A sedimentological profile of the Pahau Terrane type locality
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Pahau Terrane as part of the Torlesse Superterrane extends for over 30% of the New Zealand landmass. Although an unofficial type locality around the Pahau River has been proposed, it has never been adequately described or defined. One of the main goals of this research was to describe in detail the deposits of the Pahau River area in order to define the type locality. In total ~900m of section was logged describing four lithofacies; mudstone, interbedded mudstone - sandstone, sandstone, and conglomerate. Two formation names and a group name are proposed and intended for widespread use. The Mt Saul Formation is proposed for coarse-grained conglomeratic lithofacies whereas the Ship Spur Formation is proposed for the combined fine-grained sedimentary lithofacies. The Pahau River Group encompasses these formations. Stratigraphic logs clearly show coarsening and thickening upward, progradational sequences, at the tops of which either highly carbonaceous beds or rootlets can be observed. The bedding in the conglomerate lithofacies are dominated by bedload transport structures including large scale cross stratification, imbrication and clast support. Micro and macrofossil collection suggest a nearshore marine sequence yet rootlets suggest subaerial indicating a range from fully marine to terrestrial depositional environments, making them the only known terrestrial deposits in the Pahau Terrane. This suggests a fan-delta depositional setting. Very few bi-directional structures were observed, suggesting that these deposits were not influenced by wave reworking and tidal currents to any great degree but are fluvially dominated. Thin section analysis of fine to medium sandstones suggests a transitional to mixed arc provenance. Measurable indicators suggest a palaeoflow direction towards the east to northeast. These results are similar to previous studies indicating the suitability of the Pahau River area as a type locality for the Pahau Terrane.