Palaeogeological Map of the Lower Palaeozoic Floor Below the Cover of Upper Devonian Carboniferous and Later Formations

A PALAEOGEOLOGICAL MAP of the LOWER PALAEOZOIC FLOOR below the cover of UPPER DEVONIAN, CARBONIFEROUS AND LATER FORMATIONS with inferred and speculative reconstructions of Lower Palaeozoic and Precambrian outcrops in adjacent areas LEONARD JOHNSTON WILLS, M.A., Sc.D., Ph.D.

Not to seek in any subject greater accuracy than its nature admits. Aristotle.

Edited by PROFESSOR B. E. LEAKE

University of Glasgow

MEMOIR NUMBER 8

A JOINT PUBLICATION W I T H THE P E T R O L E U M E X P L O R A T I O N SOCIETY OF GRE A T BRITAIN

GEOLOGICAL SOCIETY LONDON 1978

Published for The Geological Society London

by

Scottish Academic Press Ltd. 33 Montgomery Street, Edinburgh EH7 5JX

First published 1978

All rights reserved. No part of this publication stored in a retrieval system, or transmitted, in means, electronic, mechanical, photocopying, wise, without the prior permission of the Burlington House, London, WIN 0JU.

may be reproduced, any form, or by any recording or otherGeological Society,

9 The Geological Society, London, 1978

SBN

7073

0226

9

Printed in Great Britain by Lindsay & Co. Ltd., Edinburgh

PREFACE It is four years since Professor Wills produced his outstanding pre-Permian Palaeogeological Map of England and Wales, published by this Society with an explanatory memoir and now to be found on the walls of many working offices across the country. He was then 89 and it was assumed that this was his final production, a remarkable achievement in the face of great physical frailty. Nevertheless he immediately moved on to a map of a stratigraphically deeper level m a palaeogeological map of the rocks following the Caledonian movements and the subsequent erosion, the surface of which would be exposed by the removal of Upper Devonian and later strata. This was completed and printed in 1975, and Wills began the equally complex task of writing an accompanying memoir to explain the way he had interpreted and applied the varied evidence of the deep structure. This process however led him to undertake still another map, that of the preDevonian rocks of a belt extending from East Anglia to South Wales, which has been completed this year. These two maps form the coloured plates accompanying this memoir. The memoir is also illustrated by a map of the Basal Elements, and by three Phanerozoic Time Scale sections, which illustrate the point which Wills has made before m that in the dominantly shelf environment of Britain the stratigraphical record is largely incomplete, with the lacunae at least comparable in time terms to the periods represented by sediments. This publication, then, represents the further remarkable achievement of a man of 93 ( n "and a half", as he emphasises - - he feels that months count now), working in a country study without technical support, his activity sharply limited by eye trouble and by a heart condition. It would have been a notable production for a fit man half his age. Both economic and academic geologists are already deeply indebted to Wills for his map of the pre-Permian surface; these further compilations add more organised data and more food for thought about the past history and deep structure of Britain. P. E. KENT, F.R.S. Fellow of the Geological Society Past Chairman, Petroleum Exploration Society September 17, 1977 Natural Environment Research Council.

