and the remainder in support. Logistics was provided by two vse menshe (Fewer and fewer blank spots, 23 February); each of 11-14 and AN-2 airplanes and of the Ml-8 helicopter. Rasstupites, Aysbergi! (Icebergs, Step aside!, 3 March). Pravda, Vesti iz ekspeditsiy (News from expeditions). Geologic work in the Shackleton Range involved mapping and detailed stratigraphic-structural studies of the Clarkson, P.D., 1972. Geology of the Shackleton Range. British Antarctic Survey. Bulletin, 31: 1-15. metamorphic complex and overlying sedimentary rocks, col- lection of a Cambrian trilobite fauna discovered by Soviet Grew, E.S., 1975. With the Soviets in Antarctica, 1972-1974. Ant- arcticJournalof the U.S., X(1): 1-8. geologists near Mount Provender in 1975-1976, and map- ping and studies of glacial deposits and geomorphologic Grew, E.S. 1976. 0 geologicheskikh issledovaniyakh v Sovetskoy ant- arkticheskoy ekspeditsii v 1972-1974 features. Field camps, which were established near Mounts gg. [Geological investigations with the Soviet Antarctic Expedition in 1972 . 1974]. Sovetskoy Provender (901 meters) (figure 5), Skidmore (864 meters), A ntarkticheskoy Ekspeditsii. Informa tsionnyy Byulleten, 93: and Wegener (1416 meters), served as bases for detailed 27-29. work; reconnaissance examination of other parts of the Stephenson, P.J. 1966. Geology 1. Theron Mountains, Shackleton Shackleton Range was carried out using the Ml-8 and the Range and Whichaway Nunataks. Trans-Antarctic Expedition, AN-2. The skimobile "Buran" was deployed with great suc- 1955-1958, Scientific Reports, 8. 79 p. cess in geologic traverses originating from the field camps. The geologists worked in the Shackleton Range from 30 December to the middle of February. I spent the season (30 December 1976 to 5 February 1977) at the Provender camp situated at about 250 meters elevation. Field work was possi- ble here on about two-thirds of the days. During this time, temperatures ranged from a few degrees centigrade above Geological comparison of the freezing in January to -13°C in early February; there were a Shackleton Range and the few days with fog, light snow, or high wind (averaging up to 18 to 20 meters per second, gusts to 25 meters per second). Pensacola Mountains during the On 5 February I returned to Druzhnaya and on the 7th I accompanied Arthur B. Ford, U.S. Geological Survey, on 22nd Soviet Antarctic Expedition an LC-130 flight from Druzhnaya to McMurdo, arriving there on 8 February. ARTHUR B. FORD I thank all the expedition members, particularly the geologists, for their support and cooperation that resulted in Alaskan Geology Branch the successful completion of my field program. I consider U.S. Geological Survey myself very fortunate to have had a second opportunity to Menlo Park, California 94025 participate in the SAE. My summer in the Weddell Sea area was a memorable and profitable experience. The research was supported by National Science Founda- An objective of the 22nd Soviet Antarctic Expedition tion grant DPP 75-17390. (SAE) at Druzhnaya Base in the summer 1976-1977 was a References geological study of the Shackleton Range (80140S. 2600W.), a study started the previous year with the establishment of this new summer-only Soviet base on the edge of the Filchner Ice Shelf. The geological studies were Bardin, V.I. 1977. S polyusom p0 sosedstvy (With the Pole as a neighbor, 13 January); K taynam Shekltona (Towards the secrets part of a larger program that included geophysical and of the Shackletons, 30 January); My—so stantsii "Druzhnaya"! geodetic surveys, and that is expected in future years to be (We—from the station "Druzhnaya"!, 14 February) Belykh pyaten expanded to other areas around the head of the Weddell Sea. The surveys of the 22nd SAE used AN-2 and IL-14 skiplanes and MI-8 helicopters for aerial support. The first reconnaissance, in 1957 (Stephenson, 1966), and later more detailed surveys, in 1968-1971 (Clarkson, 1972), 4 of the Shackleton Range by British parties demonstrated - - - - - :- - - that similarities exist in the late Precambrian and early Paleozoic records of this area and the Pensacola Mountains 300 kilometers to the southwest, a region in which I have worked six summers (960-1976). Between 17 January and 7 February 1977 I visited the Shackleton Range, as a partici- pant with the 22nd SAE, to compare the geology of the two mountain regions. An understanding of the regional geologic relations is required for interpreting the ancient (late Precambrian and early Paleozoic) configuration and tectonic history of this part of the continental margin of Antarctica. The Pensacola Mountains- Shackleton Range region is critically located in models of pre-Mesozoic Gond- wanaland, for which many have been proposed. Paleogeo- Figure 5. AN-2 biplane near geology field camp near graphic and paleotectonic relations, such as here studied, Mount Provender, Shackleton Range, December 1976. must be a principal consideration in interpreting possible

