Hydrogeological implications of fault-related folding in a Triassic braided sandstone

Abstract

Although our understanding of how fault-related folds influence groundwater flow has advanced, the understanding of the exact geometries and architecture of displacement propagation is often uncertain. This paper focuses on fault-related folds (i.e., fault bend, fault propagation, and detachment folding) within a braided sandstone and their implications for groundwater flow due to the internal fracture architecture. To achieve this, we combine lineament mapping, field observations, 3D regional kinematic geological restoration modelling, and juxtaposition analysis of a major aquifer system in the Permo-Triassic Sydney Basin. Our lineament mapping allowed for targeted fieldwork where outcrop observations unveiled a mechanical stratigraphy (i.e. variations in mechanical properties, layer thickness, and frictional properties of mechanical boundaries) controlled by the braided river depositional architecture. This results in systematically spaced fracture sets and rotational shearing following the sedimentary fine-grained depositional variations causing brittle folding of the sandstone mass. Our 3D geological modelling quantitated the existence of regional fault-related fold geometries. We re-interpret these folded zones as broad fault damage zones, which are likely to promote vertical fracture flow through a rock matrix otherwise dominated by horizontal porous flow controlled by the depositional braided river environment. The conceptual model for potential for flow within the folded sandstone is then placed into the regional study context by fault juxtaposition analysis of displacements across the major regional aquifers separated by a regional aquitard assuming the geometry of a fault propagation fold. Such insights were notably absent from previous conceptual groundwater models for fault-related folds in braided river deposited sandstone successions. Our findings enhance the understanding of how mechanical stratigraphy can strongly influence the internal structures of fault-related folds and, in turn, affect the groundwater system connectivity.