Estuarine shore platforms in Whanganui Inlet, South Island, New Zealand
Introduction
Shore platforms have been referred to as neglected coastal geomorphic features, especially when compared to other coastal landforms such as beaches and dunes (Trenhaile, 1980, Stephenson, 2000). Recently there has been a renaissance of interest in the rock coast, with investigations ranging from millimetre scale changes of platform surfaces over hourly cycles (Trenhaile, 2006, Gomez-Pujol et al., 2006) to the evolution of terraces during Quaternary eustatic oscillations (Trenhaile, 2001a, Trenhaile, 2002a). Geology and tidal range exert a major influence on broad platform morphology. Lithologies with a low hardness tend to erode faster and to a lower elevation than more resistant rocks (Gill, 1972, Kirk, 1977, Trenhaile, 1987, Dickson, 2006, Thornton and Stephenson, 2006) while on a regional scale platforms are steeper in areas of higher tidal range (Trenhaile, 2002b). There is, however, still no consensus on the relative role of wave and weathering processes in determining their final shape. Interestingly nearly all investigations of shore platforms have been conducted on the open-ocean coast, which are impacted by waves developed over a significant fetch. It is the high-energy nature of these settings that has often made data collection difficult, especially at the seaward edge of the platform.
In low-energy marine environments, such as estuaries, wave energy decreases rapidly over a short distance and, in turn, the dominant process driving landform change varies markedly (Davis and Hayes, 1984). On depositional landforms such as beaches, profile shape can be determined by wave and tidal processes even in microtidal regimes, or there may be relict morphologies only activated during storms (Hegge et al., 1996, Kennedy, 2002, Goodfellow and Stephenson, 2005). Such environments provide an ideal setting for the investigation of the role of marine and subaerial processes in driving landform evolution, as the relative role of each in eroding the shoreline changes markedly over a short distance. For shore platforms very few studies have been conducted in fetch-limited environments, and those that have are focussed within tideless lakes (e.g., Lake Boverbrevatnet, Norway, Matthews et al., 1986; Storavatnet and Bergen areas, Norway, Aarseth and Fossen, 2004a, Aarseth and Fossen, 2004b; Lake Waikaremoana, New Zealand, Allan et al., 2002). Lakes Erie and Huron, Canada, (Trenhaile, 2004) also contain well-developed shore platforms although the fetch is significantly higher than the previous examples. Despite evidence that platforms can form in sheltered marine settings (Bartrum, 1916, Dionne and Brodeur, 1988, Dionne, 1993) there is still little knowledge about rocky coasts from estuarine environments, especially those with a limited fetch. This study sets out to describe the morphology of such a system investigating the morphology of the estuarine shore platforms of Whanganui Inlet, New Zealand. It aims to infer the processes driving platform formation, and to develop a model for platform evolution within this estuary.
Section snippets
Regional setting
Whanganui Inlet is a narrow estuary 13 km long and up to 3 km wide located near the north-west tip of the South Island of New Zealand (Fig. 1). The majority of the inlet's shoreline is rocky, composed of quartzo-feldspathic shallow marine sandstones and localised conglomerates of Tertiary to Late Cretaceous age (Pakawau and Kapanui Groups respectively) (Rattenbury et al., 1998). The catchment is steep rising to an elevation of 506 m at Knuckle Hill, where Early Cretaceous granites outcrop (Fig.
Materials and methods
A total of 53 profiles was surveyed from the cliff platform junction to the seaward edge of the shore platform, using an electronic distance meter (Sokkia SET 4010). At each main study area sites were surveyed at regularly spaced intervals to allow for the parameterization of platform characteristics at that particular location. The distance between surveys was therefore determined by the occurrence of platforms along that section of shoreline. Where the seaward edge of the platform was buried
Platform morphology
Extensive platform surfaces are found within Whanganui Inlet ranging in width from 4.1 to 185.2 m with an average of 44.9 m. Almost all (83%) have a steep (> 60°) outer edge, while the remainder have little change in slope as they descend below low-tide level. In general the profiles tend to be widest on the tips of the promontories, however there is little correlation between platform width and fetch distance (r2 < 0.1) with both wide and narrow platforms occurring in the most protected and
Discussion
The shore platforms within Whanganui Inlet are all semi-horizontal, range in width up to 185 m, with the majority (77%) of surface gradients being < 1.5°, ranging up to 3.4°. In all cases the seaward portion of the platform is buried by intertidal muddy-sand flats to depths of up to 1.5 m. This means that unlike platforms found on the open-coast the seaward edge is not exposed to either waves or subaerial processes during tidal cycles. Therefore, although they are morphologically similar to
Conclusions
The extensive shore platform development found within Whanganui Inlet indicates that significant rocky coast landforms can develop in relatively low-energy marine environments. Little relation occurs between platform morphology and wave exposure, with platforms most commonly forming at or very close to the level of intertidal sediment flats that occur throughout the inlet. This corresponds to the level of surface saturation of the rocky surfaces at around mean high water neap levels. These
Acknowledgements
The authors would like to thank Rhea Dasent, Mike Henry and Hamish McKoy for assistance in the field. Victoria University of Wellington provided funding for this project. Critical reviews by Alan Trenhaile and an anonymous reviewer were appreciated.
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