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Spatial Variation in Cenozoic Volcanism of Marie Byrd Land and Ellsworth Land

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Spatial Variation in Cenozoic Volcanism of Marie Byrd Land and Ellsworth Land Spatial variation in Cenozoic volcanism western Marie Byrd Land. The Cenozoic volcanic sec- of Marie Byrd Land and Ellsworth Land tion in the Jones Mountains, for example, is roughly 500 ui thick (Craddock et al., 1963), and the thickest section in the Hudson Mountains is roughly 200 m thick (field notes, 1968-1969 Ellsworth Land Survey). WESLEY E. LEMASURIER By contrast, the basalt section at Mount Murphy is University of Colorado about 2,000 m thick; in the Crary Mountains a Denver Center 1,200-rn thickness of basaltic rock is exposed; and at Toney Mountain, seismic evidence indicates that the base of the basalt section lies 3,000 m below sea level During the past decade, field and laboratory studies or about 4,000 in the exposed top of the section have brought to light several lines of evidence that (Bentley and Clough, in press). In addition to their suggest a continuity of Andean geologic features, such great thickness, preliminary studies of vesicularity sug- as Mesozoic plutonism and Cenozoic volcanism, ex- gest that the central Marie Byrd Land basalts were tending from the base of the Antarctic Peninsula erupted at much greater depths beneath ice level than across Marie Byrd Land and Ellsworth Land to the those in Ellsworth Land (e.g., Moore, 1970). The fact Ross Sea region (e.g., Halpern, 1968; LeMasurier and that these deposits are now exposed at elevations as in Wade, 1968; Craddock, in press). In this context, one much as 1,000 to 2,000 than the coastal of the major objectives of research on Marie Byrd ranges in Ellsworth Land or western Marie Byrd Land volcanism has been to examine the possibilities Land suggests that, during Cenozoic time, central for gochernical correlations with other areas of Ceno- Marie Byrd Land was much more mobile tectonically zoic volcanism in Antarctica and to relate the charac- than adjacent regions. Evidently there were large ver- ter of volcanism to tectonic environment. It has been tical displacements associated with the rectangular sys- pointed out recently that the alkalinity of Cenozoic tem of faults that has already been described for this lavas along the Pacific continental margin of Antarc- region (LeMasurier, in press). tica represents a significant change from the tectonic In summary, the Cenozoic history of central Marie environment of Mesozoic volcanism in Antarctica, and Byrd Land appears to be characterized by extensional a significant difference from the general character of block faulting, periodic eruptions of alkali basalt that Cenozoic volcanism elsewhere in the circum-Pacific may have approached flood basalt proportions, and progenic belt (LeMasurier, 1970; Baker, in press). the development of trachytic stratovolcanoes. The en- Upon still closer examination, it appears that there tire environment has many similarities to the African are important discontinuities within the Cenozoic vol- Rift Valleys and to Iceland and neighboring parts of anic province of West Antarctica and that the im- the Brito-Arctic volcanic province. This is entirely pressions of continuity gained from reconnaissance consistent with interpretations of rifting and fragmen- mapping may be somewhat misleading. tation that have been based largely on studies of base- If one compares the volcanic rocks in the central ment geology in this region (Craddock, in press; sector of Marie Byrd Land—between longitudes Wade and Wilbanks, in press). The volcanic history 110°W. and 140°W.—with those in the adjacent suggests further, however, that rifting was more local- areas of Ellsworth Land and western Marie Byrd ized in central Marie Byrd Land, and probably the Land, two characteristics become evident: (1) the Ross Sea, than in intervening areas, and that conti- volume of basalt in the central sector is much greater nental fragmentation in this part of West Antarctica and (2) siliceous differentiates seem to be confined to was largely a Cenozoic event. the central sector. In central Marie Byrd Land, trach- If Marie Byrd Land and adjacent regions are, in ytic differentiates were produced in relatively large fact, similar in volcanic characteristics to rift prov- volumes during Cenozoic time and were erupted to inces elsewhere in the world, one might expect to find form the large stratovolcanoes that are characteristic tholeiitic basalt occupying structural depressions now of the region. No siliceous volcanics of Cenozoic age covered by ice or sea water (Lipman, 1969; Mohr, and no stratovolcanoes have been reported from Ells- 1971). It will be interesting, therefore, to see whether worth Land (Craddock et al., 1963; Wade and La- the Joint Oceanographic Institutions for Deep Earth rade, 1969; Laudon, in press) or west Marie Byrd Sampling holes that are planned for the Amundsen Land (Warner, 1945; Wade and