Calcrete conglomerate, case-hardened conglomerate and cornstone - a comparative account of pedogenic and non-pedogenic carbonates from the continental Siwalik Group, Punjab, India



The occurrence of authigenic carbonates formed in three different environmental situations, within the continental Siwalik Group, has been used to compare the lithological and petrographic characters of the contrasted lithofacies. The three lithofacies are: (1) calcrete conglomerate, (2) case-hardened conglomerate, (3) cornstone (pedogenic, nodular calcrete).

The calcrete conglomerate facies laterally intertongues with the channel conglomerates. It consists of pisolites which are interpreted to have formed from carbonate-rich spring waters emerging on to the gravelly substrate of dry, abandoned channels. The laminae characteristics of these pisolites are distinctly different from those of marine origin and also from comparable biogenic materials.

Case-hardened conglomerate occurs in the youngest part of the Siwalik stratigraphic column, in boulder conglomerates having limestone as the principal clast component. This lithofacies has resulted from cementation of the conglomerate through continued dissolution and re-precipitation of calcite, by meteoric water, downwards from the surface. It displays a coarsely crystalline, sparry calcite cement with no evidence for displacive growth or replacement by calcite.

Cornstones (nodular calcrete) occur in several sedimentary cycles of the Middle Siwalik Sub-Group. These are immature and commonly associated with thinly-bedded sandstones (levée) and red shales (overbank). This lithofacies is a result of concentration of carbonate through capillary action associated with pedogenic activity. Ooids developed in cornstone are essentially micritic in nature and usually composed of less than five indistinct laminae.