GEORGE A. DOUMANI Institute of Polar Studies, Ohio State University, Columbus, Ohio ERNEST G. EHLERS Dept. Mineralogy, Ohio State University, Columbus, Ohio
Petrography of Rocks from Mountains
in Marie Byrd Land, West Antarctica
Abstract: Geological observations made in Mane Range, the Crary Mountains, and Toney Mountain Byrd Land, West Antarctica, during 1959-1960, consist mainly of andesite, basalt, and trachyte show that the region is dominated by extinct flows. Mount Petras is composed of felsic tuffs and volcanoes in the form of isolated nunataks project- flows. The Clark Mountains consist of hornfels and ing through the ice cap. The Executive Committee felsic intrusive rocks.
CONTENTS Introduction 877 Figure Acknowledgments 879 1. Index map of Marie Byrd Land, West Antarctica 878 Executive Committee Range 879 Crary Mountains 879 Table Toney Mountain 880 1. Summary of major features of some mountains Unnamed escarpment 880 in Marie Byrd Land, West Antarctica . . 879 Mount Petras 881 Clark Mountains 881 Facing General interpretations 881 2. Per cent distribution of minerals in some rocks References cited 882 from Marie Byrd Land, West Antarctica . 882
graphic Office map (1957). Doumani (1960) INTRODUCTION gives a general discussion of the rock types in As the primary purpose of the Marie Byrd this area which is also mentioned in the U. S. Land Traverse of 1959-60 was to make seismo- G. S. Professional Paper 400-A (p. 50-52). logical, glaciological, and geodetic studies, Forty-seven thin sections of samples col- geological and petrographic observations were lected from the various ranges were studied made only when they did not hinder the with the petrographic microscope, using con- progress of the traverse. Consequently, the ventional techniques. Specimens were collected geological investigations were neither detailed from Mount Sidley of the Executive Com- nor extensive. Representative specimens were mittee Range, which was visited on a separate collected from the exposed lithologic units and traverse in February-March of 1959. Where the general geology studied in these outcrops. the grain size permitted, percentages of the (See Table 1.) various minerals present in each thin section The area traversed (Fig. 1) was mostly un- were determined with a Wentworth integrat- explored and had previously been seen only ing stage. Feldspar type and composition were on reconnaissance flights. The visits to the determined in most cases by means of the mountains along the traverse represent the Rittman zone technique on the universal stage. first attempt at any geological studies in this Table 2 shows the per cent distribution of the part of West Antarctica; further observations minerals in most of the thin sections. were made on two reconnaissance flights from The detailed descriptions of the thin-section Byrd Station over the proposed traverse route. study are filed at the Institute of Polar Studies, Neither topographic base maps nor aerial The Ohio State University, and are being photographs of the area were available; Figure prepared as a data report in the Institute's 1 is a modified version of a U. S. Navy Hydro- publication series.
