Abstract:
A broad north-east trending horst of Upper Cretaceous grey to brown argillites (Whangai facies) occupies the greatest part of the area. It is flanked on both sides by younger sediments. Down-faulted to the east, is a Lower Miocene sequence of alternating, graded-bedded sandstones and mudstones. Down-faulted still further to the east is a small outlier on the coast at Castlepoint, consisting of a Lower Pliocene mudstone, and a Lower Pleistocene limestone. Down-faulted to the west of the Upper Cretaceous belt are complexly faulted sediments ranging from early to middle Tertiary in age. These outcrop in the Middle Tinui Valley. The Eocene - Lower Oligocene sediments consist of fine-grained light-coloured argillites, including bentonites and calcareous siltstones. The Upper Oligocene is represented by a basal conglomerate and graded-bedded sandstones and mudstones, locally overlain non-conformably by basal Miocene reef-limestone and calcareous sandstone. Down-faulted further to the west are middle Miocene massive blue-grey siltstones, passing into graded-bedded sandstones and mudstones in the Upper Tinui Valley.
The structural trend is defined by continuous major north-north-east trending faults. Subsidiary west-north-west and north-trending faults are also present. The major fold structures are not as clearly defined.
