas, Guillermo Albanesi and Stig Bergström examine paleobiogeographic relationships between Laurentia, Baltica, the Argentine Precordillera, and the proto-Andean margin of Gondwana and find that the evidence does not support a Laurentian origin of the Argentine Precordillera (the Cuyania terrane), a hypothesis that has received much recent attention. Organic-rich black shales are prominent, widespread Ordovician lithofacies and commonly contain abundant graptolites, which are essential for chronostratigraphic correlations. Using modern analogues, William Berry concludes that they accumulated in settings similar to modern hypoxic-anoxic environments. The final three papers in this volume are complementary examinations of the Earth system interactions in eastern Laurentia during the Katian (early Late Ordovician or Mohawkian). Daniel Goldman and Wu Shuang-Ye conclude that a significant episode of graptolite extinction was regional and not global and likely due to cold-water masses that penetrated the epicontinental sea of Laurentia, as proposed by the ocean–climate system model of Herrmann and Haupt (this volume). Frank Ettensohn associates the influx of cold-water masses to regional tectonics that altered regional paleogeography. John Coates and co-authors demonstrate that carbon-isotope stratigraphy provides high-resolution correlation between varied lithofacies within this complex depositional setting. Thorough reviews of manuscripts submitted for this volume were provided by Leho Ainsaar, Peter Cawood, Robin Cocks, Roger Cooper, Olda Fatka, Don Gorsline, Jack Green, Christopher Holmden, Dmitri Kaljo, Thomas Kelty, Dennis Kolata, Ed Landing, Stephen Leslie, Greg Ludvigson, Michael Melchin, Godfrey Nowlan, Mark Patzkowsky, Michael Pope, Leonid Popov, John Repetski, Rong Jiayu, Matthew Saltzman, Hans Peter Schonlaub, Brian Witzke, and Jan Zalaciewicz. Their efforts are very much appreciated.

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Preface REFERENCES CITED Bergström, S.M., Chen Xu, Gutiérrez-Marco, J.C., and Dronov, A., 2009, The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages and to δ13C chemostratigraphy: Lethaia, v. 42, p. 97–107. Cooper, R.A., and Sadler, P.M., 2004, The Ordovician Period, in Gradstein, F.M., Ogg, J.G., and Smith, A.G., eds., A Geologic Time Scale 2004: Cambridge, UK, Cambridge University Press, p. 165–187. Cope, J.C.W., 2007, What have they done to the Ordovician?: Geoscientist, v. 17, p. 19–21. Finney, S., 2005, Global series and stages for the Ordovician System: A progress report: Geologica Acta, v. 3, p. 309–316. Finney, S.C., Berry, W.B.N., Cooper, J.D., Ripperdan, R.L., Sweet, W.C., Jacobson, S.R., Soufiane, A., Achab, A., and

Noble, P.J., 1999, Late Ordovician mass extinction: New perspective from stratigraphic sections in central Nevada: Geology, v. 27, p. 215–218, doi: 10.1130/0091-7613(1999) 0272.3.CO;2. Huff, W.D., Bergström, S.M., and Kolata, D.R., 1992, Gigantic Ordovician volcanic ash fall in North America and Europe: Biological, tectonomagmatic, and event-stratigraphic significance: Geology, v. 20, p. 875–878, doi: 10.1130/0091 -7613(1992)0202.3.CO;2. Ogg, J.G., Ogg, G., and Gradstein, F.M., 2008, The Concise Geologic Time Scale: Cambridge, UK, Cambridge University Press, 177 p. Ross, R.J., Jr., 1984, The Ordovician System, progress and problems: Annual Review of Earth and Planetary Sciences, v. 12, p. 307–335, doi: 10.1146/annurev.ea.12.050184.001515. Webby, B.D., Paris, F., Droser, M.L., and Percival, I.G., 2004, The Great Ordovician Biodiversification Event: New York, Columbia University Press, 484 p.

The Geological Society of America Special Paper 466 2010

Global setting of Ordovician orogenesis Cees R. van Staal† Geological Survey of Canada, 625 Robson Street, Vancouver, British Columbia V6B 5J3, Canada Robert D. Hatcher, Jr. Department of Earth and Planetary Sciences and Science Alliance Center of Excellence, University of Tennessee, Knoxville, Tennessee 37996-1410, USA

ABSTRACT The global distribution, setting, and dynamic implications of Ordovician orogenesis are reviewed. Evidence for true Ordovician continent-continent collision is absent. Orogenesis is principally due to accretion of arc terranes and/or ribbon microcontinents. Most arc terranes are ensialic and separated from the adjacent continents by backarc or marginal basins, the episodic closure of which commonly was responsible for orogenesis. Little evidence is preserved for true intra-oceanic juvenile arcs during the Early to Middle Ordovician. Instead, subduction appears to have been localized near the margins of Laurentia, Gondwana, Baltica, and Siberia, forming extensive linear orogenic belts during relatively short periods when the upper plate switched from extension to compression. Such tectonic switching appears to have taken place along the entire length of the Pacific and Iapetan margins of Gondwana (>10,000 km) from Middle–Late Cambrian to Early Ordovician time. The onset of orogenesis along Gondwana’s Pacific margin during the end of the Early Cambrian (ca. 513 Ma) coincided with subduction initiation along both margins of the Iapetus Ocean. Orogenesis and subduction initiation are causally related to a global-scale plate reorganization, probably induced by terminal amalgamation of Gondwana. During the Paleozoic, Laurentia’s Iapetan margin steadily grew in size and expanded southward owing to continuous accretion of suprasubduction zone oceanic crust, peri-Gondwanan arc terranes, and ribbon microcontinents. In contrast, the Pacific, Iapetan, and Rheic margins of Gondwana saw little addition of new, allochthonous crust. Accretion mainly involves reattachment of previously rifted-off arc terranes and small slivers of the intervening marginal basin crust. INTRODUCTION

and Eurasia. There is also minor Ordovician deformation in Africa, but this mainly relates to transpression and basin inversion induced by far-field stresses and is not relevant to this paper. What makes Ordovician orogenesis stand out from all other periods in the Paleozoic is that nowhere it appears to have involved true continent-continent collision. Some workers have invoked a Middle Ordovician collision between Gondwana and Laurentia (e.g., Dalziel et al., 1994), mainly based on orogenesis taking

Orogenic events (orogenesis) affected many parts of the world during the Ordovician, and the remnants of the mountain belts created can be found principally in the present-day continents of North America, South America, Australia, Antarctica, †

E-mail: [email protected].

Van Staal, C.R., and Hatcher, R.D., Jr., 2010, Global setting of Ordovician orogenesis, in Finney, S.C., and Berry, W.B.N., eds., The Ordovician Earth System: Geological Society of America Special Paper 466, p. 1–11, doi: 10.1130/2010.2466(01). For permission to copy, contact [email protected]. ©2010 The Geological Society of America. All rights reserved.

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place nearly coevally on both margins of Iapetus (Fig. 1) and involving accretion of Laurentian-derived crust (Precordillera) to Gondwana. However, this model was abandoned by Dalziel (1997) and replaced by a short-lived collision between a large promontory on Laurentia that bulged into the Iapetus Ocean (Texas Plateau) and Gondwana. Most other workers favor collision between a Laurentian-derived microcontinent (Cuyania/ Precordillera) and Gondwana instead (e.g., Ramos et al., 1986; Astini et al., 1995; Thomas and Astini, 1996). Irrespective of whether or not the fleeting promontory-Gondwana collision ever happened, we do not consider it a true continent-continent collision, because the area involved in this putative collision was very small compared with the length of the margins. Instead the four principal continental masses present on Earth at that time (Laurentia, Gondwana, Baltica, and Siberia; see Fig. 1 and also Cocks and Torsvik, 2002; Torsvik and Cocks, 2005) were involved in accretion of peri-cratonic arc terranes and/or ribbon-shaped microcontinents and remained separated by oceans (principally the Iapetus, Aegir, Tornquist-Ran, and Pacific Oceans). These accretionary events were precursors to major continent-continent collisions that occurred during the middle to late Paleozoic. Formation and accretion of ribbon-shaped microcontinents, principally derived from Gondwana and to a lesser extent Laurentia, and the closure of marginal oceanic basins on a global scale

during the Ordovician are important tectonic phenomena that pose important geodynamic questions. The purpose of this paper is to provide a brief review and summary of the data and ideas related to Ordovician orogenic events, mainly concentrating on their tectonic evolution and whether they are kinematically and/or dynamically linked, and if so, how and why, and finally to discuss the implications of these orogenic events. The latter is done to further our understanding of the global tectonic climate and the driving tectonic forces during the Ordovician. In this paper we have adopted the usage of the new Ordovician stage names (Finney, 2005; Bergström et al., 2006). OROGENESIS AND TIME In general, classification and correlation of structures and other evidence of tectonic events were historically (pre–plate tectonics) done on the basis of time alone, which is a hazardous process (although one sometimes has no other choice), because orogenic events are invariably diachronous, and kinematically unrelated tectonic events may take place coevally in different parts of the world (e.g., van Staal, 1994). These, by chance, may have been juxtaposed later as a result of subsequent plate motion(s) and closure of intervening oceans (van Staal et al., 1998).

Figure 1. Distribution of the main continents, oceans, subduction zones and arc-backarc systems, and micro-continents in the Early Ordovician (Tremadocian). The South Pole is situated in North Africa. The Pacific margin of Gondwana with the Ross-Delamerian belt of Australia and Antarctica that is continuous with the active proto-Andean margin, together forming a very long circum-Gondwanan subduction system, is just out of view. The Corel Draw figure originally made by Conall MacNiocaill for van Staal et al. (1998) was modified for this paper. Narrow volcanic arc terranes are indicated by ellipses. Dashed lines are used where the existence of subduction zones and/or transform faults is uncertain or contentious. Stippled structures may not have been active at this time. Arm—Armorica terrane; Aval—Avalonia; Boh—Bohemia terrane; Cadom—Paleozoic Cadomian arc terrane of Stampfli and Borel (2002); Car—Carolinia; Dashw & ND arc— Dashwoods microcontinent and Notre Dame arc; Famat arc—Famatinian arc-backarc system; Finnm arc—Finnmark arc-backarc system; Gan—Ganderia; Høl.—Hølanda terrane; Ib—Iberia terrane; Kip arc—Kipchak arc; M—Meguma terrane; Mu arc—Mugodzhar arc-microcontinent; Pc—Precordillera terrane; Penobs. arc—Penobscot arc-backarc system; S-M—Sakmara-Magnitogorsk marginal basin; Svalb—estimated position of the Svalbard and Pearya terranes.

Global setting of Ordovician orogenesis ORDOVICIAN OROGENESIS Orogenic events discussed in this paper include the Finnmarkian, Penobscottian, Taconian, Grampian, Shelveian, and the early stages of the Salinic and Scandian in the Appalachians and Caledonides of North America, British Isles, and Scandinavia; M’Clintock in Pearya of Arctic Canada and its unnamed equivalent in neigboring Svalbard; Famatinian and/or Ocloyic in South America; Sardic or Eo-Variscan in the Eurasian Variscan orogen; events in the Central Asian orogenic belt (Altaids); and parts of the Ross-Delamerian and Lachlan orogenies in eastern Australia and Antarctica (Fig. 1). Most of these orogenies are restricted to the Ordovician, but some started or mainly took place during the Middle to Late Cambrian (Finnmarkian and Ross-Delamerian), whereas others started during the latest Ordovician and climaxed during the Silurian (Salinic and Scandian). Their inclusion here is either due to diachroneity, such that their orogenesis locally extends into the Ordovician, and/or to their dynamics having had an impact on understanding Ordovician orogenesis in general. Following the definition of McKerrow et al. (2000), we restrict usage of the term Caledonian orogeny to describe all orogenic events involved in closing the Iapetus Ocean. Ordovician Paleogeography Comprehensive reviews of Earth’s geography, position of tectonic elements, and migration of terranes relevant to this paper are presented in van Staal et al. (1998), Cocks and Torsvik (2002), Stampfli and Borel (2002), Torsvik and Cocks (2005), and Cawood (2005). Although these authors do not necessarily agree on the evolution of each element, they broadly agree on the disposition of the main continental landmasses summarized in Figure 1. We adhere to these views, which are significantly different from the one proposed by Dalziel (1997). North American Appalachians and British Caledonides Ordovician orogenesis affected the Appalachians differently in different parts of the mountain chain. These orogenic events generally are diachronous along the length of the mountain chain, and to make matters more confusing, they occurred on both sides of the Iapetus ocean while the ocean was still very wide (Fig. 1), which makes the classification of Ordovician structures difficult without knowing the provenance and tectonic setting of the deformed rocks involved. The oldest orogenic events that affected the Appalachian orogen are represented by the latest Cambrian–Early Ordovician phases of the Taconian on the Laurentian side and the Penobscottian on the Gondwanan side (e.g., Colman Sadd et al., 1992; van Staal et al., 2007). Early phases of both orogenies partly overlap in time, and both involve ophiolite accretion-obduction and arc-continent collision (Fig. 1). The onset of ocean closure along opposite margins of the Iapetus Ocean (Fig. 1) signals a major plate reorganization at the end of the Early Cambrian (van Staal et al., 1998; also see below).

