Post-Depositional Transformations during Burial and Exhumation in the Neoproterozoic Evaporite Sequences, NW Himalaya, India

The Kashmir and Chamba sub-basins of the Proto-Tethys were the sites of wide spread evaporitic deposition along the northern most part of the Indian passive continental margin during the Neoproterozoic. The evaporite lithologies of the Kashmir sub-Basin in the form of layered gypsum and nodular gypsum show deformational features and largely, secondary textures. Evidence of anhydritization and compaction/deformation is present in the form of anhydrite porphyrotopes, alignment of the crystals in a plane (recrystallization) and flowage layers (the result of plastic deformation). The original sulphate precipitates have passed through stages of burial diagenesis, metamorphism and exhumation, with formation of alabastrine and porphyrotopic sulphate. The exhumation of the sulphate sequence resulted in the development of overgrowths, dissolution along the relict anhydrite crystal boundaries, and the formation of lenticular gypsum crystals.

The Chamba sub-Basin contains a more complete example of evaporite sedimentation including carbonates, sulphate and chloride sedimentation. The sulphate sequences here show alabastrine, porphyrotopic and ameboid textures suggesting dehydration and rehydration reactions simlar to Kashmir sub-Basin. The fibrous satin-Spar gypsum occurring in the fractures of the carbonates most likely formed by the gypsum-Saturated brine released from dissolution and reprecipitation of near surface sulphate evaporites.

The rock salt in the Chamba sub-Basin is comprised of banded halite together with variable amount of clay and other impurities. The halite consists of interlocking crystals with zoning and irregular internal outline due to burial and subsequent recrystallization. The ragged outer edges and coigns of the zoned crystals, and brecciated texture are suggestive of groundwater dissolution during exhumation.

The microthermometric experiments on single-Phase fluid inclusions occuring in the crystals demonstrate that fluid inclusions were formed around 45°C, the temperature at which anhydrite rehydrated to gypsum and halite reprecipitated during exhumation.