The unsuccessful search for the Shepard Island

During the past year, we have conducted studies of Marie. Byrd Land volcanic geology, focusing on the petrography of samples collected during the 1977-78 ...
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The unsuccessful search for the Shepard Island Quartzite In Marie Byrd Land

1972; Wade 1969; Wade and Wilbanks 1972). Wade (1969) and Wade and Wilbanks (1972) apply the formal name Shepard Island Quartzite to the unit. Although all U.S. geologic maps covering this part of Antarctica show the Shepard Island Quartzite, the error evidently has been corrected in the Soviet Union; Shepard Island is shown as entirely volcanic on the geological map of Antarctica prepared by the Ministry of Geology of the USSR (Ravich and Grikurov 1976). A brief description of field observations made at Shepard Island by two of us (W. F. LeMasurier and C. W. Grindley), and the geologic history arrived at by subsequent petrographic study and K-Ar dating, is presented here. Shepard Island was visited, with helicopter support, on 24 November 1977 in fine weather. One of the major objectives of the scientific party was to study the Shepard Island Quartzite. It is shown on the geologic maps as PreCretaceous metasedimentary rock, perhaps equivalent (at least lithologically) to the Swanson Group in the southern Ford Ranges. The implication of some kind of geologic continuity between these widely separated regions was a cause for considerable interest in this unit, as was the possibility that it might be a distinctive enough rock type to have some value in interpreting Pre-Cretaceous geologic relationships between Marie Byrd Land and New Zealand. The possible significance of the unit is suggested by the fact that it is represented on maps of 1:5,000,000 and 1:10,000,000 scales, even though the total area of outcrop at Mount Petinos and Worley Point together, is about one- half a square kilometer. Areas of outcrop on Shepard Island are shown on the Grant Island topographic map (Antarctica 1:250,000 Reconnaissance Series, U.S. Geological Survey) at Mount Petinos, Worley Point, Mathewson Point, Moran Bluff, and Mount Colburn (figure 1). All of these outcrop areas were visited

WESLEY E. LEMASURIER

Department of Natural and Physical Sciences, Geology Division University of Colorado-Denver Denver, Colorado 80202

GEORGE W. GRINDLEY New Zealand Geological Survey Lower Hutt, New Zealand

DAVID C. REX Department of Earth Sciences University of Leeds Leeds L52 91T, United Kingdom

During the past year, we have conducted studies of Marie Byrd Land volcanic geology, focusing on the petrography of samples collected during the 1977-78 field season and selection of samples for potassium-argon (K-Ar) dating and chemical analysis. These studies have provided an opportunity to confirm earlier conclusions, arrived at in the field, that the geology of Shepard Island is entirely volcanic. A reconnaissance report describing the occurrence of micaceous quartzite at Mount Petinos (Klimov 1967) appears to be in error, and geologic maps derived from that report should be amended in future editions (e.g., Craddock 1970,

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Figure 1. Geological sketch map of Shepard Island.

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except the last two, which could be examined fairly closely from the air but were inaccessible to ground parties. No other outcrops were discovered. Worley Point lies at the westernmost edge of Shepard Island, adjacent to Mount Petinos, and was not named on the maps available to the geologic party who last visited this area in 1966-67. Three geologic units could be recognized at Shepard Island: (1) a platy trachyte (apparently the oldest of the three units), (2) hyaloclastite tuffs and breccias, and (3) posterosional basalt flows, bombs, and scoria blocks. The platy trachyte crops out at Worley Point, and perhaps at Mount Colburn also, where dark green, crudely columnar-jointed rock makes up an east-facing cliff exposure. The trachyte at Worley Point is a dark greenish-gray rock, characterized by steeply dipping platy structure, that crops out over a large flat area near sea level (figure 2). In some outcrops the rock cleaves readily to grooved plates roughly one half a centimeter thick and has the appearance of a phyllite. This kind of structure is fairly common in lava domes (Compton 1962) and can be seen at Observation Hill, McMurdo Station. Thin-section studies of four samples collected at Worley Point confirm that the rock is volcanic. It has a trachytic- (or pilotaxitic-) textured groundmass dominated by alkali feldspar microlites and contains phenocrysts of alkali feldspar, aegerine-augite, and fayalitic olivine. Subhorizontally stratified hyaloclastites occur at Mathewson Point, Moran Bluff, and Mount Petinos, in sections up to 300 meters thick. At Mount Petinos they appear to lie stratigraphically above the trachytes at Worley Point, and they extend from the base to the summit of the mountain 500 meters above sea level. The hyaloclastites are all basaltic in composition and appear to represent eruptions beneath the ice sheet at a time when the ice level was a few hundred meters higher than it is today. Preliminary K-Ar age determinations have yielded dates of about 1.5 million years at Mathewson Point and 0.6 million years at Mount Petinos. At Mathewson Point, a thin basalt flow and a few very small cinder cones overlie the hyaloclastite. They appear to postdate the erosional dissection of the hyaloclastite and one sample has yielded a preliminary K-Ar date of approximately 0.6 million years. These studies show that Shepard Island is a small volcanic center that seems to represent Plio-Pleistocene volcanism during a period of fluctuating ice levels. We are reasonably confident that there are no exposures of prevolcanic basement rock at, or in the vicinity of, Shepard Island. We visited all accessible localities in good weather on 24 November, and again on 5 December, to observe whether summer melting had exposed any new areas of outcrop. We therefore recommend that the Shepard Island Quartzite be removed from future maps and reports.

1980 REVIEW

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Figure 2. Collecting samples at Worley Point, presumed locality of the Shepard Island Quartzite. Photo by W. E. LeMasurier.

This research was supported by National Science Foundation grants DPP 76-04396 and DPP 77-27546 to the University of Colorado. C. W. Grindley participated in 1977-78 field activities in Marie Byrd Land as a visiting scientist. D. C. Rex performed the age determinations at the University of Leeds. References Compton, R. R. 1962. Manual of field geology. New York: Wiley and Sons. Craddock, C. 1970. Geologic map of Antarctica, 1:10,000,000. In V. C. Bushnell and C. Craddock (Eds.), Geologic maps of Antarctica, Antarctic map folio series (Folio 12, Plate 20). New York: American Geographical Society. Craddock, C. 1972. Geologic map of Antarctica, 1:5,000,000. New York: American Geographical Society. Klimov, L. V. 1967. Hekotorye rezul'taty geologicheskikh issledovaniye na Zemle Meri Berd v 1966-1967 gg. [Some results of geological observations in Marie Byrd Land, 1966-1967.] Information Bulletin of Soviet Antarctic Expeditions, 66, 18-25. Ravich, M. G., and Grikurov, G. E. 1976. Geological Map of Antarctica, 1:5,000,000. Leningrad: Ministry of Geology of the USSR, Research Institute of the Geology of the Arctic. Wade, F. A. 1969. Geology of Marie Byrd Land. In V. C. Bushnell and C. Craddock (Eds.), Geologic maps of America, Antarctic map folio series (Folio 12, Plate 17). New York: American Geographical Society. Wade, F. A., and Wilbanks, J . R. 1972. Geology of Marie Byrd and Ellsworth Lands. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget.

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