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The main phase of the Taconian (Figs. 1 and 2) in the northern Appalachians is the collision between magmatic arc(s) and Laurentia, which started during the Early Ordovician (ca. 475 Ma; Humberian in Newfoundland) and lasted until the Late Ordovician. Deformation and accompanying tectonothermal activity are principally related to collision of a magmatic arc (Notre Dame–Shelburne Falls arc) built upon a ribbon-shaped peri-Laurentian microcontinent (Karabinos et al., 1998) named Dashwoods in Newfoundland (Taconic 2 suture in Fig. 1; Waldron and van Staal, 2001; van Staal et al., 2007), rapidly followed by Late Ordovician (ca. 450 Ma) softdocking of the peri-Gondwanan Popelogan–Victoria–Bronson Hill arc (Taconic 3, Fig. 2) built on a sliver of Ganderian crust (van Staal, 1994; Rogers et al., 2006 ; Zagorevski et al., 2007). This phase represents the terminal phase of the Taconian and closed the main oceanic tract of the Iapetus between Laurentia and Ganderia (Fig. 2), although a wide oceanic backarc basin (Tetagouche-Exploits backarc) and seaway (Acadian seaway) remained open, to be closed during the Silurian and Early Devonian, respectively (van Staal et al., 1998, 2007; van Staal, 2005; Valverde-Vaquero et al., 2006). In the southern Appalachians the Taconian is termed Blountian (Kay, 1942; Rodgers, 1971; Drake et al., 1989). Orogenesis here is particularly evident from the accumulation of the substantial Blount-Sevier foredeep clastic wedge on Laurentia’s continental margin near the beginning of the Middle Ordovician (ca. 468 Ma, Darriwilian), but it is also recorded by abundant metamorphic ages, formation of eclogite, and generation of arclike plutons in the internal parts of the mountain chain (central and eastern Blue Ridge and western Inner Piedmont). The most attractive model, which is virtually identical to the northern Appalachians, is one involving formation of a west-facing magmatic arc on a peri-Laurentian microcontinent (Hatcher, 1989; Miller et al., 2006; Hibbard et al., 2007). Metamorphism as high as granulite facies affected the rocks in the internal parts of the southern Appalachians between 455 and 465 Ma (Moecher et al., 2004) as a result of the arc-continent collision and associated obduction. Hibbard (2000) proposed that immediately subsequent to this collision the leading edge of the peri-Gondwanan Carolina terrane docked in the latest Ordovician–Early Silurian. If Hibbard’s model is correct, it indicates a remarkably similar kinematic/dynamic evolution for the entire Appalachian margin of Laurentia (Fig. 2) during the Ordovician (Hibbard et al., 2007), virtually eliminating the possibility of a fleeting Middle– Late Ordovician Gondwana-Laurentia collision as proposed by Dalziel et al. (1994) and Dalziel (1997). Other compelling sedimentologic, geochronologic, and field data from the southern and central Appalachians also support closure of the bulk of the Iapetus Ocean in the Late Ordovician, but these data also indicate that the trailing Rheic Ocean remained open until its late Paleozoic destruction (Bream, 2003; Merschat et al., 2005; Hatcher and Merschat, 2006; Merschat and Hatcher, 2007). Possibly several other, yet to be identified terranes outboard of Carolina were accreted to Laurentia prior to final closure of the Rheic Ocean,

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Figure 2. Paleogeography of the tectonic elements shown in Figure 1 in the Late Ordovician (450 Ma). Figure modified from van Staal et al. (1998). Abbreviations are the same as in Figure 1. Tac 2 and 3 refer to suture zones formed during the Taconian orogeny (van Staal et al., 2007). ACSW—Acadian seaway between Ganderia and Avalonia, the closure of which caused the Acadian orogeny; Hun—Hun superterrane of Stampfli et al. (2002); PVA— Popelogan-Victoria arc; RILA—Red Indian Lake arc; TEB—Tetagouche-Exploits backarc basin; Thor—Thor suture zone, forming part of the TESZ (Trans-European suture zone).

which caused major underthrusting of the “westerly terranes,” situated near the leading edge of Laurentia beneath Carolina. The Taconian also affected the central Appalachians, but its effects were generally recorded slightly later (Late Ordovician) on the foreland. Here, the Martinsburg (Martinsburg-Juniata) clastic wedge is thicker (~5 km) than the Blount-Sevier wedge (~3 km) farther to the south. Deposition of the Martinsburg in the type area started approximately at the same time as the BlountSevier wedge, but deposition continued into the Late Ordovician. These relationships attest to the diachronous and composite nature of the Taconian orogeny in the Appalachians (Shanmugam and Lash, 1982; Drake et al., 1989; Hatcher, 1989). Several exposures of the angular unconformity from Pennsylvania and New Jersey to eastern New York reveal Early Silurian (Llandovery) molasse overlying tilted Upper Ordovician clastic wedge rocks (Rodgers, 1971). Extensive peri-cratonic island arc systems fringing both the Iapetan margins of Laurentia and Gondwana until the Late Ordovician (van Staal et al., 1998), combined with paleomagnetic and faunal evidence (e.g., Cocks and Torsvik, 2002), require that both continents remained widely separated until at least the Late Devonian. Hence, we conclude that there is no evidence to support collision of Laurentia with any large continental landmass other than ribbon-shaped continental arc terranes during the Ordovician.

The Taconian is kinematically and temporally equivalent to the Grampian in the British Isles (Dewey and Shackleton, 1984). Like the Taconian, the Grampian starts with ophiolite obduction immediately prior to arc-continent collision in the Early Ordovician (Dewey and Mange, 1999). The main Grampian orogenesis is a short-lived ca. 15 Ma event (ca. 475–460 Ma), although it comprises a complex tectonometamorphic cycle involving early blueschist and eclogite facies metamorphism of the subducting Laurentian (Dalradian) margin (Friend et al., 2000; Chew et al., 2003), polyphase deformation, and medium to high grade Barrovian metamorphism of the collision complex concurrent with syntectonic intrusion of plutons (Friedrich et al., 1999; Soper et al., 1999; Dewey and Mange, 1999). In this respect, the Grampian is nearly coeval and kinematically remarkably similar to the main phase of the Taconian in central and western Newfoundland (van Staal et al., 1998, 2007). Grampian orogenesis was rapidly followed by the Late Ordovician–Early Silurian Scandian orogenesis similar in nature to the Salinic in the northern Appalachians. While Grampian orogenesis occurred on the Laurentian side, Early to Middle Ordovician orogenesis also took place on the Gondwanan side in the British Isles (e.g., Max et al., 1990; Todd et al., 2000). This orogenic event, which has no name attached to it as yet in the British Isles, is the equivalent of the Penobscottian in the Appalachians (van Staal et al., 1998).

Global setting of Ordovician orogenesis Scandinavian, Greenland, and Svalbard Caledonides Ordovician orogenesis in the Scandinavian Caledonides was, until recently, mainly thought to be represented by an early event that principally took place in the Late Cambrian along the Baltic margin (Finnmarkian; Andréasson et al., 1998; Sturt and Ramsay, 1999; Grenne et al., 1999; Roberts, 2003). However, the structurally highest allochthons, in the Scandinavian Caledonides, which have a Laurentian provenance, have preserved ample evidence for a major Ordovician tectonic event that took place prior to their Silurian (Scandian) docking with the Baltic margin (Yoshinobu et al., 2002; Roberts, 2003). The Ordovician event is kinematically equivalent to the Taconian–Grampian orogeny in the Appalachians and British Caledonides. Surprisingly, no evidence for this event has been preserved in the East Greenland Caledonides, the area from which these rocks presumably were derived. However, the presence of Middle Ordovician blueschist, eclogite, and other evidence of major tectonism in western Svalbard (Dallmeyer et al., 1990; Gee and Page, 1994) and Pearya (M’Clintock orogeny; Trettin, 1987) leave little doubt that the Taconian–Grampian was probably continuous along the entire length of Laurentia’s Iapetan margin north of the Appalachians. The lack of evidence of this event in East Greenland suggests that most Ordovician tectonism was restricted to the peri-Laurentian arc terrane preserved in the Caledonides upper allochthon (Hølanda terrane of Grenne et al., 1999), which is thought to have been present directly outboard of Laurentia at the latitude of Greenland (Fig. 1) during the Ordovician (Grenne et al., 1999). A similar relationship has been observed in Newfoundland, where most Taconian tectonothermal activity was restricted to the peri-Laurentian Notre Dame arc and its basement (the Dashwoods microcontinent of Waldron and van Staal, 2001) and largely absent in the exposed parts of the adjacent passive margin, because most of the underthrusted margin rocks were never fully exhumed (van Staal et al., 2007). The Finnmarkian event is generally thought to be mainly restricted to the latest Cambrian, but age dating revealed that orogenesis continued into the Early–Middle Ordovician in Finnmark in northern Norway (e.g., Rice and Frank, 2003). The Finnmarkian involved arc development, ophiolite obduction, arc accretion, and metamorphism to eclogite facies conditions that principally affected rocks that now reside in the middle and upper allochthons in Norway and Sweden (Gee et al., 1985). The main Finnmarkian event is thought to have been the result of collision of a peri-cratonic arc terrane with the adjacent Baltic margin (Sturt and Roberts, 1991; Grenne et al., 1999; Roberts, 2003). Holtedahl (1920) recognized a slightly younger Early Ordovician event in central Norway and called it the Trondheim event, which probably is related tectonically to Finnmarkian orogenesis. It includes ophiolite emplacement and blueschist and eclogite facies metamorphism (Eide and Lardeaux, 2002; Roberts, 2003). Ophiolites have yielded uranium lead ages of 497–482 Ma (Dunning and Pedersen, 1988; Roberts, 2003), which are similar in age to the Penobscot ophiolites preserved in Newfoundland.

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In general, the close similarities in age and style of orogenesis between the Penobscottian and Finnmarkian, both positioned on the southern side of Iapetus, invite correlation. If correct, Baltica and adjacent Gondwana (Ganderia and parts of Avalonia; Rogers et al., 2006) may have been characterized by a more or less continuous (excluding transform faults) peri-cratonic arc terrane with a length on the order of thousands of kilometers. Thus consistent to what has been observed in the British Caledonides and the northern Appalachians, Ordovician orogenesis also appears to have taken place on both sides of the Iapetus Ocean in the Scandinavian Caledonides. The Baltic–East European craton (Baltica) also converged with Ganderia and Avalonia during closure of the Tornquist Ocean, which separated Baltica from Gondwana, along the former’s southern margin (Valverde-Vaquero et al., 2006) and which led to formation of the Thor suture. The Tornquist closure was completed by the end of the Ordovician (454–443 Ma) and caused the Shelveian orogeny, whose effects on the exposed parts of Baltica (lower plate) appear to be minor (Torsvik and Rehnstrom, 2003). In general, this collision appears to have been relatively soft, with deformation and metamorphism (low grade) mainly restricted to the collsion zone (Trans-European suture zone [TESZ]; see below). Scandian orogenesis in the Scandinavian Caledonides mainly took place during Silurian times. The Scandian was principally caused by the main collision between Laurentia (Greenland) and the Baltic craton. It is coeval and is correlated with the Salinic orogeny (Laurentia-Ganderia collision) in the northern Appalachians (van Staal, 2005; van Staal et al., 2008). Since Baltica and Ganderia were already together by Late Ordovician times as a result of the Shelveian orogeny (see above and Valverde-Vaquero et al., 2006), the Salinic and Scandian orogenies are both dynamically related to collision of Laurentia (upper plate) and the composite craton comprising Baltica and Ganderia (lower plate). Early Paleozoic Orogenic Events in Eurasian Gondwana The orogenic events that involved the assembly of Paleozoic Europe comprised kinematically and temporally diverse, and in part, unrelated events that were scattered throughout the Paleozoic, culminating in the collision that formed Pangea at the end of the Paleozoic. Orogenic events that took place during the Ordovician include the Shelveian orogeny, which formed as a product of convergence among Baltica, Ganderia, and Avalonia (McKerrow et al., 1991; Pharaoh et al., 1993; van Staal et al., 1998; Torsvik and Rehnstrom, 2003; Valverde-Vaquero et al., 2006). The structures related to the Shelveian orogeny are mainly present in the unexposed basement beneath the North Sea and Denmark. Shelveian structures near the Thor suture were incorporated in a poorly understood but complex, long-lived deformation zone that characterizes the TESZ. The TESZ separates the Baltic–East European craton from younger accreted lithosphere of western Europe over a strike length exceeding 2000 km. Accretion-related deformation in the TESZ possibly had already started during the

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Late Cambrian–Early Ordovician (Sandomierz orogenic phase of Belka et al., 2000) owing to accretion of a small peri-Gondwanan crustal block to Baltica. Evidence for Early to Late Ordovician orogenic events (referred to as the Sardic or Eo-Variscan) is also preserved farther west in Europe (e.g., Armorican terrane assemblage) from the Alps to Spain and Portugal (von Raumer, 1998; Handy et al., 1999; Martínez Catalán et al., 2002; Matte, 2002; Robardet, 2002; and references therein). This evidence includes arc magmatism, ophiolite generation, polyphase deformation, and eclogite and high-grade metamorphism, which are generally thought to be related to formation of a short-lived volcanicmagmatic arc terrane(s) and backarc basin(s) fringing the Eurasian margin (Fig. 1) of the Gondwanan supercontinent (Paleozoic Cadomia of Stampfli et al., 2002) and an accretionary event. Ordovician orogenesis here does not represent a major collisional event but more of an amalgamation and/or reattachment of the peri-Gondwanan, ribbon-like arc terrane(s) to the Eurasian margin of Gondwana. According to Stampfli and Borel (2002), the reattached arc terranes were assembled into the long Hun superterrane (Fig. 2), which became the leading edge of the Eurasian margin until it rifted off Gondwana as a coherent ribbon continent during the latest Silurian and started to move north, opening part of the Paleotethys in its wake. Reattachment of the Paleozoic Cadomian arc terrane and closure of the intervening marginal basin during the Early to Middle Ordovician took place after departure of Avalonia and concomitant eastward migration of the trailing Rheic Ocean spreading center to the north of the Cadomian arc terrane. The eastward migration of the Rheic spreading center may have put the arc terrane into compression (Stampfli and Borel, 2002) and caused it to re-accrete. Final accretion of the Hun superterrane to Laurussia took place later during the Variscan (Martínez Catalán et al., 1997, 2007; Matte, 2002). The ribbon terranes fringing the Eurasian margin extended at least as far east as the Himalayan margin of India (Stampfli and Borel, 2002; Cocks and Torsvik, 2002). The latter also preserved evidence for extensive Cambrian–Ordovician orogenesis (e.g., Gehrels et al., 2006). Accretion-reattachment of a peri-Gondwanan ribbon-like terrane to the Himalayan margin of India, possibly kinematically related to coeval events along the European segment of this Gondwanan margin, is also an attractive mechanism for explaining Ordovician orogenesis here. Central Asia Orogenic Belt (Altaids) and the Urals The notion that the Ordovician globally was a time of development of a series of rapidly evolving and amalgamating pericratonic arc terranes, which, when built on continental crust commonly formed ribbon continents, may also be valid for the Central Asia orogenic belt of central Asia and the Uralides. The central Asia orogenic belt and narrow Uralides formed mainly during closing of the Aegir Ocean (Fig. 1) and separated the Siberian (Angara) craton from the Baltic–East European craton to the east and the accreted terranes of the Tethysides to the south. Ordovician orogenesis in the Central Asia orogenic belt

mainly involved closure of marginal basins and assembly of separated fragments of the ribbon-shaped Kipchak arc, according to S¸engör and Natal’in (1996). In addition, collision of part of the Kipchak arc with the Mugodzhar arc-microcontinent, which was positioned outboard off the Baltic–East European craton’s eastern margin and separated from it by the Sakmara-Magnitogorsk marginal basin (Fig. 1), may have induced oroclinal bending and strike-slip imbrication of the Kipchak arc. The Mugodzhar arcmicrocontinent was later re-accreted to the East European craton during the middle–late Paleozoic Variscan orogeny (Puchkov, 1997). Kröner et al. (2007), on the other hand, suggested that the central Asia orogenic belt is an accretionary orogen that formed by across-strike rather than by highly oblique accretionary processes, and the precursor of the Kipchak arc was not a ribbon continent. In addition, detailed structural, petrologic, geochemical, and geochronologic studies in the Ol’khon (Baikal) region in southern Siberia by Fedorovsky et al. (2005) suggest that the central Asia orogenic belt includes an Early Ordovician orogenic event. This event is represented by a polydeformed tectonic collage of ophiolite, arc volcanic, plutonic, and sedimentary rocks, produced during its accretion to the southern margin of the Siberian craton. Accretion was oblique, with strike-slip dominating over dip-slip tectonics. Metamorphic grade ranges from granulite facies against the Siberian craton, with little or no metamorphic overprint on the craton itself, to amphibolite facies to the south. Shortening is indicated by craton-vergent, ductile-brittle Paleozoic folds and thrusts in the adjacent Siberian craton, which formed during accretion of the tectonic collage to the craton. An intriguing element of this part of the orogen consists of linear to blocky bodies of marble associated with metasiliciclastic and volcanic rocks to the northwest that give way to dominance of less deformed and metamorphosed volcanic and plutonic rocks to the south. Famatinian Belt in South America Ordovician orogenesis was mainly localized along the protoAndean margin, and is particularly well preserved in north-central Argentina, adjacent Chile and southern Peru, and Bolivia. This belt probably extends through Ecuador into Colombia. Orogenesis comprises subduction-related magmatism, metamorphism, deformation, and foreland basin development, which are variably referred to as the Famatinian and/or Ocloyic orogenic cycle (Ramos, 1988). During the early Paleozoic the Famatinian belt was an active margin with development of a magmatic arc, which appears to have switched on several occasions from a compressional to an extensional state (e.g., Rapela et al., 1998). Transient compression of the magmatic arc caused localized deformation of arc plutons, which started during the latest Cambrian and/or earliest Ordovician (Pankhurst et al., 1998). Middle Ordovician compression of the Puna’s Famatinian arc-backarc basin system closed the backarc basin and led to concomitant deformation, retroarc–foreland basin development (Bahlburg and Hervé, 1997), and metamorphism in the underlying Mesoproterozoic

