LATE QUATERNARY SEDIMENTATION ON THE OUTER INDUS SHELF AND SLOPE (PAKISTAN) - EVIDENCE FROM HIGH-RESOLUTION SEISMIC DATA AND CORING

High-resolution seismic (Parasound) profiles, multibeam bathymetry (Hy drosweep), under-water photography and sediment cores are used to map the morphology and echo-facies distribution, and to describe the late Quaternary sedimentation on the outer shelf and slope off the Indus de lta (Pakistan). The morphology and origin of the Indus Canyon were stu died in detail. The upper and middle canyon (from 20- to 1350-m axial depth) is an erosional (degradational) canyon or delta-front trough wi th steep erosional walls and a meandering axial channel without levees . The lower Indus Canyon (1350- to 1500-m axial depth) is a transition al type between a degradational canyon and the aggradational channel-l evee system of the upper Indus Fan, The late Quaternary sedimentation is influenced by fluctuations of fluvial input, delta progradation, ca nyon erosion and fan aggradation. It is especially controlled by sea-l evel changes. Four episodes can be distinguished: (1) During interglac ial (?Eemian) times and a relatively high sea-level position, a layere d sequence of V-shaped, channel-like erosional features were formed be low the shelfbreak (135 m) down to an upper slope scarp by slumping or erosion as delta-front or prodelta gullies. (2) During the Last Glaci al Maximum, the sea-level lowstand caused the Indus delta to advance a cross the present shelfbreak and shed fluvial silty clays directly ont o the upper slope. This is indicated by layered, gently seaward dippin g subparallel reflectors which are interpreted as being prograded prod elta mud units. At this time the Indus Canyon experienced maximum eros ion and funnelled turbidity currents to the aggradational channel-leve e system on the Indus Fan. Major slumps and debris flows were triggere d at the continental slope. (3) During deglaciation and the beginning of the Holocene transgression, several sea-level stillstands are indic ated by the formation of biogenic sediments on the outer shelf and the build-up of shallow-water algal bioherms around 9-12 ka BP (presently at a water depth of similar to 90-100 m). (4) During the late Holocen e sea-level highstand, the shelf was flooded and the delta-front sedim ents of the Indus River were deposited on the innermost shelf. The out er shelf is characterized by a lack of deposition and erosion. The Ind us Canyon experienced ongoing (but much decreased) activity of low-den sity turbidity currents with overbank spilling. The middle and lower c ontinental slope down to similar to 2000 m is covered by hemipelagic s ediments showing a layered sequence of distinct subbottom reflectors e xplained as hemipelagic drape interbedded with turbid layer sediments and/or thin mud turbidites from spillover along natural levees borderi ng the Indus Canyon. For the past 50 ka, the continental slope (outsid e the canyon and channels) experienced no turbidite sedimentation. (C) 1997 Elsevier Science. All rights reserved.