Landscape history of the Marlborough Sounds, New Zealand

The Marlborough Sounds is a large and spectacular network of rias or drowned valleys. The Sounds were formed by stream incision and dissection of uplifting rocks followed by subsidence and marine incursion into the valleys. The evolution of the Marlborough Sounds is examined using a combination of seismic-reflection lines from the offshore Wanganui Basin, topographic data and seafloor bathymetry. Drainage incision produced valleys and ridges in the Sounds that were part of a mountainous region which reached maximum altitudes of c. 2000 m and extended northwards across the basin to the Wanganui coast. Sedimentary strata as old as c. 5 Ma infill this palaeotopography in the basin, suggesting that topography in the Sounds formed primarily during the Miocene. This topography started to submerge in the Sounds at c. 1.5 Ma when the Wanganui Basin stepped southwards by about 50 km. At the present outer edge of the Sounds, the average rates of subsidence since this time have been low (c. 0.3 mm a-1) and the resulting accommodation space in the valley system filled by sediments. As a consequence of sedimentation, the absolute altitude of valley floors remained approximately constant relative to a fixed earth datum. Thus, while subsidence was important for bringing strongly dissected topography close to sea level, isostatic sea-level rises are inferred not to have been responsible for submergence of the Sounds region. Instead, marine inundation may have been primarily driven by eustatic sea-level changes which resulted in ephemeral submergence during interglacial periods when sea levels were highest. The present coastline in the Marlborough Sounds formed c. 7 ka ago, and similar highstand sea-level configurations probably occurred about every 100 ka throughout the Late Quaternary.