Late Quaternary Geology of the Canterbury Continental Terrace

Abstract

The Late Quaternary stratigraphy and sedimentary processes are interpreted for an area of continental shelf and slope on the eastern side of the South Island, New Zealand, between latitudes 43°00's and 44°50's. Two formations are recognised in the Late Quaternary stratigraphy of the shelf: the Canterbury Bight Formation of mainly Last Glacia1 age and, locally overlying it, the Pegasus Formation of mainly Holocene age. The formations are distinguished by shelf-wide unconformities (visib1e in seismic profiles), by geomorphology, by grain-size modes, and by macrofauna. Ridge-and-swa1e topography occurs on two scales on the shelf. Very large ridges and troughs are interpreted from microbathymetry, stratigraphy, sediments and macrofauna to be the remains of Pleistocene barrier/lagoon complexes. With the aid of radiocarbon dates, four well developed shorelines between 28,000 yr and 15,000 yr old are recognised. The smaller ridges are submarine features, formed by strong currents. Those ridges that are in a zone of constricted and accelerated currents near Banks Peninsula are active, while those well removed from the peninsula constriction are fossil and date from times of lower sea level. Sedimentation on the continental shelf has reached a state of equilibrium with the modern hydraulic regime. Relict sediments of the deglacial transgressive sand/gravel sheet are being reworked in zones of high energy, principally in the region of constricted flow around Banks Peninsula. Modern-input sand (distinguished by its grain-size mode) is restricted by currents mainly to an active belt near shore, but locally it has replaced palimpsest sand on the middle shelf. The modern mud facies, being confined by zones of higher energy, has reached its maximum areal extent; its greatest thickness is in Pegasus Bay. Sea-bed drifter studies, and studies of sediment texture and provenance show that net sediment movement on the shelf and along shore during both Pleistocene and modern times has been northwards. The continental slope is dissected by submarine slide scars in the south and by submarine canyons in the north. Streams of fine sand, transported from the continental shelf to the upper slope by north-flowing currents during Pleistocene lowered sea levels, initiated the erosion of submarine canyons. Interception of littoral-drifted gravel by established canyons reaching Pleistocene strand lines probably accelerated. canyon erosion. The canyons are thought to be now effectively dormant. Deposition of fine sediment from suspension has dominated the development of the southern slope. This slope is consequently free of deeply corrasional features like submarine canyons but is prone to failure by gravity sliding. The youngest slides are less than 18,000 yr old. The history of growth of Pegasus Submarine Canyon is investigated in detail. The course of the canyon across the shelf is not fault controlled. As well as growing landwards, the canyon and its tributaries have, during Pleistocene sea level stillstands, grown southwards along shore towards the supply of littoral drifted gravel and sand. A buried tributary, of Penultimate Glacial age or older, on the canyon's west side, once brought the canyon 7 km closer to the present shore. The relative ages of the south-trending arms of the canyon are inferred from their relationship to known Last Glacial shorelines that are preserved on the shelf, and by their position with respect to a regional subsurface unconformity of Penultimate Glacial age. Canyon erosion was concentrated in the largest arm during the last deglacial rise of sea level, and shallow channels, interpreted as feeders are common around its rim.