Displacement-dependent microstructural and petrophysical properties of deformation bands and gouges in poorly lithified sandstone deformed at shallow burial depth (Crotone Basin, Italy)

Abstract

We present the results of meso-and micro-structural analyses performed on fault-related soft-sediment deformation structures affecting poorly lithified, high-porosity siliciclastic sediments in the Crotone Basin, Southern Italy. The investigated extensional fault zone has a total displacement of ~90 m and juxtaposes marine clayish sediments in the hanging wall against arkosic to lithic arkosic sandstone in the footwall. In the footwall damage zone, deformation is achieved by a network of conjugate deformation bands, whereas the foliated fault core hosts cm-thick gouges. Deformation bands and black gouges accommodated displacement between 0.2 and 20 cm. Microstructural observations and quantitative image analysis pointed out that particulate flow operated during the early stages of faulting, followed by cataclasis after significant porosity loss. Mineralogy of clasts controlled grain-scale deformation mechanism: following this, feldspar experienced extensive intragranular crushing, while quartz grains were deformed mainly by splitting and abrasion. Permeability of pristine sandstone spans from 5.4 × 104 to 1.4 × 105 mD, while inside deformation bands is reduced by 1-2 orders of magnitude, reaching 3-4 orders of magnitude within fault gouges. Permeability drop inside the fault zone is related to the accommodated displacement along each deformation structure, potentially leading to hydraulic compartmentalization of high-porosity sandstone reservoir.