Basinwide evaporites

Basinwide evaporites (aka megahalite, megasulphate, saline giant deposits) are made up of thick evaporite units >50–100 m thick composed of varying combinations of deepwater and shallow water evaporites (Warren, 2010). They retain textural evidence of different but synchronous local depositional settings, including mudflat, saltern, slope and basin (Figure). When basinwide evaporite deposition occurs, the whole basin hydrology is evaporitic, holomictic, and typically saturated with the same mineral phase across vast areas of the basin floor, as in the Dead Sea basin today.

 

The Dead Sea has a more limited lateral scale than ancient basinwides, but currently has halite forming simultaneously as; 1) decimeter-thick chevron-dominated beds on the saline-pan floor of the shallow parts around the basin edge in waters typically less than 1-10 meters deep, and 2) as coarse inclusion-poor crystal meshworks of halite on the deep basin floor that sits below a halite-saturated brine column up to hundreds of meters deep.

 

Ancient basinwide successions are usually dominated by thick massive salt beds, generally more than 100-500 m thick. Deposits tend to made up of stacked thick halite beds, but can also contain substantial volumes of thick-bedded Ca-sulphate and evaporitic carbonate, as in the intracratonic basinwide accumulations of the Delaware and Otto Fiord Basins.

 

Owing to inherent purity and thickness of the deposited halite, many halite-dominant basinwide beds are also remobilized, via loading or tectonics, into various halokinetic geometries. Some basinwide systems (mostly marine-fed intracratonic settings) entrain significant accumulations of marine-fed potash salts, as in the Devonian Prairie Evaporite of western Canada. In contrast, all Quaternary examples of commercial potash deposits are accumulating in continental lacustrine systems.

 

In the Phanerozoic, the period from the Permian through Jurassic generated the greatest volumes of both halite and total evaporites. This corresponds to the time of the assembly and disassembly of the Pangaean supercontinent. A supercontinent is the culmination of a mega-longterm (300 – 600 million year duration) tectonic pattern of continental accretion and fragmentation, whereby a supercontinent gradually assembles then rifts and drifts apart prior to reassembling (in a repeating tectonic cycle known as the Wilson Cycle).

 

Substantial accumulations of basinwide evaporites are most likely to accumulate on the floors of isolated subsealevel depressions at times of close proximity of drifting landmasses. This occurs both during the building of the supercontinent and then during times of its early disassembly. Ongoing marine drawdown incursions into marine-fed arid-zone subsealevel basins takes place both as a supercontinent assembles (collision and foreland basins) and then as it unzips (rifts and sag basins). Intracratonic evaporite basins, The third setting of substantial ancient salts deposits typically form as intracratonic deposits in the continental interior in zones of distal crustal flexure in response to tectonic interactions at the continent margins.  Smaller scale deposits can accumulate in intracontinental transform sags as in the modern Dead Sea.

 

When estimates of the volume of salt being recycled back into the oceans are added to volumes of actual salt in the various megahalite beds of the world an interesting picture emerges of actual precipitated salt volumes at various times of enhanced evaporite deposition (Hay et al., 2006). The greatest volumes of halite still remnant in sedimentary basins are defined by Jurassic and Cretaceous successions. Deposition was largely within the circum-Atlantic salt basins and tied to the early rift to drift transition during the opening of the Neo-Tethys and the northern and the Southern Atlantic and the disassembly of Pangaea. Yet when recycled halite estimates are added, these Mesozoic deposits are lesser volumes compared to the Permian and Neoproterozoic/Cambrian successions, with the greatest volume of salt deposited across the past 600 million years occurring in the various Neoproterozoic-early Cambrian Hormuz and Ara halite deposits of the Middle East.

 

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