Tournaisian Tectonic Phase With Major Carbonate Buildup Structures in the Campine Basin (Northeastern Belgium)

ABSTRACT By means of seismic interpretations, this study provides improved constraints on a major Tournaisian (lowermost Carboniferous) tectonic phase with faulting across the Campine Basin, northeastern Belgium. Faults are normal with throws below 100 m, except for some larger intra‐rift horst and graben structures with throws up to 300 m. In an asymmetric graben structure in the southern study area, an estimated average of 1000 m of Tournaisian sediments accumulated. Outside the graben, Tournaisian thicknesses are in the order of 300–500 m, which agrees with the limited available well data outside the study area of the Campine Basin. There is an uncertainty on fault strikes since the individual fault segments are short compared to the spacing between the seismic lines, but we estimate it to vary between SW–NE and WNW–ESE. The wide range of fault strikes can be related to the reactivation of pre‐existing faults in the Cambro‐Silurian basement. The SW–NE and WNW–ESE directions of the Tournaisian fault strikes have been identified as lineaments on gravimetric and aeromagnetic maps of the lower Palaeozoic Brabant Massif to the southeast and southwest of the study area, respectively. Such fault strikes imply a roughly NNW–SSE to N–S extensional stress field prevailing in the area during the Tournaisian. The range of fault strikes is very similar to the strike of contemporaneous faults in Ireland and the United Kingdom, which suggests that the NNW–SSE to N–S extensional stress field occurred throughout much of northwestern Europe. The Tournaisian succession of the Campine Basin includes numerous mound‐shaped complexes, interpreted as buildup structures. We show examples of major buildup complexes that developed in graben structures. One of them reaches a height of 750 m and is 3 km wide. Given the similarity in timing of formation and size of the buildup complexes in the Campine Basin with buildup complexes in southern Belgium and Ireland, we consider it likely that the buildup complexes in the Campine Basin represent Waulsortian mudmounds.