Journal of Conference Abstracts

Tufa Deposition in Tropical Setting, Orissa State, India; Insights from Carbonate Petrography, Water Chemistry and Stable Isotopes

Srikanta Das (satpathy@giascl01.vsnl.net.in) & Manmohan Mohanti (satpathy@giascl01.vsnl.net.in)

Dept Geology, Utkal University, Vani Vihar, Bhubaneswar, 751004, Orissa, India

Holocene tufa carbonates have been formed at waterfalls and streams in the warm and humid tropical setting of Orissa, India. Mineralogically the tufas are mostly of low Mg-calcite with local occurrences of aragonite. The occurrence of thin crusts of aragonite in older crusts indicates increase in Mg/Ca ratio in the non-hot-spring waters for short periods.

Stromatolitic tufas are composed of alternate sparry and micritic laminae, in which Phormidium, Scytonema and Shizothrix-like cyanobacteria and diatoms occur. These may represent seasonal deposition. Aragonitic laminae exhibit 'bushy' and spherulitic fabrics, which are possibly microbially controlled. Cryptic microbial structures occur in micritic to microsparry zones. SEM reveals microbial mats formed of cyanobacteria, diatoms and mucilage/EPS, which provide a template for surficial mineralization. These may also initiate crystal nucleation and favour subsequent abiotic growth of crystals. Both calcified and uncalcified filaments are associated which may suggest a microenvironmental control over calcification. Micropores in spar crystals (comparable to 'peccan fabric') encrusting the algal filaments indicate bacterial activity. Precipitation due to bacterial activity may also lead to calcification of associated cyanobacteria. Moss-rich tufas show micritic to microsparry encrustations and microbial films of epiphytic microorganisms. Phytoclastic deposits show carbonate encrustations on detrital fragments of higher plants. Voids in tufas exhibit cement fabrics indicative of vadose diagenesis. Neomorphic fabrics suggest inversion of aragonite to calcite.

The presently flowing tufa-depositing stream waters show a pH value of 8.23-8.89. Concentrations of calcium, magnesium and bicarbonate measure 20-90 mg/l, 1.95-35.14 mg/l and 150-276 mg/l respectively. The saturation index of calcite is in the range 0.39- 1.48. Carbon dioxide outgassing at the turbulent stretches leads to a high degree of supersaturation with respect to calcite. Precipitation starts at 7 times supersaturation.The modern tufas deposited are constituted of calcite. Despite positive saturation index of dolomite (0.79-2.62); the low ionic strength, non-availability of enough active Mg²⁺ , high Ca²⁺/Mg²⁺ ratio and low temperature have inhibited the precipitation of dolomite.

The range of ¹³C values (-12.56 to -2.93‰ PDB) suggest varying degrees of biogenic CO₂ input from the soil zone to the waters. As there is no marine limestone and /or dolomite in the studied area, the data indicate higher values. This is probably due to partial disequilibrium precipitation which is attributed to rapid outgassing where precipitation is faster than equilibrium fractionation of stable isotopes. Photosynthesis by algae and mosses might have locally enriched tufas in heavy isotopes. The ¹⁸O data (-7.16 to -2.00‰ PDB) indicate a low rate of evaporation. Despite disequilibrium precipitation, the approximate thermal condition during precipitation may be constructed considering several simplifying assumptions based on investigation of recent tufas (Pazdur et al., 1988). Based on the global temperature gradient in meteoric water for coastal regions, the observed differences in ¹⁸O in the tufa samples(4-5‰) may indicate a water temperature change of ca. 5 degrees centigrade (Detailed collaborative work with Anna Pazdur, Poland in progress). The variation in ¹⁸O values indicate differences in isotopic composition of the tufa-depositing waters, which is influenced by rainfall and air temperature. This may imply a climatic control over tufa deposition.

Pazdur A, Pazdur MF ,Starkel L & Szulc J, Quat. Res., 30, 177-189, (1988).

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