The recently discovered Fezouata Biota, from the Early Ordovician (late Tremadocian to late Floian) of Morocco, preserves a diverse soft-bodied fauna. While preservation is mostly of Burgess Shale-type, giant anomalocaridids also occur in siliceous concretions. Petrographic and geochemical analyses of these concretions reveal their growth history and the circumstances that led to the fossilization of nonbiomineralized anatomy within them. The large (>1 m) concretions are homogeneous in composition and geochemical characteristics, suggesting rapid, pervasive growth of mineral frameworks during decay of the large animals at, or near, the sediment-water interface. Concretions are comprised of ultrafine-grained (2–20 µm) authigenic quartz, Fe chlorite, and calcite, a composition unlike other described marine concretions. Abundant pyrite, now represented by oxide pseudomorphs, grew adjacent to the anomalocaridid carcasses, but rarely within the matrix of the concretions. This distribution indicates that sulfate reduction around the carcasses was vigorous within otherwise organic-poor sediments resulting in the establishment of prominent chemical gradients around the giant anomalocaridids that led to early precipitation of mineral overgrowth around nonbiomineralized tissues. Rapid precipitation of intergrown silica and Fe chlorite required an abundant source of silica, iron, and aluminum. These ions were likely derived from dissolution of volcanic ash in the sediments. Limited intergrown calcite (δ¹³C avg. −12.2‰, n  =  23) precipitated from bicarbonate that was generated largely by sulfate reduction of organic tissues of the carcasses. Whereas Burgess Shale-type preservation of fossils in the Fezouata biota required suppression of degradation, exceptional preservation of anomalocaridids within the siliceous concretions resulted from extensive microbial decomposition of a large volume of organic tissues. Rapid mineralization was facilitated by localization of microbial activity around the large carcasses and must have required an unusually reactive sediment composition.