Report on the Victoria University of Wellington Antarctic Expedition 1960-61: VUWAE 4
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.
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 page 7 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.
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.
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.page 8
High angle faults cut the basement rocks in many places; their displacement could generally not be determined except where they offset the lamprophyres.
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).|