Studies of late quaternary sediments Shag Point and South Oamaru coastal areas, South Island, New Zealand.

Situmorang, Mangatas.

Late Quaternary sediments in Shag Point and south Oamaru have been investigated. The Hillgrove Formation and loess deposits are distributed consistently throughout the coastal area.
The lower part of the Quaternary sequence, Hillgrove basal gravel, consists predominantly of greywacke gravels and m1nor sands and silts. Two ecologic groups of macrofauna are recognised, intertidal and subtidal species. Intertidal species are Melagraphia aethiops., Micrelenchus tenebrosus., Cellana genera and the subtidal fauna consists primarily of Ostrea lutaria. This study clearly shows that paleoecology of these Quaternary sediments has close similarities with ecology of the modern fauna. Foraminifera paleoecology is in agreement with macrofauna and indicates shallow water environment. The restricted shallow water benthic foraminiferids, Notorotalia zelandica and Elphidium charlottensis are recorded abundantly. Paleoclimatic interpretations from planktic foraminiferids and Nonionella flemingi suggest a cold climate during deposition perhaps slightly cooler than mean sea temperature in present day. This evidence in conjunction with several radiocarbon 14C dates of shells (>45,000 yr.B.P.) and stratigraphic relationship with overlying sediments (loess deposits), suggest the Hillgrove basal gravel was deposited in Otiran Glaciation under interstadial conditions.
The upper part, Hillgrove loose sand, is massive 1n exposure, always shows a gradational boundary with overlying sediments, and an absence of cross-bedding, suggesting a possibly beach origin. On the other hand sedimentological analyses including granulometric study and SEM investigation on quartz texture show strong evidence for an eolian origin.
Loess deposits were classified into several types according to stratigraphic relations, macrofeatures and sedimentological analysis. The loess BI, moderately hard silt loam, shows conspicuous jointing pattern distinctively high proportion of green-hornblende, and is texturally coarser than other loesses. Heavy mineral study, particularly through ZTR index (Zircon-Tourmaline-Rutile) implies the presence of "intrastratal solution" in heavy minerals. This reflects the increase of maturity in older loess deposits. This might be associated with diagenetic features on the surfaces of quartz grains as quartz crystal growth, solution pits and adhering particles. Clay minerals have been observed in loess however it was not possible to distinguish between loess horizons on the basis of their clay minerals. Allogenic clays are presumed to consitute most of the loess, these are obviously of illite and kaolinite. Minor authigenic kaolinite is also present. Study of grain orientation in loess deposits gains two source paleowind directions, the central Otago and eastern Otago continental shelf. This interpretation accord with heavy mineral's provenance.
Mineralogically the loess deposits are usually homogeneous, with angular monocrystalline quartz predominant. Feldspar consists mainly of plagioclase whilst potassium feldspar was found in minor quantity. Heavy minerals in loess are distinctive from other sediments (e.g. Hillgrove Fm, Recent sediments), phyllosilicate heavy minerals are in high percentages.
In beach studies, grain slze analysis and dispersal pattern of some heavy minerals suggest longshore transport of beach material in a northwards direction. Furthermore it also noticed that the Shag River supplies considerable amounts of beach material to the Shag Point beaches, more obvious in phyllosilicate heavy minerals.

245 p. : illl. (some col.) ; 30 cm.

1981Situmorang