Geological control on the morphology of estuarine shore platforms: Middle Harbour, Sydney, Australia

Middle Harbour, Sydney, Australia is a microtidal environment formed in resistant Triassic quartzose sandstone (mean N-type Schmidt Hammer rebound value of 37 ± 6). Wave energy is dominated by deep-water ocean swell which is dissipated rapidly as it propagates through the estuary mouth and is absent 1.5 km upstream. The small fetch of the harbour also means locally-generated wind waves are insignificant, generally < 1 m. Shore platforms occur in locations where deep-water waves can impact the bedrock and here surfaces up to 60 m wide are formed. They occur to an elevation of 3.5 m above mean sea level and are characterised by a vertical seaward cliff at least 1 m high, often > 2 m in the most exposed locations. All the platforms are semi-horizontal and appear to occur where a joint or bedding plane is coincident with intertidal elevations, although there is little correspondence with a particular tidal limit, with surfaces occurring at all levels between MLWS and MHWS. Several horizontal surfaces are found above the limit of modern tides and those at c. 1.5 and 2.1 m levels are likely to result from higher sea levels. Lithological structure and rock resistance therefore appear to be the primary determinants of platform morphology, with the most distinct development occurring where these planes are coincident with the intertidal zones where most erosion occurs. Some morphological inheritance from previous higher sea levels may also be found within the estuary.