A. Watchman et al., Spatial and compositional variations within finely laminated mineral crusts at Carpenter's Gap, an archaeological site in tropical Australia, GEOARCHAEOL, 16(7), 2001, pp. 803-824
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.