Lithologic Character of Chester Rocks in Illinois-Kentucky Fluorspar District

Abstract

This paper describes the lithologic character of the formations of the Chester series in the Illinois-Kentucky Fluorspar district from a study of cores obtained by diamond drilling. This study has extended over a period of more than 9 years and during the course of the work nearly 350,000 feet of core have been examined from an area about 20 by 30 miles in extent. Lithologic details not discernible in ordinary exposures are detectable in the cores. The Chester rocks vary notably in lithologic character and thickness. In parts of the district the lithologic variations are nearly uniform in a given direction but in general there is no apparent uniformity of change for any appreciable distance. The variations in thickness are not uniform either parallel with, or at right angles to, the strike and are the result of non-uniform deposition and differential erosion during the Chester epoch. The Renault formation consists of four well defined and persistent zones, a basal limestone and overlying shale, the Shetlerville member, and an upper limestone and overlying shale, the Downeys Bluff member. The Renault ranges from about 40 to 137 feet thick, the thickest sections occupying a triangular area with its apices at the Beard, Skelton, and Royal properties (Fig. 1). The thicker Renault sections are generally characterized by a prominent upper shale. The Shetlerville member in the western part of the Fluorspar district exhibits a rather uniform increase in limestone and a decrease in shale toward the south. Elsewhere there is no uniform directional variation. The Bethel sandstone contains less shale than other Chester sandstones. Its thickness is between 55 and 105 feet, the thickest sections occupying a belt from northwest to southeast across the center of the district. The Paint Creek is one of the most variable Chester formations both in lithologic character and thickness. The thickness ranges from 0 to 63 feet; the thickest sections are in the northwest and southeast parts of the district. It is almost entirely interbedded thin shale and sandstone layers in the northern and western areas. The most conspicuous lithologic change is the increase in calcareous sediments, especially limestone, toward the southeast. The Cypress sandstone, 70-125 feet thick, is similar to the Bethel except for a greater amount of shale especially in the southern and southeastern parts of the district. The shale beds do not occupy a consistent position in the section. The Cypress-Golconda contact is transitional, with a shale zone between the top sandstone of the Cypress and the lowest limestone of the Golconda in many localities. This zone is in most places non-calcareous and even sandy and might well be called Cypress rather than Golconda as has been done in this paper. The Golconda, 100-200 feet thick, consists of interbedded limestones and shales without lithologic zones which are persistent throughout the district. The Barlow sand which occurs in places in the northern and eastern parts of the district is cons stently near the middle of the formation. The Hardinsburg is a quartz sandstone with variable amounts of shale and ranges in thickness from 68 to about 140 feet. The shales occur in different parts of the sections in different localities but in general are in the middle and upper parts. It is thickest in the southeastern part of the district. The Glen Dean consists of interbedded shales and limestones much like the Golconda but generally with purer limestones than other Chester formations. It is 45-75 feet thick; the greatest thickness is in the south-central part of the district. Considerably less information about the upper Chester has been obtained by drilling. The Tar Springs consists of interbedded sandstone and shale 65-145 feet thick. The Vienna limestone and shale vary from 30 to 106 feet in thickness. The Waltersburg, 40-87 feet, consists of sandstone and shale; the shale is most conspicuous in the middle part of the formation. The Menard, 75-120 feet thick where drilled, consists of interbedded limestones and shales, which do not occur as persistent zones. Complete sections of the higher Chester formations have not been drilled. It is concluded that many of the clastic Chester sediments are of continental origin and that the marine parts of the section were deposited in relatively shallow water. End_Page 1777------------------------------