Shore platform development in a low energy marine environment: a case study from Whanganui Inlet, South Island, New Zealand

Abstract

The morphology and evolution of shore platforms eroded on rocky coastlines has been the subject in intense debate. Nearly all these studies have been conducted on high-energy open ocean environments, with rocky coasts in low-energy marine settings such as estuaries being overlooked. The shorelines of Whanganui Inlet, South Island, New Zealand, are characterised by well-developed shore platforms and provides an ideal location in which to investigate the character of such landforms in an estuarine environment. The inlet is fetch-limited and hindcasting suggests that maximum wave height rarely exceeds 0.77m and there is a semi-diurnal spring tidal range of 3.04m. 53 semi-horizontal platforms were surveyed between mean high water springs and mean low water neap levels. Two general morphologies are observed with the more common semi-horizontal platforms having a distinct seaward truncation at the outer edge in contrast to surfaces that gently slope below mean low water level with no abrupt terminus. Platform widths range from 4 to 185m and have been eroded into late Cretaceous to Tertiary shallow marine sandstones of low hardness (average N-type Schmidt hammer mean rebound value = 15.7 ± 2). Shore platforms at Whanganui Inlet are Holocene features and their intertidal elevations supports evidence for recent tectonic stability in the region. Statistical analysis showed no relationship exists between the main shore platform components (mean elevation, gradient and width) and rock hardness or wave energy. Platforms appear to planate to an elevation coinciding with the level of permanent saturation. Tidal flats are identified as a primary factor limiting wetting and drying and wave energy operating at vertical levels on platforms since the mid Holocene. Ledges found at the base of landward cliffs formed in response to a ~0.40m higher sea level that occurred during the mid Holocene. Their formation reflects the erosion efficiency of wave processes in the inlet at mean high water neap level. Mean backwearing rates are estimated from platform widths since the seaward outer edge was buried by tidal flats at ~5000 years BP and when sea level reached the outer edge at ~7000 years BP were 9.46mm/yr and 6.25mm/yr respectively. A conceptual model summarises shore platform development in Whanganui Inlet during the Holocene. Platforms experienced an initial period of rapid development following the stabilisation of postglacial sea level. Shore platforms are currently developing towards a state of decay equilibrium with the system tending to a static state. Erosion of platforms in the decay or static state is reinitiated when a subtidal channel re-exposes the outer edge and platform surface, or, from either a rise of fall in eustatic sea level.