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DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

PREPARED IN C08PERATION WITH THE 

NATIONAL SCIENCE FOUNDATION 

SIMPLE BOUGUER GRA\7l'fY AND GENERALIZED GEOLOGIC MAP AND 

AEROMAGNETIC PROFILES OF THE SCHMIDT HILLS QUADRANGLE AND 

PART OF THE GAMBACORTA PEAK QUADRANGLE, ANTARCTICA 

By 

John C. Behrendt9 William Rambo9 John R. Henderson, and 

Laurent Meister 

GEOPHYSICAL INVESTIGATIONS 

MAP GP-889 

PUBLISHED BY THE U.S. GEOLOGICAL SURVEY 

WASHINGTON. D.C. 2024.4 

1973 

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DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

TO ACCOMPANY MAP GP-889 

SIMPLE BOUGUER GRAVITY AND GENERALIZED GEOLOGIC MAP AND 

AEROMAGNETIC PROFILES OF THE SCHMIDT HILLS QUADRANGLE 

AND PART OF THE GAMBACORTA PEAK QUADRANGLE, ANTARCTICA 

By John C. Behrendt, William Rambo, John R. Henderson, and 

Laurent Meister1 

INTRODUCTION 

During the 1965-66 austral summer we made reconnaissance gravity and magnetic surveys of the Pensacola 

Mountains and the adjacent ice-covered area, Antarctica. 

The gravity survey consisted of 397 observations on 

ice or rock and seismic reflection measurements of ice 

thkkness at 18 stations away from the mountains. 

Turbine helicopters were used and the work was coordinated with simultaneous geologic and topographic 

work by the U.S. Geological Survey. Reconnaissance 

aeromagnetic profiles were also flown perpendicular to 

the general northeast strike of the Pensacola Mountains. 

The objective of the geophysical surveys was to complement the geologic mapping in an effort to understand the tectonic development of the Pensacola Mountains and their relationship to the transition from West 

to East Antarctica. Behrendt, Meister, and Henderson 

( 1966) discussed preliminary results of the geophysical 

work. 

GRAVITY SURVEY 

We used one Worden and two LaCoste and Romberg 

geodetic gravimeters. The Pensacola survey base station 

at Camp Neptune (Schmidt Hills quadrangle) was tied 

four times, over a period of several hours each, to the 

McMurdo base station (Behrendt and others, 1962). 

An additional check on the base station was a reoccupation of one of the 1957 IGY (International Geophysical Year) traverse stations on bedrock in the Dufek 

Massif (Behrendt, 1962). A difference of +0.9 mgal 

was observed when gravity at the base station was compared with that at the IGY traverse station; this difference is acceptable considering that the older data were 

all tied to North America by ship and land-surface 

vehicle (Behrendt and others, 1962) over a period of 

months and years. All gravity observations in this 

survey were tied to the Pensacola base station within a 

few hours, and errors in observed gravity are negligible. 

Absolute elevations are considered accurate within 

±25 m (±5 mgal) and relative elevation within± 10m 

(±2 mgal). Two seismic reflections from the water-ice 

contact at the base of the Ronne Ice Shelf northwest 

of the Dufek Massif allowed a determination of sealevel elevation based on hydrostatic equilibrium. Corrections were made for changes in seismic velocity due 

to density increase in the upper ice shelf (Thiel and 

Behrendt, 1959); and absolute elevation accuracy was 

estimated conservatively, at the reflection stations, as 

± 10 m. The reflection stations were tied, one by vertical 

angle and one by altimetry, to the Pensacola Mountains 

control net. The elevations at stations in the survey 

were obtained from altimeter data corrected for temperature and for barometric-pressure variations at a central control station. In addition, 97 gravity stations were at 

control points where vertical angle observations were 

made as part of the topographic mapping control. The 

standard deviation of the unadjusted altimetry data at 

the vertical angle stations is ± 12 m. All altimeter elevations were adjusted by use of the 97 vertical angle stations 

as control. 

Positions of gravity stations located at points in the 

topographic control net are accurate to tenths of seconds; 

and stations away from the mountains where graphic 

solutions of astronomical observations were made are 

accurate to about a tenth of a minute. Therefore, latitude-correction errors are negligible. 

