Quartz and clay minerals dominate the bulk sediment in the lower siliciclastic Unit III of Cretaceous to late Eocene age, as well as in the transitional Unit II of latest Eocene to earliest Oligocene age. Increased contents of opal-CT in Unit III coincide with a downhole decrease in siliceous microfossils and porosity, whereas decreased contents of opal-CT farther downhole coincide with an increase in quartz and a further decrease in porosity. Dominant quartz and clay minerals in Unit III are superseded by dominant calcite from the uppermost Unit II to the lowermost biogenic Unit I in the early Oligocene. One exception is Site 1168 on the WTM, where quartz and clay minerals decrease more slowly until the middle Miocene. Quartz, clays, and associated minerals increase significantly in the late Pliocene at northern Sites 1168 and 1172.
Clay mineral assemblages are dominated by smectite for most of the Cenozoic. Chlorite, illite, and/or kaolinite are locally important or dominant from the late Paleocene to the early Eocene and from the late middle Eocene to the earliest Oligocene. These changes coincide with intervals of active tectonism (strike-slip activity and basin formation) in the Tasmanian area. Further increases in chlorite, illite, and/or kaolinite occur in the middle and in the late Miocene, during intervals of climate change and glacial development in Antarctica. The clay assemblages change near the early/late Pliocene transition to similar assemblages at all sites, corresponding to increasing aridity in southern Australia, and a concomitant increase in wind-blown clays.