The Middle Miocene Miri Formation records part of the infill of an estuarine valley system that was incised during an early Middle Miocene drop in sea level and subsequently infiled during a transgressive episode later in the Middle Miocene time. Around the Miri town in Sarawak, part of this incised valley succession is exposed and exhibit a wide range of siliciclastic lithofacies which reflect a variety of depositional settings. Three main depositional environments are recognised: tide dominated estuary, distal lower shoreface to offshore transition and s.horeface. The estuarine lithofacies association (FA:1) is characterized by distinct and diagnostic tidal signatures - tidal dune cross-bedding with mud draped cosets and forests including mud couplets, bidirectional (herringbone) cross-bedding, rhythmic stratifications, flaser bedding, wave bedding and lenticular bedding. Distal lower shoreface to offshore transition environment (FA:2A) is represented by interbedded laminated and bioturbated siltstone/sandstone, bioturbated siltstone and laminated mudstone/hummocky sandstone interbedding. Shoreface, storm-and-wave facies association (FA:2B) are represented by swaley cross-stratified sandstones, hummocky cross-stratified sandstones, bioturbated sandstones and associated mudstones. The tidal estuarine deposits form mix aggradational to retrogradational parasequence set, which represent the transgressive valley fills resulting from the establishment of an estuarine system (Early Transgrassive System Tract: E TST) (Figure). This stage (Stage I) represents the early phase of a significant relative sea-level rise, and possibly reflects a southward migration of the paleo Miri shoreline. This stage also characterized by great wave actions coinciding with abrupt sea level rising, which resulted in the deposition of several tempestites at estuary mouth. Stage II records the complete drowning event of the estuarine system, with the development of shallow marine settings (Late Transgressive System Tract: L TST), representing the final phase of transgrassive system tract. Parasequence sets of the transgressive system tract generally display clear, landward shift in facies trend. Shallow marine deposits in the studied rock succession overwhelms the estuarine deposits, displaying a retrograditional set of parasequence. The transgressive trend that started with the establishment of an estuarine system culminated with the deposition of offshore transition facies, which represents the maximum flooding surface (MFS). At the outcrops, this MFS marks the turnaround from retrograditional stacking pattern of TST to aggradational parasequences of HST. Stage III (Highstand System Tract) is represented by a well developed shoreface succession. This stage represents the upper portion of the exposed Miri Formation, which was possibly deposited during stable high and slowly falling sea-level. HST parasequence sets show an aggradational trend, formed during stable high sea-level, to progradational trend, formed during slowly falling sea-level. HST is predominantly composed of upper to middle shoreface deposits (swaley and thick hummocky cross stratified facies) that overlie MFS.


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