88 ANTARCTIC JOURNAL joins of plates around the southernmost Atlantic, for inferred to be tilted fault blocks. Moreover, the Shackleton reconstructing the configuration of this ancient continent. Range apparently lacks evidence of early Paleozoic granitic In late Precambrian time the Pensacola Mountains- plutonism, which seems to be a characteristic feature of the Shackleton Range region was part of a seaway, on the orogen, according to the map of Craddock (1970). Thus, if margin of the craton, that appears to have extended the en- the early Paleozoic Ross orogeny occurred in this area, tire 3,000-kilometer length of the Transantarctic Moun- which seems unlikely, the effects were minor compared to tains. Lithologic and bedding characteristics of the upper those in the Pensacola Mountains. It may be speculated Precambrian Patuxent Formation in the Pensacola Moun- alternatively that the Blaiklock Glacier Group, which is tains (Schmidt et al., 1964; 1965) indicate turbidite deposi- chiefly arenaceous and locally conglomeratic, was deposited tion associated with submarine volcanism. The Patuxent in a Ross foredeep on the craton. The possible broad folding correlates approximately with Clarksons (1972) Turnpike of the Blaiklock, if it is folded, may be related to the dying Bluff Group in the Shackleton Range. The Patuxent consists out of deformation eastward over the foreland. A third ma- of rhythmically interbedded subgraywacke and slate in jor orogeny, the Weddell orogeny of Triassic(?) age, is about equal amounts, whereas the Turnpike is chiefly recorded in the Pensacola Mountains by locally strong quartzitic, with locally abundant slate, and lacks volcanic folding of a Silurian(?) to Late Permian sequence (Schmidt members and apparent turbidite features. The Turnpike et al., 1965; Ford, 1972), but possible effects of this orogeny and younger units in the Shackleton Range were deposited have not been recognized in the Shackleton Range. on an eroded surface of an older Precambrian crystalline The area of the Pensacola Mountains and Shackleton terrane that is part of the antarctic shield and dated as being Range seems to be one of the very few areas known in the at least about 1,500 million years old (Clarkson, 1972). A Transantarctic Mountains where late Precambrian and ear- crystalline basement is not exposed in the Pensacola Moun- ly Paleozoic continental margin relations to the antarctic tains (Schmidt and Ford, 1969), but a source of that type in craton can be studied. These relations are still very poorly the vicinity shed sediments into late Precambrian and known, in part owing to incompleteness of exposure and in Paleozoic basins of the area (Williams, 1969). part to the poorly fossiliferous nature of many of the rock These relations indicate a shoreward transition in the late units, which makes correlations highly uncertain. Addi- Precambrian trough, from a eugeocline or deep-water tional collections of fossils from the Shackleton Range by marginal basin in the Pensacola Mountains area to an en- Soviet geologists during the 22nd SAE may narrow some of sialic miogeocline of the Shackleton Range. A volcanic arc these uncertainties. may have lain offshore at an unknown locality outboard Studies of the above and other relations between the two from the Pensacola Mountains. The Patuxent sandstones mountain regions were carried out in cooperation with are quartz-rich and, although locally interbedded with Soviet geologists of the 22nd SAE. I was flown by LC-130 pillow lava, contain only a small amount of volcanic detritus Hercules from McMurdo to Druzhnaya and by MI-8 heli- (Williams, 1969). Chemical analyses show the sandstones to copter to a field camp in the central Read Mountains, in the contain about 75.3 percent silicon dioxide, 2.4 percent southern part of the Shackleton Range, where I joined a potassium oxide, and 1.4 percent sodium oxide (average of geological field party led by Oleg G. Shulyatin. Work from five analyses, D.L. Schmidt, unpublished