Influence of Estuaries on Shelf Sediment Texture

Abstract

Granulometric analyses of sediments on the western continental shelf of India demonstrate that the texture of inner continental shelf sediments (10 to 60 m water depth) appears to be determined by the size and number of estuaries present on the coast. Offshore from regions where there are a large number of estuaries, the inner shelf sediments are fine grained (average mean size 5.02 ø, 0.03 mm), rich in organic matter (>2%) and low in calcium carbonate (<25%). In contrast, in regions with relatively fewer estuaries, the shelf sediments are of sand size (average mean size 1.53 ø, 0.35 mm), poor in organic matter (<1 %) and rich in calcium carbonate (>30%). Echo profiles of the sea floor are characterized by sub-bottom reflections in the former and their absence in the latter. These differences are attributed to the fact that the estuaries act as regional filters which permit deposition of only fine-grained sediments on the inner shelf while trapping the coarse-grained material in the estuarine basin. An example of this filtering effect is discussed from the sediment texture distribution on the south-western shelf of India (Kerala) where it is particularly well displayed.