Acoustic character and thickness of sediments of the Indus Fan and the continental margin of western India

Abstract

Several acoustic-sediment layers, with interval velocities ranging from 1.9 to 3.8 km/sec are present in the Indus Fan; the basal layers with velocities 3.0 to 3.8 km/sec appear to follow the structural trends of the basement and may have been influenced mainly by pelagic sedimentation whereas the top layers with lower velocities were deposited by turbidite sedimentation. Although sediment thickness in the Indus Fan in general varies from > 5 sec (> 10 km) in the proximal fan to < 1 sec in the distal areas, the fan can be divided into several basins of relatively thick sedimentary fill separated by the Laxmi (Lakshmi) Ridge and its branches which form a northward continuation of the Chagos—Laccadive Ridge. Turbidity currents transported sediments since Oligocene—Miocene times through several submarine valleys and channels in the fan. However, before the turbidite sedimentation became a dominant process, there appear to have existed only two major troughs in the upper fan region which probably served as the initial conduits for turbidity current transport into the distal areas. There are also several basins along the Indian continental margin which, with sediment thickness sometimes exceeding 4 km, are parallel or subparallel to the coastline especially in the northern part. Slumping process appears to have played a significant role in sedimentation along the Indian margin, at least in the recent geological past. The velocity structure of the sediments of the continental margin and the adjacent Arabian Sea east of the Laxmi Ridge is essentially similar to that of the Indus Fan west of the ridge. Additionally, in the region east of the Laxmi Ridge, there is a 4.0–4.3 km/sec basal sedimentary layer which we believe was deposited during the early stages of rifting in the northeastern Arabian Sea.