Petrography And Geochemistry Of Shallow Subsurface Sediments Of A Part Of The Brahmaputra Valley In Assam, India

The Brahmaputra Valley comprises very thick Quaternary alluvium in Upper Assam. The alluvium had been drilled for coring up to about 50m at four sites, viz. Dhola, Dum Duma, ChotaTingrai and Naharkatiya, which are almost equally placed in a north-south direction from the river Brahmaputra towards south close to the foot of the Naga Hills. From the sediment cores of each bore hole about seven thin sections had been prepared at equal intervals by impregnating the loose sediments with binding resin in undisturbed condition for petrographic study. The petrographic study reveals that quartz grains are angular to sub rounded, elongated to equidimensional in nature and of variable in shape and size. The detrital minerals in the sediments indicate it to be lithic arenitic in composition. QFL triangular plot infers that the sediments of two wells were derived from recycled orogenic sources. Q_mFL_t triangular plot shows that the provenance of the sediments of Naharkatiya was only transitional recycled, those of Dhola was quartzose, transitional, lithic recycled and mixed, those of Dumduma and ChotaTingrai was transitional and lithic recycled, mixed and dissected arc. The plot of the recalculated percentages of undulatory, non-undulatory monocrystalline and polycrystalline quartz (> 3units) in Diamond diagram indicates that most of the sediments were derived from plutonic and metamorphic sources except in Naharkatiya sediments where the contribution from plutonic sources is not evident.

Geochemical analysis of sediments of each well was carried out from six different depths to estimate particle density, bulk density, pH, LOI, acid insolubles, Ca, Mg, Na and K. Most of these parameters show the highest values in the sediments of ChotaTingrai well.