Submerged sand ridges on the western continental shelf off Bombay, India: evidence for Late Pleistocene-Holocene sea-level changes

Abstract

The morphology, distribution and probable origin of submerged sand ridges on the western continental shelf off Bombay, India have been studied using echosounding, side-scan sonar and high-resolution seismic reflection techniques supplemented by seabed sampling. 7–8 rows of well defined, broad, symmetrical submerged sand ridges were found on the middle and the outer shelf 100–200 km offshore and between 75 and 100 m water depth, the ridges being more prominent between 85 and 90 m. These ridges are 0.5 to 10 km wide, 1.5 to 18 m high, several tens of km long, with a few kilometers spacing and a trend of NNW-SSE parallel to the shelf edge. The inter-ridge areas are characterised by long, narrow palaeo-lagoonal-like depressions. Some rounded/smooth ridges appear to be moribund while others have undergone reworking as indicated by superimposed bedforms/sand waves and reefs. Ridges also occurred on the inner shelf where they were either eroded during the Holocene transgression or covered by Recent sediments. The sand ridges originated as dunes which were submerged and reformed into very large sand bars. Further drowning and lithification favoured the development of organic growth and sand wave formation which must have taken place during a period of stillstand or slow sea level rise in the early to mid-Holocene. The superimposed bedforms reflect the prevailing flow regime on the shelf.