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Middle Ordovician ramp carbonates, Virginia, were deposited in a subsiding, foreland basin bordered on the southeast by tectonic highlands. Ramp carbonates were lithified, in part, by turbid marine cements, but major cementation was by nonferroan, clear rim, and equant cements. Zoned (defined by cathodoluminescence) clear cements consist of nonluminescent (oldest), bright, and dull (youngest) cements; the zonation relates to increasingly reducing conditions of pore waters. Zoned cements in peritidal beds, best developed in southeastern belts, have complex zonations, pendant to pore-rimming fabrics, and are associated with crystal silt (which abuts all cement zones), solutional cavities, and erosional surfaces (which locally truncate dull cement). These cements are meteori vadose to shallow phreatic. Cements in northwestern exposures of peritidal beds are dominated by non-zoned, dull cement which lacks abundant evidence of early, near-surface precipitation.
Major cementation of subtidal facies occurred under burial conditions. Burial cements, best developed in southeastern belts, have a simple zonation reflecting progressive burial (up to 7.5 km; 4.5 mi) of the carbonate ramp. Shallow burial nonluminescent cement formed from oxidizing, meteoric waters which expelled anoxic, connate marine waters. These meteoric waters were carried by aquifers from tectonic upland recharge areas on the southeastern basin margin. Bright cement formed under more reducing conditions following stagnation of the paleoaquifer with burial, and possibly was precipitated at depths of 2 to 3 km (1 to 2 mi). Burial cements in northwestern exposures of subtidal beds are dominated by dull cement, initial generations of which precipitated from downdip portions of aquif rs. Deeper burial, dull calcite and ferroan dolomite cements largely formed at burial depths of 2 to 3 km (1 to 2 mi) and temperatures of 75° to 135°C (165° to 275°F) or more, associated with hydrocarbon formation-emplacement during the late Devonian to Mississippian. Latest, clear dull cement fills tectonic fractures and was emplaced during late Paleozoic (Alleghenian) deformation, probably at temperatures of 200° to 300°C (390° to 575°F) and depths of 5 to 7 km (3 to 4.5 mi). Deeper burial diagenesis appears to be genetically linked to late Paleozoic, Mississippi Valley-type mineralization in the southern Appalachians.
Zoned peritidal and burial cements are confined mainly to southeastern portions of the ramp where cementation was influenced by meteoric waters shed from tectonic uplands on the southeast and carried northwest by paleoaquifers. Northwestern portions of the ramp were influenced very little by upland-sourced, meteoric waters and nonzoned dull cements precipitated from relatively reducing waters.
The distribution of nonluminescent cement in Middle Ordovician subtidal facies defines the regional distribution of oxidizing portions of the paleoaquifer system. Such incursion of upland-sourced, oxidizing meteoric waters into ramp carbonates should be a common feature of foreland basin carbonates deposited adjacent to tectonic uplands. Furthermore, foreland basin carbonates that undergo progressive burial should show a simple, nonluminescent-to-bright-to-dull sequence of cement zones. The close association of zoned cements and regional uplands in the Middle Ordovician sequence indicates the importance of assessing regional geologic relationships, environmental parameters, geologic history, and tectonics in understanding regional cementation patterns and cementation processes of anci nt carbonate platforms.
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