GSA Connects 2021 in Portland, Oregon

Paper No. 236-9
Presentation Time: 4:00 PM


GONTZ, Allen1, TIBBY, John2, MARSHALL, Jonathan3, BARR, Cameron2 and HOFMANN, Harald4, (1)San Diego State University, Department of Geological Sciences, San Diego, CA 92182-1020; Department of Civil and Environmental Engineering, Clarkson University, Potsdam, NY 13676, (2)Department of Geography, Environment and Population, University of Adelaide, Adelaide, SA 5005, Australia, (3)Water Planning Ecology, Science Division, Queensland Department of Environment and Science, Dutton Park, QLD 4001, Australia, (4)School of Earth and Environmental Science, University of Queensland, St Lucia, 4072, Australia

Minjerribah is one of the world’s largest sand islands built by sands transported from New South Wales via longshore drift. Dunes stabilized by vegetation with elevations > 240 masl dominate. Parabolic dunes result in depressions at various elevations. Higher depressions host lakes or wetlands that are supplied with water from small perched aquifers (i.e. Brown Lake; Welsby Lagoon). Lower depressions host lakes and wetlands that tap the regional aquifer (i.e. Blue Lake; 18 Mile Swamp). The perched systems respond to rapidly changing climate conditions, especially rainfall, while window systems are more robust and buffered by the large, regional aquifer.

In addition to their sensitivity to variations in climate, which make them ideal archives for reconstructing past environmental change, they play key roles in culture of the Quandamooka People. If Fern Gully has been persistent as a source of water for >60,000 years, then Minjerribah may have been an ideal refuge for early indigenous people and fauna.

Fern Gully, in the northwest corner of the island (27.417183oS, 153.460471oE), is at 39 masl. Sediment cores recovered thick lacustrine and palustrine sediments and adjacent morphology suggest geologic controls. Within ~75 m of the northern shoreline is a scarp with ~30 m of relief and a canyon cut into the scarp comes within 20 m of the shore and has a sill < 2 m above present wetland surface. Evidence of water flowing across the sill, cascading over the scarp and developing the canyon is evident. The morphology of the site results in several questions about the site’s architecture, hydrology and evolution:

  1. What is the morphology of the basin floor?
  2. Does Fern Gully exhibit similar stratigraphy as Welsby Lagoon – i.e. dune, wetland, early lake, late lake wetland phases?
  3. Are shoreline complexes evident – above and below current wetland elevation?
  4. With a large scarp nearby and the system developed in unconsolidated sand, how is Fern Gully retaining water?

A 2019 GPR survey of Fern Gully employed high definition 160 and 80 MHz GPR on the fire breaks north and west of Fern Gully and 160 MHz over the surface of the wetland. The data has been processed and initial results will be presented. Work to further analyze the subsurface architecture and relate GPR data to sediment cores and hydrologic maps and modeling continues.