Abstract:

Depressions in semiarid regions with a gypsiferous geological substrate commonly contain highly gypsiferous sediments, with gypsum that formed either as a synsedimentary precipitate during perennial or ephemeral lake stages or as a diagenetic precipitate during saline mudflat stages. The dry to ephemeral lake basins of the Monegros region in northern Spain are depressions of this type and provide a unique opportunity to examine whether and how conditions of gypsum formation are reflected by gypsum crystal morphology, yielding information that can contribute to a correct interpretation of the depositional environment of gypsum in ancient and modern formations. In most lake basins of the study area, the sediment fill consists of a similar sequence of three lithological units. The lower unit includes fragmented gypsum crystals in some basins, recording clastic reworking, and in other depressions it contains large lenticular crystals, formed as phreatic diagenetic precipitates. The middle unit typically contains synsedimentary gypsum with a non-lenticular morphology, modified by postdepositional processes, but it also comprises synsedimentary lenticular gypsum in some basins. In the upper unit, separated from the underlying interval by a marked hiatus, both synsedimentary and diagenetic gypsum are present, the former including layered gypsum deposits with features such as grading and variations in crystal morphology. The results show that a non-lenticular crystal habit of gypsum occurring within the groundmass (sediment matrix) indicates synsedimentary precipitation. In contrast, a lenticular crystal habit cannot be used in isolation as an indicator of the environment in which the gypsum initially precipitated. Unambiguously late-diagenetic gypsum includes fine-grained precipitates in voids created by soil fauna. These infillings consist of initially non-lenticular crystals, with a tabular-prismatic habit, recording that conditions promoting lenticular crystal growth do not always prevail outside the groundmass in soil-related settings and illustrating the importance of knowing the context of analyzed gypsum occurrences at the scale of a sample.

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