Granulometric study of placer sands across a multi-berm beach at Chavara, southwest coast of India

摘要

Heavy mineral bearing beach sediments collected along a shore perpendicular transect between the surf zone and monsoonal berm at Chavara were analyzed for their grain size distribution of heavy mineral constituents. The principal heavy mineral present in this beach are ilmenite (35- 64%), zircon (2-17%) sillimanite (4-9%), rutile (2-11%), monazite (1-2%) and leucoxene (1-2%). The heavy mineral content increases towards onshore in the upper and lower foreshore slope, but the grain size becomes finer and better sorted. Higher concentration of heavy mineral was found in the berms and upper foreshore slope. The concentration factor of heavy minerals correlates well with the specific gravity and the mean sizes suggesting that these parameters play major roles in their selective sorting and concentration in the upper foreshore slope. The heavy mineral concentrates so formed are carried to the foreshore berm as the water level rises during high tide and are left behind as the water recedes. The transport paths of heavy minerals determined from sediment trend matrix suggest that they move differently from light minerals in the beach environment and the heavy minerals are selectively deposited at the berms and upper part of the lower foreshore slope. The geological set-up of the study area, and mineralogy and texture of the constituent minerals, suggest that these heavy mineral placers are formed mainly from the palaeo-beach deposits reworked and transported by the waves during the Holocene sea level rise. The present day seasonal erosion of the heavy mineral rich barrier beach system has further concentrated the heavy minerals on the beach.