Abstract Analysis of well core and cuttings from the Black Warrior Basin in Mississippi reveals the presence of a Middle Ordovician (Whiterockian) erosional unconformity interpreted to be equivalent to the well-known Knox-Beekmantown unconformity in eastern North America. The unconformity occurs at the top of a peritidal dolostone unit known informally as the upper dolostone, whose stratigraphic placement has been the subject of a long-standing controversy. The unconformity, which represents the Sauk-Tippecanoe megasequence boundary on the North American craton, was previously thought to be short-lived or altogether absent in the Black Warrior Basin. The unconformity is characterized by subunconformity solution pipes, solution-collapse breccias, internal sedimentation, and erosional truncation of the underlying dolostone unit. This erosional surface is veneered with sand- to pebble-size, rounded and angular lithoclasts of the underlying dolostone, and rounded and angular quartz sand and silt. Extensive secondary porosity developed in the upper dolostone below the unconformity. Although much of this porosity was later occluded by internal sedimentation and pore-filling dolomite and calcite cement, porous zones remain in the upper dolostone. Based on conodont biostratigraphy from four cores and from a previous study on cuttings from a nearby well, the unconformity is middle Whiterockian in age and likely spans most or all of the Histiodella holodentata Biozone.