Evidence for pliocene erosion at Ashmore Reef (Timor Sea) from the sonic velocities of neogene limestone formations

Abstract

The sonic velocity of the Miocene Oliver Formation at the Ashmore Reef-1 well is anomalously fast with respect to other wells in the Timor Sea. In all wells except Ashmore Reef-1, velocities in the carbonate Oliver Formation follow a well-defined normal compaction trend with burial depth. Anomalously high velocity at Ashmore Reef is interpreted to be due to removal of section by erosion. The Pliocene-Recent Barracouta Formation, also a carbonate unit, unconformably overlies the Oliver Formation. The Barracouta Formation exhibits normal compaction (i.e. normal velocity/depth relation) at Ashmore Reef with respect to other wells in the Timor Sea. It is inferred that erosion of the Oliver Formation at Ashmore Reef reduced its burial depth during the time gap encompassed by the unconformity between the Oliver and Barracouta Formations. That erosion occurred during Miocene/Pliocene times, with a Pliocene date preferred on regional grounds. Sonic velocities suggest that the Oliver Formation at Ashmore Reef is approximately 1.0 km above its maximum burial depth. Since the Oliver Formation has been subject to 0.3 km of post-erosional burial, the magnitude of erosion during Pliocene times is estimated as 1.3 km. Pliocene erosion was synchronous with subsidence of the present-day Timor Trough and with uplift of the island of Timor. Uplift and erosion at Ashmore Reef is believed to be intimately linked with the collision between the Australian Continent and the Indonesian Banda Island Arc.