The graded sediments of the Mahoenui formation (King Country, north island)

Abstract

The graded beds of the Mahoenui formation (Upper Oligocene) are confined to a fairly narrow zone with a north-south trend; narrow in the north (south-east of Te Kuiti) and broadening out to the south on either side of Taumarunui. The beds are graded from fine to medium argillaceous sandstone at the base to silt or clay at the top. The orientation of plant fragments and the under surfaces of sandstones commonly show linear features which have a very marked north-south trend. Flute casts are usually weak to absent. Where present, they show anomalous directions for the source of supply of the sediments. Many flute casts suggest a sediment origin in the south, but others, combined with current bedding on the flanks of the area, and overturning of convolutions within the sandstones, indicate a northern source of supply. Considering all the available data, it is suggested that a northern source area for the sediments is more probable. The graded beds are interpreted as having been deposited, in part, by turbidity currents initiated by the flooding of a major river flowing from the north.