Stratigraphical and sedimentological studies of the tura sandstone near Tura Garo hills Assam India

Abstract

Tura and its neighbouring areas of Garo-hills district, Assam are geologically mapped. The area is covered by metamorphic rocks intruded by granite and other basic dykes and sills. These are unconformably overlain by sedimentary formations. Detailed petrological and chemical studies of these metamorphic and igneous rocks are made. The Tura sandstone formation which forms the main bulk of the sedimentary rocks is studied in detail. Grain/size composition and detailed petrographic studies are made. Heavy mineral frequency and other parameters are sta-tistically measured. The fossil contents of the Siju lime-stone stage, overlying the Tura formation, are identified and their age determined. The Rewak alternation stage, which overlie the Siju stage and consists of sandstone and shale is also studied in detail to distinguish them from the Tura sandstone formation. The metamorphic and the granitic rocks are correlated with rocks of the Shillong series and the Mylliem granite Shillong plateau respectively, while the basic dykes and sills are shown as Middle Jurassic in age. It is also shown that the Tura formation was depo-sited in river mouth and flood plains, whereas the Siju and the Rewak stages are deposited in deep sea marine condition and in a fluctuating regressive sea condition respectively. Further, it is shown that the Tura sandstone formation of the present area was deposited within a span of time from Upper Palaeocene to part of Lower Eocene.