Maunganui Bluff, New Zealand.


  The Waipoua Basalt Formation is the youngest (15 - 11 million years ago) of the three Miocene Waitakere Group volcanoes. It covers some 500 km2 between Kaihu and the Waimamaku Valley. From the coast the basalt extends inland up to 20 km to form the south-westward tilted Tutamoe Plateau, rising up 700m high. Erosion of the softer rocks under the basalt has formed bluffs ringed with talus slopes of basalt boulders. Along the Kawerua coast the basalt surface dips gently out to sea, but further south lava flows built up to 450m in the Maunganui Bluff.mb1

Waipoua Basalt comprises lava flows, pyroclastic deposits, volcanic breccia, dikes and breccia dikes. Because the basalt is deeply weathered and covered in thick bush, it is almost impossible to map units cross country.

In the Kaihu - Mangatu Stream area, Anne Wright described the Whatoro Breccia, a layer of bits of basalt in brown tuffs, with secondary limonite and zeolite mineralisation. This layer, 5 - 17m thick, is usually covered with a Waipoua Basalt lava flow.

Lava flows are the bulk of the Formation. Along the coast they are typically 5 - 15 m thick, and have been traced for up to 4 km in the north. Aa lava is in most of the flows along the coast; scoriaceous rubble half a metre to several metres thick is overlain by massive lava, which is capped by another scoriaceous layer. The scoriaceous basalt is usually oxidised to a red or mauve colour, and in the Maunganui Bluff area, where dikes have intruded, a white zeolite infills vesicles and joints. Away from the coast this rubble is less commonly seen.

Anne Wright did not find evidence of flow direction or any eruptive vents. However dikes in the southern section penetrate to the top of the sequence and may have fed the flows, which were probably terrestrial.

Geological map There are tephra beds, especially at Maunganui Bluff, up to 2 m thick. There are mafic fragments and broken crystals in a matrix that is pale brown when not baked red by overlying flows. Near a dike the whole deposit is permeated by a white zeolite.

Dike swarms penetrate the pile of lava in a zone from Maunganui bluff to Toronui Stream. The dikes vary from 0.1 to 10 m in width. Most have dark glassy edges and a central vesiculated zone. Some show evidence of multiple intrusion, and most have baked contacts.

Anne Wright described the basalt as plagioclase - olivine - augite, with phenocrysts of these minerals up to 30% of the rock.  She only found two secondary minerals, a dark green mineral lining vesicles, which she believed to be an iron-rich saponite; and chabazite, which she believed was restricted to the areas intruded by dikes.

Since then collectors have found microminerals in lavas, scorias and dikes, mainly at Maunganui Bluff and in Aranga Quarry. The Waima and Kaihu Rivers have been less visited and Sue Courtney examined minerals in coastal rocks at Kawerua.


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