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Heavy minerals in Cretaceous and Tertiary sandstones from Northland

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    Smale, D. 1988 Heavy minerals in Cretaceous and Tertiary sandstones from Northland. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report SL 18 41 p.

    Abstract: Heavy mineral suites from the Waitemata Group background sediments around Auckland city contain a high proportion of hornblende, and a few contain pyroxene. Sediments of the Waitakerie Group show greater variety, but pyroxenes are dominant in most. Rarely hornblende or biotite-epidote is dominant. Further north, in the Cape Rodney and Mt Auckland areas, sediments of the Waitemata Group (Pakiri Formation, volcanic-rich) contain a very high proportion (65-80%) of ferromagnesian minerals, including large amounts of pyroxene. A sample of the East Coast Bays Formation (volcanic-poor) contains less ferromagnesian material (47%), which is nearly all hornblende. In a sequence at Puketotara Peninsula (Kaipara Harbour) a pyroxene-rich suite distinguishes the Pakaurangi Formation from the underlying formations. Though the latter contain mostly pyroxene and hornblende, in the Timber Bay and Waihangaru Formations they are in the ratio 2:1 or 3:1, whereas in the Matapoura Conglomerate they are more nearly equal. Pyroxene tends to be particularly common in the Kaipara area and in sediments derived therefrom. Sequences from the Waipapa basement into the Waitemata Group were examined at Hays Stream, Claudes Stream, Cape Rodney, and Parua Bay. Most showed a sodeb-epidote-pumpellyite Waipapa Group association grading with an increasing volcanic component through the Kawau Subgroup into the Waitemata Group proper. However, in Hays Stream the Waipapa Group contained a suite rich in pyroxene and hornblende, strongly resembling the Waitakerie Group, yet many of the overlying sediments contained little pyroxene or hornblende. Most limestones and coal measures, and most sediments north of about Whangarei, showed a very impoverished ilmenite or ilmenite-zircon suite of little value, probably resulting from severe solution. The one area (Tangowahine River) where the Tangihua composition was assessed consisted mainly of ilmenite-magnetite-hornblende-pyroxene. Cretaceous sandstone from Parua Bay showed a strong component of sphene-epidote-apatite-biotite. Sources with this latter somewhat enigmatic (but possibly granitic) association may have influenced sparse sediments at Mangamuka Bridge, Ngatoka, and Redvale quarry

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