Dating mid-Oligocene current activity in the South Pacific - the Marshall Paraconformity from the Canterbury Basin, South Island, New Zealand

The Tengawai-1 (TNW-1) drillcore recovered Neogene and Paleogene sediments from the onshore part of the Canterbury Basin, South Island, New Zealand, at a position in line with the offshore transect drilled by IODP expedition 317. These sediments record the changing currents and sea level during the early development of the cryosphere as well as the development of the Australian-Pacific plate boundary through New Zealand. The Marshall Paraconformity is a widely recognised feature of the New Zealand Paleogene record, and has been recovered in both TNW-1 and in the farthest offshore hole of the IODP 317 transect (U1352). The TNW-1 record is typical of the onshore exposures of the Marshall Paraconformity showing Early Oligocene bathyal sediments unconformably overlain by Late Oligocene greensand, with an upper bathyal/shelfal fauna. Precise dating of the unconformity has been problematic, with most sections dated to within the resolution of biostratigraphic stages and some available Sr isotope dates. In TNW-1, the overlying greensand has a particularly strong magnetic signal, carried by fine grained magnetite. Samples from this interval consistently show reversed polarity, and are correlated with Chron C7r (25.183-25.496 Ma). This is the most precise age available for the shallow-water Marshall Paraconformity, and indicates a resumption of deposition at least 1 My later than that indicated by Sr dating at the type section of the unconformity, which lies ~30 km to the south of the drillsite. The age and lithology of sediments bounding the Marshall Paraconformity at U1352 are similar to TNW-1, although by inference from overlying sediments this site may have been at greater depth. Greensand was likely present but not recovered from directly above the unconformity at U1352 and the apparent age of the resumption of deposition is therefore later here. Both U1352 and TNW-1 show the sediment beneath the unconformity was deposited at or before ~30 Ma. There is evidence for basin-scale tectonic activity before and after the Marshall Paraconformity, but the unconformity's occurrence in a range of depositional environments supports a regional driving mechanism. The age of the Marshall Paraconformity has, in the past, been used as an indicator for the timing of onset of the Antarctic Circumpolar Current (ACC). Study of the shallow water exposures indicates that any regional event to which the Marshall Paraconformity can be attributed must have occurred between 30 - 27.3 Ma, must have begun to wane by 26.5-28.5 Ma, continued waning or fluctuating, until 25.5 Ma and have largely ceased by ∼ 25 Ma when there was widespread limestone deposition across the basin. Timing constraints of resumption of deposition in shallow water post-date the opening of the Tasman Gateway but pre-date estimates of the initiation of the full ACC. It is likely that the shallow-water Marshall Paraconformity was caused by regional surface currents, which were reorganized prior to the initiation of the ACC. The timing of deposition also coincides closely with the change between extension and transpression in New Zealand plate tectonics.