Sea level and storm control on the evolution of a chenier plain, Firth of Thames, New Zealand

Geological controls on the evolution of the Miranda chenier plain, Firth of Thames, New Zealand are investigated using high resolution stratigraphic information obtained from geophysical surveys. Two previous studies of the Miranda chenier plain have provided disparate conceptual models of its development over the Holocene: one study evokes sea level oscillations to form individual cheniers, while the other offers an alternate model based on the observed transgression of the modern chenier. Both studies agree that sea level has dropped between 4000 and 1000 years BP, but the elevation of the highstand and nature of the fall are contentious. This paper extends previous interpretations using data obtained from ground penetrating radar surveys and historical aerial photographs. Stratigraphic interpretation of the continuous, high resolution sub-surface radar data was used to infer a sea level curve for Miranda. These data enable four new inferences: (1) sea level fell from ~ 2 m around 4000 years ago (mid-Holocene highstand) to present about 1000 years ago; (2) falling sea level influenced chenier spacing, with the onset of alongshore (rather than across-shore) chenier-plain development coinciding with the termination of sea-level fall after the formation of ridge six; (3) all cheniers initially transgressed, producing similar resultant spit morphologies, but the evolution of the more recent ridges (1–5) differed from older ridges (6–13) in that transgression occurred over embayed tidal-flat sediments, while the older cheniers stabilized on foreshore mudflats; and (4) storms exert an important control in initiating chenier genesis. The Miranda chenier plain is a site of global significance for understanding chenier evolution. In this study, the high level of subsurface detail, unimagined in past decades, presents greater insight into the morphology and stratigraphy of the chenier ridges shedding new light on the role of sea level and storms on the evolution of the plain.

► GPR and air photo analysis were used to examine chenier plain evolution.
► Radar records of palaeo-beachfaces indicate storms initiate chenier formation.
► Sea-level proxy imaged by GPR implies a fall from + 2 m to MSL at 4–1 ka yrs BP.
► Forced chenier plain progradation occurred during sea-level fall.
► Progradation switched to alongshore extension when sea-level stabilized.