The Geology of the Whareama Area: A Study of Slumping and Redeposition as Applied to the Origin of Alternating Beds

Abstract

Both the sedimentation and the subsequent deformation of the Upper Cretaceous and Tertiary beds deposited in the region have been considerably influenced by the basement structures consisting of a minor trough separating two "highs" which have shown a continually recurring tendency to rise during most of post-Upper Cretaceous time. The area of deepest sedimentation coincides approximately with the present central Whareama valley, while the thinnest sequences were laid down over the crests of the upfolds. These occasionally succeeded in rising above sea-level, contributing local conglomerates to the beds being deposited around their flanks. Sedimentation, considering the length of time involved, was not great, a thickness of some 1,000-2,000 ft. of mainly fine-grained Cretaceo-Tertiary beds being laid down in the central syncline, while the thickest sequence of Miocene strata is not more than 3,000 ft. thick. These latter rocks consist predominantly of alternating sandstone and siltstone which display various types of intraformational and interstratal slump phenomena; deposition of the beds is attributed to turbidity current action. Facies changes in the Altonian provide evidence for the hypothesis that the source of supply lay towards the north-east. The major compressional phase occurring during the Kaikoura Orogeny resulted in the elevation of the "highs" and in the sliding of the Cretaceo-Tertiary beds off their flanks. Slumping took place underneath the comparatively undisturbed Miocene cover which was, however, in places pierced and uplifted by the moving material below. Bedding-plane thrusts have forced the older beds against the Miocene on the east side of the coastal "high" and an important fault zone has resulted. Recent movement has consisted of small uplifts, a series of terraces and a raised marine platform being formed.