Paleoenvironmental interpretations commonly cite the occurrence of gypsum associated with nonfossiliferous redbeds as evidence for arid conditions and high evaporation rates. Such interpretations ignore the common occurrence of gypsum in gossan, on weathered black shale surfaces, and especially in Pleistocene glacial deposits. Glaciolacustrine deltaic sequences in Ohio contain abundant large gypsum crystals that formed diagenetically as a result of oxidation of sulfides by calcium-rich, oxidizing ground water. Crystals display two forms related to the host sediment. Euhedral, tabular crystals occur in pliable prodelta mud, whereas bladed, dovetail gypsum sand crystals occur in grain-supported delta-front sand. The diagenetic non-evaporative model could explain the formation of gypsum in numerous sedimentary sequences: dune-tidal flat, beach-lagoon, reef-lagoon, deltaic, and barrier bar-lagoon. Massive, extensive sulfate accumulations probably formed under arid conditions; however, the nonevaporative, diagenetic model should be considered in the interpretation of scattered gypsum crystals and thin gypsum crusts.

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