DEJP> A.R TMENT Of THE INTERIOR 

!UNHTJED STATES GEOLOGICAL SURVEY 

GEOLOGITC MAJP OF THE 

SARATOGA TAJBLE QUADRANGLE, 

PENSACOLA MOUNTAINS, 

ANTARCTICA 

By Arthur B. Forrd, Dwight L. Schmidt, 

Walter W. Boyd9 Jrr., and Willis H. Nelson 

Prepared by the 

U.S. GEOLOGHCAL SURVEY 

under the auspices of the 

NATIONAL SCIENCE FOUNDATITON 

U.S. ANTARCTIC RESEARCH PROGRAM MAP 

Published by the U.S. Geological Survey, 1978 

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GEOLOGIC MAP SYMBOLS 

COMMONLY USED ON MAPS OF THE UNITED STATES GEOLOGICAL SURVEY 

(Special symbols are shown in explanation) 

Contact-Dashed where approximately 

located; short dashed where inferred; 

dotted where concealed 

Contact-Showing dip; well exposed at 

triangle 

Fault-Dashed where approximately 

located; short dashed where inferred; 

dotted where concealed 

Fault, showing dip-Ball and bar on 

downthrown side 

-,--..,....--.,.-~"""T"-...-- Normal fault-Hachured on downthrown side 

Fault-Showing relative horizontal 

movement 

+ + + + + + Thrust fault-Sawteeth on upper plate 

• Anticline-Showing direction of plunge; 

-E---+60 

dashed where approximately located; 

dotted where concealed 

Asymmetric anticline-Short arrow 

indicates steeper limb 

Overturned anticline-Showing 

direction of dip of limbs· 

Syncline-Showing direction of plunge; 

dashed where approximately located; 

dotted where concealed 

Asymmetric syncline-Short arrow 

indicates steeper limb 

Overturned syncline-Showing 

direction of dip of limbs 

Monocline-Showing direction of plunge 

of axis 

Minor anticline-Showing plunge of axis 

Minor syncline-Showing plunge of axis 

Strike and dip of beds-Ball indicates 

top of beds known from sedimentary 

structures 

_!!!_ Inclined EB Horizontal 

-t- Vertical -.;:.Overturned 

Strike and dip of foliation 

.31?_ Inclined --+- Vertical + Horizontal 

Strike and dip of cleavage 

~ Inclined 1--f Vertical +Horizontal 

Bearing and plunge of lineation 

'~Inclined • Vertical -Horizontal 

Strike and dip of joints 

-~ Inclined --Vertical +Horizontal 

Note: Planar symbols (strike and dip 

of beds, foliation or schistosity, and cleavage) may be combined with linear 

symbols to record data observed at 

same locality by superimposed symbols 

at point of observation. Coexisting 

planar symbols are shown intersecting 

at point of observation. 

Shafts 

~ Vertical ~ Inclined 

Adit, tunnel, or slope 

>-Accessible :>-!--Inaccessible 

X Prospect 

Quarry 

X Active 

Gravel pit 

X Active 

Oil wells 

o Drilling 

~Gas 

• Oil 

'X' Abandoned 

X Abandoned 

¢ Shut-in -¢- Dry hole, 

~ Show of gas abandoned 

~ Show of oil 
DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

TO ACCOMPANY MAP A-9 

GEOLOGY OF THE SARATOGA TABLE QUADRANGLE 

PENSACOLA MOUNTAINS, ANTARCTICA 

by Arthur B. Ford, Dwight L. Schmidt, Walter W. Boyd, Jr.! 

and Willis H. Nelson 

INTRODUCTION 

The area that includes the Saratoga Table quadrangle 

in the Pensacola Mountains was first seen and photographed on January 13, 1956, on a U.S. Navy transpolar flight from McMurdo Sound. Trimetrogon aerial 

mapping photography of the quadrangle was obtained 

in January 1958 by the U.S. Air Force. 2 The area was 

first visited by U.S. Geological Survey parties in the 

austral summer, 1965 - 66. 

