Influences of Holocene sea level, regional tectonics, and fluvial, gravity and slope currents induced sedimentation on the regional geomorphology of the continental slope off northwestern India

Abstract

Regional geomorphology derived from echosounding and bathymetric data (lines of a total of 12,000 km), depth to basement, regional deep seated tectonic features, surficial sediment distribution and Holocene sedimentation rates of the continental margin off northwestern India are presented. Results indicate that the shelf edge over most of the region (shelf break between 80 and 154 m water depth) is covered with relict carbonate facies and partially buried bioherms of Holocene age, particularly off Bombay and Saurashtra regions. Significant changes in the depth and orientation of the shelf edge adjacent to Saurashtra and the Bombay High areas in relict carbonate facies are recorded. At locations, shelf edge depth is much more (up to 150 m) than the Early Holocene eustatic sea-level still stand in this region (about 100 m). The upper boundary of the slope also varies considerably in depth: about 1450 m in the vicinity of the Indus Fan having high sedimentation and 2900 m further southwards. The gradient of the slope off the Kachchh is steepest and some regional notches are observed at the upper slope. The gentler slope off Saurashtra and the Bombay High area has large benches at depths of 180–230 m (width 2–10 km) and 650–780 m, well below the Holocene sea level. The Bombay high area has slope breaks between 400 and 600 m, whereas off Saurashtra steep breaks in the slope occur between 560 and 960 m depth. Further southwards, at the foot of the slope, elevations and depressions are present. Variations in the geomorphology of the slope off the northwestern continental margin of India are attributed to: (1) Late Pleistocene-Holocene sea levels, (2) regional tectonics, (3) fluvial sedimentation, and (4) gravity and slope currents induced sediment dynamics. The locations under influences of slumping, turbidity and tectonic movements, with altered in situ sequence of sedimentations, are delineated.