Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates (GSA Special Paper 416)

Special Paper 416

THE GEOLOGICAL SOCIETY OF AMERICA

Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates

edited by Ana María Alonso-Zarza Departamento Petrología y Geoquímica Facultad de Ciencias Geológicas Universidad Complutense de Madrid 28040 Madrid Spain Lawrence H. Tanner Department of Biological Sciences Le Moyne College Syracuse, New York 13214 USA

Special Paper 416 3300 Penrose Place, P.O. Box 9140

Boulder, Colorado 80301-9140, USA

2006

Copyright © 2006, The Geological Society of America, Inc. (GSA). 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. 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 Editor: Marion E. Bickford and Abhijit Basu Library of Congress Cataloging-in-Publication Data Paleoenvironmental record and applications of calcretes and palustrine carbonates / edited by Ana María Alonso-Zarza, Lawrence H. Tanner. p. cm.--(Special paper; 416) Includes bibliographical references and index. ISBN-10 0813724163 (pbk.) ISBN-13 9780813724164 (pbk.) 1. Calcretes. 2. Rocks, Carbonate. 3. Paleopedology. I. Alonso-Zarza, Ana María, 1962-. II. Tanner, Lawrence H. III. Special papers (Geological Society of America) ; 416. QE471.15.C27.P35 2007 552/.58--dc22

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Cover: View of laterally continuous pedogenic calcretes in the Upper Triassic (Norian) Owl Rock Formation (Chinle Group), northern Arizona. Photo by L.H. Tanner. Back cover: Recent vertical calcrete formed by the penetration of tree roots on Miocene deposits of the Madrid Basin, Guadalajara, Spain. Photo by A.M. Alonso-Zarza.

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Contents Preface .........................................................................................................................................................v Ancient Landscapes, Climate and Sequence Boundaries 1.

Calcic pedocomplexes—Regional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States ...................................................................................1 D.L. Hanneman and C.J. Wideman

2.

A Late Triassic soil catena: Landscape and climate controls on paleosol morphology and chemistry across the Carnian-age Ischigualasto–Villa Union basin, northwestern Argentina .....17 N.J. Tabor, I.P. Montañez, K.A. Kelso, B. Currie, T. Shipman, and C. Colombi

3.

Investigating paleosol completeness and preservation in mid-Paleozoic alluvial paleosols: A case study in paleosol taphonomy from the Lower Old Red Sandstone .......................................43 S.B. Marriott and V.P. Wright

4.

Calcareous paleosols of the Upper Triassic Chinle Group, Four Corners region, southwestern United States: Climatic implications .................................................................................................53 L.H. Tanner and S.G. Lucas

5.

Estimates of atmospheric CO2 levels during the mid-Turonian derived from stable isotope composition of paleosol calcite from Israel ......................................................................................75 A. Sandler

6.

Pedogenic carbonate distribution within glacial till in Taylor Valley, Southern Victoria Land, Antarctica ..................................................................................................................89 K.K. Foley, W.B. Lyons, J.E. Barrett, and R.A. Virginia

Sedimentary Environments and Facies 7.

Calcretes, oncolites, and lacustrine limestones in Upper Oligocene alluvial fans of the Montgat area (Catalan Coastal Ranges, Spain) .......................................................................105 D. Parcerisa, D. Gómez-Gras, and J.D. Martín-Martín

8.

The role of clastic sediment influx in the formation of calcrete and palustrine facies: A response to paleographic and climatic conditions in the southeastern Tertiary Duero basin (northern Spain) ..............................................................................................................................119 I. Armenteros and P. Huerta

9.

The Upper Triassic crenogenic limestones in Upper Silesia (southern Poland) and their paleoenvironmental context ............................................................................................................133 J. Szulc, M. Gradzi´nski, A. Lewandowska, and C. Heunisch iii

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Contents 10. A recent analogue for palustrine carbonate environments: The Quaternary deposits of Las Tablas de Daimiel wetlands, Ciudad Real, Spain ................................................................153 A.M. Alonso-Zarza, M. Dorado-Valiño, A. Valdeolmillos-Rodríguez, and M. Blanca Ruiz-Zapata 11. Depositional conditions of carbonate-dominated palustrine sedimentation around the K-T boundary (Faciès Rognacien, northeastern Pyrenean foreland, southwestern France)..............169 D. Marty and C.A. Meyer 12. Reworked Microcodium calcarenites interbedded in pelagic sedimentary rocks (Paleocene, Subbetic, southern Spain): Paleoenvironmental reconstruction ...................................................189 J.M. Molina, J.A. Vera, and R. Aguado Dating of Calcretes: Applications 13. Calcite cement stratigraphy of a nonpedogenic calcrete in the Triassic New Haven Arkose (Newark Supergroup) ......................................................................................................................203 E.T. Rasbury, E.H. Gierlowski-Kordesch, J.M. Cole, C. Sookdeo, G. Spataro, and J. Nienstedt 14. Calcrete features and age estimates from U/Th dating: Implications for the analysis of Quaternary erosion rates in the northern limb of the Sierra Nevada range (Betic Cordillera, southeast Spain) ...............................................................................................................................223 J.M. Azañón, P. Tuccimei, A. Azor, I.M. Sánchez-Almazo, A.M. Alonso-Zarza, M. Soligo, and J.V. Pérez-Peña

Preface The study of ancient soils continues at an accelerating pace as more geologists recognize the value of these ancient land surfaces as archives of important paleotopographic, paleoenvironmental, and paleoclimatic information. Indeed, a survey of one database yields over 600 citations containing the keyword “paleosol” for just the first half of this decade, compared to only one-fourth this number from the first half of the 1990s! Not all of these publications presented detailed descriptions and interpretations of paleosols, certainly, but many were broader studies that incorporated the description of ancient soil surfaces into examinations of tectonics, basin evolution, sedimentary processes, or climate change. Clearly, the variety of paleosols and their potential applications to geological problems is enormous. Given the breadth of this subject, we chose to focus this volume on the topic of calcretes and the closely related subject, palustrine carbonates. Calcretes are perhaps the most commonly described of paleosols, owing to their ready preservation in the rock record and relative ease of recognition. The term calcrete, synonymous with caliche, is widely applied, although it is neither the name of a soil order nor of a soil horizon. In a broad sense, calcretes are, as proposed by Watts (1980, p. 663; after Goudie, 1973), “terrestrial materials composed dominantly, but not exclusively, of CaCO3, which occurs in states ranging from nodular and powdery to highly indurated, and result mainly from the displacive and/or replacive introduction of vadose carbonate into greater or lesser quantities of soil, rock, or sediment within a soil profile.” This definition was restricted to calcretes of pedogenic origin, however Wright and Tucker (1991) later expanded the term calcrete to include, as initially recommended by Netterberg (1980), the effects of shallow groundwater. This broader sense suggests the importance of the interaction between sediments undergoing active pedogenesis and shallow groundwaters. Palustrine carbonates exhibit many similarities with calcretes. As described by Freytet (1984, p. 231), a palustrine limestone “must show the characteristics of the primary lacustrine deposit (organisms, sedimentary features) and characteristics due to later transformations (organisms, root traces, desiccation, pedogenic remobilizations).” Palustrine carbonates are common in alluvial sequences, often in association with calcretes, but their widespread recognition has been attained more slowly. Indeed, much of the research on alluvial carbonates has focused exclusively on either palustrine carbonates or calcretes, when in fact there is often a spatial transition from one to the other, revealing an interplay between pedogenic, sedimentary, and diagenetic processes. Indisputably, these deposits contain information that is significant to the interpretation of the sedimentary record and the evolution of the landscape in both recent and ancient settings (Alonso-Zarza, 2003). These terrestrial carbonates are widely distributed on floodplains and in the distal reaches of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence or changing accommodation rates, and therefore as indicators of different tectonic regimes. Although calcretes and palustrine carbonates are both commonly associated with semiarid climates, more detailed climatic information can be obtained from the depths of the carbonate-bearing horizons within paleosol profiles and from the oxygen isotope signature of the carbonate. The carbon-isotope composition, on the other hand, has been used quite successfully to track changes in atmospheric pCO2 through the Phanerozoic. Vegetation is important to the formation of many of these types of carbonates, and data on the prevailing vegetation may be obtained sometimes from the analysis of the micro- and macrofabric of the carbonate. This volume was inspired by a technical session on the topic of calcretes and palustrine carbonates (chaired by us) that was held at the 32nd International Geological Congress in Florence in August 2004. Six of the contributions presented here were first delivered at this meeting, and the volume grew with additional v

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Preface

contributions that provided both a broad overview of calcretes and palustrine carbonates and the “state of the art” of their application. The papers presented here cover a wide array of environmental settings and ages of deposits where calcretes and palustrine occur. Moreover, the papers included in this monograph discuss a number of interesting applications, including: a possible modern analogue for palustrine carbonates, the interplay between palustrine, pedogenic and diagenetic processes, the utility of radio-isotopic dating of pedogenic carbonates and its application to understanding the evolution of recent landscapes, the reconstruction of a diagenetic sequence, and the climatic and geomorphic controls on calcrete formation. The papers have been arranged in three groups. Papers that apply calcretes and palustrine carbonates to the reconstruction of ancient landscapes, climate and sequence boundaries comprise the first group. Hanneman and Wideman illustrate the utility of calcic pedocomplexes in delineating regional unconformities that are large-scale sequence boundaries. Their study, focused on the Tertiary of the Great Plains, shows that these pedocomplexes have distinct physical properties that allow their identification in seismic sections and well logs. Tabor and co-authors demonstrate that the distribution of the fluvial channel sandstones and the characteristics of the paleosols are both controlled by geomorphic evolution during deposition of the Triassic Ischigualasto Formation in northwest Argentina. The preservational bias in paleosol formation is described in the contribution by Marriott and Wright. These authors analyzed mid-Paleozoic paleosols from the Lower Old Red Sandstone and show that reactivated, truncated cumulate horizons provide a means of assessing the dynamics of floodplains, including those from before the advent of rooted vascular plants in the mid-Paleozoic. The fourth paper, by Tanner and Lucas, relates the potential climatic control on the morphology of Upper Triassic paleosols in the Chinle Group of the southwestern United States. Temporal changes in the types of paleosols and the maturity of calcretes suggest a gradual aridification across the Colorado Plateau during the Late Triassic. Sandler uses the isotopic composition of Mid-Turonian paleosol carbonate to estimate the atmospheric pCO2 level for this interval. His results, which indicate high mid-Turonian pCO2, correspond with the high temperatures that prevailed at that time. The last paper of this group, by Foley and co-authors, demonstrates that the relatively low carbonate concentrations in Antarctic polar desert soils can be attributed to the shallow active layer, low rates of weathering, and the extreme aridity of the landscape. Moreover, the differences in CaCO3 concentrations in these soils correlate with landscape position with respect to elevation and distance from the coast. Six papers dealing with the sedimentary environments and facies of calcretes and palustrine carbonates comprise the second group. These papers provide an overview of the interrelationships between calcretes and palustrine carbonates in terrestrial environments, focusing on their similarities and on problems in their interpretations. Notably, some papers discuss the lack of a recent analogue for ancient palustrine carbonates. In the first contribution to this group, Parcerisa and co-authors analyze the geochemistry of calcretes, oncolites and lacustrine limestones formed during the Upper Oligocene in two coalescent alluvial fans. They find that the trace element and isotopic composition of the limestones were controlled mainly by the fluvial regime and the lithology and altitude of the catchment areas in the sedimentary basin. Armenteros and Huerta studied calcretes and associated palustrine of the southeastern Tertiary Duero basin. The characteristics of both carbonate facies indicate their accumulation in semiarid climates with scarce clastic sediment supply, and that meter-scale cyclicity of the carbonate and siliciclastic sediments was controlled mostly by climate. The interrelationship between spring, fluvial, palustrine, and pedogenic facies is discussed by Szulc and collaborators in their study of the Upper Triassic freshwater carbonates from the Upper Silesian basin. These carbonates were deposited within a shallow swampy depression, fed by springs of deep-circulating groundwater. Alonso-Zarza and co-authors focused their study on a recent core in Las Tablas de Daimiel, Spain, one of the few freshwater wetlands preserved in southern Europe. Their studies of the core, including mineralogy, petrography, stables isotopes and pollen analyses indicates that these sediments are similar of those of ancient palustrine sequences, suggesting that Las Tablas is a suitable recent analogue for freshwater palustrine sequences. Marty and Meyer analyze in detail a palustrine sequence (Faciès Rognacien) encompassing the K-T boundary in southwestern France. The facies association indicates a seasonal, palustrine wetland system, with ephemeral ponds surrounded by vegetated areas of freshwater marshes under subarid to intermediate climates. The last paper of this group, by Molina and co-authors, describes an unusual occurrence of various types of calcarenites containing reworked Microcodium prisms. Their study of Paleocene marine deposits from southern Spain indicates that the Microcodium was reworked from exposed inland areas, thus providing evidence of emersion and clarifying the palebathymetry of the adjacent pelagic deposits.

