Ediacaran sedimentary successions are noted for the preservation of microbes and microbial textures on the surfaces of sandstones and siltstones. Although microorganisms have been preserved in coarse-grained siliciclastic sand throughout geologic history, the exceptional preservation of microbes in Ediacaran sediments suggests the potential for a unique taphonomic window. Here, we identify conditions conducive to the fossilization of filamentous cyanobacteria growing in the presence of siliciclastic sand and demonstrate that the sheaths of filamentous cyanobacteria can become coated by clay minerals within days under oxic conditions. Smooth, extensive mineral coatings develop in the presence of 5.6 to 55.6 mg/L of suspended clay and 0.1 mM or greater concentrations of dissolved silica. Thus, elevated concentrations of seawater silica and the delivery of suspended clays promote microbial preservation on sandy and silty surfaces. These factors likely facilitated microbial fossilization in coarse-grained siliciclastic sand throughout the Ediacaran Period and may have also contributed to microbial fossilization in siliciclastic deposits at other times throughout Earth’s history.