Steven L. Dorobek1
(1) Texas A&M University, College Station, TX
Abstract: Cenozoic Carbonate Platforms Across the Southern South China Sea: Relationships Between Platform Growth Histories, Regional Tectonics, and Lithospheric Structure
Numerous carbonate platforms of Cenozoic age (mostly Oligo-Miocene) are found across the South China Sea on structural topography that was largely created during Paleogene extension of continental lithosphere. Most platforms were initiated during late syn-rift to early post-rift stages (late Oligocene to mid-Miocene) when rapid thermal subsidence, coupled with long-term eustatic rise, flooded the fault-bounded basement highs. Some platforms, however, formed on flower structures and inversion-related folds that reflect post-rift compressional and strike-slip deformation. Platforms stayed fully aggraded throughout their growth, so they can be used to track the isostatic position of basement over time. This in turn, provides insight into the post-rift structure and rheological properties of underlying continental lithosphere.
On a more local scale, some platforms contain characteristic growth strata that reflect rotation of hanging walls above listric faults. Geometric analyses of growth strata allow estimation of fault geometries and amount of hanging wall rotation. Coalescence of smaller platforms into larger isolated platforms occurred where fault spacing, post-rift structural relief, platform surface area, and off-platform sediment transport were optimal for filling of inter- and intra-platform troughs.
Seismic stratigraphic relationships indicate most platform tops were at 250-400 m water depth before they were buried by progradational siliciclastics, which suggests that proximity to siliciclastic point-sources had little effect on terminating most platforms. Relatively rapid early post-rift subsidence, coupled with long- and short-term eustatic rises during mid-Miocene to Quaternary time, was more likely responsible for platform drowning. Major eustatic lowstands in the mid-Miocene and latest Miocene/early Pliocene exposed some platforms and caused intense karstification locally. The fully aggraded, flat-topped nature of many platforms was also an important prerequisite for rapid drowning in that the ‘carbonate factory’ was unable to shift to an updip position during sea-level rises.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana