Abstract of Seminar by Thomas Stieglitz
Submarine Groundwater Discharge from Paleochannels: "Wonky Holes" on the Inner Shelf of the Great Barrier Reef
The fishing industry has long known of seafloor depressions in the Great Barrier Reef lagoon called "Wonky Holes", referred to as such because they are a hazard to trawlers. There is much anecdotal evidence from fishermen of freshwater actively emanating from these Wonky Holes. A geophysical survey was carried out in Halifax Bay to investigate the "truth" behind the fishermen’s anecdotes.
Measurements of the salinity just above the seafloor and of the conductivity of seafloor sediment in Wonky Holes show the presence of fresh groundwater in the seafloor in these depressions, and discharge of this porewater into the bottom ocean water under favorable conditions. From sidescan and dive surveys, ca. 100 Wonky Holes were identified, and their morphology investigated. They are depressions with a diameter of 10 to 30m and a depth of up to 4m, and occur in ca. 20m water depth 7 to 10 km offshore from the coast. Numerous circular tunnels lead horizontally away from the walls of the depressions. Prolific sea-life is found in the holes compared to the surrounding seafloor. Seismic transects show that Wonky Holes are located on riverine/estuarine paleochannels, which appear to be continuously connected to the shore landwards from the Wonky Holes. Thus a connection from an inland hydraulic head to the Wonky Holes may exist, which would make the discharge of terrestrially derived groundwater possible. A simple geological model for groundwater transport in paleochannels will be presented, based on the collected data and other studies on regional stratigraphy. We conclude that submarine paleochannels may provide a pathway for groundwater to be discharged onto the inner shelf of the GBR lagoon.