Sahul Shoals Processes : Neotectonics and Cainozoic Environments : Cruise 122, Post Cruise Report : Project 121.41

The ship departed Berth 1 at North Quay, Fremantle at 1510 on Saturday 9 October, 1993.The safety meeting and cruise briefing was held for all AGSO and Sydney University personnel soon after the ship left the wharf. The ship then proceeded for the next six and a half days to make its way north to the survey area. The ship arrived at the survey area just after midnight of Friday, 15 October. The first current meter station was reached at 0110 hrs on Saturday 16 October. Prior to the current meter being deployed, a CTD cast was made in 195 m water to measure temperature, salinity, pH,dissolved oxygen, and light transmission. The first current meter was deployed in 200 m,followed by two others on the top of one of the banks, in water depths of 30-40 m. After the three current meters were deployed, the ship proceeded to the first seismic waypoint nearby. The 1200 m cable was deployed and balanced by late morning of Saturday 15 Octoberand shooting commenced at 1220 hrs. A total of 830 km of multichannel seismic data was recorded over 15 lines covering the area on and between the Sahul Banks and the Cartier Trough. This was the first survey that the GI gun array has been used to any extent, and the array behaved superbly, with very little down-time resulting from malfunctions in the gunarray itself. Because most of the banks have a flat top around 70 ms (TWT), with the first multiple at 140 ms, it is difficult to interpret the seismic events beneath the first multiple. However, in somecrossings it was possible to interpret lower horizons that show relief beneath the bank. This relief, while significant, is not as great as that of the present bank. However, it does appear that these carbonate platforms have been in existence since at least the Late Miocene. Most of the lines (13 out of 15) cross the inter-bank areas. Here, it is possible to tie certain horizons to wells such as Sahul Shoals 1 and Pokolbin 1. Again, because of multiples, it was only possible to tie the Top Miocene Unconformity with any confidence, and to tie from one side of a bank to the other is difficult. However, it does appear that the latest Miocene was atime of subaerial exposure, producing a regional erosional unconformity. This was presumablya result of a substantial fall in sea level at this time (the Messinian event?). The Cartier Trough has subsided substantially since the Late Miocene, and it contains a verythick Miocene to Recent sequence. Some structures within the trough appear to have resultedfrom a combination of tectonic activity and sea level change. They consist of a series of backstepping onlap features, that are structurally high. Similar structures deeper in the troughcould have some petroleum potential, if sealed adequately. What has been interpreted as theTop Miocene Unconformity can be traced with some certainty across the trough. The multichannel seismic leg of the cruise was completed at midnight on Thursday 21October.