Abstract
Holocene tufas of Orissa State, India, are formed in stream/fluvial environments. Sparry and micritic laminae in stromatolitic tufa represent seasonal deposition and contain cyanobacteria and diatoms. Micropores in spar crystals suggest bacterial activity. Aragonitic laminae show radial, bushy, and spherulitic fabrics which may be microbially controlled. Phytohermal (moss-rich) tufas show micritic to microsparry encrustations and reveal epiphytic microorganisms. CO2 degassing due to agitation from waters supersaturated with respect to calcite, is the dominant process of precipitation. Aragonite precipitation is possibly controlled by Sr concentrations in the waters. Pisoids have been formed by alternate disposition of micritic and microsparry laminae and sediment-rich zones around intraclastic tufa. Vadoids show development of fine cortical laminae around spherulites and radial needles. Microfabric of speleothem-like crusts suggest precipitation of finer crystallites and their subsequent transformation to a coarser fabric. Rhombic, needle and spike-like cements and neomorphic spar (inverted from aragonite) are developed in a vadose setting. Aragonite to calcite inversion is supported by texture-chemistry variance. The tufas have been formed in three depositional environments (1) waterfall environment, (2) slope environment and (3) river channel margin environment.
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