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

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Research Article

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Indian Journal of Geosciences


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.

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