Sea-level fluctuations cause periodic, post-glacial progradation, South Kaipara Barrier, North Island, New Zealand

Abstract

The off-shore region between Mt Egmont and the Kaipara Harbour entrance forms the off-shore portion of the Egmont—Kaipara Sand System. The late post-glacial input of sand from landward sources into this system is less than 7% of the volume of post-glacial dunes which forms its coastal deposits. The only other source for the dune sand is the sea floor. Hence it is not surprising that the mineralogy of the sea floor and dune sands is the same. Furthermore, five separate periods of progradation are recognised that are correlated with five periods of sea-level fluctuations which have occurred during the post-glacial fall in sea-level from a local maximum of +2.1 m, 4425 years ago. The volume of progradation is approximately proportional to the net fall in sea level during each fluctuation. Departures from this proportionality are due mainly to insufficient time for equilibrium to be established between sea floor and new sea level. Sea level is currently rising and is promoting deposition on the sea floor down to depths of 50 m, beyond which there is a belt of coarser sand down to an average of 100 m. The main movement of sand is between the depth of 50 m and the shore. A wedge of sand from this region with a maximum, near-shore thickness of 2.1 m (the overall post-glacial fall in the sea level) equals slightly more than the total volume of post-glacial dune sand preserved within the Egmont—Kaipara Sand System. It is concluded that the present sea floor is in partial equilibrium with sea level, and the local profile of equilibrium probably extends to depths of between 80 and 120 m.