Calcretes in semi-arid alluvial systems: formative pathways and sinks

Late Quaternary deposits in Gujarat, western India show an abundant development of calcretes. Three major sinks of carbonate in the alluvial deposits are recognized: (1) groundwater calcretes, (2) pedogenic calcretes, and (3) calcrete conglomerates. Groundwater calcretes originate from carbonate-saturated waters travelling preferentially along stratification planes. Pedogenic calcretes form through soil-forming processes typically in extra-channel areas. Calcrete conglomerates occur as ribbons, sheets and lenses due to the reworking of both pedogenic as well as groundwater calcretes. As a result a pathway of calcretization develops that has the route: groundwater calcrete to pedogenic calcrete to calcrete-conglomerate. The formation of pedogenic calcretes over sediments containing groundwater calcretes demonstrates that (1) apart from aeolian dust, river waters are also a major source of carbonate, and (2) pedogenic carbonates may attain large sizes at accelerated rates due to the presence of pre-existing groundwater calcretes. Consequently, the maturity of a soil may be overestimated if determined by following established morphogenetic sequences.