Lithostratigraphy of the subsurface Silurian rocks in western Kentucky

Abstract

Syndepositional tectonic movements and geometry of the Illinois Basin had a profound influence on thickness, lithofacies distribution and dolomitization in the Silurian rocks in western Kentucky. The northern portion of the Illinois Basin {in Indiana and Illinois) was the site of extensive development of reef trends and numerous isolated reefs with associated petroleum entrapment. However, no reefal trends have been reported from the southern portion of the Illinois Basin in western Kentucky.

The Kentucky portion of the Illinois Basin consists predominantly of the Moorman Syncline and its shelves, which are separated from the northern part of the minois Basin by the Rough Creek Fault Zone. Syndepositional Tectonic movements not only controlled the basinal geometry, but also influenced the depositional environments in the Moorman Syncline. The basinal geometry of the Moorman Syncline accounted for a unique geologic setting and a possible reefal trend, altogether different from that of the northern portion of the Illinois Basin.

Silurian rocks in the study area consist of more than 600 feet of predominantly clastic-free carbonates with a few thin shaly intervals. Percentages of grainstone/packstone facies and dolomite generally increase toward margins of the basin, whereas percentages of wackestone/packstone facies and calcite increase toward the center of the basin.

Syndepositional subsidence of the basinal block, along a segment of the Rough Creek Fault Zone in Grayson County, created a local steep shelf-margin within an otherwise gently sloping carbonate platform. This, in turn, influenced the thickness of the sedimentary-fill, facies distribution and dolomitization pattern. Thickness of the reef-bearing interval (the St. Clair and Moccasin Springs formations) increases locally on the basinal side of the syndepositional fault in Grayson County. Percentages of grainstone/packstone facies and dolomite also increase anomalously in this area.

Four different but interdependent criteria: basinal geometry, geometry of sedimentary-fill, lithology and distribution of dolomite support the existence of a probable reef trend in Grayson County. This inferred reef trend in Grayson County is the most promising site for exploration of hydrocarbons entraped within the Silurian reefs or in draped structures in the overlying rocks.