THE ALEXANDER TERRANE – A DISPLACED FRAGMENT OF NORTHEAST SIBERIA? EVIDENCE FROM SILURIAN BRACHIOPODS AND SILURIAN-LOWER DEVONIAN STRATIGRAPHY

The Alexander terrane of southeast Alaska is one of the major elements of the terrane collage comprising present-day Alaska. The terrane has recently been speculated to having had various tectonic origins, including eastern Australia, or as part of Baltica or Siberia. A new overview of the Upper Silurian brachiopod fauna of this terrane, along with the gross stratigraphic succession of Silurian and Lower Devonian rocks of this terrane indicates that it is most closely allied with the Omulevsk Mountains (Omulevka terrane) of northeast Siberia. The Late Silurian brachiopod fauna of the Omulevsk Mountains (listed and/or illustrated by A.A. Nikolaev and O.I. Nikiforova) includes a number of shared distinctive species previously reported only from Southeast Alaska by Edwin Kirk: Brooksina alaskensis Kirk, Conchidium (=Kirkidium) cf. alaskense Kirk, Harpidium insignis Kirk. While many of these genera occur in the faunally-allied Urals or Salair, none of the above-named species have been recognized in the latter areas.

The Silurian and Lower Devonian strata of the Alexander terrane mimic the gross stratal succession recognized in the Omulevsk Mountains, with Lower Silurian deep-water rocks of the Descon Formation succeeded by shallow-water carbonates of the Heceta Limestone (late Llandoverian-Ludlovian), and overlain in turn by red clastics of the Karheen Formation (of latest Silurian to Early Devonian age). The Mirninskaya Suite of the Omulevsk Mountains, represents a close analog of the Karheen and is of approximately the same age. Clastic red bed units of this age are rare or absent elsewhere in the Paleo-Pacific Ocean or in the Urals. The Omulevka and allied terranes within the Kolyma-Omolon superterrane of NE Siberia are thought by many Russian workers to represent locally derived blocks rifted from the eastern margin of the Siberian paleocontinent. The sharing of so many elements (both fauna and stratigraphy) makes the origin of the Alexander terrane from northeast Siberia very appealing.