Global setting of Ordovician orogenesis crust of the Arequipa-Antefolla terrane (Loewy et al., 2004). Farther south, orogenesis mainly involves accretion of the ribbonlike Precordillera-Cuyania microcontinent, whose provenance remains conjectural, although it is generally regarded as having been derived from Laurentia (Ramos et al., 1986; Astini et al., 1995; Thomas and Astini, 1996). Terra Australis Orogen in Australia and Antarctica Early Paleozoic orogenesis is also evident along the eastern margin of Australia, Tasmania, and the Transantarctic Mountains of Antarctica, which were positioned directly along strike of the proto-Andean margin of South America during that time. All three continents formed part of a very extensive convergentaccretionary margin that nearly circumnavigated the entire length of the Pacific margin of the Gondwana supercontinent (Terra Australis orogen of Cawood, 2005). The first phase of Ordovician orogenesis was represented by the waning stages of the predominantly Cambrian Ross-Delamerian orogen (Foden et al., 2006). However, age dating of the slaty cleavage of the most outboard Robertson Bay terrane (northern Victoria Land) suggests that deformation, at least locally, continued well into the Early Ordovician (Dallmeyer and Wright, 1992). This event was followed by the first phase of the Lachlan orogeny during the Late Ordovician (Foster et al., 1999; Foster and Gray, 2000; Collins, 2002). Both the Ross-Delamerian and Lachlan orogenies involve accretion of peri-cratonic volcanic arcs to the AustralianAntarctic cratons related to closure of the intervening marginal basins. The Ross-Delamerian orogeny may have involved more than one subduction zone (see reviews by Boger and Miller, 2004, and Cawood, 2005, and references therein) and was a tectonically complex event. Ophiolite obduction locally accompanied closure of the marginal basins. The passive margin of the Lachlan backarc basin was built partly on the foundered Delamerian Mountains and hence was filled by a large volume of clastic detritus from this orogen. This clastic wedge was incorporated into a thick accretionary wedge during Late Ordovician–Silurian backarc basin closure. The latter was achieved by subduction of marginal basin crust beneath both the passive and active sides of the basin. The Lachlan orogenic events continued into the middle Paleozoic, around the Devonian-Carboniferous boundary, which led to full development of the Lachlan segment (Foster and Gray, 2000) of the Terra Australis orogen. DISCUSSION AND CONCLUSIONS The evidence for worldwide Ordovician orogenesis clearly indicates that it was a global phenomenon, but in contrast to the Proterozoic Grenvillian, late Paleozoic Appalachian, or Tertiary Alpine-Himalayan orogenic cycles, it did not involve true continent-continent collision(s), and the major continents remained separated by wide oceans. Instead the Ordovician is marked by formation of peri-cratonic arcs of great strike-length along the margins of Laurentia, Baltica, Siberia, and Gondwana;

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linear orogenic belts, several characterized by low-grade metamorphism; marginal basins; suprasubduction ophiolites; ribbon microcontinents; global high-water stands; and black shale deposition. Evidence for formation of true intra-oceanic juvenile arcs starved of continental-derived sediment (such as found in the Pacific today) is rare or absent. The latter is significant because it indicates that subduction was localized along continental margins. In addition, prior to the start of the Ordovician, during the Middle and Late Cambrian (513–490 Ma), the margins of virtually all continental cratons underwent either the onset of orogenesis or initiation of subduction. The latter commonly led to generation of large tracts of suprasubduction zone ophiolite and associated boninite. For example, subduction had initiated between 515 and 510 Ma directly outboard of the Ganderian, Baltic, and Laurentian margins (van Staal et al., 1998, 2007) at a time when the Pacific margins of Antarctica and eastern Australia saw the onset of Ross-Delamerian orogenesis. Early Cambrian Pampean orogenesis along the West Gondwanan margin had started slightly earlier (ca. 530 Ma; e.g., Rapela et al., 1998), but may also have been causally related. Combined, these phenomena suggest a causal link to a global-scale plate reorganization. An attractive mechanism was the Early Cambrian terminal suturing between East and West Gondwana (Boger and Miller, 2004; Cawood, 2005) and final assembly of Gondwana, which undoubtedly must have had a major impact on the Early–Middle Cambrian global plate motion budget. The rapid volume increase of oceanic spreading ridges that probably accompanied the global-scale plate reorganization may also be responsible for the eustatic rise in sea level during the Cambrian. Surprisingly, subduction initiation in the Iapetus appears to have taken place near the passive margins of Laurentia, Ganderia, and Baltica. In the Phanerozoic geological record there are no obvious examples in which passive margins were directly transformed into long-lived, active margins. Margins are generally characterized by old, strong oceanic lithosphere and hence are not favorable sites for subduction initiation (Cloetingh et al., 1982). The locations of nascent subduction zones are generally thought to be controlled by the presence of a zone of weakness, buoyancy, and compressive forces, the latter related to convergence (Mueller and Phillips, 1991, and references therein). The presence of compressive forces is critical, because no combination of a lithosphere-penetrating fault and negative buoyancy forces alone would produce a subduction zone (Hall et al., 2003). For this reason, van Staal et al. (2007) proposed that subduction had nucleated on old transform faults and/or abandoned spreading centers near the Laurentian margin of the Iapetus Ocean, created by an Early Cambrian inboard ridge jump(s), whereas on the opposite side of the Iapetus, near Avalonia and Ganderia, an extensive transform fault system had been created by late ridge-trench collisions during the late Neoproterozoic and Early Cambrian, respectively (Rogers et al., 2006). However, in light of the evidence discussed above and elsewhere (Cawood and Buchan, 2007), the possibility that subduction can initiate along a passive margin should not be ruled out. Erickson (1993)

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suggested that subduction could start near a passive margin, provided that the continent-oceanic interface becomes decoupled as a result of reactivation of old, rifting-related faults during strikeslip or extension. Rollback of downgoing slabs probably was mainly responsible for the great tract of Early Ordovician peri-cratonic arcs and associated marginal basins that were present in the Iapetus, Aegir, and Pacific Oceans. In particular, the propagation of the peri-cratonic arc-backarc systems that were present along the entire length of the Pacific and Iapetan margins of Gondwana (Australia, Antarctica, proto-Andean margin of South America, Ganderia, and Avalonia) and probably also Baltica was very impressive and comparable in scale to the active margins that surrounded the Pacific Ocean during the Mesozoic and Cenozoic. Tensional forces induced by slab rollback probably were also responsible for the terrane dispersal (Avalonia and Ganderia) along Gondwana’s Iapetan margin (e.g. Schultz et al., 2008) and their subsequent motion toward Laurentia (van Staal et al., 1998). Terrane dispersal along the Laurentian margin—e.g., Precordillera (Astini et al., 1995; Thomas and Astini, 1996), several internal southern Appalachian terranes (Merschat et al., 2005; Hatcher and Merschat, 2006), and Dashwoods (Waldron and van Staal, 2001; Cawood et al., 2001)—on the other hand was probably due to an inboard ridge jump from a spreading center positioned close to the Laurentian margin. It calved off ribbonshaped slivers of continental crust from adjacent promontories (van Staal et al., 2007). During the Ordovician, peri-cratonic terranes along the Gondwanan margin underwent several short-lived periods (10– 20 Ma) during which large arc segments switched from extension to compression, commonly leading to closure of the trailing marginal basins or narrow seaways (Zagorevski et al., 2007). The latter appears to be the main cause of Ordovician orogenesis along large segments of the Gondwanan margin (Penobscottian, Famatinian-Ocloyic, Sardic, late Ross-Delamerian, and early Lachlan) and probably also the margin of Baltica (Finnmarkian). Why the stresses along these margins switched from extension to transient compression is poorly constrained and not well understood at present. Collins (2002) proposed that compression was due to short-lived periods of flattening of the downgoing oceanic slab as a result of the entrance of relatively buoyant lithosphere, such as oceanic plateaus, into the subduction zone. Although this model is at first sight appealing and may be applicable to some events, it is difficult to accept that flattening of the circum-Gondwanan subduction zones more or less happened coevally (within a period of 20 m.y.) over a strike length exceeding 10,000 km simply because of arrival of buoyant oceanic crust; neither is there much evidence preserved for accretion of oceanic plateaus during the Early Ordovician along the Pacific and Iapetan margins of Gondwana. Perhaps the switch from extension to compression along the Pacific and Iapetan margins was due to far-field stresses generated by Gondwana’s drift and/ or associated rotation during the Ordovician.

While the Ordovician mobile belts along the Gondwanan margins were dominantly non-accretionary orogens sensu stricto (i.e., little or no accretion of truly exotic terranes, but rather re-accretion of rifted-off arc terranes: the extensional accretionary orogens of Collins, 2002), the Iapetan margin of Laurentia was the recipient of intra-Iapetan and/or Gondwana-derived arc terranes and/or microcontinents (van Staal et al., 1998, 2007; Hibbard, 2000; Hibbard et al., 2007; Merschat et al., 2005; Zagorevski et al., 2006, 2007) and hence was steadily growing in size. This process continued throughout the Paleozoic and basically ended with the formation of Pangea during the Permian. ACKNOWLEDGMENTS We thank Jim Hibbard and Monica Escayola for discussions on aspects of Cambrian–Ordovician orogenesis in the Appalachians and the Famatinian and Pampean mountain belts, respectively. Conall MacNiocaill provided us with the original paleomagnetic diagrams published in van Staal et al. (1998), which became the starting points for Figures 1 and 2. Jim Monger, Peter Cawood, and an anonymous reviewer provided insightful comments, all of which improved the manuscript. RDH acknowledges U.S. National Science Foundation grants EAR-8305832, EAR8417894, EAR-8816343, and EAR-9814800. This is Geological Survey of Canada publication 2006371. REFERENCES CITED Andréasson, P.G., Svenningsen, O.M., and Albrecht, L., 1998, Dawn of Phanerozoic orogeny in the North Atlantic tract: Evidence from the Seve-Kalak superterrane, Scandinavian Caledonides: GFF, v. 120, p. 159–172, doi: 10.1080/11035899801202159. Astini, R.A., Benedetto, J.L., and Vaccari, N.E., 1995, The Early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted and collided terrane: A geodynamic model: Geological Society of America Bulletin, v. 107, p. 253–273, doi: 10.1130/0016-7606(1995)1072.3.CO;2. Bahlburg, H., and Hervé, F., 1997, Geodynamic evolution and tectonostratigraphic terranes of northwestern Argentina and northern Chile: Geological Society of America Bulletin, v. 109, p. 869–884, doi: 10.1130/0016 -7606(1997)1092.3.CO;2. Belka, Z., Ahrendt, H., Franke, W., and Wemmer, K., 2000, The BalticaGondwana suture in central Europe: Evidence from K-Ar ages of detrital muscovites and biogeographical data, in Franke, W., Haak, V., Oncken, O., and Tanner, D., eds., Orogenic Processes: Quantification and Modelling in the Variscan Belt: Geological Society [London] Special Publication 179, p. 87–102. Bergström, S.M., Finney, S.C., Chen, X., Goldman, D., and Leslie, S.A., 2006, Three new Ordovician global stage names: Lethaia, v. 39, p. 287–288, doi: 10.1080/00241160600847439. Boger, S.D., and Miller, J.McL., 2004, Terminal suturing of Gondwana and the onset of the Ross-Delamerian orogeny: The cause and effect of an Early Cambrian reconfiguration of plate motions: Earth and Planetary Science Letters, v. 219, p. 35–48, doi: 10.1016/S0012-821X(03)00692-7. Bream, B.R., 2003, Tectonic implications of geochronology and geochemistry of para- and orthogneisses from the southern Appalachians crystalline core [Ph.D. thesis]: Knoxville, University of Tennessee, 296 p. Cawood, P., 2005, Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic: Earth-Science Reviews, v. 69, p. 249–279, doi: 10.1016/ j.earscirev.2004.09.001.

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Global setting of Ordovician orogenesis Torsvik, T.H., and Cocks, R.M., 2005, Norway in space and time: A centennial cavalcade: Norwegian Journal of Geology, v. 85, p. 91–104. Torsvik, T.H., and Rehnstrom, E.F., 2003, The Tornquist Sea and BalticaAvalonia docking: Tectonophysics, v. 362, p. 67–82, doi: 10.1016/S0040 -1951(02)00631-5. Trettin, H.P., 1987, Pearya: A composite terrane with Caledonian affinities in northern Ellesmere Island: Canadian Journal of Earth Sciences, v. 24, p. 224–245. Valverde-Vaquero, P., van Staal, C.R., McNicoll, V., and Dunning, G., 2006, Middle Ordovician magmatism and metamorphism along the Gander margin in Central Newfoundland: Journal of the Geological Society [London], v. 163, p. 347–362, doi: 10.1144/0016-764904-130. van Staal, C.R., 1994, The Brunswick subduction complex in the Canadian Appalachians: Record of the Late Ordovician to Late Silurian collision between Laurentia and the Gander margin of Avalon: Tectonics, v. 13, p. 946–962, doi: 10.1029/93TC03604. van Staal, C.R., 2005, The northern Appalachians, in Selley, R.C., Cocks, R.L.M., and Plimer, I.R., eds., Encyclopedia of Geology: Oxford, UK, Elsevier, v. 4, p. 81–91. van Staal, C.R., Dewey, J.F., MacNiocaill, C., and McKerrow, W.S., 1998, The Cambrian–Silurian tectonic evolution of the northern Appalachians and British Caledonides: History of a complex, west and southwest Pacifictype segment of Iapetus, in Blundell, D.J., and Scott, A.C., eds., Lyell: The Past Is the Key to the Present: Geological Society [London] Special Publication 143, p. 199–242. van Staal, C.R., Whalen, J.B., McNicoll, V.J., Pehrsson, S., Lissenberg, C.J., Zagorevski, A., van Breemen, O., and Jenner, G.A., 2007, The Notre Dame Arc and the Taconic orogeny in Newfoundland, in Hatcher, R.D., Jr., Carlson, M.P., McBride, J.H., and Martínez Catalán, J.R., eds., The 4-D Framework of Continental Crust: Geological Society of America Memoir 200, p. 511–552, doi: 10.1130/2007.1200(26).

