Late Quaternary Shelf-Edge Deltas from Northeastern Gulf of Mexico and Eastern Borneo (Indonesia): A Comparison
H.H. Roberts, J. Sydow, R.H. Fillon, B. Kohl, 2013. "Late Quaternary Shelf-Edge Deltas from Northeastern Gulf of Mexico and Eastern Borneo (Indonesia): A Comparison", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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As the sea level low of the latest Pleistocene glacial maximum was approached, fluvial systems worldwide migrated across continental shelves and built their deltas at or near the shelf-edge. Two excellent data sets consisting of high resolution seismic profiles and cores have been acquired in recent years from two of these shelf-edge deltas in very different climatic and physical process settings, the temperate Lagniappe (Mobile River) delta of the northeastern Gulf of Mexico and tropical Mahakam River delta of the eastern coast of Borneo (Indonesia). Both of these late Pleistocene deltas were built during the latest Pleistocene falling-to-low relative sea-level conditions.
The Lagniappe Delta is defined by a complex clinoform wedge built in two simultaneous progradational thrusts resulting in western and eastern deltaic depocenters at the shelf-edge (Fig. 1). Both depocenters contain individual clinoforms sets interpreted to be both the products of autocyclic switching of dominant distributaries and a stepped fall in sea-level. Clinoforms lap down on outer shelf shale overlying an oxygen isotope stage 5 condensed section. The tops of clinoforms sets are extensively eroded resulting in a complex northeast/southwest-oriented fluvial scour. The eastern lobes of the Lagniappe Delta show evidence of wave reworking while the western depocenter is fluvially dominated.
The Lagniappe clinoforms built wedges on a relatively stable shelf under conditions of a low tide range, low wave energy, and seasonal storms. In this setting, siliclastic sedimentation dominated and carbonates had little impact on the sedimentary record. Dating (14C methods) of both the eastern and western Lagniappe depocenters indicated an age of approximately 19 Ka BP. However, a deeper clinoforms wedge that was stratigraphically younger than the western and eastern deltaic depocenters occurred at a depth of approximately 119-126 m water depth. This clinoforms wedge has not been dated, but it is part of the falling-to-low-stand progradation.
The latest Pleistocene Mahakam River Delta was deposited on a tectonically active shelf and in an equatorial setting characterized by moderate tide, extremely low wave activity, low storm activity, a strong north-to-south flowing oceanic current, and upwelling. Tropical ocean conditions and upwelling of nutrient-rich water provided environmental conditions suitable for developing a mixed siliciclastic-carbonate depositional system. Below the Pleistocene-to-Holocene ravinement surface, two deltaic depocenters developed during the latest Pleistocene falling-to-low sea-level, a northern depocenter characterized by numerous distributaries that created off-set and overlapping individual clinoforms sets through autocyclic processes and a larger southern depocenter characterized by an actuate delta front and a central northwest-southeast oriented distributary network resulting in a broad fluvial scour (Fig. 2). These Pleistocene deltaic deposits prograded to the shelf edge along most of shelf-slope break. Clinoforms of these falling-to-lowstand deposits lap down onto highly irregular shelf topography created by the growth of the calcareous green alga Halimeda. These isolated bioherms and aggregates of bioherms are seated on a prominent ravinement surface. A maximum flooding surface runs through these features. The success of Halimeda in building impressive shelf bioherms is attributed to the upwelling of nutrient-rich tropical water onto the Mahakam shelf thus stimulating plant growth as sea level rose across the shelf. Bioherm growth is terminated by siliciclastic influx as the delta progrades during a prolonged highstand or falling-to-lowstand conditions. During the lowstand turn-around, over half of the lowstand systems tract stratigraphy is related to delta plain aggradation. Modern equivalents to these delta plain sediments are found to have a lipid-rich organic content of sufficient concentration to be considered a source rock precursor.