AUTHIGENIC SMECTITE CLAY COATS AND OTHER AUTHIGENIC MINERALS IN CAPE ROBERTS DRILL CORE 3, VICTORIA LAND BASIN, ANTARCTICA

Of some 800 m of lower Oligocene marine sediments cored continuously from the seafloor in 295 m of water at Cape Roberts Site CRP-3, the lower 500 m exhibits authigenic smectite clay coats on shallow-water sandstone grains. SEM/microprobe study of fracture sections confirms that the distribution of the clay coats through the unit is not uniform or evenly distributed, but rather varies with depth, original porosity, and the kinds and abundance of source materials.

The source material deposited along with quartz sand in a glacially influenced environment is believed to be fine volcanic detritus as represented by dolerite clasts, pyroxene grains, and partially or completely skeletonized detrital-feldspar grains. In some intervals the clay coats are poorly developed (only a basal root system tangential to the grain surface is present) and authigenic zeolites may be common instead. In such cases, the abundance and type of source material seem to be the most important determining factors. Where fully developed, the clay coats are over 10 microns thick and exhibit the microporus, polygonal box-work pattern characteristic of authigenic smectite.

Calcite forms a late-stage cement in much of the section, and appears in the SEM as massive and blocky where well developed, and as small, often isolated euhedral crystals where poorly developed. The source of the large amounts of calcite required is unknown.

Smectite authigenesis, like that of opal-CT and some zeolites, occurs at temperatures and pressures above near-surface conditions but not greater than about 80 degrees C. Based on the present-day geothermal gradient, regional stratigraphy and rifting history, we estimate that the section in question achieved a burial depth of at least 800 m by the early Oligocene and that diagenesis was able to proceed until glacial erosion unroofed the sequence during the Neogene.