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Abstract

A study of individual beds of Devonian salt at Midland, Michigan, was based on descriptions of cores from closely spaced wells where few accurate subsurface data have been available. Many cycles of evaporites were defined. Cores show that salinity of depositional environments gradually vacillated from highly saline to nearly normal, resulting in gradational successions of evaporite lithofacies both vertically and horizontally. Interruptions of the sedimentary cycles, shown by missing facies, were minor, considering both map position and time.

Some of the depositional cycles have been correlated from the southeastern edge of salt beds of the Detroit River Group in east-central Michigan to the limit of preserved salt in the westernmost part of the lower peninsula of Michigan. Tentative correlations of some sulfate facies have been extended eastward to Ontario.

Conclusions are that: (1) many of the cyclic sections can be correlated; (2) correlation by cycles is a valuable adjunct to correlation strictly by lithology, and in many cases it is superior; (3) evaporite maxima and minima of correlative cyclic sections record nearly parallel time lines; (4) convergent lithologic boundaries are crossed by nearly parallel time lines in repeated instances; (5) with each cycle, space in the depositional basin was filled with evaporite lithofacies appropriate to the local salinity gradient and to map positions of gradients; (6) facies predominantly of halite changed laterally to sulfate rock with little or no halite; and (7) these strata were deposited “synchronously” and in nearly equal thicknesses across distances as great as 38 mi (61 km).

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