Adam D Switzer

Sea Level, Storm, or Tsunami: Enigmatic Sand Sheet Deposits in a Sheltered Coastal Embayment from Southeastern New South Wales, Australia

Abstract An extended period of marine-dominated back-barrier sedimentation, truncated by the deposition of two large-volume, laterally extensive sand sheets that extend up to 3 km inland from the modern coastline, is present at Minnamurra in southeastern Australia. The sand sheets are anomalous as they drape the entire back-barrier succession and infill the estuary. The lower sand sheet contains mud clasts and significant amounts of organic matter including coastal dune grasses indicating that significant overwash and erosion incorporated material from both the barrier and estuary into the lower sand sheet deposit. The first sand sheet is overlain by a poorly developed soil that records a time break before the now infilled system was inundated once more. The second sand sheet covers the poorly developed soil and incorporates a number of erosional features, such as ripped-up soil clasts, cobbles, and organic-rich sand. Along this part of the coast, large laterally extensive sand sheets are spatially and sedimentologically unique, occurring only in embayments that face southeast. This, along with a lack of evidence of wave action or beach structures, suggests that the deposits are not evidence for higher Holocene sea level but are the result of overwash sedimentation from the southeast. The sand sheets share many of the characteristics of those emplaced by tsunami waves, although the possibility of exceptionally large storm events cannot be entirely discarded. However, such a storm would have to be several orders of magnitude higher than those in recorded history.