Assessing topographic controls on flow direction in washover deposits using measurements of Magnetic Fabric

• We identify a set of overwash deposits enclosed by buried soils.
• Direction of flow during deposition is determined via Magnetic Fabric analysis.
• Flow direction is controlled by topography and river configuration.
• Tsunami is determined to be the most likely overwash mechanism.
• Flow direction is important for interpreting paleo-events from their deposits.

A set of anomalous sand deposits enclosed between paleosol layers were identified in Okains Bay on eastern Banks Peninsula, New Zealand. These were investigated to determine flow patterns during their deposition and the influence of topography on these patterns. This was achieved using a combination of sedimentary analyses, Magnetic Fabric (MF) and Ground Penetrating Radar (GPR). Two washover facies were identified, based on stratigraphic context, particle size analyses and flow direction during deposition from MF results. These deposits, which decrease in thickness with distance inland, comprise two layers of fine-medium sand and are situated 1.3 km from the coast on an estuary margin. Results show the sand sheets were deposited as overwash from the estuary, with the extent and deposition patterns controlled by local topography and the presence of relict dune ridges. MF is demonstrated as a versatile and important technique for determining flow patterns and depositional mechanisms. Flow patterns identified here from MF highlight the importance of river channels as conduits for short-lived high energy marine inundation events. Additionally, these results support the importance of topography in controlling deposition patterns, which has important implications for the reconstruction of events where the paleotopography is unknown.