Stratigraphic Architecture and Fundamental Sedimentology of Two Late Pleistocene Deltas: Gulf of Mexico and Indonesia

Harry H. Roberts: Coastal Studies Institute, Louisiana State University, Baton Rouge, LA; Richard Fillon: Earth Studies Associates, New Orleans, LA; John Sydow, BP Amoco Trinidad and Tobago, Exploration Business Unit, Port of Spain, Trinidad; and Barry Kohl: Department of Geology, Tulane University, New Orleans, LA

Abstract

The Lagniappe shelf-edge delta, built by the Mobile River, prograde to the shelf-slope break as sea level approached the latest Pleistocene glacial maximum. At the same time, the Mahakam River has built a complex shelf-edge delta on the eastern shelf of Borneo (Indonesia). Both deltas were constructed by falling-to-lowstand deposition, the former by the temperate Mobile River, the latter by the equatorial Mahakam River. Four cores provide detailed stratigraphic control for the Mobile River delta while one long boring and numerous piston and vibracores provide stratigraphic control on the Mahakam delta. Systems tracts and key bounding surfaces have been related to the eustatic sea level curve in both settings over ca. 125 ka. Sequence architectures differ significantly, an important consequence of different depositional settings. The tropical Mahakam shelf is tectonically active with low wave energy, strong oceanic currents, upwelling, and a mixed siliciclastic-carbonate depositional system. Falling-to-lowstand clinoforms of this delta downlap a highly irregular surface of isolated carbonate bioherms built above a transgressive surface that formed during the preceding sea level rise. The northeastern Gulf of Mexico is relatively stable, also with low wave energy, but dominated by siliciclastic sedimentation. Falling-to-lowstand progradation of the Mobile River delta occurred in numerous overlapping and spatially offset lobes incised by a complex channel network. Clinoforms downlap an isotope stage 5 interglacial condensed section. Both the Mahakam and Mobile River depocenters are multilobate and clearly built by autocyclic switching of depositional sites. The eastern lobes of the Mobile River delta show evidence of wave reworking while the western flank is fluvially dominated. Both the Mahakam and Mobile deltas are composed of sand-rich clinoforms and channel deposits that possess excellent potential reservoir properties.