Deposition and dolomitization of Upper Knox carbonate sediments, Copper Ridge district, East Tennessee

Abstract

The upper part of the Cambro-Ordovician Knox Group in the fault-bounded Copper Ridge zinc district, East Tennessee, consists of interbedded limestones and dolostones, with very minor sandy and cherty horizons. The nature of allochems suggests that the limestones formed in marine peritidal environments. Other features, such as granular interallochem cements, dolomite inclusions in calcite, low Na content, and low Sr/Cr ratio, suggest stabilization of the limestones in the presence of fresh water. There are two textural varieties of dolostone: (1) fine grained and (2) medium to coarser grained. Fine-grained dolostones, composed of unzoned 0.02- to 0.05-mm dolomite crystals, represent penecontemporaneous dolomitization of calcareous sediments in upper intertidal to supratidal environments. Evidence for this includes the presence of continuous cryptalgal laminae, mud cracks, birdseyes and oolitic units, the probably former presence of evaporites, and the absence of normal marine fauna. The oolite units may be analogous to modern shoreline occurrences in Laguna Madre and Baffin Bay, Texas. The low Na and Sr concentrations, as well as the nearly stoichiometric Ca/Mg ratio of the fine-grained dolostones, compared to Holocene supratidal dolomites, are attributed to the formation or neomorphism of the fine-grained dolostones in the presence of fresh water. The medium and coarser grained dolostones consist of 0.1- to 0.8-mm dolomite crystals that show textural and compositional zoning. These dolostones formed by early diagenetic replacement of limestones due to dilution of marine pore water by fresh water. A mixing-zone environment of dolomitization is indicated by the following features of the constituent dolomite crystals: textural zoning with cloudy centers and clearer rims, concentric luminescent zoning, increase of Fe content toward the rims, and low Na contents and Sr/Ca ratios untypical of the marine environment. The depositional setting within the area of study was a coastal, tidal flat environment toward the northwest, adjacent to a very shallow marine environment dotted with tidal islands toward the southeast. In the rock record, the former presence of these islands is indicated by thickening of fine-grained dolostones and a corresponding thinning of underlying limestone units.