Tahora Formation: The basal facies of a Late Cretaceous transgressive sequence, northeastern New Zealand

The Tahora Formation is the basal unit of a Late Cretaceous (Campanian-Maastrichtian) transgressive sequence in western Raukumara Peninsula, northeastern New Zealand. Two major lithofacies associations are recognised. In the west, the Tahora Formation is predominantly poorly bedded, finegrained quartzose sandstone, and is up to 500 m thick (Maungataniwha Member). To the east, the stratigraphic equivalent is relatively thin (generally < 50 m), very poorly bedded glauconitic siltstone (Mutuera Member). In the central part of the study area the two are complexly interbedded. Further to the east and northeast, laterally equivalent beds are flysch facies. Sedimentary structures, grain-size analyses, and locally common macrofossils indicate that, in the west, Maungataniwha sandstone facies were deposited in beach to shallow inshore environments, largely at water depths of less than 30 m. The Mutuera facies are inferred to have accumulated further offshore. Virtually massive 3-30 m thick Maungataniwha sandstones, which apparently interfinger with Mutuera siltstone to the northeast, may represent discontinuous, elongate shelf sand ridges, formed by tidal or storm-generated currents. A local channel-fill sequence (Houpapa Member) was probably deposited by sediment gravity flows, including turbidity currents. In the area of what is now Raukumara Peninsula, marine transgression advanced westward in the early to mid Campanian, and continued into the late Maastrichtian. Sedimentation was partly fault controlled in the west, and growing folds probably influenced depositional patterns to the east.