Preface

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The final section contains two papers on different types of calcretes to which radio-isotopic dating techniques have been applied. Rasbury and co-authors describe the importance of cement stratigraphy to the application of U-Pb dating of calcite in Triassic calcretes from the New Haven Arkose, and demonstrate further that this analysis is a useful tool for distinguishing between pedogenic versus nonpedogenic calcrete. U-Th dating of calcretes is used to constrain the evolution of the Quaternary Ranges in the Betic Cordillera by Azañón and co-authors. These authors describe how rapid incision by the rivers, and subsequent capture of the Guadix by the Guadalquivir River is constrained by initial dissection of a calcrete layer dated at 42 ka. This age is used to calculate the incision and erosion rates, demonstrating once again that calcretes play a major role in the evolution of landscape in many arid and semiarid regions. This collection of papers in its final form would not have been possible without the work of the reviewers who dedicated their time to careful reviews and revisions. We were truly lucky to have the help of the following colleagues: J. Andrews, C. Arenas, B. Barclay, J. Bockheim, G. Bowen, Ll. Cabrera, J. Casanova, E. Cheney, C. De Wet, S. Dunagan, M.A. García del Cura, P. Ghosh, R. Goldstein, A.D. Harvey, M. Joeckel, A. Kosir, J. López, G. Marion, A. Martín-Algarra, P. McCarthy, D. Nash, R. Palma, T. Peryt, N. Platt, G. Retallack, D. Royer, Y. Sánchez-Moya, P.G. Silva, A.R. Soria, R. Swennen, M. Talbot, S.K. Tandon, A. Travé, D. Valero-Garcés, D. Varrone, and J. Wilkinson. Our sincere thanks also go to our departments: Departamento de Petrología y Geoquímica de la Universidad Complutense de Madrid and the Department of Biological Sciences of Le Moyne College. We also have a special remembrance for F. Calvet, one of the pioneers in the studies of calcretes in Spain who passed away a few years ago. His ideas are tangibly present throughout this volume. We hope the reader finds this collection of papers both stimulating and informative. This collection will, ideally, constitute a base for understanding how calcretes and palustrine carbonates form an integral part of ancient and recent landscapes and contribute to the broader knowledge of continental basins and their geomorphic features. REFERENCES CITED Alonso-Zarza, A.M., 2003, Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record: EarthScience Reviews, v. 60, p. 261–298, doi: 10.1016/S0012-8252(02)00106-X. Freytet, P., 1984, Les sédiments lacustres carbonatés et leurs transformations par émersion et pédogénèse: Importance de leur identification pour les reconstitutions paléogéographiques: Bulletin Centres Rechercher Exploration-Production Elf-Aquitaine, v. 8, no. 1, p. 223–246. Goudie, A.S., 1973, Duricrusts in Tropical and Subtropical Landscapes: Clarendon, Oxford, 174 p. Netterberg, F., 1980, Geology of southern African calcretes: 1. Terminology, description, macrofeatures and classification: Transactions of the Geological Society of South Africa, v. 83, p. 255–283. Watts, N.L., 1980, Quaternary pedogenic calcretes from the Kalahari (southern Africa): mineralogy, genesis and diagenesis: Sedimentology, v. 27, p. 661–686. Wright, V.P., and Tucker, M.E., 1991, Calcretes: an introduction, in Wright, V.P., and Tucker, M.E., eds., Calcretes: IAS Reprint series 2, Oxford, Blackwell Scientific Publications, p. 1–22.

Ana M. Alonso-Zarza Lawrence H. Tanner

Geological Society of America Special Paper 416 2006

Calcic pedocomplexes—Regional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States Debra L. Hanneman Whitehall Geogroup, Inc., Whitehall, Montana 59759, USA Charles J. Wideman Professor Emeritus, Montana Tech of the University of Montana, Butte, Montana 59701, USA ABSTRACT Calcic pedocomplexes are associated with regional unconformities in the Great Plains and western United States that have approximate ages of 30 Ma, 20 Ma, and 4 Ma. In southwestern Montana, the calcic pedocomplexes are readily identifiable on the surface, and a pedocomplex typically contains several partial soil profiles. In the most complete scenario, an individual profile may contain an argillic or argillic/calcareous (Bt or Btk) horizon, a K horizon, and a C horizon. Often, however, the Bt(k) horizon is truncated or can be entirely absent from an individual profile. The K horizon contains an upper laminated zone that is underlain by an indurated carbonate sheet. Carbonate nodules and chalky micritic matrix materials underlie the sheet carbonate. The calcic paleosols display carbonate morphology ranging from stage IV to stage VI. The calcic pedocomplexes also possess distinct physical properties that aid in subsurface identification. The combined density and velocity differences between paleosols and nonpedogenic strata result in bright reflections on seismic sections and distinct well-log signatures. Although the calcic pedocomplexes and regional unconformity associations were first described within Tertiary strata of southwestern Montana, the same associations exist in numerous localities in the Great Plains and in other parts of the western United States. The extensive occurrence of the calcic paleosols and regional unconformity associations throughout this large area underscores their utility as a regional correlation tool. Moreover, the delineation of regional unconformities that are largescale sequence boundaries by pedocomplexes has broad implications for continental sequence stratigraphy. Keywords: calcic, paleosol, sequence, Tertiary, pedocomplex. RESUMEN En las Great Plains y oeste de Estados Unidos, los complejos edáficos cálcicos están asociados con las discontinuidades regionales cuyas edades aproximadas son: 30 Ma, 20 Ma, y 4 Ma. En el suroeste de Montana, estos edafocomplejos cálcicos se observan fácilmente en afloramientos de superficie y contienen varios perfiles edáficos Hanneman, D.L., and Wideman, C.J., 2006, Calcic pedocomplexes—Regional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States, in Alonso-Zarza, A.M., and Tanner, L.H., eds., Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates: Geological Society of America Special Paper 416, p. 1–15, doi: 10.1130/2006.2416(01). For permission to copy, contact [email protected]. ©2006 Geological Society of America. All rights reserved.

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Hanneman and Wideman incompletos. En los casos en los que estos complejos edáficos están más completos, un perfil individual puede contener un horizonte argílico (Bt) o argílico/calcáreo (Bt[k]), un horizonte K, y un horizonte C. Sin embargo, a menudo el horizonte Bt(k) está truncado o puede estar ausente totalmente en un perfil determinado. El horizonte K tiene una zona superior laminada que se sitúa por encima de una capa carbonática endurecida. Por debajo de la capa dura se reconocen nódulos carbonáticos y material micrítico pulverulento. Los paleosuelos carbonáticos tienen estadios morfológicos que varían entre IV y VI. Los complejos edáficos cálcicos también presentan propiedades físicas que facilitan su identificación en el subsuelo. Las combinación de las variaciones de densidad y velocidad en paleosuelos y estratos sin paleosuelos da lugar a reflexiones importantes en los perfiles sísmicos y a rasgos distintivos en sondeos. Si bien la asociación entre complejos pedocálcicos y las discontinuidades regionales se describió por primera vez en estratos Terciarios del suroeste de Montana, estas mismas asociaciones se reconocen también en muchas otras zonas de las Great Plains y en otras partes del oeste de Estados Unidos Norteamericanos. La frecuente presencia de estas asociaciones en una zona tan amplia indica su utilidad como herramienta de correlación regional. Además, la delimitación de las discontinuidades regionales que constituyen límites de secuencias de gran escala, y que están marcados por estos complejos edáficos, tiene implicaciones importantes para aplicar en la estratigrafia secuencial de cuencas continentales. Palabras clave: paleosuelos cálcicos, secuencias, Terciario, edafocomplejos.

INTRODUCTION Tertiary continental strata of the Great Plains and western United States typically contain a multitude of various types of paleosols. In southwestern Montana, Tertiary paleosols commonly contain cambic, argillic, and calcic horizons; oxic horizons occur only within the basal portions of the Tertiary section (Hanneman, 1989). Of particular interest within these Tertiary continental deposits are calcic paleosols. Because of a marked climatic change to drying and cooling conditions within much of this area from ca. 33 Ma to ca. 4 Ma (Prothero, 1994, 1998; Wing, 1998; Retallack, 1992, 1998; Retallack et al., 2000), calcic paleosols commonly occur throughout the age equivalent part of the Tertiary section. Calcic paleosols with carbonate morphology stages IV and V occur within pedocomplexes at particular times within Tertiary basin fill of southwestern Montana. These times equate to regional unconformities in the northwestern United States that occurred at ca. 30 Ma, 20 Ma, and 4 Ma (Hanneman and Wideman, 1991; Hanneman et al., 1994, 2003). Consequently, these pedocomplexes mark sequence boundaries within continental Tertiary strata in southwestern Montana (Hanneman and Wideman, 1991; Hanneman et al., 1994); the sequence boundaries noted in southwestern Montana have recently been extended into central Washington (Hanneman et al., 2003) using criteria other than unconformity-bounding paleosols. The concept of using paleosols to define sequence boundaries in nonmarine strata has also recently been applied to other geologic settings. McCarthy et al. (1999) used interfluve paleo-

sols in the Cenomanian Dunvegan Formation of British Columbia to define sequence boundaries. Weissmann et al. (2002) marked sequence boundaries in Quaternary Kings River alluvial fan strata near Fresno, California, by laterally extensive, moderately mature paleosols and incised valley bases. Demko et al. (2004) used laterally continuous, mature paleosols to delineate regional unconformities within the Jurassic Morrison Formation of the U.S. Western Interior. Specifically for calcic paleosols, Gulbranson (2004) noted that calcretes within the Chinle Formation of the southwestern United States signify unconformities and delineate a terrestrial sequence stratigraphy for members of the Chinle Formation. Tandon and Gibling (1997) observed pedogenic nodular and underlying groundwater calcretes at sequence boundaries in Upper Carboniferous cyclothems in the Sydney Basin of Atlantic Canada. The purpose of this paper is to initially describe the calcic pedocomplexes in Tertiary basin fill of southwestern Montana. Because the calcic pedocomplexes do delineate regional unconformities, we will then detail their use as sequence boundary indicators in continental strata. The utility of using calcic pedocomplexes as sequence boundary markers will be further enhanced by documenting their existence within Tertiary strata of the Great Plains and western United States. CALCIC PEDOCOMPLEXES Calcic paleosol pedocomplexes typically occur within the Tertiary basin fill of many valleys in southwestern Montana

Calcic pedocomplexes (Fig. 1). The calcic pedocomplexes contain at least two calcic paleosols that are generally separated by small thicknesses of C horizon material. We define calcic paleosols informally as paleosols that have a large amount of secondary carbonate present in the form of calcic horizons (Machette, 1985). Although calcic paleosols have been placed into classifications such as Aridosols (Retallack, 1993), Calcisols (Mack et al., 1993), or paleo-Aridosols (Nettleton et al., 2000), we have not yet identified an A horizon within individual profiles of the southwestern Montana paleosol stacks, and there is typically, at best, only a truncated part of a B horizon within the profiles. Gardner et al. (1992) also noted the absence of the A and B horizons in Neogene calcic paleosol stacks of western Nebraska. These authors suggested that their absence may result from several factors such as: (1) the horizons generally not being well developed or very thick in some Aridosols, (2) the upward growth of the calcic horizon may

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overprint the A and B horizon, and (3) the A and B horizons are more prone to erosion than petrocalcic horizons are. In any event, with the absence of a diagnostic surface horizon in the paleosol profile, we find that “calcic paleosols” is the most appropriate term for these paleosols. In former publications, we referred to the vertical configuration of calcic paleosols that we observed in southwestern Montana as calcic paleosol stacks (Hanneman and Wideman, 1991; Hanneman et al., 1994, 2003). However, instead of the term “paleosol stack,” we now prefer to use the term calcic “pedocomplexes” in accordance with the definition for pedocomplex as proposed to the Paleopedology Commission of International Union for Quaternary Research (INQUA). The proposed definition states that a pedocomplex is composed of two or more paleosols that are separated over large areas by a thin deposit of C horizon material, and are overlain and underlain by greater amounts

MONTANA

HELENA

BUTTE B Va ig H lle ol y e

Jefferson Valley

BOZEMAN

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UNITED STATES

y

SCALE

n Valle

Horse Prairie

Madiso

DILLON

0

MONTANA

Gallatin Valley

Beaverhead Valley Ruby Valley

N

d

Deer Lodge Valley

Tos to Vall n-Tow ey nse n

Bitterro ot Valley

MISSOULA

Centennial Valley

KILOMETERS

Figure 1. Location map for southwestern Montana valleys, with selected valleys identified on a digital relief image of southwestern Montana (Montana State Library, NRIS data bank, 2001).

