Lithofacies relationships in a late Quaternary gravel and loess fan delta complex, New Zealand
Abstract Coastal cliffs up to 70 m high provide a nearly continuous section along depositional strike of a small Quaternary fan delta complex south of Conway Flat, north Canterbury, New Zealand. A gentle northeasterly dip permits observation of all facies of the complex, from the offshore prodelta to an emergent interdeltaic embayment facies showing beds with abundant rhizoliths, and in situ roots and stumps of large 8000-yr old podocarp trees. Coarse gravels derived from the Hawkeswood Range (650 m high), 5 km west of the sea cliffs, were transporte largely by debris flow processes into the Pacific Ocean towards the Conway Trough (800 m deep), 10 km to the east of the cliffs. Narrow (up to 1 km wide) Gilbert-type gravel deltas built elongate lobes into channels created by slumping and sediment gravity flows which deeply incised muddy prodelta deposits. Between the fan deltas, embayment facies are dominated by loessial sediment with sand:silt:clay ratios of about 7:68:25; thin sand beds are present. Prodelta sediments also are composed largely of redeposited loess. Loessial sediment came from the distant glaciated Southern Alps, transported both in air and along major rivers that produced broad plumes of suspended sediment in the ocean which were carried northwards by the prevailing longshore currents. Laterally adjoining the fan delta lobes, on several wave-cut benches within them, and scattered in the distal embayment facies are winnowed wave-sorted bars of fine gravel and sand. The proximal embayment facies includes sparse, poorly sorted, gravelly storm bars, gravelly fluvial crevasse deposits, and shallow broad tidal channel filled with loessial sediment and fine sand. Well-displayed trace fossils permit close correlation of ichnofacies with lithofacies and extend paleoenvironmental interpretation in this young deposit. Trace fossil associations and interpretations are described in the following paper in this journal.