The origin of mysterious deep holes in the upper and central Sydney estuary, Australia

Abstract

‘Deep holes’ (DHs) were first reported in the upper and central Sydney estuary in 1973 and speculation as to the origin of these features has continued since that time. The present study established that DHs were not ‘holes’, but preserved sections of the main paleo-channel and that they formed as a result of multiple processes over an extended period. The 17 DHs discovered were the product of fluvial erosion, sediment starvation and tidal action and could be classified into four Types based on bathymetric and seismic attributes. Type A DHs contained four sedimentary units, which fully, or partially filled the paleo-valley; Type B holes exhibited a deep, eroded paleo-channel and a thick remnant sediment wedge on one flank; Type C exhibited a rocky, sediment-free flank and a mainly sediment-free base; Type D DHs were genetically different in not being associated with the main paleo-channel.Extensive erosion of Pleistocene estuarine clay and overlying fluvial sediment produced deep paleo-valleys in the upper and central sectors during long periods of Pleistocene sea-level low stand. Major erosion took place where stream velocities increased in narrow parts of the sinusoidal upper paleo-valley and where the paleo-channel was restricted by protruding headlands of the central section. During recent sea-level high stands, sediment supply from small adjoining sub-catchments in the upper estuary was minimal and bedload from the major paleo-channel was confined to the headwaters of the catchment, which resulted in sediment starvation and hole preservation. In the present high sea-level period, remnant parts of the deep, paleo-channels are being preserved by increased tidal velocities in the same localities as in the past, preserving the DHs observed today.