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VICTORIA UNIVERSITY OF WELLINGTON ANTARCTIC EXPEDITION No. 6
GEOLOGY REPORT
The Brown Hills - Darwin Mountains.
No previous geological expeditions have visited the area. Air reconnaissance of the geology of the area in early 1962 indicated that the rocks were comprised mainly of the basement complex intruded by dolerite sills with Beacon Sandstone exposed in the western part. It was hoped that a study of the basement would enable correlation to be carried out with the stratigraphic sequences established near the Koettlitz Glacier by the 1960-61 VUWAE expedition. However the geology was found to be different from the Koettlitz area and consequently little correlation was attained.
Using aerial photographs taken by the U.S. Navy the geology of the Brown Hills and the Darwin Mountains was mapped for about 400 square miles. The stratigraphic sequence established for the Beacon sediments enables correlates with other nearby areas, especially that of the Beardmore Glacier area (Grindley - in press) where the sequence is essentially the same.
Basement Complex.
Basement rooks occur in the Brown Hills region and consist mainly of granitic rocks of uncertain age, probably pre-cambrian or cambrian, cut by a series of acid and basic (lamprophyric) dykes.
(a) Metasediments.
Metasediments are very rare. The main outcrop east of Diamond Hill consists of about 100 feet of black and white bands of quartz mica schists with interbedded metaquarzite. The surrounding granite passes gradationally into the metasediments and appears to have been formed by metasomatic replacement processes. Two thin discontinuous bands of similar metasediments interleaved in granite occur elsewhere.
(b) Granitic Rocks.
Granitic rocks constitute almost all of the basement. Several distinct granite types were recognized. In some cases they were able to be mapped separately over fairly extensive areas but elsewhere they were interbedded or irregularly emplaced with respect to one another. They range in appearance from coarse grained porphyritic, with strong foliation, to uniformly fine grained varieties, and from leucocratic to melanocratic in composition. It was found that the fine grained leucocratic granite was younger than the coarser grained melanocratic granite. Samples were collected for radioactive age determinations by potassium-argon and Rubidium/strontium methods.
(c) Dikes.
Dike rocks, both acid and basic, are commonly in the basement complex. Age relationships were established in most cases between various types. Acid types are leucocratic and consist mainly of quartz and feldspar, and range from coarse to fine grained. Lamprophyres of two principal types, fine grained schistose and coarser grained non-schistose, both containing abundant biotite, are common.
Tentative age relationships of some of the granites and the dikes are as follows:-
Youngest:
- Medium grained lamprophyre with biotite as an important mineral. This cuts the Ferrer Dolerites and thus is the youngest dike rock found in East Antarctica.
- Fine grained schistose lamprophyre.
- Coarse grained leucocratic pegmatite.
- Fine grained non schistose lamprophyre.
- Fine grained leucocratic pegmatite with common garnet crystals.
- Fine grained leucocratic granite.
Beacon Series:
Over 1800 feet of low dipping Beacon sediment unconformably overlies the basement. At Bastion Hill the contact of basement and Beacon is well exposed. The granite is weathered to a depth of 6-15 feet below the unconformity. At least 60 feet of quartz pebble sandstone is exposed as the basal formation on Bastion Rill.
Ten miles to the south-west at Darwin Mountains, the most basal formation exposed is about 600 feet of well sorted, finely bedded, cross bedded orthoquartzite. Good specimens of Beaconites antarcticus Vialov are common.
The orthoquartzite is overlain by 200 feet of light green siltstone containing scattered weathered pebbles and boulders of granite and metasediments. Some horizons are made up of very finely banded alternating mudstone and siltstone - microscopic analysis of sections of these may reveal grading, indicating them to be varves. A representative collection of pebbles was made for sectioning in order to establish provenance. The formation is similar to Gridnley's tillite in the Beardmore Glacier area.
The tillite is overlain by a 1 inch to 1 foot layer of pebbles and boulders of weathered granite and metasediment similar to those scattered in the tillite below. The layer represents an erosion interval in which concentration of the pebble-boulders from at least 100 feet of the tillite occurred. Immediately above the pebble band a discontinuous carbonaceous band occurs containing poor leaf and stem impressions of Gloesopteris and Gangamopteris. This in turn is overlain by 2000 feet of massive grey sandstone with thin interbedded grit layers.
The highest Beacon examined was 50 feet of light grey finely banded mudstone and siltstone with occasional pebble bands and contraformational breccias.
Samples of Beacon were collected where favourable throughout the section for spore and pollen analysis.
Ferrer Dolerites:
Sills and dikes of the Ferrar Dolerite formation intrude the basement complex and the Beacon Sandstone. Two principal sills occur in Brown Hills - one intruding the basement and another at, or near the basement/Beacon contact. The two sills are in contact but the lower sill is chilled against the upper indicating separate Dolerite intrusions. In the Darwin Mountains dolerite is extensively intruded as irregular sills dikes and lenses (which were too irregular in most cases to serve as marker horizons.) Rafts of Beacon occur irregularly distributed within the dolerite mass both at Bastion Hill and in the Darwin Mountains.
Economic Geology:
No materials of economic importance were found.
The Taylor Valley:
Only very brief reports have been published on this area. J.D. McCraw of the New Zealand Soil Bureau has recently (1962) published a paper dealing with the distribution of scoria cones and patches of the Taylor Valley.
