Hokonui Stratigraphy of the Awakino-Mahoenui area, South West Auckland

Abstract

Summary Trias-Jura rocks of the Awakino-Mahoenui area constitute part of the western limb of the Kawhia Syncline in its southern-most exposure. The syncline is approximately meridional in alignment and pitches northwards at a low angle. Strata have dips varying from nearly vertical in the west of the area to about 15° east at the top of the succession in the east. The strata form the Herangi Range which originated as a north-south fold or the peneplained surface resulting from the post-Hokonui orogeny. This fold is believed to be faulted on its western edge, but while this is also the case on the eastern limb farther north, in the Awakino-Mahoenui area faulting seems not to have occurred at the surface. The oldest beds exposed are 7,000 ft of Oretian (lower Carnian) which are believed to be the equivalent of 2,100 ft of strata above the Moeatoa conglomerate at Marakopa. These are followed conformably by 2,200 ft of Otamitan strata (Carnian), 1,800 ft to 2,300 ft of Warepan (Norian), 2,800 ft of Otapirian (Rhaetian), 2,500 ft of Aratauran (Hettangian-Sinemurian), about 2,000 it of Ururoan (Pliensbachian-Toarcian), about 4,000 ft of Temaikan (Bajocian-Oxfordian), and 4,300 ft of Heterian (Kimeridgian), giving a total of 14,000 ft of Triassic and 12,000 it of Jurassic strata. Beds of. Heterian age make up a greater thickness than in other North Island localities. On the south shore of Kawhia Harbour there are some 500 ft of beds, including Temaikan, between the type Temaikan and the black, gritty sandstone of Totara Peninsula, whereas below the Palmer Creek Shellbed (the same horizon as the black sandstone) there are 3,500 ft of Heterian strata. The sediments are mainly bedded sandstones and siltstones, with scattered and widespread tuffaceous material. True conglomerates are rare and thin, but grits are more common. Carbonaceous beds occur in the Otapirian and Heterian and accompany thin coals in the Oretian, but no non-marine beds typical of the type Temaikan sequence were observed in strata of Temaikan age. Calcareous beds are found at various levels in the Jurassic sequence but only Aratauran strata were found to contain microfossils. Marine fossils are widespread and allow closer subdivision of some stages. Warepan beds contain numerous forms of Monotis whose stratigraphic distribution enables four subdivisions to be recognized in the area. M. richmondiana is found in the older beds; M. calvata. is confined to the upper Warepan; while middle Warepan strata contain a very varied fauna including forms recognized by Trechmann (1918) and others recorded from New Caledonia (Avias, 1953). The Otapirian Stage is divided after Campbell (1956) although his basal fauna was not recognized in the area. Ururoan beds are divided into those containing Pseudaucella (Pliensbachian) and those with Dactylioceras (Toarcian). This ammonite was not found, but the fauna enables ready correlation wi'th the beds containing it at Ururoa Point.