The geology of Pongaroa, Akitio county: a correlation of erosion and soils with lithology

Abstract

Mapping in the Pongaroa area, Akitio County, was hindered by poorly fossiliferous Tertiary formations, large areas having no fossils at all and the Cretaceous practically devoid of all fauna. Much depended on lithology and continuity of outcrop, particularly in the older formations ranging from lower Cretaceous to Landon in age. Only a few faults can be detected with certainty in the field but in some areas subtle fault traces are a feature, particularly in the Whangai argillite. Deposition was dominated in the Cretaceous by the New Zealand Geanticline which was flanked on either side by long narrow troughs but some sediments were also derived at intervals from a less significant mobile geanticline, more or less coincident with the present day east coast. The region of deepest sedimentation coincided with the tectonic axis of the present Waihi Range and the trough itself was probably part of a larger geosyncline further east. Up to Whaingaroan times stable tectonic conditions prevailed but a major change from very fine light coloured sediments to sorted beds of great thickness and well developed rhythmic banding was indicative of a renewal of orogenic action early in the Landon. Preliminary uprise along the former axis of deepest sedimentation resulted in a migration of the principal sedimentation axis to the west but the Akitio Syncline still received material up to the Tongaporutuan. Southland seas were extensive and occasionally completely overlapped the Otaian sediments but increased tectonic activity resulted in all grades of lithology from shelf S/st. and L/st. to massive m/st. and turbidite. The Akitio syncline was finally emergent in mid-Tongaporutuan times but further west in the Petone Trough, Southland Sediments were completely buried beneath those of Tongaporutuan to Castlecliffian age. The speed of deformation culminated in the Kaikoura Diastrophism when movement along newly developed dextral transcurrent faults, all of which were more or less parallel to the N.Z. Geanticline, must have presented a picture of intensely writhing land masses. Many of these faults had normal components so that older formations were exposed by uplift and subsequent erosion of younger sediments. Such faults as the Waione F., originating in the lower Tertiary were few but these too follow the NNE regional trend. Warping in the Akitio Syncline has probably been induced by differential movement along adjacent transcurrent faults with associated subsidence of strata in between, resulting in assymetrical synclines. Two such synclines were separated by a fault at depth where the sharp "ridge" so produced at their point of contact may be expressed at the surface by an anticline. Later faulting has produced tilted erosion surfaces and earthquake scarplets prominent in the thesis area. There is a distinct relationship between the soils and the lithologies on which they were formed and these units were further subdivided by the steepness of slope. The profiles of many soils were distinctive but those formed on the extensive mudstones and turbidites were often difficult to separate on the basis of physical information alone. Since profiles bear a close resemblance one to another and mottling was almost invariably prominent the soil unit must be deduced from such criteria as parent material, fertility, farming practices etc. The conversion from original forest vegetation to grassland in the 19th century was reflected in the change from nutty structure in most soil profiles to granules in the uppermost 2"-3" of the topsoil. The most serious erosion problems prevailed in the vicinity of incised streams in all rock types owing to continued downcutting to reach grade after recent tectonic movements. As a rule, the more indurated rocks were much less susceptible to erosion than softer ones but these were not necessarily the older lithologies. Within the last few years, aerial topdressing has controlled slip erosion to such an extent that lithologies can no longer be differentiated with certainty on the basis of severity of slipping alone.