Hectometer-scale, shallow buried honeycomb-like structures on the continental shelf of the Otway Basin, southeastern Australia

The Plio-Pleistocene Whalers Bluff Formation of the offshore Otway Basin is composed of mixed siliciclastic-carbonate sediments. In seismic cross sections, this formation includes an interval that consists of higher amplitude seismic reflections that display alternating depressional ponds and raised ridges. This interval is shallowly buried and lies between 40-150 ms two-way travel time below the present-day seafloor. In this study, we used 2-D and 3-D seismic datasets in combination with the available shallow subsurface well logs, to characterize the geomorphology and investigate the origin of these enigmatic features. The ponds are expressed as densely packed, circular to polygonal, and in some cases, hexagonal-shaped features in time slice maps, and closely resemble previously documented honeycomb structures. In our study area, the honeycomb-like structures (HS) are comprised of large (200 to 800 m diameter range) depressed ponds that are separated by narrow (∼20 m at the top) reticulate ridges. In total, these HS cover an area of 760 km2. GIS analysis shows that the ponds of HS, especially those in the NE of the study area, are aligned along the NW-SE trend-lines. There are several possible origins for the HS. The most probable mechanism is that the HS are resulted from the bulk contraction of soft sediment, associated with shallow burial diagenesis processes such as subaqueous dewatering of the fine-grained successions within the Whalers Bluff Formation. Interestingly, irregular furrows of various lengths on the seafloor correspond to the ridges of the HS, and we hypothesize that these furrows may have formed due to differential compaction of the underlying alternating ponds and ridges. Our results demonstrate the benefits of using seismic reflection datasets in combination with geospatial analysis to investigate the buried paleo-geomorphologic features and their impact on the present-day seafloor physiography.