Brown, G. M. 1957. Pyroxenes from the early and middlestages of fractionation of the Skaergaard intrusion, EastGreenland. Mineralogical Magazine, 31: 511-543.

Brown, G. M., and E. A. Vincent. 1963. Pyroxenes from thelate stages of fractionation of the Skaergaard intrusion, EastGreenland. Journal of Petrology, 4: 175-197.

Ford, A. B. 1970. Development of the layered series andcapping granophyre of the Dufek intrusion of Antarctica.In: Symposium on the Bushveld Igneous Complex and OtherLayered Intrusions (D. J . L. Visser and G. von Gruenewaldt,eds.). Geological Society of South Africa. Special Publica-tion, 1: 494-510.

Ford, A. B., and W. W. Boyd, Jr. 1968. The Dufek intru-sion, a major stratiform gabbroic body in the PensacolaMountains, Antarctica. Proceedings of the 23rd Interna-tional Geological Congress, 2: 213-228.

Hess, H. H. 1941. Pyroxenes of common mafic magmas.American Mineralogist, 26: 515-535, 573594.

Hess, H. H. 1960. Stillwater igneous complex, Montana: aquantitative mineralogical study. Geological Society of Amer-ica. Memoir, 80. p. 121.

Jackson, E. D. 1970. The cyclic unit in layered intrusions-acomparison of repetitive stratigraphy in the ultraniafic partsof the Stillwater, Great Dyke, and Bushveld Complexes. In:Symposium on the Bushveld Igneous Complex and OtherLayered Intrusions (D. J . L. Visser and G. von Gruenewaldt,eds.). Geological Society of South Africa. Special Publi-cation, 1: 391-424.

Page, N. J . , R. Shimek, and R. Huffman, Jr. 1972. Grain-sizevariations within an olivine cumulate, Stillwater Complex,Montana. U.S. Geological Survey. Professional Paper, 800-C,C29-C37.

Poldervaart, A., and H. H. Hess. 1951. Pyroxenes in thecrystallization of basaltic magma. Journal of Geology, 59,472-489.

Walker, K. R., N. G. Ware, and J . R. Lovering. 1973. Com -positional variations in the pyroxenes of the differentiatedPalisades Sill, New Jersey. Geological Society of America.Bulletin, 84, 89-110.

Analysis of antarctic geophysical data

C. R. BENTLEY, H. K. ACHARYA, J . L. CLAPP,

J . W. CLOUGH, H. KOHNEN, and J . D. ROBERTSON

Geophysical and Polar Research CenterDepartment of Geology and Geophysics

University of Wisconsin, Madison

Continuing analysis of antarctic geophysical data fol-lows several lines, including studies of ice properties (asrevealed by seismic and electromagnetic wave propaga-tion experiments near Byrd Station), west antarcticgravity maps, Roosevelt Island strain data, and theo-retical studies of seismic wave propagation. Appendedis a bibliography of papers on these subjects (since

Contribution number 299, Geophysical and Polar ResearchCenter, Department of Geology and Geophysics, University ofWisconsin, Madison.

Bentley et al., 1969). What follows is a summary ofrecent results not yet published.

1. Seismic velocities obtained from short refractionprofiles can be used to predict density at depths betweeno and 10 meters, with a standard error of about 0.01gm/cm3.

2. A newly recognized and extensive horizon at depthsof 25 to 30 meters, marking an apparent change in thedensification rate, has been found in West Antarctica.The horizon's existence suggests that two distinct mech-anisms successively dominate the metamorphic processbetween the depth of closest packing of snow grains andthe urn-ice boundary.

3. Measurement of P-wave attenuation in ice nearByrd Station led to the determination of a very low valuefor the internal friction. From comparison with labora-tory measurements (Kuroiwa, 1964), it appears that aslight but significant contamination of the antarctic iceby ionic impurities (Gow, 1968) and the ambient icetemperature (-28 0 C.) result in the falling of seismicfrequencies at a dissipation minimum between spectralregions dominated respectively by grain-boundary phe-nomena, and by the fundamental relaxation spectrum.

4. Analysis of electromagnetic wide-angle reflectionmeasurements shows that correction for refraction in theupper portion of an ice sheet need not be made whencalculating mean velocities. Even for a reflector as shal-low in depth as 100 meters, the error introduced byassuming straight line geometry is only about 10 nanosec,well below the time resolution of the measurements. Foraccurate measurements, however, the length of wide-angle profiles must be limited to distances correspondingto the reflection path for a ray at grazing incidence onthe surface. The amplitude of the reflected wave, whichchanges markedly along the length of a profile, can playan important role in the measurements.

5. Previously existing maps of gravity anomalies inWest Antarctica have been supplemented by new dataand have been contoured by a computer. Three-dimensional modeling has been used to prepare an Airy isostaticgravity anomaly map. This map reveals several imbal-ances, which may be caused by: up-warping of the M-discontinuity in Ellsworth Land and beneath the Hollick-Kenyon Plateau; dense, lower-crustal material unusuallynear the surface southwestward from the WhitmoreMountains; the extension beneath the Rockefeller Plateauof a pre-Cretaceous geosyncline known to exist in theEdsel Ford Ranges; the southern boundary of the Ceno-zoic volcanic province in Marie Byrd Land. A deepnegative anomaly of unknown cause exists along theBakutis Coast. On the hypothesis of a recent retreat ofthe ice sheet, no more than 40 percent (and probablymuch less) of the anomaly can be attributed to incom-plete isostatic rebound.

