Depositional environment and sequence stratigraphy of Miocene deltaic cycles exposed along the Jerudong anticline, Brunei Darussalam

Kilometer-scale prograding clinoforms associated with deltas are rarely see n in outcrop; however, one such example is found in a Miocene sand-shale se quence exposed along the Jerudong anticline in Brunei Darussalam. Regional sequence stratigraphic. interpretation shows that large clinoforms at the b ase of the Miocene Belait delta represent a succession of at least three ma jor sand-shale sequences. The stratigraphically highest and best exposed se quence exhibits large slumps and sharp-based detached sand bodies at its ba se. Accumulation of these units most likely occurred during a relative sea- level lowstand. An overlying 1-1.5 km thick shale unit is interpreted to ha ve developed during subsequent transgressive and early highstand conditions . Rapid progradation of thick sand-dominated shoreface deposits characteriz es the late highstand systems tract. The clinoforms below show similar depo sitional geometries: slumps and thin blankets of shallow-marine sandstones mark the individual bases, shales and mudstones succeed, and progradational shoreface and tidal deposits form the top of each clinoform. New sedimento logical and micropaleontological data document that all sediments (regardle ss of whether sand- or shale-dominated) formed in a shoreface to shelfal se tting in front of a mud-rich delta. This differs from previous studies inte rpreting a continental-slope to deep-marine depositional environment for al l shale-dominated units, and indicates that kilometer-scale clinoforms can develop entirely on the continental shelf in water depths less than 200 m.