Lambiase, Joseph J.¹, Abdul Razak Damit² 
(1) Universiti Brunei Darussalam, Tungko Darussalam, Brunei 
(2) Brunei Shell Petroleum, Seria Darussalam, Brunei

ABSTRACT: Depositional Systems and Facies Models for Low Energy Clastic Coastlines within Low Latitude, Semi-Enclosed Seas

With a mean tidal range of 1.7 m, modern clastic coastlines on the northern and northwestern Borneo margins have highly variable coastal morphologies, sand body geometries and facies distributions, none of which match published microtidal facies models. Facies are strongly tide-dominated where mean wave heights are smallest (0.4 m) and become progressively more wave-influenced as wave height increases. Straight coastlines are wave-dominant when mean wave height reaches 0.7 m while mixed wave and tidal facies occur at equivalent wave heights in areas influenced by an estuarine tidal prism. Narrow, low-relief barrier islands develop at a mean wave height of 1.2 m. Associated tidal inlets are not a product of modern hydrodynamics but are drowned river mouths without any tidal deltas; tidal deltas may be absent because the back barrier is completely filled by supratidal mangrove swamp, which minimizes the tidal prism and greatly reduces tidal influence. 
The outcropping middle Miocene and younger clastic successions of northern Borneo were deposited in low energy environments hydrodynamically similar to the modern systems. The tide and wave-dominant strata generally resemble their mid-latitude, oceanic coastline counterparts but with some significant differences. All sandstone facies tend to be relatively fine-grained, muddy and carbonaceous while tidal sands display little evidence of high current velocities, even in strongly tide-dominant successions. Storm beds are scattered throughout shoreface successions (rather than stacked in the lower shoreface), trough cross-bedding is uncommon or absent (presumably because wave energy was too low to generate strong longshore currents), and moderate tidal influence is common.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.