Cement Types and Cementation Patterns of Middle Ordovician Ramp-to-Basin Carbonate Rocks, Virginia: ABSTRACT

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Middle Ordovician ramp-to-basin carbonate rocks of Virginia consist of peritidal fenestral limestone, shallow subtidal cherty wackestone, shallow ramp and downslope skeletal buildups, deep ramp shaly fossiliferous wackestone, and basinal black limestone and shale. Pre-burial marine cements in buildups include turbid rim cement on pelmatozoans, isopachous pseudoacicular cement, and coarsely crystalline neospar cement occurring on polycrystalline substrates. Line cavities predate other cement types and are interlayered with internal sediments. Later, nonferroan clear rim and equant cements fill remaining pore spaces in buildups. Nonferroan equant cement and internal sediments fill fenestrae in peritidal facies. These cements consist of several cathodoluminescent zones (from oldest to youngest): (1) nonluminescent black zone (in buildups) or nonluminescent passing into subzoned dull luminescent (in tidal flats); (2) thin, brightly luminescent zone; and (3) dull luminescent zone (or hydrocarbon or dolomite cement). Petrographic relations indicate that in buildups the black and thin bright zones are burial cements formed from formation waters expelled from compacting basinal facies prior to hydrocarbon migration whereas the dull zone is deeper burial in origin and is synchronous with or postdates oil migration and emplacement. In contrast, the bulk of the peritidal cement zones are pre-burial and formed from vadose to shallow phreatic waters. This is indicated by occurrence of black and bright cements that occur as pendant crystals or line fenestrae, presence f crystal silt which abuts all zones, and by erosion surfaces that truncate the dull cement zone. End_of_Article - Last_Page 715------------