Denuded caves on the Nullarbor Plain, Australia

Denuded caves are surface expressions of cave passages due to lowering of the surface by denudation and/or breakdown propagation of caves towards the surface. They appear as fragments of channels on the surface, often filled with clastic cave sediments and occasional speleothems. Elongated shallow depressions were recently observed on the surface of the southeastern part of the Nullarbor Plain, and close association with associated collapse dolines suggests that they are denuded caves.

The Nullarbor Plain in southern Australia is among the largest karst outcrops in the world and covers a surface area of approximately 200.000 km². The bedrock comprises the middle to late Eocene Wilson Bluff Limestone, the Oligocene to early Miocene Abrakurrie Limestone, and the middle Miocene Nullarbor Limestone. The plain has been since the deposition of the Nullabror Limestone uplifted and exposed to subaerial denudation. It is renowned for outstanding flatness with collapse dolines and vertical cave openings (blowholes) being the most common surface karst features. Most of the discovered caves are “shallow caves” in Nullarbor Limestone, generally less than 30 m deep with low collapse chambers. On the other hand, “deep caves” generally extend 50-120 m below the surface in the Wilson Bluff Limestone and may reach exceptional dimensions. Deep and shallow caves are thought to be genetically unrelated, but the speleogenesis of deep caves is still not understood completely, and the recognition of denuded caves might help understand their timing of initial formation.

One hypothesis suggests the phreatic origin of deep caves after the deposition of the Wilson Bluff Limestone and its exposure on the surface during the warm wet climate around 35 to 25 million years ago. Later, around 25 to 10 million years ago, the tectonic basin underwent subsidence and sea transgression in the west-central part, which is on the present surface obscured by even lateral deposition of younger limestones. Therefore, the relatively greater number of denuded caves in the east expressed as simple uniform lines of shallow elongated depressions, the combination of a line of collapses and shallow depressions in the central part of the plain, and greater discovered number of deep caves towards the west could confirm this hypothesis. Namely, the subsidence of the Wilson Bluff in its western part (with caves already formed) could have caused thinner deposition of younger limestones towards the east and the position of cave passages relatively higher. Progressive cave collapses therefore reached the surface on the eastern side sooner (expressed as denuded caves), whilst in the west the breakdown propagation of deep caves has not yet reached the surface.