Date of Award

1996

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

Harry H. Roberts

Abstract

Late Pleistocene depositional cycles of the Mahakam shelf contain stratal geometries that indicate a progradational continuum, from falling stage in sea level through to initial rise. These findings contrast with popular sequence stratigraphic concepts which predict that large, rapid sea-level falls, typical of the Late Pleistocene, should result in sediment bypass of the entire shelf. The Mahakam shelf is a mixed siliciclastic-carbonate depositional setting. Extensive, thick carbonate buildups are produced by prolific green algal Halimeda bioherm accretion during transgression and highstand flooding of the shelf. The 60 by 200 km study area lies within 2$\sp\circ$ of the equator, in the Makassar Strait on the east coast of Kalimantan, Indonesia. Stratigraphic architecture and sedimentary facies of the shelf have been mapped using over 3000 line km of single channel seismic and side-scan sonar, plus bottom sediment samples, and 44 short cores. This offshore data complements an earlier onshore study of the modern delta by TOTAL Indonesia (Allen et al. 1979). The stratigraphic framework is subdivided into depositional cycles bounded by prominent, regional shelf-flooding surfaces. A stratigraphic model is proposed to explain the sequential development of units and surfaces within a depositional cycle. Oblique clinoforms within the last cycle transform downdip into sigmoid clinoforms. This transition reflects a relative sea level change from falling phase to initial lowstand rise. Updip horizontal extensions of sigmoid clinoforms onlap an interpreted exposure surface which truncates oblique clinoform topsets. These horizontal strata are interpreted to represent delta platform aggradation due to accommodation created landward of the prograding delta front during initial lowstand rise. Accelerated sea level rise results in lowstand-delta abandonment and a backstepping shoreline transgression. Eustasy is the dominant control on shelf accommodation and stratigraphic architecture. In addition, rapid basement subsidence, structural warping and faulting, and distribution of thick carbonates and abandoned siliciclastic depocenters influence accommodation and distribution of lowstand delta depocenters. A comparison of the P1 Depositional Cycle of the Mahakam and Mississippi-Alabama shelf reveals significant differences in the stratigraphic framework. These stratigraphic variations reflect differences in the geological settings of the two study areas.

Pages

170

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