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van Staal, C.R., Currie, K.L., Rowbotham, G., Goodfellow, W., and Rogers, N., 2008, Pressure-temperature paths and exhumation of Late Ordovician-Early Silurian blueschist and associated metamorphic nappes of the Salinic Brunswick subduction complex, northern Appalachians: Geological Society of America Bulletin, v. 120, p. 1455–1477, doi: 10.1130/B26324.1. von Raumer, J.F., 1998, The Palaeozoic evolution in the Alps: From Gondwana to Pangea: Geologische Rundschau, v. 87, p. 407–435, doi: 10.1007/ s005310050219. Waldron, J.W.F., and van Staal, C.R., 2001, Taconian orogeny and the accretion of the Dashwoods Block; a peri-Laurentian microcontinent in the Iapetus ocean: Geology, v. 29, p. 811–814, doi: 10.1130/0091-7613(2001)029 2.0.CO;2. Yoshinobu, A.S., Barnes, C.G., Nordgulen, O., Prestvik, T., Fanning, M., and Pedersen, R.B., 2002, Ordovician magmatism, deformation and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny: Geology, v. 30, p. 883–886, doi: 10.1130/0091-7613(2002)030 2.0.CO;2. Zagorevski, A., Rogers, N., van Staal, C.R., McNicoll, V., Lissenberg, C.J., and Valverde-Vaquero, P., 2006, Lower to Middle Ordovician evolution of peri-Laurentian arc and back-arc complexes in the Iapetus: Constraints from the Annieopsquotch accretionary tract, Central Newfoundland: Geological Society of America Bulletin, v. 118, p. 324–342, doi: 10.1130/ B25775.1. Zagorevski, A., van Staal, C.R., McNicoll, V.C., and Rogers, N., 2007, Upper Cambrian to Upper Ordovician peri-Gondwanan island arc activity in the Victoria Lake Supergroup, Central Newfoundland: Tectonic development of the northern Ganderian margin: American Journal of Science, v. 307, p. 339–370, doi: 10.2475/02.2007.02. MANUSCRIPT ACCEPTED BY THE SOCIETY 6 OCTOBER 2009

Printed in the USA


Ordovician explosive volcanism Warren D. Huff† Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA Stig M. Bergström School of Earth Sciences, Ohio State University, 155 South Oval Mall, Columbus, Ohio 43210, USA Dennis R. Kolata Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820, USA

ABSTRACT Explosive eruptions from volcanoes are recorded in the stratigraphic record throughout the Phanerozoic, but evidence of these eruptions in the form of preserved tephra layers appears to be concentrated at times of active plate collision and concomitant high stands of sea level. The products of volcanic eruptions are lavas, tephra, and gases. Basaltic magmas (low-silica content) are usually erupted in the form of lava flows, whereas rhyolitic magmas (high-silica content) are commonly explosively erupted as plinian and ultraplinian plumes, and associated pyroclastic flows. Fallout tephras are preserved in ancient sedimentary sequences as tonsteins, bentonites, and K-bentonites. Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era. They cover minimally 2.2 × 106 km2 in eastern North America and 6.9 × 105 km2 in central and northwestern Europe as a result of explosive volcanism, which affected both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite in northwestern Europe. Similar successions are well known in South America and China. Sedimentation rates of volcanic ejecta range from meters per year locally to ~1 mm/1000 yr in the deep sea. Volcanogenic sediments react with seawater to produce secondary phases such as zeolites and clay minerals. Studies of recent ashfall behavior suggest that the preservation potential in the stratigraphic record can be viewed as somewhat remarkable in that such sudden events are preserved at all, much less produce such a wealth of valuable geologic information. OVERVIEW

perturb the Earth’s climate on time scales of one to five years, generally resulting in net global surface cooling. Historically documented eruptions do not, however, represent the full range of intensity and magnitude of all explosive eruptions in geologic history. Deposits in the geologic record provide compelling evidence for eruptions that have been orders of magnitude larger than ones witnessed by mankind. Modern studies have shown

Explosive volcanism plays a fundamental role in the exchange of material and energy from the Earth’s interior to the hydrosphere and atmosphere, and major explosive events can †


Huff, W.D., Bergström, S.M., and Kolata, D.R., 2010, Ordovician explosive volcanism, in Finney, S.C., and Berry, W.B.N., eds., The Ordovician Earth System: Geological Society of America Special Paper 466, p. 13–28, doi: 10.1130/2010.2466(02). For permission to copy, contact [email protected]. ©2010 The Geological Society of America. All rights reserved.

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that lithofacies associations of near-vent subaerial phenomena typically include pyroclastic surge deposits, thin welded tuff beds, various lava flow morphologies, abundant erosional unconformities, and fluvial and laharic facies. While volcano-tectonic subsidence might aid in the preservation of such deposits, they generally are not well known in the geologic record. Ash layers in marine sediments, on the other hand, are the best geologic record of explosive volcanism, and many of the Phanerozoic volcanic ash layers represent enormous explosive eruptions that dispersed fine ash and aerosols through the atmosphere over tens of thousands of square kilometers. Most of the world’s southern landmasses were assembled in the supercontinent of Gondwana at the beginning of the Ordovician (Scotese and McKerrow, 1990). As Gondwana drifted gradually toward the South Pole, fragments separated and moved toward the equator (Cocks and Torsvik, 2005). These included Baltica, Siberia, Avalonia, and Laurentia, and much smaller terranes such as the Precordillera of western Argentina and several blocks in south China (Fig. 1). Laurentia and Baltica are estimated to have moved at rates up to 23 cm/yr (Gurnis and Torsvik, 1994). Much of Ordovician explosive volcanism during this time was associated with closure between these various tectonic units. In the Northern Appalachians the timing of the emplacement of the Taconian allochthons and related low-grade metamorphism is constrained by faunal control and isotopic dating, which suggests emplacement ages of latest Middle Ordovician to Late Ordovician (Sasseville et al., 2003). In the southern and central Appalachians the Taconic terrane collided in a non-orthogonal fashion, first in southwest Virginia and then later in southeast Pennsylvania, and consisted of several discrete terranes (Bronson

Hill, etc.) that collided at different times in the Middle Ordovician (Aleinikoff et al., 2003). The northward drift of the Baltoscandian plate and similar activity along the Asian margin of Gondwana produced similar collisions and resulted in dramatic changes in sea level and related climatic and biological changes (Chen and John, 1998; Nordgulen et al., 2003). Each of these tectonic events included subduction-related explosive volcanism, and each has left a record of these eruptions in the form of K-bentonite beds. The Ordovician Period was not necessarily unusual in this respect. Collision-related volcanic ash beds can be found in all periods of the Phanerozoic and well back into Precambrian time. But some of the Ordovician eruptions were exceptionally large and produced massive amounts of fallout ash that, in a contemporary setting, would have been considered disastrous. A major episode of glaciation was centered in Africa during the Late Ordovician that contributed to ecological disruption and mass extinction. Although explosive volcanism was probably not a major contributor to these events, as the extinction events occurred nearly 10 Ma following the largest eruptive events, it no doubt exacerbated already fragile atmospheric conditions and might well have been a minor factor in driving environmental shifts (Rampino and Self, 1992). Miller (1997) examined faunal diversity patterns for the Ordovician and concluded that, while it is tempting to correlate global patterns of biodiversity with global-scale environmental processes, a careful examination of the data argues more strongly for a signal that is, in reality, “an aggregation of patterns and processes that are unique to particular regions or scales” (p. 100). The closing of the Iapetus Ocean, separating Baltica, Avalonia, and Laurentia, occurred by means of the subduction of

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Figure 1. Paleogeographic map for the Late Ordovician (Tychsen and Harper, 2004).

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Ordovician explosive volcanism oceanic crust beneath, and the consequent collision of, volcanically active island arcs or microplates against the southeastern margin of Laurentia (Scotese and McKerrow, 1991). These collisions were associated with the Taconic orogeny, which began during the Middle Ordovician and produced a complex deformational and sedimentological record that has been extensively documented (Rowley and Kidd, 1981; Stanley and Ratcliffe, 1985; Tucker and Robinson, 1990) and which includes numerous K-bentonite beds in both the eastern North American, British, and Baltoscandian successions. Baltica was surrounded by a passive margin during the Middle Ordovician, but it apparently was in close proximity to Laurentia (Cocks and Torsvik, 2005; Huff et al., 1992; McKerrow et al., 1991). Consequently we attribute the origin of the ~150 Middle Ordovician ash beds in southern Sweden, including the Kinnekulle K-bentonite, to be the explosive volcanic activity in the magmatic arcs associated with the Taconic orogeny. The Ordovician record of explosive volcanism consists of examples of both near-vent pyroclastic flows and ignimbrites and distal sequences of altered fallout tephras known as K-bentonites. Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era (Huff et al., 1996). They cover minimally 2.2 × 106 km2 in eastern North America and 6.9 × 10 5 km2 in northwestern Europe as a result of explosive volcanism that affected both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite in northwestern Europe. The source vents are thought to have been near the Laurentian margin of Iapetus on an arc or microplate undergoing collision with Laurentia. The volume of ash preserved in the stratigraphic record, converted to dense rock equivalent (DRE) of silicic magma, is minimally estimated to be 943 km3 for the Deicke, 1509 km3 for the Millbrig, and 972 km3 for the Kinnekulle (Huff et al., 1996). One means of expressing the magnitude of an eruption is by use of the volcanic explosivity index (VEI). The VEI was proposed in 1982 as a way to describe the relative size or magnitude of explosive volcanic eruptions (Newhall and Self, 1982). It is a 0-to-8 index of increasing explosivity, and each successive increase in number represents an increase of about a factor of ten. In terms of the VEI these eruptions would probably be ranked between 7 and 8. The Millbrig and Kinnekulle beds are coeval and possibly equivalent, yielding a combined DRE volume of nearly 2500 km3. Some unknown but probably large amount of additional ash fell into oceanic regions of the Iapetus, but these areas became subducted, and the ash is not preserved in the geologic record. The symmetry of the thickness contours (Huff et al., 1992) is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast, in terms of Ordovician paleogeography. Well-documented examples of near-vent facies are relatively few, however, and are briefly summarized below. Therefore, the primary focus here will be on the

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stratigraphic record of distal ashes and their application to stratigraphic, tectonic, and paleogeographic reconstructions. PROXIMAL FACIES Some of the most carefully documented examples of Ordovician near-vent facies have been described in North Wales (Branney and Kokelaar, 1992; Kokelaar et al., 1984; Kokelaar and Königer, 2000), where units such as the Caradocian Pitts Head Tuff represent episodes of felsic arc magmatism. An Early Ordovician episode of subaerial arc magmatism was, in turn, succeeded by extension-related marine-marginal-basin magmatism that produced thick intercalations of volcanic rocks and coeval intrusions (Kokelaar, 1986). Two main subsequent cycles of Ordovician volcanism are distinguished within the Late Ordovician successions, the Llewelyn Volcanic Group and the Snowdon Volcanic Group. In the English Lake District the onset of volcanism occurred in the Late Ordovician and produced the Eycott and Borrowdale Volcanic Groups (Fitton et al., 1982). The Borrowdale Volcanic Group of northwest England represents part of a Late Ordovician ensialic arc related to subduction beneath the Avalonian margin of the Iapetus Ocean during late stages of closure between the continents of Laurentia and Avalonia (Branney, 1991; Millward and Evans, 2003; Millward et al., 1978). The volcanic succession includes subaerially erupted calc-alkaline basaltic, andesitic, and rhyolitic lavas, and pyroclastics with continental margin, subduction-related geochemical affinities. A similar succession known as the Komarov Volcanic Complex (Darriwilian) occurs in the Prague Basin (Chlupácˇ et al., 1998). In the eastern Lachlan Fold Belt in New South Wales, mantlederived shoshonitic magmatism commenced in the Early Ordovician and was replaced by tholeiitic magmatism by Late Ordovician time (Wyborn, 1992). The favored model for the development of the shoshonites is from melting of subcontinental lithosphere that was previously enriched in incompatible elements (notably Ba, K, Sr, and P). The prior enrichment is thought to have taken place by subduction processes in the Cambrian, when basement blocks defined by granite studies were being assembled (Gray and Foster, 2004; Keay et al., 1997). In this scenario, melting in the Ordovician subcontinental lithosphere was triggered by overturning associated with asthenospheric upwelling. Subsequent Silurian and Early Devonian volcanism became strongly felsic as a consequence of the increasing involvement of crustal melting in magma generation. In North America the Late Ordovician Ammonoosuc Volcanics and overlying Partridge Formation have been interpreted to be Ordovician island arc volcanics formed during the Taconic orogeny and emplaced as part of the Bronson Hill Anticlinorium (Hollocher, 1993; Karabinos et al., 1998; Leo, 1985; Schumacher, 1988). These units contain substantial quantities of metamorphosed bimodal mafic and felsic volcanics, including an array of pyroclastics, that have been dated at 453–449 Ma (Tucker and Robinson, 1990). They are thought to have been built on a

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margin of Avalonia that collided with the Laurentian margin in Late Ordovician or Early Silurian time (McKerrow et al., 1991). Although these rocks are of the same general age as several widespread distal airfall tephras, now altered to K-bentonites, in the eastern Midcontinent, no correlations have been made that would suggest that the Bronson Hill was a source area for these ashes. DISTAL FACIES Explosive eruptions, particularly those involving magmawater interaction and/or production of large ignimbrites, can generate huge volumes of mostly ash-grade tephra that is widely transported in the atmosphere (Walker, 1981). This material eventually settles out to form distinctive layers that can be correlated across thousands to millions of square kilometers and spanning different depositional environments to form chronostratigraphic markers (Bergström et al., 1995; Drexler et al., 1980; Huff et al., 1996; Rose and Chesner, 1987). Such tephra layers provide information on source magma composition, eruption dynamics, and chronology as deduced from mineral chemistry, granulometric properties, and thickness and distribution patterns (Fredlund et al., 2000; Ninkovich et al., 1978). Finer material (ash 5 cms-1

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Figure 2. Paleogeography of the early Late Ordovician (after Scotese and McKerrow, 1990, 1991). Light gray indicates permanently exposed land areas, and flooded areas are outlined with black lines. LA—Laurentia; BA—Baltica; SA—South America; SCE—South Central Europe; SI—Siberia; CH—China; AU—Australia; I—Iapetus; Gond—Gondwana. Note incursion of colder water masses from higher southern latitudes deflected northward toward the southern margin of Laurentia. Simulation was performed with pCO2 level of 15× preindustrial atmospheric levels (PAL) of 280 ppm. Note that 15× PAL is within the proposed range of atmospheric pCO2 during the Late Ordovician (Berner, 1994; Berner and Kothavala, 2001; Yapp and Poths, 1992). Surface ocean circulation patterns of Herrmann et al. (2004a) are indicated by arrows.

atmospheric general circulation model coupled to a 50 m thick slab ocean (Herrmann et al., 2004b). The GENESIS output is averaged (weighted average for monthly output) over the last ten years of model simulation. These surface boundary conditions, which are a function of longitude and latitude, but independent of time, are interpolated from the GENESIS model grid to the MOM grid. The model is run without flux corrections—there are no direct (paleo) observations available—from a state of rest, and no separate spin-up of the ocean is required. Using given prescribed boundary conditions, the three-dimensional (3-D) model ocean adjusts to this particular forcing and produces a unique distribution of salinity, temperature, and velocity. The model is run for an integration period of 10,000 yr. We use built-in model diagnostics, which are large-scale integral quantities like the global energy cycle, gyre components, meridional overturning, and tracer budgets and fluxes to ensure that a steady-state solution is reached. Table 1 shows main surface boundary conditions for the numerical model. Additional details of both model setups and experiments are described in greater detail in Herrmann et al.

(2004a, 2004b). The model results presented here focus on the regional conditions across Laurentia. RESULTS Our numerical model experiments indicate that the Late Ordovician surface ocean was characterized predominantly by zonal circulation patterns in the Northern Hemisphere as a result of zonal wind patterns and a lack of continental barriers. The location of most continents in the Southern Hemisphere obstructed zonal currents and led to the development of large gyre systems in the southern ocean basins between the continents. The gyre system between the eastern margin of Gondwana and Laurentia was characterized by a strong western boundary current along the eastern margin of Gondwana, which transported warm equatorial water masses southward toward higher southern latitudes. Furthermore, surface water masses with lower temperatures from the high latitudes reached subtropical latitudes in the Iapetus region along the southeastern margin of Laurentia (Fig. 2).

