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Abstract:

The lower beds of the Miocene Intermediate Unit in the southern and southeastern parts of the Madrid Basin, central Spain, are formed mainly of mudstones, marls and gypsum. The mudstones contain euhedral gypsum crystals and are interpreted as the sediments of a saline mudflat environment. Two main types of gypsum deposit have been identified: (1) chemical precipitates, which include selenite and lenticular gypsum crystals, and (2) detrital gypsum grains. The latter form prominent deposits throughout the study area and display a wide variety of hydrodynamic structures, including scours, wavy bedding, parallel stratification, ripple lamination, parting lineation, graded bedding, and low-angle cross-stratification. The detrital gypsum beds are formed of sand-sized, hemipyramidal, lenticular or prismatic crystals that commonly exhibit abraded and/or partially dissolved edges.

We recognize four facies associations. The first, formed of mudstone and gypsum-bearing mudstone, is a typical saline mudflat association. The second consists of detrital gypsum deposits interbedded with gypsum-bearing mudstones. In this association, the detrital gypsum is interpreted as sheetflood deposits derived from the reworking of the mudflat facies during periods of heavy rainfall. The other two facies associations comprise different types of detrital gypsum deposits that are interbedded with marginal and/or more central, lacustrine evaporite facies. These facies associations allow us to characterize the different sedimentary subenvironments throughout a saline mudflat-saline lake complex.

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