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Petrology of caliche-derived peloidal calcirudite/ calcarenite in the late triassic maleri formation of the pranhita-Godavari valley, South India

1988; Elsevier BV; Volume: 55; Issue: 3-4 Linguagem: Inglês

10.1016/0037-0738(88)90134-0

1879-0968

Soumen Sarkar,

Soil erosion and sediment transport

The Maleri Formation (Late Triassic) of the Pranhita-Godavari Valley, South India, is a continental red-bed sequence represented mainly by red clays with a few sheet-like bodies of channel-fill sandstone. Crudely bedded and cross-bedded peloidal calcirudite/calcarenite occurs at the base of fining-upward sequences in multistorey sand bodies and also as discrete solitary lenses within red clay. The calcirudite/calcarenite consists mainly of calcite-cemented spherical peloids which are made up of micrite and microspary calcite with microfabrics characteristic of caliche. Pure calcitic peloids are often reddened and those having a quartzose component are generally not red. The paucity of broken and abraded peloids, close association with intraformational materials and other evidence indicate a local source of the peloids. The microfabric of the peloids suggest their derivation from caliches. Available evidence favours a pedogenic origin for the peloids which mostly developed by displacive precipitation in a set-up of alternate wetting and drying. The lack of in-situ caliche profiles in the Maleri sequence and the rarity of compound grains imply that the peloids were derived from incipient caliche profiles which were localized at or near the surface and were completely stripped off by subsequent erosion. The ghost caliche profiles suggest the presence of periodically stable levels in the Maleri alluvial plain and low to moderate rates of alluviation. The residence intervals of channels were long enough to allow incipient pedogenesis in the temporarily stable inactive areas. In the present context, the predominance of smectite in fines, the poor floral content, the Maleri faunal assemblage and the paucity of evaporites point to a low seasonal rainfall in a semi-arid climatic environment.