6. A Rayleigh wave group-veloiity curve, applicableto the known velocity-depth and density-depth curves in

September-October 1973 263

West Antarctica, has been computed with the use of afinite difference technique. Results agree well with ob-served data. Comparison with calculations based onapproximations commonly made in surface wave analyses(Poisson's ratio = 1/4 ; density = constant) surprisinglyshows that the group velocities are relatively more sensi-tive to incorrect densities than to incorrect shear wavevelocities.

7. Final strain-rate calculations for a grid networkacross Roosevelt Island show a strongly asymmetricalprofile, with the longitudinal extensional strains twice asgreat on the northeast as on the southwest flank of theisland. Since accumulation rates on the two flanks areabout the same the difference in strain rates is probablyattributable to the effect of the Ross Ice Shelf.

Bentley, C. R., and J . W. Clough. 1972. Seismic refractionshooting in Ellsworth and Dronning Maud Lands. In: Ant-arctic Geology and Geophysics (R. J . Adie, ed.). Oslo, Uni-versitetsforlaget. 169-172.

Clough, J . W. 1973. Radio-echo sounding: brine percolationlayer. Journal of Glaciology, 12(64): 141-143.

Kohnen, H. 1971. The relation between seismic urn structure,temperature, and accumulation. Zeitschrift für Gletscherkundeund Glazialgeologie, VII( I-2): 141-151.

Kohnen, H. 1972. Uber die beziehung zwischen seismischengeschwindigkeiten und der dichte in firn and eis. Zeitschriftfür Geophysik, 38: 925-935.

Kohnen, H., and C. R. Bentley. 1973. Seismic refraction and re-flection measurements at Byrd Station, Antarctica. Journal ofGlaciology, 12(64): 101-111.

Kososki, B. A. 1972. A gravity study of West Antarctica. M. S.Thesis, University of Wisconsin.

Robertson, J . D. 1972. A seismic study of the structure andmetamorphism of 6rn in West Antarctica. M. S. Thesis, Uni -versity of Wisconsin.

References

Bentley, C. R., H. K. Acharya, J . E. Beitzel, and J . W. Clough.1969. Analysis of antarctic geophysical data, 1968-1969. Ant-arctic Journal of the United States, IV(5) 219.

Gow, A. J . 1968. Electrolytic conductivity of snow and glacierice from Antarctica and Greenland. Journal of GeophysicalResearch, 73(12): 3643-3649.

Kuroiwa, D. 1964. Internal friction of ice. Contributionsfrom the Institute of Low Temperature Science, HokkaidoUniversity. Series A. 18.

Bibliography

Acharya, H. K. 1970. Reflection from the free surface of an in-homogeneous media. Bulletin of the Seismological Society ofAmerica, 60(4): 1101-1104.

Acharya, H. K. 1972. Surface-wave dispersion in Byrd Land,Antarctica. Bulletin of the Seismological Society of Amer-ica, 62(4): 955-959.

Acharya, H. K. In press. Investigation of surface wave disper-sion in inhomogeneous media by the finite difference method.Proceedings of the Ninth Annual Symposium on Geophysi-cal Theory and Computer Applications.

Bentley, C. R. 1972. Seismic-wave velocities in anisotropic ice:a comparison of measured and calculated values in andaround the deep drill hole at Byrd Station, Antarctica. jour-nal of Geophysical Research, 77(23): 4406-4420.

Bentley, C. R., 1972. Suglacial rock surface topography of Ant-arctica. Antarctic Map Folio Series, 16.

Bentley, C. R. In press. Crustal structure of Antarctica. Pro-ceedings of IUMC Symposium: crustal structure based onseismic data. Tectono physics.

Bentley, C. R., and J . W. Clough. 1971. Electromagnetic sound-ing of ice thickness In: Propagation Limitations in RemoteSensing (J. B. Lomax, ed.). AGARD Conference Proceed-ings, North Atlantic Treaty Organization. 90: 18-1-18-7.

Age of the Falla Formation (Triassic),Queen Alexandra Range

G. FAURE and R. L. HILL

Department of Geology and MineralogyInstitute of Polar StudiesThe Ohio State University

A whole-rock rubidium-strontium age determinationof tuff from the Triassic Falla Formation, containingDichroidiurn odontopieroides, indicates a date of 190±9million years.

Five whole-rock specimens collected from the type sec-tion located 293 to 414 meters above the base of theFalla Formation on the northwest face of Mt. Falla,Queen Alexandra Range, were analyzed for an age de-termination by using the rubidium -strontium method.P. J . Barrett collected the samples from his section F-2(Barrett, 1968). He described these rocks as fine-grainedtuffs composed of fresh to slightly devitrified or zeolitizedglass shards, and grains of quartz and plagioclase in amatrix with low birefringence which is not optically re-solvable. Barrett (1968) reported finding Dicroidiunuodontoptet-oides in a shale bed 135 meters above thebase of the Falla Formation, at the type locality. Accord-ing to Townrow (1967), this fossil occurs elsewherein rocks of Middle to Upper Triassic age.

The samples used in this report were originally ana-lyzed by Hill (1969), who calculated a whole-rockrubidium-strontium isochron date of 203±12 millionyears, based on four of the five analyzed spçcimens. Theonly other age determination of the Falla Formation is awhole-rock potassium-argon date of 197.7±2.7 millionyears for a trachyte pebble collected 280 meters above

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