Controls on porosity and permeability in fracture-flow and conduit-flow (karst) rocks of the Knox Group, southern Appalachian fold-and-thrust belt, Alabama, United States of America

Knox Group rocks of Upper Cambrian to Lower Ordovician age are anisotropic and heterogeneous with respect to porosity, permeability, and resultant fluid flow. The Knox is an important water-supply aquifer; a host for oil, natural gas, and Mississippi Valley-type ore deposits; and has received environmental pollutants. The purpose of this study is to determine the geologic controls on porosity and permeability in these rocks, and the relative importance of those controls. Components of this study included photogeologic work; geologic mapping at various scales; logging of approximately 9,157 feet of core and stratigraphic correlation; tracer (dye) test analysis; grouting records analysis; piezometric data analysis; and statistical analysis.;Post-Alleghanian normal faulting and Alleghanian thrust faulting constitute the dominant controls. Normal faulting, and subsequent dissolution of carbonate rocks, has created porosity, permeability, and resultant groundwater flow to depths of at least 486 feet in the study area. Extension associated with normal faulting is a likely precursor to dissolution, and the normal faulting is probably associated with the opening of the Atlantic Ocean during the Mesozoic. Thrust faulting enhances porosity and permeability when positioned so water can readily gain access, either near surface or intersected by other controls such as normal faults. Individual fractures oriented in a northeasterly direction constitute a second-order or third-order structural control. Alleghanian-aged fractures, other than those associated with thrust faults, do not enhance porosity and permeability, as they are usually healed.;Stratigraphy interacts with structure to enhance porosity and permeability. Dissolution of bedrock is common within the dolomites of the study area, and resultant cavity distribution in dolomites usually mimics structure (faulting and folding). Quartz sandstone beds within approximately 110 feet of the surface usually have a 0.1- to 11.8-foot thick cavity associated with them. Algal dolomites (thrombolites and stromatolites) also appear to enhance porosity and permeability in Knox Group rocks.;Tracer tests performed in the study area provided reliable information on velocity of flow. Ground-water velocities range from 748 to 34,778 feet per day, with a mean and standard deviation of 11,361 and 5,699 feet per day, respectively.