Publications

Recent seismic and multibeam data, combined with information from land, provide the basis for a new interpretation of western Gorontalo Bay. The stratigraphy is divided into three parts. The basement (Unit A) is proposed to be Sundaland continental crust, below a major unconformity interpreted to be either Mid Eocene or Early Miocene in age. Above the unconformity is a sequence up to 6 sec TWT divided into two parts. The lower part (Units B and C) is interpreted as quartz-rich marine sediments, with little volcanic debris, derived from granites and continental basement of western Sulawesi. There is a minor unconformity at the base of the upper part (Units Dto F). We suggest Units D and E are carbonates. Unit D has major clinoforms indicating water depths less than 200m. At the top of Unit E are linear bands of pinnacle reefs which step back towards the north. They mark rapid subsidence which began at about 5 Ma and are partly buried by Pliocene–Recent deposits (Unit F) in the basin centre. This may include quartzose siliciclastic material, carbonate sediment and volcanic ash. Given the observed subsidence the absence of significant faulting in the basin is noteworthy. We suggest the cause of extension is lithosphere-scale low angle normal faulting driven by subduction roll-back at the North Sulawesi trench. This is consistent with the existence of young rapidly exhumed core complexes and low angle detachments on land. Hydrocarbons are leaking from the south side of the bay. Important considerations for a petroleum system are: a basement of predominantly highly thinned Australian continental crust accreted to the Sundaland margin in the mid Cretaceous, a quartzose lower sequence with possible sources from coals at the base or organic matter in sands, an upper sequence of shallow marine carbonates withlittle volcanic material, and a bay that has subsidedsignificantly only since about 5 Ma.