Stratigraphy and sedimentology of Tokomaru Formation (Late Miocene — Early Pliocene), eastern Raukumara Peninsula, New Zealand

Abstract

Abstract Tokomaru Formation is a thick, extensive, Upper Miocene — Lower Pliocene (Kapitean-Opoitian), sheet-form, sand-dominated sequence within the East Coast Deformed Belt, Raukumara Peninsula, New Zealand. The rocks appear to be autochthonous, lying with small unconformity on mid — upper Tertiary allochthonous rocks in the east, and overstepping with large unconformity onto complex Cretaceous strata in the west. The formation is fine grained, but has at least 1 localised, basal conglomerate lens, some medium-grained calcarenites, and a tuff bed. From composite sections, the formation can be subdivided into a lower 100 m of homogeneous, thickbedded, fine-grained sandstone, followed by at least 200 m of alternating, frequency graded, very fine sandstones and coarse siltstones. The sands are largely volcanic derived, but only 1 definite tephra (rhyolitic in composition) was found. Foraminifera indicate upper to mid bathyal water depths, but transported inner shelf forms are also present. Locally abundant, mostly fragmentary macrofossils of low diversity appear to have been transported. Abundant trace fossils include Ophiomorpha, Zoophycos, Paleodictyon, Chondrites, and a simple V-shaped Tigillites-type burrow, which are consistent with an upper to mid bathyal environment. More than 1 sediment gravity-flow mechanism was responsible for sediment emplacement. The conglomerates were possibly emplaced by high-density turbulent flows with subsequent tractional reworking. The thick-bedded sandstones and alternating sequence were deposited by many turbidity currents. Deposition took place in the arc-trench gap of the Neogene New Zealand magmatic arc.