Bachelor of Arts
Folded sedimentary strata, Buckle, Appalachian Valley, Ridge, Tonoloway limestone, United States
Large scale thrusts and imbricates overlain by folded sedimentary strata characterize structure in the Valley and Ridge Province of the Central Appalachians. The Cambrian Waynesboro Formation is a decollement zone that detached an imbricated Cambro-Ordovician sequence from an unfaulted Pre-Cambrian basement. The Ordovician Martinsburg Shale is a second zone of major detachment that de-coupled the blind thrust system in the Cambro-Ordovician carbonates from the overlying orogenic wedge. Thus, the Central Valley and Ridge deformed during the late Paleozoic Alleghenian orogeny as a three tiered system consisting of the undeformed basement, the imbricated stiff layer, and the primarily folded cover layer.
A road cut along the eastern side of Martin Mountain exposes a smaller (probably second-order) fold in the Silurian Tonoloway limestone that belongs to the cover layer. This anticline is one of a train of regular folds with wavelengths of 150 to 250 m. A small hinge region and symmetrical planar limbs characterize this angular, open anticline. Bed thickness remains relatively constant throughout the fold, making it a class IB (parallel) or IC fold. The anticlinal hinge trends 28° east of north and dips 8° to the north. A poorly exposed, smaller syncline flanks its eastern limb.
The Tonoloway Limestone formed during the late Silurian on a carbonate platform. During Paleozoic deformation, it lay under 3 km of sediments at a temperature of 250°C. This outcrop exposes only 80 m of the 550 m thick formation. Cathodoluminescence reveals that the sediments comprise only calcite, dolomite, organic rich clays, and trace amounts of quartz. These fine-grained arenites and lutites display neither noteworthy fossil content nor sedimentary structures oblique to bedding. Bed thicknesses vary from 0.5 m. to 2 m.; planar lamination defined by grain size and clay content occurs in most beds.
Slickensides on primary bedding surfaces indicate that flexural slip was an important mechanism during folding. Mesoscopic and microscopic structures accommodate strain within the layers. Two morphologically distinct cleavages within the fold are non-coaxial. Clay selvage seams indicate that both cleavages resulted from pressure solution. Whereas one spaced cleavage fans convergently around the fold hinge in the more competent beds, the second, more penetrative cleavage fans divergently around the fold hinge in the less competent beds. The non-coaxiality of these two cleavages helps define the chronology of and tectonic stress field during the shortening event that produced the cleavages and the fold.
In addition to establishing a deformation history of folding, mesoscopic and microscopic structures speak to the question of strain behavior and layer rheology of the folded Tonoloway Limestone. The persistence of pressure solution surfaces and fibrous veins throughout the fold suggest linear or Newtonian behavior in the deformed strata. In the fold limbs, twinning and undulose extinction patterns in calcite crystals signal power law or plastic behavior. Fold geometry, layer rheology, and geologic setting suggest that buckling best defines the mechanism of folding here.
In the Central Valley and Ridge, new cross sections show that the stiff tier of thrusts and imbricates controls megascopic structural morphologies in the cover layer. Regional anticlinoria and synclinoria formed by either fault bend folding or passive kinking during emplacement of imbricates in the stiff layer. The train of second order buckles, in which this exposed anticline formed, occurs, however, in a smaller scale environment, which, although it allowed for local buckling, results from the geometry of regional imbrication in the stiff layer.
Traditionally, cross sections of this region fail to provide a long enough cover layer to blanket the unthrusted stiff layer. Volume loss strain (i.e. loss through percolating meteoric fluids) in shallow deformed rocks may account for one element of this problem with balancing the two tiers together. If volume loss strain is more significant in one layer than in the other, an unraveled cross section naturally displays an unbalance between the two. Although volume loss strain directly relates to structures and strain patterns, this phenomenon has interested geologists only recently. As a result, no general methodology exists for measuring it. Volume loss strain challenges the notion that structures such as folds and cleavages develop within a closed system, in which material dissolved in areas of high stress reprecipitates locally in areas of low stress. Volume loss strain, however, requires an open system from which foreign fluids introduced by significant dewatering or infiltration remove material.
Markley, Michelle J., "Strain and Volume Loss in a Second Order Buckle Fold, Central Appalachian Valley and Ridge, U.S.A." (1990). Honors Papers. 579.