Late Quarternary Sea Level and Tectonic History of Marlborough Sounds

Abstract

The Marlborough Sounds form a distinctive region of New Zealand distinguished by its intricate coastline of submergence and "all-slopes" topography. The submergence has been variously attributed to subsidence of the landscape, to the postglacial rise in sea level or to a combination of these processes. Pelorus Sound, in the central part of the area, was chosen for study by the use of high resolution Uniboom seismic reflection profiles, archived 3.5 kHz seismic profiles, and shoreline age-elevation data interpreted from piston cores, to elucidate the late Quaternary tectonics and sea level history. The marine mud in Pelorus Sound is underlain by about 400 metres of alluvial deposits. Seismic profiles and radiocarbon dates from a core in the axial part of the Sound near Maud Island, indicate that before 12.5 kyr B.P. the area was occupied by a river system that had formed an alluvial plain. The upper surface of the alluvium comprises a major seismic unconformity that has been here given the informal name Maud Surface. By c.12 kyr B.P. the area near Maud Island was an estuary as the shoreline progressed southward into the Sound with the rising sea level. The age-elevation data indicate subsidence of the central part of the area at an average rate of 1 m kyr-1. The Sounds region is also tilting towards the centre of the South Wanganui Basin with measured tilt rates increasing from 1.1 ° Myr-1 during the Last Interglacial to 3.9 ° Myr-1 at 12 kyr B.P. with perhaps a 2° rotation in direction towards the north. The subsidence and tilting were initiated by the migration of the depocentre of the South Wanganui Basin and probably generated by frictional coupling between the subducting Pacific Plate and the overriding Australian Plate. Structural analysis shows the hinge line for the tilting located near Canvastown where the first outcropping of basement from underneath surficial sediments is observed in the upstream direction of the Pelorus River. The hinge line probably passes through the topographic high at Okaramio which forms the divide between the north-flowing part of the Kaituna River from the south-flowing part. The hinge probably terminates against the offshore Taranaki Boundary Fault to the nothwest and the Wairau Fault to the southeast. A set of paired, submerged marine terraces on either side of the channel near Maud Island, was probably formed by stillstands or fluctuations in the rising Postglacial sea level after the Last Glacial. Age-elevation data show a correlation of observed Pelorus Sound sea levels with available data for the New Zealand-East Australian region. The marine terrace elevations, when corrected for tectonic subsidence, correlate with proposed stillstand events at eustatic levels of -45 m (10.5 kyr B.P.), -33 m (c.9.5 kyr B.P.), and the -19 to -26m paracyle at 8.5 kyr B.P. The presence of the well-defined terraces and their apparent correlation with well-constrained sea level data support the model of an episodic rise of the Postglacial sea level. The episodic rise model is further supported by the c.12 kyr B.P (Libby age) shoreline which correlates with substantial sediment wedges and stillstands of this age in the New Zealand – East Australian region. The c. 14 ka calendric age of the stillstand event is also the time of the Antarctic Cold Reversal which is suggested as the cause of the stillstand or fluctuation in the general rise of the deglacial eustatic sea level at this time. There is no evidence in the area for a sea level higher than the present at any time in the Quaternary, and the absence of any marine Quaternary sediments in the Linkwater area indicates that it could not have been a seaway as had been suggested in the past.