W. John Nelson
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W. John Nelson Illinois Institute of Natural Resources CIRCULAR 523 STATE GEOLOGICAL SURVEY DlVlSlON 1981 Jack A. Simon, Chief OVER PHOTO: Herrin (No. 6) Coal offset by a high-angle normal fault (AB) and a horizontal bedding fault (CD). Located in the Cottage Grove Fauit System, at a mine in southern Illinois. Ruler is 6 feet iong. Editor: Mary Szpur Draftsman: Craig Ronta Nelson, W. John Faults and their effect on coal mining in fliinois. - Champaign, 111. : [Ilinois State Geological Survey Division, 1981. 40 p. : ill. ; 28 cm. - (Circular / Illinois State Geological Survey ; 523) Glossary : p. 33 1. Mines and mining. 2. Faults (Geology). I. Title. ti. Series. Printed by authority of the tate of ///in~is/2# . John Nelson ILLlNOlS STATE GEOLOGICAL SURVEY Natural Resources Building 615 East Peabody Drive Champaign, I L 61820 ABSTRACT Slickensides and mullion Slickensides on lower surface of shale FAULTING-BASIC TERMS AND CONCEPTS Sample of breccia from Rend Lake Fault System Definition of faulting Reverse fault Nomenclature of faults Faults forming an en echelon pattern Types of faults Configurations of faults dangerous to mining Features of faults Portion of the Orient No. 6 Mine, about 1000 feet Origin of faults east of the Rend Lake Fault System Encountering faults when drilling EFFECTS OF FAULTS ON COAL MINING Determining the throw of a fault by comparing strata Displacement of coal seams Mining through a large fault Weakening of roof in underground mines Placement and angling of crosscuts in a faulted area, Influx of water and gas along faults where faults cross mine headings at right angles Impurities in coal Placement and angling of crosscuts in a faulted area, where faults cross mine headings at oblique angles RECOGNIZING AND PREDICTING FAULTS Faults and related structures in southern Illinois Recognizing faults in mines The Cottage Grove Fault System Recognizing faults when drilling Cross section of part of the Cottage Grove Seismic exploration for faults Fault System Major structures of the Wabash Valley Fault MINING IN FAULTED AREAS System Portion of Eagle No. 2 Mine of Peabody Coal Company TECTONIC FAULTS IN THE COAL FIELDS Detailed structure of the top of the Herrin (No. 6) OF ILLlNOlS Coal in Wabash County Cottage Grove Fault System Rend Lake Fault System, Du Quoin Monocline, Wabash Valley Fault System Centralia Fault, and related structures Rend Lake Fault System Shawneetown Fault Zone, Eagle Valley Syncline, Du Quoin Monocline, Dowell Fault Zone, and associated structures and Centralia Fault Crown II Mine of Freeman United Coal Mining Co. The Shawneetown Fault Zone Geologic structures of lllinois Faults in the Eagle Valley Syncline Concretion in black shale above the Herrin (No. 6) Faults in southeastern Saline County Coat Other tectonic faults in Illinois Sketch of a typical roll Mineralized "goat beards" at a fault in the NONTECTONIC FAULTS IN COAL-BEARING STRATA Herrin (No. 6) Coal OF lLLlNOlS Clay dike in the Springfield (No. 5) Coal Compactional faults in a surface mine Clay-dike faults Vertical clay dike in black shale above the Gravitational slumps and slides Springfield (No. 5) Coal in an underground mine Origin of clay dikes and clay-dike faults RECOMMENDATIONS Clay-dike faults Portion of the Orient No. 6 Mine showing outline REFERENCES of the shear body North-south cross section through the shear body at GLOSSARY the Orient No. 6 Mine FIGURES TABLE 1. Fault nomenclature 3 1. Characteristics of tectonic and nontectonic faults 2. Types of faults 4 ABSTRACT Faults are fractures in the earth's crust along which move- ment (slippage) has occurred. They are one of many types of geologic disturbances that affect coal seams. Faults, which are common in coal seams of Illinois, have consid- erable effects on coal mining, such as: offsetting of the coal seams, creation of grades too steep for mining equipment to follow, weakening of roof and ribs, admission of water and gas into workings, and introduction of clay and other impurities into the coal. Faults can be grouped into tectonic faults, which are caused by forces acting in hard rock deep within the earth, and nontectonic faults, which are formed by localized disturbance of incom$etel lithified sediments. The presence of most tectonic faults can be predicted before mining begins, and their location can be determined by drilling, seismic exploration, and other means. Most nontec- tonic faults, in contrast, are too small to detect or predict much beyond the working face. The major systems of tectonic faults in Illinois that influence coal mining are located in the southern part of the state. They include the Cottage Grove, Wabash Valley, and Rend Lake Fault Systems, the Dowel1 Fault Zone and Centralia Fau It, the Shawneetown Fault Zone, and faults in the Eagle Valley