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Abstract

The modern deltas of NW Borneo have long been regarded as wave-dominated, as typified by the Baram River Delta. However, the sedimentary facies associated with several modern deltas within Brunei Bay are strongly tide-dominated. A notable example is the Trusan River Delta, which occurs within a subsiding sub-basin and has an intertidal morphology and facies distribution that indicates tidal dominance, although the shoreline geometry suggests significant fluvial influence. The succession becomes sandier upward from embayment muds through mixed sand and mud flats to tidal-channel and bar sands near mean sea level, and then fines upward to tidal sand flats and finally mud flats near the high-tide shoreline.

Tide-dominated sediments also are common in the outcropping strata of the mid-Miocene and younger Belait Formation. The Seven-Up Beach succession is interpreted as the distal ends of progradational lobes of a tide-dominated delta that coarsen upward from brackish-water mudstone through muddy and sandy tide-dominated mouth-bar deposits to tidal-channel and bar sandstones. The tidal-flat and tidal-channel and bar sandstones and interbedded tidal-flat mudstones of the Jalan Sungai Akar succession are more proximal deposits of a similar delta. Examples of distributary channels that eroded into lobe deposits during a relative sea-level fall and were backfilled during the subsequent relative sea-level rise are exposed in the Tanjong Batu succession. All three outcropping successions are strongly aggradational.

A model for tide-dominated deltas in NW Borneo derived from the modern facies distribution and outcrop stratigraphy indicates that stratigraphic architecture is controlled primarily by the interaction of tectonic and compaction-driven subsidence, a relatively high rate of sediment supply, and basin hydrodynamics. The result is a largely aggradational, sandy intertidal succession on the delta plain and muddy delta-front deposits, with aggradational stacking of individual delta lobes.

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