Pensacola Mountains Geologic Project*DWIGHT L. SCHMIDT and A. B. FORD

U.S. Geological Survey (Denver)

The U.S. Geological Survey has been conductinglaboratory studies and compiling field data on thegeology of the Pensacola Mountains since the lastfield investigation in the area during the 1965-1966 austral summer (Huffman and Schmidt, 1966).

The discovery by Schmidt and Willis H. Nelson(Schmidt and Ford, 1966) of folded sedimentaryrocks containing a glossopterid flora in the northernPensacola Mountains indicates an orogeny of latestPaleozoic or younger age that is not recognized else-where in the Transantarctic Mountains (a part ofnorthern Victoria Land may be an additional excep-tion). This folding suggests a structural tie withthe Ellsworth Mountains, where glossopterid-bearingrocks are also folded.

James M. Schopf's correlation of the Gale Mud-stone in the Pensacola Mountains and the WhiteoutConglomerate in the Ellsworth Mountains with theBuckeye Tillite in the Ohio Range seems valid(Nelson et al, in press). Plant microfossils suggestan Early Permian age for fluvial beds near themiddle of the Buckeye. The glossopterid-bearingcoal measures of the Pensacola Mountains correlatewith the Polarstar Formation of the EllsworthMountains and the Mount Glossopteris Formationof the Ohio Range.

A. B. Ford and W. W. Boyd, Jr., believe that theDufek stratiform intrusion of post-Permian age inthe northern Pensacola Mountains was probably in-volved in at least the later part of the folding ofthe adjacent Upper Paleozoic sedimentary rocks(Schmidt and Ford, 1966). The interpretation ofaeromagnetic anomalies by J. R. Henderson andJ. C. Behrendt indicates that the Dufek gabbrounderlies an area of at least 9,500 km2 , making itone of the world's largest stratiform bodies (Beh-rendt et a!, 1966). The stratiform body is consid-erably more than 4 km and perhaps as much as 7km thick. The intrusion consists predominantly ofpyroxene gabbro interlayered with lesser amountsof anorthosite and pyroxenite, and it is capped by atleast 300 in granophyre. Neither the base nortop of the body is exposed; lateral contacts are dis-cordant against folded Paleozoic sedimentary coun-try rocks.

Systematicmagmaticdifferentiationduringcrystallization of the intrusive gabbro is indicated by

*publication authorized by the Director, U.S. GeologicalSurvey.

mineralogic and petrographic studies by Ford andBoyd; the anorthite content of the plagioclase de-creases upward, whereas the quartz and alkali-feld-spar content increases upward, as do the iron-oxideminerals, chiefly magnetite. Studies of the pyroxenesand isotopic age determinations are in progress.

The direction and intensity of remanent mag-netization and magnetic susceptibility are beingstudied by Myrl E. Beck, Jr., assisted by Nancy C.Lindsley (Beck et a!, in press). Such measurementsaid in stratigraphic correlations within the stratiformbody. At least two zones of opposite magnetic po-larity occur within the body. The gabbros of theDufek Massif consistently indicate a paleomagneticpole position that is significantly different from otherpublished paleomagnetic-pole positions for Ant-arctica.

References

Beck, M. E., Jr., A. B. Ford, and W. W. Boyd, Jr. Inpress. Paleomagnetism of gabbroic rocks of the Dufekstratiform intrusion, Pensacola Mountains, Antarctica.Science.

Behrendt, J. C., J. R. Henderson, and J. R. Meister. 1966.Airborne geophysical study in the Pensacola Mountainsof Antarctica. Science, 153 (3742): 1373-1376.

Huffman, J. W., and D. L. Schmidt. 1966. Pensacola Moun-tains Project. Antarctic Journal of the United States, 1(4): 123-124.

Nelson, W. H., D. L. Schmidt, and J. M. Schopf. In press.Structure and stratigraphy of the Pensacola Mountains,Antarctica. Geological Society of America Special Pa-per. (Abstracts to Geological Society of America Meet-ing, Santa Barbara, March 22-25, 1967).

Schmidt, D. L. and A. B. Ford. 1966. Geology of thenorthern Pensacola Mountains and adjacent areas. Ant-arctic Journal of the United States, 1 (4) : 125.