Geological, geophysical, and geodetic surveys of the 

entire map area, as well as over many other parts of the 

Pensacola Mountains, were made by the U.S. Geological 

Survey during the 1965 - 66 field season (Huffman and 

Schmidt, 1966). The surveys used close support of 

U.S. Army UH - 1 B turbine helicopters operating from a 

base camp in the central Neptune Range where the field 

party was placed by U.S. Navy LC- 130F (Hercules) aircraft from McMurdo Station. This work completed the 

geologic mapping of the entire 540-km-long chain of the 

Pensacola Mountains, a project begun in 1962- 63 at 

the south end of the mountains (Schmidt and Ford,l969). 

Additional geologic studies were made in January 1974 in 

the vicinity of Mount Lechner (Cameron and Ford, 1974) 

during investigations of possible ice-runway aircraft landing sites by the U.S. Army Cold Regions Research and 

Engineering Laboratory (Kovacs and Abele, 1974). 

The major geologic feature of this quadrangle is the 

south end of the Dufek intrusion (Ford and Boyd, 1968), 

an immense differentiated stratiform mafic igneous body 

that was discovered in 1957 by an International Geophysical Year (IGY) traverse party visiting Dufek Massif, a 

northern range of the Pensacola Mountains (Aughenbaugh, 

1961; Walker, 1961; Neuburg and others, 1959). Our 

1965 - 66 studies showed that the intrusion makes up all 

of Dufek Massif (Ford and others, 1978) and virtually all 

of the Forrestal Range. Geophysical surveys show that 

most of the body is covered by ice sheets adjoining the 

ranges and that its area is of the order of half that of the 

Bushveld intrusion of South Africa (Behrendt and others, 

1974). 

Access to this remote part of the continent is difficult 

except by ski-equipped aircraft. The ice terrain near the 

Forrestal Range is gen~ally suitable for ski-plane landings 

n"r two seasons' experrence in the area showed summer 

1 Present address, Geological Survey of Finland, Helsinki 

2 Aerial photographs of the Pensacola Mountains can be 

ordered from the U.S. Geological Survey, National Center, 

Reston, Virginia 22092 

weather conditions to be mostly fair, but strong winds 

were common near mountain escarpments. Summer temperatures during our work in the area ranged from about 

-l9°C (-2°F) to -7°C (20°F) and averaged about -12°C 

(10°F). 

Work on the Dufek intrusion in 1965 - 66 was chiefly 

by A.B. Ford and W.W. Boyd, Jr. and was continued in 

January 1974 by Ford. Study of the sedimentary country 

rocks was mainly by D.L. Schmidt and W.H. Nelson. 

This report briefly outlines geologic relations and reviews 

work done in the quadrangle and adjacent regions. 

Acknowledgments. -This work was part of the 

U.S. Antarctic Research Program of the Office of 

Polar Programs, National Science Foundation. General 

logistic and fixed-wing aircraft support was provided 

by the U.S. Navy Air Development Squadron Six. 

Helicopter support was provided by the U.S. Army 

Aviation Detachment. We gratefully acknowledge the 

help of all support personnel, of Jerry Huffman, National 

Science Foundation, in coordinating the field logistical 

support, and the assistance by Steven W. Nelson in 

laboratory studies related to this project. 

SYNOPSIS OF GEOLOGIC STUDIES 

The map area is near the north end of the Pensacola 

Mountains, a mountain group that forms part of the 

transcontinental chain of the Transantarctic Mountains 

(fig. 1B). Principal ranges of the Pensacola Mountains 

are shown in figure lA. 

The principal rock exposures in the map area are units 

of the Dufek intrusion. This layered igneous body, which 

is Jurassic in age, intruded moderately to highly deformed 

Precambrian to Permian sedimentary rocks during the time 

of emplacement of sills of Ferrar Dolerite throughout the 

Transantarctic Mountains (Grindley, 1963). The poorly 

exposed sedimentary country rocks are visible only in 

small, isolated nunataks in the southern and southwestern 

parts of the Forrestal Range. Relations between sedimentary formations are inferred to be the same as in the Neptune Range, about 60 km southwest of the Forrestal Range, 

where the units are much better exposed (Schmidt and 

others, 1978). General geologic relations between ranges 

of the Pensacola Mountains are shown on the 1: 1,000,000-

scale map of Schmidt and Ford (1969). Formal rockstratigraphic nomenclature for the Dufek intrusion is presented by Ford (1976)"and for sedimentary rocks is given 

by Schmidt and others (1964, 1965), Schmidt and Ford 

( 1969), and Williams ( 1969). The summary below is 

drawn largely from these sources. 
50 KILOMETERS 

I I 

~. 

