Regional‐scale development of opening‐mode calcite veins due to silica diagenesis

Abstract

The formation and distribution of natural fractures in Cretaceous‐Paleogene strata in Jordan are strongly tied to diagenetic processes, which in turn reflect the lithology of the host material. Observations collected from subsurface cores show that widespread fracturing began before compaction of the host sediment was complete, based on ptygmatic folding of one set of mineral‐filled fractures (veins). Nonfolded veins are preferentially developed within heavily cemented layers. Calcium carbonate is the greatest volumetric component of the host sediment, and most fractures are at least partially filled by calcite. Dolomite‐bearing and silica‐bearing fractures are present in dolomitized and silicified host beds, respectively. Horizontal veins are filled by cone‐in‐cone calcite or, rarely, silica or dolomite. The stratigraphic arrangement and degree of compaction around ptygmatically folded calcite veins and chert nodules suggest that silica diagenesis was an important driver of early fractures. Nevertheless, those fractures were filled with carbonate cements as they opened, based on crack‐seal texture of the vein fill. The volume loss associated with silica diagenesis created fracture porosity, which was filled coevally by carbonate cements. The distribution of later veins reflects embrittlement of host layers by cementation and is consistent with crustal deformation as the primary fracture driver.

Fractures in mudrocks: Advances in constraining timing and understanding mechanisms

“…An important example is ptygmatic folding of steeply dipping cement-filled fractures in sedimentary rocks. Such ptygmatic folds have been interpreted to form by compaction of the fracture, implying that the sediment fractured well before the consolidation process was complete (Hiscott, 1979;Karner and Shillington, 2005;Gale, 2014;Hooker et al, 2017a).…”

“…Fluid overpressure from organic matter maturation, derived from local organic components, drove fracture opening in layer-parallel fibrous calcite veins (Jochum et al, 1995;Zhang et al, 2016;Meng et al, 2017a) and at least assisted the opening of vertical fractures, whose orientation was likely controlled by regional tectonics (Hooker et al, 2017b). Volume loss from early silica diagenesis drove the opening of vertical, mudrock-hosted calcite veins (Hooker et al, 2017a). Meng (2017b) showed that the intensity of vertical and layer-parallel fibrous gypsum veins correlates directly with the amount of sedimentary gypsum in the host strata.…”

“…However, for mudrock-hosted, opening-mode fractures that are isolated from faults, the preponderance of evidence supports closed-system fluid behaviour. This evidence includes (i) fill textures indicating that cement kept up with fracture opening (i.e., fibrous fill), so that the fracture was never particularly porous throughout growth (Hilgers et al, 2001); (ii) fill mineralogy and fracture intensity that both vary sensitively with the local stratigraphy (Franks, 1969;Hooker et al, 2017a), suggesting that fracture propagation and fluid flow produced little cross-stratal migration; (iii) isotope geochemistry of fracture and host-rock minerals (Hooker et al, 2017b), whose correspondence generally supports a low-fluid, rock-buffered environment of fracturing; (iv) blunt tips, such that the fracture aperture decays rapidly beyond the end of the median zone, implying little propagation during widening (Meng, 2017a); and (v) a timing that commonly predates significant tectonic deformation (Foreman and Dunne, 1991;Maher et al, 2016;Ukar et al, 2017), during which fluids commonly advect (Evans and Fischer, 2012). Gasparrini et al, 2013;c: Heimhofer et al, 2016;d: Hooker et al, 2017b;e: Hudson et al, 2001;f: Li et al, 2013;g: Meng et al, 2017a;h: Parnell et al, 2013;i: Yu et al, 2015;j: Zhang et al, 2016. Hudson et al, 2001;b: de Haller et al, 2011;c: Gasparrini et al, 2013;d: Li et al, 2013;e: Parnell et al, 2013;f: Yu et al, 2015;g: Heimhofer et al, 2016;h: Zhang et al, 2016;i: Hooker et al, 2017b;j: Meng et al, 2017a.…”

“…Mudrock fractures are more commonly layer-parallel and filled by fibrous crystals, which may or may not include host sediment. These observations, along with mudrocks' low porosity, high organic content, and high content of relatively soluble minerals, have led previous researchers to attribute mudrock-hosted fracturing as much to diagenesis (Hooker et al, 2017a) or catagenesis (Jochum et al, 1995) as to tectonically induced stresses.…”

The Role of Chemistry in Fracture Pattern Development and Opportunities to Advance Interpretations of Geological Materials

“…Although volume changes associated with reactions are likely to be unimportant in many natural fracture systems in which the most volumetrically significant fracture cements are quartz and carbonate minerals, host rock volume changes may be important in some fine‐grained, porous carbonate rocks and shales. Numerical modeling (Okamoto & Shimizu, ) and mineralogical observations (Hooker et al, ) suggest that fractures resulting from volume‐decreasing reactions can increase permeability and facilitate continuance of reactions causing fracture, via increasing reactive surface area in contact with fluids. In other words, fracture strain makes physical space for fluid access and reactant minerals.…”