The Kirkpatrick Basalt, Mesa Range, northern Victoria Land

The Kirkpatrick Basalt, Mesa Range, northern Victoria Land D. H. ELLIoT, M. SIDERS, C. FAURE, and K. S. TAYLOR Institute of Polar Studies and Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210

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As part of the Northern Victoria Land Project, we examined the Kirkpatrick Basalt of northern Victoria Land between early November 1981 and mid -January 1982. We measured and sampled stratigraphic sections at Solo Nunatak, Pain Mesa (Mills Valley), Tobin Mesa, and Sheehan Mesa (figure). In addition, we examined the Kirkpatrick Basalt at Agate Peak and the Ferrar Dolerite at Section Peak, Vantage Hills, and Exposure Hill (figure). The base of the basalt sequence is exposed only at the southern end of Gair Mesa, where it rests on Beacon strata that were, subsequent to lava eruption, intruded by diabase sills. No lavas are present west of the Rennick Glacier in the Section Peak to Vantage Hills area, contrary to earlier reports (Gair 1967). At Agate Peak (Nathan and Schulte 1968) the lowest exposed lava, a pillow basalt, is overlain by a thick pyroclastic bed containing blocks of sediment similar to the Mawson Formation that separates rocks of the Beacon Supergroup from the Kirkpatrick Basalt in southern Victoria Land. It is possible that the Agate Peak rocks are close to the base of the Kirkpatrick Basalt. Continuous stratigraphic sections in the Mesa Range are more than 670 meters thick, and the total stratigraphic thickness in the range may well exceed 800 meters. As many as 40 flows are present in several places where sections were measured. If a tentative correlation between Solo Nunatak and the Mills Valley is confirmed by later study, the total thickness will be more than 1,000 meters, and the number of flows more than 50. Flow thicknesses range from less than 1 meter to possibly as much as 100 meters. Sills may be present within the lava sequence, but none have been positively identified. Sedimentary interbeds occur sporadically in the section, but no widespread units have been identified; all sedimentary interbeds found so far in situ consist of reworked volcanic material. Lake beds such as occur at Carapace Nunatak (Ball et al. 1979) and in the Beardmore Glacier region (Tasch 1970) have been found only as disrupted blocks of conchostracan-bearing sediment caught up in the pillow basalt at Agate Peak. Fossil wood has been found in a number of places. The presence of a 60-meter (200-foot) sedimentary interbed that was recorded by Gair on Silva Ridge has not been confirmed. Correlation of flows within the Mesa Range is possible only where flows have distinctive characteristics. Two that appear to be very widespread have been identified; one is a thick black basalt that caps much of the Mesa Range, and the other is a composite flow with a remarkable wavy unit at its top which occurs near the base of Tobin Mesa. Correlation from the Mesa Range to other outcrops is more problematic, but may be aided by information about the chemical and isotopic compositions of the flows. Tentative identification of flows suggests that the Sheehan Mesa section is largely encompassed by that of the Mesa Range. If this is correct, the outcrops west of the Rennick

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Glacier (Sheehan Mesa to Monument Nunataks) may not be stratigraphically higher than the Mesa Range. Petrographically the lavas are typical tholeiites, having two pyroxenes, plagioclase, and opaques set in a groundmass that ranges from glassy through cryptocrystalline to microcrystalline aggregates of quartzofeldspathic minerals, with or without ferromagnesian minerals (principally pyroxene). Zeolites (stilbite and probably others) and other secondary minerals including quartz (chalcedony) and calcite are a common occurrence, filling vesicles in the upper parts of flows, as well as forming masses within the rubbly flow tops and between the pillows at Agate Peak and elsewhere. Selected samples collected this season will be analyzed geochemically for major- and trace-element, rare-earth, and strontium-isotopic data. The study of zeolites and other secondary minerals will include identifying mineral and establishing a zonation (if possible) and the relationship to temperature distribution and groundwater circulation in the lava pile. Major faulting has been postulated for the Rennick Glacier region (Gair 1967; Gair et al. 1969), and it has been suggested that the Kirkpatrick Basalts are situated within a graben, the "Rennick Graben" (Kyle and Cole 1974). There is no compelling evidence for a major fault at the head of the Rennick Glacier between Vantage Hills and Exposure Hill. The lavas have a very low-angle regional dip to the west or northwest, and it is clear that a fault may exist between Sheehan Mesa and Section Peak. Therefore, the postulated fault must have a scissorslike character, opening to the north. This study was supported by National Science Foundation grant DPP 80-21401. 19

