9417907 Soja The accretion of "exotic" crustal fragments, or terranes, to continental margins is the tectonic process believed by many geologists to have been responsible for continental growth along western North America in the last 60-100 million years. Because of the complex geologic relationships produced by accretionary tectonics, the complete geologic history of this tectonic collage in western North America is still unknown. Paleobiogeographic research is of critical importance in Alaska because fossils preserved there occur within this complex zone of accreted terranes and occupy a present geographic position intermediate between the faunal provinces of North America and the circum-Pacific and circum-Arctic regions. To gain a better understanding of the origins and geologic histories of terranes in Alaska, paleontologists have been seeking to identify distinctive suites of fossils that, on the basis of shared affinities in faunas which occupied similar habitats, link certain terranes together or establish biogeographic connections with continental source areas. Using paleobiogeographic data in this way has enormous potential for resolving the controversies reflected in competing models that describe the tectonic evolution of western North America, in general, and Alaska, in particular. The proposed project aims to build upon the PI's previous research by investigating Silurian reef and reef-related carbonate deposits in southeastern Alaska to determine their geologic relationship to Silurian limestones in southwestern Alaska. Rocks exposed in southeastern Alaska belong to the Alexander terrane, which existed during the Early-Middle Paleozoic as volcanically active islands at an unknown site in the ancient Pacific Ocean, whereas deposits in southwestern Alaska (Nixon Fork terrane) represent an original, although dismembered part of the North American continent. Strata in these two areas share in common distinctive Silurian sponge faunas preserved in unusual stromatolite ree fs. In southeastern Alaska, Silurian carbonates have been examined in detail at scattered localities but principally only in one relatively small area in the southern part of the Alexander terrane. The proposed research will enable detailed investigations of Silurian rocks that are exposed in other parts of the Alexander terrane, especially where reconnaissance studies have revealed well-preserved, but as yet unstudied biotas. Field work involving undergraduate research students will focus on describing rocks and collecting fossils systematically from measured and photographed stratigraphic sections. Samples will be shipped from the field for the extraction of fossils and to be prepared in the lab as thin-sections and polished slabs for petrographic purposes. Compilation of petrologic, petrographic, and paleontolgic data will form the basis for determining the paleoecology, environmental setting, and faunal affinities of the preserved biotas and for identifying the degree of similarity in rock and fossil composition with Silurian rocks in southwestern Alaska. Demonstrating that a diversity of fossils from more than one relatively small part of southeastern Alaska is shared in common with southwestern Alaska will represent an important advancement in our understanding of the early geologic history of the Alexander terrane. These data will help to document supporting evidence for the hypothesis that the Alexander terrane was in close proximity to northwestern North America allowing faunal communication between the two localities. Such data would be important in providing a definitive paleogeographic link between the Alexander terrane and North America proper. Because similar Silurian sponge deposits also occur in the Ural Mountains, Russia, the proposed study will lay critical groundwork for exploring the idea that a seaway enabled migration of organisms along the northern rims of North America (Laurentia) and Europe (Baltica) in the Late Silurian.