A 

EXPLANATION 

Bedrock exposure 

f1'rtf 

Escarpment 

Hachures on down slope side 

Direction of ice flow 

$~ ..... ,,, 

Crevasse zone 

B 

Figure 1. -Index maps. A, Major ranges of the Pensacola Mountains and location of Camp Neptune. 

B, Location of Pensacola Mountains in Antarctica. 

SEDIMENTARY ROCKS 

The sedimentary units exposed in this quadrangle belong chiefly to the youngest of three depositional sequences 

known to the south and west in the Neptune Range. The 

sequences in the Neptune Range are clearly seen to be separated by angular unconformities. Fossils are scarce in or 

absent from most units, but available paleontologic and 

radiometric age data suggest that ages of the sequences 

are ( 1) Precambrian, (2) Middle and Late(?) Cambrian, and 

(3) Ordovician(?) to Permian. Rocks correlative with the 

oldest of these sequences are exposed only on small nunataks in the westernmost part of the Saratoga Table quadrangle but are probably much more extensive under the 

cover of ice. Correlatives of the second sequence are not 

known but could occur beneath the ice cover. Rocks of 

the third sequence form the principal nunataks south and 

west of the Dufek intrusion in this area. 

The Patuxent Formation, of late Precambrian age 

(Schmidt and others, 1978), makes up the oldest sequence 

and consists in the Neptune Range of at least about 

2 

10,000 m of metasubgraywacke and slate locally interbedded with voluminous basaltic and felsic volcanic materials. Radiometric dating (Eastin and Faure, 1972) suggests that some of these volcanic units may be as old as 

1,210 m.y. Diabase sills locally intrude the formation. 

The igneous units of the formation are not known in the 

Saratoga Table quadrangle. The Patuxent sediments were 

deposited in a high-energy environment, probably a volcanically active eugeosyncline, as indicated by lithologies 

and abundant sedimentary structures such as graded bedding, channel features,and sole markings. The Patuxent 

was widely metamorphosed to the greenschist facies during an orogeny near the end of the Precambrian. 

The second sequence, which is not known to occur in 

and may be absent from the Saratoga Table quadrangle, 

consists of a fossilferous Middle Cambrian (Palmer and 

Gatehouse, 1972) limestone and overlying silicic volcanic 

and clastic sedimentary units that have a composite thickness of at least 700 m. Some of the units are known to 

thin northward from the southern Neptune Range, and 
as none are exposed in adjoining areas, they are believed 

to be absent and are not shown in the cross sections. 

Accordingly, the single angular unconformity shown is 

equivalent to the two pre-Devonian angular unconformities that are known below the Neptune Group in the 

central and southern parts of the Neptune Range. 

The third sequence consists, in ascending order, of the 

Neptune Group, Dover Sandstone, Gale Mudstone, and 

the Pecora Formation,a succession that is at least 4,000m 

thick elsewhere in the Pensacola Mountains. The Neith 

Conglomerate, a coarse, poorly sorted unit at the base of 

the Neptune Group, is not exposed but probably underlies the Elbow Formation. An upper sandy part of the 

Neith probably correlates with the Elliott Sandstone, and 

a lower conglomeratic part with the Brown Ridge Conglomerate, in the type area of the Neptune Group. The 

Dover Sandstone, which overlies the Neptune Group, is 

disconformably overlain by the Gale Mudstone, a diamictite of probable continental glacial origin (Schmidt and 

Williams, 196 9; Schmidt and Friedman, 19 7 4 ). The contact of the Gale with the overlying Pecora Formation is 

not exposed in the Pensacola Mountains, but the two 

formations appear to be concordant. Rare diagnostic 

fossils indicate a Devonian age for the Dover and a Permian age for the Pecora (Schopf, 1968). Depositional 

environments varied from high energy in late orogenic to 

early postorogenic time (Ordovician? to Silurian?) during 

deposition in local basins of the basal part of the Neptune 

Group, to low energy during Devonian to Permian deposition of later sediments, which probably took place on 

a slowly subsiding continental shelf or terrestrial plain. 