The largest source of error, and the most difficult to 

evaluate, is the terrain effect. Terrain corrections could 

not be made in the usual manner because of insufficient 

detail on the best maps ( 1 :250,000-scale, 200-m contour 

interval) available. A large unknown effect due to subglacial terrain could not have been corrected for, even 

if larger scale maps had been available. The corrections 

could be as great as several tens of milligals in certain 

cases, but experience suggests they should be no more 

than 10 mgal for most stations. We attempted to allow 

for terrain effects in contouring the maps by assuming 

that all corrections for stations on rock would be positive and that complete Bouguer anomaly (unknown) 

must be at least as positive as simple Bouguer anomaly. 

MAGNETIC SURVEY 

Total magnetic-intensity measurements were made 

along the flight paths indicated on the magnetic map by 

means of an Elsec Wisconsin proton precession magnetometer (Wold, 1964), flown in an LC-117 aircraft at a 

constant barometric elevation of 2,100 m. Trimetrigon 

photography provided position control in the area of 

the mountains. Dead reckoning and solar observations, 

1 Geophysical Services International, London, England. 
adjusted to the photographically identified indicated 

points, were the only controls at the northwest and 

southeast ends of the lines over the featureless ice sheet. 

Consequently, position errors are variable but are minimal near the control points. Some lines crossed no 

identifiable rock outcrops and these have greater position errors. A reliable estimate of quantitative error is 

difficult to obtain but probably errors at the ends of 

the lines are several kilometers. Diurnal control was 

obtained from a base line connecting the profiles. No 

flights were made during magnetic storms. 

Because of the widely spaced flight paths, position 

uncertainties, and relatively low amplitude and areal 

extent of the anomalies in this quadrangle, profiles 

rather than contours provide the most geologic information. Because the flight lines approximately 

parallel the main magnetic field contours, no regional 

gradient was removed. 

SUMMARY OF GEOLOGY 

Schmidt and Ford ( 1969) have described the geology 

in this quadrangle. A composite stratigraphic section 

for the Pensacola Mountains, from Schmidt and Ford 

( 1969) is included here. 

The oldest rocks are those of the Precambrian 

Patuxent Formation; these compose most of the exposed rock. The Patuxent is a thick section (at least 

10 km) of terrigenous turbidite sedimentary rock, 

interbedded, in the Williams Hills area, with pillow 

lavas and basalt flows. Large amounts of diabase and 

minor amounts of felsic magma were injected into the 

sedimentary rocks. The diabase, in the form of sills, 

is found mostly in the Schmidt Hills and Williams Hills 

where Schmidt and Ford ( 1969) indicated thicknesses 

of 2-150 m. They gave a ratio of diabase to sedimentary rock in the Schmidt Hills of 1:2. 

After deposition of the Patuxent Formation and 

the intrusion of diabase, the rocks were uplifted and, 

in Early Cambrian time, eroded. During subsequent 

marine transgression, Cambrian limestones, felsic 

flows and pyroclastic rocks. silt and mud beds, and 

volcanic rocks were deposited. At the end of Cambrian time the Cambrian rocks were folded, the Precambrian rocks were refolded, and granitic rocks were 

emplaced in the form of sills and rhyolite porphyries 

and, in the Torbert Escarpment area, as a thick pluton 

(granite of Median Snowfield). Schmidt and Ford 

( 1969) reported an Rb-Sr age of 510 million years 

(Early Ordovician) for the time of cooling of this 

granite pluton. 

After a period of erosion, the Paleozoic Neptune 

Group was deposited. This consists of about 300m 

of quartzose sandstone, about 300m of interbedded 

argillaceous siltstone and fine-grained quartz sandstone, 

about 700 m of carbonate-cemented coarse-grained 

sandstone (including thin interbeds of shale and conglomerate), and a variable thickness (0-1,000 m) of 

coarse conglomerate. 

