Cementation and dolomitization of Mississippian limestones, Kentucky and Virginia

Abstract

The Mississippian Newman Limestone (0-30 m thick) in eastern Kentucky contains pink-staining, aquifer-related cements (up to 750 ppm Mn+2); up to 1000 ppm Fe+2; δ¹⁸ -7.5 to -6.8 per mil; δ¹³C 1.7 to -6.8) that is non-luminescent (low Fe and Mn) in the recharge area, but becomes uniformly dully luminescent downdip. This aquifer developed toward the end of Newman limestone deposition during two major regressions (Late Mississippian and Mississippian-Pennsylvanian time).

Shallow burial cementation was less common to the south-east into the Appalachian Basin, where the Mississippian limestone is up to 1000 m thick. Here, phreatic meteoric diagenesis in more distal parts of the aquifer caused high-Mg calcite and aragonite grains to be leached, while isotopically light, fine dolomite (δ¹⁸O -1.7 to -6.7 per mil; δ¹³C 2.7 to -5.3 per mil) replaced muddy carbonates in a paleomixing zone. As the aquifer evolved, low-iron, dully luminescent calcite was precipitated from reducing pore waters. With increasing burial, compaction caused spalling of ooid cortices; iron rich saddle dolomite (δ¹⁸O -5.5 to -11.2 per mil; δ¹³C 0.9 to 1.4 per mil), and moderately ferroan (purple-staining) calcite cement (0-1200 ppm Mn2+; 1000-3000 ppm Fe2+; δ¹⁸O -4.6 to -10.6 per mil; δ¹³C 2.7 to -6 per mil) precipitated in pores and fractures from waters that were increasingly dominated by warm, basinal, oil-bearing fluids expelled from dewatering Paleozoic shales. These coarse dolomites overgrew early fine dolomite of reservoirs, while the purple staining calcite filled intercrystal porosity outside of the reservoirs. At or near deepest burial, Fe-rich (blue-staining) calcite (up to 1200 ppm Mn2+; 3000-7000 ppm Fe2+; δ¹⁸O -3. 8 to -7. 8 per mil; δ¹³C 1. 8 to -1. 5 per mil) precipitated in much of the remaining void space in the limestones. During uplift of the sequence late calcite cements with decreasing Fe contents were precipitated from increasingly oxidizing fluids that penetrated the section through fractures and remaining pore spaces.