Influence of system controls on the Late Quaternary geomorphic evolution of a rapidly-infilled incised-valley system: The lower Manawatu valley, North Island New Zealand

Abstract

The Manawatu incised-valley estuary was rapidly infilled between 12,000–4700cal.yrBP. A combination of empirical measurements of sedimentation rates, a reconstruction of relative sea-level (RSL) change, and digital elevation models of key surfaces within the Holocene sedimentary fill of the valley were integrated to produce a numerical model to investigate the influence of the system controls of sea-level change, sediment flux, and accommodation space on the rapid infilling history of the palaeo-estuary. The numerical model indicates that sediment flux into the palaeo-estuary was greatest during the Holocene marine transgression between 12,000–8000yearsBP. The average rate of sediment deposition in the estuary during this period was ~1.0Mm3yr−1. This rapid rate of sedimentation was controlled by the rate of accommodation space creation, as regulated by the rate of sea-level rise and the antecedent configuration of the valley. By the time sea levels stabilised c. 7500cal.yrBP, the palaeo-estuary had been substantively infilled. Limited accommodation space resulted in rapid infilling of the central basin, though sediment flux into the estuary between 7100 and 4500cal.yrBP was at a lower rate of 234,000m3yr−1. The limited accommodation space also influenced hydrodynamic conditions in the estuarine central basin, driving export of fine-grained sediment from the estuary. Once the accommodation space of the estuarine basin was infilled sediment bypassed the system, with a consequent reduction in the sedimentation rate in the valley. More accurate partitioning of the sources of sediment driving the infilling is necessary to quantify sediment bypassing. Post-depositional lowering of RSL index points from the valley is driven by neotectonics and sediment compaction.