Kentucky Geological Survey Report of Investigations


The Mascot Dolomite, the upper unit of the Cambrian-Ordovician Knox Group, is a major host for Mississippi Valley-type ore deposits and petroleum in south-central Kentucky. The Mascot was deposited on a broad, shallow platform that exhibited unusually uniform conditions of deposition, ranging from supratidal to shallow subtidal environments. The formation has a complex diagenetic history, including several stages of dolomitization, silicification, solution, and brecciation. Diagenesis and, to some extent, deposition were influenced by Early Ordovician tectonic activity. Tectonic activity influenced development of a regional unconformity during Middle Ordovician time. Evidence of this tectonic activity includes up to 300 feet of erosional relief on the Mascot surface and thinning in the interval between the internal marker beds across the Cincinnati and Waverly Arches.

The development of an erosional unconformity at the top of the Knox Group and its resulting paleoaquifer and karst system created diagenetic changes, migration avenues, and solution-collapse breccias that controlled subsequent fluid migration and accumulations of base metals and hydrocarbons. The location of breccia-hosted ores was influenced by major and minor structural features and paleotopography. Breccia development is related to the presence and subsequent dissolution of limestone (now represented by dolomite) and a dolomite-limestone transition zone. Base-metal mineralization (dominantly sphalerite) is associated with late-stage brecciation and stromatolitic units, which commonly are silicified.

Several oil pools produce from Knox paleotopographic highs, and production seems to be related to the presence of fractures and diagenetically enhanced porosity. A distinct positive relationship exists between ore and petroleum accumulation. Locations of ore deposits are related to circular features that can be recognized on remotely sensed Landsat imagery, and areas of petroleum accumulation appear to be related to paleolinear trends.

Publication Date



Series XI

Report Number

Report of Investigations 4

Digital Object Identifier (DOI)

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