Pahau Terrane type locality: Fan delta in an accretionary prism trench‐slope basin

Abstract

The sediments of the Pahau Terrane, part of the Torlesse Superterrane, have never been adequately described or formally denned, although an unofficial type area in the Pahau River exists. This research formalises the type locality and interprets the sedimentology and tectonic setting of the deposits. Four well‐defined lithofacies are able to be distinguished in the area: mudstone, interbedded mudstone‐sandstone, sandstone, and conglomerate. We propose the Lochiel Formation to encompass these lithofacies and the Mt Saul Member for the conglomeratic bodies found within the Lochiel Formation. The Pahau River Group is to encompass the Lochiel Formation and future formations defined within the Cretaceous Torlesse Supergroup and is envisioned to extend over a large part of the Pahau Terrane. A shallow‐water fan delta model is proposed for the Lochiel Formation in the type area, based primarily on coarsening and thickening upwards sequences, mixed terrestrial and marine depositional setting indicators such as rootlets and dinoflagellates, and conglomeratic bodies with deposits indicative of both bedload and debris flow transport. A fan‐delta model is chosen over a coarse‐grained delta as local relief had to be high enough to produce debris flows in the delta plain. The presence of rootlets directly under a debris flow deposit indicates a subaerial environment. Paleocurrent indicators from 14 locations suggest a paleoflow direction towards the east‐northeast corrected for regional rotation to east‐southeast. The rare occurrence of bi‐directional and wave ripples suggests that these deposits were not greatly influenced by tidal currents or wave reworking, but were dominated by fluvial unidirectional currents. The tectonic setting for the Pahau Terrane is interpreted as distal forearc to accretionary prism. QFL analysis for the Lochiel Formation suggests a transitional arc provenance, similar to other analyses in the Pahau Terrane. Conglomerate clast rounding and the wide variety of clast compositions indicate a large catchment area, suggesting that the relief needed to produce the debris flows was provided locally by a large fault scarp. This was likely to have been a thrust fault with the Pahau River fan delta deposited in a trench‐slope basin on top of the accretionary prism.