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Middle Miocene formational stratigraphy (Mokau-Mohakatino Groups) at Waikawau, northeastern Taranaki Basin margin, New Zealand

1990; Taylor & Francis; Volume: 33; Issue: 4 Linguagem: Inglês

10.1080/00288306.1990.10421377

1175-8791

Scott D. Nodder, Campbell S. Nelson, Peter J.J. Kamp,

Geological and Geophysical Studies

Abstract The Waikawau region occupies a critical position near the junction between onland North Wanganui Basin and mainly offshore Taranaki Basin, affording a unique, albeit narrow, onshore “window” into a portion of the mainly offshore geology in North Taranaki Graben of Taranaki Basin. The sedimentary geology at Waikawau has previously been ascribed to the Middle Miocene Mohakatino Group, and undifferentiated as to formations. Although probable correlations with formations in both the Mohakatino and Mokau Groups elsewhere can be made, four new formations are defined and their distribution presented in a geological map. The lowermost, the Nukuhakari Formation, is dominated by Altonian to upper Lillburnian-Waiauan, massive, blue-grey calcareous mudstone, which either overlies localised Oligocene Te Kuiti Group strata, or, more commonly, onlaps Mesozoic basement rocks. Nukuhakari Formation correlates partly with Mokau Group, but mainly represents time missing in a prominent unconformity in Mokau-Mohakatino rocks to the south near Awakino. The three overlying formations are all intra-Waiauan in age. The Tirua Formation varies from massive, calcareous sandstone beds in the south to thinly bedded, thoroughly bioturbated (Thalassinoides-Scolicia-Palaeophycus-Planolites) sandy limestone sheets in the north; it correlates with Mangarara Sandstone elsewhere. The Ngarupupu Formation includes six variably volcaniclastic and interbedded sandstone and mudstone lithofacies, some with abundant trace fossils (Zoophycos-Chondrites-Planolites), and is correlated with Purupuru Volcanic Sandstone to the south. The Waikaretu Formation is essentially nonvolcaniclastic. and is dominated by well-sorted, massive, reddish-brown, fine-grained sandstone units, separated by thin layers of grey mudstone. The formation is a possible correlative of Ferry Sandstone and, more tentatively. Tawariki (Mudstone) Siltstone elsewhere. The four formations at Waikawau are a record of mainly hemipelagic and gravity flow deposition in an outer shelf to more typically slope setting located between basement Herangi High to the east and evolving andesitic volcanic massifs offshore to the west. The Waikawau region has simple structure. The Miocene formations dip generally to the WSW or northwest at 3–11°, with discernible discordances of 1–3° between formations, and subtle eastward wedging within some formations. Their outcrop distribution is strongly influenced by two sets of mainly high-angle normal faults oriented either northeast-southwest or east-west.