Stratigraphy, Structure, And Graptolites of an Ordovician and Silurian Sequence in the Terra Cotta Mountains, Alaska Range, Alaska By Michael Churkin, Jr., and Claire Carter Geology and paleontology of three newly named Paleozoic formations and descriptions of six new graptolites UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1996 U.S.DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director For sale by U.S. Geological Survey, Information Services Box 25286, Federal Center Denver, CO 80225 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Text and illustrations edited by Mary Lou Callas Camera lucida fossil illustrations by Claire Carter Other illustrations by Michelle Coveau Layout by Mary Lou Callas Library of Congress Cataloging-in-Publication Data Churkin, Michael. Stratigraphy, structure, and graptolites of an Ordovician and Silurian sequence in the Terra Cotta Mountains, Alaska Range, Alaska I by Michael Churkin, Jr., and Claire Carter. p. cm. - (U.S. Geological Survey professional paper ; 1555) Includes bibliographical references and index. Supt. of Docs. no.: 119.16: 1555 1. Graptolites-Alaska-Terra Cotta Mountains. 2. Paleontology-Ordovician. 3. Paleontology-Silurian. 4. Animals, Fossil-Alaska-Terra Cotta Mountains. 5. Geology-Alaska-Terra Cotta Mountains. 6. Geology, Stratigraphic-Ordovician. 7, Geology, Stratigraphic-Silurian. I. Carter, Claire. I1 Title. I11 Series. QE840.5.C48 1995 563'.993-dc20 94-33018 CIP CONTENTS Abstract ................................................................................................................... Introduction ............................................................................................................. Previous studies ....................................................................................................... Acknowledgments ................................................................................................... Stratigraphy and structure of a graptolite-rich Ordovician and Silurian sequence in the Terra Cotta Mountains. Alaska Range. by Michael Churkin. Jr., and Claire Carter ....................................................................................................... Regional geologic setting .............................................................................. Stratigraphic succession ................................................................................ Post River Formation ..................................................................................... Lithologic succession and members .................................................... Lower siltstone member ....................................................................... Mudstone member ................................................................................ Upper siltstone member ....................................................................... Graptolite Canyon Member ................................................................. Limestone member ............................................................................. Terra Cotta Mountains Sandstone ............................................................... Lower limestone member .................................................................. Middle limestone member ................................................................. Upper limestone member ................................................................... Barren Ridge Limestone .............................................................................. Lower limestone member .................................................................. Siltstone member ................................................................................ Upper limestone member ................................................................... Structure of the Terra Cotta Mountains ....................................................... Regional structural setting ................................................................. Structure and tectonics of mapped area ............................................. Structure sections ......................................................................................... Section H-1 ......................................................................................... Section J-K-L-M ................................................................................. Graptolite fauna of the Terra Cotta Mountains, Alaska Range, by Claire Carter Introduction .................................................................................................. Acknowledgments ....................................................................................... Ordovician graptolite zones and correlations .............................................. Adelograptus Zone ............................................................................. Tetragraptus approximatus Zone ....................................................... Tetragraptus fruticosus Zone ............................................................. Oncog raptus Zone .............................................................................. Paraglossograptus tentaculatus Zone ............................................... Diplograptus? decoratus Zone .......................................................... Dicellograptus Zone .......................................................................... Clirnacograptus bicornis Zone .......................................................... Climacograptus tubuliferus Zone ...................................................... CONTENTS Silurian graptolite zones and correlations ................................................... Lagarograptus acinaces and Coronograptus cyphus Zone ............... Monograptus convolutus Zone .......................................................... Monograptus sedgwickii Zone ........................................................... Monograptus turriculatus Zone ......................................................... Monograptus spiralis Zone ................................................................ Cyrtograptus sakmaricus.C . laqueus Zone ...................................... Cyrtograptus centrifugus Zone .......................................................... Cyrtograptus lundgreni Zone ............................................................ Monograptus digitatus Zone .............................................................. Monograptus ludensis Zone ............................................................... Early Ludlovian .................................................................................. Systematic descriptions ............................................................................... Class Graptolithina ............................................................................. Order Dendroidea ..................................................................... Family Anisograptidae .................................................... Order Graptoloidea ................................................................... Family Dichograptidae .................................................... Family Isograptidae ......................................................... Family Glossograptidae .................................................. Family Nemagraptidae .................................................... Family Dicranograptidae ................................................. Family Diplograptidae .................................................... Family Retiolitidae .......................................................... Family Monograptidae .................................................... Family Cyrtograptidae .................................................... References cited .................................................................................................... Index ...................................................................................................................... PLATES [Plates are in pocket] 1. Geologic map of part of the McGrath A.1. A.2. B- 1. and B-2 Quadrangles. Alaska 2 . Map showing. fossil localities. type sections. reference sections. and field camp locations. Terra Cotta Mountains 3 . Correlation chart of graptolite successions in central Alaska and measured section of the type Post River Formation FIGURES Index map showing location of Terra Cotta Mountains ............................................................................... 3 Panoramic view across Terra Cotta Mountains showing general stratigraphy and structure ...................... 6 Photograph of type section of the Post River Formation ............................................................................. 8 Guide to graptolite faunas and other fossils in Terra Cotta Mountains ....................................................... 9 Photographs of: 5. Lower siltstone member of the Post River Formation ........................................................................ 10 6 . Lower part of the Graptolite Canyon Member of the Post River Formation at type section ............. 10 7 . Resistant siliceous mudstone in upper part of Graptolite Canyon Member ....................................... 11 Photomicrograph of thin section of mudstone from the Graptolite Canyon Member ............................... 11 Columnar sections of limestone member and upper