Structures and Mechanics of Shear Enhanced Compaction Bands, Provence, France

Abstract

Deformation bands are zones of strain localization in porous sandstone characterized by their microstructural deformation modes and by their amount of compaction and shear components from pure dilation, pure shear to pure compaction. The mechanism for their formation remains debated, especially the role of burial depth and host rock properties on the formation of compaction bands. In this study, we describe a set of shear enhanced compaction bands formed in the porous Montmout sandstone (Provence, France) under shallow burial conditions, i.e. less than 500 m. These bands are organized in conjugate and strongly distributed network. They are located within the coarse sandstone units, i.e. with a mean grain size around 0.6 mm, whereas few or no bands are observed in the fine sandstone units, i.e. with a mean grain size less than 0.3 mm. The mechanical approach using calculated stress path and cap envelopes in Q-P diagram confirms that these compaction bands can only form within the coarse sandstone units, i.e. only the cap envelopes of these units are reached by the stress path. This analysis underlines the importance of the grain size of the host sandstone for compaction band formation under shallow burial conditions.