Calcretes from the Quaternary alluvial deposit of Purna basin, central India: Lithological and climatic controls

Abstract

The Purna alluvial basin of central India, infilled largely with areno-argillaceous sediments of Quaternary age, is also marked by occurrences of both pedogenic and non pedogenic calcretes of diverse shape and size. The previous includes nodular, laminar, hardpan and root calcretes along with rhizolith balls and rhizospheres, whereas non pedogenic type is represented by groundwater calcretes. These calcretes have been investigated for their morphological details, lithological associations in both vertical and lateral profiles of the alluvial deposit and paleoclimatic reconstructions. This study shows that the area under investigation has undergone through subtropical, arid and semiarid climatic conditions having low to medium humidity, high rainfall and evaporation that favored formation of various types of calcretes. Nodular calcretes, the most dominant morphotype, are preserved in a wide spectrum of lithology and fluvio-lacustrine environment. To some extent, their formation is controlled by parent material, irregular rainfall and mild temperature. Rapid accumulation of carbonate material, in and around the plant roots giving rise root calcretes of various shapes and size is a common phenomenon in warm and dry climatic conditions. Both hardpan and groundwater calcretes are formed due to CaCO3 precipitations in humid condition having heavy rainfall. Besides, the later also indicates arid to semiarid conditions facilitating the movement of Ca rich solution in the unconsolidated sediments through capillary action that later deposited calcium carbonate in the voids during wet and dry conditions. Well preserved rhizolith balls and rhizospheres in the palaeosols, constituting together the lower part of succession in almost entire alluvial basin, acts as potential paleoclimatic indicator.