C/S geochemistry of Beacon Supergroup rocks, from southern Victoria Land and the Ohio Range, Transantarctic Mountains, Antarctica

Abstract

Carbon/sulphur (C/S) ratios are used in this study to indicate the marine or nonmarine nature of Beacon strata from southern Victoria Land and from the Ohio Range. Beacon Supergroup strata in the Transantarctic Mountains comprise the Taylor Group, a sequence of dominantly quartzose sandstone beds of Devonian age, and the Victoria Group, a heterogeneous sequence of glacial beds, sandstone, shale, conglomerate and coal from Late Carboniferous to Early Jurassic age. The Beacon is largely considered alluvial in origin, but lower Taylor Group strata in southern Victoria Land have been interpreted as both marine and nonmarine, and those in the Ohio Range are clearly marine from the presence of marine body fossils. C/S ratios from fresh water to brackish conditions from Lake Wairarapa and Lake Onoke were examined to add more detail to the transition from fresh water to brackish water and brackish water to normal marine. The C/S data from this study supports previous work by Berner et al. and Berner and Raiswell (1983) which showed that normal marine C/S ratios were generally less than 4, brackish from about 4 to 11 and fresh water generally greater than 11. Organic carbon lost due to burial maturation and thermal alteration must be taken into account before interpretations based on C/S ratios of Beacon strata can be made. Beacon strata from southern Victoria Land have lost a minimum of 68 % of the original organic carbon that was present at the time of early diagenesis and the onset of thermal maturation. Vitrinite reflectance data from Taylor Group rocks in the Ohio Range indicate as much as 80 % of the organic carbon has been lost primarily due to burial diagenesis in the lower part of the sequence and from thermal alteration due to dolerite in the upper part. C/S analyses from southern Victoria Land support previous interpretations for an entirely nonmarine setting for both the Taylor Group and Victoria Group rocks. These findings are particularly important for the Taylor Group, over which there has been some debate. C/S results from the Terra Cotta Siltstone and the Junction Sandstone (Altar Mountain Sandstone correlative) support a fresh water interpretation for these strata. C/S data from the Ohio Range indicates fresh water to brackish conditions for carbonaceous shales from the Devonian Horlick Formation. The presence of psilophyte plant remains in the shales had earlier been interpreted as indicating fresh water to brackish swamps or lagoons (Long, 1965; Doumani et al., 1965), though more recent interpretations favour an entirely marine setting except for isolated fluvial sandstone deposits (McCartan and Bradshaw, 1987). C/S ratios from the Permian strata in the Ohio Range indicate largely nonmarine deposition. Though some data from the middle and lower part of the Mount Glossopteris Formation provide evidence to support deposition in brackish to freshwater conditions. This supports the interpretation of Bradshaw et al. (1984) for a marginal marine setting for the middle of the formation, based on the presence of the trace fossil Monocraterion and "wave ripples". C/S data from the upper part of the formation indicates fresh water deposition which is consistent with previous interpretations based on sedimentary features, fossil wood remains and a fresh water fossil fish fauna.