Sand textural control on shear-enhanced compaction bands in poorly-lithified sandstone

Abstract

Sand textural control on SECB (shear-enhanced compaction band) formation is analyzed combining field observations, detailed material characterization and mechanical testing for poorly lithified sandstone units in Provence (France). Field observations show that SECBs are densely distributed in a coarse-grained unit with moderate porosity (27%), whereas few SECBs are developed within the overlying fine-grained, high-porosity (39%) unit. Results from textural characterization show that the main difference between the two sand units is grain size and sorting, whereas they are similar with respect to composition and grain angularity. Packing density is introduced as an important parameter for comparing the compaction properties independent of the textural variations between the two units. Compaction experiments show a slightly faster compaction of the coarse-grained sand as compared to the fine-grained sand, and more pronounced grain crushing is observed in the coarse-grained unit. The results indicate that the preferential localization of SECBs to the coarse-grained unit is controlled by a slightly denser packing of the coarse-grained material at the time of band formation together with higher stress concentrations on grain contacts. Hence, this study emphasizes that porosity alone is an insufficient parameter for predicting deformation band evolution in sand (stone).