Geology of autochthonous and allochthonous strata in the Omahuta area, northern New Zealand

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Brook, F.J.; Isaac, M.J.; Hayward, B.W. 1988 Geology of autochthonous and allochthonous strata in the Omahuta area, northern New Zealand. Lower Hutt: New Zealand Geological Survey. Record / New Zealand Geological Survey 32 40 p.

Abstract: In the Omahuta area, northern North Island, highly deformed and weakly metamorphosed Paleozoic-Mesozoic basement (Waipapa Group) is unconformably overlain by a thin, northwest-dipping, transgressive shelf to upper bathyal sequence of Late Eocene age (Whakateterekia Subgroup; Te Kuiti Group). The Tertiary sequence incorporates a lower glauconitic sandstone (Ruatangata Sandstone) of Bortonian to early Kaiatan age, and an upper calcareous mudstone (Mangapa Mudstone) of early Kaiatan to Runangan age. The Te Kuiti Group is truncated at a bedding parallel/subparallel decollement surface, above which are a series of nappes constituting a >2500 m thick allochthonous unit. The allochthon incorporates mid Cretaceous-Paleocene submarine basaltic volcanics, gabbroic and doleritic intrusives, and hemipelagic/pelagic sediment of the Tangihua Complex; Late Cretaceous-Early Eocene predominantly non-calcareous, terrigenous, and hemipelagic/pelagic sediment of the Mangakahia Complex; and Early Eocene-Oligocene calcareous hemipelagic/pelagic and terrigenous sediment of the Motatau Complex. Macroscopic structures in rocks of the allochthonous units indicate that nappe transport was predominantly towards the southwest quadrant. None of the nappes have a root zone in the mapped area, indicating a minimum transport distance of 40 km. The nappes were probably emplaced in the Omahuta area by gravity sliding, in the latest Oligocene to earliest Miocene (Waitakian-Otaian). Basement and covering strata were uplifted and tilted to the northwest in the Early Miocene (Otaian), following allochthon emplacement. Basalt was erupted from several centres near Mangapa in the late Pliocene to early Pleistocene (Kerikeri Volcanic Group). Some normal faults in the southeast of the mapped area were probably active during the Quaternary, and the Omahuta area has undergone regional uplift during the late Quaternary

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