FOREWORD During the preparation of the map in Memoir No. 7 I made sections across the map with the object of seeing whether the limits of the stratigraphical units shown in the sections agreed with the boundaries shown or implied by the reconstructions given in my Palaeogeographical Atlas, 1948. Nothing, however, came from this except the invention of the P.T.-S. Section, now known as a hiatograph, two of which were incorporated in Memoir No. 7 as Figs. 1 and 2. The present Fig. 2 shows the top of the 'Floor' and the bottom of the 'Cover' by ornamentation which shows how the stratigraphical age of the 'basal elements' varies from place to place; and how the 'Floor' on which a particular Basal Element rests may also vary. In the text of Mem. No. 7 no significance was attached to these features. Thus I failed to recognise the significance of 'Basal Elements' as a pile o f tattered carpets, each carpet giving a stratigraphical date to the particular part or parts of the Floor on which it lies. The sections are often very conjectural below the Carboniferous, but Fig. 2 shows clearly the well-known unconformity between the Upper and Lower Old Red Sandstone-Devonian in South Wales, which in my ponderings, I became convinced recorded the surface of the 'Old Red Sandstone Continent'. The same unconformity had long ago been discovered by James Hutton in Scotland. It is strikingly displayed by the break between Lower Palaeozoic rocks and the Carboniferous Limestone of the Pennines and North Wales. Here I thought was the surface of the very first continental land - - surely a suitable subject for a second palaeogeological map of England and Wales. I found however that there were far fewer boreholes than were available for the map in Memoir No. 7. There was also a dearth of papers (Bott's 1967 paper on the structure of the North of England is a brilliant exception) and a want of agreement on, for example, radiometric ages. As the map stands, it looks simple enough, but one should expect this since all the Upper Palaeozoic, Mesozoic and Tertiary complications of sedimentation, tectonics, magmatic intrusions and erosion are assumed to have been omitted. The Memoir however has to take account of these latter factors, which necessitate the inclusion of two supplementary maps and a complicated diagram (Fig. 1) illustrating the various manifestations of cyclicity throughout Phanerozoic Time, and five Hiatographs which demonstrate how much of Phanerozoic Time is unrecorded by sediment from place to place. However the map that I produced was welcomed by the few geologists and geophysicists who saw it as an attempt to depict the geology of England and Wales at the end of the Caledonian orogeny. It was redrawn beautifully by Mr A. A. Miles, Senior draughtsman of British Petroleum Ltd., at their expense, and colour printed at the expense of the Petroleum Exploration Society of Great Britain. Even before this, I discovered that the surface that I thought was Middle Devonian in age, was in reality the Palaeozoic Platform fashioned by erosion at seven different geological dates ranging from Upper Devonian to Gault! This involved the drawing of two more figures (Figs. 1 and 2). Later another map, Plate 2, became necessary to demonstrate the putative interpretation of the geology concealed by the outcrops of Lower and, in places, Middle O.R.S.-Devonian shown on Plate 1 in South Wales and S.E. England. These figures have been redrawn from my originals by Mr Colin Knipe. At long last the text of Memoir No. 8 has been completed about a map whose very title is wrong! It should read - - 'A palaeogeological map of the Palaeozoic Platform below the Cover of Upper Devonian and younger Formations'.

Partly as an outcome of this work I am now the proud recipient of the Honorary Fellowship of the Geological Society of London and of the Honorary Membership of the Petroleum Exploration Society of Great Britain, and I take the opportunity again to express my gratitude to the two Societies and to many individual members who have helped me, in particular Sir Peter Kent and Dr L. V. Illing, and others named in the acknowledgements. Work on the maps and memoir has taken an old man (I went to Cambridge in 1903) into the complexities of a good many modern 'ologies' such as sedimentology, ecology, radiometric dating and plate tectonics. The reader will notice that the memoir makes no reference to many modern theories although some must have inevitably invalidated many of the conceptions on which I was brought up. Ave atque Vale L. J. WILLS 1977

ACKNOWLEDGEMENTS

and T H A N K S

The author is deeply indebted for information on deep borings from the following Companies: British Gas Corporation, per K. W. Barr and V. S. Colter. British Petroleum Company Ltd., per Sir Peter Kent and F. Howitt. Burmah Oil Company. Cambrian Exploration Ltd., per J. A. C. Gage. Esso Exploration and Production U.K. Ltd. Shell U.K. Exploration and Production Ltd., per J. M. Bowen. Superior Oil Co. Ltd. Trend Cheshire U.S. Ltd., per J. Q. Stokstad. Tricentrol International Ltd., per C. A. Fothergill. Ultramar Exploration Ltd., per L. V. Illing. And for general assistance and advice from: Sir Peter Kent, N.E.R.C. L. V. Illing (of V. C. Illing and Partners). P. Lovelock (of Shell U.K. Exploration and Production Ltd.). Isles Strachan, Geology Dept., University of Birmingham. G. Bennison, Geology Dept., University of Birmingham. A. A. Miles (of British Petroleum Company Ltd.). C. Knipe (of Johnson, Poole and Bloomer, Civil and Mining Engineers, Dudley). Mrs D. Rae (Geology Dept., University of Glasgow). Mrs M. Darley (late of Geology Dept., University of Birmingham). Mrs P. A. Pyatt, East Worcestershire Waterworks Company. Miss S. Hodge (Geology Dept., University of Birmingham). Financial assistance towards the cost of publication has generously come from: British Gas, British National Oil Corporation, and the Petroleum Exploration Society of Great Britain.

CONTENTS PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

FOREWORD ..................................................................

4

ACKNOWLEDGEMENTSand

6

CONTENTS a n d

THANKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LIST OF ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

1.

Explanatory Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

2.

