Sedimentation in Hoopers and Papanui Inlets (Marine Science).

Rayns, WD

Hoopers and Papanui Inlets have resulted from the infilling of two Pleistocene river valleys drowned during the Holocene marine transgression on the Otago Peninsula (South Island, New Zealand). A southward pointing barrier spit protects each inlet from the sea. Both lack fluvial input and each is connected to the sea via a single, narrow tidal channel. They exist on a microtidal ( < 2m) coast. At present the two inlets (each approximately 3.5 km2 ) have an average water depth of less than 1.5 m.
The distribution of surface sediments is tidally dominated in Papanui Inlet, and tide and wind dominated in Hoopers Inlet. The major sediment type in both inlets is a well to very well sorted, fine to very fine sand with zero to fine skew. Mineralogical studies indicate that the major sediment sources for the quartz and feldspar rich sands are the Clutha River catchment and the Fiordland-Foveaux Province, both of which lie to the south of the study area. The Dunedin Volcanic Complex forms a minor secondary source.
Aerial photography was used to determine patterns of erosion and deposition in the inlets and along the barrier beaches. Papanui Inlet was found to have been primarily a site of erosion over the past 40 years and to exhibit a highly dynamic sedimentary surface. Hoopers Inlet was determined to be a site of deposition over a similar period. The barrier beaches have both exhibited similar sedimentary patterns during the last forty years with deposition occurring at either end and little movement or slight erosion about the centre of each.
Longshore currents proved to be predominantly toward the ends of each barrier beach in both northeasterly and southwesterly wind-wave regimes with a maximum recorded velocity of 44m/min-1. Current directions indicated sediment transport away from present areas of erosion and toward areas of deposition.
Thirty-seven cores (ranging from 0.30m to 1.75m in length) provide information on late Holocene ( <6000 years BP) stratigraphy. Two types of coring device were used; for cores of <lm a length of PVC pipe was used, for 2 and 4m cores a portable coring unit with a telescopic tube and stationary piston mounted on a tripod frame was built. The sedimentary sequence is uniform consisting of sand similar in texture and mineralogy to the present day environment. Coring showed that ripples and other minor internal structures are rarely preserved; this is thought due to bioturbation. In many cores a single major shellbed was recognised (dominated by Chione stutchburyi [Cl Bivalvia] and Amphibola crenata [Cl Gastropoda]) with an associated microfossil-mica rich zone. 14C dating of C. stutchburyi shells supported the view that the shellbed consisted of a multiage population due to recurring cycles of erosion and deposition over the past 6000 years. The average sedimentation rate of Papanui Inlet ( 0. 47mmyr -1 ) was 1.25 times that of Hoopers Inlet (0.37mmyr -1 ).
Mica and microfossils are thought to be shelf derived due to incipient microfossil phosphatisation and the presence of mica rich sands at shallow (20 to 40m) depths off the Peninsula.
Several localised raised beach sequences in Southern Papanui Inlet supplied information on late Pleistocene deposition, The raised beaches are proposed to be of last interglacial (approximately 125,000 BP) and are 3 to 8 metres above the present-day inlet surface. Sediment texture and mineralogy were similar to that of today.
Beneath one raised beach site a peat outcrop was observed, and is thought to be an interglacial, cool-temperature Silver Beech (Nothoiv fagus menzisii) forest which developed behind a barrier during a sealevel stillstand, extending out onto the inlet sandflats. Continued sealevel rise buried and preserved the forest building an interglacial raised beach above it.

xvi., 272 p., ill, 30 cm.

1985Raynes