Carboniferous and Permian strata of the Tepuel-Genoa Basin, Patagonia, Argentina: a near-continuous, deep-water record of Polar Gondwana during the Late Paleozoic Ice Age

ISBELL, J.; GULBRANSON, E.L.; TABOADA, A. C.; PAGANI, M.A.; LIMARINO, C.O.; FRAISER, M.L.; PAULS, K.N.; HENRY, L.C.

Albuquerque

Congreso; Carbonifersous-Permian Transition Conference; 2013

The Pampa de Tepuel and Mojón de Hierro formations in the Tepuel-Genoa Basin exposed in the Sierra Tepuel between Tecka and Gobenador Costa, Argentina constitutes a ~4000 m thick, near-continuous Mississippian (late Viséan) to Permian (Sakmarian-early Artinskian) succession that contains at leasts 6 glaciomarine and glacially-influenced marine horizons separated by non-glacial intervals. During much of the late Paleozoic, Patagonia was located near or within the South Polar circle. Thus, these strata represent the thickest and most complete record of polar conditions for the late Paleozoic ice age (LPIA).The Tepuel-Genoa Basin was interpreted by López-Gamundí and Breitkreuz (1997) as a foreland basin and by Forsythe (1982) and Ramos (2008) as either a forearc or a peripheral foreland basin. Regardless of tectonic scenario, rapid basin subsidence was required to produce such a thick Carboniferous to Permian succession.Dropstone- and fossil-bearing strata of the Pampa de Tepuel and lower portion of the Mojón de Hierro formations were previously interpreted by González-Bonorino (1992) and González and Díaz Saravia (2010) as intercalated glacial and marine strata deposited in subglacial to littoral/near-shore marine environments. The occurrence of striated and grooved surfaces was used as evidence for either terrestrial subglacial conditions or as evidence of grounded ice advance into a shallow glacimarine setting. Conversely, López-Gamundí and Limarino (1984) interpreted strata in the Pampa de Tepuel Formation as submarine debris flow, sandy turbidity current, and hemipelagic deposits.In our ongoing investigation, we identify facies consisting of wave rippled and hummocky cross-stratified sandstones; thick fossil-bearing mudrocks; lonestone-bearing mudrocks; discontinuous blocks of sandstone overlying shear planes; folded and brecciated sandstones overlying shear planes; thin-bedded sandstones; massive sandstones; massive, stratified, and thin-bedded diamictites; graded, reverse-graded, and massive clast-supported conglomerates; and deformed conglomerates resting on shear planes. These units are here interpreted as wave-influenced shelf, neritic, hemipelagic, iceberg rafted, glaciomarine meltwater plume, mass movement (slide blocks and slumps), turbidite/bottom current, sandy debris flow, muddy debris flow, and submarine channel deposits. Analysis of grooved and striated surfaces within the succession suggest formation as slide and slump block glide planes and iceberg keel marks. Clinoforms occur at the Tepuel Hill Section, which suggest that strata at this site were deposited within the basin near the shelf-slope break.  There, wave rippled and hummocky cross-stratified sandstone and associated mudrocks within topset beds and at the top of clinoforms indicate deposition on the outer shelf and upper slope. Thick successions of hemipelagic mudrocks , horizons of slump/slide blocks, dropstones, diamictites and channelized and sheet turbidites within clinoforms suggest deep-water sedimentation on the slope and possibly on the basin floor. The arrangement of the deposits suggest that during relative sea-level low stands, clastic systems, including glaciers, transited the shelf and shed clastics into the deep basin. Fossil-bearing horizons occur above “low-stand” deposits and may represent condensed zones that developed during “transgression/high-stand” and/or retreat of glaciers and clastics across the shelf. Plant fossils and deltaic deposits located in the upper Mojón de Hierro and in the Río Genoa formations indicates shallowing of the basin and disappearance of glaciers by the late Early Permian (Artinskian).