Lithological Character and Structural Geology of the Cooks River Area with Focus on the M8 Tunnels

Helen Baxter-Crawford

The M8 is a 9 km long dual road tunnel excavated south of Sydney connecting the M5 at Kingsgrove to the future M4- M5 link and Sydney Gateway at St Peters. The tunnel traverses beneath the Cooks River, one of the main sources of sediment for the Botany Basin. The Cooks River has been reconfigured by land reclamation, so the palaeotopography is no longer reflected. A 40 km2 palaeotopographic surface was developed during the M8 design phase to assist in understanding the sub-surface soil and rock mass, and locations faults may be intersected. The mainline and access ramp tunnels for the M8 also spanned the stratigraphic profile from the top of the Ashfield Shale to 80 m into the Hawkesbury Sandstone, allowing a detailed understanding of the facies profile for this part of the Sydney Basin to be formed. During excavation, several fault zones were confirmed with data collected providing geological insight to the fault character in various lithologies. One of these faults is inferred to correlate with the regional fault zones/joint swarms of Och, Pells and Braybroke, 2004 in the CBD, with a further three inferred to continue to the south based on the palaeotopographic surface generated for the M8 project. Several dyke systems were also intersected, correlatable eastwards to the coast, inland (westwards) using the palaeotopographic surface and one correlatable via the palaeotopography to a diatreme located to the north.