Abstract The origin of peloidal textures common to submarine substrates lithified by magnesium calcite has been a subject of controversy. A review of the proposed origins of this intriguing and diagnostic feature of magnesium calcite submarine cements reveals four suggested sources for these peloids—algal, sediment, replacement and precipitate origins. A reconsideration of both the petrographic and chemical characteristics of these peloidal textures indicates that they probably originate primarily in chemical processes that may involve repeated nucleation around centers of growth. Several factors support this hypothesis: the peloids have a generally limited size range; their shape is typically spherical; the best-developed peloidal textures occur in restricted microcavities; peloidal textures exhibit well-developed zonation in many cement crusts; peloids and associated magnesium calcite rim cements have a similar chemical composition; and textural and depositional fabric characteristics are similar in magnesium calcite peloids and in precipitated spheroidal aggregates of pyrite. The pelletizing action of cryptic organisms and calcification of algal filaments also play a role—although a somewhat more limited one—in the formation of a peloidal texture in these magnesium calcite cements. With the recognition that some calcite peloidal textures can be formed chemically, caution is urged in interpreting the origin of “pelleted” limestones.

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.