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Paper No. 47-3
Presentation Time: 9:00 AM-5:30 PM
REDISTRIBUTION OF MATERIAL IN MICROFABRICS ASSOCIATED WITH MACROSCOPIC FOLDING: OBSERVATIONS FROM THE JACKSONBURG LIMESTONE, PENNSYLVANIA
The Ordovician Jacksonburg micrite limestone in eastern Pennsylvania, outcropping in the hanging wall of the Kutztown thrust fault, is characterized by three distinct micro-structures and fabrics. The spatial-temporal relationships among these structures enable their relation to mapped macrostructures on a kilometer scale. This study uses petrographic microscope analysis to identify and quantitatively characterize the microfabrics. Earliest-stage deformation is recorded by a 0.07-0.14mm amplitude crenulation cleavage. This fabric is crosscut at a perpendicular angle by an axial planar cleavage. The cleavage plane is characterized by an ~0.5mm thin clay residue after pressure dissolution. The axial planar cleavage planes form several stepovers, 3-4 mm apart. The zone between the stepovers is characterized by local extension through the formation of micro-fractures forming a light grey band. This zone contains larger calcite crystals compared to the rest of the sample and causes refraction of the other two cleavages. These larger euhedral calcite crystals have a high aspect ratio and maintain a consistent angle relative to the orientation of the light gray band. Calcite is overgrown by quartz, and then clay. We interpret the axial cleavage planes to be part of a micro-shear zone due to a second deformation event and the band to accommodate extension within the stepover zone. The region of localized extension allows for reprecipitation of calcite, quartz, and clays, and thus indicates material redistribution during folding is influenced by microfabrics that are associated with the macroscopic deformation events. This study is part of an ongoing project that analyzes the timing, preservation, and interplay of microstructures in fine-grained carbonate rocks that are highly susceptible to dissolution. Such microscopic interactions may collectively affect processes such as fluid flow within macroscopic structures.