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.

尽管通常假设裂缝源于物理构造变形，但裂缝可能是对化学过程的反应。在宾夕法尼亚州中部的阿巴拉契亚盆地内，奥陶纪石灰岩层中存在多组裂缝。一组裂缝主要是平行层，这是由于页岩的机械各向异性和流体超压趋势而在页岩中普遍观察到的特征；然而，这些含裂缝的石灰岩缺乏明显的机械各向异性。层状裂缝呈层状垂直裂缝，走向多样，呈箱形状。构造应变是一个有问题的机制，因为裂缝位于单个岩层中，相对于其他岩层而言，缺乏明显的机械意义，露头中未破裂的石灰岩层。此外，由于解释的海侵古环境和假设矿物的缺乏，与压裂相关的其他成岩过程，如干燥、膨润土膨胀和白云石化，不太可能发生。

我们使用 X 射线衍射确定了裂缝胶结物和主岩样品的组成；我们使用现场照片的点数来量化断裂强度；我们使用光学岩相学来帮助识别和确定矿物相的时间。相对于未破裂的石灰岩层，破裂的石灰岩层中的二氧化硅含量持续减少。我们认为裂缝驱动机制涉及生物二氧化硅向石英的成岩转变，基于二氧化硅以生物颗粒以及胶结物和碎屑颗粒的形式存在，裂缝被方解石胶结物填充。二氧化硅从压裂层迁移解释了在建议的水平压裂机制中从裂隙层损失的体积，其中主岩收缩但被阻止垂直收缩。或者，