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

摘要

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 break is defined by a prominent fault scarp approx 130 m below sea level. Beyond, the seabed descends at an average of 2-3 deg to the Borneo Trough at >2750 m depth. The outer-shelf Quaternary succession is locally >1 km 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' reflectors, traced with the aid of 1500 line-km of high-resolution seismic profiles, are downlap surfaces beneath clinoforms on the shelf. Two of these reflectors directly overlie fluvial channels. These are interpreted as erosional ravinement surfaces that formed during shelf-crossing postglacial transgressions, and correlate with surfaces of maximum regression in the slope succession. Acoustic facies on the shelf, calibrated by 15 60-100 m-long geotechnical borings, are interpreted as muddy prodelta, transgressive, and incised-valley deposits; forced-regressive and lowstand fluvial channel sands; shelf-edge sandy deltas; and reworked sands along ravinement surfaces. Muddy prodelta deposits drape the slope. Isopach maps of strata between 'key' reflectors reveal shingled highstand to lowstand delta lobes. A widespread 4th-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 contemporaneous Valley-Canyon Sequence and is filled with lower FRST to Lowstand Systems Tract fluvial deposits, and upper backstepping deltaic deposits of the retreating Baram Delta.