Occurrence of in situ thick soil over resistant quartzite and thin soil over more weatherable granitoid gneiss in central Shillong Plateau, NE India

Article Type

Research Article

Publication Title

Indian Journal of Geosciences

Abstract

The central Shillong Plateau comprises dominantly of the Archaean gneiss and Palaeo- to Mesoprotcrozoic quartzitc/ phyllitic quartzitc. The plateau experiences extreme humid conditions, sincc Miocenc-Pliocenc and currently, it is the wettest placc on earth. It is also seismically and technically one of the most disturbed parts on the present Earth. These offer a unique situation to study the weathering of two different quartz-rich rocks. In this contribution wc highlight that the in situ weathering profiles devel¬oped on quartzitc (>92% quartz, 5-8% mica and devoid of feldspar) and granitoid gneiss (>50% quartz, >25% feldspar, >20% mica, ~2-3% acccssory minerals) in a distance of -20km apart, are remarkably different. It is intriguing that ~2-3m thick in situ soil stay at the top of the weathering profiles of quartzitc, a known resistant rock to weathering, whereas, only ~0.5- 0.8m thick in situ soil is capping thick saprolite (-12m) over more weatherablc granitoid gneiss. The presence of ~12m thick saprolitc suggests that thicker soil had developed over gneiss, but stripped off the profile plausibly due to the higher erosion rate. This paradoxical weathering of two different quartz-rich rocks brings to light that, differential rate of erosion and weathering may persist, within a short distance, in a given tectonic setting and climatic conditions.

First Page

337

Last Page

344

Publication Date

10-1-2018

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