Engineers Australia

Pilbara Cenozoic detrital sequences and associated geohazards

Helen Baxter

Abstract

This paper presents two potential geohazards associated with Cenozoic detrital sequences in Western Australia’s Pilbara region.

The first considers carbonate rich calcrete layers, often reflecting the position of historic and current water tables. Observations of geochemistry of the calcrete layers compared with textural features in the resulting rock suggests a link between CaO, MgO and Loss on Ignition (LOI) abundance and the likelihood of cavities existing.

Geohazards in the form of sinkhole or doline formation may result from changes to groundwater by dewatering for mining or town-water extraction. Contributing factors that increase this likelihood are high water flow, presence of dispersive soils in the blanketing layer, a geochemical signature of >20% CaO in the calcrete and commensurate thicknesses of the calcrete layer (and potential void space) and the blanketing layer.

Relic rock slides have been recognised in several detrital valleys in the southern Pilbara and represent the second potential geohazard. The slides appear to represent a specific marker horizon in the detrital stratigraphy, attributed to a high rainfall, global warming climatic event in the Miocene. The slides consist of large rafted slabs/blocks of Archean bedrock with lesser cobbles and clasts of high strength rock. The voids between blocks are infilled with high plasticity, firm to hard kaolinitic clay, thought to be derived from subsequent lacustrine deposition. The preserved unit thickness varies from 10 to 80 m and can be buried by over 100 m of younger detritals.

The slides present a unique geohazard to mining operations, not simply due to the variability in rock mass strength which can impact slope design. A high variability in void and matrix size and distribution is noted, though size and distribution of these zones is typically too small to be “mapped” by infill drilling. The size is however sufficiently large to cause trafficability issues on haul roads.