Sediment source and movement in Tauranga Harbour and nearshore Bay of Plenty

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Smale, D. 1993 Sediment source and movement in Tauranga Harbour and nearshore Bay of Plenty. Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 93/25 20 p.

Abstract: Heavy minerals from Bay of Plenty beaches and offshore regions, from Tauranga Harbour, and from streams flowing into the harbour, are almost exclusively products of eruption from the Taupo and Coromandel Volcanic Zones. Opaques, orthopyroxene and hornblende dominate, with lesser amounts of cummingtonite, clinopyroxene, biotite, aegirine, and olivine. Traces of epidote, apatite, titanite and granite are probably from indurated Mesozoic sediments outcropping to the south-east. Proportions of the dominant minerals vary sufficiently to allow distinction statistically into six clusters, two of which account for most of the samples. One of these two, characterised by a low ratio of magnetite to ilmenite, includes most of the samples within the northern part of Tauranga Harbour and adjacent offshore area. The other includes samples from the Wairoa River, the south-eastern part of Tauranga Harbour, and adjacent offshore area, and probably results from the Wairoa River (by far the largest in the area) flowing into this part of the harbour. Most stream sediments contain high magnetite/ilmenite ratios, in strong contrast with nonhern harbour sediments. Probably little of the sediment in the nonhern part of the harbour is stream-derived, most being brought in from outside the harbour by littoral drift, or from direct air-fall ash. One of the smaller clusters, representing a group between Waihi Beach and Whangamata, may reflect a greater prominence of material from the Coromandel Volcanic Zone. There has been overall a tendency for progressively younger ash showers to be smeared along the coast. Variations due to beach sorting, and to grain size (use of 2-3 phi instead of 3-4 phi), can produce effects that must be taken into consideration; variation due to sampling is minor. (auth)