Evidence for temporally changing mechanical stratigraphy and effects on joint-network architecture

Previous studies of jointing in sedimentary rocks have shown that joint-network architecture is controlled by mechanical stratigraphy, which is described by (1) the thickness and rigidity of stratigraphic units and (2) the nature of interfaces between beds. Using joint pattern to infer rigidity contrast between beds is complicated by the possibility that material properties of the beds may change between fracturing events. Observation of both an early bed-contained joint set and a differently oriented late throughgoing joint set in the same beds of growth strata in the Oliana anticline in the Spanish Pyrenees may reflect changing mechanical properties (i.e., via diagenesis) during the folding process. Using a Schmidt hammer, we assess the rigidity contrast between the individual units and show that the present-day contrast is not great enough to terminate joints at interfaces. This result is consistent with an interpretation that the late-stage throughgoing joints formed in strata with conditions similar to those of the present day and that the early bed-contained joints formed when the rigidity contrast between beds was significantly greater than that of the present day. For example, differential diagenesis rates between layers of differing grain size would produce temporally changing mechanical stratigraphy. We propose that changes in mechanical stratigraphy may have previously been unrecognized in other stratigraphic sections and that these changes affect joint-network architecture and subsequent fluid-flow pathways.