Development of a chenier plain, Firth of Thames, New Zealand

Abstract

A chenier plain is a prograded coastal plain in which ridges, composed of coarse material moved by longshore drift, are stranded overlying finer-grained marine or littoral sediments. Subsurface data from the Miranda coastal plain, Firth of Thames, New Zealand, confirm that shell ridges between Kaiaua and Miranda are true cheniers. Surveys of a modern chenier indicate that coarse sediment and shell form sand bars on the foreshore and that these migrate landwards through swash action. Subsequently a shell ridge is built on the sand bar from shell derived from offshore and reworked from the north, and storm events form a storm crest above high spring tide level. Deposition of fine-grained sediment occurs in an embayed tidal flat behind the chenier and within this environment the bivalve Mactra ovata is found. These sediments can accrete to an elevation at which mangrove and salt marsh are able to colonise. The chenier may continue to transgress landwards, though at a slower rate, retreating over the embayed tidal flat, mangrove or marsh, and compressing these sediments. Erosion of these compressed muds on the face of the beach leaves specimens of Mactra in their growth position exposed on the modern chenier, at an elevation of 0.62–0.80 m above MSL. Stratigraphical observations along a west-east profile across the coastal plain reveal that sandy-muddy sediments are underlain by peat, clayey-tephra and older marine sediments and 4 radiometric dates of > 30,000 yrs B.P. have been determined on these. Shell ridges on the plain are underlain by blue muds within which occurs a band of Mactra ovata tristis in growth position. These sediments are considered analogous to the embayed tidal flat sediments behind the modern chenier. The continuity of the shell layer, radiometrically dated to 3650 ± 60 yrs B.P. to landward and 1260 ± 50 yrs B.P., to seaward, implies that there have not been oscillations of sea level during this regressive coastal phase while the shell ridges have developed. Mactra were also found in situ at the north and south of the coastal plain dated to 4510 ± 70 and 200 ± 60 yrs B.P., respectively. From elevations on sub-fossil Mactra it is implied that sea level was 0.70–0.90 m above present 3600 years ago and that it fell gradually until 1200 years ago when it was close to present. Erosion, deposition and migration of a chenier can occur simultaneously and long-term variations in sea level or in sediment supply do not appear necessary to explain the episodic formation of chenier ridges.