Sedimentology and development of a passive- to convergent-margin unconformity: Middle Ordovician Knox unconformity, Virginia Appalachians

The Knox unconformity, central and southern Appalachians, marks the transition from passive-margin to convergent-margin sedimentation, possibly during global sea-level lowering. The unconformity developed on Early to Middle Ordovician Knox-Beekmantown carbonates, and it has >140-m erosional relief in southwest Virginia, decreasing to <20 m in northern Virginia. Decrease in erosional relief is accompanied by rapid depositional thickening of Lower Ordovician and earliest Middle Ordovician units into a depocenter in Pennsylvania.

Paleokarst features include topographic highs (tens of metres relief), breccia- and mud-filled sinkholes and caves that extend down to 65 m below the unconformity, and subconformity intraformational dolomite breccias that formed after dissolution of limestone interbeds. Detritus on the unconformity surface formed veneers of regolith, sub-aerial debris flows, and mud-flat deposits, and locally it was reworked during transgression.

The unconformity influenced the distribution of postunconformity carbonates, including Middle Ordovician build-ups. It also influenced later Zn mineralization and possible localization of petroleum reservoirs in the basin. Development of regional unconformities at passive-to-convergent–margin transitions is common in other orogens, reflecting gentle warping and uplift of the shelf prior to foundering and burial beneath synorogenic clastics.