Spatial and compositional variations within finely laminated mineral crusts at Carpenter's Gap, an archaeological site in tropical Australia

Mineralogical and geochemical variations in ten rock surface crusts are des cribed from a large rock shelter known as Carpenter's Gap 1 in the southern Kimberley, formed at the junction between a sandstone floor mid an overlyi ng limestone roof. The finely laminated crusts, containing quartz, clays, o xalate and phosphate minerals, and sulfate salts (bassanite, gypsum, epsomi te), formed over quartz-rich sandstones. Mineralogical analyses of crusts, determined by X-ray diffraction, and major element geochemical trends, in t he form of element maps of cross sections, show a complex distribution of h emi-, mono-, and dihydrated sulfate and oxalate minerals vertically and lat erally within the shelter. These mineralogical changes have occurred under fluctuating moisture and temperature regimes, which follow a general drying trend spatially with distance from the back wall and temporally over a per iod of about 20,000 years. Documenting these mineralogical changes is a fir st step in understanding the nature of microclimatic controls on the format ion of rock surface coatings in shelters and their implications for palaeoe nvironmental reconstructions and for dating engravings covered by these cru sts. (C) 2001 John Wiley & Sons, Inc.