LATE TRIASSIC (MIDDLE NORIAN) VERTEBRATE FOSSILS FROM HOUND ISLAND, ALASKA

Late Triassic (210 Ma) marine reptile bones are exposed in the modern intertidal zone on Hound Island, Keku Straits. The tectonic setting is associated with the Alexander Terrain prior to its northern translation from lower latitudes to the current latitude. The analysis of such a setting containing vertebrate fossils allows for the opportunity to understand better the depositional, paleoenvironmental, and biogeographic history of Southeast Alaska. Fossils occur in thinly bedded, dark gray, calcareous shale and conglomeratic limestone. Centimeter-scale field and petrographic observations on the lithologic and faunal composition, and sedimentary texture and structure indicate that the interbedded layers of conglomeratic limestone have a chaotic internal fabric with poor to no graded bedding. Clasts are typically matrix-supported and are interpreted to have been transported as debris flows that resulted from slumping and redeposition of slope sediments. All of the observed bone is disarticulated and shows abrasion and breakage, indicating transport or reworking prior to burial. Preliminary findings on a number of vertebral centra, teeth, and rib fragments, indicate that several taxa are present. Some vertebrae are amphicoelus and appear to belong to the marine Ichthyosauria. Other vertebrae are platycoelus suggesting Sauropterygian affinity. Since the late Triassic depositional interval that preserved the Hound Island faunas, the Alexander terrane was transported from a low paleolatitude (~15º) to its final position and accreted to the western margin of North America no earlier than the Middle Jurassic.