Paleomagnetic investigation in the Ellsworth Mountains

(Bockheim and Wilson 1979). The Ross Sea glaciation has been dated by Stuiver, Denton, and Hughes (1980) at 17,000 to 21,000 years ago. In conjunction with the findings of Denton, Rutford, and Andersen (Antarctic Journal of the U.S., this volume), these data suggest that most of the peaks in the Heritage Range were overrun by ice from the expanded west antarctic ice sheet during the last glaciation (about 18,000 years ago). Preliminary X-ray diffraction analysis of salt encrustations revealed the widespread occurrence of gypsum (CaSO4 21-120) in the Heritage Range. Other salt minerals include calcite (CaCO 3 ) and thenardite (NaSO 4). Soda niter Table 2. Weathering of surface bounders on moraines in the Ellsworth Mountains

Surface boulder frequency % of quartzite boulders (per 314 Site square meters) Striated Polished SpaUed Pitted 79-15 525 14 90 24 2 79-16 710 0 34 4 7 79-17 525 37 92 14 28 79-19 205 3 93 25 3 79-20 310 0 97 18 17 79-23 405 0 93 47 24 79-24 420 0 69 30 1 79-25 710 1 34 20 0 79-26 575 0 55 18 7 79-32 250 0 32 4 4 79-35 365 0 81 60 31 79-36 750 0 45 6 0

Paleomagnetic investigation in the Ellsworth Mountains DOYLE R. WArrs

and

ANDREW M. BRAMALL

Department of Earth Sciences University, Leeds, LS2 91T United Kingdom

Many scientific activities took place at Camp Macalester in the Ellsworth Mountains during the 1979-80 austral field season. One was a systematic collection of oriented drill cores from Cambrian through Devonian sedimentary rocksin the northern Heritage Range and the southern Sentinel

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(NaNO3) was discovered on Dickey Peak (78°2(YS 84°30'W) in the Sentinel Range. Chemical and mineralogical analysis of 150 soil samples and salt encrustations are in progress. This work was supported by National Science Foundation grant DPP 74-20991 to G. H. Denton. We are grateful to civilian and VXE-6 personnel at Camp Macalester for logistic support. H. Conway and M. Prentice provided field assistance.

References Birkeland, P. W. 1974. Pedology, weathering, and geomorphological research. New York: Oxford University Press. Bockheim, J . C. 1977. Soil development in the Taylor Valley and McMurdo Sound area. Antarctic Journal of the U.S., 12(4), 105-108. Bockheim, J. C. 1979. Relative age and origin of soils in eastern Wright Valley. Soil Science, 128(3), 142-152. Bockheim, J. G., and Wilson, S. C. 1979. Pedology of the Darwin Glacier area, Antarctica. Antarctic Journal of the U.S., 14(5), 58-59. Campbell, I. B., and Claridge, C. G. C. 1975. Morphology and age relationships of antarctic soils. Quaternary Studies, New Zealand Royal Society Bulletin, 13, 83-88. Denton, G. H., Rutford, R. H., and Andersen, B. G. 1980. Glacial history of the Ellsworth Mountains. Antarctic Journal of the U.S., 15(5). Pastor, J . , and Bockheim, J. G. 1980. Soil development on moraines of Taylor Glacier, lower Taylor Valley, Antarctica. Soil Science Society of America Journal, 44(2), 341-348. Stuiver, M., Denton, G. H., and Hughes, T. J . 1980. History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis. In G. H. Denton and T. J. Hughes (Eds.), The last great ice sheet. New York: Wiley-Interscience.