Wilbanks, in press). and Ross Sea areas recover basalt with tholeiitic af- The closest trachytic stratovolcanoes appear to be finities. those in the western Ross Sea region, 1,400 km to the During the past year I have had the opportunity to 1west, and Deception Island, 2,200 km to the northeast. study three collections of Ellsworth Land volcanic The basalt sections that overlie basement rock in rocks through the courtesy of Drs. C. Craddock, T. S. central Marie Byrd Land appear to be five to 10 times Laudon, and F. A. Wade. The progress reported here thicker than the basalt sections in Ellsworth Land or owes a great deal to their cooperation. September-October 1971 187 References Baker, P. E. In Press. Recent volcanism and magmatic variation in the Scotia Arc. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. Bentley, C. R., and J . W. Clough. In press. Seismic refrac- tion measurements of antarctic subglacial structure. In: Antarctic Geology and Geophysics. Oslo, Universitets- forlaget. Craddock, C. In press. Antarctic tectonics. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. Craddock, C., T. W. Bastien, and R. H. Rutford. 1963. Geology of the Jones Mountains area. In: Antarctic Geol- ogy. Amsterdam, North-Holland. p. 171-187. Halpern, M. 1968. Ages of antarctic and Argentine rocks bearing on continental drift. Earth and Planetary Science Letters, 5: 159-167. Laudon, T. S. In press. The stratigraphy of eastern Ells- worth Land. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. LeMasurier, W. E. 1970. Tectonic environment of circum- Pacific volcanism in Marie Byrd Land, Antarctica (ab- stract). American Geophysical Union. Transactions, 51: 824. LeMasurier, W. E. In press. Volcanic record of Cenozoic glacial history in Marie Byrd Land. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. LeMasurier, W. E., and F. Alton Wade. 1968. Fumarolic Labyrinthodont amphibian skull from McGregor Glacier area. activity in Marie Byrd Land, Antarctica. Science, 162: 352. Lipman, P. W. 1969. Alkalic and tholeiitic basaltic vol- cier. It had been intended to extend the work of that canism related to the Rio Grande depression, southern season to the region of McGregor Glacier, some 240 Colorado and northern New Mexico. Geological Society km distant, but weather and circumstances prevented of America. Bulletin, 80: 1343-1354. Mohr, P. A. 1971. Ethiopian rift and plateaus: some it. Consequently, this aspect of the field campaign was volcanic petrochemical differences. Journal of Geophysical resumed during the 1970-1971 season. Paleontological Research, 76: 197-1984. - prospecting and collecting was carried on by Mr. Moore, James G. 1970. Water content of basalt erupted on James W. Kitching of the Bernard Price Institute for the ocean floor. Contributions to Mineralogy and Petrology, 28: 272-279. Palontology, University of the Witwatersrand, Jo- Wade, F. A., and K. E. LaPrade. 1969. Geology of the hannesburg, South Africa, assisted by Mr. John King Peninsula, Canisteo Peninsula, and Hudson Moun- Ruben of the University of California at Berkeley, and tains areas, Ellsworth Land, Antarctica. Antarctic Journal for a short time by Mr. Thomas Rich of the American of the U.S., IV(4): 92-93. Museum of Natural History in New York. Wade, F. A., and J . R. Wilbanks. In press. The geology of It had been hoped that well preserved fossils would Marie Byrd Land and Ellsworth Land, Antarctica. In: Antarctic Geology and Geophysics. Oslo, Universitets- be found in the McGregor Glacier region, since pre- forlaget. vious studies had indicated that this area was up the Warner, L. A. .1945. Structure and petrography of the south- paleoslope from Coalack Bluff, and thus might be ern Edsel Ford Ranges, Antarctica. American Philosophical close to the source of fossil burials. The hope was fully Society. Proceedings, 89: 78-122. justified: whereas at Coalsack Bluff the fossils, though numerous, consisted of isolated and rolled bones de- posited in coarse sands and even conglomerates, at McGregor Glacier the specimens consisted of articu- lated skeletons and partial skeletons contained within Triassic tetrapods rather fine-grained siltstones (see photo). As a result, from McGregor Glacier our knowledge of early Triassic antarctic tetrapods has been augmented and expanded. EDWIN H. COLBERT The work of this past season was aided by good Museum of Northern Arizona weather, a contrast to the inclement weather that plagued the paleontologists during the previous col- During the antarctic field season of 1969-1970, as lecting field season. readers will recall, a considerable collection of Lower The initial fossil of this past season, an imprint of a Triassic tetrapods was made from the Fremouw For- complete skeleton of the mammal-like reptile Thri mation at Coalsack Bluff in the Transantarctic Moun- naxodon, was found on the first day in the field tains, immediately to the east of the Beardmore Gla- Dr. James Collinson of The Ohio State University, 188 ANTARCTIC JOURNAL.
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