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Report on the Victoria University of Wellington Antarctic Expedition 1960-61: VUWAE 4
SECTION C REPORTS
SECTION C REPORTS
GEOLOGY REPORT
General Statement
The primary mission of the Victoria University of Wellington Antarctic Expedition, 1960-61, was to investigate the geology of the ice-free terrain bordering the western edge of the Koettlitz Glacier. Early reconnaissance expeditions had indicated the presence of three roughly parallel bands of rock in this area: a strip of deformed and metamorphosed sedimentary rocks closest to the coast, a belt of granites farther inland and finally a belt of younger sediments and dol rites constituting the Royal Society Range. The first two comprise the so-called “basement complex;” it is with these rocks that the present expedition was chiefly concerned.
Using the base map compiled by Trans-Antarctic Expedition survey parties at a scale of four miles to the inch, and aerial photographs taken by the U.S. Navy, geologists of the expedition mapped about 500 square miles of largely ice-free territory during the field season. The main accomplishments were, in addition to completion of the mapping programme, establishment of a metamorphic stratigraphy and analysis of the relation of the granitic rocks to the metasediments. Rocks of the Beacon series and Ferrar dolerites in the Royal Society Range were reached at two localities; field studies were also made of Quaternary volcanics present in the southern half of the area. Observations on glacial geomorphology and surficial deposits were made throughout, particularly by the senior members of the expedition working in coastal areas.
Basement Complex
The basement rocks of the Koettlitz area consist of schists, quartzites, marbles, and granitic rocks of uncertain age, which are cut by numerous acid and lamprophyric dikes and sills. The granites lie for the most part in a loosely-defined zone extending north-south through the central part of the area between the coast and the Royal Society Range; their overall trend seems to parallel that of the main range, separating the metasedimentary rocks into two main bodies. Tentative field correlations were made between metasediments on either side of the granites, and on this basis three metamorphic formations are distinguished.
| (a) | Hobbs Formation - The type locality for the Hobbs Formation is a ridge in the northernmost part of the area studied, between the Hobbs and Blue Glaciers. The unit consists of about 12,000 feet of metamorphosed conglomeratic greywackes, quartzites, and argillites. It becomes calcareous in its basal portion, passing through a transition zone of interbedded marbles and schists into the underlying Salmon marble. |
| (b) | Salmon Marble - Several thousand feet of coarse white and grey, massive to thin-bedded marble occupy most of the terrain between the Salmon and Hobbs Glaciers, and are designated the Salmon Marble Formation after the feature known as Salmon Hill in the type locality. Bands of lime-silicate material interbedded with the marble contain garnet, diopside, wollastonite, and a variety of related minerals which indicate a high metamorphic rank. The discovery of possible fossil remains is suggestive of a lower Paleozoic age for these rocks, rather than Pre-Cambrian as had been generally believed. |
| (c) | Marshall Formation - This term designates a varied assemblage of micaceous, amphibolitic, and calcareous schists, with interbedded marble, paragneiss, and schistose quartzite, outcropping in the vicinity of the Marshall Valley. On the basis of present knowledge the formation is inferred to underlie the Salmon Marble. Several members were distinguished in the mapping but were not traceable over large distances. The schists
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attain very coarse textures in the southwestern portion of the area, where they are interbedded with granites. |
| (d) | Granitic rocks - Granitic rocks are the principal basement type in terms of outcrop area. They range from coarsely porphyritic, often gneissose, to uniformly fine-grained varieties, and in composition from aplogranites through adamellite to diorite. The different types are in many cases interleaved. Where finer-textured are in contact with coarser, it can be demonstrated that the first-named are the younger. Samples were collected for absolute age determinations by the potassium-argon method. In the neighbourhood of Hidden Valley the granites fade into metasediments along strike and appear to have been formed by metasomatic replacement processes. Elsewhere there are criteria diagnostic of intrusive emplacement; the granites spectacularly interbedded with schists on Mount Dromedary seem to have been emplaced by both processes. Inasmuch as the origin of granites is a subject of widespread controversy, the results of the current study should be of considerable interest. |
| (e) | Dikes - Both acid and basic, or lamprophyric, dike rocks are abundant in the basement complex. The acid varieties consist mainly of quartz and feldspar, and are often coarsely pegmatitic - a single microcline crystal was found to measure 29 inches in diameter. The lamprophyres are of two principal types, a schistose biotite-rich variety and hornblende-feldspar porphyry; some contain a high admixture of more acidic material. These rocks also occur as sills, but in general they cut the country rock at high angles following predominantly northeast trends. The acid dikes were injected prior to, during, and after the period of deformation and granite emplacement, while the lamprophyres exclusively postdate such activity. |
Beacon Series - Ferrar Dolerites
Rocks of Upper Paleozoic age, correlative with units named by expeditions working north of the Koettlitz area, form the high scarp at the western margin of the area studied. Owing to their elevation and remoteness from the coast they were not a principal target of the current study. However, the contact of these rocks with metamorphics of the basement complex was reached at two localities, both at elevations above 8,000 feet. There were no visible contact effects of the lowermost dolerite sill where it rests on basement quartzite, although specimens collected for petrographic analysis may reveal their existence. The basal Beacon is a medium-grained arkosic sandstone. Some evidence was found for the presence of a major unconformity a few hundred feet above the base of this series.
