Carbonate Ramp-to-Basin Transitions and Foreland Basin Evolution, Middle Ordovician, Virginia Appalachians

Abstract

Middle Ordovician carbonate ramp and foreland basin facies in Virginia developed under the influence of depocenters (located in Pennsylvania and Tennessee), which appear to have been inherited from the passive Cambrian-Ordovician shelf. Carbonate ramp and basin deposition (Mosheim, Lenoir, and Paperville beds) associated with the southern depocenter was initiated, in southwestern Virginia, with downwarping and transgression of exposed Ordovician Knox shelf carbonates. In northern Virginia, deposition may have been continuous from the lower Middle Ordovician Beekmantown beds. Continued downwarping extended the basin into northern Virginia and probably was accompanied by marine transgression from the northern depocenter. A transgressive sequence was deposited, consisting of peritidal carbonates (Mosheim/New Market Limestone) bordered on the southeast by subtidal-ramp cherty wackestones (Lincolnshire/Lenoir Limestone) and onramp and downslope buildups (Effna/Rockdell Limestone) that passed seaward (southeast) into black limestone and shale, slope, and anoxic basin facies (Liberty Hall and Paperville beds). These passed into submarine fan clastics (Knobs Formation) which were shed from tectonic highlands of Middle Ordovician to Cambrian (and Precambrian?) rocks that bordered the basin on the southeast. The basin deepened southwestward, toward the southern depocenter. Continued submergence in southeastern belts caused drowning of the shallow ramp and deposition of deep ramp shaly skeletal limestones (Benbolt Formation). These were subsequently overlain by a so theastward-prograding, upward-shallowing sequence of shallow-ramp skeletal and fenestral carbonates (Wardell and Witten Limestones), while basinal deposition continued in southeastern belts. Filling of the southern basin allowed southeastern-derived deltaic and coastal plain clastics (Bays/Moccasin beds) to be deposited on the carbonate ramp, terminating limestone deposition in southwest Virginia. In northern Virginia, carbonate ramp sedimentation continued in northwestern belts (Nealmont-Collierstown Limestone), while basinal deposition occurred on the southeast, associated with the northern depocenter. The close association of downslope buildups and anoxic-basin source beds suggests that similar peri-cratonic ramps that extend into foreland basins might be important areas for petroleum exploration.