A

E

PC

PC

B

IP

IP

IP

Kl

PC

Ku

F

IP

IP

IP

D Figure 2. Calcic pedocomplexes and individual paleosols within the pedocomplexes, southwestern Montana (detailed locations for calcic pedocomplexes are given in Table 1). (A) Calcic pedocomplex located in lower Madison Valley, Madison Bluffs area. Maximum outcrop thickness of pedocomplex (PC) in photograph is ~6 m. (B) Closer view of individual calcic paleosols (IP) within the Madison pedocomplex. Note irregular boundaries on paleosols and the presence of C horizon material between the individual paleosols. Backpack at base of outcrop is 0.5 m in height. Maximum outcrop thickness of pedocomplex in photograph is ~5 m. (C) Calcic pedocomplex located in the upper Ruby Valley. (D) Individual calcic paleosols within the Ruby pedocomplex become welded when traced laterally. Global positioning system (GPS) unit at base of section is 15 cm in length. (E) Calcic pedocomplex located in the central Deer Lodge Valley. Outcrop thickness is ~5 m. (F) An individual calcic paleosol from the Deer Lodge pedocomplex that contains laminae within the upper part of the K horizon (Ku) and is underlain by a well-indurated carbonate sheet in the lower K horizon (Kl). Portion of Jacob staff in photo is 1 m in length.

IP = Individual Paleosol Ku = Upper Part of K Horizon in Individual Paleosol Kl = Lower Part of K Horizon in Individual Paleosol

PC = Pedocomplex

Legend

C

Calcic pedocomplexes of strata that contain weak to no evidence of soil development (Catt, 1998). Additionally, individual paleosols within a pedocomplex often are “…discontinuous, being in places truncated or cut out by small disconformities and/or amalgamated with other paleosols” (Morrison, 1998, p. 31). The term “pedocomplex” is synonymous with other terminology used in paleosol studies, such as compound and multistory paleosols. Figure 2 depicts calcic pedocomplexes found in some valleys of southwestern Montana. Each pedocomplex (Figs. 2A and 2C) contains at least two calcic paleosols and occurs between thick sections of nonpedogenically modified strata. And, as previously noted, individual paleosols (Figs. 2B, 2D, and 2F) may be discontinuous and/or amalgamated (amalgamated is synonymous with the terms “welded” and “composite”; see North American Commission on Stratigraphic Nomenclature, 1983; Morrison, 1998, p. 31) even when traced laterally over short distances. Nonetheless, the pedocomplex itself may be traced over a considerable distance. Surface Calcic Pedocomplex Paleosol Profiles The pedocomplexes characteristically contain several partial soil profiles. An individual profile may include in the most idealized scenario, in descending order, (1) an argillic (Bt) horizon, (2) an argillic/calcareous (Btk) horizon, (3) a K horizon, and (4) a C horizon (Fig. 3A). An argillic diagnostic subsurface B horizon (Bt) may be present in an individual soil profile of a pedocomplex. Bt horizons contain blocky structure; illuviated clays form bridges between grains and coat ped faces. The majority of Bt horizons in southwestern Montana Tertiary deposits are developed within tuffaceous mudstone, and thus their color range is very similar to pedogenically unmodified mudstone beds with very pale brown (10YR 5/4) to yellow gray (10Y 5/2) colors. In a few sections, Bt horizons are developed on sandy parent material, and the color range is more varied, from light reddish brown (5YR 6/3) to light brown (10YR 7/3). Root traces are common within the Bt horizon. Although the root casts and rhizoconcretions are typically calcareous, they may be also be composed of silica or sediments. Where these root structures are calcareous and are numerous, the horizon is better termed a Btk horizon. Root traces are from 0.1 cm to 2 cm in diameter and range up to 30 cm in length. The Bt(k) horizon is commonly truncated within the pedocomplex and can be entirely absent from a soil profile within the pedocomplex. However, where the horizon is preserved, it has a maximum observed thickness of 0.3 m. The K horizon (Fig. 3B) is the locus of secondary carbonate accumulation within the profile. As originally noted by Gile et al. (1965, p. 74) the carbonate is “present as an essentially continuous medium. It coats or engulfs, and commonly separates and cements skeletal pebbles, sand, and silt grains….” This type of carbonate is a K-fabric, and according to the definition originally set forth by Gile et al. (1965), a K horizon must have more than 90% K-fabric. Even though the K horizon has never

5

been formally accepted as a master horizon into Soil Taxonomy (Soil Survey Staff, 1975), we find it extremely helpful for use in separating the more weakly developed calcic horizons (Bk) from those horizons with major authigenic carbonate accumulations. The uppermost part of the K horizon contains laminations that range in thickness from 0.2 cm to 3 cm. The laminated part of the K horizon attains a maximum thickness of 0.3 m. A wellindurated sheet of carbonate occurs below the laminated zone. Floating skeletal grains, clasts, pisoliths, root casts, and some laminations are contained within the carbonate sheet (Figs. 3C and 3D). The hardpans are often fractured and brecciated. Maximum thickness of the hardpan part of the K horizon is 1 m. Powdery to indurated carbonate nodules are often present below the carbonate sheet (Fig. 3E). The nodular zone may also include micrite matrix material. More commonly, the chalky micritic matrix horizon underlies the nodular zone. This K horizon profile is similar to the pedogenic calcrete idealized profiles detailed by Esteban and Klappa (1983), Goudie (1983), and summarized by Alonso-Zarza (2003). Secondary silica, in the form of nodules, stringers, and silicified root traces commonly occurs in association with the K horizons (Fig. 3F). The silica nodules range from 5 to 20 cm along the long axis; the stringers vary from 1 to 5 cm in thickness. Both the nodules and stringers are usually located in the K-C horizon transition zone. The silicified root traces occur throughout the K to upper C horizon. Contact of the K horizon with the underlying C horizon is gradational. As stated already, the paleosol profile described here and shown in Figure 3 is an idealized profile. Not all features noted for the profile are typically found in every southwestern Montana calcic paleosol. The upper surface of the K horizon can be extremely irregular (Fig. 2B), and the entire paleosol can even be truncated when traced laterally. Individual paleosols become welded with other paleosols (Fig. 2D) within some pedocomplexes. However, there are usually enough profile characteristics present in field exposures to identify calcic paleosols. Subsurface Calcic Pedocomplexes Hanneman et al. (1994) documented the identification of calcic paleosol stacks, now termed calcic pedocomplexes herein, in the subsurface of the Deer Lodge Valley, southwestern Montana (Fig. 4A). Calcic pedocomplexes with accumulated thickness in excess of 10 m appeared in the subsurface as a collection of several relatively thin, high-velocity–high-density zones within the basin fill. Zone thickness ranged from 1 to 1.5 m. Density varied within the zones by as much as 0.6 g/cm3, and differed by as much as 0.9 g/cm3 from material immediately above these zones. Velocity differed by as much as 10 ft/ms (3.3 m/ms) from the overlying material and caused bright reflections on seismic sections. Synthetic seismograms were used to tie well-log and seismic data (Fig. 4B). The high-velocity–high-density zones in the Cenozoic basin fill were interpreted to be calcic paleosols based on data extracted

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Hanneman and Wideman

B Horizons

K Horizon Laminations

C

K Horizon - Floating Clasts in Carbonate Sheet

D

K Horizon - Root Casts

Bt or Btk K

Legend Blocky Peds

K-C

transition

C

Rootlets Laminations Floating Grains Nodules Silica Stringers Mudstone

F

K - C Transition Zone Silica Stringers

E

K Horizon - Chalky/Nodular Zone Figure 3. (A) Idealized calcic paleosol profile (detailed locations for features in profile are given in Table 1). (B) Laminations in upper part of K horizon, lower Madison Valley. (C) Floating skeletal clasts in micrite of indurated sheet portion of K horizon, central Deer Lodge Valley. Lens cap is 67 mm in diameter. (D) Root casts in indurated sheet portion of K horizon, central Deer Lodge Valley. Film cap is 35 mm in diameter. (E) Chalky/nodular zone (indicated by arrow) present beneath indurated sheet of K horizon, lower Madison Valley. Quarter for scale. (F) Silica stringers (indicated by arrows) of the K horizon–C horizon transition zone, Jefferson Valley. Hammer is ~0.45 m in length.

A

from a suite of well logs that included sonic, density, resistivity, neutron, and lithology logs, and from well-cutting analyses (Fig. 4C). The pedogenic origin of the zones was shown by (1) wellcutting chips from the high-velocity–high-density zones that exhibited pedogenic features associated with calcic paleosols, (2) paleosol horizonation interpreted from well-log analysis, (3) the absence of minerals normally associated with lacustrine deposits, and (4) comparison with surface paleosols (Fig. 4D).

phologies as outlined by Machette (1985, p. 5; Table 1 therein). The stage IV morphology characteristics include laminae up to 1 cm in thickness in the upper part of the K horizon, with some laminae draped over fracture surfaces. Laminae of stage V are up to 3 cm in thickness. Fractures in the K horizon are typically coated with laminae, and pisolites are present. Thickness of the K horizon ranges from 0.5 to 1.5 m. Lateral Variation within Paleosol Stacks

Morphology of Calcic Paleosols The calcic paleosols within the calcic pedocomplexes of Tertiary basin fill in southwestern Montana have calcium carbonate morphologies consistent with the stage IV to stage V mor-

Although a calcic pedocomplex can be traced for several miles within a basin, lateral variation commonly occurs. The variance may be within individual paleosol profiles of the pedocomplex, in the vertical succession of horizons within a

Calcic pedocomplexes

A

7

B

MS 1-25 0.7

N

S

1.0

Two-Way Travel Time

1.5

0

Resistivity 70 ohms CNL 0.13 NPHI 0.62 1

Measured Surface Section Calcic Pedocomplex Deer Lodge Valley 100

915

Meters

930 10

945 Argillic Horizon K Horizon C Horizon

0

20

Depth (m)

945 MSP 1-25

100

930

Depth (m)

930

0

100 % Quartz

K Horizon Non-K Horizon

% Calcite Interpreted Pedocomplex

0 % Calcite

Depth (m)