Range. The investigation, which took place between December 1979 and January 1980, is part of a larger program using paleomagnetic methods to determine the tectonic relationship between West Antarctica and East Antarctica and to document relative motions (or lack thereof) between the crustal units that comprise West Antarctica (Alley and Watts 1979; Elliot, Watts, Alley, and Gracanin 1978; Watts in press). The sampling program in the Ellsworth Mountains was planned on the basis of a preliminary study of the magnetic properties of a suite of unoriented rock samples provided by C. Craddock of the University of Wisconsin-Madison. In the preliminary study, it was demonstrated that the argillites of the Upper Heritage Group and the hematitic quartzites of the Crashsite Quartzite Group were especially promising targets for detailed investigation. Samples were collected from more than 70 sites spanning a distance of 224 kilometers through the two ranges that comprise the Ellsworth Mountains. The sampled units

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range in age from Cambrian through Devonian. Efforts were focused on sections where the style of deformation was relatively simple and the development of a strain fabric was not evident or was very limited. Areas of great complexity and uncertain attitude and areas where cleavage and other strain fabric were prominent were not sampled. The style of sedimentation and structural deformation allowed the application of a number of field tests to determine the geologic age of magnetization. An attempt was made to sample specific horizons of different attitudes in both local and regional folds. This allows the application of the classic fold test of Graham (1949) to determine if the magnetization is pre- or post-tectonic. In certain sections, slump folds or load casts penecontemporaneous with deposition were found in abundance. The slump folds were sampled along both limbs, providing a powerful fold test that will reveal if the magnetization is penecontemporaneous with deposition. Cosets of planar crossbeds were sampled to apply the crossbed test of Elston and Purucker (1979). In this test, an inclination error in the crossbeds is interpreted as evidence of a detrital origin of the magnetization. Preliminary thermal demagnetization and measurements at the Paleomagnetic Laboratory of the University of Leeds (U.K.) from three sites has proved the worth of the preliminary study. Two sites are from argillites of probable Upper Cambrian age collected from a subhorizontal section on the flank of Pipe Peak (79°09'S 86°14'W) and the third site from a vertically dipping section of rocks of similar lithology near the same stratigraphic horizon exposed at

the southern end of Reuther Nunataks (79 0 12'S 85055'W). The age of the horizon is known because of the discovery of a trilobite fauna by the paleontologists working from Camp Macalester. Magnetic components with blocking temperatures on the order of 660°C were isolated from these three sites by thermal demagnetization techniques. Figure 1A is an equal-area stereographic projection of the directions of the magnetic components before structural correction; figure lB shows projections after structural correction. Clearly, there is a reduction in the scatter of the paleomagnetic directions from these units after the structural correction is applied, demonstrating the pretectonic age of magnetization of these rocks. Assuming that the high-blocking temperature pretectonic magnetizations are of Upper Cambrian age, they may be compared to results of Late Cambrian and Early Ordovician age from the east antarctic craton. The mean paleomagnetic declinations from the Ellsworth Mountain result and results from the Sor Rondane Mountains (Zijderveld 1968), Mirnyy Station (McQueen, Scharnberger, Scharon, and Halpern 1972), and Taylor Valley (Manzoni and Nanni 1977) are plotted in figure 2. The data are consistent with a 90° rotation of the Ellsworth Mountain block with respect to the east antarctic craton first postulated by Schopf (1969) and elaborated by Stump (1976), deWit (1977), and Daiziel (in press). The preliminary nature of this result must be emphasized, and the difficulties in interpretation must be outlined. The Paleozoic paleomagnetic field for Gondwanaland is complex and poorly documented. The possibility that the N

T.

. 0000•7

Reuther Nunataks - Before structural Pipe Peak, - correction high blocking temperature components A

0

- ..

0

- 0

Reuther Nunataks - After structural Pipe Peak, - correction high blocking temperature B \ crnPonents

Figure 1. Equal-area projection of high-blocking temperature paleomagnetic components (A) before and (B) after structural correction, showing considerable reduction of scatter with tectonic correction. The closed circles denote the intersection of the north-seeking polarity with the lower hemisphere, and the open circles denote the intersection with the upper hemisphere.

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35

900 difference in declination is due to rapid changes in the paleomagnetic field with respect to Antarctica during the Lower Paleozoic cannot be discounted at the present level of coverage. A solid data base from the Paleozoic of the east antarctic craton will be required before the problem is fully resolved. Furthermore, the results for the Ellsworth Mountains presented here represent only 3 sites of the total of 70. At this stage in the investigation, the data are most consistent with a theory proposing a microplate nature of West Antarctica.

50

'JII'M,"

Sr ndon.

C>.

