Denuded caves of the Nullarbor Plain

The Nullarbor Plain in southern Australia is one of the largest contiguous karst area in the world, constructed from horizontal beds of three distinct units of limestone. Around 14 million years of exposure under relatively arid climate, this extremely flat plain was a subject of cave formation, grouped into less than 30 m deep “shallow caves”, and genetically unrelated “deep caves”, extending 50-120 m below the surface with large passages and chambers, sometimes collapsed to the extent to form collapse dolines on the surface. Detailed topographical TandemX data recently became available, and our preliminary discovery of peculiar shallow and hardly detectable elongated depressions suggests the presence of denuded caves, most likely associated to deep caves. Also known as unroofed or roofless caves, denuded caves are the final stage of cave development, when the ceiling has been completely removed either due to chemical denudation or due to the collapse of the ceiling. Their discovery on the plain with the absence of orogenesis, glaciation or extensive sediment deposition offers a unique controlled environment to study their formation and their comparison to the worldwide occurrence where the formation of individual denuded cave may be blurred by a complex landscape evolution, so our first objective is to advance our understanding of the processes that lead to denuded caves on a global scale.

Yet there is more to the story. Whilst the plain experiences homogeneous climate from west to east, the denuded caves and often associated collapse dolines show an unusual distributional pattern: on the east, denuded caves occur as continuous and straight channels several kilometres long. In the central part they become fragmented and associated with clusters of collapse dolines. Denuded caves eventually disappear on the western half of the plain, whilst collapse dolines do continue to occur but become sporadically rarer and eventually disappear on the far western quarter of the Plain. This unexpected pattern is a paradox and in contrast to what is expected and has remained unsolved.

Our hypothesis is, that deep caves formed prior to the deposition of the youngest limestone unit, and whilst the tectonic basin suffered subsidence and subsequent sea transgression before, the subsidence was uneven and the “plate” of the older limestone experienced a minor tilt on the eastern side of the plain. This would cause, that the original cave passages in the oldest limestone, which should primarily form at the same depth, became higher elevated in the east, causing central and western part to be deposited by a thicker amount of younger limestones, and that eventually progressive cave collapses therefore reached the surface on the eastern side of the plain sooner, followed by central side and eventually the western side, which has yet to experience such events. The decades long enigma of when the deep caves on the Nullarbor Plain were formed could thus be resolved, and, as currently the evidence of geomorphological processes of the plain has been observed (only) from the Middle Miocene to the present day; the antiquity of original cave formations has a potential to span this evidence further in the past. Our second objective is to expand the Nullarbor landscape evolution record from current 14 Ma to up to 30 Ma.

Our work includes mapping and spatial analysis, lidar scanning, electrical resistivity tomography and seismic radar scanning, sequence stratigraphical analyses, application of cosmogenic nuclide methods for sediment burial and estimation of denudation rate of the surface. Whilst we would be able to reconstruct tectonical evolution and past environmental events based on the denuded caves study, we can apply this knowledge to predict a potential environmental evolution in the future, which is of global significance.