A multidisciplinary investigation of a rock coating at Ngaut Ngaut (Devon Downs), South Australia

Abstract

This paper presents the results of a multidisciplinary investigation into a dark rock coating at the Ngaut Ngaut heritage complex in South Australia (SA) using geological and botanical examination, Raman microscopy, x-ray powder diffraction, scanning electron microscopy and infrared analyses. The coating analysed contains a mixture of calcite, quartz, gypsum and weddellite. The presence of calcite and quartz can be explained by the underlying clastic fossiliferous limestone, while the most probable explanation for the origin of the gypsum is via ground water. The weddellite was likely formed from solutions derived from the reaction of calcite with oxalic acid through the intervention of surface microflora, such as algae. This article provides the first record of weddellite in any context in SA. These findings have a number of implications-one being that the oxalate mineral in the rock coating could potentially be used to conduct accelerator mass spectrometry radiocarbon analysis and thereby refine our understanding of the rock art chronology at Ngaut Ngaut. A greater understanding of the rates of accumulation may also be useful for management purposes, as the nature of the rock coating may be contributing to long-term exfoliation. Indeed, it is argued that algal colonisation of the limestone (together with other probable microfloral activities) is likely involved in the production of a film over the porous surface, leading to salt weathering.