UPPER TRIASSIC SHALLOW-WATER MARINE FAUNA FROM THE ALEXANDER TERRANE (SOUTHEAST ALASKA) AND ITS PALEOBIOGEOGRAPHIC IMPLICATIONS

This study focuses on identification, relative diversity, paleoecologic, and paleobiogeographic relationships of benthic sessile organisms such as scleractinian corals, sphinctozoid sponges, gastropods, bivalves, and brachiopods from Upper Triassic strata of the Alexander terrane, southeast Alaska. Shallow-water silicified marine fossil organisms used in this study were collected from Upper Triassic (Norian) limestones of the Hyd Group on Kuiu and Kupreanof islands as well as two correlative Upper Triassic units to the south on Gravina Island.

Preliminary findings on the corals suggest a taxonomic relationship with the Wallowa terrane. Similar coral and sponge genera include Rhaetiastraea, Kuhnastraea, Gablonzeria, Retiophyllia, Distichophyllia, Heptastylis and Spongiomorpha tenuis. Other shallow-water invertebrate fossils were collected and include gastropods, brachiopods, echinoids, bivalves, nautiloids, ammonoids, aulacocerids and the distinctive hydrozoan, Heterastridium. Gastropods are common at a number of stratigraphic horizons and show faunal ties with Farewell and Chulitna terranes of southern Alaska. Shared gastropod taxa are also present with the Wrangellia and Wallowa terranes (notably Spinidelphinulopsis whaleni and some neritopsoidean species which had teleplanic larvae), but no linkages are shown among more primitive archaeogastropod groups characterized by direct larval development. Gastropod evidence suggests that while the Alexander, Wallowa, and Wrangellia terranes were all probably situated at a warm, low latitude in the eastern part of the Panthalassa Ocean, significant distances must have separated the Alexander from both the Wallowa and Wrangellia terranes to account for faunal differences among groups characterized by direct larval development.

Preliminary results help to resolve discrepancies in the paleogeographic position for the Alexander terrane, as well as add a wealth of knowledge about silicified Triassic corals, sponges, and gastropods. Furthermore, taxonomic relationships and assemblages were compared with previously determined conodont zones in order to confirm relative fossil ages. Studied fossil assemblages infer a warm, humid (supported by fossil wood) tropical position for this terrane during the Upper Triassic (Norian) time.