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

Home , Queen Alexandra Range

West Antarctica, has been computed with the use of a Bentley, C. R., and J . W. Clough. 1972. Seismic refraction finite difference technique. Results agree well with ob- shooting in Ellsworth and Dronning Maud Lands. In: Ant- served data. Comparison with calculations based on arctic Geology and Geophysics (R. J . Adie, ed.). Oslo, Uni- versitetsforlaget. 169-172. approximations commonly made in surface wave analyses Clough, J . W. 1973. Radio-echo sounding: brine percolation 4 (Poissons ratio = 1/ ; density = constant) surprisingly layer. Journal of Glaciology, 12(64): 141-143. shows that the group velocities are relatively more sensi- Kohnen, H. 1971. The relation between seismic urn structure, tive to incorrect densities than to incorrect shear wave temperature, and accumulation. Zeitschrift für Gletscherkunde velocities. und Glazialgeologie, VII( I-2): 141-151. Kohnen, H. 1972. Uber die beziehung zwischen seismischen 7. Final strain-rate calculations for a grid network geschwindigkeiten und der dichte in firn and eis. Zeitschrift across Roosevelt Island show a strongly asymmetrical für Geophysik, 38: 925-935. profile, with the longitudinal extensional strains twice as Kohnen, H., and C. R. Bentley. 1973. Seismic refraction and re- great on the northeast as on the southwest flank of the flection measurements at Byrd Station, Antarctica. Journal of Glaciology, 12(64): 101-111. island. Since accumulation rates on the two flanks are Kososki, B. A. 1972. A gravity study of West Antarctica. M. S. about the same the difference in strain rates is probably Thesis, University of Wisconsin. attributable to the effect of the Ross Ice Shelf. Robertson, J. D. 1972. A seismic study of the structure and metamorphism of 6rn in West Antarctica. M. S. Thesis, Uni- versity of Wisconsin.

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

Bentley, C. R., H. K. Acharya, J . E. Beitzel, and J . W. Clough. 1969. Analysis of antarctic geophysical data, 1968-1969. Ant- G. FAURE and R. L. HILL arctic Journal of the United States, IV(5) 219. Gow, A. J . 1968. Electrolytic conductivity of snow and glacier Department of Geology and Mineralogy ice from Antarctica and Greenland. Journal of Geophysical Institute of Polar Studies Research, 73(12): 3643-3649. The Ohio State University Kuroiwa, D. 1964. Internal friction of ice. Contributions from the Institute of Low Temperature Science, Hokkaido University. Series A. 18. A whole-rock rubidium-strontium age determination of tuff from the Triassic Falla Formation, containing Dichroidiurn odontopieroides, indicates a date of 190±9 million years. Bibliography Five whole-rock specimens collected from the type section located 293 to 414 meters above the base of the Falla Formation on the northwest face of Mt. Falla, Acharya, H. K. 1970. Reflection from the free surface of an in- Queen Alexandra Range, were analyzed for an age de- homogeneous media. Bulletin of the Seismological Society of termination by using the rubidium -strontium method. America, 60(4): 1101-1104. P. J Acharya, H. K. 1972. Surface-wave dispersion in Byrd Land, . Barrett collected the samples from his section F-2 Antarctica. Bulletin of the Seismological Society of Amer- (Barrett, 1968). He described these rocks as fine-grained ica, 62(4): 955-959. tuffs composed of fresh to slightly devitrified or zeolitized Acharya, H. K. In press. Investigation of surface wave disper- glass shards, and grains of quartz and plagioclase in a sion in inhomogeneous media by the finite difference method. matrix with low birefringence which is not optically re- Proceedings of the Ninth Annual Symposium on Geophysi- cal Theory and Computer Applications. solvable. Barrett (1968) reported finding Dicroidiunu Bentley, C. R. 1972. Seismic-wave velocities in anisotropic ice: odontoptet-oides in a shale bed 135 meters above the a comparison of measured and calculated values in and base of the Falla Formation, at the type locality. Accord- around the deep drill hole at Byrd Station, Antarctica. jour- ing to Townrow (1967), this fossil occurs elsewhere nal of Geophysical Research, 77(23): 4406-4420. in rocks of Middle to Upper Triassic age. Bentley, C. R., 1972. Suglacial rock surface topography of Ant- arctica. Antarctic Map Folio Series, 16. The samples used in this report were originally ana- Bentley, C. R. In press. Crustal structure of Antarctica. Pro- lyzed by Hill (1969), who calculated a whole-rock ceedings of IUMC Symposium: crustal structure based on rubidium-strontium isochron date of 203±12 million seismic data. Tectono physics. years, based on four of the five analyzed spçcimens. The Bentley, C. R., and J W. Clough. 1971. Electromagnetic sound- . only other age determination of the Falla Formation is a ing of ice thickness In: Propagation Limitations in Remote Sensing (J. B. Lomax, ed.). AGARD Conference Proceed- whole-rock potassium-argon date of 197.7±2.7 million ings, North Atlantic Treaty Organization. 90: 18-1-18-7. years for a trachyte pebble collected 280 meters above

