The Middle Ordovician Knox unconformity, Virginia Applachians: transition from passive to convergent margin

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Virginia Polytechnic Institute and State University

The Knox unconformity in the central and southern Appalachians is developed on Lower to early Middle Ordovician Knox/Beekmantown carbonates. The unconformity marks the transition from Cambro-Ordovician shelf carbonate deposition on a passive margin to carbonate and clastic deposition in a foreland basin associated with a convergent margin, possibly during a time of global sea-level lowering.

Erosional relief on the unconformity decreases from over 140 m in southwest Virginia to 20 m or less in northern Virginia. This corresponds with a marked decrease in stratigraphic relief in the same direction. Paleokarst features that formed on the unconformity include topographic highs that extend up to 30 m into overlying Middle Ordovician peritidal carbonates, sinkholes and caves that extend down to 65 m below the unconformity and are filled with detritus from the unconformity and breccia from host carbonates, and sub-unconformity dolomite breccia bodies that formed by collapse after dissolution of limestone interbeds. Coarse detritus on the unconformity surface formed thin to thick veneers of regolith; locally this material was reworked by fluvial and marine processes. Much fine dolomite detritus was reworked and deposited as alluvial fan and playa mud-flat sediments in lows on the unconformity surface.

The unconformity influenced the regional distribution, composition and thickness of some post-unconformity peritidal carbonates. This is evidenced by lithoclastic supratidal sheets that cap cycles in these beds up to 100 m above the unconformity. Unconformity highs also may have controlled later Middle Ordovician buildup distribution in Virginia.

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 beneath synorogenic clastics. Such unconformities may localize hydrocarbons and base metal deposits (Pb-Zn), by controlling the distribution of permeable horizons adjacent to the unconformity.