The influence of bedrock faulting and fracturing on sediment availability and Quaternary slope systems, Talla, Southern Uplands, Scotland, UK

In bedrock-dominated upland terrains, local heterogeneity in the erodibility of rock masses is a critical but under-explored factor constraining sediment erosion, mobilisation and transport. Here we examine how fault-related fracturing controls variations in the erodibility and grain-size of bedrock source material at the hillslope-scale. We then assess how this influences the evolution of slope sediment systems using a case-study from the Southern Uplands, Scotland, UK. Faults are associated with fracture densities that are an order of magnitude greater than background joint- and bedding-related fractures in weakly metamorphosed sedimentary rocks. Thus, fault zones are enhanced source areas yielding more abundant, smaller clasts. They are associated with enhanced erosion, gullying and debris flows, and the development of blanket colluvium on steep open hillsides. The orientation at which faults intersect the hillslope constrains the evolution of the sediment system. Faults with trends closely aligned to the direction of slope are associated with higher erosion via confined-channel debris flow activity in strongly coupled gullies. Faults that are oblique to slope direction disrupt and segment gully systems developed on minor transfer faults. Overall, faults that are oblique to slope direction are associated with lower erosion and give rise to decoupling within debris flow systems. Inclusion of geological weighting parameters in the formulation of a sediment connectivity index to characterise the effect of faulting on the erodibility and mobility of source material improves correspondence of the model with observations and provides a simple approach that could be adapted for other sources of geological heterogeneity.