DECIPHERING THE INFLUENCE OF SEDIMENT SUPPLY AND ACCOMMODATION SPACE ON BARRIER EVOLUTION, NEW ZEALAND

Sea-level, sediment supply and accommodation space are recognized as primary variables controlling barrier evolution. Therefore, to understand how barriers will evolve in a global regime of accelerated sea-level rise it is imperative to decipher the role of sediment supply and accommodation space. Singling out the influence of any one variable is difficult due to their complex interactions during barrier development. The northern coast of New Zealand offers an opportunity to achieve this, however, due to local variability in sediment supply regimes and antecedent geology. For this work, we used ground-penetrating radar (GPR) to produce detailed 3-D models of stratigraphy for five prograding barriers in New Zealand.

New Zealand has 11,000 km of coast, of which 15% of it is fronted by barriers. Moreover, within close proximity are coasts that differ in terms of geologic and oceanographic setting. This stark contrast is particularly pronounced in Northland where the high-energy west coast is linear and dominated by large Pleistocene-Holocene composite barriers, and on the lower energy east coast barriers are confined to coastal compartments along a predominantly rocky indented coastline. Despite the variation around the Northland coastline, the same sea-level history exists offering a constant. The distinguishing variable between the two coasts is sediment supply, as determined by the switching of the Waikato River between these coasts in late Pleistocene time. Since ca. 18 ka the Waikato has discharged to the west coast, resulting in a sediment-rich littoral system; in contrast the east coast has been sediment starved, thus creating an inter-coastal variable. For intra-coastal variability we use accommodation space as a determinant on barrier evolution, which on this coast is determined by varied preservation of Pleistocene depositional sequences and varied bedrock topography. Ultimately, by comparing barrier stratigraphy along and across shore, a comprehensive study of evolution from a spectrum of sediment-rich with poor accommodation space to sediment starved with good accommodation space has been completed. The resulting 3-D GPR records all show detailed stratigraphy of regressive facies succession with differences in beachface geometry and dune morphology.