Paper No. 4
Presentation Time: 9:00 AM
THE BASHKIRIAN GLACIATION OF THE SAN RAFAEL BASIN AND OTHER DIACHRONOUS GLACIAL EVENTS ACROSS WESTERN ARGENTINA
The El Imperial Formation of the San Rafael Basin records a succession of depositional environments during the latest Mississippian to earliest Permian that span before, during, and after the glaciation of west central Argentina. At the base of the formation, a restricted marine environment is recorded in mudstone containing marl and rippled and deformed sandstone beds. This unit, or sequence 1, is incised by a deltaic facies association composed of cross-bedded sandstone and conglomerate that form at least 5 stacked Gilbert deltas. The deltaic facies association grades upward into the glacially-influenced facies association, made up of stratified diamictite, mudstone with dropstones, and massive deformed sandstone, indicating deposition by wet-based tidewater glaciers that calved icebergs into the basin, with contributions from mass movement processes. The glacially-influenced facies association is overlain by mudstone and horizontally laminated and cross-bedded sandstone of the post-glacial open marine facies association, recording post-glacial transgression followed by relative sea level fall. The deltaic, glacially-influenced, and post-glacial open marine facies associations comprise sequence 2. Sequence 2 is incised by fluvial conglomerates of the upper (third) sequence of the formation.
Based on palynomorph dating, glaciation in the San Rafael Basin occurred during the mid- to late Bashkirian, after the end of glaciation in the Protoprecordilleran Basins to the north, and extending later into the Bashkirian than the corresponding glacial interval in the Tepuel Basin to the south. Glaciation did not return to west central Argentina after the Bashkirian, however, glacial intervals continued in the Tepuel Basin into the Sakmarian-Artinskian, when LPIA glaciation was at its global maximum. Continued glaciation in the Tepuel Basin may have been the result of its higher latitudinal position during the Pennsylvanian–Early Permian and higher altitude due to either tectonic convergence of the Patagonian microplate or convergence along the Panthalassan margin of southwestern Gondwana.