Shallow caves and blowholes on the Nullarbor Plain, Australia — Flank margin caves on a low gradient limestone platform

The Nullarbor Plain of southern Australia is a very extensive limestone platform with relatively few large caves for its size, but contains thousands of blowholes, sub-circular vertical shafts up to 1–2 m in diameter, which often connect to similar-sized sub-horizontal passages. Recent detailed systematic surveys of large areas of the Nullarbor Plain have provided new distribution data showing that blowholes and associated shallow caves are concentrated in a 25–30 km-wide band located > 75 km inland. The known density of these features (up to 43/25 km2) underestimates the cave porosity because the strong draughts blowing from many of the blowholes indicate that they connect to extensive cave systems of small passages. These shallow caves are relict phreatic features; their entrances (blowholes) were opened as the land surface was lowered by denudation. The band of blowholes and caves is located along the Late Miocene (~ 6 Ma) shoreline across the Nullarbor, when there was a eustatic stillstand. The caves formed in the zone of enhanced dissolution at the seaward margin of the freshwater lens along the carbonate coastline, and can therefore be regarded as flank margin caves on a low gradient limestone platform; a flank margin setting relatively unknown prior to this study. The width of the band of flank margin caves, which is substantially greater than previously documented for this cave type, reflects the very low gradient of both the ground surface and water table, together with the influence of tidal fluctuations and regression of the shoreline. Flank margin cave development stopped when the sea retreated rapidly in the Late Miocene–Early Pliocene due to a period of tilting and uplift. The band of flank margin caves has high permeability and substantial porosity, and would therefore form an excellent, largely overlooked, type of palaeokarst petroleum reservoir.