The origin, evolution and life cycle of rock art shelters in the Kimberley Basin of NW Australia

Abstract

The Paleoproterozoic Warton and Wunaamin Miliwundi Sandstones in the Kimberley Basin of NW Australia contain an abundance of rock shelters hosting a striking succession of rock paintings of immense cultural and archaeological significance. The evolution of these shelters has not previously been studied in detail, yet provides the ultimate control on the long-term survival of rock art within them. The rock shelters develop initially on near-vertical sides of remnant sandstone blocks on an etched landscape exhumed from a formerly pervasive deep lateritic weathering zone of probable Neogene age. The two nearly flat-lying sandstone formations are highly cemented orthoquartzites characterized by brittle behaviour revealed in a landscape-scale pattern of etched joint planes and small strike-ridge scarps along which shelters develop. Consistent features of these rock shelters reflect their mode of origin and subsequent evolution through a life cycle lasting tens of thousands of years or more. These include horizontal bedding-plane ceilings and fractured back walls cutting through sandstone beds, which host most of the rock paintings. Fractures mostly dip back towards the deepest part of the shelter near the floor. The first stage in shelter development involves undermining by crushing of a relatively thin incompetent bed to form a recessed bedding cave in the absence of normal erosional agencies. Overlying massive sandstone beds are left unsupported and progressively collapse in one or more intact slab falls. Further falls lead to gradual enlargement and a rocky floor piled with fallen slabs. The geometry of the fractured back walls suggests that both tensile and shear failure are involved in shelter growth in a series of mass wasting events. Subsequent spallation and dilational flaking on sandstone surfaces on the lowermost sandstone faces modify the shelter walls. Eventually, the shelter may be destroyed by toppling forward due to continued undermining at the base.