Microtextures on detrital quartz grains of upper Maastrichtian-Danian rocks of the Cauvery Basin, Southeastern India: implications for provenance and depositional environments

摘要

Quartz grains from the Kallamedu (late Maastrichtian) and lower Niniyur (Danian) Formations, Cauvery Basin, Southeast India were examined under a scanning electron microscope, and twenty three distinct microtextures were observed. These microtextures have been grouped into three modes of origin, viz. mechanical (thirteen features), mechanical and/or chemical (five features) and chemical (five features) origins. Quartz grains from the Kallamedu and lower Niniyur Formations show conchoidal fractures, straight steps and arcuate steps which are the characteristic microtextures of quartz grains derived from crystalline source rocks. Quartz grains from the upper Kallamedu and lower Niniyur Formations show angular to subangular outline, whereas those from the lower Kallamedu Formation show subangular to rounded outline. The dominance of angular to subangular grains and the presence of straight and arcuate steps suggest that these clastic sediments were undergone short transportation and rapid deposition. The presence of rounded grains in the lower part of the Kallamedu Formation suggests that significant amounts of quartz grains are of recycled origin. Vs, straight scratches and curved scratches, the characteristic features of marine environment, are common on the quartz grains from the lower Niniyur Formation, which is consistent with the previous interpretation. However, quartz grains from the lower Kallamedu Formation exhibit very low frequency of these features, whereas they are moderately present in the upper Kallamedu Formation, suggestive of a change in depositional conditions with time. The lower Kallamedu Formation exhibits planar and trough cross-beddings, which suggests the fluvial depositional environment. The unfossiliferous upper Kallamedu Formation contains appreciable amount of illite and mixed-layer (illite/smectite) clay minerals, which suggests that the saline conditions prevailed during its deposition. Microtextures on the quartz grains coupled with dominant clay mineral types suggest the deltaic depositional environments for the upper Kallamedu Formation.