The Alexander Terrane Of Alaska - A Displaced Fragment Of Northeast Russia? Evidence From Silurian-Middle Devonian Megafossils And Stratigraphy

There has been various speculation on the palaeogeographical origins of the Alexander Terrane of Southeast Alaska, the suggestions including eastern Australia Baltica or even Siberia Close affinities have previously been noted between the Silurian-Devonian fossil faunas of the Alexander terrane and those of the Farewell terrane (primarily with the Nixon Fork subterrane) of west central and southwestern Alaska An overview of the Silurian to Middle Devonian faunas of the Alexander terrane, along with its gross stratigraphic succession, indicates that it is also closely allied with the Omulevsk Mountains (Omulevka terrane) of Northeast Russia The late Silurian brachiopod fauna of the Omulevsk Mountains includes a number of distinctive species otherwise reported only from Southeast Alaska The Silurian to Middle Devonian stratigraphic succession of the southern part of the Alexander terrane (i e western Prince of Wales Island) mimics the gross stratal succession in the Omulevsk Mountains In the Alexander terrane lower Silurian deep-water rocks of the Descon Formation are succeeded by shallow-water carbonates of the Heceta Limestone (late Llandovery-Ludlow) and overlain in turn by red bed elastics of the Karheen Formation (interpreted here to be of latest Silurian age in its type area) The Mirninskaya Suite of the Omulevsk Mountains represents a close analog of the Karheen and is of the same age Clastic red bed units of this age are rare or absent elsewhere in the Palaeo-Pacific Ocean or in the Urals The Lower and Middle Devonian brachiopod and gastropod faunas of the Alexander terrane also demonstrate close alliance with those of the Farewell terrane, which in turn are also closely allied with contemporaneous faunas of Northeast Russia (especially from the Omulevka terrane) The Omulevka and allied terranes within the Kolyma-Omolon superterrane of Northeast Russia are thought by many Russian workers to represent locally derived blocks rifted from the eastern margin of the Siberian palaeocontinent during Late Devonian-early Carboniferous time The sharing of so many elements (both fauna and stratigraphy) makes the rift origin of the Alexander terrane from Northeast Russia at the same time a very appealing hypothesis