Late Early Devonian (Late Emsian) eospiriferinid brachiopods from Shellabarger Pass, south-central Alaska, and their biogeographic importance; further evidence for a Siberian origin of the Farewell and allied Alaskan accreted terranes

Abstract

Two eospiriferinid taxa,Myriospirifer n. sp. aff.M. myriofilaHavlicek 1978 andJanius cf.J. vetulus (Eichwald 1860), are described from an unnamed late Emsian (late Early Devonian) limestone unit in the Shellabarger Pass area, Talkeetna C-6 Quadrangle, south-central Alaska. These biogeographically distinctive species, along with other co-occurring brachiopod taxa such as the gypidulinid genusIvdelinia and the rhynchonellidSibirirhynchia alata (Khodalevich) indicate strong biogeographic affinities with Emsian faunas of Siberia (including Kolyma, Taimyr, and the Kuznetsk Basin) and the Urals. This Alaskan fauna is from the Farewell Terrane, an accreted terrane, which also shows strong Siberian affinities in the Middle Cambrian, Late Ordovician, Late Silurian-Lochkovian, early Emsian (late Early Devonian), and Permian time-intervals, suggesting that the Farewell Terrane is a continental margin sequence rifted away from the Siberian continent. Based on their close faunal ties with Siberia and with the Farewell Terrane, Siberian origins are also indicated for other major Alaskan terranes: the Alexander Terrane of southeastern Alaska, the Arctic Alaska Superterrane, and the York Terrane of northern Alaska. The only truly North American portion of Alaska is east-central Alaska (including the Nation Arch), bounded on the north by the Porcupine River and on the south by the Yukon River and Tintina Fault. Faunas from the latter area are nearly identical to those from miogeoclinal and cratonic strata of northwestern and Arctic Canada, and are very distinct from those present in the accreted terranes making up the greater portion of Alaska.