Evaporites in time

Uniformitarianism is an essential tenet of geological understanding. Yet, when we look at evaporite volumes and depos-itional settings across deep time, we see that the diversity of modern evaporite analogues is constrained by a deficit in two conditions, specifically; 1) the current lack of greenhouse eustasy; contemporaneous atmospheric conditions and sea levels are controlled by the earth’s current icehouse climate mode and have been for the last 10-12 million years, and 2) the current lack at the plate-edge scale of marine seepage into large hydrographically-isolated oceanic sump basins (Warren, 2010). Both situations circumscribe different hydrologies and eustasies compared to continental-fed in-flows that typify the world's current larger evaporite basins.

Today, and across the Quaternary, the largest and thickest salt stacks, with areal extents up to 10,000 km2 and thickness up to 900 m, tend to precipitate in the lower parts of supra-sealevel intermontane lacustrine sumps located in tectonically active parts of continental interiors, such as Salar Atacama and Salar di Uyuni in the Andean Altiplano. Most ancient evaporites are marine-fed and deposited in huge hydrographically-isolated subsealevel marine-seepage sumps located in intracratonic basins or within rifts or compressional sutures. Often, the areal extents of these ancient systems were more than 250,000 km2, this is more than two orders of magnitude larger than any Quaternary evaporite deposit. Bedded (pre-halokinetic) thicknesses in ancient basainwide/megahalite basins in some localities were more than a kilometre.

Modern (Quaternary) evaporite deposits accumulated during an icehouse mode world-scale climate regime that began as polar icecaps began to develop in the Late Miocene. The volumes and areas of Quaternary evaporites are much less than most ancient salt deposits.

Evaporite deposits are climate sensitive, and across the Phanerozoic tend to form in arid belts located in atmospheric zones beneath the cool dry descending arms of Hadley cells. But the style, mineralogy, and volume of  deposits changes according to times of world-scale greenhouse versus icehouse as well as to times of particular tectonic conditions associated with plate-edge regions being in continent-continent proximity within arid climate belts (Warren, 2010, 2016)

The intensity and frequency of these various eustatic and tectonic features varies across time and so does the volume and type of evaporite deposited at a particular time in earth history




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