IMPACT OF DYNAMIC GONDWANAN GLACIATION ON THE STRATIGRAPHIC ARCHITECTURE OF THE AMSDEN AND TENSLEEP FORMATIONS (PENNSYLVANIAN/NORTHERN WYOMING)

The dynamic character of the Late Paleozoic Ice Age is evident from near-field, glacial deposits, but its impact on climate and sea-level variations in low-latitudes is not well constrained. In this study, we analyze how the stratigraphy of a succession deposited in an arid paleotropical setting was affected by Gondwanan glaciation. To evaluate vertical and horizontal facies variations, five closely spaced stratigraphic sections of the early Bashkirian to Kasimovian Amsden and Tensleep Formations were measured from surface outcrops in the western Bighorn Mountains of northern Wyoming, USA. Sections were then compared with available wireline logs through the same strata in the central Big Horn Basin. The Amsden Formation is characterized by a basal sandstone member overlain by red siltstones containing occasional evaporites and pisolitic layers. The upper member consists of dolomite beds containing marine fossils. The Tensleep Formation contains a wider range of facies that abruptly varies, both vertically and horizontally. They consist of alternations between eolian sandstones, shallow marine dolomites and sabkha deposits. The shallow marine dolomites are interpreted as the deposits of a sea level highstand, while eolian accumulation and preservation happened during sea level lowstands. These fluctuations imply high-amplitude variations of relative sea-level. The amplitudes of these fluctuations appears to have been less pronounced during the deposition of the older Amsden Formation, explaining the limited range of vertical facies variations. Based on the presence of the fusulinid Wedekindellina, these high-amplitude sea-level fluctuations could be connected to a major phase of glaciation in the high-paleolatitudes, recorded as the C4 glacial event of eastern Australia (Latest Bashkirian-Middle Moscovian). Our preliminary results indicates that the Amsden and Tensleep Formations record a change from low to high-amplitude relative sea-level variations that could be related to changing glacial conditions in southern Gondwana. These results provide new insight into the paleotropical response to the waxing and waning of Gondwanan glaciers.