Rangitīkei River Terraces Formed During the Otira Glaciation: Lithostratigraphic Type Sections, Review and Conceptual Model

The Rangitīkei Valley in the lower North Island of New Zealand contains flights of river terraces that are influenced by changes in sediment supply, stream power and base level associated with Quaternary climate events over the last ∼350 thousand years. Despite providing a valuable record of past environmental change, most work on the Rangitīkei terraces remains unpublished, and a majority of the stratigraphic units lack formal definition. Our study presents a review of the Rangitīkei River terraces and new reference and type sections for the lithostratigraphic units formed during the Otira Glaciation, building on the seminal work of Derek Milne. Site selection is based on outcrop quality, ease of access and health and safety considerations whilst aiming to preserve historic precedence and geographic proximity to associated place names. A conceptual model of Rangitīkei terrace formation is presented, highlighting the transition out of full glacial conditions where reduced sediment supply and increased stream power result in incision, upstream knickpoint migration, reworking of alluvium and terrace formation. This model provides an update on earlier work by incorporating concepts around knickpoint retreat and the complex response of the fluvial system to produce multiple terrace levels within a single terrace set.