Interpretation of Bedrock Topography within the Port Jackson (Sydney Harbour) Region using Marine Seismic Reflection

Abstract

An interpretation of the bedrock topography of virtually the whole of Sydney Harbour has been undertaken to estimate the bedrock configuration, identify paleochannels, and determine the nature and thickness of overlying sediment layers. The interpretations are based on an extensive series of marine seismic reflection data recorded in 1975 covering the harbour entrance and tributaries, and data recorded in August 2000 covering the harbour entrance and adjacent areas. Sydney Harbour has a bedrock configuration defined as a meandering channel that follows a generally deepening path as it reaches the harbour entrance with the bedrock configuration deepening to 85 m below sea level (b.s.l.) at the harbour entrance. Within the harbour, the paleochannel maintains a depth of about 30-40 m b.s.l. in the southern region near Point Piper and in Middle Harbour, and a depth of about 40-70 m b.s.l. in the middle region around the Sow and Pigs reef. In the northern survey area, paleochannel depths of 40-50 m b.s.l. were found in the Manly region. In Parramatta River, bedrock depths occur between 20-30 m b.s.l. in upstream sections of the channel and 40-45 m b.s.l. in downstream sections of the channel. In Lane Cove River, the depth of the paleochannel extends to 20-35 m b.s.l. Meandering paleochannels occur throughout Sydney Harbour with tributary paleochannels clearly defined. Sydney Harbour sediments interpreted as Holocene marine sand were found to be thickest (65 m) near the harbour entrance. Average estimated sediment thicknesses were 17.4, 10.8, 14.3 and 13.6 m in Port Jackson, Parramatta River, Middle Harbour and Lane Cove River respectively.