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Sedimentology of a debris-rich, perhumid valley glacier margin in the Rakaia Valley, South Island, New Zealand



The sedimentology and stratigraphy of a multi-phase glaciation sequence dating to Marine Isotope Stage 6 in the Rakaia Valley, South Island, New Zealand, is presented. This outcrop presents an example of the depositional signature of an end member of temperate valley glaciation, where voluminous sediment supply in a tectonically active setting combines with high annual temperatures and low seasonality to generate significant year-round glacifluvial activity. Such glacial systems produce geological–climatic units that are dominated by thick sequences of aggradational gravels and proglacial lake sediments trapped behind outwash heads during deglaciation. At Bayfields Cliff, outwash sequences record an oscillating glacier margin marked by a sequence of glacier-fed, Gilbert-type deltas. The deltas are cut by numerous small-scale, syndepositional, normal faults indicating both loss of glacier support and melt-out of buried ice. A larger-scale thrust fault system reflects late-stage ice overrun. Braid plain gravels and chaotic disturbed glacial lake sediments are also recorded. A notable feature of these systems is the virtual absence of till in an environment with much other evidence for proximal ice. Cumulatively we regard these sediment–landform associations as diagnostic of debris-laden, perhumid, temperate valley glacier systems. Copyright © 2012 John Wiley & Sons, Ltd.