ORIGIN AND TIMING OF CHERT NODULES IN THE PERMIAN FLORENCE LIMESTONE IN KANSAS

This study investigates the origin and timing of chert nodules in the Permian Florence Limestone in Kansas to determine the mineralogy, crystallinity, and paragenetic relations of silica within the host rock, a bioclastic wackestone with abundant brachiopods, bryozoans, corals, gastropods, and calcispheres (rarely sponge spicules). Understanding the formation of chert is important not only because of its use in prehistoric tools such as spear- and arrowheads but also because some of the earliest lifeforms on Earth were preserved in chert. Additionally, the presence of chert nodules in the limestones controls the topography due to its low susceptibility to weathering, generating the rolling hills topography characteristic of the Flint Hills region in Kansas. Field studies indicate that the nodules are 4- to 6-cm across and up to several feet long, as they are distributed along horizontal, bifurcating layers, locally connected vertically, forming networks that closely resemble Ophiomorpha burrows. The nodule boundaries are either diffuse or abrupt, with different internal textures (homogeneous, rimmed, or wood-grained). The nodules are composed of three distinct phases of silica: opaline, microcrystalline quartz (chert) and chalcedony, and rarely minute (~2.5 µm) bipyramidal quartz. Paragenetic relations indicate that opaline silica extensively replaced micrite and was later replaced by chert, which also partially or completely replaced carbonate bioclasts. Chalcedony spherulites locally replace chert. Calcite rhombs replace both the original micrite and diagenetic silica. Dissolution pores are partially filled with silica rosettes, and rock fractures with silica rosettes and chert. Our petrographic analysis, thus, demonstrates that the Florence Limestone underwent a multiphase silicification, starting with the precipitation of opaline silica along burrow pathways during eodiagenesis, possibly under evaporative conditions. Later phases may have resulted from the percolation of deep-burial solutions and/or recrystallization of metastable opaline silica into more stable phases during mesodiagenesis.