Siliciclastic-Carbonate Facies Interactions in a Tropical Deltaic Complex: Mahakam Delta, Indonesia

Harry H. Roberts, Johan Sydow

The Mahakam Delta of East Kalimantan, Indonesia, is located on the equator where it builds into the Makassar Strait. At present this delta is a combined river and tide dominated type and is confined to the inner 20-30 km part of a ~60 km wide shelf. Modern prodelta deposits are generally restricted to the inner shelf and are best developed off the more fluvially influenced southern distributaries. Middle to outer shelf areas are dominated by biohermal buildups, ranging from small mounds with a few meters relief, to buildups of over 30 m. Surface samples suggest the mounds are constructed largely of Halimeda particles in a terrigenous mud matrix. Similar features have been found growing on carbonate platforms further south in the Makassar Strait and attributed to nutrient- ich upwelling.

Halimeda buildups are established on top of abandoned, lowstand, shelf-edge deltas. Seismic data indicate that such buildups separate regressive delta cycles. Vertical stacking of regressive cycles is modulated by glacio-eustacy, further modified by tidal scour and lateral shifting of lowstand-delta depocenters. The lowstand deltas are characterized by well-developed clinoforms that indicate progradation to the shelf edge in all sectors. Clinoform geometries suggest that both fluvial and fluvial-tidal lowstand deltas are represented in the sedimentary architecture of the shelf. Thick delta plain deposits incised by fluvial distributaries and numerous tidal channels characterize the more tidally dominated deltas. Faulting on the outer shelf abnormally thickens delta deposits, steepens the shelf margin, and in some cases contributes shallow water sediments to deep water settings.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995