TECTONIC EVENTS 

The Pensacola Mountains lie in a mobile belt with a 

long record of recurrent deformation. The folding of 

angular unconformities between the three sedimentary 

sequences shows clearly that episodes of deformation in 

this region were superimposed. The area lies at or near 

the intersection of the multiply deformed Transantarctic 

Mountains fold belt and a younger, probably early Mesozoic, fold belt adjoining the Weddell Sea (Ford, 1972). 

Major orogenies in the Pensacola Mountains occurred 

(1) near the end of the Precambrian; (2) in the early Paleozoic, probably in Late Cambrian and Ordovician time; 

and (3) in the Triassic. Weak deformation during or after the Jurassic may be indicated by the broad synclinal 

flexure of the layered Dufek intrusion and by the pervasive system of microshears in the Dufek rocks that parallel a regional fracture pattern (Ford, 1968). The highangle Enchanted Valley fault zone along the northern 

base of Dufek Massif may be related to Cenozoic uplift 

of the Pensacola Mountains. 

The two pre-Devonian orogenies in the Pensacola 

Mountains probably correspond to the Beardmore 

(580-650 m.y .) and Ross(500-530m.y.) orogenies, which 

'''ere widespread in the Transantarctic Mountains (Elliot, 

1975). The third major event, locally termed the 

Weddell orogeny (Ford, 1972), is not recorded by folding in other parts of the Transantarctic Mountains. 

This event is dated in the Pensacola Mountains as hav3 

ing occurred at some time between the Late Permian 

and Middle Jurassic. However, the change in paleoslope directions at the beginning of Triassic time in 

other parts of the Transantarctic Mountains is attributed 

to uplift in the Pensacola Mountains (Barrett and others, 

1972), which thus probably dates the orogeny as Early 

Triassic. Deformation of about this age is widely recorded elsewhere near the Weddell Sea (Craddock, 1972; 

Dalziel and Elliot, 1973). 

All major folds shown on the map were produced 

during the Triassic orogeny. Intensity of the folding 

varies widely in different parts of the Pensacola Mountains. In the Neptune Range, rocks of the third sedimentary sequence in places are folded isoclinally and in 

other places are little deformed. In the Saratoga Table 

quadrangle, the folds are open and upright. The rocks 

are moderately folded about gently plunging or horizontal axes trending about N. 15-30° E. The slight 

change in axial trends between areas south of Haskill 

Nunatak and areas to the north may indicate the disruption of N. 30° E. regional trends by emplacement of 

the Dufek intrusion. 

The Patuxent Formation is much more intensely folded 

than the Paleozoic rocks but is poorly exposed in the map 

area. In the Neptune Range, axial trends of folds in the 

Patuxent generally parallel those of the younger rocks. 

THE DUFEK INTRUSION 

The first work on the Dufek intrusion by Aughenbaugh 

(1961) and Walker (1961) demonstrated the similarity of 

the body to other major layered mafic intrusions. The 

similarity was documented more strongly by our 1965-66 

work, which showed the presence of a differentiation 

trend toward strong iron enrichment in the upper part of 

the body and a final concentration of a granophyric residuum (Ford and Boyd, 1968; Ford, 1970}. The differentiation trend generally follows that of other stratiform 

complexes summarized by Wager and Brown (1968). Our 

work has emphasized the sedimentlike nature of most of 

the layered rocks. Owing to the stratigraphic character 

of the layered igneous succession, our field studies were 

carried out largely with techniques used in the study of 

a stratified sedimentary sequence. In this investigation, 

10 partial stratigraphic sections were measured by altimetry and hand leveling in the area of Dufek Massif, and 

11 were measured in the Forrestal Range. Those in the 

Forrestal Range are all in the map area. Most sections 

within each range are intercorrelated using one or more 

conspicuous marker layers or horizons. All major areas 

of outcrop were visited or viewed closely from aircraft. 

Approximately 1,000 samples collected are representative of all major exposed units. Nearly a third of the 

samples were oriented for petrofabric and paleomagnetic study. 

