Cement types and cementation patterns of Middle Ordovician ramp- to-basin carbonates, Virginia

Abstract

Middle Ordovician ramp carbonates were deposited in a rapidly subsiding foreland basin bordered on the southeast by tectonic highlands. Marine, turbid cements include neospar, bladed and turbid rim cements; they are associated with synsedimentary hardgrounds and erosional surfaces. Neospar cement is isopachous and locally botryoidal, and is neomorphic after acicular and botryoid aragonite. Bladed cement has a well-defined crystal morphology, and has non-recrystallized fabrics that suggest a high-Mg calcite precursor. Turbid rim cement syntaxially coats pelmatozoans; its substrate specificity suggests a high-Mg calcite precursor. Marine cements occur throughout the sequence but are abundant in nonreefal buildups, where syndepositional lithification was important in stabilizing buildups.

The bulk of the later, non-marine cements are nonferroan, clear rim and equant cements. Zoned (defined by cathodoluminescence) clear cements consist of nonluminescent (oldest), bright and dull (youngest) cements; the sequence relates to increasingly reducing conditions of pore waters. Zoned peritidal cements are best developed in southeastern belts and 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 truncates dull cement). These cements are meteoric vadose to shallow phreatic in origin. Cements in northwestern exposures of peritidal beds are dominated by nonzoned, dull cements which lack abundant evidence of early cementation.

Major cementation of subtidal facies occurred under burial conditions. Burial cements in southeastern belts have a simple zonation reflecting progressive burial (up to 3000 m) of the carbonates. Shallow burial, non-luminescent cement in southeastern belts formed from oxidizing waters which expelled anoxic, connate marine waters, and meteoric waters were carried by aquifers from tectonic upland recharge areas on the southeastern basin margin. Burial cements in northwestern belts are dominated by dull cement, initial generations of which precipitated from downdip portions of aquifers. Deeper burial, bright and dull calcite and ferroan dolomite cements formed at burial depths (2000 to 3000 m) and temperatures (75 to 135°C or more) associated with hydrocarbon formation - emplacement. 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 portions of the ramp where cementation was influenced by meteoric waters from tectonic uplands and was carried northwest by paleoaquifers. Northwestern portions of the ramp were little influenced by these meteoric waters, and nonzoned dull cements precipitated from relatively reducing waters. The close association of zoned cements and regional uplands in the Middle Ordovician sequence indicates the importance of assessing regional geologic relationships, geologic history and tectonics in understanding regional cementation patterns and cementation processes of ancient carbonate platforms.