Upper Triassic Terrane Faunas and Their Use in Paleogeographic Comparisons within the Cordilleran Region

University Of Montana, Missoula MT

Investigators

Abstract

Upper Triassic Terrane Faunas and Their Use in Paleogeographic Comparisons within the Cordilleran Region George Stanley, Robert Blodgett EAR0229795 University of Montana ABSTRACT This project will employ a combination of field work, laboratory, and museum collections research to assess invertebrate faunas during a critical time in the Late Triassic. It seeks to document the taxonomic composition, paleoecology and paleogeographic significance of various tropical to subtropical, shelly fossils from Alaska, western Canada, Nevada, and Mexico and to make comparisons with coeval fossils from other regions of the world, such as western Europe and parts of the former Tethys. Fossils to be studied are corals, sponges, hydrozoans, articulate brachiopods, bivalves and gasropods. These are abundant and widespread but little is known about the species taxonomy. The study will assess both taxonomy and paleoecological associations. During the Triassic time, some of these invertebrate faunas inhabited the western shores of North America but many others inhabited volcanic islands, now regarded as displaced terranes. The terranes from which faunas will be extracted, include the Alaskan portion of Wrangellia and the Farewell terrane, as well as Chulitna, Wallowa, Alexander and Junco terranes. In Triassic time, these terranes were located at unspecified distances off the western coast of the craton and their faunas provide unparalleled opportunities to test and better resolve the paleogeography, specifically to work out the paleobiogeographic affinities and relationships between some key terranes such as Wrangellia and the Alexander terrane. Many terranes represent active island arcs which, during and following cessation of volcanism, subsided and were replaced by sedimentary deposition, thus fostering rich invertebrate marine associations. Some of the fossils to be studied occur in thick carbonate deposits and even reefal associations. These organisms once developed at tropical to subtropical latitudes somewhere in the ancient Pacific ocean. Because many biotic assemblages proposed for study are silicified, fossils will be etched in dilute acids which are expected to free large numbers of fossils for study. Taxonomic study these silicified faunas will provide a wealth of new and important data. The project also is expected to benefit by collaboration with a number of outside specialists. Statistical paleobiogeographic methods will be employed and the project also will benefit from the development of an electronic paleontological database for many of the Triassic taxa. This paleontological project is expected to provide the necessary data for a general paleobiogeographic synthesis and to lay the foundation for testing some tectonic postulates for terrane relationships.