Magnetic Signature of Rocks fromEllsworth Land

PETER J. WASILEWSKIDepartment of Earth and Planetary Sciences

University of Pittsburgh

One of the most important objectives of antarcticgeoscience research should be the clear definitionof geologic provinces and their boundaries. Thisneed becomes clear when one considers the proposedreconstructions of the southern continents whereinWest Antarctica cannot be accommodated. Onlywhen South America and the Antarctic Peninsulaare considered separately do the reconstructionsmake any sense. Thus the definition of geologicprovinces, particularly those of West Antarctica, isessential to understanding both the intra- and inter-

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continental relationships that exist. The authorbelieves that the Antarctic Peninsula, with its kindredintrusives, extends into Ellsworth Land and can bedefined as a geologic province. Its boundaries canbe established by reference to the magnetic signaturesof the Ellsworth Land intrusives and associatedsedimentary and volcanic rocks. The Ellsworth Landintrusives, which invade Jurassic sedimentary strataand are associated with probable Jurassic or olderdacitic volcanics, describe the southern terminus ofpeninsular intrusives and may reveal that the westernboundary lies along a NNW-SSE line east of the nowinactive Eights Station.

The above assessments were made on the basisof rock samples and magnetometer data obtainedduring the 1965-1966 traverse into the Ellsworthnunatak area (about 160 kin of Eights Station).The rock samples were collected at 35 locations,and the magnetometer data were taken over a dis-tance of about 600 km.

Fixing the magnetic signature of the region hasincluded making determinations of scalar and vectormagnetic properties. The available chemical datafrom representative igneous rocks suggest a simpletrend in chemical differentiation for the Ellsworthintrusives. A plot of total iron content against silicacontent shows a linear trend related to silica content,but no clearly defined trend is observed in a plotof titania content against silica content. The iron andtitania are of particular importance because theopaque oxides of the Ti0 2-FeO-Fe203 ternary sys-tem are responsible for magnetization in rocks. Themeasured magnetic properties do not correlate withthe chemical trends, and it is suggested that themagnetic properties reflect the particular physico-chemical environment of the plutonic rock bodyduring emplacement.

The magnetic crust in Ellsworth Land consists ofthe following components: (1) magnetic and non-magnetic plutons of acid-to-basic composition (acid,> 65 percent Si02 ; intermediate, < 65 percent but> 52 percent Si0 2 ; basic, < 52 percent Si02);(2) nonmagnetic volcanics of acid-to-intermediatecomposition; and (3) local extremes in magnetic andnonmagnetic signature due to iron enrichment andresidual silica-rich intrusives, respectively. Intensitiesranging from 10-1 to 10-6 emu/cm3 and suscep-tibilities ranging from 10-2 to 10 emu/cm3 havebeen measured.

Measurements of many cores drilled from theoriented samples show, within the same sample,normal and nearly reversed directions of magneti-zation as well as directional variability related tocore depth. Significant intensity and directionalresponse to small shocks have been recorded,

suggesting that hammering on samples and drillingcores can seriously affect the magnetization. Becausea fundamental precept of paleomagnetic research(i.e., the dipole approximation) is violated in somecores, the usefulness of such cores for paleomagneticanalyses is being investigated.

Owing to these conditions and the extremevariability of magnetic stability in cored specimens,an alternative approach to studying the magneticproperties of rocks is being developed. In thisapproach, which at present we refer to as "mag-netic petrology," primary significance is placed onthe mineralogy associated with magnetic properties.This enables us to plan experiments according towhat we "see." In this connection, a magnetitecolloid was developed to reveal a clear distinctionbetween magnetic and nonmagnetic opaque grainsand phases (Wasilewski and Carleton, 1967).

A short-term secular variation of the geomagnetictotal force is suggested from data collected duringthe 1961-1962 and 1965-1966 field seasons (Wasi-lewski, 1966). An approximate lOOy decrease intotal force is apparent, in accordance with trendssuggested by Nagata (1964) and Orlov (1965).

Certain tentative conclusions can be drawn frominvestigations conducted to date:

Ellsworth Land, specifically the area from EightsStation eastward, is a separate tectonic element ofWest Antarctica.

A series of intrusive bodies is responsible for thebroad, high-amplitude magnetic anomalies, and theEllsworth intrusives mark the southern terminationof such bodies (if one assumes that the Ellsworthintrusives are kin to the peninsular intrusives).

There is a wide range in the magnetism of thecrust with magnetic and essentially nonmagneticplutonics of intermediate-to-basic character.

The types of magnetic crustal segments seem tobe related to pre-intrusive volcanism.

The ice sheet was at least 400 m thicker at sometime in the past than it is at present, and thesculpting of exposed rocks is due to glacial action.

The secular-variation tendency for most of WestAntarctica, as determined on the basis of short-termobservational data, is - 100 -y/year, which is inagreement with Nagata (1963), Orlov (1965), andSlaucitajs (1966).

Chemically, the Ellsworth intrusions resemble adifferentiated suite, but magnetically they bear norelation to basicity, the reason for which is not yetapparent.

A geologic province which is bounded on the westat about 80°W., on the south at 76°S., and on theeast at the boundary of the Filchner Ice Shelf breaksWest Antarctica into distinct provinces. Unfortu-

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nately, most of the detail necessary for more completedelineation of these provinces is not available.

ReferencesNagata, Takesi. 1964. Magnetic field at the Poles. Re-

search in Geophysics, 1: 423-453.Orlov, V. P. 1965. The leading trends of the secular varia-

tion investigation. Journal of Geomagnetism and Geo-electricity, 17: 277-286.

Slaucitajs, L. 1966. On geomagnetic secular variation inthe region of Antarctic Peninsula and Weddell Sea. Journalof Geomagnetism and Geoelectricity, 18:103-104.

Wasilewski, P. J. 1966. Geomagnetic secular variation inEllsworth Land, Antarctica. Journal of Geoinagnetis,'nand Geoelectricity, 18: 489-491.

Wasilewski, P. J. and B. J. Carleton. 1967. Insight intothe magnetic mineralogy of antarctic rocks. Journal ofGeotnagnetism and Geoelectricity, September issue.

Seismology at Byrd and South PoleStations

J. F. LANDER

U.S. Coast and Geodetic SurveyEnvironmental Science Services Administration

Since 1957, seismographs have been operated bythe Coast and Geodetic Survey at Byrd and SouthPole Stations. These sensitive instruments have shownAntarctica to be the least active seismically of allof the continental land masses. No earthquakes havebeen located instrumentally in Antarctica, althoughminor tremors have occasionally been recorded inthe vicinity of Mount Erebus. The seismic ob-servatories have significantly added to the number ofepicenters which can be located in the SouthernHemisphere. Of even more importance is the factthat they have added southern control, thus greatlyenhancing the accuracy of the locations.

In February 1963, South Pole Station wasequipped with the sophisticated instruments of theWorld-Wide Standardized Seismograph Network,which consists of 114 stations with matched in-strumentation. This network has been a cornerstonefor research in seismology because it provides,from one source and at a nominal cost, copies ofseismograms from stations around the world. Theseismograph at South Pole Station, being at aseismically quiet site, has been able to sustain amagnification setting of 100,000, the most sensitiveat which seismographs have ever operated on theContinent and among the highest obtainable any-where.

The data from Byrd and South Pole Stationshave been used to study storm-generated micro-seisms, continental structures of Antarctica asdeduced from surface-wave dispersion, and thecircum-Ant arctic a belt of seismicity related to theoceanic rises. The two stations support largerinvestigations of the velocity distribution of seismicwaves in the Earth's mantle and core, worldseismicity, travel-time anomalies, and the magnitudeof seismic events. The data obtained, which arereported by telegram to permit the rapid locationof epicenters, are published later in the AntarcticSeismological Bulletin.

Geophysical Investigations*

JOHN C. BEHRENDT

U.S. Geological Survey (Denver)

Gravity increase at the South Pole. Measurementsmade between December 1957 and January 1966of the gravity difference between the McMurdoSound pendulum station, which is on bedrock, andthe South Pole station, which is on the antarctic icesheet, show that the gravity at the South Pole hasincreased by 0.11 milligals per year. The mostlikely hypothesis is that the increase is being causedby ice flowing downslope across a gravity gradientand by the sinking of South Pole Station becauseof the ice accumulation. An alternate hypothesis,that the gravity increase is being caused by a decreasein ice thickness of about 40 cm per year, is theoreti-cally possible but is not supported by direct evidence.

Magnetic maps of Antarctica. Data on absolutetotal magnetic intensity collected in the area south of55°S. by expeditions of Australia, Great Britain,Japan, New Zealand, the United States, and theU.S.S.R. have been used to compile a map ofresidual total magnetic intensity. Weighted means ofresiduals of observed data compared with the 1965epoch map (U.S. Naval Oceanographic Office),corrected for secular variation, were computed fortwo-degree squares. Residual anomalies exceeding+ 400 and - 600 gammas extend over large areas.Generally, West Antarctica appears to have a morepositive residual anomaly than East Antarctica.There is a transition zone from positive to negativewhich is roughly coincident with the TransantarcticMountains; other geophysical evidence indicates thatthese mountains may be related to a structural dis-continuity in the crust.

*publication authorized by the Director, U.S. GeologicalSurvey.

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