data). Thus, they that camp and a later one in the La Grange Nunataks was correspond compositionally to the variety believed by carried out by ski and motor toboggan traverses, sup- Schwab (1975) to be characteristic of "Atlantic-type" con- plemented by ski-equipped AN-2 biplane and MI-8 tinental margins. ("Andean-type" margins are marked by helicopter flights for reaching distant mountain areas. sandstones with a potassium oxide to sodium oxide of less Following this work I returned by AN-2 skiplane to than 1, and with volcanic arc-derived components at least as Druzhnaya and by LC-130 to McMurdo, accompanied by abundant as those from the craton.) abundant rock samples for further study, in the laboratory, Orogeny played an important role in the development of of petrographic, petrologic, and geochronologic relations of this part of the ancient antarctic continental margin. The the two mountain areas. late Precambrian deposits in both the Pensacola Mountains I am deeply grateful to M.G. Ravich, assistant director, and the Shackleton Range were strongly folded and and G.E. Grikurov, leader of the 21st SAE at Druzhnaya metamorphosed in the greenschist facies in latest Precam- Base, of the Research Institute of Arctic and Antarctic brian time, during an orogeny probably corresponding to Geology, Leningrad, for making this visit to the Shackleton the Beardmore orogeny elsewhere in the Transantarctic Range possible. I thank the Soviet aircraft crews and other Mountains. Following erosion and Cambrian deposition of a support personnel at Druzhnaya (translated, friendly), sequence of limestone, rhyolitic volcanics and shale, the which I found to be very appropriately named, for their Pensacola Mountains area was involved in a second major unsparing help in all stages of my fieldwork; and I thank the orogeny, in the late Cambrian and Ordovician, correspond- United States Hercules crews, under Commanders Dan ing to the Ross orogeny elsewhere. The Pensacola Moun- Desko and James Jaeger, pilots, for the flights to and from tains Cambrian sequence is strongly folded and locally in- this base, where perennial seafog makes conditions for truded by a large granitic pluton dated as being 510 ± 30 skiplane landings far from the most favorable. I especially million years old (Schmidt et al., 1965). Whether the thank Valeriya Masolov, base leader at Druzhnaya, for ar- Shackleton Range also belongs to the Ross orogen, as in- ranging field logistical support; and KAPSh tent-mates Oleg dicated by Craddock (1970), is uncertain. The Cambrian(?) Shulyatin, Slava Michailov, and Grisha Klyametzeonok, of and Ordovician(?) (Thompson, 1972) unit in that area—the the Soviet Union, and Hans Paech, of the German Blaiklock Glacier Group of Clarkson (1972)—occurs only in Democratic Republic, for their generosity and hospitality in homoclines, some of which can be interpreted to be limbs of the field. I thank D.L. Schmidt and W.H. Nelson (U.S. folds, according to Clarkson. The homoclines might also be Geological Survey), who were previous field companions in

October 1977 89 study of the Pensacola Mountains, for much helpful discus- was established at the LC-130 ski-plane landing site near sion of the geologic relations here reported. Walker Peak, from which motor toboggan and ski traverses This research is supported by National Science Founda- were made to sites of investigations. The massif and nearby tion interagency agreement DDP 76-21663 with the nunataks are underlain by layered mafic igneous rocks of Geologic Division, U.S. Geological Survey. cumulus origin that make up the lower part of the Dufek intrusion, a Jurassic stratiform complex of immense size in the northern third of the Pensacola Mountains. References The U.S. Geological Survey (USGS) made reconnaissance studies using Army helicopters of all outcropping parts of the intrusion in the summer 1965-1966. This work showed Clarkson, P.D. 1972. Geology of the Shackleton Range: a that all of the Forrestal Range, located about 50 kilometers preliminary report. British Antarctic Survey Bulletin, 31: 1-15. southeast of Dufek Massif, is also underlain by part—the Craddock, Campbell. 1970. Tectonic map of Antarctica. Antarctic iron-enriched upper part - of the Dufek intrusion (Ford and Map Folio Series, 12: plate XXI. Boyd, 1968), and that the body probably has an area of at Ford, A.B. 1972. The Weddell orogeny—latest Permian to early least 24,000 square kilometers (Behrendt et al., 1974). As Mesozoic deformation at the Weddell Sea margin of the Trans. the thickness is estimated to be on the order of 8 to 9 antarctic Mountains. In: Antarctic Geology and Geophysics kilometers (Ford, 1976), the original magma volume may (Adie, R.J., editor). Oslo, Universitetsforlaget. 