Quaternary Volcanics
Numerous cinder cones of olivine basalt penetrate bedrock and moraines throughout the Koettlitz area from Marshall Valley southward, and associated lava flows cover large tracts in the vicinity of Mount Dromedary. The basalt contains inclusions of morainal material as well as olivine nodules of unusual dimensions. Volcanic activity apparently commenced subsequent to the ice maximum but recurred sporadically during and between later glaciations. Moraine was found interbedded with lava in the gorge of the Walcott metlwater stream, where an excellent stratigraphic section is exposed. It may be that the characteristic presence of hydrogen sulfide gas in the basement marbles is related to the episode of Quaternary volcanism.
Structure
The metamorphic rocks of the Koettlitz area have been folded into a number of broad anticlines and synclines of several mile amplitude. Northeast of the central granitic mass the bedding attitudes and fold axes strike predominantly northwest-southeast, normal to the coastline and into the granites, this trend continuing as foliation in the granitic bodies. The trends become more nearly east-west in the central and southwestern portions, but their interpretation is progressively more tenuous. The beds are steeper and in some cases overturned. Minor plications are characteristic structures in the Salmon Marble, especially near its contact, with schist and paragneiss.
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High angle faults cut the basement rocks in many places; their displacement could generally not be determined except where they offset the lamprophyres.
Economic Geology
No materials of economic importance were found in the area. The only metallic mineral encountered was pyrite, which occurs in negligible quantities in the metasediments.
Work in Progress
| 1. | Preparation of geologic map. By the use of oblique aerial photographs, a new semi-controlled base map has been constructed for the entire Koettlitz dry valley area. Geologic data has been transferred to this map. In view of its improved accuracy the new map should be of considerable value not only for the current work but also for future expeditions. |
| 2. | Petrography - Over 100 thin sections are in course of preparation and examination, for the purpose of studying mineralogy, petrogenesis, and metamorphic facies of rock specimens from the Koettlitz area. A number of thick rock slices will be cut, etched and stained in order to determine their modal content. |
| 3. | Formal presentation of results - A paper concerning the general geology of the Koettlitz area is planned for publication in the New Zealand Journal of Geology and Geophysics. In addition, separate papers will deal with specific aspects of metamorphism (Dr. Blank), granitic and dike rocks (Mr. Cooper), and Quaternary volcanics (Mr. Willis). |
GLACIOLOGY REPORT
Much of the work of the two earlier Expeditions, to the ice-free Wright and Victoria Valleys was concerned with observations of the glacio-geomorphology. From the re-location of remnant glacially-cut benches and the distribution of moraines of different ages in the two valleys, history of the retreat of the ice has been inferred.
The glaciological work of the present Expedition has been mainly a continuation and extension of these observations. The glaciers which formerly filled the Wright and Victoria Valleys were of a different type from these which previously filled the tributary valleys in the area north of the Koettlitz Glacier; in the Wright and Victoria Valleys the glaciers were carrying ice from the west of the Victoria Land ranges, through the valleys to McMurdo Sound, while in the Koettlitz area most of the small glaciers were supplied completely from local névé fields. The Koettlitz Glacier itself is fed from a larger nevé which however is comparatively small compared to, say, the Skelton Glacier which cuts the Koettlitz off from the Antarctic ice plateau.
