Alluvial fans and fan deltas in the Paparoa Formation, Greymouth Basin: a new rift model

ABSTRACT Sedimentary facies analysis of conglomerate deposits in the Greymouth Rift Basin has identified the latest Cretaceous to Paleocene alluvial fan and fan delta environments on the northwestern side. The Gravelly Braided River Facies Association is interpreted as high energy, braided river streamflow in a streamflow-dominated alluvial fan environment. The Gravelly Delta Front Facies Association was deposited by high bedload mouth bars and channel avulsion. The Gravelly Prodelta Facies Association is interpreted as high-density turbidity currents, and subaqueous debris flows in a fan delta slope environment. Bedding geometries suggest the fan deltas were Hjulström-types formed on lower angle slopes. The gradual decrease in conglomerate thickness from northwest to southeast indicates that the primary basin bounding fault was located offshore to the northwest, most likely the Cape Foulwind-Canoe Fault Zone. Overall facies distribution and paleoflow directions indicate the Greymouth Basin formed as a half-graben in a purely extensional setting with no strike-slip movement. The presence of contemporaneous sub-basins in the West Coast region suggests they likely experienced similar depositional history to the Greymouth Basin. Our findings agree with previous models from the Taranaki Basin that indicate that rifting was purely extensional, suggesting that the West Coast-Taranaki Rift System recorded primarily orthogonal extension.