Near-Surface to Deeper Burial Cementation Patterns and Foreland Basin Evolution, Middle Ordovician Ramp Carbonates, Virginia: ABSTRACT
Middle Ordovician ramp carbonates, Virginia, were deposited in a subsiding foreland basin bordered by developing tectonic highlands. Ramp carbonates are largely occluded by nonferroan, clear rim, and equant cements which contain cathodoluminescent zones consisting of nonluminescent (oldest), bright and dull (youngest) cements. The zonation largely relates to increasingly reducing conditions of pore waters. Zoned cements in peritidal beds have complex zonations, pendant to pore-rimming fabrics, and are associated with vadose silt (which abuts all cement zones); these cements are vadose to shallow phreatic. Major cementation of subtidal facies occurred under burial conditions. Zoned burial cements have a simple zonation reflecting progressive burial (up to 3,000 m) of carbo ates. Shallow burial nonluminescent cement formed from oxidizing, meteoric waters which expelled anoxic, connate marine waters; meteoric waters were carried by aquifers from tectonic upland recharge areas. Deeper burial, bright and dull cements formed at depths (2,000 to 3,000 m) and temperatures (75 to 135°C) associated with hydrocarbon emplacement during the Late Devonian or Mississippian. Final, clear dull cement fills tectonic fractures and was emplaced during late Paleozoic deformation. Deeper burial diagenesis appears to be genetically linked to late Paleozoic, Mississippi Valley-type mineralization. Zoned peritidal and burial cements are mainly confined to southeastern parts of the ramp, where cementation was influenced by meteoric waters from developing uplands on the southe stern margin of the foreland basin and carried northwest by aquifers. Cements in northwestern peritidal and subtidal ramp facies are dominated by nonzoned dull cements, where cementation was little influenced by upland-source meteoric waters. The close association of zoned cements and regional End_Page 574------------------------------ uplands in the Middle Ordovician sequence indicates the importance of assessing regional geology, geologic history, and tectonics in understanding regional cementation patterns and cementation processes of ancient carbonate platforms. End_of_Article - Last_Page 575------------