Syncline. Smaller tectonic faults are known or may exist in other coal mining areas of the state. Many tectonic faults occur in regions where the rock layers are folded, but some exist in unfolded areas. Nontectonic faults are found in every mine in the state, although they are more troublesome in some areas than in others. The major classes of nontectonic faults include compactional faults, clay-dike faults (associated with clay dikes), and gravitational slumps and slides. Many nontectonic faults are strongly controlled by lithologic patterns in the rocks above a coal seam. The relationships of nontectonic faults to lithology often can be mapped so that the presence of the faults can be predicted a short distance ahead of the face. Mining plans should be as flexible as possible to allow adaptation to local conditions in faulted areas. FAULTS AND THEIR EFFECT ON MINING IN ILLINOIS paring a second report that will cover the other types of geologic disturbances in the coal seams of Illinois. Faults are responsible for many difficulties in coal mining in Illinois. They cause major losses in production; they increase the danger, difficulty, and expense of mining; and FAULTING-BASIC TERMS AND CONCEPTS sometimes they force the abandonment of large blocks of coal reserves, or even of entire mines. Every coal mine in Definition of faulting the state is affected to some degree by faults, and some of the most productive coal-mining areas are also the most A fault is defined as any break or fracture in the earth's heavily faulted. Therefore, a better understanding of faulting crust along which slippage has occurred. The slippage may could be highly beneficial to the mining industry. range from a barely perceptible amount to many miles. A fault, in geologic terms, is a break in the earth's Faults produce displacement of rock layers; a coal seam crust along which slippage or displacement has occurred. that has been faulted will be broken, and the broken pieces Faults are only one of many types of geologic disturbances will have moved relative to one another. The movement that can interrupt coal seams and interfere with mining may be up and down, lateral, or any combination of the operations. Unfortunately, many people who work in two. mining in Illinois and elsewhere possess a limited under- Joints are fractures along which noslippage has occurred. standing of the variety of geologic disturbances that affect Joints are found in almost all hard, brittle rocks, and coal measures. Frequently, all discontinuities in the coal are especially pronounced in coal and black shale. Some of are labeled as "faults," and no attempt is made to identify the joints in coal are frequently called cleat. Joints can have the true nature or origin of the disturbances. Thus some significant effects on coal mining, but since they do not structures that are not true faults are mistakenly treated produce slippage, they are not faults and will not be con- as faults, which leads to improper mining procedure-often sidered any further in this report. Some features of joints with enormous costs in lost production, wasted effort, and are covered in Krausse et al. (1979); joints also will be hazards to workers. discussed in my upcoming paper on disturbances in coal This report defines faults, describes their effects, seams not related to faulting. explains how they can be identified and predicted, and suggests procedures to ensure optimum mining in faulted Nomenclature of faults areas. The major fault systems in the coal fields of Illinois are presented in maps and text, and unmined areas where The surface of a fault, along which slippage has occurred, faults can be expected to occur are identified. Although is known as the fault surface. A fault surface may be curved, this report is aimed primarily at the mine operator in Illinois, but commonly it is planar or nearly so; then it is termed many of the principles presented are valid for other coal a fault plane (fig. 1). Fault planes can have any orientation, fields as well. ranging from horizontal to vertical. The orientation of a A glossary of terms related to faults is included in the fault plane is described in terms of strike and dip (fig. 1). back of this publication. The strike is the trend of a horizontal line in the plane of This report does not cover all the technical aspects of the fault; that is, it is the direction shown by the fault on faulting or present all the details known about the various a map (horizontal faults do not have a strike). The dip is fault systems in Illinois. For more complete discussions the angle the fault plane makes with a horizontal surface. of individual fault systems, the reader should consult the Faults with a dip of 0 to 45O often are called low-angle other published reports available on these systems. Among faults; those dipping 45O to vertical are high-angle faults.