Carmen M Fraticelli¹, John H Wrenn²
(1) Rice University, Houston, TX
(2) Louisiana State University, Baton Rouge, LA

Abstract: Differentiating depositional environments in the Mahakam Delta, East Kalimantan, using siliceous phytolith and diatom distributions

Differentiating depositional environments based on well cuttings is typically achieved with marine microfossils, such as foraminifera and nannoplankton. They are however, of little use for facies differentiation within proximal deltaic environments, where many potential reservoirs are found. Although palynology has proven to be a valuable paleoenvironmental indicator within deltaic settings, siliceous phytoliths are a new, potentially useful tool for increasing the resolution of paleoenvironmental determinations. Their utility in this context was tested in the Mahakam Delta, East Kalimantan.

The Mahakam Delta is a tropical, composite delta whose central delta plain is tidally influenced, whereas the northern and southern regions are fluvially dominated. Plant assemblage zones change from head-of-passes to the delta’s distal margin on the shore of the Makassar Strait. Many of the tropical plants produce distinctive and diagnostic siliceous phytoliths, which permit differentiation of the plant assemblages.

Diatoms are abundant in distributary and tidal channels, and their assemblages reflect salinity variations that range from freshwater to fully marine conditions. Similar to terrestrial plant assemblages, diatom assemblages change in composition and abundance from the head-of-passes to the distal margins of the delta.

The sedimentary processes acting on the delta influence the distribution of both siliceous microfossils groups by controlling not only the dispersal of their remains, but also the distribution of the source material (i.e., the plants or diatoms living in any one area). Within the Mahakam Delta, tidal channel, fluvial channel, tidal mouth, and distributary mouth bar facies are recognized based on abundance/diversity data and presence/absence data of specific forms.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana