Can belemnite distribution reveal pressure-solution processes along faults? A case study in the chalk of the Mons Basin, Belgium

A detailed study of normal faults in the calcarenites and chalks of the Mons Basin reveals that pressure-solution processes play a significant role in fault zones, which affects the determination of the amounts of extension. Drag folding associated with normal simple shear and rock dissolution concurred to produce layer flexuring in the hanging wall of the normal faults, whereas the footwall remained almost intact. Analysing the distribution of fossil belemnite rostra in and away from fault zones enabled us to evaluate these effects. The sense and amount of belemnite tilt constrain the minimum thickness of dissolved hanging wall fringe, revealing significant pressure-solution in addition to drag folding. We developed 2-D and 3-D analytical models of along-fault belemnite rotations induced by drag folding and dissolution, and we applied these models to our data from the Mons Basin. The results show that the pressure-solution effect is significant, implying a smaller ratio between horizontal and vertical components of motion than expected. It follows that the across-fault amount of extension is smaller than that predicted from the single fault geometry, suggesting that although the existence of Campanian–Maastrichtian–Danian extension in the Mons Basin area is beyond doubt, its amplitude should not be overestimated.