Lambiase, Joseph J.¹, Abdul Razak Damit² 
(1) Universiti Brunei Darussalam, Tungko Darussalam, Brunei 
(2) Brunei Shell Petroleum, Seria Darussalam, Brunei

ABSTRACT: Facies, Morphology and Sedimentary Processes on Two Modern Deltas in NW Borneo: Implications for Subsurface Deltaic Successions

The Baram and Trusan deltas lie in close proximity and in similar hydrodynamic environments on the NW Borneo margin, yet their morphology, facies distribution and sand body geometry are markedly different. The Baram Delta is a mixed tide and wave system while facies on the Trusan Delta are strongly tide-dominated. Facies distribution accurately reflects hydrodynamic regime on both deltas but morphology does not match hydrodynamic dominance on either delta. The Trusan Delta has a lobate, fluvial-dominant morphology; the active lobe features an active distributary channel that exhibits levees and a small mouth bar. Conversely, the Baram Delta, formed by a much larger river, has a wave-dominant shoreline geometry and a large, dominantly tidal, mouth bar. On both deltas, morphology and facies distribution appear to be strongly responsive to the rate of sediment supply relative to the rate of redistribution by marine processes. 
The apparent mismatch between facies distribution and delta morphology indicates that subsurface sand body geometry cannot be predicted accurately from stratigraphic succession. Sand body geometry in the preserved succession of the Trusan Delta will mimic its fluvial morphology whereas the facies succession predicts strongly tidal sand bodies. Marine processes dominate the Baram Delta and deposition is limited to the mouth bar; preserved sand body geometry will resemble the mouth bar but not the overall delta morphology. This implies that deltaic successions within transgressive and highstand systems tracts are more likely to have Baram-like sand body geometries while regressive and lowstand systems tract deltas will have more Trusan-like geometries.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.