Depositional sequences controlled by high rates of sediment supply, sea-level variations, and growth faulting: the Quaternary Baram Delta of northwestern Borneo

The shelf off the Baram Delta is 50-70 km wide, and is underlain by 8-9 km of post-Eocene upper slope, prodelta, delta-front, fluvial, shelf-shoreface , and estuarine (incised-valley and tidal-embayment) deposits. The shelf br eak is defined by a prominent fault scarp similar to 130 m below sea level. Beyond, the seabed descends at an average of 2-3 degrees to the Bomeo Trou gh at > 2750 m depth. The outer-shelf Quaternary succession is locally >1 k m thick. Uppermost Quaternary units then thicken by a factor of 2-5 across an en echelon set of shelf-edge growth faults. Five widespread 'key' reflec tors, traced with the aid of 1500 line-km of high-resolution seismic profil es, are downlap surfaces beneath clinoforms on the shelf. Two of these refl ectors directly overlie fluvial channels. These are interpreted as erosiona l ravinement surfaces that formed during shelf-crossing postglacial transgr essions, and correlate with surfaces of maximum regression in the slope suc cession. Acoustic facies on the shelf, calibrated by 15 60-100 m-long geote chnical borings, are interpreted as muddy prodelta, transgressive, and inci sed-valley deposits; forced-regressive and lowstand fluvial channel sands; shelf-edge sandy deltas; and reworked sands along ravinement surfaces. Mudd y prodelta deposits drape the slope. Isopach maps of strata between 'key' r eflectors reveal shingled highstand to lowstand delta lobes, A widespread 4 eh-order Lowstand-Bypass Sequence developed during the 120-10 ka sea-level cycle. It reaches 400 m thickness and resembles 3rd-order sequences of Vail and coworkers, but with a thinner Transgressive Systems Tract and a thick Forced Regressive Systems Tract (FRST). The FRST includes sandy shelf-edge deltas deposited by small rivers after the Baram system began to bypass the shelf through an incised valley. The incised valley belongs to a contempor aneous Valley-Canyon Sequence and is filled with lower FRST to Lowstand Sys tems Tract fluvial deposits, and upper backstepping deltaic deposits of the retreating Baram Delta. (C) 2001 Elsevier Science B.V. All rights reserved .