Chert nodules in pelagic limestones as paleo-stress indicators: A 3D geomechanical analysis

The objective of this study is to explain the occurrence and paleo-stress significance of 3D joint clustering in chert nodules (inclusions) within a layered pelagic limestone sequence. The difference in stiffness between chert and limestone is about one order of magnitude. Field observations show that fracture localization occurs mostly in chert nodules as opposed to the limestone matrix. We show with a novel three-dimensional geomechanical modelling analysis how the inclusion (ellipsoid) axes ratio influences fracture intensity and propagation within and outside the chert nodules and how the nodules record different deformation phases under different remote stress conditions. From field observations, we recognize two joint sets in the chert nodules: joints parallel and normal to the plane containing the two major axes of the nodule (bedding plane). In the nodules, the normal-to-bedding joints are interpreted to be younger than the parallel-to-bedding ones. The modelling of the 3D Eshelby solution for the stress field inside the chert nodule and in the surrounding matrix is consistent with our field observations and it suggests a strong differential stress during deformation (sigma(r)(max)/sigma(r)(max) < 0.3). Chert nodules in a deformed carbonate sequence, therefore, can provide important clues on the paleo-stress conditions, the temporal sequence of events, and fracture distribution heterogeneity.