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the base of the formation on Mount Falla (Barrett and Elliot, 1972; Barrett, 1972). FALLA FORMATION We redetermined the rubi di urn and strontium concen - Sr 87 QUEEN ALEXANDRA RANGE trations of all of Hills samples by using X-ray fluores- cence and U.S. Geological Survey rock standards to Sr 86 obtain a calibration curve. The results (table) are a combinaton of Hills data and the new analyses and 0.760 E_- represent our best estimates of these values at this time. }324 0.750 An isochron, shown in the figure, was applied to the data by using the regression method of York (1966). The 0.740 date indicated by the slope of the isochon is 190-+-9 323 million years (A Rh = 1.39 >< 10 11 per year). 0.730 The absolute time scale for the Triassic period still is ± 9 M.Y. in doubt (Tozer, 1 96i). According to the Geological 0.720 •321 Society of London time scale (Harland ci al., 1961), the 325 Sr87 0.710 = 0.7128 ±0.0096 age of the lowermost Triassic period (Tnduan) is 225

million years. More recently, Webb and McDougall 1 1 0.700- I I (1967) proposed an age of 235 million years for the 0 2 4 6 8 10 12 14 16 18 Permian-Triassic boundary. The age of the Triassic-Juras- sic boundary was placed by KuIp ( 1961 ) at 181 million Rb87 ! Sr 86 years and by Holmes (1959) at 180 million years. Per- haps the best-established date for the Upper Triassic Barrett, P. J . , and D. H. Elliot. 1972. The early Mesozoic Vol. (Norian) is the age of the Palisade sill of New Jersey, caniclastic Prebble Formation, Beardmore Glacier area. In: to which KuIp ( 1961 ) assigned a date of 193±3 million Antarctic Geology and Geophysics (R. J . Adie, ed.). Oslo. Universitetsforlaget. 403-409. years based on the potassium-argon method. The age of Barrett, P. J . 1968. The postglacial Permian and Triassic Beacon the Falla Formation consequently appears to be Upper rocks in the Beardmore Glacier area, central Transantarctic Triassic. Mountains, Antarctica. Ph.D. dissericion, The Ohio State This research was supported by National Science Foun- University. 5101-1. dation grant GA-898X. Harland, W. B., A. G. Smith, and B. Wilcock. 1964. The Phanerozoic time-scale. Quarterly Journal of the Geological Society of London, 1205: 458. Hill, R. L. 1969. Strontium isotope composition of basaltic References rocks of the Transantarctic Mountains, Antarctica. M.Sc. thesis, The Ohio State University. 87p. Barrett, P. J . 1972, Stratigraphy and petrology of the mainly Holmes, A. 1959. A revised geological time-scale. Edinburgh fluviatile Permian and Triassic part of the Beacon Super- Geological Society Transacilo nc, 17: 183-216. group, Beardmore Glacier area, In: Antarctic Geology and KuIp, J. L. 1961. Geologic time-scale. Science, 133: 1105-1114. Geophysics (R. J . Adie, ed.). Oslo. lJniversitetsforlaget. 365- Townrow, J . A. 1967. Fossil plants from Allen and Carapace 372. Nunataks, and from the upper Mill and Shackleton Glaciers,

