Discussion and Conclusions-Tectonic Evolution | Table of Contents

Regional Sedimentation
The sedimentary succession in the Tasmanian region generally records three major phases of sedimentary deposition:

1. Paleocene to early upper Eocene rapid deposition of shallow-water siliciclastic sediments during rifting between Antarctica and Southeast Australia, a time of minimal or no connection between the southern Indian and Pacific Oceans.
   
2. A transitional interval of slow sedimentation, with shallow-water upper Eocene glauconitic siliciclastic sediments giving way suddenly to lowermost Oligocene pelagic carbonates (often clayey initially). The transition was caused by the activation of bottom currents during the late Eocene as the Tasmanian Gateway opened and deepened during early drifting. Cooling reduced precipitation and the flow of siliciclastic sediment, and currents swept the shelves. Sedimentation no longer kept up with subsidence, and pelagic carbonate deposition took over in the early Oligocene.
   
3. Oligocene through Quaternary deposition of pelagic carbonates in increasingly deep waters and in increasingly open-ocean conditions as the Southern Ocean developed and expanded with the northward flight of the STR and the Australian continent. The sedimentary sequence seems to record an integrated history of interplay between decreasing continental influence, rifting and subsidence of the rise, Antarctic Circumpolar Current development and Antarctic cooling, and other related factors.

In general, sedimentation rates throughout the region changed dramatically in relation to these distinct phases of sedimentation. These were rapid (~10 cm/k.y.) during the siliciclastic sedimentation of the early rifting phase of the late Paleocene to early late Eocene; slow to condensed during the Eocene-Oligocene transition, when more glauconitic sediments were deposited, and generally slow during the biogenic sedimentation from the earliest Oligocene to the present day. There were periods of minimal sedimentation or erosion affecting the late Oligocene and late Miocene sequences.

Considerable variation in thickness is evident for different time periods among the sites. The two major sediment types, the pelagic carbonates of the Neogene and the shallow-water siliciclastic mudstones of the Paleogene and Late Cretaceous, are dealt with separately below. In regard to the pelagic carbonates, the Quaternary is thickest at the two southern sites (Sites 1170 and 1171).swas protected from scouring by currents from the west by its great depth and the ridge of the TFZ. The Miocene is affected by the onset of the strong Antarctic Circumpolar Current, with the thickest and most complete section in the protected northern Site 1168. The other northern site (Site 1172) has a fairly complete but thinner section. The lower Miocene is especially thin at the southern Sites 1169, 1170, and 1171, probably because of current scouring. The Oligocene, too, was strongly affected by the Antarctic Circumpolar Current and/or the East Australian Current, with the protected northern Site 1168 as the only one with a thick and reasonably complete sequence.

In regard to the siliciclastic mudstones, the upper Eocene is thin everywhere, apparently because fine-grained sediments were swept away by the newly forming Antarctic Circumpolar Current in the shallow-water depths at all sites. In contrast, the lower and middle Eocene are thick everywhere because siliciclastic supply was rapid and there was little current erosion. The same interpretation probably applies to the Paleocene at the southern Site 1171, whereas there is little Paleocene at the northeastern Site 1172. The uppermost Cretaceous mudstones penetrated at Site 1172 may be quite thick judging from seismic evidence only.