OÖLITE DEFORMATION IN THE SOUTH MOUNTAIN FOLD, MARYLAND

Abstract

The South Mountain fold is a large asymmetrical overturned anticline. Its axial plane dips to the southeast, and its crest is the western slope of South Mountain. Cleavage dips steeper in the upper than the lower limb thus forming a fan which opens to the northwest. All parts of this fold participate and reveal an identical deformation plan: fold axes are nearly horizontal, cleavage dips southeast, lineation is in the cleavage plane normal to the fold axes, also dipping east. All formations including the volcanics participate. Deformation axes have been determined at several hundred localities by systematic measurement of ooid distortions in Cambero-Ordovician oolites. The directions are shown in maps and diagrams. Intensity of deformation varies greatly within the fold depending on (1) physical properties of materials (2) location within the fold and (3) geographical location. Undeformed crystalline micropebbles and detrital carbonate grains within highly distorted oolites seem to indicate that deformation affected rather soft and pliable and maybe little consolidated rocks. Undistorted growth aprons on deformed ooids and crystallization of matrix are postkinematic and serve to date deformation and consolidation. Approaching South Mountain from the west intensity grows gradually, is strongest in the lower limb, decreases toward the crest and the upper limb. Abrupt changes as would be expected in large-scale thrusting were not observed, the deformation seems to have been absorbed within a much wider complex. Cleavage is invariably defined by the maximum (a) and mean (b) axis of distortion and not by a shear plane. This also points toward flowage as distinct from shearing of solids. The fold is interpreted as a large “shear” fold as distinct from flexures. Deformation is thought to be due to laminar flow on subparallel planes. The presence of flow planes and the fact that ooids are extended at large angles to bedding show that stratigraphic thicknesses as now seen are not equivalent to depths of deposition. Calculations indicate that the latter amounted to less than one half of the present thickness. Analysis of South Mountain fold requires reconstructions not usually applied in tectonic analysis. If deformation affected soft strata, exaggeration of thicknesses would go unnoticed but for painstaking structural observations, not commonly undertaken in stratigraphic research at present. Careful microscopic investigation of the constituents of the oolites permits the dating of deformation, crystallization, consolidation, and, generally, the relation between diagenesis and deformation. Lateral shortening cannot be determined by straightening the strata into the horizontal if cleavage is present, but reconstruction becomes much more complicated in a “shear” fold. The author has attempted such a reconstruction using ooid distortion.