The age models for the early Oligocene-early Miocene interval for ODP Sites 1168 and 1170-1172 were independently tested on the basis of a number of paleoceanographic data sets, including stable isotopes and carbonate content. With the exception of Site 1172, our integrated approach proves largely consistent with the magnetostratigraphic outline (as of August 2002, we propose to follow magnetic reversal hypothesis 1 in the case of Site 1168). The development of these integrated age models forms the basis for a study of the hydrographic system around Tasmania. One of the questions addressed is the relationship between the sites and their respective location on the Pacific Ocean (Sites 1171-1172) or Indian Ocean (Sites 1168 and 1170) side of the South Tasman Rise.
Changes in sedimentation rates (Figs. F3, F11) suggest that Site 1168's location is rather different in character from Sites 1170-1172, due to its continental margin location. The Marshall Paraconformity in the early Oligocene is one of the most important hiatuses in the Southern Ocean region. The base of this hiatus is generally at ~33 Ma in deep-sea sections (Shipboard Scientific Party, 1999a, 1999b) and is at 32.4 Ma at the onland type location (Fulthorpe et al., 1996). Its duration at the type area is 3.4 m.y. (up to 29 Ma), but at Site 1122 it lasts until ~20 Ma (Shipboard Scientific Party, 1999a). Despite the significance of this hiatus for other sites, and here at Sites 1170-1172 from ~33 to 30 Ma, at Site 1168 it is apparently represented only by an interval of reduced sedimentation rate. Sedimentation rates return to high values throughout the Oligocene and Miocene.
Sites 1171 on the east South Tasman Rise and 1172 in the Tasman Sea are clearly most strongly affected by the Marshall Paraconformity. Even though the hiatus appears to end at ~30 Ma, sedimentation rates remain low for most of the Oligocene. At Site 1170 on the west South Tasman Rise, sedimentation in the early Oligocene is relatively high. However, this does change following the early/late Oligocene boundary, when we observe a decrease similar to but more pronounced than that at Site 1168. At Site 1168 we observe a change in the biogenic components of the sediment. The transition from opaque and sometimes branched spicules to clear, tiny, straight spicules with a canal might imply a change in water depth or ocean circulation. The Oligocene/Miocene boundary is marked by a strongly condensed section or small hiatus/core gap in Sites 1170-1172 from just before the boundary to ~22.8 Ma, but the overall decrease in sedimentation rate across this interval is greatest at Site 1168. The problem of the mismatch of the Mi-1 event (and/or the adjacent magnetic reversals) at Sites 1168 and 1170 requires closer investigation, as two key tenets of deep-sea stratigraphy appear to be in conflict.