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The Late Triassic Bivalve Monotis in Accreted Terranes of Alaska

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The Late Triassic Bivalve Monotis in Accreted Terranes of Alaska The Late Triassic Bivalve Monotis in Accreted Terranes of Alaska By N.J. Silberling, J.A. Grant-Mackie, and K.M. Nichols U.S. GEOLOGICAL SURVEY BULLETIN 2 15 1 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997 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 Library of Congress Cataloging-in-Publication Data Silberling, N. J. (Norman John), 1928- The Late Triassic bivalve Monotis in accreted terranes of Alaska / by N.J. Silberling, J.A Grant-Mackie, and K.M. Nichols. p. cm.-(U.S. Geological Survey bulletin ; 2151) Includes bibliographical references. Supt. of Docs. no. : I 19.3: 2151 1. Monotis-Alaska. 2. Paleontology-Triassic. 3. Animals,Fossil- Alaska. I. Grant-Mackie, J.A. 11. Nichols, K. M. (Kathryn Marion), 1946- 111. Title. IV. Series. QE75.B9 no. 2151 [QE8 12.M6] 557.3 sdc20 [564'.11] 95-25680 CIP CONTENTS Abstract ........................................................................................................................... Introduction ..................................................................................................................... Characterization of Alaskan Monotis .............................................................................. Genus Monotis ................................................................................................................ Subgenus Monotis ........................................................................................................... Subgenus Pacimonotis .................................................................................................... Subgenus Entomonotis .................................................................................................... Subgenus Eomonotis ....................................................................................................... Occurrence of Monotis in Alaska .................................................................................... Northern Alaska ...................................................................................................... East-central Alaska ................................................................................................. Central and Southern Alaska ................................................................................... Southwestern Alaska ............................................................................................... Southeastern Alaska ................................................................................................ Age and Biostratigraphy of Alaskan Monotis Faunas ..................................................... Paleogeographic Significance of Alaskan Monotis Faunas ............................................ References Cited ............................................................................................................. Appendix 1. Locality Descriptions ................................................................................ PLATES [Plates follow appendix] 1 . Alexander terrane . 2 . Peninsular terrane . 3. Wrangellia terrane . 4 . Wrangellia, Susitna. Chulitna terranes . 5 . Nixon Fork and McKinley terranes . 6 . Non-accretionary Upper Yukon region . 7 . Upper Yukon region and Endicott Mountains subterrane . 8 . Endicott Mountains subterrane . 9 . Endicott Mountains subterrane . 10. North Slope subterrane . 1 1 . North Slope subterrane . FIGURES 1 . Index map showing location of accreted terranes containing Monotis-bearing strata in Alaska and adjacent Canada .............................................................................................................................................................. 2 2 . Diagrammatic sketch of left valve of Monotis illustrating morphologic features and terminology ............................................................................................................................................................... 4 3 . Chart showing relative stratigraphic ranges of Alaskan Monotis species and subspecies ................................................................................................................................................................. 15 4 . Checklist showing occurrence of Monotis species in accretionary and non-accretionary Alaska. and in the Stikinia terrane of Canada .............................................................................................................................. 16 THE LATE TRIASSIC BIVALVE MONOTIS IN ACCRETED TERRANES OF ALASKA By N.J. Silberling, J.A. rant-~ackie,' and K.M. Nichols ABSTRACT INTRODUCTION Late Triassic bivalves of the genus Monotis occur in at Thin-shelled, pecten-like bivalves of the genus Monotis least 16 of the lithotectonic terranes and subterranes that are widespread and locally abundant in Upper Triassic rocks together comprise nearly all of Alaska, and they also occur of Alaska (fig. 1). These easily recognized fossils have been in the Upper Yukon region of Alaska where Triassic strata found in 16 of the 50 accreted tectonostratigraphic terranes are regarded as representing non-accretionary North Amer- and subterranes recognized by Jones and others (1987) as ica. On the basis of collections made thus far, 14 kinds of forming nearly all of Alaska. Each of these terranes and sub- Monotis that differ at the species or subspecies level can be terranes is fault-bounded and records a distinct geologic his- recognized from Alaska. These are grouped into the subgen- tory; their tectonic displacement relative to each other and to era Monotis (Monotis),M. (Pacimonotis),M. (Entomonotis), the North American craton ranges from tens to thousands of and M. (Eomonotis). In places, Monotis shells of one kind or kilometers (Jones and others, 1986). Not only are Monotis another occur in rock-forming abundance. faunas useful for dating and correlating the strata of the ter- On the basis of superpositional data from Alaska, as ranes in which they occur, but the paleolatitudinal distribu- well as from elsewhere in North America and Far Eastern tion of different species of Monotis varied, and thus they also Russia, at least four distinct biostratigraphic levels can be provide important paleogeographic constraints for interpret- discriminated utilizing Monotis species. Different species of ing the displacement histories of the various terranes. The M. (Eomonotis) characterize two middle Norian levels, both major biogeographic conclusions that can be drawn from probably within the upper middle Norian Columbianus Alaskan Monotis faunas have previously been discussed by Ammonite Zone. Two additional levels are recognized in Tozer (1982) and Silberling (1985). the lower upper Norian Cordilleranus Ammonite Zone uti- The purposes of this report are to enlarge upon these lizing species of M. (Monotis)or M. (Entomonotis), both of general conclusions and to provide needed documentation at which subgenera are restricted to the late Norian. a refined taxonomic level of the Monotis faunas presently An attached-floating mode of life is commonly attrib- known from Alaskan accreted terranes. Additionally, we uted to Monotis; thus, these bivalves would have been hope that the illustrations, diagnoses, and identification key pseudoplanktonic surface dwellers that were sensitive to sur- will enable Alaskan geologists to use this report as a field face-water temperature and paleolatitude. Distinctly differ- guide to the various species of Monotis. Not only are the ent kinds of Monotis occur at different paleolatitudes along various species useful in the field for general age and biogeo- the Pacific and Arctic margins of the North American craton graphic interpretation but, in places such as northern Alaska, inboard of the accreted terranes. Comparison between these they also occur in profusion in an easily recognized succes- craton-bound Monotis faunas and those of the Alaskan ter- sion of different species. In such places, they thus can be a ranes indicates that all of the Monotis-bearing terranes in valuable aid in deciphering the structure of intricately southern Alaska south of the Denali fault were paleoequato- deformed strata. rial in latitude during Late Triassic time. Among these ter- The Alaskan Monotis faunas on which this study is ranes, the Alexander terrane was possibly in the southern based are part of the U.S. Geological Survey paleontological hemisphere at that time. Terranes of northern Alaska, on the collections currently housed in the Denver, Colorado, field other hand, represent middle, possibly high-middle, northern center. These collections and their supporting locality and paleolatitudes. stratigraphic data represent the effort of many geologists involved with U.S. Geological Survey research programs in Alaska since the early 1900's. In all, Monotis is known from 'university of Auckland, Auckland, New Zealand. several hundred geographically or stratigraphically different CHARACTERIZATION OF ALASKAN MONOTIS 3 Alaskan localities. Specimens figured herein that were col- Taxonomic assignments adopted here are strictly cir- lected by the senior author are from localities originally dis- cumscribed by means of measureable parameters and the covered and put into stratigraphic context by E.E. Brabb, presence
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