UPPER ORDOVICIAN-MIDDLE DEVONIAN PALEOBIOGEOGRAPHIC SIGNALS FROM SHELLY FAUNAS OF THE ALEXANDER TERRANE, SOUTHEAST ALASKA

The Alexander terrane of Southeast Alaska contains diverse shelly faunas of Late Ordovician-Middle Devonian age with strong paleobiogeographic signatures. Gastropods from latest Ordovician carbonate rocks on Prince of Wales Island include the gastropods Daidia and the operculum of Maclurites. Maclurites is cosmopolitan in North America, but Daidia is only known from the York Terrane in Alaska and eastern North America. Co-occurring sphinctozoan sponge includes several species previously known only from the Eastern Klamath terrane of northern California.

Upper Silurian brachiopods include a number of distinctive pentameroid genera (Brooksina, Harpidium, Cymbidium) as well as widespread Kirkidium alaskense. Other brachiopod genera present include Skenidioides, Alaskospira, Gracianella, Atrypoidea, Howellella, Lissatrypa, and Morinorhynchus. The brachiopods are most similar to those of Northeast Russia (Omulevka terrane). Upper Silurian gastropods not known from cratonic North America include Retispira, Pachystrophia, Beraunia, Kirkospira, Bathmopterus, Medfracaulus, Coelcaulus, and Morania. The occurrence of Medfrazyga and Medfracaulus suggests a faunal link between the Alexander and Farewell terranes of Alaska. Aphrosalpingid sponges are abundant in Upper Silurian microbial reefs of the Alexander terrane. These sponges are otherwise known only from the Farewell terrane of SW Alaska, the Urals and Salair region of Russia.

Lower Devonian brachiopods include highly provincial Pragian and Emsian faunas which show their closest relations to the Farewell terrane of SW Alaska, Northeast Russia (Omulevka terrane) and Taimyr. None of these species appears in coeval strata of western Laurentia. Eifelian (Early Middle Devonian) age brachiopods and gastropods include a number of endemic species known only from the Farewell terrane.

These highly distinctive faunas are unlike those known in Laurentia, but closely approach faunas known from other accreted Alaskan terranes (notably the Farewell terrane), Siberia, and the Urals. Based on this, as well as on strong lithostratigraphic similarities, we believe that the Alexander terrane, along with the Farewell terrane and the Omulevka terrane, were rifted from the eastern margin of the Siberian paleocontinent.