Geological Society of America Bulletin, v. 73, p. 877-882, 1 fig., July 1962 877
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the Executive Committee Range is presently ACKNOWLEDGMENTS in preparation. This expedition was organized by the U. S. Antarctic Research Program under a grant CRARY MOUNTAINS from the National Science Foundation. Naval The Crary Mountains comprise four ice- Air Development Squadron Six (VX-6) pro- covered mountains rising 3670 m above sea vided logistical support. The U. S. Navy level. They extend roughly southeast-north- Hydrographic map used in this report was west between lats. 76.5° and 77° S. and longs. modified on the basis of a field survey by 117° and 119° W. Two mountains in the center William E. Chapman, U. S. Geological Survey, are most prominent. The eastern mountain Division of Special Maps. was seen from a distance; it is isolated from
TABLE 1. SUMMARY OF MAJOR FEATURES OF SOME MOUNTAINS IN MARIE BYRD LAND, WEST ANTARCTICA
Position Geo- Per cent Mountains latitude graphic rock Apparent structure Lithology longitude trend exposed Executive Committee 76°-77° S N-S <5 Dipping gently south- Basalt; lapilli tuff; Range 126° W east; craters agglomerates; moraines 76.5°-77° S Dipping gently south- Basalt (aa-type) flows; mafic Crary Mountains 117°-119° W NW-SE <5 east; probable faulting dikes; agglomerates; coars: on north side pyroclastics 75.5° S Trachyandesites ; Toney Mountain 1H0-U7°W E-W <5 Craters; plugs agglomerates 76° S Probable faulting; Olivine basalt; light-colored Unnamed escarpment 123°-125°W E-W <5 minor folding tuff; tuff-breccia; rhyolite; diorite 76°-76.5° S NW-SE 20-25 High-angle faulting; Aphanitic to coarse felsite, Mount Petras 128°-130°W dipping southeast rhyolite, latite, dacite High-angle faulting; Clark Mountains 77.3° S NW-SE 90-95 gentle dipping south- Aplitic to pegmatitic gran- 142° W east; jointing ite; diorite; metasediments
the main chain and is considerably lower in EXECUTIVE COMMITTEE RANGE elevation (2400 m), although apparently con- This group of mountains consists of four, nected to the main range above ice level. The separate, apparently extinct volcanoes; the western mountain is a continuous ridge which highest, Mount Sidley, attains an elevation of includes several low peaks,with crescent-shaped about 4181 m. The investigation at Mount gorges (perhaps large ice-carved cirques or Sidley showed a succession of pyroclastic rocks remnants of craters) on the northern slopes. as well as trachytes and andesites, character- The ice plateau on the south side of the ized by large phenocrysts of feldspar. The Crary Mountains is approximately 1900 m andesites contain plagioclase phenocrysts, which above sea level and joins the gently inclined closely resemble the so-called "anorthoclase" heavily crevassed ice that covers the moun- of Mount Erebus, Antarctica (Smith, 1954, tains. In the outcrop on the south slope, the p. 48). The phenocrysts are euhedral to sub- chief rock type is a highly vesicular, black, hedral and vary in size up to 8 mm. Compo- olivine basalt with intratelluric inclusions. sitions range from Ani0 to An2e. Most grains Highly contorted bombs up to 1 m in diameter show gradational zoning and both albite and occur over most of the outcrop. Surface pericline twins. moraines contain boulders of trachyte and Overlying the trachytes and andesites is a andesite. The trachyte boulders are character- series of lapilli tuffs and agglomerates, ranging ized by large phenocrysts of anorthoclase up to between 20 and 300 m in thickness. Their 5 cm long, embedded in a grayish-black matrix; significance is beyond the scope of this report, the andesites are light to dark gray, showing and the detailed geology and volcanology of varicolored flow-lamination and banding.
