PALEOECOLOGY AND CHEMOSTRATIGRAPHY OF THE AMANSAIR AND TSAGAANBULAG FORMATIONS, GOBI-ALTAI TERRANE, SHINE JINST, MONGOLIA

Mongolia’s geologic history as an accretionary zone of allochthonous terranes offers an excellent opportunity to study the origin of central Asia. This study investigates the Late Silurian to Early Devonian paleoecology and depositional environments of the Amansair and Tsagaanbulag Formations at two locations near Shine Jinst in the Gobi-Altai terrane, Mongolia. Field observations of the Amansair Formation describe 33 meters of mostly fossiliferous wackestones and packstones overlain by 10 meters of green shale at the top of the section, where there is a fault contact with Devonian conglomerate. The two sections examined in the Tsagaanbulag Formation cover 40 and 76 meters, respectively, and exhibit two distinct facies. The lower section is fossiliferous wackestones and packstones, and the upper section comprises fossiliferous wackestones-packstones interbedded with brown-weathered limestones and shales. Coarse-grained sand is present in the limestone near the top of the upper section just below the contact with the Devonian conglomerate. Preliminary analysis suggests these rocks formed in a shallow marine setting in a tropical environment. The Amansair and upper Tsagaanbulag formations have been dated previously as Lockhovian in age, but the age of the lower Tsagaanbulag Formation remains uncertain. Carbon isotope analysis will be performed to see if the well known Klonk positive isotope excursion, which marks the Silurian-Devonian boundary in marine limestones worldwide, is evident. Preliminary data show that d13C values over the first 20 meters are rather variable, fluctuating between +1.3‰ to +2.8‰, but then steadily increase from +2.4‰ to +3.3‰ over the next 4 meters. The excursion may be present here, but more data are required to interpret the isotope profile. This detailed study of the Gobi-Altai terrane contributes to an understanding of the paleogeographic location of central Asia and is a component of the Keck Geology Consortium Mongolia Project, a Mongolia-US collaboration to create a comprehensive picture of the paleobiogeographic setting of the Gobi-Altai terrane from the Ordovician to the Devonian-Carboniferous boundary.