Geometry, Facies, and Development of Middle Ordovician Carbonate Buildups, Virginia Appalachians

Abstract

Large carbonate buildups (from 1 to 60 km wide and 20 to 250 m thick) in the lower part of the Middle Ordovician foreland basin sequence in Virginia include shallow ramp and downslope types. The buildups overlie peritidal and lagoonal carbonates. Shallow-ramp buildups are overlain by deep-ramp nodular limestone; downslope buildups are overlain by slope and basin limestone and/or shale. Lime wackestone and/or mudstone (commonly with stromatactoid structures) occurs in cores of buildups and as abundant small pods enclosed in skeletal sand. Bindstone is rare, but bryozoan bafflestone (locally marine cemented) is common in downslope cores and pods. Pelmatozoan-ramose bryozoan sands and rudstone, locally with abundant mud pods, may form sheets beneath, flanking, and above core facies. They are planar bedded or, less commonly, have large-scale foresets. Black, shaly, skeletal limestone, containing siliceous sponges, brachiopods, bryozoans, and pelmatozoans, forms deep flank beds surrounding downslope buildups and grades outward into basinal limestone and/or shale. The buildups are carbonate banks that developed in a transgressive setting involving sea-level rise and subsidence of the foreland basin. They formed as a result of high rates of skeletal production by bank biotas and by baffling action of stalked and branching organisms; widespread marine cementation converted the loose bank sediments into rigid structures. Some downslope buildups were initiated on unconformity highs, but most buildups overlie thick sequences of shallow-water facies. Some shallow-ramp buildups may be localized above a linear high (Tazewell arch). Shallow-ramp buildups were killed by influxing mud following incipient drowning of the shallow ramp. Downslope buildups were killed following basin subsidence when they were covered by anoxic basin waters. Exploration models involving buildups on carbonate ramps should recognize the potential for both shallow-ramp and downslope buildup trends. Such downslope buildups are particularly important because buildup facies are closely associated with potential reservoir rocks of the slope and basin.