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The northern slopes of the Crary Mountains sighted from the air and indicates volcanic are steeply inclined and are overridden by activity in this region. heavily-crevassed blocky ice-falls, but the per- Most of the mountain is covered with ice; centage of exposed rock is considerably higher rock exposures probably do not exceed two than that of the southern slopes. A steep almost per cent. The western outcrop is a dense, dark- vertical exposure is composed chiefly of strati- gray to brown, alkaline lava (Table 2); it is fied lava flows dipping gently to the southeast. aphanitic, with minute vesicles and iron oxide The lava is light- to reddish-brown, aphanitic, stains in the fractures. Some specimens show clinkery basalt. Clinkers of the Hawaiian breadcrust-bomb structure and baked clinkery aa-type lava are highly scoriaceous and characteristics. Farther east andesite occurs in fragmental and exhibit a caterpillar mode of a well-exposed outcrop; it is highly vesicular, flow. Massive agglomerates, breccia, and coarse dark gray to black, and porphyritic with pyroclastics occur in contact with the basaltic phenocrysts of zoned plagioclase. lava. They are characterized by poorly sorted, dark-red, basalt fragments of all sizes, mixed UNNAMED ESCARPMENT with pellets of rounded and subangular highly "Escarpment" is the term which has been vesicular scoria, indicative of proximity to the applied to this topographic feature since its source of eruptive activity. discovery on the reconnaissance flight of The absence of any visible volcanoes or other October 1959. It comprises an ice-covered land forms in the vicinity of the Crary Moun- ridge and two major mountains, trending east tains indicates that volcanic activity probably between longs. 123M250 W., slightly south originated in the Crary Mountains themselves. of lat. 76° S. The highest peak is about 2515 m, This assumption is also supported by the with a relief of about 800 m on the north side. character of explosion debris, which could not In contrast, the ice to the south is apparently have come from very far away. The steep walls dammed up behind the mountains and spills of the northern slopes, particularly the north- over the "escarpment" as blocky ice-falls. western section, may be remnants of craters or This points to the possibility of a fault with collapse calderas. the up-thrown block on the south side; hence, Two intermediate to mafic dikes, about 1 m the term "escarpment." in thickness, vertically transect the almost- Approximately 20 per cent of each of the horizontal flows of the main outcrop. The two mountains is exposed rock. Light-colored thickest of the two is composed of three layers: tuffs and tuff-breccias are dominant to the east. the central dike, about 50 cm thick, and on They range from greenish-white, massive, ashy either side contact zones, 25 cm thick. A tuffs to dark-green, thinly laminated, hard- larger dike about 3 m thick cuts the icy slope welded, highly brecciated material. Defor- and the underlying flows. The resulting small mation and post-depositional folding are clearly hogback ridge extends up the slope to about displayed where the strata are dipping steeply, 300 m above the ice plateau. The rock is almost vertically, and in some places are over- greenish and glassy, with platy crystals of turned. Like most rocks in cold regions, the plagioclase in certain specimens; it is generally tuff exhibits intense mechanical weathering, andesitic in composition. The contact zone is wind abrasion, and exfoliation caused by frost- contorted and exfoliated, resulting in a slaty wedging. The cracks and planes of exfoliation appearance and cropping out in tombstone are abundantly encrusted with calcite pre- fashion. cipitated from solution. Hard, fine-grained, rhyolitic rocks, distinctly darker than the tuffs, TONEY MOUNTAIN crop out in the form of tombstone pinnacles, Toney Mountain trends roughly east from probably the result of intense mechanical its highest peak of 3570 m, and gradually weathering. merges with the ice plateau. Like those of the The western mountain is largely granodiorite Crary Mountains, the southern slopes of Toney overlaid unconformably by olivine basalt. Mountain are gently inclined, and the northern Intratelluric inclusions of olivine and pyroxene slopes are precipitous. The eastern part of the crystals in the basalt occur in the form of mountain is apparently the crater of an extinct tightly-packed lenticular clusters up to 15 cm volcano. No aerial photographs were obtained in length. Some clusters occur as discrete of this crater, but to the east of it a circular rounded to subangular fragments, the edges outcrop, barely visible through the ice, was and outer surfaces molded and smoothed by the
