Cluster Analysis Approach to the Problem of Facies Classification of the Carbonate Rocks in the Krol Series

The Krol series in the Sub-Himalayas, include six lithological units, namely Infra Krol, Krol A.B.C.D. and E. Among these, A.C.D. and E. incorporate increasing thicknesses of carbonate rocks from A to E, with minor shale intercalations. Cluster analysis of 150 carbonate samples drawn from these four formations in five stratigraphic profiles between Mussoorie and Lansdowne, based on six petrographic attributes, has shown that the samples cluster into four meaningful groups. Interpretation of these groups reveals that the carbonate rocks of the Krol series fall into two broad facies or depositional mechanisms, one Krol A+C, of micrites and sparites, and the other, Krol D+E of calcicdolo and dolocalcicmicrites, and their variants. The earlier classification of Krol carbonates into four lithological units, A.C.D and E, from a facies point of view, seems to be unjustified. The depositional mechanism of A+C is succeeded by Krol D+E in the vertical profile. This supports the earlier view of Rao that the Krols represent a phase in the development of a basin centre evaporite sequence.