The general geography of Caledonian England and Wales . . . . . . . . . . . . . . . . . . . . . . . . . .

21

A.

The geosynclines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

B.

The English Microcraton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

3.

The Precambrian Basement, ridges and troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

4.

The topography of the pre-Phanerozoic Basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

26

5.

Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

6.

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

32

7.

Appendix: Explanation of Plate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34

PLATES Plate 1. England and Wales. A palaeogeological map of the Lower Palaeozoic floor below the cover of Upper Devonian, Carboniferous and later formations with inferred and speculative reconstructions of the Lower Palaeozoic and Precambrian outcrops in adjacent areas. Scale: 1/625,000 or about ten miles to one inch. Completed October 1975. N.B. The crimson colour for the Silurian has been inadvertently omitted from the map between the putative interpretation and the coastline in Yorkshire. Bobbing should be 9 km S.W. of Sheerness B.H. Fobbing, add Cambrian? below M.Devonian. Plate 2. An inferred palaeogeological map of Wales and of England south of National grid Latitude 40 North, assuming the removal of all O.R.S.-Devonian and later formations, to reveal the outcrops at the close of Silurian times. The map records the sites of some 111 boreholes that reached the L. Palaeozoic or Precambrian formations, plus the sites of certain magnetic anomalies and other features. For a list of these, see Appendix. Scale: 1/625,000 or about ten miles to one inch. Completed August 1977.

TEXT-FIGURES Fig. 1. Fig. Fig. Fig. Fig. Fig. Fig.

2. 3. 4. 5. 6. 7.

Key to the Hiatographs and to the cyclic control of sedimentation throughout Phanerozoic time. Map of the basal elements or 'carpets' of the post/Mid-Devonian cover. Hiatograph A1, St. Davids to Norfolk coast. Hiatograph A2, Anglesey to Tunbridge, Kent. Hiatograph B1, Northumberland to Portsmouth, Hants. Hiatograph B2, Chesterfield to Brighton, Sussex. Hiatograph across the Shelve-Longmynd-Church Stretton-Wrekin country.

10 12-13 14-15 16 17 18 19

CONTENTS PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

FOREWORD ..................................................................

4

ACKNOWLEDGEMENTSand

6

CONTENTS a n d

THANKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LIST OF ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

1.

Explanatory Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

2.

The general geography of Caledonian England and Wales . . . . . . . . . . . . . . . . . . . . . . . . . .

21

A.

The geosynclines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

B.

The English Microcraton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

3.

The Precambrian Basement, ridges and troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

4.

The topography of the pre-Phanerozoic Basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

26

5.

Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

6.

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

32

7.

Appendix: Explanation of Plate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34

PLATES Plate 1. England and Wales. A palaeogeological map of the Lower Palaeozoic floor below the cover of Upper Devonian, Carboniferous and later formations with inferred and speculative reconstructions of the Lower Palaeozoic and Precambrian outcrops in adjacent areas. Scale: 1/625,000 or about ten miles to one inch. Completed October 1975. N.B. The crimson colour for the Silurian has been inadvertently omitted from the map between the putative interpretation and the coastline in Yorkshire. Bobbing should be 9 km S.W. of Sheerness B.H. Fobbing, add Cambrian? below M.Devonian. Plate 2. An inferred palaeogeological map of Wales and of England south of National grid Latitude 40 North, assuming the removal of all O.R.S.-Devonian and later formations, to reveal the outcrops at the close of Silurian times. The map records the sites of some 111 boreholes that reached the L. Palaeozoic or Precambrian formations, plus the sites of certain magnetic anomalies and other features. For a list of these, see Appendix. Scale: 1/625,000 or about ten miles to one inch. Completed August 1977.

TEXT-FIGURES Fig. 1. Fig. Fig. Fig. Fig. Fig. Fig.

2. 3. 4. 5. 6. 7.

Key to the Hiatographs and to the cyclic control of sedimentation throughout Phanerozoic time. Map of the basal elements or 'carpets' of the post/Mid-Devonian cover. Hiatograph A1, St. Davids to Norfolk coast. Hiatograph A2, Anglesey to Tunbridge, Kent. Hiatograph B1, Northumberland to Portsmouth, Hants. Hiatograph B2, Chesterfield to Brighton, Sussex. Hiatograph across the Shelve-Longmynd-Church Stretton-Wrekin country.

10 12-13 14-15 16 17 18 19

1.

