Post-glacial sea-level control and sequence stratigraphy of carbonate–terrigenous sediments, Wanganui shelf, New Zealand

Abstract

Abstract Shallow-marine, mixed carbonate–terrigenous sediments have been accumulating in water depths of 20–110 m on Wanganui shelf, central-western New Zealand, during the latest Quaternary post-glacial sea-level rise since about 18,000 years BP. Cores show an upward change from coarse, mostly terrigenous-dominated sands and gravels, to shell-rich sediments, and then to terrigenous-dominated muddy, very fine sands and sandy muds. These post-glacial carbonate–terrigenous sediments are a modern analogue for similar episodes of cyclothemic shelf deposition that characterise onland Plio–Pleistocene sections in central-western New Zealand. Like their ancient counterparts, the latest Quaternary sediments can be interpreted in a sequence stratigraphic context. A transgressive systems tract, 4–15 m thick, overlies a transgressive erosion surface evident on seismic reflection records, and is represented by slightly shelly coarse sands and gravels at the base of several cores, and as palimpsest surficial deposits on the modern shallow shelf out to a depth of about 50 m. The overlying shelly deposits, both shell-rich and matrix-rich, are equivalent to mid-cycle or backlap shellbeds described elsewhere. They comprise a diverse in situ or locally transported bivalve molluscan and bryozoan biota typical of temperate-latitude cool-water carbonate shelf facies. These condensed section shellbeds are up to at least 1.5 m thick, gradationally overlie the transgressive sediments in cores, and occur as modern surficial shelf facies between about 25 and 100 m water depth. Cores are locally capped by up to a few metres of terrigenous sands or muds that represent highstand systems tract deposits fed onto the shelf from opposing onshore and offshore directions. In the nearshore region, North Island-derived felsic fine sands are beginning to prograde seawards (westwards) across the shellbeds, while in the offshore zone, South Island-derived micaceous very fine sands and muds are encroaching upon, and covering, the shellbeds from the west and south.