Rates and forms of erosion on intertidal platforms at Kaikoura Peninsula, South Island, New Zealand

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Abstract
Shore platform development and processes are examined for Kaikoura Peninsula on the north-east coast of the South Island, New Zealand. The environment is exposed to high energy storm and swell waves, is mesotidal (mean range 1.36 m, maximum 2.57 m) and has a temperate climate with moderate rainfall (average 865 mm yr-1). Shore platforms range from 40 m to over 200 m wide and are cut in Tertiary mudstones and limestones. Most of the profiles display a prominent low water cliff, an outer rampart and channels developed along lines of structural weakness. The inner margins are extensively mantled in beach, hillslope, and lagoonal sediments which are being rapidly eroded except for three locations where there is an active marine cliff. Net tectonic uplift of the peninsula is thought to be of the order of 108 m during the Quaternary and 2 m during the last 1000 years. The shore platforms are thus clearly polycyclic and contain “inherited” morphological features but are being actively rejuvenated by removal of cover deposits. Data on intertidal platform lowering rates are derived from 31 micro-erosion meter sites located on seven profiles around the peninsula margin. Each site produces three values per observation and they were measured at two monthly intervals for two years from 1973–1975 yielding 672 values for surface lowering, a 60%measurement repeatability. Mean lowering rate for the two years was 1.53 mm yr-1 (minimum 0.38 mm yr-1, maximum 7.03 mm yr-1, standard deviation 1.45 mm yr-1) and rates were generally higher on mudstone than on limestone and where eroding cover deposits yield abundant abrasional materials. Backwasting rates were estimated from analysis of air photographs over the period 1942–1974 for 41 sites on the peninsula and ranged up to 1.49 m yr-1 for eroding beach complexes and between 0.24 and 0.33 m yr-1 for active mudstone cliffs. Lowering rates were approximately 3% of backwasting rates. No data was obtained on either recession rates of the low water cliff or block disintegration of platform rock. An approximate minimum littoral sediment budget revealed that the intertidal area of 0.72 km2 contributes between 4–5000 m3 of eroded material annually 34% of which is derived from the limited stretches of active marine cliff and 39% of which comes from platform lowering. A parallel retreat model was applied to platform erosion rates with limited success and planation ages of between 102–103 years were estimated. Despite the short term nature of the erosion data both the rates and planation times are likely to be of order of magnitude accuracy. Both erosion rates and planation ages lie within the ranges of other studies and are consistent with dated shoreline events on the Kaikoura Peninsula. Four phases of activity are identified for the last 5–6000 years involving changing tectonic-eustatic levels, platform processes and erosion episodes in the hinterland. The platforms are thus rapidly evolving features which reflect both contemporary processes and recent history. Analysis of variation in lowering rates across the shore revealed upper and lower zones of relatively more intense erosion separated by a central zone of lesser intensity. The upper zone is thought to be essentially sub-aerial and supra-littoral in character, while the lower one may be dominated by marine and sub-littoral processes. There is thus a gradient from sub-aerial processes to true marine processes across the shore so that it is unlikely that a single dominant mode controls platform development and morphology.