Middle Ordovician Knox Unconformity, Virginia Appalachians: Transition from Passive to Convergent Margin: ABSTRACT

Abstract

The Knox unconformity, central and southern Appalachians, is one of the major targets for petroleum exploration in the Appalachians, as well as being closely associated with base metal mineralization. It developed on Lower to Middle Ordovician Knox/Beekmantown carbonates, and marks transition from passive margin carbonate deposition to deposition in a foreland basin (convergent margin), possibly during global sea level lowering. Erosional relief on the unconformity is over 140 m (460 ft) in southwest Virginia, decreasing to 20 m (66 ft) or less in northern Virginia. Decrease in relief is accompanied by rapid depositional thickening of Lower Ordovician and earliest Middle Ordovician units into the Pennsylvania depocenter. Paleokarst features include topographic highs (tens of meters relief), breccia, and mud-filled sinkholes and caves that extend to 65 m (215 ft) below the unconformity, and sub-unconformity intraformational dolomite breccias that formed after dissolution of limestone interbeds. Coarse detritus on the unconformity surface formed thin to thick veneers of regolith that were locally reworked by fluvial and marine processes. Dolomite detritus also was deposited in alluvial fan and playa mud flats i lows on the unconformity surface. Under the luminoscope, the detrital dolomite is evidenced by corroded nonluminescent detrital cores overgrown by luminescent dolomite. The unconformity influenced the regional distribution, composition, and thickness of some post-unconformity peritidal carbonates and may have localized some Middle Ordovician downslope buildups. The unconformity also localized deposition of Pb-Zn ores, which commonly are associated with unconformity related intraformational breccias that remained open during deeper burial diagenesis. Development of regional unconformities on shelf sequences of passive margins immediately beneath foreland basin sequences is common in other orogens, reflecting gentle warping of the shelf prior to foundering and burial beneath synorogenic clastics. Such unconformities may localize hydrocarbons and base metal deposits (Pb-Zn) by developing permeable zones adjacent to the unconformity. End_of_Article - Last_Page 523------------