ROBERTS, HARRY H.¹, BARUN K. SEN GUPTA², JOHN H. WRENN², and JOHAN SYDOW^3
1Coastal Studies Institute, LSU, Baton Rouge, LA
²Department of Geology and Geophysics, LSU, Baton Rouge, LA
³BP Exploration, Mozambique

Abstract: Facies and Surfaces in a Mixed Deltaic and Carbonate Shelf FallingStage Systems Track: Late Quaternary Mahakam Delta (Indonesia)

A study of modern and late Pleistocene deltaic and shelf sediments in the offshore Mahakam Delta of Indonesia indicates that both deltaic siliciclastic sediments and massive carbonates were preserved in the falling-stage systems track. Field data obtained for the 3-year project include over 3000 line km of high resolution seismic data from an area approximately 60 X 200 km. A total of 380 bottom samples and 97 vibracores/piston cores were used to relate Holocene lithofacies/biofacies to seismic facies so that extrapolations could be made to the Pleistocene.

The latest falling-stage systems track incorporates entrenched fluvial networks and prograded deltaic deposits in all sectors of shelf These deltaic deposits bury massive carbonate bioherms (10-30 in relief) composed of the calcareous alga Halimeda, and prograde to the shelf edge where stacking of alternating deltaic deposits and carbonates have built a steep shelf-to-slope transition. The brittle nature of the shelf edge, coupled with the highly tectonic setting of Indonesia, promotes shelf edge instability that results in faulting and mass-wasting on many scales. Each cycle starts with bioherm development following rising-stage shelf flooding. A prominent ravinement surface underlies the bioherm facies and a maximum flooding surface runs through these features. Falling-stage deltaic sediments preclude Halimeda growth and encase individual and fused bioherms. Clinoform packages of the falling-stage thicken dramatically as they cross shelf edge faults. They also fill canyon-like features in the southcentral shelf edge area. Clinoform stacking at the shelf edge creates accommodation space that fills with subparallel units that are interpreted from seismic records as organic-rich delta plain sediments.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas