Age of the Falla Formation (Triassic), Queen Alexandra Range · 90: 18-1-18-7. Age of the Falla Formation (Triassic), Queen Alexandra Range G. FAURE and R. L. HILL Department of Geology

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

264 ANTARCTIC JOURNAL

the base of the formation on Mount Falla (Barrett andElliot, 1972; Barrett, 1972).

We redetermined the rubi di urn and strontium concen -trations of all of Hill's samples by using X-ray fluores-cence and U.S. Geological Survey rock standards toobtain a calibration curve. The results (table) are acombinaton of Hill's data and the new analyses andrepresent our best estimates of these values at this time.An isochron, shown in the figure, was applied to the databy using the regression method of York (1966). Thedate indicated by the slope of the isochon is 190-+-9million years (A Rh' = 1.39 >< 10 11 per year).

The absolute time scale for the Triassic period still isin doubt (Tozer, 1 96i). According to the GeologicalSociety of London time scale (Harland ci al., 1961), theage of the lowermost Triassic period (Tnduan) is 225million years. More recently, Webb and McDougall(1967) proposed an age of 235 million years for thePermian-Triassic boundary. The age of the Triassic-Juras-sic boundary was placed by KuIp ( 1961 ) at 181 millionyears and by Holmes (1959) at 180 million years. Per-haps the best-established date for the Upper Triassic(Norian) is the age of the Palisade sill of New Jersey,to which KuIp ( 1961 ) assigned a date of 193±3 millionyears based on the potassium-argon method. The age ofthe Falla Formation consequently appears to be UpperTriassic.

This research was supported by National Science Foun-dation grant GA-898X.

References

Barrett, P. J . 1972, Stratigraphy and petrology of the mainlyfluviatile Permian and Triassic part of the Beacon Super-group, Beardmore Glacier area, In: Antarctic Geology andGeophysics (R. J . Adie, ed.). Oslo. lJniversitetsforlaget. 365-372.

FALLA FORMATIONQUEEN ALEXANDRA RANGE

E_-}324

323

321 ± 9 M.Y.• 325 Sr87 = 0.7128 ±0.0096

0.700- 11 I I

024681012141618

Rb87 ! Sr 86

Barrett, P. J . , and D. H. Elliot. 1972. The early Mesozoic Vol.caniclastic Prebble Formation, Beardmore Glacier area. In:Antarctic Geology and Geophysics (R. J . Adie, ed.). Oslo.Universitetsforlaget. 403-409.

Barrett, P. J . 1968. The postglacial Permian and Triassic Beaconrocks in the Beardmore Glacier area, central TransantarcticMountains, Antarctica. Ph.D. dissericion, The Ohio StateUniversity. 5101-1.

Harland, W. B., A. G. Smith, and B. Wilcock. 1964. ThePhanerozoic time-scale. Quarterly Journal of the GeologicalSociety of London, 1205: 458.

Hill, R. L. 1969. Strontium isotope composition of basalticrocks of the Transantarctic Mountains, Antarctica. M.Sc.thesis, The Ohio State University. 87p.

Holmes, A. 1959. A revised geological time-scale. EdinburghGeological Society Transacilo nc, 17: 183-216.

KuIp, J . L. 1961. Geologic time-scale. Science, 133: 1105-1114.Townrow, J . A. 1967. Fossil plants from Allen and Carapace

Nunataks, and from the upper Mill and Shackleton Glaciers,

Sr 87Sr 86

0.760

0.750

0.740

0.730

0.720

0.710

Concentrations of rubidium and strontium and Sr 87/Srratios of tuff from the Falla Formation.

Number Rubidium Strontium Rb(parts per million) Sr

321 (17219) 144.6

* Corrected for isotope fractionation to Sr 86 /Sr 8 = 0.1194Elmer and Amend strontium isotope standard, Sr"'/Sr'(' = 0.7082

322 (F228) 240.2

323 (F229) 209.2

324 (F240) 368.9

325 (F245) 307.9

123.7 1±69± 0.027

46.44 5.1724-0-047

58.67 3.566± 0.113

58.77

6.277± 0.340

114.7 0.470± 0.011

Sr 87*

RbSr8°

Sr so

0.7215 3.389± 0.078

0.7554

1504± 0.137

0 .7 394

1036-1:0.328

0.7574 18.26± 0.989

0.7161 1.132±0.032

September-October 1973 265

Antarctica. New Zealand Journal of Geology and Geophysics,10(2): 456-473.

Tozer, E. T. 1964. The Triassic period. Supplement to theQuarterly Journal of the Geological Society of London, 1205:207-209.

Webb, A. W., and I. McDougall. 1967. Isotopic dating evidenceon the age of the Upper Permian and Middle Triassic planetearth. Science Letters, 2: 483-488.

York, D. 1966. Least-squares fitting of a straight line. CanadianJournal of Physics, 44: 1079-1086.

Geophysical investigations of thePensacola Mountains and adjacent

glacierized areas

JOHN C. BEHRENDT, JOHN R. HENDERSON, andWILLIAM RAMBO

U.S. Geological Survey

LAURENT MEISTER

Geophysical Service International

Recent analyses of aeromagnetic, gravity, and seismicreflection measurements made in 1965-1966, in the Pen-sacola Mountains of Antarctica, have extended knowl-edge of the geology beneath areas covered by thick ice.There is a broad regional Bouguer anomaly with grad-ients parallel to the northwest edge of the PensacolaMountains block. Bouguer anomaly values decrease from82 milligals to —90 milligals across this transition fromWest to East Antarctica. Theoretical profiles fitted to thegravity data indicate the presence of either an abnormallythin crust on the west antarctic side, or a normal cruston the west antarctic side with a steep step-like transi-tion from West to East Antarctica. This transition sug-gests that a fault extends from the crust-mantle boundaryto near the surface in the vicinity of Schmidt Hills.Gravity, magnetic, and seismic data suggest the existenceof a thick section of low-velocity, low-density, nonmag-netic, presumably sedimentary rock beneath the ice north-west of the Pensacola Mountains.

A least square regression of the Bouguer anomalies,compared with elevation in the Pensacola Mountainsarea, suggests that the amplitude of the gravity anomalyassociated with the Dufek layered gabbroic intrusion isabout 85 milligals. This corresponds to about 8.8 to 6.2kilometers thickness for the intrusion, assuming reason-able density contrasts. Magnetic anomalies approaching2,000 gammas in amplitude are associated with the in-trusion. The decrease in amplitudes of one to two orders

Publication authorized by the director of the U.S. GeologicalSurvey.

of magnitude from the northern Forrestal Range to thesouthern Dufek Massif is consistent with measuredmagnetic properties (including normal and reversedremanent magnetization). This interpretation is sup-ported by theoretical magnetic models that suggest thepresence of a 4-kilometer fault across the front of theDufek Massif, down to the northwest. Models fittedto 100 to 200 gamma anomalies over the southern DufekMassif require either a basal section 1 to 2 kilometersthick, of higher magnetization than that measured fromrocks in the lowest exposed part of the section, or in-finitely thick bodies of the low magnetization actuallyobserved. The first hypothesis is most reasonable andsuggests a possible basal ultramafic layer.

Magnetic and gravity data suggest a great extensionbeneath the ice of the Dufek intrusion. The magneticdata indicate a minimum areal extent of about 24,000square kilometers and gravity data outside the magneticsurvey suggest an additional 10,000 square kilometers.This gives a total minimum estimate of 34,000 squarekilometers, at least half of the area of the Bushveldcomplex. Other magnetic data suggest that the Dufekintrusion possibly continues as far north as BerknerIsland.

A number of magnetic and gravity anomalies of lim-ited areal extent are associated with small scale geologicsources within the Pensacola Mountains and beneath theice sheet. Precambrian diabase intrusions in the SchmidtHills area are inferred to be the sources of 50 gammaamplitude magnetic anomalies. A —200 gamma mag-netic anomaly and a positive Bouguer anomaly in theWeber Ridge area at the north end of the Patuxent Rangeare thought to be caused by a mafic intrusion. There isa negative anomaly of at least —30 milligals amplitudeover the Median granite and Beacon(?) sedimentaryrocks in the Torbert Escarpment area relative to thePatuxent Formation in the Neptune Range.

The free air anomaly data and the Bouguer anomaly-elevation regression calculation suggest that the area isin regional isostatic equilibrium.

New data for a Cenozoic history ofWright Valley, southern Victoria Land

M. J . MCSAVENEY

The Institute of Polar StudiesThe Ohio State University

Recent reinterpretations of the pecten locality inWright Valley, south Victoria Land (Webb, 1972a,1972b; McSaveney and McSaveney, 1972; Brooks, 1972;Vucetich and Topping, 1972), investigations of high-level till deposits at Shapeless Mountain and Mount

266 ANTARCTIC JOURNAL