924

Kilometers 1.0

Lithology Log MSP 1-25

D

945

0 MSP 1-25

C Figure 4. Hanneman et al. (1994) used well-log data, seismic data, and well cuttings analyses to define calcic paleosols and pedocomplexes in the subsurface of the Deer Lodge Valley, southwestern Montana. (Figure was modified from Hanneman et al., 2003.) (A) Geologic setting of the Deer Lodge Valley, southwestern Montana. Location of Montana State Prison (MSP) 1-25 well and seismic line of 1B are also shown. (B) Seismic-reflection line from the Deer Lodge Valley. Synthetic seismogram generated from well-log data of MSP 1-25 is tied to bright reflectors that occur on the seismic data at 1.0–1.1 s (~930–980 m in depth). (C) Paleosol profile delineated by resistivity and neutron log data. Argillic paleosol horizons are interpreted to have low resistivity; K horizons are interpreted where porosity is low on the neutron log. The overlay of these two logs depicts individual profiles within the mature pedocomplex. The K horizons also correspond to the interval’s high calcium content on the lithology log. CNL—compensated neutron log; NPHI—neutron porosity. (D) Matrix identification depth plot correlated with a surface pedocomplex measured in the northern Deer Lodge Valley. The thickness and frequency of increased calcite-content zones compare reasonably well with the K horizons of the surface pedocomplex.

complex, and in the overall thickness of the pedocomplexes (Fig. 5). Within individual profiles, soil descriptive features such as texture, color, root trace concentration, and horizon boundary distinctness often vary laterally, particularly within the Bt(k) horizons (Fig. 5A shows K horizon termination; Fig. 5B shows scoured K horizon top). These changes can be related to local soil-forming controls, such as topography, parent material texture, and scour events (McCarthy and Plint, 1998; McCarthy et

al., 1999). Lateral changes that affect soil horizon succession and overall pedocomplex thickness may be correlated to calcic profile initial development position and the variable deposition and/or erosion events associated with calcic profile formation. Typically, soil profile development begins on stabilized areas within a basin, such as interfluves or distal portions of alluvial fans (Alonso-Zarza et al., 1998; McCarthy et al., 1999). However, in order to generate a pedocomplex, episodic sedimentation

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needs to occur. Periodic deposition of sediment over the initial calcic soil profile provides more space for plant growth, and new cycles of calcic soil formation are begun. Given time, calcic soils will form over large parts of a basin, wherever surfaces become inactive. With episodic sedimentation, calcic pedocomplexes will eventually build over this larger area. The result of this soil-forming activity on a basin-wide scale is that (depending on a variety of factors, such as differential subsidence, climate, hydrology, parent materials) over time, thicker sections of calcic soils, and soils with somewhat different profiles, may exist in different areas. Alonso-Zarza et al. (1998) documented this pedocomplex variability on Pleistocene alluvial fan surfaces of the Campo de Cartagena–Mar Menor Basin, Murcia, southeast Spain. These authors noted the formation of different calcrete profiles in proximal and distal fan areas. These differences resulted from the interplay of erosion and deposition on the fan surfaces. In proximal fan areas, some soil horizons were stripped from a profile, while on the more stable areas, reworking and brecciation occurred, which would presumably produce a brecciated horizon. Conversely, in the distal fan areas, episodic sedimentation initially disrupted calcic soil formation, leading to another cycle of calcic soil formation. Collectively, these differences in the individual soil horizons of the various fan areas resulted in complex composite profiles being thicker in the distal fan areas than in proximal areas. CALCIC PEDOCOMPLEXES AS SEQUENCE BOUNDARY INDICATORS The calcic pedocomplexes present in the Tertiary basin fill of southwestern Montana developed over extended periods of time as evidenced by their advanced carbonate morphology stages. Soil development ceased for brief intervals because of sediment

influx, but then resumed, adding yet another soil profile to the pedocomplex. Collectively, the individual paleosol profiles contained within a pedocomplex represent significant breaks within the Tertiary basin-fill record. Consequently, the calcic pedocomplexes mark unconformities that occur between large-scale sedimentary packages. The age of each unconformity is constrained by paying strict attention to well-documented fossil vertebrate and radioisotopic age data taken from units occurring on both sides of the unconformity. The regional unconformities marked by calcic pedocomplexes occur at ca. 30 Ma, 20 Ma, and 4 Ma. The magnitude of each hiatus represented at these regional unconformities in southwestern Montana is estimated to be ~3–4 m.y. where all sequences are present. Because age data are derived not directly from the bounding surface itself but from strata that occur at some distance above and below pedocomplexes, there is yet a degree of uncertainty that exists for exact ages of the sequence bounding surfaces. Consequently, we are constantly looking for better age constraints on the regional unconformities. Montana Unconformity-Bounded Sequences Five unconformity-bounded sequences were initially delineated within continental Tertiary strata in southwestern Montana (Hanneman and Wideman, 1991; Hanneman et al., 2003). The sequences have upper and lower bounding surfaces that are unconformities of regional extent. We refer to these unconformity-bounded sequences as large-scale sequences, because they contain sizeable packages of basin-fill material. The unconformity-bounded sequences can include several hundred meters of strata, many different lithologies, and represent several million years of the geologic record. Calcic pedocomplexes mark the unconformities that separate four of these unconformity-bounded

A

B

Scoured K horizon K horizon termination Glove for scale

Figure 5. Examples of lateral variability that occurs within calcic pedocomplexes located in the lower Madison Valley of southwestern Montana (location of pedocomplex given in Table 1). (A) K horizon termination in a 20 Ma calcic pedocomplex. Terminated K horizon is ~0.3 m in thickness. (B) Scoured K horizon top (laminar zone and a part of the carbonate sheet). Glove is 24 cm in length.

Calcic pedocomplexes sequences. The regional unconformity-bounded sequences delimited by calcic paleosol stacks are informally designated as: sequence 2—middle/late Duchesnean to Whitneyan (ca. 38–30 Ma), sequence 3—Arikareean (ca. 27–20 Ma), sequence 4—Barstovian to Blancan (ca. 16–4 Ma), and sequence 5—early Quaternary (ca. 1.8 Ma) to the present (Fig. 6). Locations for examples of these calcic pedocomplexes and unconformity associations are given in Table 1. There are some differences among the calcic pedocomplexes that occur on the upper bounding surfaces of sequences 2, 3, and 4 in southwestern Montana. Where sequence 3 directly overlies sequence 2, pedocomplex development at the top of sequence 2

Epoch

Ma

Holocene

0.01 Pleistocene 1.75 Pliocene 5.3

North American Land Mammal Ages Rancholabrean Irvingtonian Blancan

Ma

0.1 1.75 4.9

Ma

9

typically contains a maximum of three paleosol profiles. The K horizon in these profiles has carbonate morphology equivalent to stage IV. However, in many locations, sequence 2 is overlain directly by sequence 4. In these areas, calcic paleosol stacks have several paleosol profiles and K horizons attain a carbonate morphology stage V. Calcic pedocomplexes at the top of sequence 3 have several paleosol profiles, and the K horizons in each profile reach a carbonate morphology stage V. It should be noted that in some past studies of southwestern Montana Tertiary basin fill, paleosols at this same stratigraphic level have been described as “red, saprolitic, and kaolinite-rich” (Thompson et al., 1982, p. 415;

Southwestern Montana

Washington

Sequence 5

High Cascade

Sequence 4

Walpapi

5

Hemphillian

9 Clarendonian

Miocene

Barstovian Hemingfordian

23.8

11.5 15.9

Calcic pedocomplexes 4 Ma

15

20 Ma

19

Arikareean

30 Ma

25

Sequence 3

Upper Kittitas

35

Sequence 2

Lower Kittitas

Oligocene 33.7

Whitneyan Orellan

30 32 33

Chadronian

Legend

Approximate time and duration of hiatus Newly defined hiatus

37 Duchesnean

40

Eocene Uintan Bridgerian

Challis

45 Sequence 1

47 50.4

Wasatchian

54.8 Clarkforkian

55.5 56.2

55

Tiffanian

Paleocene

60.5 Torrejonian

65

Puercan

63.5 65

65

Figure 6. Correlation of southwestern Montana sequences with central Washington (CW) sequences. The dashed lines within the CW Kittitas represent the newly recognized 30 Ma to 27 Ma hiatus. The gray area in between some of the wavy lines represents the estimated magnitude of the hiatus. Age estimates for the Cenozoic epochs are ones proposed by Berggren et al. (1995). Age estimates for Paleogene North American Land Mammal Ages (NALMA) are based on those given by Prothero (1995). Age estimates for Neogene NALMA are those delineated by Woodburne and Swisher (1995).

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Hanneman and Wideman

TABLE 1. LOCATIONS FOR EXAMPLES OF TERTIARY CALCIC PEDOCOMPLEXES (PC) AND ASSOCIATED UNCONFORMITIES IN SOUTHWESTERN MONTANA Remarks Approximate age of Valley Section, township – NAD 1927, Zone 12, UTM U.S. Geological Survey 7.5ƍ unconformity location range easting/northing coordinates (Ma) quadrangle 4 Jefferson– SE ¼ section 34, T 387150 m E; 5031420 m N Beaverhead Rock, Vertebrate fossils below PC are Biltmore area 4 S, R 7 W Montana Hemphillian; mapped Quaternary gravels are above PC. 20 Upper Ruby NE ¼ section 3, T 9 406085 m E; 4992985 m N Belmont Park Vertebrate fossils below PC are Valley S, R 5 W Ranch, Montana late Arikareean; vertebrate fossils above PC are Barstovian. 20 Central Deer NW ¼ section 31, T 363500 m E; 5142162 m N Conleys Lake, Barstovian fossils are above Lodge 3 N, R 9 W Montana PC; mapped Arikareean strata are below PC. ? 20 (Lower Lower SW ¼ section 34, T 463440 m E; 5070289 m N Manhattan SW, Barstovian vertebrate fossils boundary age Madison– 1 N, R 2 E Montana are above PC; no age unconstrained) Madison constraints were found below, Bluffs area so they could range from Chadronian to Arikareean in age (37–19 Ma). 30 Jefferson– SE ¼ section 29, T 422640 m E; 5082290 m N Black Butte, Vertebrate fossils below PC are Golden 2 N, R 3 W Montana Chadronian; fragmentary Sunlight Mine oreodont fossils above PC most likely are Arikareean (W. Coppinger, July 2004, personal commun.). 30 Jefferson– NE ¼ section 28, T 413542 m E; 5073180 m N Whitehall, Vertebrate fossils below PC are Renova area 1 N, R 4 W Montana Chadronian; thin veneer of Quaternary loess is above PC.

Fields et al., 1985). Strata at the locations sampled for the oxic horizon were originally thought to be ca. 21–17 Ma. Later mapping with more-detailed age control revealed that strata at these sample localities are much older than previously thought. Consequently, recent work has shown no evidence for an oxic horizon at this stratigraphic level and that the regional unconformity at ca. 21–17 Ma is instead marked by calcic pedocomplexes (McLeod, 1987; Hanneman, 1989; Hanneman and Wideman, 1991; Portner and Hendrix, 2004). Calcic pedocomplexes that mark the upper surface of sequence 4 are similar to those at the top of sequence 3. However, these pedocomplexes are often absent in the southern areas of southwestern Montana, where there are no reported uppermost Tertiary strata and there are scant Quaternary age sediments. It may well be that much of this part of the section (including the calcic pedocomplexes) has been stripped from the basins due to recent uplift of the Yellowstone–Snake River Plain area of Idaho, Wyoming, and Montana. Washington Unconformity-Bounded Sequences The Cenozoic unconformity-bounded sequences identified in Montana extend into central Washington based upon work originally done by Cheney (1994, 2000). Hanneman et al. (2003) recognized that there are equivalent interregional unconformity-bounded sequences in this area: Lower Kittitas—ca.

