Geology of the Dangerous Grounds, South China Sea, and the Continental Margin off Southwest Palawan: Results of SONNE cruises SO-23 and SO-27

Multichannel reflection-seismic, magnetic, and gravity data were recorded along 51 profiles in the southeastern part of the South China Sea, predominantly in the Dangerous Grounds and in the Palawan Trough, during SONNE cruises SO-23 in 1982 and SO-27 in 1983. These investigations were combined with geological sampling. Five unconformities of regional extent were recognized in the study area: a Miocene-Pliocene unconformity, a Middle Miocene unconformity which coincides with the end of seafloor spreading in the South China Sea basin, a Lower Miocene unconformity which often marks the top of a carbonate platform in the South China Sea, a Middle to Upper Oligocene unconformity representing the transition from the rift to the drift phase in the South China Sea and a Cretaceous-Paleocene unconformity interpreted as the onset of rifting. The Dangerous Grounds and possibly also parts of the Palawan Trough are underlain by a stretched crust of continental origin. The oldest rocks sampled in the Dangerous Grounds are of Mesozoic age (Upper Triassic to Jurassic). Marine geophysical data indicate the existence of an Upper Oligocene to Lower Miocene carbonate platform that extends from the Dangerous Grounds eastward beneath the Palawan Trough and beneath the central and south Palawan shelf. Hence central and south Palawan are interpreted as being part of a unique microcontinent which incorporates the Dangerous Grounds/Reed Bank, north Palawan, and the Calamian block. The eastern edge of this carbonate platform is overlain by a wedge of chaotically deformed allochthonous sediments which have been overthrust from the south onto the carbonate platform. These results imply that the Palawan Trough does not represent the location of an ancient subduction zone but is an expression of the elastic downwarp of the crust due to isostatic compensation for the thick overthrust allochthonous wedge.