Spatial variation in Cenozoic volcanism of Marie Byrd Land and ...
Spatial variation in Cenozoic volcanism of Marie Byrd Land and Ellsworth Land WESLEY E. LEMASURIER University of Colorado Denver Center During the past decade, field and laboratory studies have brought to light several lines of evidence that suggest a continuity of Andean geologic features, such as Mesozoic plutonism and Cenozoic volcanism, extending from the base of the Antarctic Peninsula across Marie Byrd Land and Ellsworth Land to the Ross Sea region (e.g., Halpern, 1968; LeMasurier and Wade, 1968; Craddock, in press). In this context, one of the major objectives of research on Marie Byrd Land volcanism has been to examine the possibilities for g'ochernical correlations with other areas of Cenozoic volcanism in Antarctica and to relate the character of volcanism to tectonic environment. It has been pointed out recently that the alkalinity of Cenozoic lavas along the Pacific continental margin of Antarctica represents a significant change from the tectonic environment of Mesozoic volcanism in Antarctica, and a significant difference from the general character of Cenozoic volcanism elsewhere in the circum-Pacific progenic belt (LeMasurier, 1970; Baker, in press). Upon still closer examination, it appears that there are important discontinuities within the Cenozoic volanic province of West Antarctica and that the impressions of continuity gained from reconnaissance mapping may be somewhat misleading. If one compares the volcanic rocks in the central sector of Marie Byrd Land—between longitudes 110°W. and 140°W.—with those in the adjacent areas of Ellsworth Land and western Marie Byrd Land, two characteristics become evident: (1) the volume of basalt in the central sector is much greater and (2) siliceous differentiates seem to be confined to the central sector. In central Marie Byrd Land, trachytic differentiates were produced in relatively large volumes during Cenozoic time and were erupted to form the large stratovolcanoes that are characteristic of the region. No siliceous volcanics of Cenozoic age and no stratovolcanoes have been reported from Ellsworth Land (Craddock et al., 1963; Wade and Larade, 1969; Laudon, in press) or west Marie Byrd Land (Warner, 1945; Wade and Wilbanks, in press). The closest trachytic stratovolcanoes appear to be those in the western Ross Sea region, 1,400 km to the 1west, and Deception Island, 2,200 km to the northeast. The basalt sections that overlie basement rock in central Marie Byrd Land appear to be five to 10 times thicker than the basalt sections in Ellsworth Land or September-October 1971
western Marie Byrd Land. The Cenozoic volcanic section 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). By contrast, the basalt section at Mount Murphy is about 2,000 m thick; in the Crary Mountains a 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 or about 4,000 in the exposed top of the section (Bentley and Clough, in press). In addition to their great thickness, preliminary studies of vesicularity suggest that the central Marie Byrd Land basalts were erupted at much greater depths beneath ice level than those in Ellsworth Land (e.g., Moore, 1970). The fact that these deposits are now exposed at elevations as much as 1,000 to 2,000 in than the coastal ranges in Ellsworth Land or western Marie Byrd Land suggests that, during Cenozoic time, central Marie Byrd Land was much more mobile tectonically than adjacent regions. Evidently there were large vertical displacements associated with the rectangular system of faults that has already been described for this region (LeMasurier, in press). In summary, the Cenozoic history of central Marie Byrd Land appears to be characterized by extensional block faulting, periodic eruptions of alkali basalt that may have approached flood basalt proportions, and the development of trachytic stratovolcanoes. The entire environment has many similarities to the African Rift Valleys and to Iceland and neighboring parts of the Brito-Arctic volcanic province. This is entirely consistent with interpretations of rifting and fragmentation that have been based largely on studies of basement geology in this region (Craddock, in press; Wade and Wilbanks, in press). The volcanic history suggests further, however, that rifting was more localized in central Marie Byrd Land, and probably the Ross Sea, than in intervening areas, and that continental fragmentation in this part of West Antarctica was largely a Cenozoic event. If Marie Byrd Land and adjacent regions are, in fact, similar in volcanic characteristics to rift provinces elsewhere in the world, one might expect to find tholeiitic basalt occupying structural depressions now covered by ice or sea water (Lipman, 1969; Mohr, 1971). It will be interesting, therefore, to see whether the Joint Oceanographic Institutions for Deep Earth Sampling holes that are planned for the Amundsen and Ross Sea areas recover basalt with tholeiitic affinities. During the past year I have had the opportunity to study three collections of Ellsworth Land volcanic rocks through the courtesy of Drs. C. Craddock, T. S. Laudon, and F. A. Wade. The progress reported here owes a great deal to their cooperation. 