ABSTRACT

Silicified evaporite nodules from 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 shared by evaporite-replacement quartz nodules and includes both mineralogical and textural aspects. 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. Proportions of the different textures vary considerably between nodules. Anhydrite inclusions are abundant in the centers of megaquartz crystals. Megaquartz in the nodules is characterized by strongly un ulose, radial extinction. Euhedral terminations often have a cubic appearance. Many nodules contain crusts of zebraic chalcedony on the euhedral megaquartz bordering internal cavities.

Oxygen isotope analyses are useful in understanding the silicification history. Microcrystalline quartz and fibrous quartz types have O¹⁸ values which are heavier than those of megaquartz. The microcrystalline quartz nodules, associated with the silicified evaporites, formed earliest in fluids related to sea water. Length-slow fibrous quartz types formed later in water of intermediate composition. Megaquartz developed at somewhat higher temperatures and in fluids derived from meteoric water. Zebraic chalcedony in the nodules is also of meteoric origin. All silicification of these samples has probably occurred at temperatures of less than 40° C.