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hot, explosive and scoriaceous, basalt matrix. and mafic constituents was clearly seen in a Individual crystals of olivine up to 3 cm in boulder which apparently had fallen from a length and 1 cm in diameter were found in the nearby cliff. The granite is highly jointed talus. A yellowish, highly-altered acid flow laterally and vertically. The main factor in- crops out roughly south of the basalt, probably volved in the formation of a spectacular chasm overlying it. in the center of Mount Atwood was faulting, which was then aided by jointing, exfoliation, MOUNT PETRAS glacial abrasion, and wind action. The southern A group of separate peaks is situated between cliff exhibits a form of erosional hollows created longs. 128°-130° W. and lats. 76°-76.5° S. by the abrasive swirling of frost-wedged crys- The traverse route passed through the middle tals. It was observed in the field that feldspar of these peaks, and the geologists visited the plates in particular are mechanically weathered section south of the traverse route. To avoid along cleavage planes. The flakes of feldspar are confusion, only the southern section is plotted then easily lifted by the wind, and their sharp on the map (Fig. 1); the western ridge is dis- angular edges serve as tools for abrasion. cussed below. Metasedimentary rocks overlie the granite. The highest peak in Mount Petras rises 800 The hornfels still retains some characteristics of m above the ice plateau to an elevation of the sedimentary origin of the rocks. In a. few 2870 m. It is part of the eastern ridge, which, places some shale partings are discernible, and together with the western ridge, forms the the original bedding, intercalations, and gran- walls of a semicircular bowl occupied by glacier ular gradation are fairly well preserved. The ice. Almost 25 per cent of this part of the beds are intruded by veinlets of quartz and sills mountain is exposed. Steep cliffs on the western of aplite averaging 5-10 cm in thickness. slopes provide more easily accessible exposures. Mineralization occurs along fissures and con- Talus and morainic debris are abundant on the tact zones. These zones are rich in coatings of slopes. The rock fragments are mostly angular mica, pyrite, malachite stains, radiating clusters to subrounded, as a result of extensive weather- of quartz crystals, and minor quantities of ing, frost-wedging, and original jointing. other secondary products. At Mount Van Lithologically the rocks are chiefly frag- Valkenburg the hornfels exhibits pronounced mental tuffaceous pyroclastics. They range from slickensides and contortion in the saddle be- rhyolite, through quartz latite, to dacite, with tween the twin peaks. The saddle is developed considerable variation in grain size. Aphanitic on a dip-slip, normal fault that trends roughly rhyolite occurs in light-greenish, thinly lami- eastward, and dips steeply to the south. Two nated, folded layers. Sparsely disseminated granodiorite dikes intrude the granite basement pyrite occurs on fracture surfaces. The rhyolite and are well exposed on the northern slope of shows a striking resemblance to the rhyolite Mount Van Valkenburg. The granodiorite is found on the eastern mountain of the un- fine grained with a high mafic content result- named "escarpment." Zones of breccia and ing in a dark-greenish color. A contaminated slickensides are present near the summit of the zone consisting of a dark groundmass with western ridge. large, pink, zoned orthoclase (?) crystals occurs at the contact zones bordering the dikes. The CLARK MOUNTAINS metasedimentary rocks are not disturbed; no A cluster of low peaks comprising the Clark folding of significance was observed in the field. Mountains is situated at long. 142° W. and lat. They have a general northeast strike and dip 77.3° S. The individual mountains are sparsely gently to the southeast. distributed in the form of ice-free nunataks and low ridges protruding through an expanse of