The area covered in this report on the geology of Sheet V14 Maketu 1 :50 000 includes Motiti Island and a small triangular section of the Bay of Plenty coastal lowlands from the Kaituna River mouth to Pukehina Beach within the Maketu Basin, eastern North Island.
Motiti Island is situated 12 km offshore from the Bay of Plenty coast and is an eroded remnant of a Pliocene (4.3-3.4 Ma) andesitic composite cone. It is a flat-lying island surrounded by cliffs up to 30 m high, and the highest point is 57 m above sea level. The island is constructed on a base of andesitic rocks that have been divided into two new formations, the Motiti Formation consisting of thick massive or platy jointed lava flows and volcanic breccias, and the Orongatea Formation comprising thin lava flows, volcanic breccias, lapilli tuffs, tuffs, and dikes that are considered to be proximal strombolian and phreatomagmatic deposits. The andesitic formations are unconformably overlain by a 20-m thick sequence of middle to late Quaternary volcanogenic sediments (fluviatile sands, silts, and gravels), which in turn are covered by a 6-m thick blanket of late Quaternary tephras including Rotoiti Tephra Formation at the base. Originally the island would have been manifested as an andesitic composite cone and tuff ring complex, but it is now characterised by the strong angular unconformity that truncates the older andesitic rocks, which have been planed flat, presumably by marine erosion. The andesites and basaltic andesites of Motiti Island have ages and geochemical compositions similar to those of the Waihi district and southern Coromandel Volcanic Zone.
The Maketu peninsula (Town Point) forms a 67-m high headland on the Bay of Plenty coast. and is considered to be a horst, bounded by NE (035°)-striking normal faults and downfaulted blocks on both sides. The oldest rocks in the Maketu area are exposed in cliffs around the peninsula and consist of fluvial sands, silts, and gravels, aeolian sediments, and a 25-m thick lahar or lake-breakout flood deposit (Newdicks Formation, new) with a stratigraphic age of c. 0.25 Ma. The source of the lahar/flood deposit is unknown, but it contains clasts up to 7 m across of a densely welded ignimbrite that form a litter of boulders around the Maketu peninsula. The fluvial sediments are similar in rock type and stratigraphic position to those on Motiti Island, and both sequences have been included here in the Matua Subgroup. Intercalated with the sediments of the Matua Subgroup are thin Mid-Pleistocene pumiceous tephra fallout beds of the Huntress Creek and Kukumoa subgroups, and a c. 5-m thick non-welded ignimbrite identified as Mamaku lgnimbrite (c. 0.22 Ma). The uppermost part of the Matua Subgroup is marked by a well-developed, dark paleosol formed in clayey loess, and is Last Interglacial in age (c. 125 ka).
The Maketu peninsula is capped by a 15-m thick sequence of late Quaternary pumiceous and unweathered to weakly weathered tephra beds that include Rotoiti Tephra Formation, four tephras of the Mangaone Subgroup, and numerous younger tephras derived from the volcanic centres of Okataina (ten tephras), Taupo (six tephras), and Tuhua (Mayor Island) (one or more tephras), and minor loess deposits. The youngest of these tephras include the white, pumiceous rhyolitic Kaharoa Tephra erupted from Mt Tarawera in c. 1314 AD, Rotomahana Mud from the 1886 AD eruption of Mt Tarawera, and thin dustings of andesitic ash from the 1995-1996 AD eruptions of Mt Ruapehu.
In the Kaituna and Pongakawa lowlands and around the Maketu and Wai hi estuaries there is a late Pleistocene to Holocene sequence of fluvial terraces, alluvial plains, dune sands, minor loess, estuarine sands and muds, peats, and intercalated tephra layers.