PETROLOGIC NOMENCLATURE 

The layered igneous rocks formed by accumulation of 

settled crystals in a mush on the floor of a magma chamber. Rocks of this origin are termed "cumulates," and 

their textures and structures reflect sedimentary processes controlled by gravity, hydraulic parameters of the 
magma, and the presence or absence of currents (Jackson, 

1971). Following usage in sedimentary petrology, which 

distinguishes detrital grains from cement, the cumulus 

(detrital) phases form the framework of the rock, and 

the postcumulus (cement) phases are primary materials 

that crystallized in places they now occupy. 

Cumulates are named by prefixing names of settled 

minerals in order of decreasing abundance (Jackson, 

1967, p. 23). The principal cumulus phases in the Dufek 

intrusion are plagioclase, pyroxenes in the series augiteferroaugite, pyroxenes in the series bronzite-inverted 

pigeonite, and opaque oxides. Preliminary results of 

studies in progress on the chemistry and mineralogy of 

opaque oxides indicate that the most common phases 

are titaniferous magnetite and ferrian ilmenite in composite grains that probably represent unmixing of original homogeneous titaniferous magnetite (Himmelberg 

and Ford, 1975; 1977}. For brevity, the opaque oxides are 

referred to simply as magnetite. Phases that are cumulus 

in some rocks are postcumulus in others. Quartz and 

K-feldspar are known to be only postcumulus and so 

are not included in the cumulate terminology. Two 

pyroxene-plagioclase cumulate is the predominant lithology in the lower part of the intrusion, and two pyroxene-plagioclase-magnetite cumulate predominates in 

the upper part. 

Rock names used in the map explanation and text 

are of two types based on ( 1) total mode of the rock 

and (2) types and proportions of cumulus minerals. In 

our field studies, rocks were mapped on the basis of total 

mode, in which cumulus and postcumulus minerals are 

not distinguished. The map units are given formal rockstratigraphic names using standard noncumulate igneous 

rock nomenclature. The cumulate terminology of Jackson 

(1967, 1971) is used, however, in the explanation and 

text for description of the rocks. Where proportions of 

cumulus minerals are not known, or not otherwise indicated, conventional names are prefixed, as in "gabbroic 

cumulate" or "anorthositic cumulate." 

STRATIGRAPHIC NOMENCLATURE 

The rock-stratigraphic nomenclature and stratigraphy 

of the int~usion are discussed by Ford (1976) and are 

shown in figure 2. The layered succession of rocks is 

incompletely exposed and is estimated on the basis of 

structure and geophysical evidence to be of the order of 

8- 9 km thick. A lower part of the body is exposed in 

Dufek Massif. The composite section of this part, about 

1 ,800 m thick, is referred to as the "Dufek Massif section." 

The upper part of the body is exposed in the Forrestal 

Range. The composite section of this part, about 1,700 m 

thick, is referred to as the "F orrestal Range section." An 

ice-covered interval, termed the "Sallee Snowfield section," 

between the two exposed-rock sections, is estimated to 

be 2 - 3 km •thick. Geophysical studies by Behrendt and 

others (1974) suggest that an unexposed basal section is 

1.8 - 3.5 km thick. An unknown but probably small thickness of rock was eroded from the upper part of the body. 

Fine- to coarse-grained gabbro near the contact and 

dike rocks ranging from aplite to gabbroic pegmatite are 

4 

the principal nonstratified rocks of the intrusion. The 

contact gabbro is of noncumulus origin and probably 

forms an envelope around the layered sequence. It is 

exposed at only a single locality near Mount Lechner in 

the southern F orrestal Range. 

, 

All the stratified rocks are included in the Forrestal 

Gabbro Group, which is divided into four formations, 

from base to top, the Walker Anorthosite, Aughenbaugh 

Gabbro, Saratoga Gabbro, and Lexington Granophyre. 

Distinctive, mappable units of the Aughenbaugh and 

Saratoga are recognized as members. Most members 

consist of one or more mineral-graded cumulate layers. 

