Regional Cathodoluminescent Cement Zonation Related to Upland-Sourced Paleoaquifers: Devonian Helderberg Carbonates and Clastics, Central Appalachians: ABSTRACT

Abstract

Cathodoluminescent zonation in shallow burial calcite cements of the Keyser and New Creek Limestones (lower Helderberg Group, Upper Silurian-Lower Devonian) in Virginia, West Virginia, and Maryland can be mapped regionally. Regional distribution of cement zones coincides with the distribution of easterly derived sandstone tongues within the carbonate sequence. In southwestern Virginia, calcite cements in limestones subjacent and laterally equivalent to the Clifton Forge and Healing Springs Sandstones show cathodoluminescent zonation of nonluminescent (earliest cement) to bright to dull or subzoned dull (last shallow burial cement). Noneluminescent cements are Mn2+ and Fe2+ poor and precipitated from oxidizing meteoric pore fluids; bright cement (Mn2+-rich) reflects more reducing pore fluids; dull cement (Fe2+-rich) precipitated from most reducing shallow to deeper burial pore fluids. Nonluminescent-bright-dull zonation grades abruptly into correlative subzoned dull cement downdip from sandstone tongues. Further downdip, subzoned dull cement grades into dull (nonzoned) cement. Limestone adjacent to the Elbow Ridge Sandstone (West Virginia and Maryland) lacks nonluminescent and bright cement but has subzoned dull cement that grades into dull cement westward. Clear rim cement on scattered pelmatozoan grains in the Clifton Forge, Healing Springs, and Elbow Ridge Sandstones have complexly zoned nonluminescent and bright cements. Quartz grains have pressure-solved contacts and are cemented by dully luminescent, deep b rial cement. Predominance of deep burial Fe-rich cement in sandstone indicates that they remained open during shallow burial diagenesis and that sandstone tongues acted as conduits for oxidizing meteoric ground waters recharged from tectonic highlands. Downdip from sandstone tongues, subzoned dull and nonzoned dull cements precipitated from reducing meteoric ground waters distant from subareal recharge areas. Precipitation of shallow burial zoned cements ceased when recharge areas were buried by thick Devonian clastics (Ridgeley Sandstone, Huntersville Chert, Millboro Shale) or when burial removed the sediments from the effects of upland sourced meteoric ground waters. Final porosity occlusion in the limestone is by deep burial, clear, Fe-rich calcite cement (dull luminescence), Fe-ric saddle dolomite, silica cement, and rare fluorite. This study emphasizes the importance of (1) tectonic uplands as recharge areas for aquifers involved in cementation; (2) potential of interlayered sandstones in carbonate sequences to act as permeable conduits for meteoric ground waters because they are less susceptible to calcite cementation than associated lime grainstones; and (3) documenting regional cathodoluminescent zonation of porosity-occluding shallow burial calcite cements in potential reservoir facies prior to hydrocarbon migration. End_of_Article - Last_Page 452------------