"BASEMENT" SHELF PHYSIOGRAPHY - ITS ROLE IN SHAPING STRATIGRAPHIC ARCHITECTURE: AN EXAMPLE FROM THE SOUTH CHINA SEA

Nearly 1,900 km of high resolution seismic reflection data and more than 800 km of chirp and side-scan sonar data were acquired on the northeastern margin of the South China Sea in an investigation of the impact of variation of sediment supply, physiography and base level (both tectonics and eustasy) on the architecture of depositional sequences. Physiography (controlled by earlier carbonate deposition) of the margin alters the drainage of overlying siliciclastic systems. In the middle shelf, hard carbonate rock creates a topographic high (20 km wide and almost 60 km long) that trends parallel to the shelf edge and there is a slightly less continuous high area near the shelf-edge. As sea level fell, down-dip transport of siliciclastic sediment across the shelf was blocked by the middle shelf high. This caused the channelized Falling Stage siliciclastics to flow toward the northwest, nearly perpendicular to depositional dip, until they encountered an area where the shelf edge trends updip, intercepting the mid-shelf depression, creating a conduit for transport of the siliciclastic sediment over the shelf edge. The flow of the siliciclastic fluvial systems expands at the shelf edge creating an outer-shelf delta. In the southwestern portion of the study area, the mid-shelf carbonate topographic high is absent and the Falling Stage fluvial systems moved directly down-dip across the shelf. So where the carbonate “reefs” are absent, Falling Stage systems move directly down dip, and in areas where they are present, the systems move parallel to strike. Shelf facies in areas around the “highs” consist of thick chaotic to channelized facies and areas above the “highs” consist of thin flat-lying facies. In mixed carbonate/siliciclastic systems, the topography of underlying carbonates may have a tremendous impact on the routing of siliciclastics across the shelf and therefore the locus of slope sedimentation.