UPLIFT AND EROSION ON THE ASHMORE PLATFORM, NORTH WEST SHELF: CONFLICTING EVIDENCE FROM MATURATION INDICATORS.

Abstract

The Ashmore Platform is situated to the north of the Browse Basin, on the North West Shelf, off the coast of Western Australia. Apatite fission track analysis (AFTA™), vitrinite reflectance and fluo­rescence alteration of multiple macerals (FAMM) measurements were undertaken on drill cuttings material recovered from the Late Triassic sequence of the oil exploration well, Ashmore Reef-1.

Vitrinite reflectance measurements indicate that the Late Triassic sequence is currently experienc­ing maximum temperature. However, reflectance methods were suspected of being unreliable due to suppression of the reflectance, a common problem when dealing with marine influenced sediments. The FAMM technique was used to provide an alter­native maturity estimate using the same speci­mens. The FAMM results suggested that vitrinite reflectance is suppressed and that the true matu­rity is higher than conventional reflectance mea­surements predict.

The results also suggest that some of the cuttings material from the sampled level is contaminated by material from higher in the Late Triassic sequence. Both the AFTA™ and FAMM data show bi-modal populations from some depths. It was possible to distinguish between the two populations and esti­mate the maturity of the caved material. FAMM and AFTA™ results together imply that maximum palaeotemperature was reached in the Mid-Creta­ceous, corresponding to a major unconformity in the well.

The FAMM results do not agree with published maturity estimates based on conodont alteration indices (CAI), which suggest that temperatures have only recently and rapidly reached current levels. The AFTA™ results can also be interpreted to support this model. Furthermore, sonic velocity data in Miocene limestone suggests post-Miocene erosion, which would be expected to be associated with a temperature drop.