GENERAL INTERPRETATIONS solid blue ice. The highest peak attains an ele- The Executive Committee Range, the Crary vation of 1223 m, and the general relief does Mountains, and Toney Mountain contain not exceed 300 m. similar rock types, among which are picrite Pink, perthitic, alkaline granite intrudes a basalt, tholentic basalt, ohvine andesite, and sequence of dark, spotted hornfels, apparently trachyte. Trachyte is the most common mem- of sedimentary origin. At Mount Atwood the ber of this group. Nepheline and analcite, as hornfels overburden is absent. The exposed well as sanidine and anorthoclase are present in granite is generally coarse grained, but varies some of the rocks. Nodules of peridotite occur from aplitic to pegmatitic. Variation in texture in at least one of the basalt flows. Although the
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evidence for formulating generalizations is gist) suggest a form of archipelago, dotted with meager, it is possible that these assemblages in- isolated volcanic islands rising separately from dicate the continental members of the alkaline the floor of an ocean basin. basalt suite. The unnamed "escarpment" mainly consists The alkaline basalt suite is characteristically of rhyolitic tuffs and flows, with relatively located near or is preceded or followed by dif- small amounts of granodiorite and picrite ferent rock types in the same region. This ap- basalt. Mount Petras is composed entirely of pears to be the case in both the Executive Com- acid tuffs, rhyolites, dacites, and quartz-latites. mittee Range and the Crary Mountains, where The Clark Mountains area contains hornfels, large amounts of andesite and minor amounts granodiorite, and granite, with no extrusive of rhyolite are present. The volcanic suite of equivalents. The granite, as indicated by its andesite, rhyolite, and olivine basalt is typical perthitic character, is of the hypersolvus type, of those continental areas which are associated probably indicating a relatively small intrusion with orogeny. This group of rocks is common rather than the deep-seated core material throughout the folded belts surrounding the typical of folded mountain regions. The area is Pacific Ocean. The morphology and physio- probably one of related small intrusives with graphic distribution of the volcanoes of this local metamorphism of pre-existing sedimen- part of West Antarctica as well as seismic pro- tary deposits. files made by F. K. Chang (traverse seismolo-
REFERENCES CITED
Doumani, G. A., 1960, Geological observations in West Antarctica during recent oversnow traverses: Am. Geophys. Union Trans., v. 41, p. 706-710 Smith, W. C., 1954, The volcanic rocks of the Ross Archipelago, British Antarctic ("Terra Nova") Expedition, 1910: Geology, v. 2, no. 1, p. 1-107 U. S. Geological Survey, 1960, Synopsis of geologic results: U. S. Geol. Survey Prof. Paper 400-A, 136 p. U. S. Navy Hydrographic Office, July 1957, Antarctica, map no. V30-SP5, 3d ed.
MANUSCRIPT RECEIVED BY THE SECRETARY OF THE SOCIETY, APRIL 4, 1961
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Miscellaneou s Rock name Plagioclas e Sericit e
pecimen Plagioclas e Orthoclas e Pyroxen e Magnetit e Muscovit e Hydrate d iro n oxid e Hornblend e Analcit e Sanidin e Anorthoclas e Groundmas s Olivin e Iddingsit e Kaolinit e Chlorit e Carbonat e 6> < « Hematit e N £ 2 D 599 85 31-21 Tr 5v 5 5 Tr 5v 5v a Olivine andesite D 5910 80 10 20x 20x 20x 20x 20x 2 Ox ,. u Andesite 5911 10 25-29 5 85v 85y 85x Tr 85v > a Andesite 5913 70 12-14 Tr 1 5 5 h 3 E Andesite 5918 Tr 15 15 i Tr 9 77 % § Trachyte 5920 8 15 1 3 3 Tr Tr 75 WQ Trachyte 5921 10 10 18 10 6 64 5fi Rhyolite 5929 75 11 15v 10 15x ISv Andesite 5930 Tr ~5 8 4 Tr 6 80 H Trachyte 5932 15 3 Tr 5-10 Tr 15 60 Trachyte 5934 60 60 10 5-10 15 8 Tr h 2 a Picrite basalt 5935 45 52 Tr 15 Tr 35 5 Tr °° Basalt 5938 51 7 5 Tr 44 Tr 88 s Andesite 5940 65 7 35x 35x 35x 35x 35v Andesite a-6001 60 5 Tr Tr 1-2 Tr 30 Rhyolitic tuff b-6001 1 1 1 Tr Tr Tr Tr Tr 97 Rhyolitic tuff 6003 -> 5 30 15 Tr Tr Tr Tr Tr Tr Tr Tr Tr 75 Rhyolitic tuff 6004 3 7 7 4x 4x 4x 4x 4x 4v 7P Rhyolitic tuff 6005 3 15 28 20 5x 5x 5x 5x 5x 5v 57 4-1 Rhyolitic tuff 6008 1 4 25 4 Ix 1v Ix Ix Ix 90
DOUMANI AND EHLERS, TABLE 2 Geological Society of America Bulletin, volume 73 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/7/877/3427453/i0016-7606-73-7-877.pdf by guest on 26 September 2021