Two members of the Saratoga are characterized by an 

abundance of cognate inclusions composed of noncumulus anorthosite and leucogabbro. The chief lithologic 

difference between the Aughenbaugh and Saratoga is the 

much greater abundance of cumulus magnetite in the 

Saratoga. Other mineralogic differences are discussed 

subsequently. 

MINERAL DISTRIBUTION AND COMPOSITION 

The Forrestal Gabbro Group contains highly variable 

proportions of cumulus minerals. The stratigraphic distribution of these minerals and of postcumulus minerals 

is shown in figure 2. Cumulates of gabbroic composition 

generally consist of subequal amounts of plagioclase and 

pyroxene. Magnetite is a chief modal variant occuring 

in only minor amounts near the top of the Aughenbaugh 

Gabbro and as a major constituent throughout the Saratoga Gabbro. In many thin layers and lenses in the Saratoga, it is the only or the greatly predominant mineral, 

commonly at the base of thicker mineral-graded layers. 

The upward stratigraphic increase in amounts of magnetite in the intrusion as a whole results in an overall upward increase in average rock density (Ford and Nelson, 

1972) and in great differences in aeromagnetic intensities 

measured over the two ranges (Behrendt and others, 1974 ). 

Layers in which cumulus plagioclase is concentrated occur throughout the intrusion, as in the Walker Anorthosite, Spear Anorthosite Member, and Stephens Anorthosite Member; whereas layers in which cumulus pyroxene 

is concentrated occur only in the Dufek Massif section, 

as in the Neuburg3 and Frost Pyroxenite Members. Most 

of these layers are of mineral-graded type. Apatite is a 

common though minor cumulus phase in the highest 

cumulates of the Saratoga Gabbro and is a noncumulus 

constituent of the Lexington Granophyre. Quartz and 

K-feldspar are minor postcumulus minerals in the upper 

part of the Saratoga and are dominant constituents of 

the Lexington. 

The chemistry of bulk rocks and of individual mineral 

phases shows progressive stratigraphic changes generally 

similar to those of other stratiform mafic intrusions described by Wager and Brown (1968). As determined by 

preliminary optical and X-ray studies, plagioclase compositions range from about An 80 _ 85 in anorthositic cumulates near the base of the Dufek Massif section to about 

3 This name, derived from Neuburg Peak, was incorrectly 

spelled "Neuberg" in previous reports (Ford, 1976; 

Himmelberg and Ford, 1976) 
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Figure 2 - Rock-stratigraphic nomenclature of the Dufek intrusion and distribution of minerals. Solid vertical lines, cumulus 

minerals; dashed lines, noncumulus minerals (in granophyre) and postcumulus minerals. Pig+, lowest occurrence of inverted 

cumulus pigeonite. After I\ord (1976) and Himmelberg and Ford (1976) 

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A MAFIC INDEX B CATION RATIO Fe/(Fe + Mg) 

• Mafic rocks ... Ca-rich pyroxene 

o Felsic rocks • Ca-poor pyroxene 

Figure 3 - Stratigraphic variations in chemical composition in the 

Dufek intrusion. A, Mafic index, (FeO + Fe2 03 ) x 100/(FeO + 

Fe2 0 3 + MgO), of whole rocks (after Ford, 1970). B, Cation ratio, 

Fe/(Fe + Mg), of the two pyroxene series (after Himmelberg and 

Ford, 1976). 

seen to pinch out downward. Thicknesses of different 

dikes range from a few millimeters to several meters. 

Compositions of the dikes seem to show changes related 

to stratigraphy and compositional changes of cumulatehost rocks, which suggests that they formed by the 

filling of fractures with late intercumulus liquids from 

local sources at various stages during consolidation of 

~-

An45- 55 in gabbroic rocks and granophyre near the 

top of the F orrestal Range section (Ford and Boyd, 

1968). Electron microprobe analyses show that Ca-rich 

cumulus pyroxenes in the series augite-ferroaugite range 

from Ca 36.4 Mg 48.7 Fe 14.9 to Ca 30.0 Mg 23.5 Fe46.5, 

and that Ca-poor cumulus pyroxenes in the series bronzite-inverted pigeonite range fromCa3_5Mg69.1Fe27.4 

to Ca 11.4 Mg34.0 Fe54.6 (Himmelberg and Ford, 1973, 

1976). The upward stratigraphic increase in the ratio 

Fe/(Fe + Mg) in both pyroxene series and the increase in 

modal magnetite are reflected in the general upward increase in the mafic index of bulk-rock compositions 

the intrusion (Ford, 1970). Granophyric dikes high in 

the intrusion may be related to development of the 

Lexington Granophyre at the end stage of differentiation. 

