Date of Award
Master of Science
John L Isbell
Lindsay J McHenry, Mark T Harris
Chemical Index of Alteration, Deep-water deposits, Late Paleozoic Ice Age, Mojon de Hierro Formation, Paleo-oxygenation, Permian
The Earth has had multiple Phanerozoic glacial intervals but the Late Paleozoic Ice Age (LPIA) was its longest and most extensive, lasting from the Late Devonian (~372 Ma) until the Late Permian (~254 Ma). The LPIA is the last complete climate shift from a greenhouse to icehouse and back to a greenhouse state and the only one to occur on a biologically complex Earth. Therefore, it provides perspectives on deep-time climatic transitions, the parameters controlling them, and the Earth’s physical, chemical and biological responses to such climate changes. Research on mid to high-latitude deposits in Gondwana provides evidence that the LPIA had a highly dynamic climate, with multiple ice sheets and ice caps, that fluctuated asynchronously and diachronously across the supercontinent as it drifted across the paleo South Pole. Numerous questions remain on the spatial and temporal extent of the ice centers, the timing of expansion and contraction of the ice sheets and the distribution of ice across Gondwana through time. The Tepuel-Genoa Basin, in Patagonia, Argentina, was situated within the paleo South Polar Circle as part of Gondwana and contains a nearly continuous sedimentary succession of Carboniferous to Lower Permian strata. The Mojón de Hierro Formation has been described as both non-glacial and glacial in origin. This study investigates a unit within the Mojón de Hierro Formation which was used in a detrital zircon analysis which implies a large ice sheet covered the basin at the time of deposition, which extended from an ice center in the Ellsworth Mountains all the way to the Panthalassan Margin in west-central Gondwana. These strata contain mudrocks, some with outsized clasts, diamictites and sandstones. Five stratigraphic sections were measured at Arroyo Garrido and the strata were categorized into five lithofacies associations; 1) laminated mudrock, 2) laminated mudrock with dispersed clasts, 3) graded rhythmites, 4) laminated and bedded diamictites, and 5) deformed bedded sandstones. The strata were deposited in a basinal slope environment from suspension settling, ice rafted debris, sediment gravity flows and mass transport complexes. The mass transport complexes were deposited on a lower slope and formed deep-water topography that resulted in a ponded mini basin behind the sand blocks. Paleo-flow is indicated as flowing to the northwest. The Chemical Index of Alteration (CIA) indicates average marine shale values and that muds are of temperate terrestrial sediment sources. A glacial source could not be substantiated using CIA values. Vanadium/chromium ratios indicate dysoxic values in samples between the mass transport sand blocks and oxic values in samples above the filled ponded mini basin. The detailed facies analysis and geochemistry data do not indicate the Arroyo Garrido strata to be of glacial origin. However, facies analysis data suggest that icebergs from a distant source transited the basin during emplacement of the mass transport and sediment gravity flow deposits.
McNall, Natalie Beatrice, "Sedimentological and Geochemical Analysis of Deep-Water Deposits in the Mojón De Hierro Formation at Arroyo Garrido, Tepuel-Genoa Basin, Patagonia, Argentina" (2019). Theses and Dissertations. 2226.