The formation of the pinnacle karst in Pleistocene aeolian calcarenites (Tamala Limestone) in southwestern Australia

A spectacular pinnacle karst in the southwestern coastal part of Western Australia consists of dense fields of thousands of pinnacles up to 5m high, 2m wide and 0.5–5m apart, particularly well exposed in Nambung National Park. The pinnacles have formed in the Pleistocene Tamala Limestone, which comprises cyclic sequences of aeolian calcarenite, calcrete/microbialite and palaeosol. The morphology of the pinnacles varies according to the lithology in which they have formed: typically conical in aeolianite and cylindrical in microbialite.Detailed mapping and mineralogical, chemical and isotopic analyses were used to constrain the origin of the pinnacles, which are residual features resulting mainly from solutional widening and coalescence of solution pipes within the Tamala Limestone. The pinnacles are generally joined at the base, and the stratigraphy exposed in their sides is often continuous between adjacent pinnacles. Some pinnacles are cemented infills of solution pipes, but solution still contributed to their origin by removing the surrounding material. Although a number of pinnacles contain calcified plant roots, trees were not a major factor in their formation.Pinnacle karst in older, better-cemented limestones elsewhere in the world is similar in morphology and origin to the Nambung pinnacles, but is mainly influenced by joints and fractures (not evident at Nambung).The extensive dissolution associated with pinnacle formation at Nambung resulted in a large amount of insoluble quartz residue, which was redeposited to often bury the pinnacles. This period of karstification occurred at around MIS 5e, and there was an earlier, less intense period of pinnacle development during MIS 10–11. Both periods of pinnacle formation probably occurred during the higher rainfall periods that characterise the transition from interglacial to glacial episodes in southern Australia; the extensive karstification around MIS 5e indicates that the climate was particularly humid in southwestern Australia at this time.