Effects, Influences and Controls of Sedimentology, Stratigraphy, Tectonics, Paleogeography and Diagenesis on Hydrocarbon and Mineral Accumulations in the Cambrian-Ordovician Knox Group in Kentucky
Gooding, Patrick J.
The Cambrian-Ordovician Knox Group in Kentucky exceeds 3,500 feet in thickness and signifies one of the largest buildups of carbonates identified in the United States. Evidence supports the interpretation that these sediments were deposited on a broad, gently sloping, continental shelf in a warm, shallow, low-energy marine environment during a period of relative structural tranquility and represent accumulations in a uniform sequence of subtidal, intertidal, and supratidal conditions. Vertical distribution of lithologic types indicates a shallowing-upward sequence with abundant depositional cycles. This pronounced sequence indicates that the rate of deposition generally kept pace with subsidence of the craton. The porosity characteristics of these carbonates are secondary due to chemical and physical changes such as dolomitization, hydrothermal activity, solution channels or fractures. Disturbed and destroyed bedding due to chemical alteration and bioturbation are common in addition to fractures which are extensive and complex.
Many geomorphic processes were active during subaerial exposure of these carbonate sediments. The paleogeography is characterized by numerous residual hills; sinkholes; interrupted, elongate and steep sided valleys of limited extent with a lack of a well-developed drainage pattern all indicating karstification. A well-developed brecciated zone containg angular re-worked Knox fragments and bentonite is also present at the base of the Middle Ordovician with relief on the paleosurface varying about 500 feet, further supporting karst.
Significant amounts of petroleum and natural gas have been produced from Cambrian-Ordovician sediments throughout the United States. In Kentucky, hydrocarbon entrapment occurs at or near the unconformable surface and is closely associated with residual highs, fractures and faults. Hydrocarbons generated from Devonian Black Shales and driven by pressures exerted by fluids and gases migrate from deep in the Appalachian and Illinois Basins both vertically and horizontally, through faults, fractures, joints, weakened bedding planes, vugs, breccias, unconformable surfaces and along the flanks of the Cincinnati Arch to accumulate in Knox reservoirs. Brecciated and fracture zones related with the unconformity also serve as a host for mineralization and these deposits contain varying amounts of quartz, galena, sphalerite, barite, calcite, and fluorite resulting from various episodes of hydrothermal action.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013