This study aims to determine the geometry of a soapstone body quarried in central Virginia for dimension stone. The soapstone is thought to have originated as an ultramafic igneous rock. Hydrothermal metamorphism metamorphosed the ultramafic rock into soapstone, which is primarily talc – one of the softest minerals known – and serpentine. The ultramafic body has been hypothesized to have been emplaced as an igneous dike, sill, or have been emplaced as an ophiolite sequence. Field work as well as mapping was done to determine the geometry of this body. Cores were drilled at the Old Dominion Quarry, and the Climax & Nash Quarries and were logged for color, texture, and mineralogy. They were also analyzed with the X-ray Fluorescence Analyzer (XRF) and Schmidt Hammer (SH). The XRF measured the chemistry of the core, and the SH measured the compressive strength, or “toughness” of the rock. After the analyses were complete, lithologic columns, a series of graphs, maps, and a cross section were made to determine the geometry of the body. The cross section drawn with the lithologic columns gave one of four possible outcomes for the emplacement and geometry of the soapstone. The cross-section was drawn along the plane of cores CL1 and CL2, with core CL3 being further “behind” the cross-section plane. Core CL1 has a repeating sequence of soapstone, to mylonitic gabbro, to gabbrodiorite, to soapstone, to mylonitic gabbro, to soapstone, and ended with Lynchburg Group. Core CL2 has Lynchburg Group on top, then goes to felsite, to soapstone, so felsite, to soapstone. Core CL3 starts with Lynchburg Group, goes to soapstone with felsite dikes, to mylonitic gabbro, to soapstone. The placement of the Lynchburg Group on cores CL2 and CL3 is bizarre due to the proximity to core CL1. With this data the emplacement and geometry comes down to one of four possibilities: these cores were drilled at the edge of the sill, faulting thrusted the Lynchburg Group away from the top of core CL1 and caused the repeated sequence, there was regional folding with an isoclinal hinge that repeated the sequence, or a chunk of the Lynchburg Group sank into the magma as it was cooling, freezing it in place. Based on the data, it is concluded that the soapstone originated as an igneous sill with normal faulting that thrusted the Lynchburg Group from the top of core CL1 thus causing a repeated sequence of lithology.
Mojica, Gabriel, "Structural Geometry of Mafic/Ultramafic Bodies Near Schuyler in the Eastern Blue Ridge, Central Virginia" (2021). Geology Senior Theses. Paper 8.
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