Opening history and porosity evolution of fractures in sandstone, Triassic to Jurassic La Boca Formation, Northeast Mexico

Sandstone outcrops of the Triassic to Lower Jurassic La Boca Formation, NE Mexico, offer an unusually well preserved example of cements within opening-mode fractures (veins and joints). Although all fractures in these outcrops that can be reliably attributed to subsurface deformation contain cements, some retain open pore space whereas others have been sealed. Quartz cement within otherwise open fractures contains crack-seal textures that record fracture opening history. Elsewhere otherwise identical fractures are sealed with calcite. Outcrops contain fractures that range over three orders of magnitude in size quantified by kinematic aperture. Kinematic apertures of sealed microfractures and open and sealed macrofractures have distributions that can be described by power law scaling. Fracture size and cements deposited during and after fracture opening govern fracture porosity evolution. Although similar patterns have been found in many cored fractures from low-porosity sandstones, my study is the first to document these patterns in outcrop. I used aperture measurements along scanlines, petrography, fluid-inclusion analysis and high-resolution scanning electron microscope-based cathodoluminescence (CL) mosaics to characterize cement and fracture populations. CL imaging of quartz cement reveals crack-seal textures that show cement precipitation while fractures were opening. Using recent cement precipitation models and maps of cement deposit patterns within crack-seal cement bridges, I infer that cement deposition proceeded at rates comparable to overall fracture pattern formation but slower than individual cracking episodes. My observations are consistent with a model of quartz precipitation in fractures proposed by Lander et al. (2002) in which thermal exposure history and rock surface area govern cement accumulation rate and patterns.Geological Science