The hinuera formation and associated quaternary events

Abstract

Deposition of the Hinuera Formation in the south Auckland district began during the Last Glaciation. There were two main periods of sedimentation which culminated during two cool periods–one slightly more than 20,000 years ago and the other at some time between 12,000 and 16,000 years ago The slopes on the surface of the Hinuera Formation are much steeper than the present graded reaches of the Waikato River and steeper than the surface on the Taupo Pumice Alluvium. The steeper Hinuera stages were due partly to increased rates of erosion and, possibly, partly to increased volcanic activity during the cool climates. Periods of increased volcanism could have resulted from reduction of hydrostatic pressure during low sea levels. Log-log graphs of the surface profiles are useful in emphasising changes in slope. An appendix describes observations of an experimentally built alluvial fan, which led to several conclusions,including the following: When the load being transported by a stream is reduced, erosion starts and subsidiary fans are built out of the material eroded from the primary fan. At the same time terraces are cut upstream of the head of the subsidiary fan; these may be matching or non-matching, depending on the presence or absence of variations in load during their formation