Stratigraphy, structure and geological history of mid-Cretaceous sedimentary rocks across the Torlesse-like/non Torlesse boundary in the Sawtooth Range-Coverham area, Marlborough.

Abstract

This thesis describes the geology of an approximately 100km2 area lying between the Clarence River and Kekerengu. The objectives were to determine the relationship of the "Torlesse-like" sawtooth Group to the late Early Cretaceous Coverham Group; to determine the relationship between the coeval Split Rock and Burnt Creek Formations within the Coverham Group; and to investigate the nature of Cretaceous events which led to the traditional differentiation into older Torlesse type "basement" and younger Cretaceous "cover". Geological mapping indicates the presence of three packets (Glencoe, Pikes and Coverham Blocks) of sedimentary rocks separated by the major Ouse and Pikes Faults. These packets comprise probable submarine fan flysch, massivEð€„’ sandstone, massive siltstone, acid tuffs and conglomerate of Sawtooth Group (Torlesse-like Urutawan - Motuan) unconformably overlain by probable slope basin flysch, massive siltstone, Inoceramus shellbed, and conglomerate of Coverham Group (non-Torlesse). The unconformity is most commonly angular but in a few places is a more subtle paraconformity. A further minor unconformity occurs at the base of the Ouse Member within the Split Rock Formation of the Coverham Group and is thought to reflect the presence of the growing Ouse Anticline. The Coverham Group rocks have similar Motuan - Teratan ages on each side of the Ouse Fault. The Split Rock Formation, previously used only for rocks in the middle Clarence Valley, has been extended to the Coverham area and used for rocks west of the Ouse Fault. The partly coeval Burnt Creek Formation east of the Ouse Fault was probably deposited some distance from the Split Rock Formation in a different basin separated by a structural high. They were juxtaposed by low angle reverse movement on the Fault in the Late Cretaceous. structural/deformation characteristics cannot be used as criteria for separating the Torlesse-like rocks from non-Torlesse rocks in the study area. It is dangerous to assume that 'Torlesseness' is a certain and particular state of deformation. Both the Torlesse (Sawtooth) and Coverham Group rocks exhibit a whole spectrum of deformation from 'broken formation' to more or less undisturbed beds. The pattern of deposition and deformation suggests an accretionary prism setting for these rocks. Sawtooth Group rocks are likely to represent 'younger' Pahau Terrane rocks which were deformed by a single intra-Motuan event either tectonic or perhaps a huge submarine slide, creating widespread unconformity between them and the Coverham Group slope deposits. Continuing instability is likely to have led to growing folds and further minor unconformities. The termination of the Rangitata Orogeny occurred in a progressive and evolutionary way representing a mid-Late Cretaceous change from a compressional subduction regime to a tensional rifting regime. Andesitic-rhyolitic volcanism was common in the late Early Cretaceous.