(fig. 3). Although major-element contents of the oxide 

minerals show no regular variation with height, the minor 

elements vanadium and aluminum in ilmenomagnetites 

generally decrease with height and thus are useful indicators of fractionation trends during the stage of oxidemineral crystallization (Himmelberg and Ford, 1977). 

DIKE ROCKS \ 

Felsic dikes with wide range in textural and chemical 

characteristics are the only dikes observed in the map 

area. Mafic dikes similar to those in the Cordiner Peaks 

(Ford, 1974; Ford and others, 1978) were not found. 

The dikes are all within the Dufek intrusion except for 

two thin aplite and pegmatite dikes less than 10 em 

thick that cut the Pecora Formation on Grob Ridge. 

Within the intrusion they occur at many stratigraphic 

levels. Dike rocks are mainly felsite, aplite, and felsic 

pegmatite and include alaskite, leucogabbro, gabbroic 

pegmatite, and granophyre. Zoning occurs locally. The 

dikes are vertical or su bv:ertical and in places can be 

6 

REGIONAL CORRELATION 

Sedimentary formations in the Pensacola Mountains 

are apparently correlatable for great distances in the 

Transantarctic Mountains and probably in the Ellsworth 

Mountains. All sedimentary units in the Saratoga Table 

quadrangle probably are part of the Beacon Supergroup, 

a thick platform sequence of mainly arenaceous rocks 

ranging in age from Devonian or older to Jurassic in the 

Ross Sea region (Barrett and others, 1972). Sedimentation in the Pensacola Mountains area apparently terminated in the Permian, however, owing to orogenic 

uplift. The Neptune Group and Dover Sandstone probably correlate with the Taylor Group near the Ross Sea. 

The Dover may also correlate with the Crashsite Quartzite in the Ellsworth Mountains (Schopf, 1969). The 

Gale Mudstone and Pecora Formation probably correlate with the lower part of the Victoria Group near 

the Ross Sea. Late Paleozoic glaciation was widespread 

in Antarctica, but dating is poor in the general absence 

of diagnostic fossils. The Gale Mudstone probably also 

correlates approximately with the Darwin Tillite, Buck-
eye Tillite, and Pagoda Formation elsewhere in the Transantarctic Mountains (Barrett and others, 1972) and with 

the Whiteout Conglomerate in the Ellsworth Mountains 

(Schopf, 1969). Units in the Pensacola Mountains probably also have equivalents in the Shackleton Range 

(Stephenson, 1966; Clarkson, 1972) and possibly in 

western Queen Maud Land (Juckes, 1972). 

The Patuxent Formation, consisting of metasedimentary rocks of pre-Beacon age, probably is also widely 

correlatable, but paleontologic dating is poor, and interpretation of radiometric ages (Eastin and Faure, 1972) 

is complicated .by resetting. 

Basaltic igneous activity, both intrusive and extrusive, 

was widespread in Antarctica in mid-Mesozoic time. The 

Dufek intrusion correlates approximately with the Ferrar 

Group of Grindley (1963), which in the Ross Sea sector 

of the Transantarctic Mountains includes the Ferrar Dolerite and the Kirkpatrick Basalt. Similar mafic intrusive 

rocks of this age are reported from the Shackleton Range 

area (Stephenson, 1966) and western Queen Maud Land 

(Juckes, 1972). The Dufek intrusion is by far the largest 

single differentiated intrusive body of Ferrar age known 

in Antarctica. However, smaller layered intrusions are 

known in the Ferrar Group (Gunn, 1963; Grapes and 

Reid, 1971 ). 

