ABSTRACT

Sedimentary differentiation is defined here as the process by which initially heterogeneous mixtures of sediments are split into diverse end products with different textural and mineralogical composition. The study of a suite of arkosic sandstones from the Barakar Formation (Lower Permian) of the South Karanpura coalfield, India reveals distinct linear trends of textural evolution when the grain-size-distributions of the sandstones are treated as three-component systems and plotted on triangular diagrams. Similar linear trends of mineralogical evolution are revealed when their compositions are plotted on a quartz-feldspar-clay triangle.

The size triangle shows two converging linear trends, one ranging from a poorly-sorted gravel-sand-clay mixture to a well-sorted coarse sand and the other ranging from a poorly-sorted sand-silt mixture to the same coarse sand end member. The composition triangle again shows two clearly separated linear trends, one starting from a quartz-clay mixture and ending in the pure quartz end (orthoquartzite). The other starts from a quartz-feldspar-clay mixture and ends in a quartz-feldspar mixture (arkose). In both the cases the maturity of the sandstones increases from one end to the other by gradual elimination of clay.

From the above data, and a few other observations the entire history of the sediments from the source rock to the final lithology of the sedimentary rock has been traced.