Age constraints on Oligocene sedimentation in the Torquay Basin, southeastern Australia

Abstract

The Otway, Gippsland and central coast basins (including the Torquay Basin, Sorrento Graben and Port Phillip Basin) preserve one of the most complete records of Mesozoic and Cenozoic sedimentation in southern Australia. However, robust age constraints on sedimentation are scarce. Strontium isotope analysis of calcitic bioclasts from the Jan Juc Marl of the Torquay Group, Torquay Basin, gives a range of possible ages between 24.2 and 27.9 Ma for the Jan Juc Marl, and an age of 24.24 Ma (+1.3 Ma, −1.2 Ma) for the base of the Point Addis Limestone. These data demonstrate that the Jan Juc Marl and the lower Point Addis Limestone are age-equivalent facies representing deep- and shallow-water sedimentation, respectively. 40Ar/39Ar whole-rock analysis of the underlying Angahook Formation basalt of the Demons Bluff Group gives an eruption age of 28.7 ± 0.2 Ma. The upper boundary of the Angahook Formation is a subaerial exposure surface and paleoshore platform over which younger marine sediment onlapped during subsequent marine transgression. These new age constraints provide an improved age datum, of around 28.7 Ma, for the base of the Janjukian Stage. This age is very close to the European Rupelian–Chattian boundary, which is the accepted boundary between the Early and Late Oligocene with an estimated age of 28.4 Ma.