Late Palaeocene – Eocene tectono-sedimentary evolution of North Westland, South Island : an analysis of the Brunner Coal Measures and their basal contact. (2015)
AuthorsMonteith, Fraser Danielshow all
The Late Palaeocene to Middle Eocene geological history of New Zealand is characterised by tectonic quiescence the development of an extensive unconformity. This period represents the time between cessation of spreading in the Tasman Sea Basin in the Late Palaeocene and commencement of extension of the Challenger Rift System in the Late Eocene. The unconformity itself and the Middle Eocene Brunner Coal Measures (BCM) which directly overlie it are used to study the tectono-sedimentary evolution of this time period in detail. Field research has shown a deep leached horizon underlies the unconformity in almost all cases. This horizon formed through prolonged subaerial chemical weathering in the warm subtropical climate at this time. The degree of weathering is directly related to the duration of subaerial exposure but is occasionally enhanced through leaching of overlying coals. Truncation, partial or complete removal of the weathering profile indicates the presences of palaeovalleys and highs respectively. Two occurrences of continuous deposition through the Late Palaeocene – Early Eocene are recognised. The Ikes Peak Formation and the Papahaua Formation, both within the Paparoa Group, are newly defined in this thesis. Both are related to faulting in the Late Palaeocene, potentially reflecting late stage tectonism related to the Tasman Sea spreading. Stratigraphic columns created from measured sections across coastal North Westland have identified three distinct regions for the depositional history of the Middle Eocene Brunner Coal Measures (BCM). Deposition in the Greymouth Region predominantly records infilling of relict topography of the Late Cretaceous fault bounded basin in which the Late Cretaceous to Palaeocene Paparoa Group accumulated. The Middle Eocene time equivalent of the BCM is the near shore marine Island Sandstone. In the Buller Region, deposition is influenced by small scale syndepositional faulting and constitutes the thickest BCM. The remaining area between, the Charleston Region, is characterised by deposition related to marine transgression over a palaeohigh and the BCM here are thin, younger and often show hints of marine influence. Palaeographic reconstructions were then used to produce a unified tectono-sedimentary model for North Westland which indicates strong NNE trending structural control and diachronous marine flooding. This suggests that the period of tectonic quiescence was not purely passive and that deposition of the Brunner Coal Measures records the initiation of the Late Eocene diffuse extension of the Challenger Rift System.