Coast Care [New Plymouth District Council] Webpage
- Author
- New Plymouth District Council
- Publisher / Organisation
- New Plymouth District Council
- Summary
- Raglan Harbour, a drowned river valley system lying in a structurally depressed fault-block, has a maximum depth of 18 m and covers about 33 km2, 70% of which consists of tidal flats up to 1 km wide and dissected by channels up to 5 m deep. Salinities range mainly from 30 to 33%0, and truly estuarine conditions are restricted to the tidal reaches of small streams entering the harbour. Sediments range from clean, well sorted sands in the beaches, dunes, channels, and bars in the lower harbour to mainly muddy sands, sandy muds, and muds in the extensive tidal flats of the middle and upper harbour. Gravelly sediments occur sporadically as the product of shore line erosion, and as lag and dump deposits in channels. Sediment transport and deposition is primarily under the control of cyclically variable, multidirectional tidal flows, complicated by a complex bottom topography. TidaJ currents attain surface speeds of 50-150 cm.s-1 and effect mainly a net seaward drift of suspended sediment but a net upharbour transport of bed-load material. Harbour sediments consist of quartz, feldspar, and clay minerals supplied by streams and by shore line erosion from hinterland Mesozoic to Quaternary sedimentary and volcanic rocks; of titanomagnetic and ferromagnesian minerals derived mainly from coastal ironsands by wind, wave, and current action; and of skeletal aragonite and calcite supplied by an intrabasinail, predominantly molluscan benthos. Subrecent to Recent diagenetic phosphatic concretions occur locally in the harbour. A large proportion of the tidal flats in Raglan Harbour are simply sediment-covered rock platforms formed by active erosion by wetting and drying of coast line montmorillonitic mudstones; cliff recession rates of about 2 cm.y-1 are indicated. Only a thin veneer of sediment covers the modern shore platform which is up to 100 m wide and has been cut during the last 5000 years, since when Raglan Harbour has acted mainly as a sediment trap. A -10 m platform beneath the tidal flats is up to 600 m wide and was eroded during an interstadial sea level stand, possibly from 85 000 to 60 000 years ago, with contemporaneous deposition of the montmorillonite-dominated detritus on the continental shelf.