Above the Neptune Group is about 2,000 m (Schmidt 

2 

and Ford, 1969) of sandstone, siltstone, shale, and a 

mudstone bed containing some tillite. The younger rocks 

of this sequence are of Permian age. D. L. Schmidt 

( unpub. data) considers these rocks and the Neptune 

Group to be generally correlative in age and lithology 

with Beacon strata. These rocks seem to be of lower 

density than the Cambrian and Precambrian sedimentary 

rocks (as discussed below). After the Permian rocks 

were deposited, tectonic activity deformed th0 sedimentary rocks into broad and locally tight to overturned 

folds. 

A fourth period of structural deformation folded and 

faulted the rocks over a broad region. 

DISCUSSION OF GRAVITY MAP 

The most apparent feature of the gravity map is the 

existence of anomalies of greater amplitude than the 

probable errors caused by terrain effects. The Bouguer 

values decrease regionally from northwest to southeast 

from+20 to -60 mgal. Local anomalies over the Neptune 

Range are as low as -70 and -90 mgal. The regional 

gradient is the result of crustal thickening crossing the 

transition from West to East Antarctica, as discussed by 

Behrendt, Meister, and Henderson ( 1966). The contours 

separating the Schmidt Hills from the Neptune Range 

are too closely spaced to be caused by deep crustal 

effects alone; the gradient here is about 4 mgal/km as 

compared with the general regional gradient of about 

2 mgal/km. D. L. Schmidt (unpub. data) concludes 

that a fault separates the Schmidt Hills from the Neptune 

Range, as indicated on our map. This fault is probably 

associated with the source of the steep gravity gradient, 

but that is also partly due to crustal thickening. The 

diabase sills emplaced in the Patuxent Formation in the 

Schmidt Hills no doubt have increased the mean density 

of the upthrown western side of the fault. Probably 

the Patuxent rocks in the Schmidt Hills are representative of a deeper part of the crust. Allowing for the 

regional gradient, there seems to be about 20-mgal difference across the fault. The elevation is about the same 

on both sides of the fault. The diabase intrusions in the 

Patuxent Formation of Williams Hills also are on the 

high-gravity side of the fault; the gravity gradient, however, is not as linear in this area. 

Estimates of the vertical displacement on this fault 

could be made by fitting the observed data with theoretical models. But uncertainty about the density contrasts between the various rock units make this approach 

rather speculative. The relative masses required to produce a Bouguer anomaly relief of 20 mgal would be 

4.8 and 2.4 for density contrasts of 0.1 and 0.2, respectively. 

The higher peaks of the southern Neptune Range on 

which Cambrian sedimentary rocks crop out have an 

associated 10- to 20-mgal positive anomaly relative to 

the Patuxent Formation (Precambrian in the northern 

Neptune Range and about 20- to 30-mgal positive 

anomaly relative to the Beacon(?) rocks (Paleozoic) 

mapped on the flanks. There is a negative anomaly on 
the west side of the southern Neptune Range over 

Beacon(?) rocks. Because a density contrast greater 

than 0.1-0. 2 gj cm3 between different Paleozoic sedimentary rocks is unlikely, a 10- to 30-mgal difference 

in gravity corresponds to 2.4-7.2 km for L).p = 0.1 

gfcm3 and half as much for L).p = 0.2 gjcm3 . These 

values are suggestive of the relative variation in thickness of the Beacon(?), Cambrian, and Patuxent sedimentary rock units. 

A small positive anomaly over the Cambrian sedimentary rocks of the Hill Nunatak area southeast of 

the Neptune Range indicates that the density of these 

rocks is higher than that of the Beacon(?) rocks. 

The anomaly over the granite of Median Snowfield 

and Beacon(?) sedimentary rocks in the Torbert 

Escarpment area is at least 30 mgalless than that over 

the Patuxent Formation in the Neptune Range. The 

granite must have a lower density than the Patuxent 

Formation if it is the source of this anomaly. Densities 

for granite as low as 2.5 g/cm3 are not uncommon 

(Woollard, 1962), so this is a reasonable explanation. 

It is not easy to separate the effect of the Beacon(?) 

rocks from that of the underlying granite in the Torbert 

Escarpment area. The combined thickness could be 

as great as 7.2 km for L).p = 0.1 gjcm3 relative to the 

Patuxent Formation. 