References Ball, H. W., Borns, H. W., Hall, B. A., Brooks, H. K., Carpenter, F. M., and Delevoryas, T. 1979. In B. Laskar and C. S. Raja Rao (Eds.), Fourth International Gondwana Symposium (Vol. 1). Delhi: Hindustan Publishing Corporation. Gair, H. S. 1967. The geology from the upper Rennick Glacier to the coast, northern Victoria Land. New Zealand Journal of Geology and Geophysics, 10, 309-344.

Gair, H. S., Sturm, A., Carryer, S. J., and Grindley, G. W. 1969. The geology of northern Victoria Land (Folio 12, Plate 12). In V. C. Bushnell

Paleomagnetic results from the Kirkpatrick Basalt Group, Victoria Land WILLIAM C. MCINTOSH New Mexico Bureau of Mines and Mineral Resources Socorro, New Mexico 87801

PHILIP R. KYLE Department of Geoscience New Mexico Institute of Mining and Technology Socorro, New Mexico 87801

and C. Craddock (Eds.), Geologic maps of Antarctica 1:100,000, Antarctic maps folio series. New York: American Geographical Society. Kyle, P. R., and Cole,

J.

W. 1974. Structural control of volcanism in the

McMurdo volcanic group. Bulletin Volcanologique, 38, 16-25.

Nathan, S., and Schulte, F. J. 1968. Geology and petrology of the Campbell-Aviator Divide, northern Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 11, 960-975. Tasch, P. 1970. Antarctic and other Gondwana conchostracans and insects: New data; Significance for continental drift. In S. H. Haughton (Ed.), Second Gondwana Symposium. Pretoria, South Africa: Council of Scientific and Industrial Research.

samples from 15 sites in 15 flows) (figure 1). Paleomagnetic results show that the samples possess strong normal remanence, with stability equal to or greater than that of Ferrar Supergroup dolerites and gabbros. The natural remanent magnetization (NRM) includes a small viscous component that is readily removed by alternating field demagnetization in fields of 20 to 50 millitesla. The mean VGP of the David Glacier samples lies at 55°S 138°W, which is nearly coincident with the average of VGP's previously reported from the Ferrar Supergroup. The mean VGP of the Mesa Range samples, however, lies significantly farther south at 64°S 150°W (figure 2) (page 22). It is unlikely that the anomalously high latitude of the mean of the Mesa Range samples is an effect of unremoved viscous or chemical components, tectonic tilting or rotation, or incomplete averaging of paleosecular variation. In addition, an VGP

ERIC M. CHERRY and HALLAN C. NOLTIMIER 162E

Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210

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Several previous paleomagnetic studies have been made of Jurassic-age antarctic igneous rocks, including (as summarized and referenced in the table): (1) Ferrar Dolerite sills from four localities along the Transantarctic Mountains; (2) Forrestal Gabbros from the Dufek Stratiform Intrusion; (3) Kirkpatrick Basalt lava flows from the Queen Alexandra Range in the central Transantarctic Mountains; and (4) tholeiites from Queen Maude Land. The results have shown generally high stabilities and predominantly normal polarities. The average virtual geomagnetic pole (vcP) is near 55°S 140°W, in good agreement with Jurassic VGP's of Africa, South America, India, and Australia in their Gondwanaland configurations (Schmidt 1976). The purpose of the present study was to investigate further the paleomagnetism of the Kirkpatrick Basalts. Oriented samples were collected from Kirkpatrick Basalts in the David Glacier Area, southern Victoria Land (55 samples from 25 sites in 24 flows), and at the Mesa Range, northern Victoria Land (60 20

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