THERMAL MODELLING IN THE EASTERN OTWAY BASIN

Abstract

Vitrinite reflectance data across the Otway Ranges yield a thermal maturity map that indicates the presence of a broad NE-SW trending anticline with strong vergence to the southeast. Surface Rv max values for the central part of the ranges are >1.5 whilst those on both limbs decrease to <0.5. Vitrinite reflectance profiles of the Olangolah-1 well in the central Ranges indicate ~2.5 km of erosion and, when combined with apatite fission track analysis data, indicate elevated Cretaceous heat flows. Vitrinite reflectance data from wells along the coast, such as Anglesea-1 and Krambruck- 13 show minor uplift, whilst those from wells in the flanking embayments show no significant uplift.

Apatite fission track analysis of seven Eumeralla (Lower Cretaceous) samples from the coast around Wye River yields an AFTA age of 90±5 Ma, consistent with similar cooling ages measured around most of the SE Australia margin. Modelling of the data is consistent with a stratigraphic age of -100 Ma, rapid heating to −80°C for −5 Ma followed by cooling from 95–80 Ma and further cooling in the Miocene.

Geologically the very large asymmetric anticline can be explained by inversion of a thick rift sequence along a major, listric northwest-dipping fault, perhaps soling at mid-crustal levels. This is consistent with structures observed offshore in the Torquay Embayment and is being tested by the AGSO deep seismic profile BMR 920T1. Observed gravity highs in the Otway Ranges may be associated with inverted high density sediments. Fission track analysis indicates that the major cooling was at −90 Ma, which is therefore likely to be the time of inversion. However, this implies compression during continental breakup, the mechanisms of which are not fully understood.