Coevolution of crack-seal texture and fracture porosity in sedimentary rocks: cathodoluminescence observations of regional fractures
This paper examines evidence of coupled diagenetic and mechanical processes within growing fractures in sandstones: crack-seal texture and associated, concurrently produced fracture porosity. Crack-seal textures in narrow mineral bridges associated with fracture porosity are common in regional fractures formed at moderate to great depth (>1000-̃6000 m) in quartz-cemented sandstones that otherwise lack significant structure. Use of SEM-based cathodoluminescence systems and superposition of images collected using color filters accurately delineate crack-seal increments in fracture-bridging quartz cement. Bridges and crack-seal texture mark competition between cement precipitation and opening rates during opening-mode fracture growth. These structures document episodic fracture growth that can include tens to hundreds of widening increments in fractures having apertures of a few tens of microns to several millimeters or more. These structures are not the product of unique circumstances in burial history and fluid flow but, rather, reflect the confluence of rock-dominated geochemistry that is widespread in time and space and fracturing caused by a spectrum of loading conditions.