Clay Fabric and its Relation to Structural Properties in Mid-Continent Pennsylvanian Sediments

Abstract

ABSTRACT Many structural properties of argillaceous sediments correlate closely with clay fabric; the degree of preferred orientation of clay particles. Shaly structure is associated with a significant degree of clay particle orientation, massive structure with a more random clay particle arrangement. Fissile structure occurs in shales where there is a high degree of orientation of clay minerals and other flaky constituents. It is best developed in shales containing a high percentage of organic material and where organic material is concentrated in bands. Organic material and in some cases, carbonate minerals, exert a secondary influence on fabric and structural properties. In ancient sediments that contain a significant percentage of organic material, the preferred orientation of clay minerals generally increases as the percentage of organic matter increases. Sediments with more than 10 percent carbonate minerals commonly show a poor degree of clay particle orientation. Little relation appears to exist between fabric or structural properties and clay mineral composition, except perhaps in sediments with appreciable montmorillonite. Poor particle orientation has been shown to exist in sediments in which montmorillonite is the major clay mineral component. No apparent relation was found between fabric or structural properties and total clay content (above 20 to 30 percent), geologic age, or depth of burial. Variations in orientation of clay particles, from essentially parallel to nearly random, occur within a few vertical inches in some thin stratigraphic sections. This, and the close relation of particle orientation to structural properties suggest that the orientation of clay minerals (fabric) in argillaceous sediments is not dominantly controlled by compaction caused by the weight of overlying sediments. These relations appear to reflect physicochemical conditions in the environment of deposition that influence the mode of clay mineral sedimentation, and/or geochemical factors that may promote particle rearrangement soon after deposition.