Microstructural characterization of hybrid dissolution-driven compaction bands in low porosity calcarenite from Cotiella Basin, south-central Pyrenees, Spain

Deformation bands form in high porosity sedimentary rocks. They are defined as tabular zones of strain localisation that may act as barriers or baffles to fluids. In nonporous rocks, the functionally analogous structures are the pressure-solution stylolites. A main difference between these two features is their dominant mechanisms; stylolites are predominantly steered by dissolution and reprecipitation whereas deformation bands are characterised by grain reorganisation due to sliding, rotation and/or cataclasis. Although the porosity of the host rock appears to influence the occurrence of deformation bands and/or stylolites, herein we document ‘hybrid dissolution-driven compaction bands’ in a non-porous calcarenite of the Upper Cretaceous Maciños Unit in the Cotiella Massif, South-Central Pyrenees, Spain. This observation raises the question: under which conditions do these hybrid bands develop in low porosity rocks? To answer this question, we combine field observations with different experimental laboratory-scale methods, in order to understand the processes for their formation. The host rock is a bioclastic calcarenite with fine, angular to sub-rounded quartz grains. The observed hybrid bands developed in wandering branches and their strike is perpendicular to the N-S shortening of the Pyrenean Orogeny. Destructive methods have facilitated the microstructural description of the hybrid bands (optical microscope), the crystallographic characteristics of quartz grains inside and outside these bands (EBSD analysis) and the chemical analysis of cementation and visualization of the internal structure of the bands (microprobe analysis with cathodoluminescence detector). Non-destructive methods (Xray micro-tomography) have been used to visualise the spatial distribution of these hybrid bands within the selected samples and quantify their textural characteristics in relation to the nearby stylolites and the host rock. Our initial results show that the accumulation of strain in quartz grains is higher inside the hybrid band than within the stylolite. This observation, combined with the increased number of fractured and more angular grains inside the deformation band -compared to the stylolite and the host rock- indicates that these hybrid, dissolution-driven compaction bands were formed in an intermittent rate. This transition is bounded by the prevalence of cataclasis of quartz in the deformation band, in contrast with the dominance of dissolution in the stylolites. Data suggest that local porosity was created due to local dissolution of the matrix, so quartz grains were placed in contact and then fractured due to the tectonic compression. This conceptual model suggests that deformation bands can develop in low porosity rocks, by creating local porosity through dissolution.