Magnetic and Sedimentological Studies of Netravati and Gurpur River-Bed Sediments, West Coast of India

Magnetic susceptibility and sedimentological studies have been carried out on Netravati and Gurpur river-bed sediments along the west coast of India to throw light on the distribution and movement of magnetic minerals in the riverine environment. The sediments studied are coarse to fine sands, platy to leptokurtic and poorly to well sorted. Granule to medium size sands dominate in the upstream segment. whereas very fine sand and silt+clay are either absent or of negligible importance. Netravati river-bed sediments have a two-fold higher bulk susceptibility than Gurpur sediments because the Netravati drainage basin has magnetite quartzites, amphibolites and garnetiferous biotite gneisses in contrast to granitoids found in the Gurpur drainage basin. This trend of susceptibility is seen for the 13 particle size fractions studied.

Magnetic minerals are concentrated in poorly sorted, fine grained sediments of Netravati river and in leptokurtic sediments ofGurpurriver. They are particularly enriched in the size range of 0.063-0.125 mm and to a lesser extent in the >4 mm fraction. Downstream decrease in susceptibility of both bulk samples and particle size fractions reflects on the vicinity or otherwise of sampling sites to the source of magnetic minerals in the drainage basin.

On the basis of bulk susceptibility data, it is.estimated that about 87% of sediment in the Netravati main river is derived from Kumaradhari river and 13% from Netravati (upstream) tributary. However. about 78% of particles >0.35 mm in size in Netravati main river are contributed by Kumaradhari river and 22% by Netravati (upstream) tributary. The bulk susceptibility of Ourpur river sediments shows that the Venoor hole is the dominant sediment contributor (83%) to the main river when compared to Hosangadi hole (17%). In contrast, Hosangadi hole contributes ca. 63% of the fine particles (0.55-1.0 mm) to the Gurpur main channel. This discrepancy could be attributed to the higher susceptibility of coarse fraction (> 1 mm) of Gurpur (main) river when compared to its tributaries.

Magnetite is the most important mineral that accounts for nearly a half of the susceptibility. Magnetite weight percentage and susceptibility are significantly correlated (r=O.50). About 21 % of the suspectibility is explained by ilmentite and garnets. Amphiboles, pyroxenes and other weakly magnetic minerals account for ca. 27% of the susceptibility. The residual fraction, censtituting ca. 60% of sample weight, explains only 8% of the susceptibility of samples.