419-425. have been on the order of 300,000 cubic kilometers. Schmidt, D.L., J.H. Dover, A.B. Ford, and R.D. Brown. 1964. The 1976-1977 summer fieldwork was the beginning of a Geology of the Patuxent Mountains. In: Antarctic Geology (Adie, detailed geologic study to obtain data on parts of the intru- R.J., editor). New York, John Wiley. 276.283. sion not adequately covered in earlier field studies and to in- Schmidt, D.L., P.L. Williams, W.H. Nelson, and JR. Ege. 1965. vestigate questions raised by the reconnaissance and ensuing Upper Precambrian and Paleozoic stratigraphy and structure of laboratory studies. Principal results of that background the Neptune Range, Antarctica. U.S. Geological Survey profes- work are (1) a 1:1,000,000-scale geologic map showing the sional Paper, 525 D. D112.D119. . regional setting of the intrusion (Schmidt and Ford, 1969), Schmidt, D. L., and A.B. Ford. 1969. Geology of the Pensacola and (2) two 1:250,000-scale geologic maps showing major rock- Thiel Mountains. Antarctic Map Folio Series, 12: plate V. stratigraphic units within and near the intrusion (Ford et Schwab, F.L. 1975. Framework mineralogy and chemical composi- al., in press a, in press b), (3) the definition of units and tion of continental margin-type sandstone. Geology, 3: 487.490. measurement of general stratigraphy of the layered se- Stephenson, P.J. 1966. Geology. 1. Theron Mountains, Shackleton quence (Ford, 1976), (4) the recognition of the cumulus Range and Whichaway Nunataks. Trans-Antarctic Expedition origin of the layered rocks (Ford and Boyd, 1968), (5) the 1933-1938 Scientific Reports, 8: 1.79. analysis of rock magnetic properties and paleomagnetism of Thompson, M.R.A. 1972. Inarticulate brachiopoda from the the body (Beck and Griffin, 1971; Beck, 1972), (6) the deter- Shackleton Range and their stratigraphical significance. British mination of a strong iron-enrichment trend in rocks Antarctic Survey Bulletin, 31: 17-20. stratigraphically upward in the sequence (Ford, 1970), (7) Williams, P.L. 1969. Petrology of upper Precambrian and the determination of iron-enrichment fractionation trends Paleozoic sandstones in the Pensacola Mountains, Antarctica. Journal of Sedimentary Petrology, 39: 1455-1465. in orthopyroxene, inverted pigeonite, and augite- ferroaugite (Himmelberg and Ford, 1973, 1976), (8) using coexisting pyroxene geothermometry, the calculation that primary crystallization occurred in the temperature range 1180 0 to 1074°C (Himmelberg and Ford, 1976), (9) the Geological studies of the Dufek determination that compositions of the iron-titanium oxide minerals largely reflect subsolidus recrystallization in a intrusion, Pensacola Mountains, calculated temperature range of about 900° to 600°C, therefore showing little systematic change related to frac- 1976-1977 tionation except for apparent systematic variation in vanadium and aluminum concentrations in ilmeno- magnetites (Himmelberg and Ford, 1975, in press), (10) the ARTHUR B. FORD, CHRISTINE CARLSON2, determination of the crystallization sequence bytownite- or- GERALD K. CZAMANSKE, WILLIS H. NELSON, thopyroxene, labradorite-pigeonite- augite, labradorite- and CONSTANCEJ. NUTT2 pigeonite- augite-magnetiteilmenite, labradorite-ferro- augite- magnetite- fayalite-apatite (Himmelberg and Ford, U.S. Geological Survey 1976), and (11) the determination of a radiometric age of Menlo Park, California 94025 168 million years for the intrusion (R.W. Kistler, USGS, un- 2Department of Geology published data), showing it to be coeval with tholeiitic Stanford University Stanford, California 94305 Formed by gravity settling of minerals and their accumulation on the floor of the magma chamber. Such rocks are termed Geological studies by the authors and geophysical studies "cumulates" and named by prefixing names of settled minerals, in by A.W. England (USGS, Reston, Virginia) were focused in decreasing order of abundance, as in pyroxene-plagioclase the western part of the Dufek Massif (82 136S. 52130W.) cumulate" (Jackson, 1971). Common rock name may also be used, between 30 November and 30 December 1976. A base camp as in "gabbroic cumulate", if settled phases not specified.

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