Abstract: The Modern Mahakam Delta (Indonesia): Sedimentary Facies and Reservoir Geometry in a Low Wave Energy, Mixed Tide and Fluvial-Dominated Delta

George P. Allen, Robert Gastaldo

The Mahakam delta, affected by low wave energy, mesotidal range and a relatively constant fluvial discharge exhibits features common to tide- and fluvial-dominated environments. The subaerial delta exhibits a symmetrical lobate geometry, with an intertidal delta plain composed of organic clay incised by distributary and tidal channels. Distributary channels are rectilinear and sand-filled, while the tidal channels are sinuous and contain only organic-rich mud as they are unconnected to the fluvial inflow. In their distal

extremity the distributaries are more tide-dominated and the sediments grade to sandy mud. There is a conspicuous absence of alluvial levees, fluvial overbank and splay deposits on the delta plain. This is due to the effects of tides and the relatively constant fluvial discharge.

Distributary sand accumulates as lateral bars within the channel. These coalesce to form narrow rectilinear sand ribbons with a width/thickness ratio between 50-100. The sands accumulate as 5-15 m thick channel-fills which grade upward into massive delta plain clay. Sedimentary structures are predominantly small scale cross strata and ripple laminae; tidal clay laminations are common and fluvial flood deposits are absent. In the distal extremity of the distributaries lateral bars are isolated within the channel and sand occurs as discontinuous pods.

Delta front sediments consist of fine sand, sandy muds and organic muds. Facies reflect the low energy waves and currents, bioturbation is abundant and stratification is rarely preserved. Sand accumulates as triangular-shaped distributary mouth bars at channel bifurcations. These bars are not in longitudinal contact with the distributary channel sands and are composed of stacked, thin (1-3 m), upward coarsening fine-grained sand sheets which coalesce laterally and thin distally and laterally into prodelta muds. The bars are dissected by mud-filled channel-fills which form localized clay plugs. Little or no sand is transported seaward to the prodelta by density currents.

The mouth bars form sand deposits up to 8 m thick, with a width/thickness ratio between 1000-3000. Their internal architecture is complex with superimposed sheet-like sand units, however numerous interconnections exist within the central core of the bar so that the entire bar system would form a single reservoir.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994