CHARACTERIZING WETLAND DEVELOPMENT AND INFILLING USING GPR ON NORTH STRADBROKE ISLAND (MINJERRIBAH), AUSTRALIA

Palustrine and lacustrine wetlands are often listed as components of “young” landscapes yet the world’s largest sand islands (Fraser (K'Gari), North Stradbroke (Minjerribah) and Moreton (Mulgumpin) Islands) which may be older than 1 Myr, all host numerous such wetlands. The wetlands on these islands are in dune morphosequences that have been shown to predate ~150 kya and previous work has dates from basal or near basal wetland sediments that exceed ~130 kya. So how does an island composed of mobile, unconsolidated sand host wetlands that are persistent and have existed through an entire glacial cycle?

During October 2018 and 2019, high-resolution and deep penetrating ground penetrating radar surveys were conducted over Brown Lake (Bummiera), Welsby Lagoon, Fern Gully and 18 Mile Swamp on North Stradbroke Island in an effort to better understand the context of several cores from these locations. An 80 and 160 MHz GPR system was hand dragged or vehicle towed around the margins of the sites to provide a geologic context of the basin. The 160 MHz GPR system was also hand dragged over wetland surfaces while both 80 and 160 MHz were rafted over open water for understanding water depth, sediment infill and pre-wetland basin morphology. The 80 MHz antenna was effective to below ground depths of 40 m on land and 25 m over water while the 160 was effective to 25 m over land and about 18 m over wetland sediment and water.

The surveys revealed a great deal of insight about the basins and infill. On the margins, there are clear examples of buried shoreline deposits that range from strandline beaches to prograding beaches and migrating dunes. Infilling sediments were observed to have characters that ranged from draped to ponded. Horizons of disturbance were observed throughout the wetland interval and thought to correlate to periods of burning observed as charcoal peaks in cores. Below the wetland units and on land surrounding, sedimentary units correlative to podsol soil sequences were observed.

Work is ongoing to correlate the GPR data with the sediment core archive to create a better understanding of the basin development as well as a better understanding of past water levels through mapping buried lowstand beaches and changes to depositional style observed on the GPR, but not visible at the sediment core scale. These insights will allow better conceptualization of the drivers of current wetland water regimes and climate controls on the system.