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Herrmann and Haupt TABLE 1. SURFACE BOUNDARY CONDITIONS FOR NUMERICAL MODEL Boundar y conditions Atmospheric CO2 4200 ppm (15× preindustrial level) Other trace gas concentrations Methane: 0.65 ppm; N2O: 0.275 ppm; CFCs: 0 ppm Land-sea distribution Paleogeography of Scotese and McKerrow (1991) Orbital parameters Eccentricity: 0.06; obliquity: 22.0; precession 270, perihelion to N.H. vernal equinox Solar luminosity 95.5% of present-day value Note: For additional details on model setup and boundary conditions, see Herrmann et al. (2004a, 2004b).

The global ocean of the early Late Ordovician is typified by warm surface waters and cold bottom waters (Fig. 3A and 3B). The thermal structure is caused by strong vertical downwelling of water masses in the higher southern latitudes (Fig. 4). A less significant downwelling occurs in the high latitudes of the Northern Hemisphere because of fresher (less saline) and thus less dense waters compared with the high latitudes in the Southern Hemisphere. The strong deepwater formation in the Southern Hemisphere led to a predominantly zonal distribution of temperature and salinity, especially in the Northern Hemisphere. Deepwater masses below 2000 m had a temperature range of ~4–5 °C (Fig. 3A). On the shallow epicontinental sea of Laurentia, temperatures were above 20 °C (Fig. 3B). The water temperatures in the deeper ocean basins surrounding Laurentia decreased with depth to ~10 °C at a depth of ~1000 m. DISCUSSION The lithological and geochemical changes in the Mohawkian of Laurentia include (1) a shift from tropical-type to temperatetype carbonates, (2) an increase in fine-grained siliciclastics, (3) an increase in the amount and distribution of phosphatic sediments, and (4) a positive shift in the δ13C of carbonates and organics (Lavoie and Asselin, 1998; Patzkowsky and Holland, 1996; Patzkowsky et al., 1997; Pope and Read, 1997; Saylor et al., 1997). Global interpretations for this transition suggest that the lithologic and oceanographic changes in North America are the result of the onset of icehouse conditions that eventually led to a glaciation in the latest Ordovician (Lavoie and Asselin, 1998; Pope and Read, 1997). Kolata et al. (2001) identified the Sebree Trough as a potential passageway through which cold water from the southern (margin) opening could penetrate deeply into the shallow epicontinental sea of Laurentia. The presence of cold, open-ocean water masses close to the vicinity of the Sebree Trough was subsequently demonstrated by Pope and Steffen (2003). Pope and Read (1997) had speculated previously about the presence of cold-water masses along the southern margin of Laurentia that were carried into lower latitudes. Our simulations of the global ocean circulation support the hypothesis that cold ocean-water masses must have reached the southern margin of Laurentia during the Late Ordovician (Fig. 2). Using pCO2 values as high as 15× the preindustrial level, our modeled climate scenario indicates that the southern part of

the epicontinental sea, where the cooling event of the TurinianChatfieldian has been identified, had surface waters with temperatures below 28 °C. Temperatures above 28 °C occur only much farther north, closer to the equator. The average marine tropical seawater temperature required for precipitation of most carbonate particles ranges from 28 °C to 30 °C (James, 1997; Lees, 1975; Lees and Buller, 1972). At these temperatures, abiotic precipitation of carbonate is possible, and both pellets and ooid/aggregate grains can form. At the lower temperatures predicted by our modeling, carbonate precipitation rates would have been greatly reduced, consistent with the observed lithological and faunal changes in the Late Ordovician. These changes therefore could have been the result of the onset of the global cooling trend. Our numerical model results show that the global ocean was characterized by cold, deep ocean-water masses of 4 °C, with warmer water near the surface. This is in contrast to geochemical data from the Taconic foreland basin, which suggest that the epicontinental ocean was inversely stratified (Railsback et al., 1990). Based on paleoecological studies (Cisne et al., 1982; Cisne and Rabe, 1978), Railsback et al. (1990) suggested that along a depth transect across the eastern margin of Laurentia, warmer, saline bottom waters underlay colder, brackish surface waters. Two alternative explanations can be used to resolve this disagreement. First, the observed geochemical signal from the Taconic foredeep might be related to the outflow of saline waters from the extensive epicontinental shelf of Laurentia, similar to present-day outflow of warm saline waters from the Mediterranean Sea into the Atlantic Ocean (e.g., Johnson, 1997) and from the Persian Gulf into the Indian Ocean (e.g., Pratt et al., 1999). Second, studies by Railsback et al. (1990) and Ackerly et al. (1993) focused on the brachiopod genus Paucicrura. Ackerly et al. (1993) correlate the number of punctae in Paucicrura rogata with water depth and temperature, and showed that water temperature increased with depth. However, dysoxic conditions within the foreland basin may have contributed to reduced growth rates of brachiopods. Dysoxic conditions would have increased the packing density of punctae per unit area, and therefore packing densities would not have been a good predictor for any depthdependent temperature variations. The cold-water conditions would have existed in the epicontinental sea of Laurentia despite high pCO2 values of 15× preindustrial levels. Using an atmospheric general circulation model, Herrmann et al. (2003) showed that at those pCO2 levels,

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accompanied by a high sea level, no ice sheets will form. This indicates that despite the predominance of cold-water masses within the low-latitude shallow epicontinental sea and global cooling, high-latitudinal ice sheets are not necessary (Tobin et al., 2005). Until unambiguous glacial deposits are found, the early Late Ordovician should be referred to as a transition from hothouse to icehouse, rather than the onset of glaciation. CONCLUSIONS Although Laurentia was situated in tropical to subtropical latitudes during the Late Ordovician, lithological evidence indicates that part of this time period was characterized by deposition under cool-water conditions. Whereas Turinian strata of eastern North America contain abundant warm-water carbonates, the Lower Chatfieldian marks a shift to cool-water carbonates, an increase in siliciclastic influx, and phosphatic sediments. Numerical modeling results show that global cooling had a major impact on sea surface temperatures in the shallow epicontinental sea of Laurentia during the Late Ordovician. The presence of cold-water masses that originated in the higher latitudes and were brought toward the southern and eastern margins of Laurentia could have led to the observed stratigraphic changes. Our current global climate model does not include a turbidity function, and this alternative hypothesis cannot be directly tested with our model setup. Future modeling studies therefore need to extend the modeling approach by using high-resolution regional sediment-transport models that include turbidity functions. ACKNOWLEDGMENTS This paper was much improved thanks to the careful reviews by M. Saltzman and M. Pope, and to the editorial work of S. Finney. L. Wasylenki provided useful comments that greatly improved this manuscript. ADH expresses his gratitude for financial support from The Sol and Esther Drescher Faculty Development Grant. REFERENCES CITED Ackerly, S.C., Cisne, J.L., Railsback, L.B., and Anderson, T.L., 1993, Punctal density in the Ordovician orthide brachiopod Paucicrura rogata: Anatomical and paleoenvironmental variation: Lethaia, v. 26, p. 17–24, doi: 10.1111/j.1502-3931.1993.tb01506.x. Barnes, C.R., 2004, Ordovician oceans and climate, in Webby, B.D., Paris, F., Droser, M.L., and Percival, I.G., eds., The Great Ordovician Biodiversification Event: New York, Columbia University Press, p. 72–76. Beauchamp, B., and Desrochers, A., 1997, Permian warm- to very cold-water carbonates and cherts in northwest Pangea, in James, N.P., and Clarke, J.A.D., eds., Cool-Water Carbonates, v. 56: Tulsa, SEPM (Society for Sedimentary Geology), p. 327–347. Bergström, S.M., Finney, S.C., Xu, C., Goldman, D., and Leslie, S.A., 2006, Three new Ordovician global stage names: Lethaia, v. 39, p. 287–288, doi: 10.1080/00241160600847439. Berner, R.A., 1994, GEOCARB II: A revised model of atmospheric CO2 over Phanerozoic time: American Journal of Science, v. 294, p. 56–91. Berner, R.A., and Kothavala, Z., 2001, Geocarb III: A revised model of atmospheric CO2 over Phanerozoic time: American Journal of Science, v. 301, p. 182–204, doi: 10.2475/ajs.301.2.182.

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The Upper Ordovician Guttenberg δ 13C excursion (GICE) in North America and Baltoscandia: Occurrence, chronostratigraphic significance, and paleoenvironmental relationships Stig M. Bergström† School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, USA Birger Schmitz Department of Geology, GeoBiosphere Science Centre, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden Matthew R. Saltzman School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, USA Warren D. Huff Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA

ABSTRACT Two prominent, and apparently globally distributed, δ13C excursions have been documented from the Upper Ordovician, namely the early Katian Guttenberg isotope carbon excursion (GICE) and the latest Ordovician Hirnantian isotope carbon excursion (HICE). The former excursion, which has lower δ13C values than the HICE, is now recorded from dozens of localities in North America and Baltoscandia, and it appears to be present also in China. In North America the GICE ranges from the uppermost Phragmodus undatus Midcontinent Conodont Zone to near the top of the Plectodina tenuis Midcontinent Conodont Zone, an interval corresponding to the lower part of the Diplacanthograptus caudatus Global Graptolite Zone. The base of the GICE lies somewhat above the Millbrig K-bentonite. In Baltoscandia the GICE occurs in the upper Diplograptus foliaceus through the lower Dicranograptus clingani Graptolite Zones, and in the upper Amorphognathus tvaerensis Conodont Zone. Its base is a few meters above the widespread Kinnekulle K-bentonite. In Baltoscandia and in Oklahoma the GICE ranges through a part of the Spinachitina cervicornis Chitinozoan Zone. In North America the GICE is regionally in a transgressive-regressive succession. The bathymetric conditions in the GICE interval in Baltoscandia were somewhat complex and have been the subject of different interpretations, but there is no obvious correlation between the GICE and apparent sea level changes. A review of the relations between the GICE and potential climatic and water temperature

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Bergström, S.M., Schmitz, B., Saltzman, M.R., and Huff, W.D., 2010, The Upper Ordovician Guttenberg δ13C excursion (GICE) in North America and Baltoscandia: Occurrence, chronostratigraphic significance, and paleoenvironmental relationships, in Finney, S.C., and Berry, W.B.N., eds., The Ordovician Earth System: Geological Society of America Special Paper 466, p. 37–67, doi: 10.1130/2010.2466(04). For permission to copy, contact [email protected]. ©2010 The Geological Society of America. All rights reserved.

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Bergström et al. indicators, such as lithofacies, faunas, and 18O geochemistry, does not suggest a close correlation to specific environmental conditions. The cause of formation of the GICE is enigmatic, but there is no direct evidence that it was coeval with a period of extensive glaciation in the Gondwana. The GICE is a powerful chemostratigraphic tool that is useful for detailed local and even transatlantic correlations.

INTRODUCTION Conspicuous, geographically widespread variations of δ13C values in the stratigraphic record have in recent years proved to have great utility as tools in chemostratigraphic research. Such changes, known as excursions, frequently provide new insights into stratigraphic relationships at both the local and regional scales. In the case of the Ordovician, two major, apparently global, positive δ13C excursions are currently recognized, namely the early Late Ordovician Guttenberg isotope carbon excursion (GICE) and the latest Late Ordovician Hirnantian isotope carbon excursion (HICE). Although additional Katian excursions have been recognized in both North America (Ludvigson et al., 2004) and Europe (Kaljo et al., 2004), these are mostly of smaller magnitude and are as yet not documented outside these regions. The HICE, which is a prominent positive excursion, has been studied more intensely than the GICE and is documented from North America, the British Isles, China, South America, and Baltoscandia (Bergström et al., 2006). It has been widely employed in Hirnantian chronostratigraphy. The GICE, which is a somewhat less conspicuous, but still prominent, excursion in the Ordovician δ13C curve, has so far been recorded from a substantial number of sites in North America and northern Europe, and it also occurs in China (Bergström et al., 2009). The literature on the HICE has expanded rapidly during the last decade and is considerably more extensive than that dealing with the GICE. A recent paper (Bergström et al., 2006) reviewed several aspects of the HICE, especially its global chemostratigraphic significance and its putative relations to the Gondwana glaciation. The purpose of the present paper is to summarize pertinent data on the GICE from North America and Europe, including new information from sections in Sweden, and assess its local and transatlantic chemostratigraphic significance. This review, the first to deal in some detail with both North American and European records of the GICE, also includes brief discussions of its relations to depositional environments (especially bathymetric conditions) and possible glacial periods as well as proposed hypotheses to explain its formation. DISCOVERY AND CONCEPT OF THE GICE The discovery of the GICE dates back to the mid-1980s, when Hatch et al. (1987) found elevated δ13C values in oils and rock samples from nine drill cores from Iowa and Kansas. Samples from the Guttenberg Member of the Decorah Formation (Fig. 1) showed heavy δ13C values of up to +4‰ from baseline values

ranging from –3‰ to +0.5‰. This isotope excursion was in an interval a few meters thick well above the Millbrig K-bentonite in a highly condensed section. Ludvigson et al. (1996) presented a large amount of information in a comprehensive sedimentological, chemical, and petrographic study. Later, Ludvigson et al. (2000a) published additional important δ13C data based on rock samples from three outcrops through the same stratigraphic interval in Iowa, Wisconsin, and Missouri. Further information was provided by Ludvigson et al. (1990, 2000b, 2001a, 2001b) and Smith et al. (2000). Finally, Ludvigson et al. (2004) published a comprehensive chemostratigraphic and lithologic investigation of the excursion interval in which they formally labeled the positive excursion just above the Elkport K-bentonite within the Guttenberg Member of the Chatfieldian Decorah Formation as the Guttenberg excursion. This concept of the Guttenberg excursion (GICE) has been followed herein and is the same as that used by several other authors such as Saltzman et al. (2001, 2003a, 2003b), Bergström et al. (2001, 2004), Young et al. (2003a, 2003b, 2004a, 2004b, 2005) and Barta et al. (2003, 2004). It is important to recognize this concept of the GICE because Ludvigson et al. (2004) distinguished and named four older, apparently not global and hence regionally less significant, excursions in the succession just below the GICE in eastern Iowa. These should not be confused with the GICE, and they are apparently more local in their distribution than the GICE, as will be discussed below. Likewise, Kaljo et al. (2004) and Bergström et al. (2007) recently named several excursions above the GICE in the Estonian and North American successions, respectively. GICE IN NORTH AMERICA Since the initial discovery of the GICE, this excursion has been recorded at many localities in more than a dozen states and provinces in North America (Fig. 2). It has been particularly intensely studied in the Upper Mississippi Valley, but its currently known distribution area includes a large part of the North American Midcontinent as well as the Appalachians and the Great Basin in Nevada. In all probability, future work will show that it is present in all parts of the continent where rocks of suitable age and facies are present. Format restrictions of the present paper do not permit discussion of all North American GICE records, and we have therefore selected for review only particularly important occurrences, generally 1–3 for each outcrop region. Each of these records is discussed separately below and illustrated by the pertinent δ13C curve. For a general classification of the North American stratigraphic units discussed, see Figure 3. It should be noted

Upper Ordovician Guttenberg δ13C excursion (GICE) in North America and Baltoscandia

