Does porosity really matter? A first model for dissolution-enabled deformation bands in low porosity rocks based on microstructural analysis of calcarenite from Cotiella Basin, Spain.

We report for the first time deformation features functionally described as deformation bands developed in low porosity rocks. This observation contradicts existing knowledge that deformation bands develop only in highly porous rocks. The studied formation is a bioclastic calcarenite of the Upper Cretaceous Maciños Unit in the Cotiella Massif. It is part of a megaflap adjacent to a salt diapir that has experienced extensional tectonics before the Pyrenean contraction. The bands present thickness variations, and in places they imitate the appearance of stylolites. This observation raises the question: how do deformation bands form in low porosity rocks?

To answer the question, we combine field observations with microstructural analysis to identify the occurring processes for the formation of deformation bands within low porosity rocks. After using optical microscopy and cathodoluminescence spectroscopy to conduct a petrographic study, we observe that the rock underwent multiple diagenetic cycles before the deformation stage, confirming that its porosity was significantly reduced before the deformation stage. Subsequently, we characterized the quartz grains inside the host rock and the dissolution-enabled deformation bands, using non-destructive imaging techniques. Three-dimensional image analysis from X-ray microtomography scans shows low grain size variations between the quartz grains in the host rock and in the bands, suggesting minor grain fracturing along the bands. However, grain reorientation has been reported for the quartz grains inside the bands, in relation to those in the host rock. Strain analysis was performed from Electron Backscattered Diffraction measurements, revealing higher strain along the quartz grain contacts inside the deformation band, compared to those in the host rock and stylolites. Our current data suggest that new porosity was created from local dissolution of the matrix, so the less soluble quartz grains were placed in contact. By wrapping-up the above observations, we propose a conceptual model that demonstrates the genesis and evolution of dissolution-enabled deformation bands in low porosity rocks, through local dissolution of the micritic matrix and transient porosity increase.