Torlesse greywacke and Haast Schist source for Pliocene conglomerates near Reefton, New Zealand

Abstract

Late Miocene‐Quaternary conglomerates in the Inangahua valley record provenance changes which can be related to tectonic activity in the northern South Island. Conglomerate clasts within the early Miocene Rotokohu Coal Measures comprise mainly metamorphosed quartzose greywacke with subordinate granitoids, consistent with an essentially local clastic source from west of the Alpine Fault. In contrast, conglomerates of the late Pliocene Old Man Group (Larry Schist and Cronadun Conglomerates) are dominated by Rakaia Terrane (Older Torlesse) greywacke and Haast Schist clasts from east of the Alpine Fault, which is c. 50 km southeast of the study area. A complete range of textural and metamorphic grades is present, confirming that oligoclase zone Alpine Schist was being exhumed during Pliocene time. The 50–70°‐dipping Pliocene beds are unconformably overlain by subhorizontally dipping late Quaternary gravels (Tophouse and Speargrass Formations), which, like the Miocene conglomerates, are composed of quartzose greywacke and granitoid rock types. Significant Quaternary deformation isolated the Inangahua valley area from the Southern Alps and resulted in a return to a local clastic provenance.