SYNOPSIS OF GEOPHYSICAL STUDIES 

GRA.VITY AND AEROMAGNETIC DATA 

Results of gravity and aeromagnetic surveys in the Pensacola Mountains are discussed by Behrendt and others 

(1974). The geologic map of the Saratoga Table quadrangle should be used in conjunction with the aeromagnetic intensity and simple Bouguer gravity maps of 

Behrendt and others (1973), which are also at 1:250,000 

scale, as an aid in interpreting the geology of the extensive ice-covered regions. The location of the contact of 

the Dufek intrusion on this sheet is based largely on the 

geophysical anomaly maps. 

A broad, regional Bouguer gravity gradient crosses the 

Pensacola Mountains. Values range from 82 mgal in the 

northwest, over Ronne Ice Shelf, to -90 mgal in the northern Neptune Range. Values over the mapped extent of 

the Dufek intrusion range from 40 mgal in the northwest 

to -60 mgal in the southeast. Theoretical models based on 

the gravity data suggest either crustal thinning northwestward across the west front of the Pensacola Mountains, 

or the presence of a steep, faultlike step at the crust-mantle interface along this front. 

The mass effect of the Dufek intrusion is superimposed 

on and distorts the regional gravity gradient. The exact 

gravity effect of the intrusion cannot be calculated from 

available data, but the maximum effect is estimated to 

be about 85 mgal. Assuming a reasonable range of density contrast of 0.27-0.33 gfcm3,this corresponds to 

about 8.8- 6.2 km for total thickness of the Dufek intrusion, according to Behrendt and others (1974, p. 13). 

Using gravity data from the southwest end of Dufek 

Massif, where the lowest part of the body is exposed, 

7 

and an assumed density contrast of 0.27 g/cm3, the basal unexposed part of the intrusion is estimated to be 

1.8- 3.5 km thick. 

Aeromagnetic anomalies of nearly 2,000 gammas, 

from peak to trough, are associated with the Dufek intrusion. In the Saratoga Table quadrangle, the maximum magnetic intensity of 1,1 00 gammas occurs near 

the southeast edge of Lexington Table about 3 km southwest of Mount Mann. Minimum intensities of -200 to 

-500 gammas occur in a narrow belt from Henderson 

Bluff to near Mathis Spur. The high magnetic anomalies, 

accordingly, are clearly associated with the magnetitebearing to magnetite-rich gabbroic cumulates of the 

upper part of the intrusion. 

The aeromagnetic anomalies extend northward to the 

edge of the surveyed region in the Davis Valley quadrangle; thus the full extent of the body is not known. 

The magnetic survey indicates that the intrusion is at 

least 24,000 km2 in area, and gravity data suggest that 

it has an additional area of 10,000 km2 (Behrendt and 

others, 1974). Behrendt and others (1973) believe that 

the areas of 200-gamma magnetic intensity and high 

gravity anomaly shown southeast of the Forrestal Range 

indicate a possible buried extension of the body. If 

their interpretation is correct, the intrusion would extend 

much farther southeastward than indicated on the geologic 

map and cross sections, and the outline of the body would 

be much more irregular than here inferred, as the sedimentary country rocks from Ray Nunatak to Coal Rock 

would form a deep embayment into the body. Although 

this is possible, especially if concealed faults are present, 

we believe that the geophysical anomalies are more likely 

related to a different body of mafic rock. Because of 

this uncertainty, both alternative contacts in this ice-covered region are shown on the map. 

PALEOMAGNETIC DATA 

Paleomagnetic properties of rocks of the Dufek intrusion are discussed by Beck and others ( 1968), Griffin 

(1969), Beck and Griffin (1971) and Beck (1972). Intensity of natural remanent magnetization and magnetic 

susceptibility generally increase with height in the intrusion. Both normal and reversed polarities are present 

and may have resulted from either a self-reversal or a 

geomagnetic field-reversal mechanism. The paleomagnetic pole position for the body is at lat 56.5° S., long 

168° W., near poles reported for the Jurassic Ferrar 

Dolerite from elsewhere in the Transantarctic Mountains. 

The paleomagnetic studies reported by Beck and 

Schmidt ( 1971) show that regional heating apparently 

associated with the Triassic Weddell orogeny in the 

Pensacola Mountains resulted in the resetting of remanent magnetization directions of Cambrian volcanic 

rocks in the southern Neptune Range. 
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