Abstract

Late Pleistocene-to-modern stratigraphy of Mahakam River delta deposits records a progradational continuum from falling stage in sea level, through initial rise, to modern highstand deposition. These results, from two field data collection periods (May-June 1993 and 1994), contrast with popular sequence stratigraphic concepts which predict that large, rapid sea level drops, typical of the latest Pleistocene, should result in sedimentary bypass of the entire shelf.

Stratigraphic architecture of Late Pleistocene-to-modern deposits reflects the interactions and impacts of eustacy, subsidence, syndepositional structural deformation, deltaic deposition, prolific carbonate accretion, and physical oceanographic processes. Over 3000 km of high resolution acoustic data, >137 bottom samples, 44 vibracores/piston cores, and 3 geotechnical borings (avg. 130 m long) provided the data base to map seismic relationships and sedimentary facies. The last glacioeustatic cycle produced a stratigraphic succession of falling stage deltaic deposits, lowstand turn around-to-initial rise deltaic deposits, and transgressive sediments dominated by impressive carbonate buildups. Falling-to-lowstand deposits of this latest sea level cycle are organized into two distinct depocenters in the northern and southern sectors of the shelf. Clinoform dip orientations indicate that the broad delta front of the southern depocenter (P1S) was sourced by the Mahakam River while the lobate progradational centers of the smaller northern depocenter (P1N) were sourced both by the Mahakam and the smaller northern Santan River. Oblique clinoforms of P1S transform downdip into sigmoid clinoforms. Updip extensions of these clinoforms onlap an exposure surface formed as sea level dropped to the lowstand maximum. This sigmoid clinoform/horizontal stratal unit results from accommodation space created behind a shelf edge delta prograding during initial sea level rise. The horizontal strata represent delta plain accretion units. Because of a smaller sediment supply, aggradational horizontal strata are poorly represented in P1N. Carbonates, dominated by Halimeda (calcareous green alga), create mounds, pinnacles, and platforms on the transgressive surface stratigraphically above the lowstand delta deposits. Holocene highstand deltaic sediments are now downlapping carbonate buildups on the inner shelf. Seismic data indicate that the sedimentary architecture of the shelf is dominated by vertical stacking of aforementioned depositional sequences related to successive glacioeustatic cycles.