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Variations in Syndepositional Fault Properties as a Function of Stratigraphic Position in High-Relief Carbonate Platforms

Abstract

Recent studies highlight the structurally dynamic nature of many carbonate platforms and show the significance of syndepositional deformation on permeability evolution and long-term fluid flow. However, questions remain on how fault properties vary in response to stratigraphic hierarchy. To address this we compare two well-documented syndepositional fault systems in the Permian of the Guadalupe Mountains that developed below, and above, a major subaerial exposure surface. The older faults, developed in composite sequence 13 (CS13), are positioned below a subaerial exposure surface at the CS13-14 boundary. They are enlarged by extensive early dissolution with 10-25 m wide paleo-caverns developed along the faults and a petrophysical “damage zone” that extends 5-25 m further. The paleo-caverns contain complex siliciclastic-rich fills with varying amounts of breccia. In contrast younger faults, developed in CS14, show only minor dissolution and limited siliciclastic fill. Instead, these faults zones are filled by coeval carbonate cements and sediment, and evaporites. Fault zones in this setting consist of a 0.1-1.5 m wide aperture, flanked by 1-15 m wide damage zones marked by increased fracturing, brecciation, and modest dissolution. The geometry, offset, and associated growth strata of the two fault systems are comparable, so it appears that structure was not a major control on fault zone attributes. Instead, variations in fault-zone width are interpreted to reflect the degree of early dissolution. Early dissolution is related to meteoric or mixing-zone dissolution, the relative intensity of which is dictated by stratigraphic position. Given their position below the CS13-14 boundary, meteoric dissolution is interpreted to be the dominant control the CS13 fault properties. In contrast, no major exposure event has been identified in CS14. Here mixing zone dissolution, and to a lesser degree, meteoric dissolution associated with high-frequency exposure are interpreted to affect fault properties. The contrasting fill of the resultant fault zones cavity systems is also primarily related to stratigraphic position. Fault fills reflect the abundance of clastic material in CS13 and at the CS13-14 boundary, and limited siliciclastic sedimentation within CS14. The results presented here highlight the importance of understanding the timing of deformation relative to stratigraphy when characterizing syndepositional fault/fracture properties in carbonate strata.