The question of post-Rangitata Peneplantation in New Zealand

• Main Author: Gair, H. S.
• Language: en
• Published: University of Canterbury. Geology 2010
• Online Access: http://hdl.handle.net/10092/4966

This thesis is an investigation of the question of peneplanation following the Rangitata Orogeny. Two opposed views had been put forward:- A. That the Rangitata Orogeny was followed by peneplanation and that both were of New Zealand-wide extent, B. That the Rangitata Orogeny was not of New Zealand-wide extent and that on the contrary, in the East Cape and Marlborough, the sequence was continuous and hence there was no peneplanation. The investigation thus necessitated the examination of Cretaceous sequences and their relationship to older rocks in certain key areas viz. East Cape, Marlborough, North Westland and Northwest Nelson. It is shown that:- A. Physiographic evidence of peneplanation in the form of exhumed fossil peneplain surfaces, and stratigraphic evidence in the form of deep weathering (=leaching) on an old landscape, exist in most places throughout New Zealand and the outlying Chatham and Campbell Islands. The apparent absence of exhumed fossil plain remnants in areas of thick Cretaceous marine sediments is considered to be due to the high dip of the cover beds and not that peneplanation did not take place. The dip is appreciably greater than the natural declivity of hill slopes and any fossil plain surface below would thus tend to be truncated rather than exhumed by erosion processes. The absence of stratigraphic evidence of peneplanation (i.e. leaching and quartzose coal-measures) in these areas is considered to be due to an increased tempo of diastrophism resulting in rapid marine transgression that eroded-off the leached zone completely in most places and was too rapid to allow the formation of coal-measures. B. The stratigraphic position of the peneplain is below the Paparoa and Pakawau beds of the Greymouth and Northwest Nelson areas respectively and not immediately below the quartzose coal-measures (Brunner-type beds). C. In all the areas examined major metamorphic unconformities are present either between Cretaceous sequences. The terms “covering strata” and “undermass” are valid and all the beds recognised as cover are everywhere younger than the youngest undermass rocks. D. The Korangan Stage is not valid and the Urutawan Stage is redefined to include the fossils formerly considered to be diagnostic of the Korangan. E. The lithological correlation of the Korangan Stage (= Koranga Sandstone in part) with the Taitai Sandstone at Mt Taitai is wrong and the Taitai Series is thus restricted to apply to only the Taitai Sandstone and the Mokoiwi Mudstone and their age correlatives when proved by fossils. F. The oldest known cover beds are ?Aptian and the youngest undermass ?Hauterivian. The amount of time missing is of the order of 20 million years which is considered to be sufficient for peneplanation to have taken place. It is thus concluded that both the Rangitata Orogeny and subsequent peneplanation were New Zealand-wide in extent and that at the time New Zealand was part of a much larger landmass that included the Chatham and Campbell Islands.