Coastal sands of Northland and Auckland

Abstract

Eight types of sand facies are recognised. The Waikato River and the marine Hauraki (A) sand facies are derived from a rhyolitic provenance and are characterised by included pumice, Hauraki (B) Sand Facies is a coastal derivative of Hauraki (A), in which rock fragments and mafic minerals are decreased and pumice has disappeared. Its predominantly andesine feldspar content has increased to between 50% and 75%. The Parengarenga Sand Facies is high in silica, 95%+, and is derived from podsols. The Bay of Islands and Orewa sand facies are localised where there is little coastal progradation. The former has a high proportion of both shell and rock fragments whereas the latter with 70% quartz is derived by erosion of Waitemata sandstones. The West Auckland Sand Facies has been formed by mixing of the Parengarenga, Orewa, Bay of Islands, and Waikato River fades by strong west coast currents. At Muriwai it is also mixed with the Egmont Blacksand Facies. The region of greatest east-coast progradation lies between Marsden Point and Tokatu Point, where almost all the sand is derived from the sea floor, and where progradational rates are a function of sea-level fluctuations. Progradation along the west coast, since at least the Pliocene, has built the very large Manukau, South Kaipara, and North Kaipara barriers and Aupouri Tombolo. Despite periods of erosion during the periods of rising sea level, they owe a continued preservation to an overall drop in sea level of several hundred feet during the Quaternary—a falling sea level being one that favours progradation by supplying sand from the sea floor. The south-eastward directed North Kaipara Barrier and smaller barriers to the north, and the north-westward directed South Kaipara and Manukau barriers have been built by the opposing oceanic West Auckland and Westland Currents that converge opposite the Kaipara Harbour entrance.