ABSTRACT

The Mississippian Newman Limestone in Kentucky was studied to assess the effects of unconformities within and above the sequence on calcite cementation. Early cements are low in Fe and show a nonluminescent to dull to nonluminescent zonation. Early nonluminescent cement mainly formed in updip areas from oxidizing, meteoric waters in a regional paleoaquifer that formed during regional regression in the Late Mississippian. During Late Mississippian transgression, waters became reducing to form early dull cement (up to 600 ppm Mn, up to 2,700 ppm Fe but averaging 570 ppm). The aquifer underwent later recharge during Mississippian-Pennsylvanian regression to form a second nonluminescent cement. This event continued into the Pennsylvanian when the sequence underwent increasing burial. Earl cements have ¹³O values that become lighter downdip, perhaps reflecting a downdip temperature increase due to burial. ¹³C values are lightest in the north (postulated recharge area) and reflect input of organically derived CO₂ in waters there. Most calcite cementation appears to have been associated with two post-Newman unconformities which generated regional aquifers and reflects a shift to a wetter climate in Late Mississippian-Pennsylvanian time. Early cement abundance decreases away from postulated recharge areas.

Later iron-rich burial cements (up to 2,200 ppm Mn, up to 11,000 ppm Fe) fill remaining pore spaces, compaction-induced fractures, spalled cement rims, and tectonic fractures lined with saddle dolomite and pyrite. They formed above 60°C at depths of 1 to 3 km. ¹³O values are compatible with derivation by pressure solution of preexisting calcites in isotopically heavy waters highly modified by burial diagenesis. Light ¹³C values reflect isotopically light organic carbon in the deltaic waters entering the subsurface, or being expelled from shales or buffering of waters by light C, early calcite cements. The ¹³C calcite cements become heavier downdip by equilibration of waters with host limestone. Newman sediments may have been subjected to late, postcementation(?) migration of warm fluids based on secondary fluid inclusions. These late-stage fluids most likely were chemically complex, warm brines (7-17 equiv. wt. % NaCI, 50° to 160°C) that resided in the formation for sufficiently short times that conodont CAI values were not affected.