EXPLANATORY

DESCRIPTION

THE FLOOR The first truly-continental land surface in Phanerozoic Britain is that of the Old Red Sandstone continent which forms the subject of the present essay and map (P1. 1). As there is in west England and South Wales a great unconformity between Upper Devonian and Lower Old Red SandstoneDevonian formations, with the Lower O.R.S.-Devonian following the Silurian with little or no break; and the Upper Devonian merging upwards into the Lower Dinantian (Tournaisian), MidDevonian is taken as the date of the land surface, here termed the 'Ground Floor' (often referred to in the sequel as 'The Floor'), on which the overlying rocks of "The Cover' rest. There are four more newer Continental Floors indicated on Fig. 1. The geophysical properties of these Floors are very different; and this fact makes it imperative to distinguish them in present day exploration for coal, oil and gas. The term 'Basement Floor' should be restricted to the pre-Phanerozoic Floor, and the term Floor should be qualified as follows: 'Ground Floor' is pre-U. Devonian-Lower Carboniferous (here under discussion). This was the subject of the Yorkshire Geological Society's 1967 Symposium on the Sub-Carboniferous Basement. The '1st Floor' refers to the Sub-U. Permian (Zechstein) Floor described by Wills (1973) and which has also been referred to as 'the Basement' without any agequalification e.g. in Woodland (1975). The '2nd Floor' refers to the sub-U. Cretaceous Floor, the subject of Sir Aubrey Strahan's Geological Society Presidential Address 1913 in which it is rightly and consistently referred to as the 'Palaeozoic Platform '. Since completion of the Map, it has been shown that the supposed Mid-Devonian surface is composite, being made up of a patchwork of erosion surfaces ranging in age from Upper Devonian to Cretaceous, see Fig. 2. In fact the supposed Mid-Devonian Floor has turned out to be part of Strahan's Palaeozoic Platform. Likewise, the map of Wills (1973) ought to be a Palaeogeographical map of the Palaeozoic Platform below the Zechstein, etc.

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Fig. 1 Key to the hiatographs (Figs. 3--7), to the hypothesis of cyclic sedimentation and to deductions made from P1. 1. Cycles i--iv pre-Middle Devonian are deduced after removing the effects of the later cycles 1--9. Shaded strips show sedimentation due to prolonged rising ocean-level, v. volcanic rocks. Columns 1 - - 4 explained on Fig. Column 4 is constructed mainly from the following transitional areas: St. Davids-W. Dyfed; Llandeilo-Llandovery-Builth; Long Mynd-Church Stretton-Caradoc; Wrekin-Lilleshall-Edgmond; Furness-Cautley-Ingleborough-Horton. Column 5 shows the sequence on the English Microcraton. Column 6 Eo-North Sea Caledonian geosyncline, southern extension after Stormer (1967) and Bogdunoff et al. (1964). Evidence on P1. 1 and 2 and in Appendix. Columns 3 - - 6 The Ground Floor or the surface of the Palaeozoic Platform with three expanses of O.R.S.-Devonian as shown P1. 1. One underlies the North Irish Sea, the second stretches from the Severn Estuary to Herefordshire being an Anglo-Welsh delta and the third is the S.E. England Gulf which may have joined Kent with the Ardennes and the marine Devonian of the North Sea (Kent 1965b, Pennington 1975). The second Continental Floor was explained by Wills (1973). Column 7 Complete plantation to within 30m of ocean-level by terrestrial erosion. Column 8 Evidence available that present ocean-level is close to ocean level at these times. Column 9 Long suites of (a) plankton-bearing mudstones (graptolitic shales); coccolith limestones (chalk); (b) benthonic shelf-deposits-sandstones, mudstones, limestones, delta conglomerates, and Carboniferous cyclothems. Both (a) and (b) indicate long-continued rise of ocean-level with maintenance of the same environment. Column lOa Lower Palaeozoic cycles in the Floor (Roman numerals). Column lOb Cycles in the Cover, Nos. 1 to 7 being Basal Elements. Cycles of various lengths of time are shown by the curves on Fig. 1, ranging from c. 60 m.y. to a very short cycle e.g. Carboniferous cyclothems (Ramsbotton (1976), Calver (1969)). The base of each cycle represents sediment deposited on a virtually horizontal surface produced by erosion and planation of the preceding one which was graded to the contemporary ocean-level (often quite near to present ocean level). Column 11 Isostatic movements are due to movements in the asthenosphere (Bott 1967).

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