Silica diagenesis and natural fracturing in limestone: An example from the Ordovician of Central Pennsylvania

Abstract

Although fractures are commonly assumed to stem from physical, tectonic deformations, fractures may instead form in response to chemical processes. Within the Appalachian Basin in Central Pennsylvania lie multiple sets of fractures hosted within layers of Ordovician age limestone. One set of fractures is dominantly 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. The layer-parallel fractures are found with layer-perpendicular fractures having various strikes, forming a boxwork-like pattern. Tectonic strain is a problematic mechanism because the fractures are hosted in individual beds lacking apparent mechanical significance relative to other, unfractured limestone beds in the outcrop. Furthermore, other diagenetic processes associated with fracturing, such as desiccation, bentonite swelling, and dolomitization, are unlikely because of the interpreted transgressional paleoenvironment and a deficiency of the hypothesized minerals.We determined the composition of fracture cement and host-rock samples using X-ray diffraction; we quantified fracture intensity using point counts of field photographs; and we used optical petrography to aid in the identification and timing of mineral phases. Silica content is consistently depleted in fractured limestone layers relative to unfractured limestone layers. We interpret that the fracture driving mechanism involved the diagenetic transition of biogenic silica to quartz, based on silica being present as biogenic grains as well as cement and detrital grains, and fractures being filled with calcite cement. Silica migration away from fracturing layers explains the volume lost from fractured layers in a proposed horizontal fracturing mechanism whereby the host rock shrinks but is prevented from contracting vertically. Alternatively, or simultaneously, silica precipitation may have decreased permeability, thus promoting fracturing via fluid overpressure.