The Modern Mahakam Delta: Ongoing Delta Abandonment on a Tide-Dominated Margin

Husein, Salahuddin¹; Lambiase, Joseph²; Nirsal, Nadia³
¹Department of Geological Engineering, Gadjah Mada University, Jogyakarta, Indonesia.
²Petroleum Geoscience Program, Chulalongkorn University, Bangkok, Thailand.
³Pearl Energy Indonesia, Jakarta, Indonesia.

The only quantitative study of sediment dynamics on the Mahakam Delta concluded that the delta is currently in the process of being transgressed and that its morphology is produced by tidal modification of an originally fluvial-dominant delta morphology. This morphology develops because waves are exceedingly small and have no significant influence on the delta and tidal currents are not strong enough to completely rework delta morphology.

As transgression proceeds, the distributaries subside and become too deep for the size of the Mahakam River, allowing a wedge of fully marine water to penetrate 10s of km upstream. This configuration causes flooding tides to lift river water out of the distributaries, deposit mud on levees and in interdistributary areas and promote the growth of Nypa. Tidal scouring converts active meandering fluvial channels into the wider and straight distributaries that occur on many SE Asian deltas, whilst previously-abandoned distributaries become estuaries with flared mouths at their seaward end. The straightening and widening process presently is occurring up to ~13 km upstream of the delta apex. "Lateral bars" within the upper reaches of the distributaries are fluvial point bars that are being reworked by tidal currents.

Ongoing transgression also explains why the break in slope between the gently-dipping delta plain and the delta front is ~10 km offshore; the break in slope marks the point of maximum progradation prior to the onset of transgression that probably began ~2000 BP by analogy with other SE Asian deltas. The low-relief "subaqueous distributaries" that extend offshore to the break in slope are drowned distributaries that have been nearly filled in and the sandy "mouth bars" that lie several km offshore adjacent to drowned distributaries and are separated by up to 20 km of mud from the nearest sand in those distributaries, are remnants of interdistributary areas that have subsided into the intertidal zone.

The incomplete reworking of progradational deposits by tidal current results in a preserved stratigraphic record of the transgressive deposits. These consist primarily of back-filled distributaries that have increasing marine influence upward. Outcrop examples of back-filled distributaries deposited by the paleo-Mahakam Delta suggest that similar transgressive events have contributed significantly to the delta's stratigraphic architecture.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012