Stratigraphy, sedimentology and structure of the Ihungia decollement, Raukumara Peninsula, North Island, New Zealand

Abstract

The 75 km² Ihungia catchment, in the extreme northeast of the North Island of New Zealand, includes part of the northeast-trending, complex belt of Cretaceous and Tertiary rocks forming the eastern continental margin sequence of New Zealand (the East Coast Deformed Belt). Upper Cretaceous “autochthonous” rocks, exposed in the north of the region, are mapped as extensions of the Ngaterian Hikurangi Beds(formation)and overlying Teratan–Haumurian calcareous flysch of Tapuaeroa Formation. The Hikurangi Beds contain 2 new units—the lower Mangarakeke lithofacies(300 m thick, alternating noncalcareous sandstone and mudstone), and the Rangikohua lithofacies(200 m thick, massive, noncalcareous sandstones separated by thin siltstone lenses). These rocks were deformed by north–southtrending isoclinal folding before becoming a repository area for 3 thrust sheets, emplaced by gravity sliding from the north over the “autochthonous” material. The lowest thrust sheet, emplaced following Late Oligocene time, is composed predominantly of Mokoiwi Formation mudstones, including blocks of Taitai Sandstone Member (Motuan, Albian age), and blocks of Mangatu Group lithologies, including Whangai and Weber Formations (Dannevirke–Landon, Paleogene),enclosed in bentonite. New lithologies in the Mangatu Group are divided into sandstone, flysch, or greensand lithofacies. The middle thrust sheet contains thick Upper Tertiary mudstone and flysch of the Ihungia Formation, including a 10-15 m thick, normally graded, igneous conglomerate lithofacies, and a 100 m thick, laminated bryozoan, coquina limestone lithofacies. Emplacement postdates the emplacement of the lowest thrust and occurred later than Middle Miocene. Mangatu Group lithologies recur in the uppermost thrust sheet, emplaced after the middle thrust. All lithologies have been influenced by multiphase ENE-WSW to east-west faulting and folding episodes.