Figure 1. GICE curve in the Guttenberg Member of the Decorah Formation in the Cominco Millbrook Farms SS-9 drill core from eastern Iowa, one of the sections in which the GICE was first recorded (Hatch et al., 1987). The δ13C curve is based on Pancost et al. (1999), as interpreted by Ludvigson et al. (2004). Note the GICE peak of ~+1.5‰ in the Guttenberg Member as compared with δ13C baseline values of ~ –1‰ to 0‰ in the Platteville and Dunleith Formations. Also note that the Deicke and Millbrig K-bentonites are not present in this core, apparently because this interval is not preserved at the PlattevilleDecorah unconformity of Ludvigson et al. (2004, Fig. 5).

that for series classification we use the recently approved global standard series in which the provincial North American Mohawkian Series is referred to the global Upper Ordovician Series. Upper Mississippi Valley To date, the GICE has been most intensely studied in the Upper Mississippi Valley, a region that includes the important Upper Ordovician outcrop areas in Iowa, Wisconsin, Illinois, Minnesota, and Missouri. More than a dozen GICE sites have been recorded, and major references include Hatch et al. (1987), Hasiuk et al. (2001), Ludvigson et al. (1996, 2000a, 2000b, 2001a,

39

2001b, 2004), Smith et al. (2000), Patzkowsky et al. (1997), Pancost et al. (1998, 1999), Simo et al. (2003), Saltzman et al. (2003a, 2003b), and Witzke et al. (2000). A typical GICE curve from this region is illustrated in Figure 4. As recently described in detail by Ludvigson et al. (2004), the GICE occurs in a stratigraphic interval characterized by much condensation and the presence of significant stratigraphic gaps that locally may include the excursion interval. When present, the GICE interval is generally +2‰. In the next 50 m there are numerous fluctuations in δ13C values between +1‰ and near +3‰, with average values of ~+2‰. The curve, based on samples from the uppermost Trenton Limestone, above the covered interval, shows a notable decline down to values of 50 m thick in the Kullsberg mound facies, hence some 10 times as thick as at Fjäcka that represents the inter-mound facies. However, the end of the excursion is currently poorly established in the mound facies, because at Skålberget there is a stratigraphic gap at the top of the mound lens. However, based on conodonts, and especially the important species Hamarodus europaeus, the Skålberg Limestone that overlies the Kullsberg Limestone correlates with the post-GICE part of the Moldå Formation at Fjäcka, and the end of the GICE therefore may be at the top of the Kullsberg Limestone. Smedsby Gård, Östergötland The well drilled by the Geological Survey of Sweden in 1946 at Smedsby Gård near the town of Motala (Fig. 13) penetrated the GICE interval as well as older and younger strata (Jaanusson, 1962; Wikman et al., 1982). Because the succession corresponding to the Skagen and Moldå Formations is uniform lithologically in this region, Jaanusson (1982) introduced the

Kinnekulle K-bentonite

-6

-8

0

1

2

13

C

carb.

Amorphognathus complicatus Hamarodus europaeus

80

85

Kinnekulle K-benonite

Baltoniodus alobatus

-4

SLANDROM LS.

?

Amorphognathus tvaerensis B. alobatus not named

-2

Amorphognathus tvaerensis


SKAGEN FM.

Core missing

2

0

DALBY FM.

75

GICE

?

m

74

FREBERGA FM.

4

53

SMEDSBY GÅRD CORE

DALBY FM.

6

Am. superbus

Amorphognathus complicatus Hamarodus europaeus

Depth m

GICE

MOLDÅ FM.

SLANDROM LS.

FJÄCKA

CONODONT ZONE CONODONT SUBZONE


90

0

1

2

13

C

carb.

Figure 17. Comparison of the GICE in the Fjäcka and Smedsby Gård sections, based on data from the present study. Note that the baseline value of the δ13C curve is ~+1‰, and the GICE peak value approaches +2‰ in both successions.

term Freberga Formation for this interval, and this designation is used herein. Although Wikman et al. (1982) briefly referred to the Freberga Formation portion of the core, this interval has not yet been described in any detail. In the lower 5 m of the Freberga Formation the δ13C values are between 0‰ and +1‰ (Fig. 20), which are considered baseline values. At about the 83 m core depth, the values show a distinct increase to nearly +2‰, and we regard this as the beginning of the GICE. For a stratigraphic thickness of ~5 m, the δ13C values range between +1‰ and +2‰ before decreasing to baseline values of ~1‰ at 78–79 m core depth, which marks the end of the GICE. As seen in Figure 17, the GICE curve is closely similar to the Fjäcka curve, and the excursion has a similar thickness. Also the biostratigraphy of the Freberga Formation resembles closely that of the Skagen and Moldå Formations at Fjäcka. The uppermost part of the Dalby Formation contains conodonts of the Baltoniodus alobatus Subzone of the Amorphognathus tvaerensis Zone. As in the Fjäcka section, the conodonts Hamarodus europaeus and Amorphognathus complicatus appear just after

the end of the GICE in the upper part of the Freberga Formation at virtually the same level as in the Fjäcka section. The Freberga Formation is slightly thicker (~13.3 m) compared with ~10 m of the combined thickness of the Skagen and Moldå Formations at Fjäcka, but the δ13C chemostratigraphy and biostratigraphy are remarkably similar. GICE IN ESTONIA AND LATVIA An exceptionally large amount of δ13C work has been carried out in recent years in the Ordovician of eastern Baltoscandia, and especially in Estonia (Ainsaar et al., 2004a, 2004b; Kaljo et al., 1999, 2004). In fact, the stratigraphical and regional δ13C variations in that system are better known in Estonia that in any other region of similar size in the world. Furthermore, biostratigraphy, lithofacies, faunas, and lithostratigraphy have been investigated in great detail in both outcrops and many drill cores. For a useful general summary of non-isotopic information, see Raukas and Teedumäe (1997).

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Bergström et al.

N. ENTRANCE

SKÅLBERGET & AMTJÄRN

N

m SKÅLB. KULLS

50

Fjäcka Sh.

Boda Lst. Tunnel section

30 25 20 15 10 5 0

S. entrance section 0

covered

30

AMTJÄRN SOUTH

sampled section

5

DALBY FM.

m

Figure 18. Sketch map of the Skålberget locality (after Jaanusson, 1982), which is now a nature preserve. Our sampled section through the 50+-m-thick Kullsberg Limestone at the south margin of the quarry is marked by a thick black line.

GICE

Sla nd rom

Kullsberg Lst.

N. entrance section

35

SKÅLBERGET SOUTH

water-filled pit

KULLSBERG LIMESTONE

40

Skålberg Lst. Lst .

no ou tcr op s

45

0

covered

0

1

2

3

13

C

carb.

The Upper Ordovician succession in eastern Baltoscandia includes mostly calcareous epicontinental rocks deposited in shallow-water environments at temperate to subtropical latitudes. By and large, water depth increased in a southern direction in terms of present latitudes, with the shallowest environments being in northern Estonia and somewhat deeper waters in southern Estonia and Latvia. Although the Upper Ordovician succession in both northern and southern Estonia has several stratigraphic gaps (Nõlvak and Grahn, 1993; Ainsaar et al., 2004a; Kaljo et al., 2004, Fig. 2), these tend to be relatively minor, and as a whole the portion of the succession pertinent to the present study is remarkably complete stratigraphically. A significant δ13C excursion in the upper Keila and Oandu Stages, referred to as the mid-Caradoc excursion by Kaljo et al. (2004), is here identified as the GICE on the basis of biostratigraphic and other evidence. The report by Ainsaar et al. (1999) is apparently the first to compare this Estonian excursion with the Guttenberg excursion (GICE) in North America. As indicated by drill core investigations, the excursion is best developed in southern Estonia and Latvia. This may at least partly

Figure 19. Composite δ13C curve, based on our Skålberget and Amtjärn sections. The baseline δ13C values are likely to be ~+1‰, and the GICE peak values through the Kullsberg Limestone are extraordinarily uniform, being mostly between +2‰ and +2.5‰. Note that the top of the GICE is not yet well established at Skålberget, but the topmost part of the GICE curve may well be cut out by the unconformity at the top surface of the Kullsberg Limestone mound.

reflect the widespread stratigraphic gap at the Keila-Oandu contact that may cut out parts or all of the upper Keila GICE interval in many sections in northern Estonia. For southern Estonia, instructive excursion curves have been published from, for instance, the Ristiküla-174 (Meidla et al., 1999; Ainsaar et al., 2004a), Viljandi (Kaljo et al., 2004), Männamaa F-367 (Ainsaar et al., 2004a), Mehikoorma-421 (Martma, 2005), and Valga–10 (Ainsaar et al., 2004a) drill cores. In Latvia, the excursion is present in the Kandava (Brenchley et al., 1996) and Jurmala (Ainsaar et al., 2004a) drill cores. Because of its stratigraphic completeness and informative δ13C curve, we select the


74 75 Core missing

406.7

KEILA

80

GICE

402.5

GICE

FREBERGA FM.

401.1

85

90

0

1 C

13

2

carb.

424.6

Kinnekulle K-b.

0

HALJALA

Kinnekulle K-benonite


DALBY FM.

RAKVERE

Depth m

OANDU

m

NABALA ESTONIAN STAGES

RISTIKÜLA 174 CORE, SOUTHERN ESTONIA Amorphognathus complicatus Hamarodus europaeus

SLANDROM LS.

SMEDSBY GÅRD CORE

55

1 C

2

13

carb.

Figure 20. Comparison between the GICE in the Smedsby Gård, Sweden, and the Ristiküla, southern Estonia (Ainsaar et al., 1999), drill cores. Note the close similarity of the δ13C curves, despite the fact that these sites are more than 600 km apart. In both successions the baseline values are between 0‰ and 1‰, and the GICE curve peak values are near 2‰. Based on chemostratigraphic correlation, the base of the Oandu Stage would correspond to a level at ~79 m in the Smedsby Gård drill core.

Ristiküla-174 drill core as an instructive illustration of the GICE in eastern Baltoscandia. Ristiküla-174 This core was drilled by the Estonian Geological Survey and was studied by Ainsaar et al. (1996, 1999), who provided extensive faunal and lithologic data. In this drill core an 8–9-m-thick interval above the Kinnekulle K-bentonite, which represents the lower half of the Keila Stage, shows baseline δ13C values between 0‰ and +1‰, with most values being ~+0.5‰ (Fig. 20). Slightly higher stratigraphically there is a conspicuous rise in the δ13C curve to values near +2‰, which mark the beginning of the GICE. These high values persist for the next few meters before the δ13C curve drops to values between +1‰

and +1.5‰; these are followed, slightly higher up, by even lighter values of ~0‰, which are taken as marking the end of the GICE. As a whole, the Ristiküla δ13C curve shows a remarkably close similarity to that of the Smedsby Gård, allowing a close correlation between these successions (Fig. 20). There is also an obvious similarity between the Estonian curve and the Fjäcka one (Fig. 17). This close chemostratigraphic correlation is of special stratigraphic interest, because in this stratigraphic interval the precise relations between the Estonian standard regional stages and the Scandinavian succession have long been uncertain and even controversial (see, e.g., Jaanusson, 1976). Assuming that no significant gaps occur in the Keila-Rakvere succession in the Ristiküla drill core, the chemostratigraphic data suggest that the base of the Oandu Stage corresponds to a level 1–2 m above the base

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of the Moldå Formation in its type section at Fjäcka, and that the top of the Oandu Stage is coeval with a level in the middle part of the Moldå Formation. As noted by several authors, for instance, Ainsaar et al. (1996), the boundary between the Keila and Oandu Stages is in large parts of eastern Baltoscandia marked by a stratigraphic gap that reflects a widespread regression. In the Fjäcka succession there is no obvious lithologic evidence of such a gap, although the more common occurrence of limestone beds with a higher carbonate content in the Moldå Formation compared with those of the Skagen Formation perhaps may be interpreted as indicating a shallowing in the depositional environment. We attribute the apparent lack of the Keila-Oandu boundary gap in the Fjäcka succession as due to the fact that the latter was deposited at a water depth large enough to prevent minor regressions to leave a recognizable imprint in the geological record. δ13C EXCURSIONS ABOVE AND BELOW THE GICE INTERVAL Several, in most cases relatively small, positive shifts in the δ13C curve above and below the GICE interval have been distinguished and named in both North America and northern Europe. However, the question as to whether or not these are local or global excursions has not been adequately addressed in the literature by regional comparisons. The detailed δ13C curves now available from a considerable number of sections provide a timely opportunity to assess the regional distribution and stratigraphic significance of these δ13C perturbations. In their detailed study of the δ13C chemostratigraphy of the Decorah Formation and associated strata in the Upper Mississippi Valley, Ludvigson et al. (2004) recognized four positive pre-GICE δ13C excursions in the late Turinian Stage, namely, in descending order, the Spechts Ferry, Quimby’s Mill, Grand Detour, and Mifflin excursions; three of these are shown in Figure 21. Presumably, owing to the existence of stratigraphic gaps in the succession, not all of these excursions are recognized in the δ13C curve of each locality, but they are best displayed at their localities 2–4 (Ludvigson et al., 2004, Figs. 7–9). The Spechts Ferry excursion, which lies typically between the Deicke and Elkport K-bentonites, is characterized by a positive shift in the δ13C curve just below a negative shift beneath the point of the beginning of the GICE (Fig. 21). However, in most sections the excursion curve differs little in this interval from the baseline values above and below the GICE. The stratigraphic position of the Spechts Ferry excursion just above the Deicke K-bentonite facilitates direct comparison with North American sections outside the Upper Mississippi Valley. For instance, the Tyrone Limestone at Boonesborough, Kentucky (Fig. 21), the Nealmont Limestone of Dolly Ridge, West Virginia (Fig. 6), and Reedsville, Pennsylvania (Fig. 9), and the Carters Formation of central Tennessee (Fig. 7) do not show a significant excursion between the two major K-bentonites. On the other hand, there is a positive shift of ~+1.5‰ between these K-bentonites at Hagan,

Virginia (Fig. 6). In Baltoscandia there is no notable positive excursion directly below the Kinnekulle K-bentonite that could be interpreted as the Spechts Ferry excursion. Hence, based on the evidence at hand, it appears that this excursion is a local, rather than global, shift in the δ13C curve. The stratigraphically next lower excursion recognized by Ludvigson et al. (2004, Figs. 9 and 10), the Quimby’s Mill excursion, was described from the uppermost part of the Platteville Formation just below the Deicke K-bentonite. This excursion is identified in only a few drill cores, but its absence in other sections in the Upper Mississippi Valley may be due to the absence of the excursion interval at the very prominent unconformity at the top of the Platteville Formation. In the Boonesborough, Kentucky, succession (Conkin and Conkin, 1983), there is a minor positive shift in the δ13C values from 0‰ to ~+1‰ within an interval a few meters thick beneath the Deicke K-bentonite (Fig. 21) that might correspond to the Quimby’s Mill excursion in eastern Iowa. However, in the coeval interval in other studied sections outside the Upper Mississippi Valley there appears to be little deviation from the δ13C baseline values, and we consider this excursion to be a local, rather than regional, feature. The lowest two excursions recognized by Ludvigson et al. (2004), the Grand Detour and Mifflin excursions, are characterized by shifts of up to ~2‰ from the baseline values in the δ13C curve (Fig. 21), and we are uncertain about their regional significance. If regionally distributed, they ought to be present in the interval of the Tyrone-Oregon Formations in Kentucky, but the biostratigraphic resolution currently available is insufficient to establish their precise positions within this interval in which there appears to be no significant shift from the baseline values (Fig. 21). At Hagan, there is a minor positive shift (up to ~+1.5 ‰) in the δ13C curve ~30–50 m below the Deicke K-bentonite that might represent one of these excursions, but further studies are needed to confirm this. The contact between the Platteville and the Decorah Formations marks a prominent gap of regionally variable magnitude in the Upper Mississippi Valley succession (Ludvigson et al., 2004, Fig. 3). In some areas, for instance, in northern Illinois, the entire GICE interval is missing. There is no evidence of the corresponding unconformity in the upper Turinian interval below the Deicke K-bentonite in Kentucky. In Baltoscandia, equivalents to the North American Turinian Stage ought to be present in the middle and upper Dalby Formation in Sweden, and in the lower Adze and Dreimani Formations in Estonia, which represent an interval referred to the Baltoniodus alobatus Subzone, and possibly the B. gerdae Subzone, of the Amorphognathus tvaerensis Zone (Bergström, 1971). This interval, below the Kinnekulle K-bentonite, is characterized by a prominent negative δ13C shift in some published δ13C curves from Sweden, Latvia, and Estonia (Ainsaar et al., 2004b, Fig. 1; Meidla et al., 2004, Fig. 1). In the recent description of the Kergula (565) drill core from northern Estonia, Martma (2006, Fig. 4) showed this interval as representing the upper part of the


57

Figure 21. Comparison of the Turinian δ13C excursions recognized in eastern Iowa (Ludvigson et al., 2004) and the δ13C curve from the coeval interval in the thicker, and presumably stratigraphically more complete, succession through the upper High Bridge Group at Boonesborough, Kentucky. Whereas the Iowa succession shows δ13C value fluctuations from near –3‰ to ~+0.5‰, most Kentucky values are between 0‰ and ~+1‰. This makes safe comparison of the Iowa δ13C excursions with the Kentucky succession difficult. Interestingly, correlation between the two curves is improved if one assumes that the prominent gap between the Platteville and the Decorah Formations in Iowa corresponds to ~6 m of section represented just below the Deicke K-bentonite in the Kentucky section, where there is no evidence of an unconformity.

