The Great Rann of Kutch (Kachchh), India

The Great Rann of Kutch (Kachchh) is a marine-influenced evaporitic sump some 300 km long and 80−100 km wide on the margin of the Thar Desert in arid western India. In India's summer monsoon, this flat desert of salty clay and mudflats, which averages 15 meters above sea level, fills with standing water. In very wet years, a wetland extends from the Gulf of Kutch on the west through to the Gulf of Cambay on the east. In the dry season the wetland transform into a series of saline pans covered with salty effloresences and halite crusts. Todat the Rain occupies one of the hottest areas of India – with summer temperatures peaking at 49.5 °C. Winter temperatures reduce dramatically and can go below 0 °C.

The intraplate position, south of the Himalayan suture, creates a unique arid capillary salt landscape inundated during the monsoon by wind-driven marine storm surges, sourced on its seaward side. The Kutch depression is bound by the Nagar Parker Fault (NPF) on the north and Kachchh Mainland Fault (KMF) on the south. Capillary evaporitic sedimentation in the Great Rann occurs in a siliciclastic matrix with deposition controlled by an interplay between tectonic, continental and marine processes. IN the early Holocene, the area was a vast shallow of the Arabian Sea until continuing geological uplift closed off the hydrographic connection with the sea, creating a vast lake that was still navigable during the time of Alexander the Great. The Ghaggar River, which presently empties into the desert of northern Rajasthan, formerly emptied into the Rann of Kutch, but the lower reaches of the river dried up as its upstream tributaries were captured by the Indus and Ganges thousands of years ago. 

Glennie and Evans (1976) concluded that the Great Rann was probably occupied in the Early Holocene by a shallow marine gulf following the rise in sea level after the last glacial epoch, whereas continental sediment influx dominated during the transgressive phase. According to them, the clay mineral matrix assemblages are dominated by illite, kaolinite, chlorite and montmorillonite, with little or no carbonate, suggesting the dominant influence of the Indus River drainage system on the matrix mineralogy. They argued that most of the matrix was derived from the combined Indus and Nara Rivers, which extended southwards towards the present Indus delta, with some of its waters escaping into the Arabian Sea via Kori Creek. There is also the possibility that an ancient mighty river, called the Saraswati, received water runoff and sediment from the Himalaya and once flowed into the Great Rann of Kutch.

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Great Rann of Kutch.

The sabkha-capped Western Great Rann is known for its tectonic instability; this has created numerous episodes of liquefaction and seismite overprint in the sediments of the Rann as evidenced in the 2001 earthquake. As well as causing severe structural damage, this 7.7 magnitude event killed more than 13,000 people, caused collapse or severe damage to over a million houses and an economic loss in the region of US$10 billion. Paleoseismological investigations in the vicinity of the earlier 1819 Allah Bund earthquake indicate the occurrence of an even older earthquake around 800–1000 years ago, both relate to the surface expression of a growth anticline atop an active (but hidden) thrust fault.

Much of the halite crust is recycled each year via flooding by marine waters and so does not attain any great thickness in the sediment column. Halite layers with thicknesses of 90-120 cm were, however, described by Oldham (1893) from near Sindree. The lenticular gypsum that has grown within the sediment will probably remain and may gradually increase in percentage with time to preserve a displacive salt sabkha signature in the more evaporitic sediments.


Pan edge of Rann of Kutch

Although the total proportion of the Holocene sediment volume in the Rann of Kutch is that is made up of capillary gypsum is less than 5-10% it is an interesting marine-associated evaporitic example for a number of reasons. It is a saline sedimentary system, formed by hydrological changes superimposed on tidally-influenced marine clastics, which are the primary signature in the matrix (mostly tidal flat heterolithics with some tidal channel sands).

There were no evaporitic salt beds at the time the tidally-influenced matrix of the Rann of Kutch saline flat was deposited. The transition to a capillary evaporite hydrology, came about some 1000-2000 years ago, as a diagenetic evolution driven by tectonics, via the formation of a hydrographic barrier tied to the early stages of the growth of the fault-controlled Allah Bund high. This makes it an interesting example of a Holocene marine sabkha created by tectonics, not eustasy. A significant question generated by its current sedimentological scenario is how we would recognise an ancient counterpart? The pervasive seismically-induced liquefaction that occurs in the Kutch sediments with each significant seismic event (every hundred or so years) probably means little or no primary matrix structure (tidal sedimentation) will be preserved in ancient counterparts. What would be preserved is likely a few anhydrite nodules (after gypsum) floating in a relatively structureless or intensely seismite-overprinted siliciclastic matrix (see Warren, 2016; chapter 3 for detail).


Overview of Rann of Kutch, India

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