Sinkholes (Dolines) in lateritised sediments, western Sturt plateau, Northern Territory, Australia

Sinkholes up to 50 m diameter and 15 m deep are developed in the lateritised Mullaman (Lower Cretaceous) quartzitic and argillaceous beds of the western Sturt Plateau, in the monsoonal north of the Northern Territory, Australia. Some sinkholes are degraded and of some antiquity: though their age is not precisely known, they formed during the late Cenozoic following the uplift and desiccation of the latertte. Others are fresh and a few have formed in living memory. Some sinkholes may be due to collapse of the Mullaman beds into voids in the underlying Cambrian limestone, but most have evolved wholly within the highly siliceous Mullaman beds, which have been dissolved or altered, creating either voids or compartments filled with uncosolidated material. Subsequent water-table lowering and collapse of the overlying beds into the low-density zones caused the formation of sinkholes.Fractures have significantly influenced the location, pattern and detailed plan shape of sinkholes. Sinkholes also occur within and at the margins of old drainage lines. The water table stood higher in the late Pleistocene, bringing alkaline groundwaters into contact with the country rock. A late Cenozoic fall in regional water table caused slacking or disaggregation. Litter derived from a Eucalyptus-dominated woodland facilitated iron mobilisation. Biogenic agents may have accelerated the weathering of the silica, silicates and iron oxides of which the country rock and laterite are largely composed.