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Diagenesis in Polar Shelf Sediments: Insights From the Cenozoic Succession of McMurdo Sound, Antarctica

Abstract

As hydrocarbon exploration expands into the polar latitudes, understanding the processes and products of diagenesis in glaciomarine settings becomes increasingly critical. Although protected by the Antarctic Treaty, the Cenozoic succession beneath McMurdo Sound, Antarctica provides a useful analog for exploration targets elsewhere. This work investigates the origin and distribution of diagenetic phases in Lower Miocene-Quaternary strata recovered in a 1138.54-m-long core, which was drilled by the international Antarctic Drilling Project (ANDRILL). The stratigraphic succession comprises a range of lithologies, including diamictite, mudrock and sandstone with minor conglomerate and diatomite. Lithofacies are arranged in cyclic patterns that record repeated advance and retreat of ice sheets. Drilling activities included analysis of pore water, providing a unique opportunity to examine relationships between interstitial fluids and diagenetic phases. Pore water data show that the upper c. 200 m of the succession contains connate seawater and that strata below that depth are saturated with hypersaline brine. Brine geochemistry indicates that it formed as a byproduct of seawater freezing along ice sheet margins during glacial advance-retreat cycles. Thin section analysis of the core reveals a complex diagenetic history. Whereas secondary carbonate is present throughout the succession, its abundance, distribution, and morphology is highly variable. Some intervals of clean sand up to 25 m thick maintain porosities as high as 41% and show little evidence of diagenetic modification. These sands formed in coastal to nearshore marine (deltaic) environments during ice minima, when substantial volumes of meltwater were released from glacier termini. At the other end of the spectrum, some intervals are densely cemented with various forms of carbonate cement, ranging from microgranular to blocky, to spheroidal, and radiaxial bladed rims on grains. In addition, microcrystalline calcite appears to be replacing the fine-grained matrix of mudrocks and diamictites. Stable isotopic analysis of the carbonate phases indicates that most precipitated from the hypersaline brine. These observations suggest that reservoir character in this succession is governed by a range of factors that include not only changing facies distributions brought about by glacial advance-retreat cycles, but also the propensity of the glaciomarine system to produce dense, carbonate-saturated brine.