Authigenic smectite in Cape Roberts Project cores, Antarctica: implications for environmental, basin and tectonic history

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0126218WiseThis award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, provides funds for a study of clays (specifically smectite) in cores from the Ross Sea, Antarctica, toward an understanding of the age and origin. One of the most surprising discoveries of the recently concluded geological drilling of the Cape Roberts Project (CRP) in the eastern Ross Sea, Antarctica, was the presence of authigenic smectite clays in lower Oligocene (or older) sandstones in the lower portion of Hole CRP-3, which was diamond-cored continuously to 941 meters below sea floor with 98% recovery. This is the first confirmed report of authigenic smectite in sandstones from the Antarctic continental margin. Situated along the eastern margin of the Victoria Land Basin, adjacent to the rising Transantarctic Mountains, CRP-3 was drilled to elucidate the paleoclimatic and tectonic history of the region; however, the base of the Cenozoic section is poorly dated, with limited constraints from biostratigraphy and paleomagnetics. This project is designed to shed additional light on the spatial extent, source, timing, and mode of emplacement of the authigenic smectite, which were probably emplaced shortly after deposition of the host sediments. This will help constrain the age of the basal Cenozoic sediments and the environment in which the smectite formed. This in turn will lead to a better understanding of the tectonic processes at work during the early history of the basin.Because this serendipitous discovery was not anticipated prior to drilling, only a pilot study could be undertaken within the time and funding constraints dictated by the science plan for the project. Work to date allows the definition of three multiple working hypotheses to account for the source and mode of emplacement of the authigenic smectite: 1) Burial diagenesis with necessary components sourced from volcanogenic materials and heavy minerals within the Cenozoic sequence or from a dolerite intrusion in the Paleozoic sandstones below; 2) Precipitation from hydrothermal waters associated with possible igneous intrusion(s) and nearby faults; 3) Mobilization and injection of fluids along a nearby fault that bounds a major graben parallel to the Transantarctic Front. The data gathered form the preliminary study was insufficient, however, to arbitrate among these three possibilities.This award supports work to test, to the extent possible, these three hypotheses and to allow completion of the study through: 1) a thorough examination of the core by scanning electron microscope/energy-dispersive microprobe analysis to determine the extent of the smectite in CRP-3 and the nearby CRP-2 core (also predominantly of Oligocene age), and 2) radiometric dating of the authigenic clays and potential source rock(s) by the 40Ar/39Ar method. Completion of the study will help answer fundamental tectonic an paleoenvironmental questions posed by the CRP project, will help guide future exploration of the Antarctic margin (such as by the ANDRIL, SHALDRIL, and IODP projects), and will impact major Antarctic initiatives such as ANTEC, MARGINS, and STRATAFORM.