“Upper Kukruse low” in the δ13C curve. Hence, in Baltoscandia there is currently no obvious evidence of the existence of the three Turinian positive δ13C excursions recognized in the Upper Mississippi Valley. Investigations after the initial submission of the present study (Bergström et al., 2007, 2009) have shown that the several positive δ13C excursions recognized between the GICE and HICE in Estonia (Kaljo et al., 2004) have their counterparts in the Cincinnati region reference successions of the Cincinnatian Series, the Upper Ordovician in the North American classification. These interesting results, which will be published in some detail elsewhere, indicate that not only GICE and HICE, but these excursions also, are of a global nature.

GICE RELATIONSHIP TO EUSTASY It is generally agreed that the latest Ordovician Hirnantian δ13C excursion (HICE) was associated with a eustatic regression caused by the lodging of very large amounts of water in the continental ice sheets on the Gondwana continents. Although recent studies (Bergström et al., 2006) suggest that this event might have involved more than a single glacial period, the correlation between the positive δ13C excursion and regression(s) appears well established, as it is based on investigation of a considerable number of successions on several continents. The possible relation between GICE and a Gondwana glaciation is more controversial and has been discussed in several

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recent papers (see, e.g., Kaljo et al., 2004; Tobin et al., 2005; Young et al., 2005), but finding a conclusive relationship between GICE, glaciation, and eustatic regression has been elusive. It can be assumed that the development during GICE time of a significant continental glaciation with large ice sheets would result in a glacio-eustatic regression that is likely to have left a recognizable record, especially in the shallow-water cratonic sedimentary successions. Unfortunately, most Late Ordovician sea level curves are based only on a particular region (see, e.g., Ross and Ross, 1992; Nielsen, 2004), and published global sea level curves tend to have limited stratigraphic resolution. Furthermore, it is often difficult to separate regressive events caused by more or less local epeirogeny from those resulting from eustasy. As noted by Tobin et al. (2005), in order to separate epeirogenic effects from eustasy, it is useful, if not necessary, to examine coeval sea level events on several continents. Despite these problems, in a regional study like the present one it is of interest to examine the relations between the GICE and bathymetric indications in the rock record. If it could be shown that this excursion was globally associated with a eustatic regression similar to the regression present in the HICE interval, it would strongly suggest the existence of an ice age during GICE time. Unfortunately, the evidence used in support of the existence of such a glacial period during GICE time in northern Africa is highly controversial (Hamoumi, 1999, 2003) and is not considered conclusive by most workers. Abundant evidence indicates that in the North American Midcontinent, late Turonian strata, such as the Tyrone Formation of Kentucky (Cressman and Noger, 1976), the Bromide Formation of Oklahoma (Amsden and Sweet, 1983), the Platteville Formation and coeval formations of the Upper Mississippi Valley (Witzke and Bunker, 1996), the Carters Formation of central Tennessee (Holland and Patzkowsky, 1998), the Eggleston Formation of Virginia, the Nealmont Formation of central Pennsylvania, and the Black River Group of New York–Ontario (Brett et al., 2004), represent very shallow-water (mostly peritidal) facies that has been described as having “layer-cake” stratigraphy. The upper part of this succession represents the M4 sequence of Holland and Patzkowsky (1996, 1998). Commonly, the top of this sequence is an omission surface (Kolata et al., 1998) that locally, as in Kentucky, has a relief of several meters (Cressman, 1973). The strata overlying this unconformity, which were regarded as the lower part of their M5 sequence by Holland and Patzkowsky (1996, 1998), are known by different formation names in different states, such as the Lexington Limestone in Kentucky, the Viola Springs Formation in Oklahoma, the Decorah Formation in the Upper Mississippi Valley, the Dolly Ridge Formation in West Virginia, and the Hermitage Formation in central Tennessee. All these units show evidence of having been formed in deeper water than the underlying Turinian peritidal carbonates, and clearly they represent sediments deposited during a sea level rise. The GICE starts in the Logana Member of the Lexington Limestone in Kentucky and in a coeval position in other sections near, or at, what has been described as a maximum flooding surface (Brett

et al., 2004). Hence, the major part of the GICE occurs in a highstand systems tract in Kentucky, and the same is the case in the other regions just listed. The upper part of the GICE is present in strata laid down in a shallowing depositional environment characterized by the occurrence of grainstones, etc. A similar trend can be recognized in the central Nevada succession, where the moderately deep-water Copenhagen Formation with the GICE is overlain by the very shallow-water Eureka Quartzite (Saltzman and Young, 2005). To summarize, in North America at least a significant part of the GICE appears to be present in a transgressive, rather than regressive, part of the Ordovician sedimentary record. In this respect it would seem to differ from the HICE. The bathymetric conditions in Baltoscandia were less uniform than in North America during the GICE interval, and two recently proposed interpretations of water depth are partly contradictory. Kaljo et al. (2004, Fig. 5) described the initial part of the GICE succession in the upper Keila stages of Estonia as having been deposited during a regression that culminated at the end of Keila time as shown by the unconformable Keila-Oandu contact, which is associated with a stratigraphic gap, especially in northern Estonia (Nõlvak and Grahn, 1993). The upper part of the GICE interval was interpreted as representing a minor transgression in early-middle Oandu time. Hence, in this bathymetric interpretation, assuming that the transatlantic correlation of the GICE is correct, the Estonian sea level changes would be opposite to those displayed in the North American Midcontinent. Nielsen (2004) proposed a partly different interpretation of the sea level history in Baltoscandia during this particular time interval. Based primarily on the Caledonian foreland basin succession in the Oslo region of southeastern Norway, his Keila Drowning Event in the lower-middle Keila Stage was followed by a lowstand in the upper Keila and Oandu Stages, the Frognerkilen Lowstand Event. On the other hand, Kaljo et al. (2004) interpreted most of the Oandu deposits as representing a deepening event. Most of the following, post-GICE, Rakvere Stage was shown by Nielsen (2004) to represent his Nakkholmen Deepening Event, whereas Kaljo et al. (2004) showed this as a lowstand episode. Hence the interpretations by Kaljo et al. (2004) and Nielsen (2004) are in conflict with each other in the GICE interval, and neither of these alternatives fits very well with the sea level history of the North American Midcontinent, although Nielsen’s (2004) interpretation would appear to be closest to the bathymetry of the latter region. Nielsen and Meidla (2004) tried to resolve some of the problems in the correlation of sea level curves between Norway and Estonia, but there are still differences in interpretation. In Sweden the GICE starts in the upper part of the Skagen Formation and in the coeval basal part of the Kullsberg Limestone. For a recent discussion of the general bathymetric conditions in the Siljan region, see Tobin et al. (2005). The strata of the GICE interval were deposited in substantially deeper water than coeval rocks in Estonia and hence were less affected by relatively minor bathymetric changes. Although estimating the

Upper Ordovician Guttenberg δ13C excursion (GICE) in North America and Baltoscandia precise water depth is difficult, the topographic relations between the Kullsberg mounds and the surrounding level bottom where the Fjäcka succession was deposited may provide a minimum local depth figure. As shown above, the GICE occurs through the >50-m-thick Kullsberg Limestone and through ~5 m of the non-mound Fjäcka succession. According to Tobin et al. (2005), the depth in the Kullsberg depositional environment was likely to have been several tens of meters. Assuming a relatively constant rate of deposition in both mound and non-mound facies during the GICE interval, it would seem that during at least late GICE time, the water depth at Fjäcka must have been >50 m and perhaps as much as 75–100 m, based on the elevation of the mound above the surrounding sea bottom on which the non-mound Skagen and Moldå Formations were deposited. In Scania, southernmost Sweden, the Skagen Formation with the GICE appears to have been deposited during a brief shallowing episode, as shown by the change from dark shale to limestone facies, and in some other areas there is a significant stratigraphic gap at the top of the Skagen Limestone (see Nõlvak and Grahn, 1993) that may mark the peak of the regression. Whether this gap cuts out the upper part of the GICE, as one would expect on the basis of the stratigraphically more complete succession in the Siljan area, is not yet known and requires further study. In view of these problems the only conclusion that can be drawn is that at least at the present time there is no very convincing regional evidence from Baltoscandia and North America that overall the GICE was closely correlated with a eustatic regression. This would make it unlikely that the GICE was somehow related to a significant glaciation episode in the Gondwana region. If there was a glaciation at that time, it would appear unlikely that it involved the formation of large continental ice sheets that trapped substantial amounts of seawater (Tobin et al., 2005). GICE AND CLIMATIC INDICATORS It is a widely held view that overall, the Ordovician Period, except its latest part (Hirnantian Stage), was characterized by a greenhouse-state climate with high CO2 and relatively low O2 (see, e.g., Barnes, 2004). Although this may be true as a generalization, it may not necessarily be valid for relatively short time periods such as the GICE interval. The possible presence of continental ice sheets in the Gondwana region in the Late Ordovician prior to the Hirnantian has been discussed by several authors (see, e.g., Hamoumi, 1999, 2003; Pope and Steffen, 2003; Saltzman and Young, 2005), but the supporting evidence for a glaciation has been equivocal. The apparent absence of undisputed glacial deposits in the well-exposed successions of early Katian age in the Gondwana region has been taken by some authors as important negative evidence for the existence of such a glacial period. Recently, Tobin et al. (2005), on the basis of geochemical evidence, suggested that the GICE was associated with a cooling event without formation of a continental ice sheet. In the following section we will briefly discuss some lithologic, faunal, and geochemical indications from North America and Baltoscandia that

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may have potential for interpretation of the climate, as reflected by the seawater temperature, during the GICE interval. This in turn may help clarify whether or not it is likely that the GICE was associated with a glacial period, as was the case with HICE. Lithofacies During the early part of the Katian Stage, which corresponds approximately with the beginning of the GICE, shallow seas with dominating carbonate deposition covered most of the North American Midcontinent from the Mississippi Valley region to the Appalachian foreland basin. Paleomagnetic data indicate that, for instance, Kentucky lay at ~10°S latitude (Bergström and Noltimier, 1982), and the data at hand suggest that the entire Midcontinent apparently was in the tropical zone. Many limestones are bahamitic, indicating deposition at water temperatures of >22–25 °C (Jaanusson, 1973). Evaporites are uncommon in these strata, and the diverse and abundant benthic faunas suggest normal seawater salinities and good water circulation. In central Kentucky the lithofacies of the GICE interval (essentially the Logana Member of the Lexington Limestone) has been described as being a cool-water type with phosphaterich limestones that differ in some respects from the underlying peritidal bahamitic limestones in the Tyrone Formation (Holland and Patzkowsky, 1996). It has also been proposed that the lithofacies of these carbonate sediments was affected by upwelling from the nearby Sebree Trough (Kolata et al., 1998, 2001), which was an elongate basinal structure that is thought to have had an oceanic connection to the south. But the carbonates of the GICE interval in Kentucky, as well as elsewhere in the Midcontinent, are of a tropical type and differ substantially from the subtropical-temperate cold-water limestones in Baltoscandia. Thus we conclude that the lithofacies of the Laurentian carbonates does not provide evidence of a marked lowering of the seawater temperature that could be used as evidence for the presence of a glaciation. It is also significant that, as noted above, the GICE is in that part of the Katian succession that represents a flooding event in both the Mississippi Valley and in the eastern Midcontinent, which is opposite to what would be expected from a glacioeustatic regression. The lithofacies of the sediments in Baltoscandia corresponding to the early Katian Stage in North America, especially those in Sweden and the Baltic States, have been investigated extensively, and a large amount of information has been published. During GICE time, Baltoscandia occupied a subtropical-temperate position (Spjeldnaes, 1961; Bergström and Noltimier, 1982; Cocks and Torsvik, 2005) at a latitude of 30–35°S. Jaanusson (1973) and Lindström (1984), among others, discussed various environmental parameters bearing on the deposition of the Ordovician limestones in Baltoscandia. Although these authors differed in their interpretation of the water depth in the depositional environment, they agreed that most pre-Hirnantian Ordovician limestones in Baltoscandia were deposited in relatively cold water. Bahamitic carbonates are unknown in the GICE interval in

