Sequence boundary T4, South China Sea

Marginal basins of the western South China Sea were formed by rifting in the Eocene and filled during the Early and Middle Tertiary. Seismic profiling, drilling, and mapping for over a decade has revealed numerous structural and stratigraphic sequences, many bearing gas or oil. Chen et al. (1993) published a sequence stratigraphy for the Qingdongnan Basin, proposing correlations to the global onlap cycles of Haq et al. (1987). Although more than 30 wells have been completed, chronostratigraphy has been largely proprietary. Sequence boundary T4 is a regionally extensive seismic reflector. Major regressive sedimentary facies, shelf margin incision, and basin fan formation indicate that it is a type I sequence boundary. Study of calcareous nannofossils and planktonic foraminifera in wells penetrating T4 away from clastic deposits date the event as early Late Miocene (N14/NN8). In most sections, however, the magnitude of the hiatus varies considerably: erosion below the unconformity may expose units as old as the Late Oligocene (P21/NP24) and non-deposition following the unconformity may continue up to the Early Pliocene (N18/NN12). Microfossil paleobathymetry indicates that basinal sedimentary depositi on following T4 occurred at bathyal or deeper water depths. Because the T4 sequence boundary and subsequent deep-water sediments can be consistentlymore » recognized throughout the South China Sea-Phillippines-Indonesia region, T4 can be regarded as a major hydrocarbon seal.« less