Morphology, Internal Structure, Seismic Stratigraphy, and Sedimentation of Indus Fan

Abstract

The upper Indus Fan (1,600-3,600 m) is characterized by up to several hundred meters relief that resulted from the aggradation of large channel-levee complexes; gradients greater than 1:500; a distinct 3.5 kHz echo character with several continuous subbottom reflectors; and by fine-grained sediments, except within the channels where coarse-grained materials are inferred. The lower fan (4,000-4,500 + m) has a smooth relief with channels and levees of relatively small dimensions; overall gradients of less than 1:1,000; prolonged 3.5 kHz echo character with few or no subbottom reflectors; and a dominantly sandy lithology. The characteristics of the middle fan are intermediate between those of the upper and lower fans. Seismic records reveal at least three canyon complexes on the shelf, each of which gave rise to several leveed channels on the fan. The canyons and channels migrated extensively in time and space across the fan, and channel abandonment and avulsion were very common. Seismically, the canyon fill consists of several reflection-free zones overlain by inclined reflections of moderate amplitudes which are inferred to indicate fine-grained sediments. The channel fills consist of high-amplitude, random reflections overlain successively by reflection-free zones and weak to moderate-amplitude continuous reflections. These characteristics suggest coarse-grained deposits at the base fining upward to the top of the channel fill. The channels, especially on the upper and middle fan, are flanked by wedge-shaped, concave-upward reflection packages characteristic of levee-overbank deposits. Sea level changes and the Himalayan orogenies have profoundly affected the Indus Fan sedimentation since the Oligocene-early Miocene. Sedimentation was dominantly by channelized turbidity currents with overbank deposition on the upper fan, and by both unchannelized and channelized turbidity currents on the lower fan during the lowstands of sea level.