Series Developments in Petrology l.K.R. Mehnert MIGMATITES AND THE ORIGIN OF GRANITIC ROCKS 2. V. Marmo GRANITE PETROLOGY AND THE GRANITE PROBLEM 3. J. Didier GRANITES AND THEIR ENCLAVES The Bearing of Enclaves on the Origin of Granites 4. J.A. O'Keefe TEKTITES AND THEIR ORIGIN 5. C.J. Allègre and S.R. Hart (Editors) TRACE ELEMENTS IN IGNEOUS PETROLOGY 6. F. Barker (Editor) TRONDHJEMITES, DACITES, AND RELATED ROCKS 7. C.J. Hughes IGNEOUS PETROLOGY 8. R.W. Le Maître NUMERICAL PETROLOGY Statistical Interpretation of Geochemical Data 9.M. Suk PETROLOGY OF METAMORPHIC ROCKS Developments in Petrology 10 SHALE-SLATE METAMORPHISM IN SOUTHERN APPALACHIANS CHARLES E. WEAVER and Associates School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, GA 30332 (U.S.A.) ELSEVIER, AMSTERDAM-OXFORD-NEW YORK-TOKYO 1984 ELSEVIER SCIENCE PUBLISHERS B.V. Molenwerf 1 P.O. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, N.Y. 10017 Library of Congress Cataloging in Publication Data Weaver, Charles E. (Charles Edward) Shale-slate metamorphism in southern Appalachians. (Developments in petrology ; 10) Bibliography: p. Includes index. 1. Metamorphism (Geology)—Appalachian Mountains. 2. Shale—Appalachian Mountains. 3. Slate—Appalachian Mountains. I. Title. II. Series. QEiiT5.A2W39 1981+ 552'. h 33-25292 ISBN 0-l*M-U226U-l ISBN 0-444-42264-1 (Vol. 10) ISBN 0-444-41562-9 (Series) © Elsevier Science Publishers B.V., 1984 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopy- ing, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 330, 1000 AH Amsterdam, The Netherlands Printed in The Netherlands This book is dedicated to my parents Kathryn Best Weaver and Clair W. Weaver who wanted me to have the opportunity they never had. VII PREFACE This book is based on the investigations of seven people. In an attempt to recognize the contributions of each, authors are given for each chapter. The work includes portions of four M.S. theses (B.R. Broekstra, P.B. Highsmith, R.A. Sedivy and D.M. Bean) and one special problem (A.E. Stone, Jr.). Sup- plementary data were obtained for most chapters. In order to produce a co- herent, internally consistent story, the entire book was synthesized, rewritten, and many of the data and ideas reinterpreted by C.E. Weaver. There was no other way. My sincere thanks to all who participated in this adventure, par- ticularly Dr. J.M. Wampler who worked closely with the students and rewrote my rewrite of the K-Ar chapter. Dr. James W. Smith first brought the Cona- sauga Formation and Great Smoky fault to my attention and collected some preliminary samples. Dr. Paul F. Williams kindly examined some of the thin sections and reviewed an early draft of The Petrology Chapter. Marilyn Thoroman patiently spent many hours obtaining chlorite concentrates. Wade Mullin prepared many of the SEM samples. Dr. K.C. Beck made several chem- ical analyses. The manuscript was typed by Ms. Annette Plunkette and Ms. Pat Rice. I apologize to the metamorphic petrologists for any misuse of their terminology. Much of the support for this study was supplied by Battelle Memorial Institute, Office of Nuclear Waste Isolation, under contract with the De- partment of Energy. The objectives of the program were to develope a paleothermometer for "shales" and to be able to predict the reactions that would occur in response to an increase in temperature. Chapter 1 INTRODUCTION CHARLES E. WEAVER The diagenetic changes that occur in muds and shales between the tem- perature range of 20° and nearly 200°C have been well documented (Burst, 1959, 1969; Weaver, 1959; Dunoyer de Segonzac, 1964,1970; Teodorovich and Konyukhov, 1970; Perry and Ho wer, 1970; Weaver and Wampler, 1970; Weaver and Beck, 1971; Moort, 1971; Foscolos and Kodama, 1974; Heling and Teichmüller, 1974; Hower et al., 1976; Aronson and Hower, 1976; Boles and Franks, 1979). The features of the greenschist faciès, the be- ginning of "true" metamorphism, have been described in considerable detail by metamorphic geologist. Less is known of the reactions that occur during very-low-grade metamorphism, approximately 200° to 400° C, though these rocks have been studied in some detail primarily by European geologists (Kubier, 1967. 