HOLOCENE MICROBIAL TUFAS - ORISSA STATE, INDIA

Holocene microbial tufas, dominantly of stromatolitic crusts, have bee n formed in a tropical setting of Orissa State, India These occur as s tratified crusts in various dispositions and are also associated with moss-rich deposits. Morphologically, these vary from planar laminated crusts to columnar growths. Stromatolitic tufas are built with alterna te lighter sparry and relatively darker micritic laminae in which poss ibly 'Phormidium', 'Scytonema' and 'Schizothrix' occur. Fibrous aragon ite also occurs in microbial crusts radiating normal to laminae and at places forms bushy and spherulitic fabrics. Other microorganisms invo lved are diatoms, coccoid cyanobacteria, fungi and (?) bacteria. Stabl e carbon and oxygen isotope values indicate precipitation from ambient temperature waters, possibly in partial disequilibrium condition. A b iogenic contribution from the soil zone to the carbon pool seems very likely. The deposition is largely effected due to inorganic precipitat ion. Calcification of the microbial community due to heterogeneous nuc leation and trapping by mucilaginous cyanobacteria has resulted in sur ficial mineralization. Precipitation differences associated with cyano bacteria have formed alternate sparry and micritic laminae. These may indicate a seasonal variation in deposition.