Diagenetic controls on the evolution of fault-zone architecture and permeability structure: Implications for episodicity of fault-zone fluid transport in extensional basins

Abstract

Diagenesis demonstrably changes both the mechanical and hydrologic properties of sediments. However, the effect of these changes on the distribution and types of structures that develop in fault zones over time, and their impact on fault-zone fluid flow has not previously been systematically investigated. We explored the impact of diagenesis on the mechanical and hydrologic properties of initially unlithified sand cut by a syndepositional normal fault in an extensional basin. Field, microstructural, and geochemical data document a diagenetic record of initially continuous fluid flow through a hanging-wall damage zone up to 10 m wide. In this zone, initial deformation via particulate flow is recorded by a foliation defined by a grain shape preferred orientation, preserved by subsequently precipitated, pore-filling calcite cement. A transition to episodic fluid flow is demonstrated by calcite veins that crosscut the previously cemented, foliated sandstone within relatively narrow (≤5 m from the fault core) segments of the older damage zone. Unlike the widespread record of particulate flow, veins are restricted to the vicinity of a mapped relay zone between overlapping fault segments, and an inferred, partially covered relay zone. Vein microstructures record repeated fracture opening and sealing. Veins in breccia zones have δ 13 C values as high as +6.0‰, suggesting degassing of CO 2 - and/or CH 4 -charged fluids. These data collectively suggest that relatively early damage zone cementation strengthened and stiffened the foliated damage zone, affecting the localization of brittle fractures and fluid flow in relay zones. Our results highlight systematic changes in the character and locus of deformation and fluid flow during the development of normal faults and provide a basis for predicting how these structures may act to trap or transmit fluids during the development of extensional basins.