Abstract:
Beach and dune sands have been sampled at regular intervals from Muriwai to Kaipara South Head, and examined for mineral content, relative abundances of minerals, and grain size characteristics.
Granulometric studies show that mean grain size falls in the medium to fine sand grade for beach sands, and in the fine to very fine sand grade for dune sands. Beach sands are usually well sorted and fine sands are very well sorted. No trends in grain size and sorting were discerned along the beach from Muriwai to Kaipara South Head, but inland from low tide to backdune locations mean grain size decreases, and quality of sorting improves.
Each sample examined for mineral content was separated into light and heavy mineral fractions with heavy liquids. These fractions were then sieved separately at screen intervals into sub-fractions which were made into thin, and polished thin, sections.
Mineral content shows a similarity with that of the rest of the West Coast 'black sands', the dominant ferromagnesian being 'diopsidic' augite. Titanomagnetite shows exsolution textures of titanhematite, characteristic of North Island ironsands. The amount of titanomagnetite decreases rapidly northwards along the coast, with enrichment in foredune and backdune areas.
Quartz is the dominant light mineral and increases in abundance along the coast. Plagioclase of the composition An30-70 is the second light mineral in abundance. Other minerals of importance include hornblende, garnet and hypersthene in the heavy mineral fractions and orthoclase in the light mineral fractions.
The author suggests that the range of minerals show at least two source lithologies; 1. an igneous volcanic source. 2. an amphibolite facies terrain - probably derived from basic igneous rocks.
