STRUCTURAL DIAGENESIS OF FAULTS IN SANDSTONE (Invited Presentation)

Structural diagenesis, the study of the relationships between deformation or deformational structures and chemical changes to sediments, is an essential approach to understanding fault and fracture attributes in sedimentary rocks. I illustrate the interaction of diagenesis and structure with an example from meter- to decimeter-displacement oblique-slip faults that cut latest Precambrian lithic arkose to feldspathic litharenite and Cambrian quartz arenite sandstones in NW Scotland. Despite common slip and thermal histories during faulting, the two sandstone units have different fault-core and damage-zone attributes, including fracture length and aperture distributions, and location of quartz deposits. Fault cores are narrow, less than 1 meter, low-porosity cataclasite in lithic arkose/feldspathic litharenites. Damage zone-parallel opening-mode fractures are long, meters or more, with narrow ranges of lengths and apertures, are mostly isolated, have sparse quartz cement, and are open. In contrast, quartz arenites, despite abundant quartz cement, have fault cores that contain porous breccia and dense, striated slip zones. Damage-zone fractures have lengths ranging from meters to centimeters or less, but with distributions skewed to short fractures, and have power-law aperture distributions that tend to be sealed by quartz cement. Inhibited authigenic quartz accumulation on feldspar and lithic grains, which are unfavorable precipitation substrates, and favored accumulation on fractured detrital quartz accounts for these differences. In quartz breccia, macropores >0.04 mm wide persist where surrounded by slow-growing euhedral quartz. Differences in quartz occurrence and size distributions are compatible with cement deposits modifying the probability of fracture reactivation. Existing fractures readily reactivate in focused growth where quartz accumulation is low and porosity high. Only some existing, partly cemented fractures reactivate and some deformation is manifest in new fracture formation in partitioned growth where quartz accumulation is high. Consequences include along-strike differences in permeability and locus of fluid flow between cores and damage zones and fault strength.