Large-scale slumping deduced from structural and sedimentary features in the Lower Palaeozoic Anglo-Brabant fold belt, Belgium

Abstract

Distinguishing slump folds from tectonic folds in poorly exposed areas can be difficult, especially when the scale of the slump folds exceeds outcrop scale. In the southeastern part of the single-phase deformed, Lower Palaeozoic Anglo-Brabant fold belt a comparison of cleavage/fold relationships and stratigraphic polarity shows that a 200 m thick interval of middle Caradoc fine-grained turbidites in the core of a large synform was overturned prior to tectonic deformation. This overturning is attributed to large-scale slumping, which was most likely a result of middle Caradoc seismic activity. The exposed portion of the large slump sheet contains only a few small slump folds and intraformational breccias, making up less than 5% of the exposed thickness. If the beds were not overturned, large-scale slumping would never be suspected and the small slump folds would probably be interpreted as localized features in an overall ‘stable’ sedimentary pile. This may explain why so few ancient large-scale slides and slumps have been reported: the small amount of internal deformation makes them very difficult to recognize, especially when dealing with poorly exposed areas. As such, large ancient slides and slumps may be more common than suggested by the geological literature.