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 refraction measurements of antarctic subglacial structure. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. 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 Geology. 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 circumPacific volcanism in Marie Byrd Land, Antarctica (abstract). 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 activity in Marie Byrd Land, Antarctica. Science, 162: 352. Lipman, P. W. 1969. Alkalic and tholeiitic basaltic volcanism related to the Rio Grande depression, southern Colorado and northern New Mexico. Geological Society of America. Bulletin, 80: 1343-1354. Mohr, P. A. 1971. Ethiopian rift and plateaus: some volcanic petrochemical differences. Journal of Geophysical Research, 76: 197-1984. Moore, James G. 1970. Water content of basalt erupted on
the ocean floor. Contributions to Mineralogy and Petrology,
28: 272-279. Wade, F. A., and K. E. LaPrade. 1969. Geology of the King Peninsula, Canisteo Peninsula, and Hudson Mountains areas, Ellsworth Land, Antarctica. Antarctic Journal of the U.S., IV(4): 92-93. Wade, F. A., and J . R. Wilbanks. In press. The geology of Marie Byrd Land and Ellsworth Land, Antarctica. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. Warner, L. A. .1945. Structure and petrography of the southern Edsel Ford Ranges, Antarctica. American Philosophical Society. Proceedings, 89: 78-122.
Triassic tetrapods from McGregor Glacier EDWIN H. COLBERT
Museum of Northern Arizona During the antarctic field season of 1969-1970, as readers will recall, a considerable collection of Lower Triassic tetrapods was made from the Fremouw Formation at Coalsack Bluff in the Transantarctic Mountains, immediately to the east of the Beardmore Gla188
Labyrinthodont amphibian skull from McGregor Glacier area.
cier. It had been intended to extend the work of that season to the region of McGregor Glacier, some 240 km distant, but weather and circumstances prevented it. Consequently, this aspect of the field campaign was resumed during the 1970-1971 season. Paleontological prospecting and collecting was carried on by Mr. James W. Kitching of the Bernard Price Institute for Palontology, University of the Witwatersrand, Johannesburg, South Africa, assisted by Mr. John Ruben of the University of California at Berkeley, and for a short time by Mr. Thomas Rich of the American Museum of Natural History in New York. It had been hoped that well preserved fossils would be found in the McGregor Glacier region, since previous studies had indicated that this area was up the paleoslope from Coalack Bluff, and thus might be close to the source of fossil burials. The hope was fully justified: whereas at Coalsack Bluff the fossils, though numerous, consisted of isolated and rolled bones deposited in coarse sands and even conglomerates, at McGregor Glacier the specimens consisted of articulated skeletons and partial skeletons contained within rather fine-grained siltstones (see photo). As a result, our knowledge of early Triassic antarctic tetrapods has been augmented and expanded. The work of this past season was aided by good weather, a contrast to the inclement weather that plagued the paleontologists during the previous collecting field season. The initial fossil of this past season, an imprint of a complete skeleton of the mammal-like reptile Thri naxodon, was found on the first day in the field Dr. James Collinson of The Ohio State University, ANTARCTIC JOURNAL
western Marie Byrd Land. The Cenozoic volcanic section 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). By contrast, the basalt section at Mount Murphy is about 2,000 m thick; in the Crary Mountains a 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 or about 4,000 in the exposed top of the section (Bentley and Clough, in press). In addition to their great thickness, preliminary studies of vesicularity suggest that the central Marie Byrd Land basalts were erupted at much greater depths beneath ice level than those in Ellsworth Land (e.g., Moore, 1970). The fact that these deposits are now exposed at elevations as much as 1,000 to 2,000 in than the coastal ranges in Ellsworth Land or western Marie Byrd Land suggests that, during Cenozoic time, central Marie Byrd Land was much more mobile tectonically than adjacent regions. Evidently there were large vertical displacements associated with the rectangular system of faults that has already been described for this region (LeMasurier, in press). In summary, the Cenozoic history of central Marie Byrd Land appears to be characterized by extensional block faulting, periodic eruptions of alkali