DISCUSSION OF THE MAGNETIC PROFILES 

There are no high-amplitude anomalies in these profiles such as those exceeding 1 ,ooo, associated with 

the magnetic upper part of the Dufek mafic intrusion 

to the north (Behrendt and others, 1966). A line of 

50-1 OOy anomalies is associated with the fault separating Schmidt Hills from the Neptune Range. These 

anomalies may be due to diabase intrusions in the 

lower part of the Patuxent Formation. 

Most of the magnetic field is smooth over the thick 

section of sedimentary rocks in the Neptune Range. 

A broad negative anomaly is apparently associated 

with these nonmagnetic rocks. 

Near the east end of line profile 38 above Loren 

Nunataks there is a 40-y anomaly of unknown origin; 

this same feature can be seen farther south on lines 

39, 40, and 41. The width of this anomaly on line 

40 south of Hill Nunatak suggests that a fairly magnetic 

body of substantial thickness must underlie the sedimentary rocks at a depth of several kilometers. Possibly 

the anomaly marks the location of a mafic intrusion 

similar in age and character to the Dufek intrusion, 

but this is speculative. The southeast end of line 41 

shows short-wave length anomalies of about 1 00-y 

amplitude whose sources must be close to the base 

3 

of the ice. Probably the sedimentary rocks are thin or 

absent there, and the source of these. anomalies is the 

same as that of the broader anomaly to the northwest. 

A broad 150y anomaly south of the Neptune Range. 

crossed by line 42 may be a part of this same intrusion 

although this would require a curve to the west, southwest of line 41. Additional anomalies on lines 43 and 

44 also suggest mafic intrusions. 

ACKNOWLEJXiMENTS 

We thank D. L. Schmidt and W. H. Nelson, geologists, 

and M.A. Weber, topographic engineer, for making 

gravity observations at points vistited during their other 

work. R. E. Wanous assisted in the aeromagnetic survey. 

The topographic control party made the position and 

vertical angle elevation determinations. The U.S. Navy 

and U.S. Army provided logistic support. The National 

Science Foundation supported the research. C. R. Bentley 

of the University of Wisconsin Geophysical and Polar 

Research Center loaned the gravimeters, magnetometer. 

and seismic reflection equipment. 

REFERENCES 

Behrendt, J. C., 1962, Summary and discussion of the 

geophysical and glaciological work in the Filchner 

Ice Shelf area of Antarctica: Wisconsin Univ., 

Geophys. and Polar Research Center, Research Rept. 

62-3, 66 p. 

Behrendt, J. C., Meister, L. J., and Henderson, J. R., 1966, 

Airborne geophysical study in the Pensacola Mountains, 

Antarctica: Science, v. 153, no. 3742, p. 1373-1376. 

Behrendt, J. C. Wold, R. J., and Laudon, T. S., 1962, 

Gravity base stations in Antarctica: Geophys. Jour., 

v. 6, no. 3, p. 400-405. 

Schmidt, D. L., and Ford, A. B., 1969, Geology of the 

Pensacola and Thiel Mountains, in Bushnell, V. C., and 

Craddock, Campbell, eds., Geologic maps of Antarctica: 

Am. Geog. Soc., Antarctic Map Folio Ser., Folio 12, 

pl. 5, sheet 5. 

Thiel, Edward, and Behrendt, J. C., 1959, Seismic studies 

on the Filchner Ice Shelf, Antarctica, in Thiel, Edward, 

Bentley, C. R., Ostenso, N. A., and Behrendt, J. C., 

Oversnow traverse programs, Byrd and Ellsworth stations, 

Antarctica, 1957-58: Am. Geog. Soc., Internat, Geophys. 

Year Glaciology Rept. Ser., No.2, Group V, p. V 1-Y 14. 

Wold, R. J., 1964, The Elsec Wisconsin digital recording 

proton precession magnetometer system: Wisconsin 

Univ., Geophys. and Polar Research Center, Research 

Rept. Ser. 64-4, 83 p. 

Woollard, G. P., 1962, Relation of gravity anomalies to 

elevation, topography and geology, Pt. 2 of The relation 

of gravity anomalies to surface elevation crustal structure 

and geology: Wisconsin Univ., Geophys. and Polar 

Research Center, Research Rept. 62-9, 78 p.