Petrography, Geochemistry, and Origin of Burial Diagenetic Facies, Siluro-Devonian Helderberg Group (Carbonate Rocks), Central Appalachians

Abstract

Petrographic and geochemical studies of pore-filling cements and replacement products in the Helderberg Group (Upper Silurian-Lower Devonian, central Appalachians) document the diagenetic history of these rocks. Shallow-ramp skeletal limestones and buildups were partially lithified by marine cements. However, most pore-filling cement formed under shallow (<300 m) to deep burial (300-4,000 m) conditions. Regional cathodoluminescent zonation patterns in early, calcite cements indicate meteoric ground waters were involved in shallow burial cementation. Early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone, that pass downdip into subzoned dull cements (interlayered bright and dull laminae), and then into nonzoned dull cement in basinward limestone. The regional distribution, stable isotopic compositions, and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip (at least 150 km). Helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones. Late, void-filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4,000 m. Deep burial pore fluids were dilute to saline Na-Ca-Cl brines (from primary fluid inclusion data) with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields. Void-filling dull calcite cements occluded nearly all remaining porosity before late Paleozoic fracturing and deformation. Latest diagenetic phases include dolomite, fluorite, silica, and fracture-filling dull calcite. This study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution.