3D SEISMIC CHARACTERIZATION OF DOWNSTEPPING PLEISTOCENE RST (REGRESSIVE SYSTEM TRACT) FLUVIAL DEPOSITION ON A LOW GRADIENT COASTAL FLUVIAL PLAIN, SUNDA SHELF, SE ASIA

Though the FSST (Falling Stage Systems Tract) is well documented for prograding subaqueous sediment wedges in the ancient record as a consequence of falling relative sea level, its depositional equivalent on the shelf, fluvial terraces, are less well recognized within a sequence framework. We propose fluvial terraces on stable tectonic coastal planes to represent the product of a falling base level which can be inserted into the sequence cycle as the geomorphic representation of the RST (Regressive System Tract).

Some 4000 km² high-resolution offshore 3D seismic data from the Gulf of Thailand calibrated to radiocarbon sea levels reveal the high-frequency sequence stratigraphy of the Pleistocene Sunda Shelf coastal plain: RST, LST, TST, and HST deposits. With respect to the RST, 3D geobodies constructed from successive 2ms seismic stratal slices reveal the fluvial terrace and depositional architectural elements of four stream geomorphic characters types which vary with position in the relative sea level cycle

During sea-level falls, the low-gradient coastal fluvial plain is characterized by erosion or non-deposition, while preserved terraces in the incised valley and tributary valleys formed in a synchronized fashion in response to changes in climate and/or gradient and/or mechanical load grain size. In this study, three distinct fluvial terrace levels along region’s larger rivers can be correlated and are interpreted to represent RST fluvial deposition. The dimension and geometric parameter relationships among facies architectural elements are analyzed to demonstrate the spatial and temporal variability. Coastal inundation owing to sea level rise on this gentle coastal ramp were at times at extremely fast, up to 16.6m per year of transgression.

The results of this Pleistocene seismic study of the three dimensional geometry of these fluvial geomorphic features shed light upon our understanding of fluvial facies architecture as a process response of sea level change.