Gravelly Gilbert-type fan delta on the Conway Coast, New Zealand: Foreset depositional processes and clast imbrications

Abstract

Quaternary Gilbert-style fan deltas of the Conway Coast, New Zealand, are formed and exposed by the continuing uplift and erosion of the Hawkswood Range due to transpression associated with the modern New Zealand plate boundary. The fan deltas are exposed in 50–70 m high cliffs and crop out for over 5 km along the coast and up to 2 km inland. Depositional processes on the foresets were dominated by sediment gravity flows originating from hyperpycnal river flow and gravity induced slumps. Slumps may originate at any depth on the fan delta foreset and are common in the deposits. Foreset beds are interpreted as the deposits of sediment gravity flows with preserved foreset beds that are preserving normally graded, inversely graded or massive, and change abruptly laterally and down slope. Gravity induced slumps are the dominant mode of deposition on the foresets along the Conway Coast and presumably all Gilbert-type fan deltas. This study concludes that the sediment gravity flows developed basal traction carpets as evidenced by the regular appearance of ∼ 25° imbricated clasts in nearly all foreset beds. Behind slump deposits, there is a strong zone of ‘backstacking’ clast fabrics that prograde upslope into backset beds. These clast fabrics provide information about downslope slumps. Excellent exposures of distal foreset and nearshore marine deposits show the dynamic interface within a fan delta complex.