Silicified evaporite nodules from the Mississippian rocks of southern Kentucky and northern Tennessee




Milliken, K. L.

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Silicified evaporite nodules from the Mississippian rocks of south-central Kentucky and adjoining Tennessee typify in many respects silicified evaporite nodules which have been reported in rocks of widely differing ages and diagenetic histories. The silicified evaporite syndrome is an assemblage of characteristics diagnostic of an evaporate-replacement origin. This syndrome is well developed in the nodules from the study area and includes both mineralogical and textural characteristics. The sequence of textures in most nodules begins with interlocking quartzine spherules near the outer nodule edge and ends with a mosaic of equant megaquartz in the nodule center. Variations of the sequence result primarily from varying proportions of the different textures. Anhydrite inclusions are abundant in the centers of megaquartz crystals and in zones in the megaquartz of spherules. Megaquartz in the nodules is characterized by strongly undulose, radial extinction. Euhedral terminations on the megaquartz are often pseudocubic. Many nodules contain crusts of chalcedony or zebraic chalcedony on the euhedral megaquartz bordering internal cavities. The nodules examined in this study can be placed in four categories on the basis of the external characteristics: pit and ridge nodules, cauliflower nodules, spherulite-covered nodules, and beekite nodules. The first two types formed by silicification which began at the outermost edge of the original evaporite nodule and have surfaces which reflect the form of the earlier evaporite nodule. The other two types result from silicification which began away from the edge of the evaporite nodule and have surfaces reflecting the form of the replacement silica. The evaporite nodules which preceded the present quartz nodules formed early in the sediment's history as evidenced by the displacement of bedding and elongate aliochems around the nodule surface. "Exploded" fossils provide evidence that nodule formation was truly displacive. Expansion of the invertebrate skeletons could have been accomplished either by anhydrite formation, hydration of gypsum to anhydrite, or some repetitive combination of both processes. The nodules and their enclosing host rocks differ in several important ways from nodules and sediments found in Holocene evaporitic environments. Modern sabkhas do not provide a strictly analogous model for the formation of these rocks. Perhaps evaporitic conditions were developed only intermittently, causing the formation of the evaporite nodules, but leaving the subtidal character of the sediments essentially unchanged. Fibrous quartz types have [handwritten symbol] 0¹⁸ values which are heavier than those of megaquartz. The fibrous types probably formed earliest in fluids related to sea water; megaquartz probably formed later, at somewhat higher temperatures and in fluids derived from meteoric water. All of the silica in the nodules probably formed directly as some form of quartz, without disordered precursors. There is no known source for silica concentrations sufficiently high to have produced disordered silica types in the nodules.