Origin of Chert Horizons in the Permian Florence Limestone, Flint Hills, Kansas

ABSTRACT The ubiquitous chert horizons found in Permian limestones in the Flint Hills, Kansas, confer the rolling‐hills topography of the region and have been used by prehistoric communities to make tools. The source of silica and mode of formation of these ‘layers’, however, remained elusive. The integration of fieldwork, petrographic analysis, x‐ray diffraction, laser ablation inductively‐coupled plasma mass spectroscopy (LA‐ICPMS), Raman spectroscopy and oxygen isotopic analysis in the silica‐bearing Florence limestone revealed that the chert horizons are coalescing nodules formed during multiple silicification events. Early diagenetic precipitation of opal along burrow pathways was followed by the precipitation of, and replacement by, microquartz, chalcedony and subordinately megaquartz at later stages. The germanium‐silica ratios of 0.4 × 10 6 to 1.25 × 10 6 , obtained for the different silica phases (opal, microquartz, chalcedony and megaquartz), are compatible with a biogenic origin for the silicifying solutions, corroborated by the presence of siliceous bioclasts (sponge spicules and radiolarians) in the limestones. Oxygen isotope data estimated the temperature of precipitation to be between 22°C and 24°C, in agreement with the eogenetic conditions interpreted from the paragenetic relations. The identification of moganite, along with the presence of halite moulds and sulphate nodules in the succession, indicates that evaporitic conditions contributed to silica supersaturation. Silicifying solutions derived from the dissolution of siliceous organisms, augmented by highly evaporative conditions, precipitated initially opal along burrow networks on the Permian seafloor, followed by cementation and replacement by microquartz, chalcedony and finally megaquartz during eogenesis.