The main object of this phase of the expedition was to map the rocks of the basement complex in order to correlate the geological work completed to the north by previous VUWAW expeditions (McKelvey and Webb, Allen and Gibson), and mapping work completed in the south in the Koettlitz region by VUWAE No. 4. Expectations were achieved and the work in the Taylor Valley essentially completes the geological mapping programme of Victoria Land Dry Valley region. A map of the geology of southern Victoria Land can now be compiled, editing all previous mapping work by Antarctic expeditions.
Using the base map compiled by U.S.G.S. on a scale of 1 cm. to 1 kilometer and aerial photographs taken by the U.S. Navy, an area of 600 square miles was mapped (fig. 3).
Basement Complex:
Rocks belonging to the basement complex constitute most of the Taylor Valley. Like the Koettlitz area they consist of schists, quartzites, marbles and granitic rocks of uncertain age (probably Pre-Cambrian), which are cut by numerous acid and lamprophyric dikes and sills. Three belts of northerly-striking rocks occur, separated from one another by granitic masses.
(a) Metasediments.
The metasedimentary rocks were divided into two formations. The stratigraphic relationship of these are unknown.
| (i) | A series of interbedded white massive and finely bedded granular marbles, quartzo-felspathic schists, paragneiss and massive schistose metaquartzites, occur in two separate belts. An inland belt near the snout of the Taylor Glacier is 500 feet thick. The other belt, several thousand feet thick, strikes across Mt. Nussbaum in mid Taylor Valley. Bands of calc silicate minerals occur interbedded with the marble bands. No fossils were found and order of superposition could not be established. Constant strike, steep dips and repetition of sections indicate that the metasediments have been isoclinally folded. This formation is correlated with the Asgard formation distinguished to the north (McKelvey and Webb, 1962; Allen and Gibson, 1962) and the basal part of the Hobbs distinguished in the south in the Koettlitz region (Blank, Cooper, Willis - in press). |
| (ii) | Several thousand feet of coarse white and grey, massive to thinly-bedded marble occurs from Hjorth Hill north-east to Marble Point. Very subordinate pure massive metaquartzite and quartzo-felspathic schist bands are interbedded. Bands of calcsilicate minerals are commonly associated. The structure is complex, partly due to extensive intrusion or fine-grained leucocratic granite into the marble. |
(b) Granitic rocks.
Granitic rocks include the following types:
| (i) | A type with large orientated phenocrysts of orthoclase set in a finer-grained matrix, giving the granite a conspicuous foliation. |
| (ii) | A type which is medium to coarse grained and homogeneous. |
The foliation in the granites is parallel to the dip of the metasediments. Orbicular granite occurs in situ near the snout of the Taylor Glacier, near a granite-metasediment contact. It forms two discontinuous bands which are parallel to the foliation of the granite and the dip of the metasediments. Orbs average about 5 inches in diameter and each is encrusted with a dark hornblendic layer. The degree of packing of the orbs varies from widely dispersed to closely packed, but not close enough to distort the fairly regular forms.
Dikes:
The basement rocks are cut by a series of dike rocks, ranging from acid to basic in composition. Tentative age relationships are as follows:-
Youngest:
Medium-grained basic dikes with biotite as an important mineral. This is similar to the dike rocks in the Brown Hills in composition and in that it cuts the Ferrar Dolerite formation.
"Railway line porphyry" - of probable composite origin where dark porphyritic bands, 4-5 feet in width border an internal 6 foot leucocratic band. The contact between the two types is sharp with possible baking of the outer portion by a later leucocratic middle.
Fine grained schistose to non schistose lamprophyre.
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Fine grained leucocratic pegmatite.
Aplite.
"Bonney Porphyry" - porphyry with uncommon phenocrysts and a dark fine grained groundmass.
"Vida Granite" - medium grained homogeneous rook with pink orthoclases.
"Varda Porphyry" - grey coarser grained groundmass with mafic blebs and larger phenocrysts.
Oldest:
"Taylor Porphyry" - grey, with small needle or rhomb-shaped phenocrysts, the phenocrysts being more abundant than the groundmass. The relative ages of the latter two are unknown.
Other dike rocks cutting granite in which the age relationships have not been established are:- Finegrained "dioritic" dike with euhedral crystals of biotite and much felspar.
Beacon Series:
Beacon sandstone outcrops on the bigger peaks west of the snout of the Taylor Glacier. The contact of the basement with the Beacon Series was in no place observed as an extensive dolerite sill followed the contact. Over 2,000 feet of Beacon Sandstone occur in the upper Taylor Valley. It consists of finely bedded to massive current bedded ortho quartzite, with occasional pebbles and manganese nodules, and abundant worm casts throughout. Near the base discontinuous red and green bands occur. No carbonaceous remains were observed.
Ferrar Dolerites:
The Ferrar Dolerite formation consists of widespread sills intruding the basement and Beacon groups. Two major sills are present, one at the basement-Beacon unconformity and the other within the Beacon Group itself. The lower dolerite sill dips at about 40 to the west.
Quarternary Volcanic:
Numerous cinder cones of olivine basalt occur within the upper part of the valley. Their distribution has recently been described by J.D. McCraw.
Economic Geology:
No materials of economic importance were found.