Oceanographic Controls on Sedimentology and Geomorphology of a Modern Shallow Marine Carbonate Platform: Pulau Layang-Layang, South China Sea

Abstract

Isolated carbonate platforms (ICP) contain numerous productive hydrocarbon reservoirs. Although the general stratigraphy and depositional facies of ICPs are well known, details of the controls, patterns, and variability of sediment accumulations are less well constrained. This project characterizes and numerically models a modern atoll by integrating satellite, field, petrographic, and granulometric observations with oceanographic data and hydrodynamic modeling. Better understanding of oceanographic and sedimentary processes controlling ICP can improve conceptual models of variability in analogous reservoirs.

In the southern South China Sea offshore from Borneo, Pulau Layang-Layang is an east-west elongated (7 x 2 km) atoll. Roughly 180 sediment samples, field observations, satellite and photographic imagery capture the diverse range of geomorphology and sedimentology. A suite of sensors measured the in situ physical and chemical oceanographic parameters. Numerical hydrodynamic modeling calculates wave, tidal, and current forces and estimates sediment transport.

Remote sensing observations document the geomorphology of the annular reef, reef sand apron, and lagoon. Field observations reveal a diverse, healthy and productive coral framework with encrusting red algae form the bulk of the reef margin. Many corals recovered from a 2008 crown-of-thorn outbreak and continue to expand in shallow water of the reef margin. Immediately inboard, the reef sand apron varies from 50 to 1,500 m wide and encloses a deeper (up to 18 m) 4 x 2 km lagoon. Coralgal and molluskan skeletal fragments compose most of the well sorted coarse sand and gravel reef sand apron sediment, transitioning to finer, poorly sorted sediment in the lagoon with abundant coralgal, Halimeda, and foraminifera grains. Close to the equator, the region is largely clear of tropical cyclones and has reversing monsoon winds from northeast (winter) and southwest (summer). Waves average 1.3 m (maximum 6 m) with directions aligned to the monsoons. Diurnal tides range 2 m and the reef grows to sea level, exposed only during the lowest of tides. Measured lagoonal currents average 3 cm/s, with a maximum 15 cm/s. Wider reefs and sand aprons to the northeast and to the west southwest face the direction the larger monsoon-generated waves. Continued sedimentologic and hydrodynamic modeling analysis provides qualitative and quantitative insights on the controls and heterogeneity of ICP sediment accumulation.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018