basalt that may have approached flood basalt proportions, and the development of trachytic stratovolcanoes. The entire environment has many similarities to the African Rift Valleys and to Iceland and neighboring parts of the Brito-Arctic volcanic province. This is entirely consistent with interpretations of rifting and fragmentation that have been based largely on studies of basement geology in this region (Craddock, in press; Wade and Wilbanks, in press). The volcanic history suggests further, however, that rifting was more localized in central Marie Byrd Land, and probably the Ross Sea, than in intervening areas, and that continental fragmentation in this part of West Antarctica was largely a Cenozoic event. If Marie Byrd Land and adjacent regions are, in fact, similar in volcanic characteristics to rift provinces elsewhere in the world, one might expect to find tholeiitic basalt occupying structural depressions now covered by ice or sea water (Lipman, 1969; Mohr, 1971). It will be interesting, therefore, to see whether the Joint Oceanographic Institutions for Deep Earth Sampling holes that are planned for the Amundsen and Ross Sea areas recover basalt with tholeiitic affinities. During the past year I have had the opportunity to study three collections of Ellsworth Land volcanic rocks through the courtesy of Drs. C. Craddock, T. S. Laudon, and F. A. Wade. The progress reported here owes a great deal to their cooperation. 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 refraction measurements of antarctic subglacial structure. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. 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 Geology. 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 circumPacific volcanism in Marie Byrd Land, Antarctica (abstract). 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 activity in Marie Byrd Land, Antarctica. Science, 162: 352. Lipman, P. W. 1969. Alkalic and tholeiitic basaltic volcanism related to the Rio Grande depression, southern Colorado and northern New Mexico. Geological Society of America. Bulletin, 80: 1343-1354. Mohr, P. A. 1971. Ethiopian rift and plateaus: some volcanic petrochemical differences. Journal of Geophysical Research, 76: 197-1984. Moore, James G. 1970. Water content of basalt erupted on
the ocean floor. Contributions to Mineralogy and Petrology,
28: 272-279. Wade, F. A., and K. E. LaPrade. 1969. Geology of the King Peninsula, Canisteo Peninsula, and Hudson Mountains areas, Ellsworth Land, Antarctica. Antarctic Journal of the U.S., IV(4): 92-93. Wade, F. A., and J . R. Wilbanks. In press. The geology of Marie Byrd Land and Ellsworth Land, Antarctica. In: Antarctic Geology and Geophysics. Oslo, Universitetsforlaget. Warner, L. A. .1945. Structure and petrography of the southern Edsel Ford Ranges, Antarctica. American Philosophical Society. Proceedings, 89: 78-122.
Triassic tetrapods from McGregor Glacier EDWIN H. COLBERT
Museum of Northern Arizona During the antarctic field season of 1969-1970, as readers will recall, a considerable collection of Lower Triassic tetrapods was made from the Fremouw Formation at Coalsack Bluff in the Transantarctic Mountains, immediately to the east of the Beardmore Gla188
Labyrinthodont amphibian skull from McGregor Glacier area.
cier. It had been intended to extend the work of that season to the region of McGregor Glacier, some 240 km distant, but weather and circumstances prevented it. Consequently, this aspect of the field campaign was resumed during the 1970-1971 season. Paleontological prospecting and collecting was carried on by Mr. James W. Kitching of the Bernard Price Institute for Palontology, University of the Witwatersrand, Johannesburg, South Africa, assisted by Mr. John Ruben of the University of California at Berkeley, and for a short time by Mr. Thomas Rich of the American Museum of Natural History in New York. It had been hoped that well preserved fossils would be found in the McGregor Glacier region, since previous studies had indicated that this area was up the paleoslope from Coalack Bluff, and thus might be close to the source of fossil burials. The hope was fully justified: whereas at Coalsack Bluff the fossils, though numerous, consisted of isolated and rolled bones deposited in coarse sands and even conglomerates, at McGregor Glacier the specimens consisted of articulated skeletons and partial skeletons contained within rather fine-grained siltstones (see photo). As a result, our knowledge of early Triassic antarctic tetrapods has been augmented and expanded. The work of this past season was aided by good weather, a contrast to the inclement weather that plagued the paleontologists during the previous collecting field season. The initial fossil of this past season, an imprint of a complete skeleton of the mammal-like reptile Thri naxodon, was found on the first day in the field Dr. James Collinson of The Ohio State University, ANTARCTIC JOURNAL
