The geology of the northern Aorangi Range and part of Palliser Bay; sheet N165

Abstract

The rocks in the mapped area can be divided into two major groups: a basement of Mesozoic greywackes and argillites, and an unconformably overlying group of Upper Tertiary and Pleistocene rocks. About half of the Mesozoic rocks can be divided into three formations,the remainder are undifferentiated sheared greywackes and argillites. The Te Munga Formation (new name) is a medium to coarse green-grey sandstone with bands of fine partly igneous conglomerates. It is faulted against other "greywackes" of the area. It's age is not definitely known but it is thought to be Lower Cretaceous and a possible correlative of the Taitai Sandstone. The Moikau Formation (new name) consists of thin bedded purplish-black argillites and greywackes that are very contorted and sheared and contain common blocky calcareous concretions,and that are interbedded with occasional thick bands of greywacke. The age is uncertain but is probably Lower Cretaceous. The youngest Mesozoic formation is the Whatarangi Formation (new name) which consists of well developed graded beds passing up into conglomerates,grits and coarse sandstones,all with abundant plant fragments. Concretions identical to those found in the Moikau Formation are found as boulders in the conglomerates. Sparse fossils indicate a Neocomian to Albian age. Volcanic rocks are interbedded in the Moikau Formation and the undifferentiated "greywacke". Boulders of dyke rocks (camptonites?) found in streams are assumed to be derived from dykes in the rocks classed as undifferentiated "greywacke". No Upper Cretaceous,Lower or Middle Tertiary rocks were found and the area was possibly emergent for much of this time. About the end of Waiauan time the essentially non-marine Putangirua Fanglomerate (new name) was deposited. In early Tongaporutuan time a marine transgression commenced in the south-west of the mapped area. It reached the north-east part of the area in Kapitean (upper-most Miocene). At Palliser Bay, Lower and Middle Tongaporutuan fossiliferous grits and sands were succeeded by siltstones and mudstones until the end of Opoitian and Waipipian time when the transgression was at it's maximum. These were succeeded by sandy siltstone in Waitotaran and sandstone in Nukumaruan time. In the north-east basal coquina limestones were deposited as the sea advanced and these were succeeded by deeper water siltstones in Waitotaran and sandstones and limestones in Nukumaruan times. Subsequent filling of the Wairarapa Basin to the west and regional uplift caused regression,and the sea had retreated from most of the area by about the end of Nukumaruan time. Fluvial gravels have since been deposited over large areas. The older "greywacke" formations are intensely deformed, the Whatarangi Formation steeply dipping but less deformed. At least one period of deformation occurred in the early Cretaceous. Others probably occurred between then and late Miocene,but no record of them exists. Faulting occurred just prior to Lower Tongaporutuan time and the Putangirua Fanglomerate was deposited as a result. The present dominant faults were not important until Nukumaruan time when regional uplift occurred and folding of Upper Tertiary and Pleistocene rocks took place.