1968; Kisch, 1968, 1974, 1980a, b; Dunoyer de Segonzac, 1970; Frey, 1970, 1974, 1978; Frey and Niggli, 1971; Weber, 1976; Frey et al., 1980; etc.). The present study was undertaken to examine in detail the mineral, chemical, isotopic, pétrographie and structural changes that occur during late-stage diagenesis, very-low-grade metamorphism and low-grade metamor- phism. The Upper Cambrian Conasauga shale in the Valley and Ridge Pro- vince of Georgia, Tennessee, and Alabama was selected for study (Fig. 1). The diagenetic-metamorphic reactions that occurred in the Conasauga are influenced both by burial depth (temperature) and dynamic factors (thrust- ing). The study had two main objectives. One was to determine the sequence of reactions as a function of temperature so as to be able to predict what reac- tions would occur when the physilites were subjected to an increase in tem- perature due to the emplacement of high-level radioactive waste. The other objective was to develop one or more paleothermometers so as to be able to determine the maximum burial temperature a shale or a shale-like rock had been exposed to. The terminology used to describe the degree of metamorphism is based on that suggested by Kubier (1967): zone of diagenesis, anchizone (very-low- grade metamorphism), epizone (low-grade metamorphism, greenschist facies). The boundaries are based on the crystallinity index (Kubier Index) of the 10-Â X-ray peak of the micaceous material. The K.I. values have been correlated with those of Kubier using standards provided by him. The boundary values in this study are 3.0 and 1.5 (peak-width at half-height of 10 Â peak in mm). 2 The terminology used to refer to the sheet silicate minerals and the rock with a high content of sheet silicate minerals is that suggested by Weaver (1980). The term physil refers to sheet silicate minerals and has no size connotations (most of the sheet silicates in shales and slates are coarser than clay-size and therefore are not truly clay minerals); the term physilite refers to rocks which are composed predominantly of physils. Chapter 2 GEOLOGIC BACKGROUND CHARLES E. WEAVER The Cambrian Conasauga shale forms one unit of a thick mass of mostly unmetamorphosed Paleozoic Appalachian basin sediments that are exposed in the Valley and Ridge Province of northwestern Georgia, northeastern Alabama, and eastern Tennessee (Fig. 1). The sediments of the Valley and Ridge in these areas extend in a roughly linear band, trending northeast- southwest, bounded to the northwest by the predominantly undisturbed, horizontal sediments of the Cumberland Plateau. To the east, the Valley and Ridge is limited by the crystalline metamorphic rocks of the Piedmont and Blue Ridge provinces, which have been upthrust along the southeast- ward-dipping Cartersville-Great Smoky fault. The southern boundary is formed by the nearly horizontal Emerson fault (Cressler et al., 1979) which was earlier thought to be a continuation of the Great Smoky fault. Move- ment along the latter fault preceded movement along the Emerson fault. A second major fault, the Rome-Saltville fault, extends throughout the Valley and Ridge of this area, separating rocks predominantly of Ordovician to Pennsylvanian age from Cambrian and Ordovician age rocks to the south- east (Chowns, 1977). A number of authors have obtained isotopic apparent ages in the range 360—420 m.y. from crystalline rocks immediately adjacent to the Carters- ville-Great Smoky fault (Long et al., 1959; Davis et al., 1962; Smith et al., 1969). Dalimeyer (1975) reported an undisturbed 40Ar-39Ar age of 379 m.y. apparent age from a biotite, which is in accord with the K-Ar results of Smith et al. (1969). Dallmeyer concluded that the data reflect Ar retention following a pre-Middle Ordovician metamorphic event. Russell (1976) reported Rb-Sr apparent ages ranging from about 355 to 375 m.y. for