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

REPORT ON GEOMORPHOLOGICAL PROJECT

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REPORT ON GEOMORPHOLOGICAL PROJECT

Introduction

Between 29th November-8th January I carried out the following programme within the framework of the VUWAE party:

Although my original intention had been to investigate soli-fluction and/or talus slopes various circumstances inclined me to examine meltwater channels and alluvial fans. Eventually, it would seem that the results obtained will be more worthwhile, some attempt being made to relate geomorphic processes to a variety of postglacial events, particularly in the Vf right Valley. The completed project will include studies of talus slopes; hopefully in the Wright Valley some features of slope development will be tied to an absolute chronology.

At this stage complete answers cannot be expected. A variety of samples have been collected which should aid understanding of a number of slope and fluvial processes, both present and past. Further fieldwork, reading and laboratory analysis will be necessary before even approximate answers can be found. For these reasons the present report is interim and general.

Three groups of interrelated phenomena were examined: meltwater channels, deltas, and alluvial fans.

Meltwater channels

Little attention has been paid to the role of meltwater channels in landsurface sculpture in Victoria Land. As rainfall is negligible, almost all water in channels is meltwater. Only general observations (of both channel walls and channel bed) have been made to date. Little that is new or original has come from this general study other than recognition that processes associated with meltwater affect enormous area of the valley floors, particularly on the southern side of Lake Fryxell (Taylor Valley). No attempt has yet been made to map the extent and character of various meltwater deposits and forms (this mapping constitutes part of next year's field programme).

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The following features have been noted:
1.Fluvial erosion and deposition
2.Boulder Levees (steep hillslope channels and fans)
3.Talus deposition (on steep channels - gravitational sliding and rockfall processes)
4.Micro-talus deposition (channel wall slopes)
5.Sandflows (channel walls)
6.Silt flows and fine sand flows- (from finer-textured beds and with very different length-breadth ratios to sandflows)
7.Boulder pavements (channel floors)
8.Downslope creep (on talus from delta/fan surfaces)
9.Flows at talus base (on delta/fan slopes)
10.Low-angled fan formation (channel floors)
11.Pro-talus ramparts (channel floors)
12.Boulder terraces (channel floors)
13.Micro-boulder terraces (on steeper hillslope channels and sometimes fan surfaces)
14.Eolian deposits and snowdrift (channel walls and floor)
15.Salt weathering (largely below surface).

At the present time the processes and mechanisms leading to the formation of these features are imperfectly understood. A number of samples have been collected which may help to elucidate some problems. Further understanding of present-day processes and morphology should lead to reappraisal of the history of the meltwater channels, particularly in relation to old fan and delta systems formed during higher lake levels around both Lake Fryxell and Lake Vanda.

At present it is impossible to rank the processes in order of magnitude or relative importance. Indications are that processes are slow. Those that occur at readily observable rates such as sandflows, siltflows, low-angled fans, etc. are probably ineffectual in maintaining any gross modification of channel form. Detailed reexamination and re-measurement at regular intervals over lengthy periods will be required before each process can be placed in perspective but such observations must be undertaken if rates of land-surface sculpturing in Victoria Land are to be determined.

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Deltas

A series of deltaic deposits along the southern margin of Lake Fryxell was also examined in some detail.

Several workers have described these features as deltaic deposits formed at the time of higher lake levels. Evidence that these features are in fact deltas, includes: fairly horizontal fine bedding, presence of beds of finely laminated silts, general surface morphology, etc. All these criteria are, however, also consistent with fan formation and in my view it is not yet possible to determine whether these features formed inside or outside the lake margin, although there is little doubt that most are “graded” to old lake levels. Similar comments are applicable to the deltaic deposits formed on the margin of ancestral Lake Vanda.

The slopes connecting one deltaic surface to another (the riser) are of interest. Whilst these deposits are very unstable, the slope deposits only veneer the original deltaic/fan deposition. This suggests that, either the delta/fans are very young or that downslope movement, even on very unstable slopes, is extremely slow. Slopes on deltas above meltwater channels, where the channel has trenched through the deposits, also have only a veneer of slope deposits parallel with the present slope, lying unconformably on truncated horizontal sediments. While these slopes may be actively undercut by meltwater, they are similar to those which have been unaffected (except by downslope movement and wind) since the lake level fell. Furthermore, the importance of snow drifting and wind has not yet been ascertained.

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.

Acknowledgements

I would like to thank Mr R.B. Thompson, Superintendent of Antarctic Division, D.S.I.R., and Professor R.H. Clark of V.U.W. for their respective parts in arranging for me to undertake fieldwork in Antarctica. My thanks also to Burt Murrell for his cheerful companionship, culinary capabilities and critical comment, while in page 31 the field. The other members of V.U.W.A.E. 13, in particular Dr Peter Webb, have been of great assistance. Finally, my thanks to Robin Foubister, Scott Base Leader, and the personnel of Scott Base, Vanda Station and VX-6 for logistical support during fieldwork.