EARLY AND MIDDLE DEVONIAN BRACHIOPODS FROM SOUTHERN ALASKA – IMPLICATIONS FOR PALEOBIOGEOGRAPHY AND TERRANE ACCRETION

Early and Middle Devonian brachiopods from southern Alaska’s accreted terranes (notably Farewell, Livengood, and Alexander terranes) contain a number of paleobiogeographically distinctive elements. The Pragian and Emsian Stages are especially useful for biogeographic studies, as they represent the acme of faunal endemicity within the Devonian.

Alexander terrane: The Wadleigh Limestone of the Alexander terrane contains an early Pragian brachiopod fauna on Heceta Island which is most similar to that of NE Asia, Taimyr, and Farewell terrane (Soda Creek Limestone). Eifelian (Early Middle Devonian) Wadleigh Ls. brachiopods and gastropods are essentially identical to those of the Farewell terrane. The limestone member of Cedar Cove Fm. on Chichagof Island contains a late Emsian and/or early Eifelian fauna with Warrenella praekirki. The latter species is widespread throughout early Eifelian strata of western and northern Laurentia. Givetian brachiopods (dominated by stringocephalids) occur in both the Wadleigh Ls. and Coronados Volcanics. The latter unit contains Stringocephalus axius and other new stringocephalid genera and species.

Farewell terrane: The Farewell terrane contains a diverse early Pragian brachiopod fauna in the Soda Creek Ls. which is most closely related to faunas from NE Asia, Taimyr, and the Alexander terrane. Late Emsian brachiopods from Shellabarger Pass in the central Alaska Range contain a number of distinctive Eurasian brachiopod taxa (not known from Laurentia), i.e., Myriospirifer, Ivdelinia, Janius, and Sibirirhynchia alata Khodalevich, The Eifelian Cheeneetnuk Ls. (Farewell terrane) contains some distinctive Eurasian elements which are also occur in the Alexander and Livengood terranes, but also contains several species in common with the Denay Ls. of Nevada.

In summary, Pragian and Emsian brachiopods of southern Alaska clearly show strong linkage with NE Russia and Taimyr, with no species known which co-occur in western Laurentia. During the Eifelian, both Eurasian and Laurentian species are observed in the southern Alaskan terranes. Givetian brachiopod faunas of the Alexander terrane include stringocephalids such as Stringocephalus axius and new taxa which suggest a strong faunal alliance between western Canada, Northeast Asia and Nevada.