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Sweden but are present locally in the Oandu Stage in Estonia (Vasalemma Formation) and particularly in coeval shallow-water strata (Mjøsa Limestone) in the Mjøsa region in southeastern Norway (Jaanusson, 1976). These unique successions, which include reef-like structures, were interpreted by Jaanusson (1973) as representing a brief invasion of warm waters into parts of the Baltoscandic depositional basin. Judging from the Estonian succession, this warm-water invasion occurred during the final phase of the GICE. The absence of coeval bahamitic strata in Sweden has been attributed to the fact that the depositional environment was too deep and too cold at the bottom for deposition of such carbonates. The local presence at relatively low latitudes in Baltoscandia of warm-water carbonates in the GICE interval, in a succession otherwise characterized by cold-water limestones, is not what would be expected if the GICE was associated with a cold climate and a glaciation. Hence, the lithofacies evidence from North America and Baltoscandia is not very indicative of a substantial climatic cooling. A general conclusion emerges: namely that the GICE is not restricted to a particular carbonate lithofacies but is present in a spectrum of carbonate rock types representing a variety of depositional environments from warm, tropical, shallow-water seas to subtropicaltemperate, relatively deep and cool epicontinental sites. Faunas It is a well-established fact that the Ordovician is one of the Phanerozoic periods with the most conspicuous biogeographic differentiation in the faunas (Jaanusson, 1976; Jaanusson and Bergström, 1980; McKerrow and Scotese, 1990; Webby et al., 2004). A primary reason for this is clearly the vast latitudinal spacing of the continents during this time period, with large regions at different latitudes with different climates from the equator to the poles. The GICE interval provides an instructive illustration of the faunal differentiation at different latitudes with associated differences in climate and seawater temperatures. The Midcontinent megafossil faunas in the North American GICE interval are of tropical types and are dominated by brachiopods and bryozoans and with relatively less abundant occurrences of other shelly fossils such as trilobites, pelecypods, and corals. For useful reviews of the megafossil faunas in the Upper Mississipi Valley, see Sloan (2005), and for those in Kentucky and nearby states, see Pojeta (1979, and the several other U.S. Geological Survey monographs in the series “Contributions to the Ordovician Paleontology of Kentucky and Nearby States”). Useful information is also available in Feldmann and Hackathorn (1996). These faunas have very little in common, even at the generic level, with most coeval megafossil faunas in Baltoscandia. For extensive information about the latter faunas, especially the diverse ones in Estonia, see Rõõmusoks (1970). Also the Baltoscandian microfossil faunas in the GICE interval differ strikingly from those from the North American Midcontinent (Bergström, 1990) apart from the presence of a few pandemic and in most cases long-ranging taxa. However, there is

one important exception to this statement—the conodont fauna of the Mjøsa and Svartsaetra Limestones in Norway and, to a lesser extent, that of the Saku Member of the Vasalemma Formation of Estonia (Bergström, 1997; Bergström et al., 1998). The conodont fauna of the Mjøsa Limestone is of a unique type in Europe and is closely similar to the tropical one from the Lexington Limestone in Kentucky (Bergström et al., 1998); but it is strikingly different from those of the coeval Skagen and Moldå Formations in Sweden and corresponding strata in Norway (Hamar, 1966). Interestingly, these condont faunas are associated with some shelly fossils of Laurentian type and occur in shallow-water bahamitic limestones (see above). There is no significant faunal change that correlates closely with the GICE interval. In the North American Midcontinent succession, the Turinian very shallow-water shelly and conodont faunas differ in several respects from the Chatfieldian ones, but this change occurs in the uppermost Turinian, hence well below the beginning of the GICE. For instance, in the well-known succession in the Upper Mississippi Valley, the faunal change is near the Deicke K-bentonite (Sloan, 2005), and it might be at a corresponding interval in Kentucky and adjacent states (Patzkowsky and Holland, 1996, 1997), although the paucity of fossils in the uppermost Tyrone Limestone and coeval strata makes it difficult to recognize details in the faunal succession. At any rate, this faunal change is attributed to the marked environmental change from peritidal to deeper, although not very deep, conditions at the beginning of the M5 sequence. In an informative review, Ainsaar et al. (2004a) summarized various faunal changes in the Keila-Oandu succession in northern Estonia. The most significant is that at the top of the Keila Stage, which is commonly an unconformity and a stratigraphic gap of regionally variable magnitude, not only in northern Estonia but also in parts of eastern and south-central Sweden (Nõlvak and Grahn, 1993). It is important to note that this gap, which was caused by a regression in the latest Keila sea, is in the middle to upper part of the GICE curve (Fig. 20). If one assumes that the GICE curve covers the same chronostratigraphic interval in Baltoscandia and in North America, as suggested by a variety of evidence presented above, the faunal change in Estonia occurred substantially later than the one near the top of the Turinian Stage in North America, and the North American M4-M5 sequence boundary is clearly older than the latest Keila regression in northern Estonia. Hence, these faunal and depositional changes are not coeval, as suggested by Ainsaar et al. (2004a, p. 128), but diachronous. Accordingly, they are not very useful evidence for a possible global oceanographic change in the GICE interval, or for a contemporaneous eustatic sea level change, in Baltoscandia and in North America. 18

O Geochemistry

Important information about seawater temperature in ancient environments can be obtained by investigations of the δ18O isotope in well-preserved calcareous fossils and carbonate

Upper Ordovician Guttenberg δ13C excursion (GICE) in North America and Baltoscandia sediments. This method is based on the fact that the difference in δ18O/δ16O ratios between the CaCO3 and the seawater from which the calcium carbonate is precipitated is related to the temperature, with a decrease in temperature of the water leading to an increase in the calcite δ18O value. This paleothermometer is widely used in geologically relatively young (Cenozoic and Cretaceous) deposits, and it has often proved to produce valuable, and seemingly reliable, data on past seawater temperatures (Anderson and Arthur, 1983). However, many attempts to use this technique on Paleozoic marine limestones and fossils have been unsuccessful, because the δ18O values have been reset owing to diagenesis (Marshall, 1992) and do not represent the original δ18O signature. The idea that in general, Paleozoic δ18O values are unreliable (see, e.g., Kaljo et al., 2004), if not entirely useless, has resulted from the unfortunate practice of leaving out δ18O values in many 13 C publications. Most δ18O values through the GICE interval in Kentucky are between –4.8‰ and −6‰, in the Hagan (Virginia) and Dolly Ridge (West Virginia) successions between –5.7‰ and –6.5‰, and in the Fittstown (Oklahoma) section between –4.5‰ and −5.2‰ (Young et al., 2005). Shields et al. (2003) reported similar values, based on brachiopods from Missouri. Several authors (e.g., Marshall and Middleton, 1990; Marshall et al., 1997; Shields et al., 2003) recorded δ18O values as high as ~ −3‰, or even ~ −1‰, from Hirnantian brachiopods. Such high values are interpreted to reflect low seawater temperatures during the Hirnantian Gondwana glaciation. The fact that similar high δ18O values have not been recorded from the GICE interval in North America may be interpreted in at least two ways: They may indicate that the GICE was not associated with a significant lowering of the seawater temperature in the tropical region, or that the δ18O values have been diagenetically reset so the original δ18O signature has been lost. Carbonate δ18O values from Estonia recorded by Kaljo et al. (2004), who regarded them as diagenetically reset, range between −6‰, with most between –4.5‰ and −5‰, hence values similar to those in North America. An isotopic study of the GICE interval in the Kullsberg Limestone in central Sweden was recently carried out by Tobin et al. (2005). Unlike most other investigations, this study was centered on what appeared to be diagenetically relatively unaltered carbonate cements, which in the upper part of the Kullsberg Limestone produced δ18O values as high as −3.6 ‰ to –2.8 ‰, with one value being –1.8‰. Brachiopods from the lower part of the same formation show δ18O values between –4‰ and –6‰ (Marshall and Middleton, 1990), which are similar to the baseline values in the underlying Dalby Formation. Many additional samples processed during the present study have δ18O values between –4‰ and –6‰, with a few 3‰ above baseline values. There is also a regional variation in baseline values. In the Upper Mississippi Valley these values are ~ –1‰ to ~0‰, whereas in the eastern Midcontinent they are ~0‰ to +1‰. The relations between the GICE and eustasy are not straightforward. In most North American sections the GICE starts in a transgressive interval and ends in a regressive interval. The bathymetric conditions in the coeval interval in Baltoscandia are somewhat controversial, but at least part of the GICE occurred during a prominent regression at the top of the Keila Stage and during the Oandu Stage. It appears difficult to link the GICE to a significant eustatic regression such as the one associated with a major glaciation. In this respect the GICE seems to differ from the latest Ordovician Hirnantian δ13C excursion (HICE). Lithofacies evidence indicates that the GICE is present in tropical warm-water carbonates (bahamites) in the Midcontinent as well as in subtropical to temperate carbonates in Baltoscandia. The fact that the GICE is associated with warmer-water macro- and microfossil faunas in North America, and with quite different colder-water faunas in Baltoscandia, suggests no obvious direct correlation between the excursion and the water temperature. Geochemical indications from δ18O investigations on carbonate are inconclusive, as most δ18O values probably have been diagenetically reset. However, a recent δ18O study on conodont apatite in the GICE interval in Kentucky and Minnesota indicates no marked lowering of the seawater temperature. The various data at hand do not provide strong support for the idea that the GICE was closely correlated with, and in some way perhaps due to, a climatic deterioration of a similar magnitude as that associated with significant glaciations such as those during Hirnantian time. A preliminary assessment of the minor δ13C excursions recently recognized in the lowermost Chatfieldian and upper Turinian in the Upper Mississippi Valley indicates that these positive shifts in the δ13C curve, several of which are not present in every section even in that region, are not global but regional or local in their distribution. On the other hand, the several positive excursions recognized between the GICE and the HICE in Estonia have been recently identified also in the type Cincinnatian Series in its type area in the Cincinnati region, suggesting that these excursions are of a global nature.

ACKNOWLEDGMENTS We are indebted to T. Ainsaar, G.A. Ludvigsson, and S.C. Finney for careful reviews of the mauscript and useful suggestions for its improvement, and to S.A. Young for valuable

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POSTSCRIPT Since the present paper was finally submitted in 2006, several studies of aspects of the GICE have been published. It is not possible to review the results presented in these papers in any detail, but it is appropriate to list the most important of these publications. In North America, recent GICE investigations include Panchuk et al. (2005, 2006) and Fanton and Holmden (2007), which were based on Upper Mississippi Valley sections. Young et al. (2008) compared δ13Corg and δ13Ccarb GICE curves from exposures in North America and China. Finally, in a study of the relations between several Katian δ13C excursions in North America and Estonia, Bergström et al. (2007) compared the biostratigraphic positions of the GICE in the Cincinnati region standard succession and in an important drill core in Estonia. Postscript References Bergström, S.M., Young, S., Schmitz, B., and Saltzman, M.R., 2007, Upper Ordovician (Katian) δ13C chemostratigraphy: A trans-Atlantic comparison: Acta Palaeontologica Sinica, v. 46, suppl., p. 37–39. Calner, M., Lehnert, O., and Joachimski, M., 2009, Carbonate mud mounds, conglomerates, and sea-level history in the Katian (Upper Ordovician) of central Sweden: Facies, v. 56, p. 157–172, doi: 10.1007/s10347-009 -0192-6. Fanton, K.C., and Holmden, C., 2007, Sea-level forcing of carbon isotope excursions in epeiric seas: Implications for chemostratigraphy: Canadian Journal of Earth Sciences, v. 44, p. 807–818, doi: 10.1139/E06-122. Goldman, D., Leslie, S.A., Nõlvak, J., Young, S., Bergström, S.M., and Huff, W.D., 2007, The Global Stratotype Section and Point (GSSP) for the base of the Katian Stage of the Upper Ordovician Series at Black Knob Ridge, southeastern Oklahoma, USA: Episodes, v. 30, p. 258–270. Kaljo, D., Martma, T., and Saadre, T., 2007, Post-Hunnebergian Ordovician carbon isotope trend in Baltoscandia, its environmental implications and some similarities with that of Nevada: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 245, p. 138–155, doi: 10.1016/j.palaeo.2006.02.020. Panchuk, K.M., Holmden, C., and Kump, L.R., 2005, Sensitivity of the epeiric sea carbon isotope record to local-scale carbon cycle processes: Tales from the Mohawkian Sea: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 228, p. 320–337, doi: 10.1016/j.palaeo.2005.06.019. Panchuk, K.M., Holmden, C., and Leslie, S., 2006, Local controls on carbon cycling in the Midcontinent region of North America with implications for carbon isotope secular curves: Journal of Sedimentary Research, v. 76, p. 200–211, doi: 10.2110/jsr.2006.017. Young, S., Saltzman, M.R., Bergström, S.M., Leslie, S.A., and Chen Xu, 2008, Paired δ13Ccarb and δ13Corg records of Upper Ordovician (Sandbian-Katian) carbonates in North America and China: Implications for paleoceanographic change: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 270, p. 166–178, doi: 10.1016/j.palaeo.2008.09.006. MANUSCRIPT ACCEPTED BY THE SOCIETY 6 OCTOBER 2009

Printed in the USA


The Ordovician brachiopod radiation: Roles of alpha, beta, and gamma diversity David A.T. Harper† Natural History Museum of Denmark (Geological Museum), University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark

ABSTRACT The Ordovician Period (ca. 488–444 Ma) witnessed profound changes in the biodiversity and biocomplexity of marine life, marked by the installation of a benthos dominated by suspension-feeding animals, most notably the brachiopods. The Ordovician brachiopod fauna was dominated by rhynchonelliformeans in contrast to that of the underlying Cambrian System, characterized by a diversity of various nonarticulated groups. Over an interval of some 25 m.y., accelerating γ (inter-provincial), β (inter-community), and α (intra-community) diversity was initiated by high diversities among Early Ordovician brachiopod faunas associated with the dispersal of the continents and the high frequency of volcanic arcs and microcontinents (γγ diversity). During the Early and Middle Ordovician, community types expanded particularly into deeper water and around carbonate platforms and structures (β β diversity). Moreover, during the period the α diversity of individual assemblages increased from 70 m (Zhou et al., 1998a). During the late Darriwilian–early Sandbian, as the transgression progressed, the stagnant shelf-basin migrated southward to the Kalpin area, resulting in deposition of the Saergan Formation, which is a condensed succession of carbonaceous and

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Figure 14. Tremadocian–Dapingian (Early to early Middle Ordovician) paleogeography of Tarim, modified from Zhou et al. (1995a, 1996b) and Ni et al. (2001). STL—Southern Tarim Land; P—platform; B—stagnant shelf basin; S—slope.

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S

Kuqa Luntai

Aksu Dawangou Kalpin

Xingeer

PM

Korla

Uligezitag

LN46 Queerqueke

YM2

PM

DT

Yijianfang

Yaerdang

WM1

Tangwangcheng Bachu TD1

TZ10

Kashi

Ruoqiang

P

S

IN TA N U

Qiemo

Yecheng

UN

MO

ALT

STL

Hotan

Biocalcarenite

Minfeng

Measured section Borehole

Siltstone

Shale

Land

Carbonate buildups Calcilutite/ biocalcilutite

Figure 15. Darriwilian–early Katian (Middle to middle Late Ordovician) paleogeography of Tarim, modified from Zhou et al. (1995a, 1996b) and Ni et al. (2001). STL—Southern Tarim Land; P—platform; S—slope; DT—deep trough; PM—platform margin.

100

Chen Xu et al.

240km

0

N

S Luntai Kuqa

CH1

Korla

PM

LN46

Xingeer Uligezitag

Aksu TH1

Tierekeawati

YM2

Queerqueke

Kalpin Bachu

ST

Lianglitag

Kashi

PCF

TD1

TZ10

Ruoqiang

LB Qiemo

STL

Yecheng

Sandstone

Siltstone

Shale/ mudstone

Land

Minfeng Hotan Measured section

Silty shale

Borehole Biocalcarenite

Carbonate buildups

Figure 16. Late Katian (Late Ordovician) paleogeography of Tarim, modified from Zhou et al. (1995a, 1996b) and Ni et al. (2001). STL— Southern Tarim Land; LB—littoral belt; PCF—platform carbonate flat; S—slope; ST—shallow trough; PM—platform margin.

siliceous shale with abundant graptolites and trilobites. However, following the climax of transgression during the middle–late Sandbian, stagnant conditions ended abruptly, to be replaced by deposition of the partly rhythmic sediments of biocalcilutite, shale, and siltstone of the slope facies, recorded in the Kanling and Qilang Formations. The trilobite fauna in the biocalcilutite, dominated by nileids and cyclopygids, indicates water depths to 100 m. By the early Katian the slope became shallower, with deposition of the Yingan Formation, which consists of alternating beds of carbonaceous, silty, and calcareous shale intercalated with biocalcilutite with graptolites and trilobites of the Raphiophorid biofacies, which indicate a water depth of

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