ABSTRACT The northern limb of the Garhwal synform features a sequence of rock units belonging to the Jaunsar, Baliana, Krol, and Tal groups. The contact between the Krol and Tal groups marks a significant geological transition within the Garhwal synform. The Upper Krol (Kaudiyala Formation) comprises phosphatic nodules hosted in the calcareous shale, limestone, dolomite, siltstone, and quartz arenite. The Lower Tal (Deo ka Tibba Formation) comprises bedded chert interbedded with black shale and phosphate. Phosphate nodules show well-developed, onion-like whorls and circular depressions. The nodules are greyish-black and break with a conchoidal surface. Petro-mineralogical examination reveals the presence of apatite minerals, pseudo-ooliths, and pellets of collophane. XRD and SEM analysis indicate that carbonate-fluorapatite and apatite are the predominant phosphate minerals. EPMA results indicate that the pyrite grains have iron (Fe) content ranging from 45.96 to 47.50 wt% and sulfur content ranging from 51.96 to 53.97 wt%. Ca-phosphates exhibit P2O5 content ranging from 0.01 to 11.06% and CaO content ranging from 0.04 to 37.48 wt%. Pyrite is categorised into two varieties: syngenetic pyrite and diagenetic pyrite. Syngenetic pyrite forms at the boundary between oxic and anoxic conditions, settling on sediment surfaces when it can no longer remain suspended. Diagenetic pyrites form within the porewater of sediments when the boundary between oxygenrich and oxygen-poor conditions shifts into the sediment layer. The pyrite framboid and microcrystal sizes (Py1 with D5 μm and d≤0.4 μm) suggest reducing depositional environments. The cross-shaped sponge spicules in the phosphate-bearing black shales indicate a crucial role of microorganisms.
Kumar et al. (Mon,) studied this question.