Abstract
An 11 m thick subhorizontal beach deposit rests on steeply dipping Cretaceous bedrock. Sediments, ranging from a basal boulder bed to upper sands, are poorly sorted and negatively skewed, indicating pronounced winnowing of fine material. Impact features on quartz grain surfaces attest to high‐energy turbulent environments, and are similar to those found on clasts from modern nearby beaches. The Haast River was source to some of the sand and gravel. Heavy minerals from the Dun Mountain Ophiolite Belt were transported 85 km by fluvial/glacial and longshore drift processes, necessitating caution when using apparent lateral separation of source material for estimating strike‐slip displacement rates on the Alpine Fault. An optical luminescence age estimate of 123 ± 7 ka for Knights Point beach sands dates to the last interglacial (MIS 5e). A shore‐platform altitude of 113 m a.s.l. requires tectonic uplift of the Australian plate of 0.86 mm/yr, an order of magnitude less than the nearby Pacific plate.
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