APPLYING TERRANE ANALYSIS IN ALASKA TO UNRAVELING THE PALEOZOIC PALEOBIOGEOGRAPHY OF MONGOLIA

Circumscribing the Alexander terrane (AT) to the Northern Hemisphere – along the Uralian Seaway – in the mid-Paleozoic required careful documentation of the paleobiogeographic affinities of Paleozoic fossils, the paleoenvironments they inhabited, and the patterns of sedimentation associated with accreted oceanic islands. Field research undertaken by scores of individuals on three continents over two decades or more – in the AT (primarily in the Prince of Wales Island area and Glacier Bay), the Farewell terrane (southwestern Alaska), and Russia (Pay-Khoy, Ural Mountains, and western Siberia) – helped establish that these widely disparate regions developed strikingly similar reefs in the Late Silurian. The distinctive microbial-sponge biotas confirm that each of these areas must have bordered the Uralian Seaway, which as a marine corridor fostered the transmigration of organisms between the AT, northern Laurentia, eastern Baltica, and Siberia in the Late Silurian.

Central Asia, with Mongolia at its geographic center, experienced a similar process of dynamic crustal growth in the late Paleozoic-early Mesozoic through the protracted accretion of dozens of allochthonous terranes onto Asia's cratonic core. Analysis of Ordovician-Silurian deposits exposed in two Mongolian terranes will help refine paleobiogeographic models of Mongolia's terranes. The Mandalovoo terrane comprises a nearly continuous Paleozoic island-arc sequence of mildly deformed volcanic and sedimentary rocks. Well-preserved fossils from shallow- and deep-marine environments record the growth and expansion of island-fringing carbonate platforms. Higher in the section, limestone-shale turbidites appear to have formed along the slope of the adjacent Gobi Altai terrane. Research on the Ordovician-Silurian shallow-water facies in both terranes should help ascertain their proximity to other terranes and whether Uralian Seaway conditions extended into Mongolia in the Late Silurian.