36–30 Ma, Upper Kittitas—ca. 27–22 Ma, Walpapi—ca. 20–4 Ma, and High Cascade—ca. 4 Ma to present (Fig. 6). Although Cheney (1994, 2000) emphasized the importance of changes in lithology and provenance in initially delineating the Washington unconformity-bounded sequences rather than using the identification of pedocomplexes, the literature reports a caliche constraining the upper surface of the Walpapi Sequence at the Hanford Site (Pasco Basin) in south-central Washington. The caliche is developed on the Miocene-Pliocene Ringold Formation, and middle to late Pleistocene sediments overly it. The thickness of the caliche ranges from 0 to 20 m, and the unit is bounded by irregular surfaces having as much as 25 m of relief. The number of carbonate layers differs with the thickness of the deposits. Carbonate morphology of the layers varies from stage I to stage V. The caliche is interpreted to be pedogenic, although some modification to the paleosols by groundwater processes may have occurred (Slate, 1996). EXTENSION OF SEQUENCE BOUNDARIES DELINEATED BY CALCIC PEDOCOMPLEXES INTO THE GREAT PLAINS AND OTHER WESTERN U.S. AREAS The unconformity-bounded sequences cited above have been extended into the western United States and the northern Great Plains in previous studies by Hanneman and Wideman

Calcic pedocomplexes (1991), Cheney (1994, 2000), and Hanneman et al. (2003). Constenius et al. (2003, see their Fig. 19) expanded on these investigations and documented age-equivalent unconformity-bounded sequences throughout the Cordilleran orogenic belt that extends from southern Canada to Mexico. Based upon the interpretation of extensive structural data, Constenius et al. (2003) showed that the unconformity-bounded sequences record plate-tectonic interactions and continental deformation. Because age-equivalent Cenozoic unconformity-bounded sequences can be extended throughout the Great Plains and western United States, we expect that where equivalent soil-forming conditions prevailed, calcic pedocomplexes should delineate regional unconformities. A recent literature search revealed the likely identification of the paleosol–regional unconformity associations. Several occurrences of the calcic paleosol–regional unconformity associations at ca. 30 Ma, 20 Ma, and 4 Ma from

WASHINGTON

11

these areas are listed next. It is possible that many other occurrences of paleosol–regional unconformity associations are present in these areas. The locations of the paleosol–regional unconformity associations are shown in Figure 7. Regional Unconformity at ca. 30 Ma Pinnacles Lookout, Badlands National Park, Southwestern South Dakota Pinnacle Series paleosols occur in the top of the Poleside Member (early Oligocene) of the Brule Formation. The Pinnacle Series contains calcic paleosols that have prominent horizons of hard calcareous nodules at shallow depths. Elongate calcareous concretions that are interpreted as rodent burrows are also abundant in the paleosols. The calcic horizons probably only correspond to a stage II or at maximum stage III carbonate

NORTH DAKOTA

MONTANA

LEGEND Calcic Pedocomplexes/ Regional Unconformity Associations

SOUTH DAKOTA OREGON

4 Ma

IDAHO

20 Ma

WYOMING NEBRASKA

Southern High Plains with 4 Ma association

NEVADA COLORADO

UTAH

30 Ma

KANSAS

CALIFORNIA

OKLAHOMA

Southwestern Montana - All Associations Are Present

ARIZONA NEW MEXICO

TEXAS

0

500 Kilometers

N

Figure 7. Locations of calcic pedocomplexes and regional unconformities in the Great Plains and western United States. Details of locations and age constraints for the calcic paleosol pedocomplexes and regional unconformities are given in text.

12

Hanneman and Wideman

morphology. The Rockyford Ash (which has a radioisotopic age of close to 29 Ma) of the Sharps Formation unconformably overlies the Poleside Member of the Brule Formation (Retallack, 1983). Banner County, Western Nebraska The top of the upper Eocene to early Oligocene Brule Formation is locally cemented here with pedogenic nodular caliche. The Brule is overlain by gravel of the Neogene Ogallala Group (Gardner et al., 1992). Medicine Lodge Creek Valley, South-Central Idaho Nodular limestone armors the top of Medicine Lodge beds present in the Medicine Lodge Creek Valley, in Clark County, Idaho, and in sparse locations to the southwest as far as the southern Lemhi Range (Hodges and Link, 2002). The nodular limestone is ~2 m thick at the head of the south fork of Deep Creek, where it lies stratigraphically above a tuffaceous mudstone unit that has yielded a 40Ar/39Ar age of 30.23 ± 0.45 Ma. The nodular limestone is most likely pedogenic in origin (Hodges et al., 2004; M.K.V. Hodges, 2005, personal commun.). Regional Unconformity at ca. 20 Ma South Killdeer Mountains, Southwestern North Dakota (Medicine Hole Plateau, Dunn County) The Arikaree Formation contains a ledge-forming bed of carbonate/sandstone that is ~9 m in maximum thickness. This unit is known as the burrowed marker unit because it contains abundant fossil burrows (Forsman, 1986; Murphy et al., 1993). Delimata (1975) noted that this bed is an exceptional stratigraphic marker for the South Killdeer Mountains. He described the unit as containing tuffaceous limestone, nodular limestone, and banded limestone. Although Delimata interpreted the burrowed marker unit as a lacustrine deposit, its described features are more consistent with a pedogenic interpretation for the unit. The same marker bed may be present at White Butte, North Dakota, ~130 km southwest of the South Killdeer Mountains (Murphy et al., 1993). Presently, the burrowed marker unit is age constrained by: (1) a fission-track age of 25.1 ± 2.2 Ma taken from the base of the burrowed marker unit, and (2) the occurrence of two genera of oreodonts, Merychyus and Merycochoerus, located ~27 m above the stratigraphic position of the fission-track age. The range zones of these oreodonts overlap in the latest Arikareean to earliest Hemingfordian (Hoganson et al., 1998). Monroe Canyon, Nebraska The “terminal” paleosol at the head of Monroe Canyon, along the high rim, is ~4.6 m in thickness and is developed on the Harrison Formation. The paleosol appears to be a silcretecalcrete intergrade (Nash and Shaw, 1998), and it contains concentrations of rhizoliths and burrows, an upper laminar petrocalcic horizon, and a surface cemented as silcrete. Remnants of this

paleosol, the terminal Harrison paleosurface, are on flat-topped hills and buttes from Monroe Canyon west to the NebraskaWyoming state boundary (a distance of ~20 km). The Eagle Crag Ash, with a fission-track age of 19.2 ± 0.5 Ma, overlies the Harrison paleosurface by ~2 m; the Agate Ash, with a 40K/40Ar age of 21.9 Ma, occurs ~10 m below the Harrison paleosurface at Agate National Monument, in the Hoffman channel section (Hunt, 1990; MacFadden and Hunt, 1998). Regional Unconformity at ca. 4 Ma Kimball and Banner Counties, Western Nebraska Pedocomplexes of calcareous paleosols are present in the uppermost Neogene Ogallala Group, at the top of the Ash Hollow Formation, western Nebraska. The pedocomplexes are ~12 m thick, contain up to four paleosols, and each paleosol is ~1 m thick. The uppermost calcic paleosol in a pedocomplex has reached stage IV carbonate morphology, and the lower paleosols are between a stage III and IV carbonate morphology (Gardner et al., 1992). Hagerman Fossil Beds National Monument, Southwest Idaho A caliche is developed on Pleistocene-Pliocene gravels and forms a cap rock in most of the monument and the surrounding area. The caliche averages several meters in thickness, but thins to less than a meter locally. It is a very dense layer and contains vertical fractures that are often recemented (Farmer and Riedel, 2003). Southern High Plains, Texas and New Mexico The uppermost late Tertiary Ogallala Formation typically includes a stage V paleosol or up to two stage IV “caprock” calcic paleosols, and may have a stage VI calcic paleosol where the Quaternary Blackwater Draw Formation overlies it. Where the Blackwater Draw Formation is only a thin veneer or is entirely absent (as is the case in large portions of the western High Plains), the Ogallala calcic paleosol cap rock is 1.5–10 m thick, and has stage VI carbonate morphology. In these areas, it is probable that the pedogenic carbonate accumulations present within “…numerous buried calcic soils and the surface calcic soils of full sections of the Blackwater Draw have been welded onto the uppermost Ogallala calcrete” (Gustavson, 1996, p. 37). It is also possible that in certain areas, the Ogallala cap rock may range in age from late Miocene to late Quaternary. Roswell-Carlsbad, Southeastern New Mexico Stage VI calcic paleosols are developed on the top of the Ogallala Formation in this area. The age of the calcic paleosol is thought to be late Pliocene (Bachman, 1976; Machette, 1985). Morman Mesa, Southeastern Nevada The Morman Mesa calcic paleosol is ~2.5 m thick and has stage VI carbonate morphology. It is developed on red quartz sand

Calcic pedocomplexes of the Muddy Creek Formation. The age of the calcic paleosol is thought to be late Pliocene (Gardner, 1972; Machette, 1985). Vertebrate fossil remains of medial Hemphillian (late Miocene) age have been reported for the Muddy Creek Formation in the Morman Mesa area (Williams et al., 1997). Vidal Junction, Southern California A stage VI calcic paleosol is developed on the top of the Miocene–early Pliocene Muddy Creek Formation in this area. The age designated for this calcic paleosol is late Pliocene (Bull, 1974; Machette, 1985). DISCUSSION The significant areas of discussion that follow from our work on calcic pedocomplexes and their association with regional unconformities center on the usefulness of calcic pedocomplexes, or in fact, any type of mature paleosol, as sequence stratigraphic tools. Even with lateral variation of pedocomplexes, sequence boundaries can be defined when one combines other techniques for mapping unconformities. Additionally, although the primary control on the sequences described herein is tectonic, higherresolution work on the pedocomplexes, their adjacent strata, and better age constraints will help in understanding secondary controls of sequence and pedocomplex formation. Calcic pedocomplexes and calcic paleosols with stage IV to stage VI carbonate morphology are associated with regional unconformities of ca. 30 Ma, 20 Ma, and 4 Ma from the Great Plains through a large part of the western United States. These paleosol–unconformity associations mark large-scale regional sequence boundaries and consequently aid in surface and subsurface mapping of regional sequences. The calcic paleosols are easily identifiable in surface sections and have distinct physical properties that can be recognized in various types of geophysical data. Where basins contain several thousand feet of fill, and only have basin margins sections exposed, the ability to identify calcic pedocomplexes and use them to separate the subsurface geology into at least large-scale unconformity-bounded sequences is extremely advantageous in basin research. Additionally, the widespread extent of the calcic paleosols–regional unconformities associations enhances their utility as a regional correlation tool. It is important to note that the regional hiatuses recognized at ca. 30 Ma, 20 Ma, and 4 Ma are marked by many different sets of calcic pedocomplexes. Some of these pedocomplexes are laterally extensive over large areas, such as the Great Plains, but others formed within discrete depositional basins. Depositional basins began to form in the Cordilleran foreland fold-and-thrust belt by ca. 49 Ma (Hanneman, 1989; Hanneman and Wideman, 1991; Constenius, 1996; Constenius et al., 2003; O’Neill et al., 2004). Thus, pedocomplexes that formed in discrete depositional basins may be physically traced only within a particular basin. The pedocomplexes that mark these regional hiatuses were probably developed at similar times in various locations due to regional tectonic and climatic controls.

