Microstructural characterization of deformation bands in shallow porous carbonates of Apulian Platform, Southern Italy

Deformation may exert a positive or negative impact on rock strength, stiffness and porosity depending on the initial properties of the rock. In porous rocks, deformation bands (DBs) reduce both porosity and permeability, and potentially increase rock strength and stiffness. In low porosity (tight) rocks, deformation occurs via shear and opening-mode fractures, thus enhancing permeability and deteriorating strength and stiffness.

In this study we describe field observations and laboratory analyses to characterize sub-vertical deformation bands in the porous carbonates of the Calcarenite di Gravina Fm. exposed in Matera and Gravina in Puglia, Southern Italy. Matera and Gravina in Puglia are located at the boundary between the Apulian foreland and the foredeep of the Southern Apennines thrust belt. Both study areas consist of an asymmetrical horst structure involving the Cretaceous (Senonian) tight carbonates of the Apulian platform (Calcare di Altamura Fm.) unconformably overlain by Plio-Pleistocene shallow-marine coarse-grained lithic sandstone and grainstone (Calcarenite di Gravina Fm.). The Calcarenite di Gravina Fm. is dominated by pervasive DBs organized into 2 main sets dipping at high angle and striking N-S and NNE-SSW, except in some limited areas where the DBs form a complex network with the presence of a secondary set, striking NW-SE, showing mutual crosscutting relationships. The DBs have a positive relief, due to their relatively higher resistance to erosion, and appear whitish, tabular with some slight undulations, 10’s of meters long in map view, continuous and steeply inclined (from 75° to 90°). These structures have thicknesses from few mm up to a few cm depending on the facies of the calcarenite. At the outcrop scale the DBs don’t seem to accommodate significant shear offsets. A total of 53 oriented samples were collected for thin sectioning, petrophysical, and microstructural analysis. We acquired porosity measurements using a Hg-intrusion porosimeter, which showed that the DBs have an average porosity of 11%, while for the host rock is 38%. 29 Blue-impregnated thin sections of host rocks and DBs were analysed by standard microscopy, cathodoluminescence (CL), and a scanning electron microscope (SEM). The analysis of the CL images shows that the clast size distributions are similar in the DBs and host rock, and they are not crushed or fractured. Furthermore, while in the host rock the clasts are randomly oriented, in the DBs they are iso-oriented with the long axis describing low angles to the band.

Our work shows that these DBs are characterized by the absence of grain size reduction with a strong preferred orientation of long axes without significant fragmentation. These data may indicate that DBs formed in high porosity conditions, at very shallow depths. Moreover, given the strong difference between the petrophysical properties of the host rock and DBs and the apparent abundance and continuity of the DBs, they would provide a significant anisotropy in the flow pathways of the calcarenite.