Geochemical and mineralogical data suggest that bat guano has made a major contribution to the development of several separate sedimentary layers, in the predominantly clastic sand and coarse silt deposits of the Naracoorte Caves, South Australia. A 20 cm thick group of reddish fine silt sediments located in Robertson Cave contains the phosphate minerals whitlockite and apatite, significant concentrations of SO 3, P 2O 5, and CaO, and organic matter with low C/N ratios and highly enriched δ 15N values. These data suggest that accumulations of bat guano and weathered cave limestone contributed significantly to the red silts formation. Sedimentary deposits located in Wet and Blanche Caves also contain horizons that exhibit increased concentrations of SO 3, P 2O 5 and CaO, relatively lower C/N ratios and enriched δ 15N values, suggesting that bat guano may have also contributed to their formation. Physical attributes evident for both these horizons further support this interpretation. A milky-white material that appears to be whitlockite is present in the Wet Cave horizon, while similarities are identified by thin section petrography, between the Blanche Cave horizon and the guano layer in Robertson Cave. The development of such horizons relies on minimal sediment input from outside the cave system; hence their presence may serve as an indicator of periods of landscape stability in the palaeoenvironmental record. © AAP.