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.

First Page Preview

First page PDF preview
You do not currently have access to this article.