Weathering of charnockites and sediment production in the catchment area of the Cauvery River, Southern India

Abstract

In the Cauvery catchment area, charnockites are making high hills compared to gneisses. In situ, static weathering of a charnockitic boulder shows lesser extent of chemical weathering relative to that developed into a profile. This difference is seen both in clay mineral composition, their REE patterns and in the A-CN-K and A-CNK-FM triangular plots. Based on the chemical changes relative to Ti, three groups of elements are recognized. Ba, Si, K, Na, Sr, Ca and Mg show increasing depletion as the weathering intensity increases in listed order. Al remained relatively immobile, whereas, Fe followed by Mn, Cr, Ni, REE and P show a variable extent of enrichment with increasing chemical weathering. The sediment occurring on the foothills of the charnockite hills have a very uniform composition and texture without any depositional features. Reworking of these sediments in a first order stream has caused only minor compositional changes by the local segregation of heavy minerals on the channel bed and by the removal of small amount of clay materials as suspended load. The REE pattern of the reworked channel bed sand is nearly identical to that of the channel wall sediments, but with lower (by 25%) abundance. Although the rocks are >2500 Ma and, now located in tropical latitudes and probably had experienced several periods of warm and wet climate, the soils and the sediments derived from them are chemically very immature suggesting their exposure to denudational processes very recently by tectonic uplift.