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Because the regional unconformities defined at ca. 30 Ma, 20 Ma, and 4 Ma can be recognized throughout the Great Plains and the western United States, other types of mature paleosols should mark these same unconformities where climatic conditions differed. For example, in the Painted Hills of central Oregon, the 30 Ma regional unconformity separates the upper Big Basin Member of the John Day Formation from the overlying Turtle Cove Member of the John Day Formation. Mature ironrich paleosols are in the middle Big Basin Member and within the Big Basin Member, and the last one is located at the contact of the Big Basin Member and the overlying Turtle Cove Member of the John Day Formation (Bestland, 1997; Retallack et al., 2000). Even though a pedocomplex can be traced for up to several miles within a basin, lateral variation commonly occurs. The variance may be within individual paleosol profiles of the pedocomplex, in the vertical succession of horizons within a pedocomplex, and in the overall thickness of the pedocomplexes. The lateral variation is most likely related to factors such as the location of initial pedocomplex development within a basin, or the complex interplay of erosion and deposition rates (Tandon and Gibling, 1997; Alonso-Zarza et al., 1998; McCarthy et al., 1999; Weissmann et al., 2002). Although the lateral variance is easily recognized on the surface, the resolution of subsurface data may mask these differences. Where calcic paleosols or pedocomplexes are not present, angular stratal relationships, abrupt changes in provenance or lithologies, and the bases of incised valleys can also define sequence boundaries. These features can be mapped on the surface, and geometric patterns as indicators of unconformities can be recognized on seismic data. Collectively, these data types can be combined with paleosol information to complete the delineation of a sequence boundary. The calcic paleosols observed in southwestern Montana at the 30 Ma boundary are not as well developed (in regard to carbonate morphology and number of soil profiles within a pedocomplex) as those that mark the 20 Ma and 4 Ma regional unconformities. This appears to be a consistent feature of those boundaries throughout the Great Plains and western United States. The cause for this may be related somehow to a broad range of climate and/or tectonic controls, but presently, the actual reason for this difference in degree of paleosol development is not known. As stated previously, Constenius et al. (2003) have shown that the large-scale unconformity-bounded sequences defined in the northwestern United States by Hanneman and Wideman (1991), Cheney (1994, 2000), and Hanneman et al. (2003) are tectonically controlled sequences. However, future high-resolution work on these sequences will probably lead to an understanding of other secondary controls on their formation. The ages of the regional unconformities are given as approximate ages and are based upon currently available age constraints derived from radioisotopic age data and vertebrate faunal assemblages initially established in southwestern Montana. The ages appear to be fairly consistent across the Great Plains and western United States, but there is some range to these age designations. Historically, radioisotopic age

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data have been acquired in these geographic areas in order to place constraints on defining North American Land Mammal boundaries, the Cenozoic time scale, and the Cenozoic magnetopolarity time scale. Sparse attention has been paid to refining age constraints on regional unconformities. With additional age constraints, it may become apparent that there are timing differences among the regional unconformities. If the timing differences exist, they may be correlated to the time sweep on unconformity-bounded sequences boundaries noted by Constenius et al. (2003) and linked to regional tectonic events, or they may be indicators of timing differences in regional climatic change. In relation to the development of calcic pedocomplexes that mark regional unconformities, we find it of interest to contemplate the many Cenozoic relict calcic soils listed by Machette (1985, p. 11, Table 2 therein) for regions within the southwestern United States. These calcic soils may be young examples of the much older Tertiary calcic pedocomplexes. They may represent the different soils that could become pedocomplexes in a future geologic record. SUMMARY Calcic pedocomplexes with a maximum carbonate morphology of stage VI are associated with regional unconformities that have approximate ages of 30 Ma, 20 Ma, and 4 Ma. The calcic paleosols are easily identifiable in surface sections and have distinct physical properties that can be recognized in various types of geophysical data in the subsurface. The recognition of the calcic paleosol–unconformity association enables the separation of Cenozoic basin fill into at least large-scale unconformitybounded sequences, which can greatly enhance both surface and subsurface basin research. Although the 30 Ma, 20 Ma, and 4 Ma calcic pedocomplexes–regional unconformity associations were initially described in southwestern Montana, they can be traced throughout the Great Plains and western United States. The widespread extent of the calcic paleosols–regional unconformities associations enhances their utility as a regional correlation tool. Because the pedocomplexes delineate regional unconformities that are also large-scale sequence boundaries, the identification of the pedocomplex–unconformity association has broad implications for continental sequence stratigraphy. REFERENCES CITED Alonso-Zarza, A.M., 2003, Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record: Earth-Science Reviews, v. 60, p. 261–298, doi: 10.1016/S0012-8252(02)00106-X. Alonso-Zarza, A.M., Silva, P.G., Goy, J.L., and Zaza, C., 1998, Fan-surface dynamics and biogenic calcrete development: Interactions during ultimate phases of fan evolution in the semiarid SE Spain (Murcia): Geomorphology, v. 24, p. 147–167, doi: 10.1016/S0169-555X(98)00022-1. Bachman, G.O., 1976, Cenozoic deposits of southeastern New Mexico and an outline of the history of evaporite dissolution: U.S. Geological Survey Journal of Research, v. 4, p. 135–149.

Berggren, W.A., Kent, D.V, Aubrey, M.P., Swisher, C., III, and Miller, K.G., 1995, A revised Cenozoic geochronology and chronostratigraphy, in Berggren, W.A., Kent, D.V, Aubrey, M.-P., and Hardenbol, J., eds., Geochronology, time scales and global stratigraphic correlations: Unified temporal framework for an historical geology: Tulsa, Oklahoma, Society for Sedimentary Geology, SEPM Special Publication 54, p. 129–212. Bestland, E.A., 1997, Alluvial terraces and paleosols as indicators of early Oligocene climate change (John Day Formation, Oregon): Journal of Sedimentary Research, v. 67, no. 5, p. 840–855. Bull, W.B., 1974, Geomorphic tectonic analysis of the Vidal region, in Information Concerning Site Characteristics, Vidal Nuclear Generating Station [California]: Los Angeles, Southern California Edison Company, Appendix 2.5B, amendment 1, 66 p. Catt, J.A., 1998, Report of working group on definitions used in paleopedology: Quaternary International, v. 51/52, p. 84, doi: 10.1016/S1040-6182(98)902324. Cheney, E.S., 1994, Cenozoic unconformity-bounded sequences of central and eastern Washington: Washington Division of Geology and Earth Resources, Bulletin, v. 80, p. 115–139. Cheney, E.S., 2000, Tertiary geology of the eastern flank of the central Cascade Range, Washington, in Woodsworth, G.J., Jackson, L.E., Nelson, J.L., and Ward, B.C., eds., Guidebook for Geological Field Trips in Southwestern British Columbia and Northern Washington: St. John’s, Newfoundland, Canada, Geological Association of Canada, Cordilleran Section, p. 205–227. Constenius, K.N., 1996, Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt: Geological Society of America Bulletin, v. 108, p. 20–39, doi: 10.1130/0016-7606(1996)1082.3.CO;2. Constenius, K.N., Esser, R.P., and Layer, P.W., 2003, Extensional collapse of the Charleston-Nebo salient and its relationship to space-time variations in Cordilleran orogenic belt tectonism and continental stratigraphy, in Raynolds, R.G., and Flores, R.M., eds., Cenozoic Systems of the Western United States: Denver, Colorado, Society of Economic Paleontologists and Mineralogists, p. 303–353. Delimata, J.J., 1975, Petrology and geochemistry of the Killdeer carbonates [Ph.D. dissertation]: Grand Forks, North Dakota, University of North Dakota–Grand Forks, 256 p. Demko, T.M., Currie, B.S., and Nicoll, K.A., 2004, Regional paleoclimatic and stratigraphic implications of paleosols and fluvial/overbank architecture in the Morrison Formation (Upper Jurassic), Western Interior, USA: Sedimentary Geology, v. 167, p. 115–135, doi: 10.1016/j.sedgeo.2004.01.003. Esteban, M., and Klappa, C.F., 1983, Subaerial exposure environments, in Scholle, P.A., Bebout, D.G., and Moore, C.H., eds., Carbonate Depositional Environments: American Association of Petroleum Geologists Memoir 33, p. 1–96. Farmer, C.N., and Riedel, J., 2003, Water Resources Management Plan, Hagerman Fossil Beds National Monument: Hagerman, Idaho, National Park Service, 77 p. Fields, R.W., Rasmussen, D.L., Tabrum, A.R., and Nichols, R., 1985, Cenozoic rocks of the intermontane basins of western Montana and eastern Idaho, in Flores, R.M., and Kaplan, S.S., eds., Cenozoic Paleo-Geography of West-Central United States: Denver, Colorado, Society of Economic Paleontologists and Mineralogists, p. 9–36. Forsman, N.F., 1986, Documentation and diagenesis of tuffs in the Killdeer Mountains, Dunn County, North Dakota: North Dakota Geological Survey Report of Investigations 87, 13 p. Gardner, L.R., 1972, Origin of the Mormon Mesa caliche, Clark County, Nevada: Geological Society of America Bulletin, v. 83, no. 1, p. 143–156. Gardner, L.R., Diffendal, R.F., Jr., and Williams, D.F., 1992, Stable isotope composition of calcareous paleosols and ground-water cement from the Ogallala Group (Neogene), western Nebraska: Contributions to Geology, v. 29, no. 2, p. 97–106. Gile, L.H., Peterson, F.F., and Grossman, R.B., 1965, The K horizon: A master horizon of carbonate accumulation: Soil Science, v. 97, p. 74–82. Goudie, A.S., 1983, Calcrete, in Goudie, A.S., and Pye, K., eds., Chemical Sediments and Geomorphology: London, Academic Press, p. 93–131. Gulbranson, E.L., 2004, Calcretes and palustrine carbonates as indicators of climatic shift, Late Triassic and Early Cretaceous of the Southwestern US [abs.]: Geological Society of America Abstracts with Programs, v. 36, no. 5, p. 368. Gustavson, T.C., 1996, Fluvial and eolian depositional systems, paleosols, and paleoclimate of the Upper Cenozoic Ogallala and Blackwater Draw Formations, southern High Plains, Texas and New Mexico: Austin, Texas, Texas Bureau of Economic Geology, Report of Investigations no. 239, 62 p.

Calcic pedocomplexes Hanneman, D.L., 1989, Cenozoic basin evolution in a part of southwestern Montana [Ph.D. dissertation]: Missoula, Montana, University of Montana, 347 p. Hanneman, D.L., and Wideman, C.J., 1991, Sequence stratigraphy of Cenozoic continental rocks: Geological Society of America Bulletin, v. 103, p. 1335–1345, doi: 10.1130/0016-7606(1991)1032.3.CO;2. Hanneman, D.L., Wideman, C.J., and Halvorson, J., 1994, Calcic paleosols: Their use in subsurface stratigraphy: American Association of Petroleum Geologists Bulletin, v. 78, p. 1360–1371. Hanneman, D.L., Cheney, E., and Wideman, C.J., 2003, Cenozoic sequence stratigraphy of northwestern USA, in Raynolds, R.G., and Flores, R.M., eds., Cenozoic Systems of the Western United States: Denver, Colorado, Society of Economic Paleontologists and Mineralogists, p. 135–156. Hodges, M.K.V., and Link, P.K., 2002, Oligocene-Miocene Medicine Lodge beds, northeastern Snake River Plain: Correlative to the Renova and Sixmile Creek Formations, Montana: Geological Society of America Abstracts with Programs, v. 34, no. 4, p. A48. Hodges, M.K.V., Link, P.K., and Miggins, D.P., 2004, The Medicine Lodge beds: New radiometric ages from Idaho Medicine Lodge Valley restrict deposition to the Paleogene basin-forming event: Geological Society of America Abstracts with Programs, v. 36, no. 5, p. 546. Hoganson, J.W., Murphy, E.C., and Forsman, N.F., 1998, Lithostratigraphy, paleontology, and biochronology of the Chadron, Brule, and Arikaree Formations in North Dakota, in Terry, D.O., LaGarry, H.E., and Hunt, R.M., Jr., eds., Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River and Arikaree Groups: Geological Society of America Special Paper 325, p. 185–196. Hunt, R.M., Jr., 1990, Taphonomy and sedimentology of Arikaree (lower Miocene) fluvial, eolian, and lacustrine paleoenvironments, Nebraska and Wyoming: A paleobiota entrenched in the fine-grained volcaniclastic rocks, in Lockley, M.G., and Rice, A., eds., Volcanism and Fossil Biotas: Geological Society of America Special Paper 244, p. 69–111. MacFadden, B.J., and Hunt, R.M., Jr., 1998, Magnetic polarity stratigraphy and correlation of the Arikaree Group, Arikareean (late Oligocene–early Miocene) of northwestern Nebraska, in Terry, D.O., LaGarry, H.E., and Hunt, R.M., Jr., eds., Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River and Arikaree Groups: Geological Society of America Special Paper 325, p. 143–165. Machette, M.N., 1985, Calcic soils of the southwestern United States, in Weide, D.L., and Faber, M.L., eds., Soils and Quaternary Geology of the Southwestern United States: Geological Society of America Special Paper 203, p. 1–21. Mack, G.H., James, W.C., and Monger, H.C., 1993, Classification of paleosols: Geological Society of America Bulletin, v. 105, p. 129–136, doi: 10.1130/00167606(1993)1052.3.CO;2. McCarthy, P.J., and Plint, A.G., 1998, Recognition of interfluve sequence boundaries: Integrating paleopedology and sequence stratigraphy: Geology, v. 26, p. 387–390, doi: 10.1130/0091-7613(1998)0262.3.CO;2. McCarthy, P.J., Faccini, U.F., and Plint, A.G., 1999, Evolution of an ancient coastal plain: Paleosols, interfluves and alluvial architecture in a sequence stratigraphic framework, Cenomanian Dunvegan Formation, NE British Columbia, Canada: Sedimentology, v. 46, p. 861–891, doi: 10.1046/j.1365-3091.1999.00257.x. McLeod, P.J., 1987, The depositional history of the Deer Lodge Basin, western Montana [M.S. thesis]: Missoula, Montana, University of Montana, 61 p. Morrison, R.B., 1998, How can the treatment of pedostratigraphic units in the North American Stratigraphic Code be improved?: Quaternary International, v. 51/52, p. 30–33, doi: 10.1016/S1040-6182(98)90197-5. Murphy, E.C., Hoganson, J.W., and Forsman, N.F., 1993, The Chadron, Brule, and Arikaree Formations in North Dakota: Bismarck, North Dakota, North Dakota Geological Survey, Report of Investigation no. 96, 144 p. Nash, D.J., and Shaw, P.A., 1998, Silica and carbonate relationships in silcrete-calcrete intergrade duricrusts from the Kalahari of Botswana and Namibia: Journal of African Earth Sciences, v. 27, no. 1, p. 11–25. Nettleton, W.D., Olson, C.G., and Wysocki, D.A., 2000, Paleosol classification: Problems and solutions: CATENA, v. 41, p. 61–92, doi: 10.1016/S03418162(00)00109-0. North American Commission on Stratigraphic Nomenclature, 1983, North American Stratigraphic Code: American Association of Petroleum Geologists Bulletin, v. 67, p. 841–875. O’Neill, J.M., Lonn, J.D., Lageson, D.R., and Kunk, M.J., 2004, Early Tertiary

