Motivation

Morphological and sedimentological evidences indicate that the deserts of the world have experienced significant episodic expansions and contractions during Quaternary period. During this period, the sedimentological record of arid zone expansion and contraction is often complemented by morphological evidences, in the form of features such as degraded or fossilized sand dunes, alluvial fans and paleo-lakes. Like dunes, evaporite deposits in paleo-lakes are an important component of many arid and semi arid environments.
Given the nature of hydrological input in an arid and semi arid region, it is not surprising that many are characterized by endorheic drainage and lack integrated surface drainage. Under these circumstances surface depressions become important local and regional foci for accumulation of water in lakes. Frequently these water bodies, termed as playas are highly saline and in some cases supersaturated with salt. The precipitation of these salts in playa environments can result from variation in evaporation rates and surface water conditions.
Although the minerals in playa salt deposits are somewhat dependent upon chemistry of inflowing waters, differences in evaporite deposits can yield information about the degree of salinity (palaeo-salinity) and hence evaporation rates during past period of playa sedimentation.

The Thar desert, located in the Rajasthan state of India and parts of Pakistan is the home to a number of saline playas (lakes), the origin of which is one of the most debated topics of Quaternary Geology of India. These commercially important playas in this part of the world vary in size from very small depressions of few tens of square meters to massive basins which may exceed hundreds of km². These playas have been ascribed a great variety of origins, ranging from simple desiccation of depressions filled by seasonal rainfall, particularly under humid paleoclimatic conditions, to tectonic subsidence, aeolian deflation, segmentation of palaeochannels by mobile sand dunes or excessive siltation at the river confluence due to the onset of aridity. Given the range of origins and scales these basins display a bewildering degree of variability in characteristics such as mineralogy (evaporites) and geochemistry.

Location map showing Thar desert, India.

This Ph.D thesis has been designed to report the different assemblages of evaporites present in the playa sediments of Thar desert, India. An attempt is being made to understand the different phases of water level condition, to infer the source of sediments of the playas and to understand the Holocene climatic change of the region from the integrated study of mineralogy, trace and major elements, and isotope (C, S, O) geochemistry of playa sediments of the region.

The brine is being used for commercial salt production	Evaporites precipitating from the hyper saline brine water in one of the playa lake, Thar desert