Structural evolution of carbonate-hosted cataclastic bands adjacent to a major neotectonic fault, Sellicks Beach, South Australia

Abstract

Deformation bands are common fault-related strain features found in upper crustal granular highly porous sedimentary rocks. However, the majority of research has focused on siliciclastic-related case studies, resulting in a knowledge gap surrounding the nomenclature and implications of carbonate-hosted cataclastic bands. We present a systematic case study of carbonate-hosted deformation bands in the Oligocene–Miocene Port Willunga Formation (PWF) exposures along the wave-cut platforms at Sellicks Beach, South Australia. Sellicks Beach provides an exceptional location to study carbonate-hosted deformation bands because of the unique juxtaposition of the carbonate grainstone PWF and the NE-SW striking Willunga Fault. The Willunga Fault is a steep SE-dipping active reverse fault that previously accommodated deposition in a Cenozoic half-graben. We have conducted a comprehensive structural analysis of 269 PWF deformation bands that formed as a result of strain induced by the Willunga Fault. Our model identifies three sets of shear-enhanced compaction bands (SECBs), and one set of pure compaction bands (PCBs) within the PWF. The PWF deformation bands display a range of cataclasis intensity defined by the percentage of microscopic fine-grained groundmass (gm), named here as crush micro-breccia (0–10% gm) and protocataclastic bands (10–50% gm), resulting from comminution, stylolite formation, and cementation. Petrophysical analysis indicates that crush micro-breccia bands reduce permeability between 0.0–0.75 orders of magnitude, and protocataclastic bands reduce permeability between 0.25–1.75 orders of magnitude relative to host rock. Our results indicate that PWF deformation bands are a useful proxy for cataclasis intensity, forming the foundation for a new classification scheme to describe cataclastic deformation bands, specifically applied to carbonate rocks.