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GEORGE A. DOUMANI Institute of Polar Studies, Ohio State University, Columbus, Ohio ERNEST G

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GEORGE A. DOUMANI Institute of Polar Studies, Ohio State University, Columbus, Ohio ERNEST G 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 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 878 DOUMANI AND EHLERS—PETROGRAPHY OF ROCKS OF WEST ANTARCTICA o 0, 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 ACKNOWLEDGMENTS 879 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. 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 880 DOUMANI AND EHLERS—PETROGRAPHY OF ROCKS OF WEST ANTARCTICA 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.
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