Seismites in Miocene gypsum microbialites: Multiproxy tools for paleoenvironmental reconstruction of saline lakes

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This work provides the first evidence of seismites within Miocene gypsum microbialites deposited in inland lakes. Seismites are observed on good quality outcrops in two separated successions of the Madrid and Duero basins (central Spain) that are sited nearby strike-slip faults. The successions consist of sandy gypsum facies interbedded with gypsiferous mudstone and marlstone. The association of microbial induced sedimentary structures (MISS) with mineralogical biosignatures such as intrasediment-grown lenticular gypsum, dolomite with inner biomolds and other biological induced minerals, supports that gypsum beds represent microbialites. Gypsum microbialites are characterized by parallel-lamination, cross-bedding and laminated leveling structures. The rapid mineralization of the microbial mats and biostabilization processes may play a role in the preservation of the depositional structures in sulfates. In addition to MISS, several categories of large-scale soft-sediment deformation structures (SSDSs) reflecting ductile and brittle behavior repeat vertically through the successions. These are classified as load casts, pillows, dish structures, convolute bedding and lamination, plastic intrusions, fluid escape tubes, mud/sand volcanoes, Neptunian dikes and fractures. The deformation structures in crystalline sediments are interpreted to be caused by seismic shocks of magnitude over 5 related with syndepositional fault movements. Micromorphological analysis of gypsum beds reveals distinctive features such as cracked crystals, oriented crystals filling dikes and horizontal microstylolites that also indicate seismic shocks. Comparison between the two basins shows some differences in the type and vertical distribution of structures, and this suggests that earthquakes were less frequent and of higher magnitude in the Madrid Basin. The results confirm that microbial gypsum deposits have both high susceptibility to earthquakes and high potential to preserve a wide range of information of the sedimentary environment, including paleoecology, paleogeochemistry, paleohydrodynamics, and seismogenic behavior of microbial sediments and faults
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