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Faults in High Porosity Carbonates

Abstract

Deformation bands are the most common strain localization structures found in deformed granular porous rocks. Numerous documented studies on deformation bands in siliciclastics found that deformation bands create barriers to flow as a result of the following mechanisms: i) grain reorganisation, ii) cataclasis, and iii) dissolution and cementation. The former two mechanisms occur during faulting, whereas the latter mechanism is a post-deformational process that tends to occur during mesodiagenesis (i.e. >80°C). Carbonates are more susceptible to dissolution and cementation and they constitute different type/shape/size grains, thus mechanisms acting upon deformation band formation within carbonate rocks differ from the ones within siliciclastics. In this study, faults were investigated in various locations in the south of mainland Italy and on Sicily in granular high-porosity Upper Pliocene-Lower Pleistocene bioclastic calcarenites, as well as in the Upper Cretaceous chalk in the UK and other porous carbonate rocks in the Mediterranean. The variability of these deformation bands was investigated by outcrop, microstructural and petrophysical analyses. Detailed maps of study areas were produced by aerial photographic documentation using a drone, and then overlapping the stitched images with a map drawn by hand in the field. The deformation bands were found to have a wide range of distributions and architectures throughout the localities. Overall, the faults were divided into three groups: single deformation bands, clustered deformation bands and zoned deformation bands. Permeability measurements show reduction by up to 5 orders of magnitude across the bands as compared to the undeformed rock. Host rocks and deformation bands from each locality were systematically analysed and compared in order to classify different types of deformation bands in carbonates based on their known properties. The microstructure of the deformation bands also proved to vary significantly throughout different outcrops. In some bands, peloid mechanical alteration and subsequent aggrading neomorphism are the most dominant mechanisms responsible for a reduction in poroperm properties. In others only cataclasis is apparent, whilst some bands show dilational components and cementation.