Weathering history of rock art on Burrup Peninsula, Western Australia
Brad Pillans1, Stephen Eggins1, Tony Eggleton1, Keith Fifield2 and Richard
Roberts3
1 Research School of Earth Sciences, Australian National University, Canberra,
ACT 0200, Australia
2 Research School of Physical Sciences & Engineering, Australian National
University, Canberra, ACT 0200, Australia
3 School of Earth & Environmental Sciences, University of Wollongong,
NSW 2522, Australia
Figure 1.
In July 2007 the renowned Aboriginal rock art of the Dampier Archipelago (including Burrup Peninsula) in Western Australia was included in the National Heritage List. Some hundreds of thousands of rock engravings (petroglyphs) were made on weathered rock surfaces by pounding, pecking, abrading and scoring using rock tools. It is claimed that the Dampier Archipelago contains the largest known rock art gallery in the world. Industrial development at the nearby port of Dampier has stimulated research to underpin conservation strategies for the rock art.
The weathered, outer layers of rocks (dominantly granophyre) on the Burrup Peninsula consist of a thin, discontinuous surface varnish (up to ~200 microns thick) and underlying weathered zone or rind (up to ~1 cm thick), mainly composed of hematite, kaolinite, quartz, K-feldspar and phosphates. These are the typical insoluble residues from rock weathering and they are at the surface simply because they are very slow to dissolve in rain water. The dark reddish- to blackish-brown colour of the rock varnish contrasts with the pale brown colour of the underlying weathering rind. The pale weathering rind is exposed in the majority of petroglyphs, providing a distinctive colour contrast with surrounding dark coloured varnish.
Laser ablation-ICPMS depth profiling of the rock varnish indicates geochemical microlamination that may be related to changing long-term environments as described by Liu & Broecker (2008) in their study of rock varnish microlamination in the western USA. Together with our field observations, the geochemistry of the varnish is consistent with an origin from direct chemical precipitation of dissolved elements in rain water, rather than from leaching of the underlying rock or from slow diagenesis of dust particles deposited on the rock surfaces - see discussion by Thiagarajan & Lee (2004).
From our field observations, we identify three modes
of physical rock breakdown each of which impinges on the long term stability
of rock surfaces and associated petroglyphs:
1. Flaking of thin (mm-scale) surface layers associated with the development
of a weathering rind and/or rock varnish.
2. Fracturing along major rock joints (cm- to m-scale), resulting in
block fall from steep slopes and cliffs. Note, however, that in between
the very infrequent block fall events, erosion will likely be dominated
by mm-scale flaking.
3. Fire-induced fracturing around the margins of
rock outcrops caused during burning of adjacent vegetation (Dragovitch
1994).
Overall, our results indicate that the weathered granophyre rock surfaces
containing petroglyphs, on Burrup Peninsula, are extremely resistant
to erosion over thousands of years. Major contributing factors include
low rainfall, resistant rock and the presence of stable secondary minerals
on rock surfaces. We are currently undertaking a program of cosmogenic
nuclide measurements to quantify rates of erosion on rock surfaces associated
with petroglyphs.
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