Paleozoic Carbonate Buildups
The Middle Ordovician limestone sequence, Virginia Appalachians, formed on a carbonate ramp that lay northwest of a subsiding foreland basin. Large carbonate buildups (I to 60 km (.6 to 37 mi) wide and 20 to 250 m (66 to 820 ft) thick) occur in the lower (onlap) part of the sequence. The buildups are underlain by peritidal and lagoona! carbonates, and they occur as isolated buildups in two well-defined belts on the ramp. Shallow ramp buildups (Rockdell, Ward Cove Limestone) were buried by argillaceous limestone deposited during regional drowning of the shallow ramp. Downslope buildups (Effna, Murat Limestone) occur on the deep ramp and basin slope. They were killed by anoxic bottom waters (indicated by local sulfide and/or phosphorite crusts on buildup crest) and by burial beneath aggrading basinal shale and/or limestone.
The middle Ordovician buildups have similarities to late Paleozoic Waul-sortian-type buildups. They commonly have cores of stromatactoid-bearing mud-stone and/or wackestone and minor boundstone. Cores are underlain, overlain and flanked by bedded, pelmatozoan-bryozoan grainstone and packstone. Buildups generally lack robust frame-builders. They are best described as carbonate banks that formed by high rates of skeletal carbonate accumulation under a transgressive setting, baffled by pelmatozoans and ramose bryozoans, and widespread marine cementation.
Buildups were initially lithified by three major marine cement fabrics. These include turbid isopachous neospar cement and isopachous cement with local botryoidal habit, turbid rim cement which is an inclusion-rich syntaxial cement on echinoderm grains, and bladed cement which is turbid to clear with a well-defined crystal habit. These syndepositional cements were of profound importance in constructing and stabilizing the non-reefal buildups. Remaining spaces were filled by nonferroan, clear rim and equant cement together with solid hydrocarbon (in downslope buildups), and ferroan dolomite cement. The clear cements are zoned, as defined by cathodoluminescence, and consist of nonluminescent (oldest), bright and dull (youngest) cements; the zonation relates to increasingly reducing conditions of pore waters. The zonation is simple, reflecting progressive burial of the buildups by 3000 m (9843 ft) of Middle Ordovician and Mississippian sediments.
Figures & Tables
Carbonate buildups have long been a focus of intense geological study. An underlying reason is the importance of carbonate buildups as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record created by carbonate buildups with emphasis on lithofacies, stratigraphy of buildups and their surrounding deposits, geometry, “reef”-building and sediment-producing organisms, and diagenesis and porosity evolution