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Abstract

Caliche crusts are widespread and mainly well developed on siliceous substrates from different localities in Texas. The microbially mediated structures in the caliche crusts are formed in similar ways to those in shallow marine siliciclastic sediments, including through leveling, biostabilization, grain separation, and trapping and binding as well as through biotically induced mineral precipitation.

Caliche crusts, ranging from millimeters to meters in thickness, were formed on alluvial sediments in south Texas, eolian deposits in northwest Texas, and volcanic rocks in west Texas. Those mature crusts generally display three facies; from the bottom upward they consist of a (1) massive horizon, (2) laminar crust, and (3) scattered to coalesced pisoids. Microbially induced sedimentary structures (MISS) are common and particularly abundant in the upper two facies. At the microscopic scale, finely laminated stromatolite-like to concentric structures, alveolar–septal fabrics, and clotted peloidal textures are characteristic microbially induced features. These constituents are composed of filaments, needle fiber calcite, spherulites, peloids, and microrods and nanospheres. The various constituents are the biotically induced products of bacteria, cyanobacteria, algae, and fungi.

Climate has an evident control (i.e., moisture) on the variety and abundance of microbially induced structures in the caliche crusts. Although MISS are readily evident in modern caliche crusts, poor preservation can result in an underestimation of the role of microbes and, therefore, of the reported abundance of MISS in the geologic record.

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