Status: this preprint was under review for the journal SE but the revision was not accepted.
Silica diagenesis-driven fracturing in limestone: an example from the Ordovician of Central Pennsylvania
Emily M. Hoytand John N. Hooker
Abstract. Fracture patterns, interactions, and crosscutting relationships are tools for interpretation of fractures as paleostress indicators for past tectonic events and as past or present-day fluid-flow networks. In the Appalachian Basin in Central Pennsylvania along Mount Nittany Expressway Route 322 lies a significantly stratified fracture set hosted in Ordovician age limestone. Tectonic strain is a problematic mechanism for these fractures because they are hosted in individual beds lacking apparent mechanical significance relative to other limestone beds in the outcrop. Many of the fractures are layer-parallel, a characteristic commonly observed in shales, due to shales' mechanical anisotropy and tendency to develop fluid overpressures; however, these fracture-hosting limestones lack obvious mechanical anisotropy. Fracture orientations vary, but desiccation, bentonite swelling, and dolomitization are eliminated by an interpreted transgressional paleoenvironment and a deficiency of the hypothesized minerals.
X-ray diffraction determined the composition of samples collected, point-count quantification determined fracture intensity, and optical petrography recorded scaled petrographic photographs. Comparison between fracture intensity and host-rock minerals reveal that silica content is consistently depleted in fractured layers relative to unfractured layers. The diagenetic transition of biogenic silica to quartz is suggested to be the driving mechanism based on silica being present as biogenic grains, as well as cement and detrital grains, and fractures being filled with calcite cement. Silica migration explains the volume lost from fractured layers in a proposed horizontal fracturing mechanism whereby the host rock shrinks but is excluded from vertical contraction.
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Fractures in rock are commonly attributed to physical causes such as tectonism, but chemical reactions can cause fracturing as well. In this paper we show evidence from outcrop patterns, petrography, and rock mineral compositions that a fracture set in limestone is associated with silicate mineral reactions, and was most likely driven open by those reactions. A better understanding of fracture mechanisms will enable better predictions of how and when fractures transmit fluids in the crust.
Fractures in rock are commonly attributed to physical causes such as tectonism, but chemical...
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