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Anaconda metamorphic core complex, southwestern Montana: Canadian Journal of Earth Sciences, v. 41, p. 63–72, doi: 10.1139/e03-086. Portner, R., and Hendrix, M.S., 2004, Preliminary results of geologic mapping and sedimentologic analysis of the northeastern Flint Creek Basin, west central Montana [abs.]: Geological Society of America Abstracts with Programs, v. 36, no. 5, p. 72. Prothero, D.R., 1994, The Eocene-Oligocene transition: Paradise lost: New York, Columbia University Press, 281 p. Prothero, D.R., 1995, Geochronology and magnetostratigraphy of Paleogene North American Land Mammal “Ages”: an update, in Berggren, W.A., Kent, D.V., Aubry, M.-P., and Hardenbol, J., eds., Geochronology, time scales, and global stratigraphic correlations: Unified temporal framework for an historical geology: Tulsa, Oklahoma, Society for Sedimentary Geology, SEPM Special Publication 54, p. 305–316. Prothero, D.R., 1998, The chronological, climatic and paleogeographic background to North American mammalian evolution, in Janis, C.M., Scott, K.M., and Jacobs, L.L., eds., Evolution of Tertiary Mammals of North America, Volume 1: Terrestrial Carnivores, Ungulates and Ungulate Like Mammals: Cambridge, Massachusetts, Cambridge University Press, p. 9–34. Retallack, G.J., 1983, Late Eocene and Oligocene paleosols from Badlands National Park, South Dakota: Geological Society of America Special Paper 193, 82 p. Retallack, G.J., 1992, Paleosols and changes in climate and vegetation across the Eocene–Oligocene boundary, in Prothero, D.R., and Berggren, W.A., eds., Eocene–Oligocene Climatic and Biotic Evolution: Princeton, New Jersey, Princeton University Press, p. 383–398. Retallack, G.J., 1993, Classification of paleosols: Discussion: Geological Society of America Bulletin, v. 105, p. 1635–1637, doi: 10.1130/00167606(1993)1052.3.CO;2. Retallack, G.J., 1998, Fossil soils and completeness of the rock and fossil record, in Donovan, S.K., and Paul, C.R.C., eds., The Adequacy of the Fossil Record: Chichester, UK, John Wiley & Sons, p. 131–162. Retallack, G.J., Bestland, E.A., and Fremd, T.J., 2000, Eocene and Oligocene paleosols of central Oregon: Geological Society of America Special Paper 344, 192 p. Slate, J.L., 1996, Buried carbonate paleosols developed in Pliocene-Pleistocene deposits of the Pasco Basin, south-central Washington, USA: Quaternary International, v. 34–36, p. 191–196. Soil Survey Staff, 1975, Soil taxonomy; a basic system of soil classification for making and interpreting soil surveys: U.S. Department of Agriculture Handbook, v. 436, 754 p. Tandon, S.K., and Gibling, M.R., 1997, Calcretes at sequence boundaries in Upper Carboniferous cyclothems of the Sydney Basin: Atlantic Canada Sedimentary Geology, v. 112, p. 43–67, doi: 10.1016/S0037-0738(96)00092-9. Thompson, G.R., Fields, R.W., and Alt, D., 1982, Land-based evidence for Tertiary climatic variations: Northern Rockies: Geology, v. 10, p. 413–417, doi: 10.1130/0091-7613(1982)102.0.CO;2. Weissmann, G.S., Mount, J.F., and Fogg, G.E., 2002, Glacially driven cycles in accumulation space and sequence stratigraphy of a stream-dominated alluvial fan, San Joaquin Valley, California, USA: Journal of Sedimentary Research, v. 72, p. 270–281. Williams, V.S., Bohannon, R.G., and Hoover, D.L., 1997, Geologic map of the Riverside quadrangle, Clark County, Nevada: U.S. Geological Survey Geologic Quadrangle Map GQ-1770, scale 1:24,000. Wing, S.L., 1998, Tertiary vegetation of North America as a context for mammalian evolution, in Janis, C.M., Scott, K.M., and Jacos, L.L., eds., Evolution of Tertiary Mammals of North America, Volume 1: Terrestrial Carnivores, Ungulates and Ungulate Like Mammals: Cambridge, Massachusetts, Cambridge University Press, p. 37–65. Woodburne, M.O., and Swisher, C.C., III, 1995, Land mammal high-resolution geochronology, intercontinental overland dispersals, sea level, climate, and vicariance, in Berggren, W.A., Kent, D.V., Aubry, M.-P., and Hardenbol, J., eds., Geochronology, time scales, and global stratigraphic correlations: Unified temporal framework for an historical geology: Tulsa, Oklahoma, Society for Sedimentary Geology, SEPM Special Publication 54, p. 337–364. MANUSCRIPT ACCEPTED BY THE SOCIETY 17 MAY 2006 Printed in the USA

Geological Society of America Special Paper 416 2006

A Late Triassic soil catena: Landscape and climate controls on paleosol morphology and chemistry across the Carnian-age Ischigualasto–Villa Union basin, northwestern Argentina Neil J. Tabor Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275-0395, USA Isabel P. Montañez Kelley A. Kelso Department of Geology, One Shields Ave., University of California, Davis, California 95616, USA Brian Currie Department of Geology, Miami University, 114 Shideler Hall, Oxford, Ohio 45056, USA Todd Shipman Arizona Geological Survey, 416 West Congress, Suite 100, Tucson, Arizona 85701, USA Carina Colombi Instituto y Museo de Ciencias Naturales (CONICET), Espana 400(N) Ciudad San Juan, CP5400, Argentina ABSTRACT The thicknesses of stratigraphic sections of the Late Triassic (Carnian) Ischigualasto Formation change significantly, from ~300 to 700 m, along a 15 km transect in the Ischigualasto Provincial Park, San Juan, NW Argentina. Channel sandstone deposits dominate the thickest section, whereas pedogenically altered layers dominate the thinnest stratigraphic section. Eight paleosol types have been recognized in the study area, and they are unevenly distributed across the basin. In particular, paleosol B horizons are thinner and redoximorphic soil morphologies dominate in the thickest, whereas B horizons are thickest and argillic and calcic morphologies dominate in the thinnest stratigraphic section. These observations suggest that the geomorphic evolution of the Ischigualasto basin exerted the primary control on sediment distribution, depositional rate, soil drainage, and depth of the groundwater table through most of Late Triassic time in the Ischigualasto basin. In addition, δ18O values of paleosol calcite nodules are similar to modern soil calcites that form in frigid to cool climates between ~0 °C and 10 °C. Considering both lateral and stratigraphic distribution of paleosol morphological variability, there appears to be three different general modes of climate recorded throughout deposition of the Ischigualasto Formation: (1) Humid conditions recorded by Argillisols, Gleysols, and Vertisols in the lower quarter of the formation; (2) relatively dry conditions recorded by Calcisols, calcic Argillisols, and calcic Vertisols in the middle half of the formation; and (3) generally more humid conditions in the upper quarter of the formation recorded by Argillisols, Gleysols, and Vertisols. Keywords: paleosols, catena, paleoclimate, Triassic, Argentina. Tabor, N.J., Montañez, I.P., Kelso, K.A., Currie, B., Shipman, T., and Colombi, C., 2006, A Late Triassic soil catena: Landscape and climate controls on paleosol morphology and chemistry across the Carnian-age Ischigualasto–Villa Union basin, northwestern Argentina, in Alonso-Zarza, A.M., and Tanner, L.H., eds., Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates: Geological Society of America Special Paper 416, p. 17–41, doi: 10.1130/2006.2416(02). For permission to copy, contact [email protected]. ©2006 Geological Society of America. All rights reserved.

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Tabor et al. RESUMEN La Formación Ischigualasto del Triásico Superior (Carniense) presenta cambios importantes de espesor (de 300 a 700 m), a lo largo de una transversal de 15 km dentro del Parque Provincial de Ischigualasto, San Juan, en el noroeste de Argentina. En las zonas en las que la Formación es más potente dominan los canales de areniscas, mientras que en las zonas en las que el espesor de la Formación es menor dominan los niveles edáficos. Se han reconocido ocho tipos distintos de paleosuelos, que se distribuyen de forma desigual a lo largo de la cuenca. En particular, los horizontes B de los paleosuelos son menos potentes y presentan morfologías redoximórficas en las secciones estratigráficas de mayor espesor; por el contrario, en las secciones estratigráficas de menor espesor los horizontes B son más potentes y argílicos y en ellos son frecuentes los rasgos calcáreos. De forma conjunta, la distribución de los depósitos canalizados de areniscas y la morfología de los paleosuelos a lo largo de la Formación Ischigualasto indican que la evolución geomorfológica de la cuenca fue el principal factor de control sobre la distribución de los sedimentos, la tasa de sedimentación, el drenaje de los suelos y la profundidad del nivel freático durante la mayor parte del Triásico. Palabras clave: paleosuelos, catena, Triásico, paleoclima, Argentina.

INTRODUCTION Soil morphologies vary across the modern landscape in response to local and regional variations in depositional environment, soil drainage, and geomorphology (Jenny, 1941; Soil Survey Staff, 1975; Buol et al., 1997; Birkeland, 1999, and many others). Soil scientists have long recognized that landscape position is a primary factor of soil formation, and the term “soil catena” is reserved for specific instances where lateral variations in the development of a suite of contemporaneous soil profiles occur in accord with the position of those soils on the landscape (Steila, 1976). It is often difficult to document the catenary relationships of ancient sedimentary basins, because (1) accurate correlation and demonstration of contemporaneity between outcrops is often not feasible and (2) an ancient soil catena may be erased by the terrestrial sedimentary record’s bias toward preservation of low-lying areas. Several studies, however, provide persuasive evidence that variations in paleosol development in ancient sedimentary basins can reflect lateral changes in depositional rates across the paleolandscape, both of which can be related to landscape position and the location of the groundwater table (Allen, 1974; Leeder, 1976; Retallack, 1976, 1977, 1983; Bown and Kraus, 1981; Kraus, 1987; Besly and Turner, 1983; Atkinson, 1986; Tabor and Montañez, 2004; McCarthy and Plint, 2003; Demko et al., 2003). Although they are called by different names (e.g., pedofacies), these basin-scale studies of terrestrial sedimentary rocks demonstrate that catenary relationships can be recognized in ancient sedimentary records, even when the contemporaneity of individual paleosol horizons cannot be demonstrated. This recognition is important given the impact that paleosol morphology and composition have on paleoenvironmental and paleoclimatic reconstructions (e.g., Retallack, 1990; Mack and James, 1994).

