An unconformity of regional extent is present at the top of the Knox Group of Cambrian–Ordovician age. In south-central Kentucky, extensive paleokarst topography caused by subaerial weathering and erosion during the early Middle Ordovician developed on the upper Mascot Dolomite of the Knox Group.

Data generated from the examination of cores, well samples, geophysical logs, and drillers' logs were used to compile a paleotopographic map showing the general configuration of the eroded surface at the top of the Knox Group. This surface is characterized by sinkholes, residual hills, and interrupted, elongated, steep-sided valleys. No well-developed drainage pattern is apparent. Paleotopographic relief in the area studied is about 500 ft (150 m). The variable thickness (5–120 ft [1.5–37 m]) of the overlying Middle Ordovician Wells Creek Dolomite further supports the interpretation of karst development on top of the Knox Group.

In addition to the highly irregular karst surface, the following criteria were used in identifying the unconformity in the subsurface: a break in the stratigraphic record, with an abrupt change in lithology between the overlying Wells Creek Dolomite and the Knox Group; the occurrence of volcanic ash and a brecciated zone containing reworked Knox material at the base of the Wells Creek; and the presence of oil residues, weathered chert, and porous zones.

Shallow drilling depths, which are generally less than 2000 ft (610 m), combined with possibilities for high production, have made the Knox in this area a prime exploration target. Hydrocarbon entrapment at the Knox unconformity is related to enhancement of porosity and permeability at the weathered paleokarst surface and structural highs caused by residual hills on the eroded surface. Occurrences of oil and gas production are also associated with the presence of faults.