OÖLITE DEFORMATION IN THE SOUTH MOUNTAIN FOLD, MARYLAND 论文

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Research Article| September 01, 1947 OÖLITE DEFORMATION IN THE SOUTH MOUNTAIN FOLD, MARYLAND ERNST CLOOS ERNST CLOOS THE JOHNS HOPKINS UNIVERSITY, BALTIMORE 18, MD. Search for other works by this author on: GSW Google Scholar Author and Article Information ERNST CLOOS THE JOHNS HOPKINS UNIVERSITY, BALTIMORE 18, MD. Publisher: Geological Society of America Received: 22 Aug 1946 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1947, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1947) 58 (9): 843–918. https://doi.org/10.1130/0016-7606(1947)58[843:ODITSM]2.0.CO;2 Article history Received: 22 Aug 1946 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation ERNST CLOOS; OÖLITE DEFORMATION IN THE SOUTH MOUNTAIN FOLD, MARYLAND. GSA Bulletin 1947;; 58 (9): 843–918. doi: https://doi.org/10.1130/0016-7606(1947)58[843:ODITSM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search 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 oöid distortions in Cambero-Ordovician oölites. 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 oölites seem to indicate that deformation affected rather soft and pliable and maybe little consolidated rocks. Undistorted growth aprons on deformed oöids 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 oöids 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 oölites 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 oöid distortion. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.