In this work, we document the distribution of paleosols across the Late Triassic (Carnian) Ischigualasto Formation of the Ischigualasto–Villa Union basin, northwestern Argentina. The spatial and temporal distribution of paleosol morphologies establishes a basis for inferred lateral changes in soil drainage and depositional rates indicative of a paleosoil catena. In particular, lateral variations in the stratigraphic distribution of channel sandstones, pedogenic alteration, redoximorphic features, and pedogenic calcrete profiles appear to provide a sensitive record of changes in soil drainage during Ischigualasto Formation deposition. Overall, paleosol morphologies indicate that soils were generally better drained in areas of decreased accommodation, most likely reflecting the effects of a deeper groundwater table and generally drier conditions in the shallow subsurface away from the basin depocenter. Enhanced soil drainage and a reduction in the overall sediment-accumulation rate allowed more illuvial Ca2+ to accumulate as pedogenic calcite precipitation in paleosols developed on the alluvial landscape away from the basin center. BACKGROUND Tectonic Setting, Stratigraphy, and Age The study area is located along the southern edge of the Ischigualasto–Villa Unión basin in northeastern San Juan Province, Argentina, and lies within the boundaries of the Ischigualasto Provincial Park (Fig. 1). During Mesozoic time, oceaniccontinental plate interactions along the southwestern margin of Pangea produced a region of extensional deformation cratonward of the proto-Andean magmatic arc (Ramos and Kay, 1991; López-Gamundí et al., 1994). Extension focused along the NW-trending boundary between Paleozoic accreted terranes and

A Late Triassic soil catena

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Figure 1. Location maps of the Ischigualasto basin. (A) Map of San Juan Province, Argentina. White rectangle shows location of Figure 1C. (B) Distribution of Triassic basins in southern South America. “I.B.” marks the position of the Ischigualasto–Villa Unión basin in NW Argentina. (C) Geologic map of the southern part of the Ischigualasto–Villa Unión basin. Ischigualasto Formation measured sections 1–3 are shown in Figure 3. Figure was modified from Alcober (1996).

the Precambrian Gondwanan craton (Uliana et al., 1989). The Ischigualasto–Villa Unión basin of northwest Argentina is one of a series of continental-rift basins that developed in the region as a result of this extension (Fig. 1B) (Uliana and Biddle, 1988). Rift-related deposition in the Ischigualasto basin began during Early Triassic time as normal displacement on the paleo– Valle Fértil fault led to the development of a structural half-graben (Milana and Alcober, 1994). Deposition in the basin continued throughout Triassic time and resulted in accumulation of up to 3.5 km of nonmarine and volcanic strata (Alcober, 1996). The Triassic System in the basin consists of the Lower Triassic Talampaya and Tarjados Formations, the Middle Triassic Chañares-Ischichuca and Los Rastros Formations, and the Upper Triassic Ischigualasto and Los Colorados Formations (Fig. 2) (Stipanícic and Bonaparte, 1979). Quaternary shortening in the Andean foreland produced reverse-displacement reactivation of Mesozoic normal faults and structural inversion of Ischigualasto basin strata (Zapata and Allmendinger, 1996). The rocks of the Ischigualasto basin are exposed in the hanging wall of the Valle Fértil and Alto faults, both of which are interpreted to be reverse-reactivated zones of Triassic normal faulting (Milana and Alcober, 1994; Zapata and Allmendinger, 1996). The Valle Fértil fault is interpreted as the main basin-bounding normal fault separating Proterozoic-Paleozoic crystalline and sedimentary rocks of the footwall from the Mesozoic sedimentary rocks of the hanging wall. The paleo–Alto fault is interpreted as a W-NW–dipping normal fault (Milana and Alcober, 1994) that merged with the Valle Fertíl fault to the south (Fig. 1).

Ma Period

Stage

Formation/Member

Norian

Los Colorados

Radiometric Age (Ma)

223.0±0.4 Carnian

Ischigualasto 227.8±0.3

Ladinian

Los Rastros

Anisian

Chanares

Olenekian Induan

Figure 2. Time-stratigraphic chart for Triassic rocks of the Ischigualasto basin including relative position of radiometrically dated horizons. Time scale was adapted from Gradstein et al. (1995) and Alcober (1996).

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The focus of this investigation is the Ischigualasto Formation, which consists of ~300–700 m of mudstone, sandstone, conglomerate, and extrusive basalt (Fig. 3). The basal Ischigualasto Formation is defined as the first occurrence of pebble-cobble conglomerate-conglomeratic sandstone or smectitic mudstone overlying the carbonaceous sandstones and mudstones of the Los Rastros Formation, whereas the upper contact of the Ischigualasto Formation is positioned at the top of the final variegated mudstone below the dominantly red-colored sandstones and mudstones of the Los Colorados Formation. The Ischigualasto Formation appears to have conformable contacts with underlying and overlying formations (Alcober, 1996), and it is internally divided into four members (Fig. 3). In stratigraphically ascending order, these members are (1) Unit I, which consists of ~30–50 m of tan/gray pebble-cobble conglomerate, conglomeratic sandstones, and green/gray smectitic mudstone; (2) Unit II, which consists of 65–125 m of mudstone and sandstone with rare interbeds of bentonite and basalt; (3) Unit III, which consists of 250–470 m of mudstone and sandstone, the lithostratigraphy of which is dominated by smectitic mudstones in the east and channel and overbank sandstone deposits in the west portions of the study area; and (4) Unit IV, which consists of 35–65 m of variegated mudstone and sandstone. An Upper Triassic age of the Ischigualasto Formation is based on vertebrate fossils and radiometric ages of altered ash beds from the unit. Abundant vertebrate fossils from the lower two-thirds of the formation indicate a Carnian age of deposition (Rogers et al., 1993; Alcober, 1996). Altered ash beds in the Ischigualasto Formation have provided additional chronostratigraphic control. Sanidine crystals from a bentonite sampled ~80 m above the base of the formation yielded an 40Ar/39Ar date

Section 1

Section 2

Section 3

Unit IV 7 km

8 km

Unit III

Unit II

Ischigualasto Formation

Los Colorados Fm

Unit I

of 227.8 ± 0.3 Ma (Rogers et al., 1993), while plagioclase crystals from a bentonite ~70 m from the top of the formation yielded a date of 218 ± 1.7 Ma (Shipman, 2004). Collectively, these data support a Carnian age of deposition based on the Triassic time scale of Gradstein et al. (2004; Fig. 2). Paleoclimate Paleomagnetic evidence from intraformational basalt flows within the upper Chañares Formation, near the base of the Ischigualasto Formation, place the basin at 30°S during the Middle Triassic (Veevers et al., 1994; Valencio et al., 1975; López-Gamundí et al., 1994). Stipanícic and Bonaparte (1972) suggested that sediments in the Ischigualasto basin were deposited as a single climatic cycle, from relatively dry and seasonal, to more humid and back again, with temperatures ranging from moderate to hot. Stipanícic and Bonaparte’s (1972) reconstruction primarily relied on lithologic and flora/faunal evidence in the Agua de la Peña Group. They also considered the possibility that some of the change in the rock record could be a result of tectonic influences and that climate was relatively uniform. Volkheimer (1969) supported an interpretation of moderate to hot temperatures mostly based on paleofloral data. Previous researchers argued that the Ischigualasto Formation (Fig. 2) was subject to a water-limited, seasonal climate. Based on the morphology, mineralogy, and light stable isotope geochemistry of pedogenic minerals from a paleo-Vertisol, Tabor et al. (2004) suggested that the Lower Ischigualasto Formation was deposited in a seasonal, humid, and cool climate. Root sizes of rhizoliths in the Ischigualasto Formation indicate mean annual precipitation from 500 to 800 mm (Alcober et al., 1997). Bossi (1971) proposed a dominantly fluvial setting that experienced seasonal variations in water availability, while Martínez (1994) suggested an arid climate with seasonal precipitation, supported by lithological evidence such as secondary paleosol carbonate, slickensides, and blocky peds. Further, Spalletti et al. (1999) presented biozonation in relation to the chronostratigraphy of the basin that indicated a mixed forest plant assemblage for the Ischigualasto Formation. The landscape of the Ischigualasto Formation was sparsely forested with a dominance of herbaceous plants, although some large trees, represented by Rexoylon, are common in the upper half of the formation, and Spalletti et al. (1999) interpreted the deposition of the Ischigualasto Formation to have occurred in a dry, and moderate to hot, seasonal climate. METHODS

OLDER TRIASSIC

Figure 3. Schematic diagram of changing sedimentary thickness across the Ischigualasto Formation, position of informal lithostratigraphic members, and measured sections 1, 2, and 3. See Figure 1 for location.

Three detailed stratigraphic sections were measured through the Ischigualasto Formation in the eastern part of the Ischigualasto Provincial Park (Currie et al., 2001). The sections, each separated by ~7–8 km, are labeled 1, 2, and 3 from west to east.

A Late Triassic soil catena Paleosol tops were identified on the basis of the upper limit of observed pedogenic features or by the presence of an erosional contact with an overlying bed, whereas profile bases were delineated at the lowest occurrence of unaltered parent material. Field descriptions of paleosols (e.g., thickness, color, type, and distribution of mottling, soil structure, and mineralogy, size, morphology, and distribution of authigenic minerals) were completed following established methods (Tabor and Montañez, 2004). Paleosol and lithologic colors were identified from dry samples using Munsell color charts (Munsell Color, 1975). Paleosol classification followed the system defined by Mack et al. (1993), and the closest estimate of a modern soil taxonomic equivalent of the paleosol profiles is given within the context of the U.S. Department of Agriculture Soil Taxonomy (USDA) (Soil Survey Staff, 1975). For clay mineral identification, suspensions of the 4 on the Munsell color charts) and/or mottling consistent with modern surface-water gley in modern soil systems (see Retallack, 1990; Vepraskas, 1994; PiPujol and Buurman, 1994) are referred to as redoximorphic zones (e.g., Vepraskas, 1994). In modern soils, redoximorphic features form in seasonally saturated portions of profiles through removal of Fe and Mn from areas of low Eh (redox depletions) and reprecipitation as Fe- and Mn-oxides (redox concentrations) in more oxidized areas. Gley matrix colors indicate reduced conditions, which are typical of relatively prolonged saturation (25–50% of the year), whereas yellow-brown to reddish mottles record seasonal soil drying (Daniels et al., 1971; Duchaufour, 1982). Using modern soils as an analogy, redoximorphic features in paleosols are interpreted to have formed in seasonally saturated portions of the profile that contained sufficient organic content to have undergone reducing conditions (PiPujol and Buurman, 1994). Soil Mineralogy Quartz, feldspars, and micas are the dominant detrital siliciclastic minerals in both fluvial channel sands and paleosol

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profiles based on petrographic and X-ray diffraction analyses (Moore, 2003; Shipman, 2004). These detrital siliciclastic minerals also comprise a significant fraction of the clay mineralogy in soils that formed over sedimentary parent materials in the Ischigualasto Formation (pedotypes A–F described in next section; see also Moore, 2003). However, two different paleosol types (pedotypes G and H described in next section) apparently preserve in situ weathering products of volcanic ash and basalt and therefore have little or no detrital siliciclastic component. The mineralogical composition of each pedotype horizon is given in Table 1 based upon the response of the respective