cataclastic rocks along the Bartletts Ferry and Cartersville- Great Smoky faults, and within the Brevard zone. Russell has interpreted these data as indicative of a major period of faulting during the Acadian Orogeny. Paleozoic rock in the Valley and Ridge Province range in age from Cambrian to Pennsylvanian. They form a wedge-shaped sequence thickening from east to west. Present thickness increases from 300 m in the west to 6000 m to the east near the Great Smoky fault (Harris and Milici, 1977). A projection of Harris and Milici's data indicates that the maximum pre-erosion thickness in the eastern part of the basin was in the vicinity of 15,000 m. The Cona- sauga in this area was overlain by approximately 10,000 m of sediments. 4 KENTUCKY 0 50 MILES 50 KILOMETRES Fig. 1. Map of tectonic subdivisions of southern Appalachians. Area of study was the Valley and Ridge Province of Alabama. Georgia and Tennessee (after Hatcher and Zietz, 1979). 5 or more. To the west the maximum overburden was on the order of 3000 m. Though these thickness values are not accurate, the trend is well established and it is likely that the thickness differential on the two sides of the basin was even larger in a northwest—southeast direction with the maximum thick- ness occurring in northwest Georgia. The present study suggests that in Georgia the Great Smoky fault extended 20—40 km east of its present position and further added to the burial depth of the Conasauga Formation. The overthrusting was probably responsible for the relatively steep metamor- phic gradient in this area. The Paleozoic sediments were deposited in three episodes, each separated by regional unconformities (Harris and Milici, 1977). The Cambrian through Lower Ordovician depositional unit is a westward-transgressive sequence that gradually changes upward from dominantly clastic to dominantly car- bonate. During the Middle and Upper Cambrian, when the Conasauga was deposited, the source for clastic material was to the west and north. A re- latively deep-water lagoonal sequence of shale, siltstone and thin-bedded limestone accumulated in the western part of the Valley and Ridge Province. To the east fine-grained elastics interfingered with and were gradually supplant- ed by shallow-marine carbonate shelf units. During the Late Cambrian and Early Ordovician, the eastern carbonate shelf sequence (Knox Group) transgressed westward, eventually covering the entire Appalachian basin. Moderate uplift and erosion occurred during the Early Ordovician. In Tennessee the Conasauga sequence thickens from approximately 300 m along the western edge of the Valley and Ridge Province to approxi- mately 1000 m along the eastern edge. It is believed to be somewhat thicker in northwest Georgia (Colton, 1970). Thus, even though the samples were all collected from the Conasauga Formation, the burial depths can vary con- siderably depending on whether the top or bottom of the formation was sampled. In spite of this, a well-developed diagenesis-metamorphic gradient was established that was related to estimated burial depth. From Middle Ordovician to Pennsylvanian time, the major source for clastic sediments was to the east or southeast. The shift in source was caused by the Taconic orogeny. Another period of uplift and erosion occurred at the end of Early Devonian time. Much of the Upper Ordovician and Lower Devonian section was removed from the easternmost part of the Valley and Ridge Province. The Late Devonian to Pennsylvanian episode began with the widespread deposition of black Chattanooga shale. The overlying Mississippian clastic and carbonate-shelf complex thickens to the east. Final filling of the south- ern Appalachian Paleozoic basin was accomplished by a westward advance of a series of Upper Mississippian and Pennsylvanian, littoral, deltaic, and alluvial deposits (Harris and Milici, 1977). The final tectonic event was the Allegheny deformation which produced the thrust and folds now present in the Valley and Ridge Province. This event is not accurately dated but