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Victoria University Antarctic Research Expedition Science and Logistics Reports 1968-69: VUWAE 13

Alluvial fans

Alluvial fans

In the Wright Valley attention was focused on one alluvial fan at the north east corner of Lake Vanda. Current processes on the fan surface are similar to those outlined for other meltwater channels. This fan is of considerable interest as it offers several good exposures which are useful in reconstructing the postglacial history of Lake Vanda. The alluvial deposits of the fan are underlain by till - it is not yet known which till. There is some evidence of a higher, steeper, and older fan surface which has been dissected. The page 29 only remnant is east of the apex of the present fan. Three major channels have been cut into the present fan surface.

A series of beach ridges have been cut across the lower third of the fan to a height c. 55 m above present lake level. Wellman and Wilson (pers. comm.) have obtained an algal 14C date from this top surface of 3000 ± 50 years. A further sample from this level has been collected to check this date (sample 5). Algal samples have also been collected from four other levels. Samples 1 and 2 are from delta/fan surfaces respectively 22 and 25 metres above present lake level. These deltas have formed on the central channel of the three. As sample 5 also comes from adjacent to this channel and the fan/delta surfaces occur at many intervals down to present lake level along the channel, it is evident that this channel has been in use since the lake level was highest (i.e. approximately 3000 years). While the westernmost channel was also in use at this time it was not continuously so - deltaic/fan deposits being absent below about 45 m. At 23.5 metres on this channel an algal layer was collected from an old beach ridge. This ridge is cut in till, although there is some possibility that the deposit is actually a mudflow derived from till. If so this sample gives a minimum age for the flow as well as an age for the lake level 23.5 metres. The till shows evidence of cryoturbations/involutions. The sample thus provides a minimum age for these features even though their origin is not yet clear.

Since the algae grew at this site a frost wedge has formed on the beach ridge in the till, some of the algae having fallen into the wedge in V-shaped bands typical of ice wedge features. Sample 3 thus provides a maximum age for the ice wedge. Furthermore, it would appear that involutions and ice wedges cannot form at the same time as they almost certainly require completely different permafrost conditions. Both the frost wedge and the algae have since been covered by young streamflood/mudflow deposits; the algal date also gives a maximum date for this younger deposit. As the frost wedge is apparently now inactive, and has no surface expression, it is presumably a fossil ice wedge (as far as I know the first reported occurrence of a fossil ice wedge in Antarctica).

Sample 4 was collected near sample 3, at 20.5 metres above page 30 present lake level. This layer also marks the site of a former lake level, which has been subsequently covered by slope deposition from the steep beach ridge at 23.5 metres.

The five samples collected have intrinsic value in that they establish a chronology for the history of Lake Vanda. Dr T. Torii (Chiba Institute of Technology) has also collected 5 algal samples from the north side of Lake Vanda. While the exact elevations at which he collected his samples are not known to me, two of them come from 5-15 metres above present lake level, and one from about 45 metres. Thus a fairly complete chronology should be available.

The samples collected should also have other values (such as providing maximum ages for the overlying deposits). Samples 3 and 4 considered together provide some measure of the rate of slope development under certain circumstances. Other samples will give, on further field investigation, fairly reliable estimates of rates of deposition on the alluvial fan surface as well as indications of the periods when each meltwater channel was in use. Such interpolation will be approximate but a rough estimate of rates of erosion/deposition will still be superior to our present understanding of rates of geomorphic processes in this part of the Wright Valley.

As the lake levels which cut across the fan surface also cut across talus slopes to the west of the fan it should be possible to gain further knowledge of rates of processes from this area. General reconnaissance observations along these lines were begun but are not reported here; more detailed work was held up by the lack of precise levelling equipment, bottom samplers, etc. The details required to make full use of this chronology have been included in the outline of fieldwork for the 1969/70 field season.