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

Number Rubidium Strontium Rb Sr 87 Rb

(parts per million) Sr Sr8° Sr so

321 (17219) 144.6 123.7 1±69 0.7215 3.389 ± 0.027 ± 0.078

322 (F228) 240.2 46.44 5.172 0.7554 1504 4-0-047 ± 0.137

323 (F229) 209.2 58.673.566 0 .7 394 1036 ± 0.113 -1:0.328

324 (F240) 368.9 58.77 6.277 0.757418.26 ± 0.340 ± 0.989

325 (F245) 307.9 114.7 0.470 0.7161 1.132 ± 0.011 ±0.032

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

September-October 1973 265 Antarctica. New Zealand Journal of Geology and Geophysics, of magnitude from the northern Forrestal Range to the 10(2): 456-473. southern Dufek Massif is consistent with measured Tozer, E. T. 1964. The Triassic period. Supplement to the magnetic properties (including normal and reversed Quarterly Journal of the Geological Society of London, 1205: 207-209. remanent magnetization). This interpretation is sup- Webb, A. W., and I. McDougall. 1967. Isotopic dating evidence ported by theoretical magnetic models that suggest the on the age of the Upper Permian and Middle Triassic planet presence of a 4-kilometer fault across the front of the earth. Science Letters, 2: 483-488. Dufek Massif, down to the northwest. Models fitted York, D. 1966. Least-squares fitting of a straight line. Canadian to 100 Journal of Physics, 44: 1079-1086. to 200 gamma anomalies over the southern Dufek Massif require either a basal section 1 to 2 kilometers thick, of higher magnetization than that measured from rocks in the lowest exposed part of the section, or in- finitely thick bodies of the low magnetization actually Geophysical investigations of the observed. The first hypothesis is most reasonable and Pensacola Mountains and adjacent suggests a possible basal ultramafic layer. Magnetic and gravity data suggest a great extension glacierized areas beneath the ice of the Dufek intrusion. The magnetic data indicate a minimum areal extent of about 24,000 JOHN C. BEHRENDT, JOHN R. HENDERSON, and square kilometers and gravity data outside the magnetic WILLIAM RAMBO survey suggest an additional 10,000 square kilometers. This gives a total minimum estimate of 34,000 square U.S. Geological Survey kilometers, at least half of the area of the Bushveld complex. Other magnetic data suggest that the Dufek LAURENT MEISTER intrusion possibly continues as far north as Berkner Geophysical Service International Island. A number of magnetic and gravity anomalies of lim- Recent analyses of aeromagnetic, gravity, and seismic ited areal extent are associated with small scale geologic reflection measurements made in 1965-1966, in the Pen- sources within the Pensacola Mountains and beneath the sacola Mountains of Antarctica, have extended knowl- ice sheet. Precambrian diabase intrusions in the Schmidt edge of the geology beneath areas covered by thick ice. Hills area are inferred to be the sources of 50 gamma There is a broad regional Bouguer anomaly with grad- amplitude magnetic anomalies. A —200 gamma mag- ients parallel to the northwest edge of the Pensacola netic anomaly and a positive Bouguer anomaly in the Mountains block. Bouguer anomaly values decrease from Weber Ridge area at the north end of the Patuxent Range 82 milligals to —90 milligals across this transition from are thought to be caused by a mafic intrusion. There is West to East Antarctica. Theoretical profiles fitted to the a negative anomaly of at least —30 milligals amplitude gravity data indicate the presence of either an abnormally over the Median granite and Beacon(?) sedimentary thin crust on the west antarctic side, or a normal crust rocks in the Torbert Escarpment area relative to the on the west antarctic side with a steep step-like transi- Patuxent Formation in the Neptune Range. tion from West to East Antarctica. This transition sug- The free air anomaly data and the Bouguer anomaly- gests that a fault extends from the crust-mantle boundary elevation regression calculation suggest that the area is to near the surface in the vicinity of Schmidt Hills. in regional isostatic equilibrium. Gravity, magnetic, and seismic data suggest the existence of a thick section of low-velocity, low-density, nonmag- netic, presumably sedimentary rock beneath the ice northwest of the Pensacola Mountains. New data for a Cenozoic history of A least square regression of the Bouguer anomalies, Wright Valley, southern Victoria Land compared with elevation in the Pensacola Mountains area, suggests that the amplitude of the gravity anomaly M. J MCSAVENEY associated with the Dufek layered gabbroic intrusion is . about 85 milligals. This corresponds to about 8.8 to 6.2 The Institute of Polar Studies kilometers thickness for the intrusion, assuming reason- The Ohio State University able density contrasts. Magnetic anomalies approaching 2,000 gammas in amplitude are associated with the in- Recent reinterpretations of the pecten locality in trusion. The decrease in amplitudes of one to two orders Wright Valley, south Victoria Land (Webb, 1972a, 1972b; McSaveney and McSaveney, 1972; Brooks, 1972; Publication authorized by the director of the U.S. Geological Vucetich and Topping, 1972), investigations of high- Survey. level till deposits at Shapeless Mountain and Mount

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