Reports
Refractory clays of the Maryland coal measures
1950, Waage, K.M.
Bulletin 9
Introduction
Detailed stratigraphic study of the underclays of the Maryland coal measures by the U. S. Geological Survey in cooperation with the Maryland Department of Geology, Mines and Water Resources was made possible by U. S. Bureau of Mines core-drilling programs in the Georges Creek, Upper Potomac, and Castleman coal basins. The rocks studied include the Pottsville and Allegheny formations and the lower half of the Conemaugh formation. Results of the study are presented in two parts. Part I gives the stratigraphy of the coal measures, the details of the underclay zones, and the stratigraphic and geographic distribution of refractory clays in the Georges Creek, Castleman and northern Upper Potomac basins. Part II describes the refractory clay deposits of the Castleman basin and offers guides for future clay prospecting.
The Pottsville formation thickens appreciably from northeast to southwest in the Georges Creek and Upper Potomac basins as the number of units in the formation increases both by addition at the base of the formation and by intercalation within it. In the coal basins to the west the Pottsville formation shows greater uniformity in thickness. Strata in the Pottsville and lower part of the Allegheny formations are highly variable in thickness and lateral extent and coal beds are discontinuous; groups of coal beds rather than individual coal beds are used in correlation. As a rule the contact of the Pottsville and Allegheny formations is indefinite and the two units cannot be mapped separately. The upper part of the Allegheny formation is less variable than the lower part and includes persistent coal beds. The Conemaugh formation, which reaches its greatest thickness in western Maryland, is divided into an upper and lower member on the basis of lithology. The lower member, from the base of the formation to the top of the Barton coal, is typified by the pronounced regularity in thickness and lateral extent of its strata and by the presence of marine shales and zones of red shale and clay. The upper member has less regular strata, very few redbeds, and no marine zones.
Clays occur as the underclays of coal beds and are also present in the redbeds of the Conemaugh formation. The redbeds grade laterally into coal-bearing beds, and their clays are inferred to be genetically related to underclays. Three principal kinds of material are present in the underclay zone. Plastic clay, generally impure, is the most common and is present in all underclay zones. In the underclays of the Upper Kittanning and higher coal beds lime-pellet clay and argillaceous limestone occur as a lateral phase of the plastic clay. Indurated clays, or claystones, are commonly present in the underclay zones of the Allegheny and basal Conemaugh strata. Where all three kinds of material are present the plastic clay and its calcareous phase ordinarily overlie the claystones. Beds gradational between the underclay zone and the sandstone beneath it consist of a silty or sandy phase of whichever kind of underclay material is locally present at the base of the underclay zone. Repetition or reversal of the preferred sequence of materials in the underclay zone indicates a compound underclay; such underclays were probably formed when the cycle of deposition was interrupted and partly or completely repeated.
Two varieties of claystone (flint clay and semiflint clay) are the principal refractory clays. Plastic and semiplastic clays are rarely of refractory grade. Silt and sand, calcareous matter, and iron, chiefly in the form of siderite, are the most common impurities in the underclay zone and locally affect the value of the refractory clay. Clay of refractory grade is present in all the underclay zones in the Allegheny formation and in a few zones in the lower member of the Conemaugh formation. The character and distribution of all the underclay zones that could possibly contain minable bodies of refractory clay are described in detail.
Refractory clays have been mined from the Maryland coal measures since 1841. The Mount Savage clay bed has been the principal source of refractory flint clay; refractory and semi refractory soft clay has come from the Mount Savage, Lower Kittanning, and Middle Kittanning beds. Until 1947 production was entirely from refractory products plants located in the north end of the Georges Creek basin. In that year a new plant in the Castleman basin began production. At present (1950) the production of refractory products in western Maryland is not up to capacity and a shortage of raw clay appears imminent.
Specific data on the distribution of refractory clay deposits are given for the Castleman basin which is the least known of the coal basins that have facilities for the manufacture of refractory products. The Mount Savage clay, Bolivar clay, Middle Kittanning clay and Upper Kittanning clay, in order of their probable importance, are the only clay beds that are likely to contain bodies of refractory flint clay. The Mount Savage clay is the best prospective source. Refractory and semirefractory soft clay seldom occurs outside of the Mount Savage, Lower Kittanning, and Middle Kittanning clay beds. It is common locally in the weathered zone of outcrop of these units but seldom persists at depth. Commercial bodies of both flint and plastic clay are lenticular in shape and few in number. Reserves of refractory clay in the Castleman basin are critically low and extensive programs of systematic prospecting for new clay bodies are necessary to forestall an acute shortage of raw clay in the near future.
