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Unconformity, Karst, Hydrocarbons, Minerals, Environments, and Structures Present in the Cambrian-Ordovician Knox Group in Kentucky

Gooding, Patrick J.
Kentucky Geological Survey, University of Kentucky, Lexington, KY 

Data generated from the examination of cores, well cuttings, and geophysical logs were used to compile a paleotopographic map of the erosional surface of the Cambrian-Ordovician Knox Group. This unconformable surface is of considerable economic importance because of its relationship to hydrocarbon entrapment, and mineral deposits. In Kentucky, these predominantly carbonate rocks attained thicknesses in excess of 3,500 feet.

Hydrocarbons occur at or near the unconformity and are closely associated with residual highs, fractures and faults. Brecciated and fractured zones related with the unconformity also serve as a host for mineralization. Geochemistry of oils and rock samples show that hydrocarbons migrated, from Devonian and Ordovician sources in the Appalachian Basin, both vertically and horizontally to accumulate in these Cambrian-Ordovician reservoirs.

The irregular configuration of the Knox surface, which gives an appearance of disorder, is made up of many circular or elliptical depressions. This paleosurface is also characterized by numerous conical residual hills. Some valleys are interrupted, elongate, steep sided, and of limited extent. In addition, the unconformable surface generally lacks a well-developed drainage pattern. Relief on the paleosurface varies as much as 500 feet, and the variability of the overlying formation, which ranges in thickness from 5 to 120 feet further, supports the interpretation of karst. Other criteria that identify this unconformity are: a break in the stratigraphic record, with an abrupt lithologic change; the occurrence of a brecciated zone, made up of weathered, etched, and angular fragments of reworked upper Knox material; and the presence of oil residues, weathered chert, and porous zones.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004