CALCRETES IN SEMIARID ALLUVIAL SYSTEMS - FORMATIVE PATHWAYS AND SINKS

Late Quaternary deposits in Gujarat, western India show an abundant de velopment of calcretes. Three major sinks of carbonate in the alluvial deposits are recognized: (1) groundwater calcretes, (2) pedogenic cal cretes, and (3) calcrete conglomerates. Groundwater calcretes originat e from carbonate-saturated waters travelling preferentially along stra tification planes. Pedogenic calcretes form through soil-forming proce sses 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 calcre te to calcrete-conglomerate. The formation of pedogenic calcretes over sediments containing groundwater calcretes demonstrates that (1) apar t from aeolian dust, river waters are also a major source of carbonate , and (2) pedogenic carbonates may attain large sizes at accelerated r ates due to the presence of pre-existing groundwater calcretes. Conseq uently, the maturity of a soil may be overestimated if determined by f ollowing established morphogenetic sequences. (C) 1998 Elsevier Scienc e B.V. All rights reserved.