| 1. | At some early stage, perhaps of maximum Antarctic glaciation, all the area, with the exception of the divide of the Royal Society Range and a few nunataks to the west, was ice covered. Accordance of peak levels on the spurs along the front of the southern Royal Society Range suggests older glacially cut surfaces of a later period. There is reason to suspect that the peaks of the northern Koettlitz (Salmon District) area likewise show such an accordance of peak level. The spurs of the dividing ridges of the Koettlitz alpine glaciers also show vestigal evidence of glacial benching. These accordant height areas have since been severely fretted by alpine (valley) glaciation. In the area surveyed peaks have been ice sculpture to “horns” and the
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alpine valleys cut to almost present level: those in the south being above present sea level, those in the north below. |
| 2. | Later or even contemporaneous with this later stage of (1) such moraines as those of the Hidden-Miers ridge (red granite and Vanda porphyry, approximately 1,800 feet altitude) and definitely later moraines as on the Hobbs-Blue Glacier bench were deposited by a large and vigorous glacier. Vulcanism was occurring at this time as a large proportion of the Hobbs-Blue Glacier bench is volcanic material. |
| 3. | The valley (or alpine) glaciers retreated from their former union with the Koettlitz Glacier (possibly the Koettlitz ablated too, but there is no field evidence for us to be sure that tributary and trunk glaciers were synchronous in maxima). The prominent 1,200 feet (approximately) bench of the Koettlitz left bank may have been cut at this stage. |
| 4. | The Koettlitz Glacier re-expended whilst the valley (alpine) glaciers were at or near minimum extent and deposited moraine one-third to half way up their full valley length. Vulcanism was again synchronous as the moraine of this phase is markedly dark with basaltic materiel probably derived from the Mount Dromedary sector of the Koettlitz area. |
| 5. | Since this phase the Koettlitz Glacier has had two periods of notable deposition - a “brown phase” still in place on the glacier wall from Walcott Bay to Marshall Valley and almost equal in altitude with the phase described in (4) and a “black phase” of moraine almost entirely volcanic (Kenyite and basalt) clearly traceable from opposite Heald Island to the Blue Glacier. This is the most recent major morainal deposition of the area and is still ice-cored. To the north where the alpine valley floors are at or below sea level this moraine fills the seaward end of the valley and carries a rugged minor relief of morainal ridges and circular kettle lakes. Cape Chocolate is composed of this moraine. |
| 6. | The above are the main glaciological events of the area but in comparatively recent times the Koettlitz Glacier has again increased slightly leaving complicated evidence in Pyramid Trough where miner vulcanism and moraines are intermixed and further complicated by a “backwash” of ice from the Koettlitz main stream re-entering the Trough from Walcott Bay after passing between the Bulwark and Heald Island. A small increase in the height of the Koettlitz Glacier surface would cause re-occupation of the Trough both from the upstream (southern) and the northern (Walcott Bay) end. |
Minor surges of the Koettlitz have built up small fresh moraines on the Walcott Bay coast and on the upstream side of the barrier of Heald Island. These may be contemporaneous with the resurgence of the valley glaciers of the Koettlitz area (observed also in the Taylor Valley and even as far south as the Beardmore Glacier).
Today the glaciers of the Koettlitz area are almost stationary - that is, there has been no notable change in glacier snouts since exploration in Southwest McMurdo Sound began.
Five specimens of dried algae from dried up lakes in the moraines were collected. These will be dated by the C14 radio-active method and will give minimum ages for these moraines.
Observations of surface movement and ablation were made on the Radian Glacier. During the months of December and January the surface melted about 10 cm. This probably exceeds the local annual precipitation, and the glacier can be maintained in equilibrium only by forward movement. However (subject to recalculation) the glacier in this period did not move forward more than a few cms., and since the surface slope is small, this probably means that this glacier is out of equilibrium and is retreating.
PALEOMAGNETIC REPORT
This season a total of 72 samples were collected; 39 from Quaternary Volcanics, 6 from Beacon Sandstone and 27 from basement granite.
The granite and Beacon sandstone samples complete the collections of previous Expeditions from the Wright and Victoria Dry Valleys. The granite samples are of particular interest as, unlike those collected previously, they are not close to the dolerite intrusions and hence were not reheated by them. This reheating is thought to be the cause of the uniformity in pole direction shown by the dolerite and granite, two formations of different ages which should have different pole positions and it is hoped that this year's collection will show this to be so.
The volcanic specimens where possible were collected from a series of lavas in order that the secular variation may be measured. As the volcanics predate at least one of the glaciations of this region the pole position obtained from them may be able to be used to give an approximate age to the volcanics and the glaciation that they predate.
CARTOGRAPHY REPORT
As no triangulation surveying was done this season, due to a planned coverage of the area with vertical aerial photographs which did not eventuate, the only materials available for plotting a base map were United States Navy aerial oblique photographs. Several standard methods of plotting from oblique photographs were tried but could not be used because of the lack of ground control. Eventually a plotting method using a peak of approximate known height and distance from the camera (to determine the horizontal plane in the photograph and which was used as a reference plane) was evolved.
This method enabled an accurate base map to be plotted. This process took most of the first term with Willis and Cooper working on the basic plotting full-time. Accuracy was necessary to show on the map the complexities of the metamorphic rocks which comprise the Koettlitz area. Several runs of vertical photographs have turned up since the map was plotted and a comparison of these and the map led to a few minor alterations in ridge lines, shapes of glaciers, etc. but has proved the overall accuracy to be comparable with that of the normal practice of plotting from verticals (a slower, more elaborate method).
