LATE PLIOCENE STRATIGRAPHIC SUCCESSION AND VOLCANIC EVOLUTION OF KARIOI VOLCANO, WESTERN NORTH-ISLAND, NEW-ZEALAND

Karioi, in western North Island, New Zealand, is the northwesternmost volcano of the Alexandra Volcanic Lineament. Active from c. 2.48 to 2. 28 +/- 0.07 Ma (unmodified chronology), its products comprise the Kari oi Formation. That formation is here subdivided into the basal Te Tote Member (new) of basaltic lavas and breccias erupted from small centra l-vent volcanoes; the middle Whaanga Member (new) of shield-building b asalts and basaltic andesites erupted from fissures; and the uppermost Wairake Member (new) of cone-building basalt, basaltic andesite, and andesite lavas, tuffs, vent breccias, dikes, and valley-filling lavas and laharic deposits on the surface of the Whaanga shield. Wairake uni ts were erupted from at least two distinct central vents. Karioi is su rrounded by small monogenetic volcanoes of the Okete Formation that er upted basanitic, alkali olivine basaltic, and hawaiitic magmas and for med scoria cones, lavas, and tuff rings. Okete Formation is subdivided into units that predate Karioi volcano (Pauaeke Member, new), and tho se that postdate Karioi volcano (Marumaruaitu Member, new). Pauaeke vo lcanism occurred between 2.58 +/- 0.06 and 2.48 Ma, and Marumaruaitu v olcanism between 2.37 +/- 0.08 and 1.90 +/- 0.07 Ma. Early Okete and K arioi units erupted from vents close to sea level, onto a gently westw ard-sloping, mostly subaerial plain. This plain had been veneered by b eds referred to as Ohuka Carbonaceous Sandstone. The Karioi area has e xperienced little or no net uplift during the late Pliocene and Pleist ocene, as opposed to elsewhere along the west coast. Its stability may have been facilitated by fault-bounded detachment of the Karioi block and loading of the crust by the volcano itself.