Unconformity, Karst, Hydrocarbons, Minerals, Environments, and Structures Present in the Cambrian–Ordovician Knox Group in Kentucky: An Example from South-central Kentucky
Patrick J. Gooding, 2012. "Unconformity, Karst, Hydrocarbons, Minerals, Environments, and Structures Present in the Cambrian–Ordovician Knox Group in Kentucky: An Example from South-central Kentucky", Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia, James Derby, Richard Fritz, Susan Longacre, William Morgan, Charles Sternbach
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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 sub-aerial 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.
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Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia
The Great American Carbonate Bank (GACB) comprises the carbonates (and related siliciclastics) of the Sauk megasequence, which were deposited on and around the Laurentian continent during Cambrian through earliest Middle Ordovician, forming one of the largest carbonate-dominated platforms of the Phanerozoic. The Sauk megasequence, which ranges upwards of several thousand meters thick along the Bank's margin, consists of distinctive Lithofacies and fauna that are widely recognized throughout Laurentia. A refined biostratigraphic zonation forms the chronostratigraphic framework for correlating disparate outcrops and subsurface data, providing the basis for interpreting depositional patterns and the evolution of the Bank. GACB hydrocarbon fields have produced 4 BBO and 21 TCFG, mostly from reservoirs near the Sauk-Tippecanoe unconformity. The GACB is also a source of economic minerals and construction material and, locally, serves as either an aquifer or repository for injection of waste material. This Memoir comprises works on biostratigraphy, ichnology, stratigraphy, depositional facies, diagenesis, and petroleum and mineral resources of the GACB. It is dedicated to James Lee Wilson who first conceived of this publication and who worked on many aspects of the GACB during his long and illustrious career.