Thanks are due to Dr. Wellman, Geology Department, Victoria University of Wellington, for his guidance in working out the method of plotting used, and to Cartographic Division, Lands and Survey Department, for lending aerial photographs.
GRAVIMETRIC SURVEY REPORT
Bouguer Anomaly Map
In Figure 1 the values of Bouguer anomaly at a representative number of gravity stations are plotted, together with the isogals obtained by considering all of the station values. The Bouguer anomaly decreases from a value of 0 mgals in the northeast to −90 mgals at the extreme western point of the survey. A full consideration of the results has not yet been made but the following points may be significant.
| 1. | In the northeast part of the area the isogals are approximately north-south, as they are in the Wright Valley further north. This probably indicates that if the coastline is fault controlled, that the fault continues southwards from Cape Chocolate and does not swing southwesterly to follow the northern side of the Koettlitz Glacier. |
| 2. | The gravity gradient in the north-east part of the area is smaller than the 10 mgals/mile which occurs at the coastal end of the Wright Valley, and which has been interpreted in accord with the existence of a major fault along the coastline.page 11 |
| 3. | A gravity graidnet of about 10 mgals/mile does occur a few miles to the west, between the mouth of the Marshall Valley and the head of Miers Valley. This large gravity gradient is not associated with any major fault. |
| 4. | Heald Island appears to be gravitationally associated with the area on the mainland immediately west of it. It is unlikely that the northern side of the Koettlitz Valley is fault-controlled. |
| 5. | Values of Bouguer anomaly are significantly higher near the Pyramid, which was one of the main centres of recent volcanic activity in the area. |
| 6. | The overall east-west gradient is approximately 2 mgals/mile. This is a value similar to that found in traverses from the lowlands of Austria or Italy into the regionally isostatically compensated Alps. |
| 7. | In the western end of the Wright Valley the gravity gradient changes sign, and higher values of Bouguer anomaly are found to the west. This probably indicates that the western side of the Victoria Land mountain chain is also fault controlled and that the formation is a horst. No similar change in the gravity gradient is found in the present survey; values of the Bouguer anomaly continue to decrease westwards. It is regretted that time was not available to continue the survey into the Royal Society Range. |
Glacier Thicknesses
The route taken in this gravity traverse included the crossing of three glaciers. From the measurements of gravity on the glacier approximate estimates can be made of the thickness of ice under the gravity staton. The method has been fully described elsewhere (C. Bull and J. R. Hardy, “Gravity Measurements on a Glacier,” Journal of Glaciology, March 1957). Values of the Bouguer anomaly obtained on rock at the sides of the glacier are interpolated across rhe glacier, to determine the value of the Bouguer anomaly which would occur at the gravity stations on the glacier, if the ice of the glacier (density about 0.9 gm.cm−3) were replaced by “standard” rock of density 2.67 gm.cm−3. In fact the values of gravity on the glacier ore less than those required to give the values of Bouguer anomaly obtained by interpolation in this way; the deficit is attributed directly to the replacement of “standard” rock by the much less dense ice.
In the three glaciers measured by this method the Bouguer anomaly expected at the glacier stations (assuming that all of the glacier ice were replaced by rock) can be interpolated with some confidence to an accuracy of 2 milligals. The maximum gravity deficiencies on the three Glaciers are 25 mgals (Koettlitz Glacier), 12 mgals (Radian Glacier) and 16 mgals (Hobbs Glacier) so that the error in the estimate of thickness should not exceed 20 per cent.
The three profiles are given in Figure 2. At the position of the profile the Hobbs is a noticeably underfit glacier flowing from a deep valley into a lowland region. In the profile of the Radian Glacier there is some suggestion that the valley is a pair of parallel troughs. Since the glacier is fed by two approximately equal size tributaries this is not completely unexpected, but it is unusual for the two tributaries to maintain their identity so far down the glacier. A similar feature has been observed on a Norwegian glacier by the writer, but only within a short distance of the confluence of the two tributaries (C. Bull and J. R. Hardy, loc. cit.). The southern arm of the Koettlitz Glacier, between Heald Island and the Pyramid area, is a slow-moving or dead relic of a much larger glacier fed from the inland ice of eastern Antarctica. However, at the position of the traverse its thickness exceeds 1,100 feet and the sub-glacial surface is below sea level. The glacier volley is appreciably deeper on the northern side, the outer side of the curve of the arm of the glacier as it swings northward.
These profiles of the glaciers and their significance will be discussed further in the Report on the Glaciological work of the Expedition.
Note: The calculations of the values of gravity and all the attendant corrections have not yet been checked. These preliminary results and considerations may not remain unaltered in the light of recalculations and more mature judgement.
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