SEA LEVEL MODULATION OF CARBONATES--TERRIGENOUS CLASTIC DOMINANCE ON AN EQUATORIAL SHELF MARGIN, MAHAKAM DELTA SHELF (INDONESIA)

A study of modern and late Pleistocene deltaic and shelf deposits, offshore of the modern equatorial Mahakam delta (Indonesia), clearly indicates an alternating dominance of carbonates and terrigenous clastic sediments of the shelf and shelf margin. Modulation of these contrasting facies is linked to late Pleistocene high frequency eustatic sea level cycles. Field data were collected on a 3-year project to understand the stratigraphic and sedimentologic evolution of the Mahakam shelf and shelf margin. Critical data sets include seismic and side-scan data, plus bottom samples and vibracores/piston cores. Sediment data were used to relate Holocene lithofacies/biofacies to seismic facies so that confident extrapolations could be made to the Pleistocene. Three Pleistocene depositional cycles were seismically imaged; the latest cycle provides the most useful data for understanding shelf and shelf margin evolution in relation to changing sea level.

The latest falling-stage systems tract incorporates entrenched fluvial networks and prograded deltaic deposits in all sectors of shelf. These deltaic deposits bury massive carbonate bioherms (10-30m 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. Steep shelf-edge slopes, coupled with an active tectonic setting promote shelf edge instability resulting in faulting and mass wasting. Each depositional cycle starts with bioherm development following rising-stage shelf flooding. A ravinement surface underlies the bioherms and a maximum flooding surface runs through them. Falling-stage deltaic progradation precludes Halimeda growth and encases individual and fused bioherms in deltaic sediments. Clinoform packages of the falling-stage thicken dramatically as they cross shelf edge faults, where they may fill canyons intersecting the margin. During the lowstand turn-around, clinoform stacking at the margin creates landward accommodation space that fills with subparallel units interpreted as organic-rich delta plain sediments. Relationships between carbonates and terrigenous clastic sediments are replicated repeatedly in the subsurface as a product of cyclic eustatic sea level changes during the Pleistocene.