GOING "DOWN UNDER" TO TEST THE URALIAN SEAWAY HYPOTHESIS FOR SILURIAN PALEOGEOGRAPHY OF ALASKA’S ALEXANDER TERRANE

Paleomagnetic, detrital zircon, and paleontologic evidence suggests that se Alaska’s Alexander terrane (AT), an accreted island arc, was located close to either Gondwana or Laurentia-Baltica in the mid-Paleozoic. Similar Upper Silurian stromatolites in the AT, Farewell terrane (sw Alaska), and the Northern-Southern Urals indicate that the AT was positioned at a site in the Uralian Seaway, which facilitated larval exchange in the Late Silurian. Comparative research on Upper Silurian rocks in Australia adds new data to refute the hypothesis that the AT is a far-traveled (trans-Pacific) crustal fragment from Gondwana.

Samples were collected from the Upper Silurian Jack Hills Formation at Broken River, northern Queensland. Six lithologic units in the 55 meter-thick section form a regressive-transgressive sequence. Thinly bedded shales at the base of the section are interbedded with skeletal wackestones-packstones characterized by diverse suites of whole and abraded fossils, including domal stromatoporoids in growth position. These offshore, quiet-water deposits are overlain by microbial boundstones interbedded with skeletal packstones, which comprise oncoids with Rothpletzella-Wetheredella encrustations on diverse skeletal nuclei. The overlying quartz-mica siltstones represent the shallowest part of the sequence with the accumulation of siliciclastic debris in nearshore-terrestrial sites. A gradual marine transition is recorded in marly oolitic-peloidal grainstones, which are overlain by skeletal-peloidal grainstones, microbial boundstones, and crinoidal packstones of deep intertidal-shallow subtidal origin.

Despite the abundance of microbial organisms, these assemblages are unlike those found in Upper Silurian shallow-marine (B.A. 2-3) deposits in Alaska and the Urals. This suggests that a paleobiogeographic link must have existed between the AT, Farewell terrane (nw Laurentia or Siberia), and the Urals (Baltica) but not eastern Australia in the Late Silurian. These data, together with northern hemisphere options derived from other geologic evidence, help confirm that the AT was located along the Uralian Seaway near Laurentia-Baltica-Siberia in the Late Silurian.