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 (Hydrosweep), 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 delta (Pakistan). The morphology and origin of the Indus Canyon were studied in detail. The upper and middle canyon (from 20- to 1350-m axial depth) is an erosional (degradational) canyon or delta-front trough with steep erosional walls and a meandering axial channel without levees. The lower Indus Canyon (1350- to 1500-m axial depth) is a transitional type between a degradational canyon and the aggradational channel-levee system of the upper Indus Fan, The late Quaternary sedimentation is influenced by fluctuations of fluvial input, delta progradation, canyon erosion and fan aggradation. It is especially controlled by sea-level changes. Four episodes can be distinguished: (1) During interglacial (?Eemian) times and a relatively high sea-level position, a layered sequence of V-shaped, channel-like erosional features were formed below the shelfbreak (135 m) down to an upper slope scarp by slumping or erosion as delta-front or prodelta gullies. (2) During the Last Glacial Maximum, the sea-level lowstand caused the Indus delta to advance across the present shelfbreak and shed fluvial silty clays directly onto the upper slope. This is indicated by layered, gently seaward dipping subparallel reflectors which are interpreted as being prograded prodelta mud units. At this time the Indus Canyon experienced maximum erosion and funnelled turbidity currents to the aggradational channel-levee system on the Indus Fan. Major slumps and debris flows were triggered at the continental slope. (3) During deglaciation and the beginning of the Holocene transgression, several sea-level stillstands are indicated 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 Holocene sea-level highstand, the shelf was flooded and the delta-front sediments of the Indus River were deposited on the innermost shelf. The outer shelf is characterized by a lack of deposition and erosion. The Indus Canyon experienced ongoing (but much decreased) activity of low-density turbidity currents with overbank spilling. The middle and lower continental slope down to similar to 2000 m is covered by hemipelagic sediments showing a layered sequence of distinct subbottom reflectors explained as hemipelagic drape interbedded with turbid layer sediments and/or thin mud turbidites from spillover along natural levees bordering the Indus Canyon. For the past 50 ka, the continental slope (outside the canyon and channels) experienced no turbidite sedimentation. (C) 1997 Elsevier Science. All rights reserved.