Abstract

Potash evaporites of the lower Miocene Vorotyshcha Suite are sandwiched in salt breccias in the Ukrainian part of the Carpathian foreland basin. These breccias have traditionally been interpreted as tectonically deformed units related to the overthrust nappes of the Carpathians. Most clasts consist of intrabasinal sandstone, marlstone, halite, and anhydrite, but exotic clasts consisting of rocks that are characteristic of the Carpathian flysch rocks are also present. Breccia beds are intercalated with salt layers containing chevron crystals and cubic hopper crystals, and some layers are composed of detrital salt. These salt layers rarely display graded bedding or cross-lamination, and continue over great distances. There are also slump structures associated with the bedded salts. The salinity in the Vorotyshcha Basin was extremely high, but intermittent inflows caused brine dilution so that it could maintain halite precipitation for a long time. During long periods of evaporative concentration and reduced inflow, brines in the basin could reach the sylvite saturation level so that potash chlorides and sulfates started to precipitate. The potash precipitated subaqueously. On the basis of evidence from the study of fluid inclusions, the depth of the brine is interpreted to have been 20-30 m, in agreement with the supposed brine stratification in a basin subjected to influxes of sediment-laden waters. The breccias are debris-flow deposits. They are commonly tectonically deformed, but analysis of the relation of breccias to evaporites suggests that both were formed in water ten to a few tens of meters deep. It therefore seems that halite breccias in other foreland basins could have had a sedimentary origin.

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