90* W -

EIGHTS COAST

C)

O)

EAST ANTARCT IC CRATON IMInI

Ellsworth •

Minny S tation

I

Mine

-90•E

0

13"0 LAND

Taylor Valley !^Q

ISO.

Figure 2. Map showing major crustal units of Antarctica with mean declinations of paleomagnetic vectors from Upper! Late Cambrian and Early Ordovician rocks. The result from the Ellsworth Mountains is approximately 900 different from those of the east antarctic craton.

This research was supported by the Scott Turner Fund of the University of Michigan and by National Science FounFoundation grant DPP 78-21730. The authors wish to thank Rob Van der Voo for providing facilities at the University of Michigan for the preliminary study of unoriented samples.

Geological investigations and logistics in the Ellsworth Mountains, 1979-80 JOHN F. SPLETrSTOESSER Minnesota Geological Survey University of Minnesota St. Paul, Minnesota 55108 GERALD F. WEBERS Macalester College St. Paul, Minnesota 55105

36

References Alley, R. B., and Watts, D. R. 1979. Paleomagnetic investigation of the northern antarctic peninsula. EQS. Transactions, American Geophysical Union, 60, 240. Dalziel, I. W. D. In press. The pre-Jurassic history of the Scotia Arc: A review and progress report. In C. Craddock (Ed.), Antarctic Geoscience. Madison: The University of Wisconsin Press. deWit, M. J . 1977. The evolution of the Scotia Arc as a key to the reconstruction of southwestern Gondwanaland. Tectonophysics, 37, 53-82. Elliot, D. H., Watts, D. R., Alley, R. B., and Gracanin, T. M. 1978. Geologic studies in the northern antarctic peninsula, R/V Hero cruise 78-lB. February 1978. Antarctic Journal of the U.S., 13(4), 12-13. Elston, D. R., and Purucker, M. E. 1979. Detrital magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona. Journal of Geophysical Research, 84, 1653- 1666. Graham, J . W. 1949. The stability and significance of magnetism in sedimentary rocks. Journal of Geophysical Research, 59, 131 - 137. Manzoni, M., and Nanni, T. 1977. Paleomagnetism of Ordovician Lamprophyres from Taylor Valley, Victoria Land, Antarctica. Pure and Applied Geophysics, 115, 961-978. McQueen, D. M., Scharnberger, C. K., Scharon, L., and Halpern, M. 1972. Cambro-Ordovician paleomagnetic pole position and rubidium-strontium total rock isochron for charnockitic rocks from Mirnyy Station, East Antarctica. Earth and Planetary Science Letters, 16, 433-438. Schopf, J . M. 1969. Ellsworth Mountains: Position in West Antarctica due to sea floor spreading. Science, 164, 63-66. Stump, E. 1976. On the late Precambrian-early Paleozoic metasediments and metasedimentary rocks of the Queen Maud Mountains, Antarctica, and a comparison with rocks of similar age from southern Africa (Report 62). Columbus: The Ohio State University, Institute of Polar Studies. Watts, D. R. In press. Potassium-argon and paleomagnetic results from King George Island, South Shetland Islands. In C. Craddock (Ed.), Antarctic Geoscience. Madison: The University of Wisconsin Press. Zijderveld, J . D. A. 1968. Natural remanent magnetizations of some intrusive rocks from the Sor Rondane Mountains, Queen Maud Land, Antarctica. Journal of Geophysical Research, 73, 3773-3785.

The most ambitious field project of the 1979-80 austral summer season in Antarctica was conducted in the Ellsworth Mountains between 3 December and 12 January. The area studied included both the Sentinel Range (north) and the Heritage Range (south), from about 77 0 15'S to 80°30'S latitude and 80°W to 87°30'W longitude (see figure 1). The camp, Ellsworth Camp (also known as Camp Macalester), was located adjacent to Minnesota Glacier at 79°05'S 85°58'W, about midway between the northern and southern extremities of the mountains. Camp elevation was about 1,250 meters above sea level. This was the largest field program in the Ellsworth Mountains in the 20-year history of studies there. Participating were 42 scientists and technicians representing eight countries (U.S., 28; West Germany and New Zealand, 4

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