Upper Cretaceous and Tertiary geology of Mangatu State Forest, Raukumara Peninsula, New Zealand

Abstract

In the upper Waipaoa catchment, Raukumara Range, the oldest unit is Tikihore Formation (Arowhanan-lower Haumurian, upper Cenomanian-Maastrictian), a 400-m-thick unit of shallow, shelf deposited, alternating sandstone and siltstone beds. This is conformably overlain by Mangatu Formation (upper Haumurian-Runangan, Maastrictian-Eocene), 1100 m thick, which consists characteristically of fine-grained calcareous and quartzose siltstone, minor greensand, chert, carbonaceous siltstone, and fine-grained limestone. Unconformably overlying the Mangatu Formation is 2200 m of Miocene sediments subdivided into the Te Arai and Tokomaru Formations. Te Arai Formation (Altonian-Waiauan, Middle Miocene) comprises a coarse basal igneous conglomerate and alternating blue-grey mudstone and fine sandstone, with minor coquina limestone. This sequence is unconformably overlain by the youngest sedimentary unit, Tokomaru Formation (Tongaporutuan-Kapitean, Upper Miocene), consisting of massive, fine-medium-grained quartzolithic sandstone. Species of Inoceramus characteristic of all stages of the Raukumara and Mata Series (Cenomanian-Maastrictian) are found in Mangatu Forest. One outcrop of basalt, and the predominance of volcanic sediments as the primary sedimentary debris in Tikihore Formation, indicates a close volcanic source. The fossil evidence together with sedimentary structures suggest a shallow nearshore shelf environment, receiving volcanic material from the north rather than greywacke-derived material from the west. Mangatu Formation is thought to represent mid shelf deposition, as do both Te Arai and Tokomaru Formations. The structure is typical of decollement tectonics. The original depositional area was part of a basin much nearer East Cape, and the complete sequence has since moved southwest.Modifications of stratigraphy have been made on the basis of better field exposures on Forest Service roads and more accurate dating of units, based on more numerous macrofossil samples. Major faults and folds along a northwest-southeast axis complicate the sequence which is folded into a broad truncated anticline, Tawhiti Anticline. The mapped area is divided into 3 major fault blocks, Mangatu Fault Block, Matakonekone Fault Block, and Tikihore Fault Block. Locally a steeply eastward dipping fault, Waipaoa Fault, separates middle Tertiary formations from older formations. Parts of the overlying Te Arai Formation are faulted out, up to 350 m are missing, and in other locations the contact is unconformable on Mangatu Formation.