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The formation of the pinnacles in Nambung National Park, Western Australia

January 2012

Conference: International Geological Congress
At: Brisbane, Australia
Volume: 34

Authors:

Research Centre of the Slovenian Academy of Sciences and Arts

La Trobe University

The pinnacles in Nambung National Park in south-western coastal Western Australia are calcarenite pillars up to 5 metres high and up to 3 metres wide, with various shapes: conical, cylindrical with rounded top, mushroom-like, and multiple peaks. They occur in dense fields with an average spacing between 0.5 and 5 metres, and the pinnacle tops often define a horizontal surface that decreases in direction northwards. The pinnacles have been described as a syngenetic karst feature and interpreted as either residual features resulting from widening and coalescence of solution pipes, cemented filling of solution pipes, focused cementation or trace fossils of trees. However, based on detailed field work, morphometrical analysis of the pinnacles, thin section examination, and mineralogical, chemical and isotopic analysis of rock samples, microbial cementation appears to be one of the dominant process of lithification, i.e. the pinnacles are also organosedimentary structures. Calcrete development and karstic weathering have subsequently modified the pinnacle shapes.

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Conference Paper

Full-text available

The formation of the pinnacles in Nambung National Park, Western Australia

January 2012

The pinnacles in Nambung National Park in south-western coastal Western Australia are calcarenite pillars up to 5 metres high and up to 3 metres wide, with various shapes: conical, cylindrical with rounded top, mushroom-like, and multiple peaks. They occur in dense fields with an average spacing between 0.5 and 5 metres, and the pinnacle tops often define a horizontal surface that decreases in ... [Show full abstract] direction northwards. The pinnacles have been described as a syngenetic karst feature and interpreted as either residual features resulting from widening and coalescence of solution pipes, cemented filling of solution pipes, focused cementation or trace fossils of trees. However, based on detailed field work, morphometrical analysis of the pinnacles, thin section examination, and mineralogical, chemical and isotopic analysis of rock samples, microbial cementation appears to be one of the dominant process of lithification, i.e. the pinnacles are also organosedimentary structures. Calcrete development and karstic weathering have subsequently modified the pinnacle shapes.

Conference Paper

Full-text available

Limestone pinnacles in Western Australia – the result of karstic processes and microbial cementation

January 2012

Limestone pinnacles in Western Australia occur abundantly in Nambung National Park. They are up to 5 metres high and up to 3 metres wide and usually occur in dense fields with an average spacing between 0.5 and 5 metres. They vary in shape (conical, cylindrical with rounded top, mushroom-like, or multiple peaks) and material (laminated, bedded, palaeosoils). Most often they have been described as ... [Show full abstract] karstic feature, resulting from widening and coalescence of solution pipes. However, their formation has also been interpreted as a result of focused cementation, trace fossils of trees, or cemented filling of solution pipes. Based on detailed field work, morphometrical analysis of the pinnacles, thin section examination, and mineralogical, chemical and isotopic analysis of rock samples, karstic weathering was recognized as a process which has modified the pinnacles’ shapes multiple times during their genesis. However, the process of lithification by microbial cementation has also played the major role in the formation of the pinnacles.

Conference Paper

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The formation of solution pipes on Cape Bridgewater, Western Victoria

January 2013

Solution pipes are tubular vertical cavities formed in very porous limestones with matrix porosity, such as calcarenite. Their diameter varies from <0.2 metres to >1 metre, and their termination is usually round or slightly pointed. Average depth is 2 - 5 metres, but can reach more than 20 metres. They sometimes form vertical entrances to cave systems. Solution pipes are surrounded by rims a few ... [Show full abstract] millimetres to 5 centimetres wide that are more cemented and erosion resistant than the surrounding calcarenite. Solution pipes have been described as a syngenetic karst feature and interpreted as vertical karst voids, formed by focused vertical solution. Much discussion and many experiments have been published on triggers for focused solution in calcarenites or focused water flows in unconsolidated sediments, but opinions are still divided. Solution pipes are abundant in Pleistocene calcarenites at Cape Bridgewater in western Victoria. Based on field work at this site, combined with morphometrical analysis, thin section examination, and mineralogical, chemical and isotopic analysis, it appears that sufficient but limited dissolution (i.e. rainfall) is crucial for development of the solution pipes. The focused flows that trigger solution pipe development may be initiated in several different ways, mainly dependent on the climate, rock/sand characteristics, and flora and fauna. The shape of the solution pipes is then modified by later calcrete development and weathering.

Conference Paper

Full-text available

Aeolianite, calcrete, and pinnacle karst in southwestern Australia, as indicators of middle to late...

October 2014

Pleistocene aeolianites with calcreted and karstified surfaces are well exposed on the Swan Coastal Plain in southwest Western Australia. Field mapping, thin section examination, U/Th and OSL dates, and mineralogical, chemical and isotopic analysis identified the timing and palaeoenvironments of the sand deposition, calcrete formation, and karstification, providing an important insight into ... [Show full abstract] Pleistocene palaeoclimates in this region. Five periods of aeolianite deposition have been defined: MIS ≥13, MIS 11, MIS 9, MIS 7 and MIS 5, reflecting interglacial highstands accompanied by strong, predominantly southerly to southwesterly winds. Dating and oxygen isotope analyses of the calcrete layers overlying the aeolianites indicate deposition during the arid to semi-arid climate of each glacial period under lower mean temperatures (ΔT = 2.55° – 5.45°) than today’s mean temperature of 18.85°C. Different types of calcretes were deposited at different times, suggesting that the climate during glacial periods was not identical for each cycle, although the constant trend of aridity during these periods still remains. Dry climate during glacial periods is additionally indicated by the deposition of the quartz sand in MIS 3 (the Cooloongup Sand). Pinnacle karst was formed on the (palaeo)surface of two aeolianite members deposited in MIS 7 and 11, indicating a wet climate, probably wetter than present.