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DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

GEOLOGIC MAP OF THE 

DAVIS VALLEY QUADRANGLE 

AND PART OF THE 

CORDINER PEAKS QUADRANGLE, 

PENSACOLA MOUNTAINS, 

ANTARCTICA 

By Arthur B. Ford, Dwight L. Schmidt, and Walter W. Boyd, Jr. 

Prepared by the 

U.S. GEOLOGICAL SURVEY 

under the auspices of the 

NATIONAL SCIENCE FOUNDATION 

U.S. ANTARCTIC RESEARCH PROGRAM MAP 

Published by the U.S. Geological Survey, 1978 

G 

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V'l 

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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 

Normal fault-Hachured on downthrown side 

Fault-Showing relative horizontal 

movement 

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

~ Anticline-Showing direction of plunge; 

~20 

---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 axi~ 

Minor syncline-Showing plunge of axis 

Strike and dip of beds-Ball indicates 

top of beds known from sedimentary 

structures 

_1!_ Inclined EB Horizontal 

-+- Vertical -..J!. Overturned 

Strike and dip of foliation 

~ Inclined -+·Vertical +Horizontal 

Strike and dip of cleavage 

~ Inclined ~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 

)(' Abandoned 

X Abandoned 

¢ Shut-in -<r Dry hole, 

~ Show of gas abandoned 

~ Show of oil 
DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY 

TO ACCOMPANY MAP A-10 

GEOLOGY OF THE DAVIS VALLEY QUADRANGLE AND PART OF THE CORDINER PEAKS 

QUADRANGLE, PENSACOLA MOUNTAINS, ANTARCTICA 

By Arthur B. Ford, Dwight L. Schmidt, and Walter W. Boyd, Jr. l 

INTRODUCTION 

The area that includes the Davis Valley quadrangle 

and part of the Cordiner Peaks quadrangle in the 

Pensacola Mountains was first seen and photographed 

on January 13, 1956, on a U.S. Navy transpolar 

flight from McMurdo Sound. An International 

Geophysical Year (IGY) oversnow traverse party 

from Ellsworth Station reached Cairn Ridge, near 

Dufek Massif, December 9, 19 57, during the first 

exploration of the inner margins of the Filchner and 

Ronne Ice Shelves (Neuburg and others, 1959). 

Trimetrogon aerial mapping photography was 

obtained by the U.S. Air Force in January 1958, 

at which time nearly the full extent of the Pensacola 

Mountains was first seen.2 

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-lB 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, 1969). Additional geologic studies were made 

in January 197 4 in the Cordiner Peaks (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 the area shown on 

this map is the northern part of the Dufek intrusion 

(Ford and Boyd, 1968), an immense differentiated 

stratiform mafic igneous body that was discovered 

in 1957 by an IGY traverse party visiting Dufek 

Massif, a northern range of the Pensacola Mountains 

(Aughenbaugh, 1961; Walker, 1961; Neuburg and 

others, 19 59). Time limitations and the presence of 

crevasse fields on the J a burg Glacier limited the IGY 

studies to lower parts of the northern escarpment of 

1 Present address, Geological Survey of Finland Helsinki 

2Aerial photographs of the Pensacola Mountai~s can be 

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

Reston, Va 22092. 

Dufek Massif. Our 1965-66 studies showed that the 

intrusion makes up all of Dufek Massif and virtually 

all of the Forresta1 Range (Ford and others, 1978). 

Geophysical surveys show that most of the body is 

covered by ice sheets adjoining the ranges and that 

its areal size 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. Although the 

ice terrain near Dufek Massif is suitable for ski-plane 

landings, aircraft operations are commonly hindered 

by adverse weather conditions. Our two seasons' 

experience in the area indicates that, at least in the 

summer months, the likelihood of poor visibility 

from fog and low clouds is the major factor limiting 

aircraft activity. Winds are generally slight along the 

northern escarpment of Dufek Massif. Summer 

temperatures during our work near Dufek Massif 

generally ranged from about -16°C (4°F) to -3°C 

(26°F) and averaged about -8° C (18°F). 

Work on the Dufek intrusion in 1965-66 was 

chiefly by A. B. Ford and W. W. Boyd, Jr. Study of 

the sedimentary country rocks in the Cordiner Peaks 

and Spanley Rocks was mainly by D. L. Schmidt and 

W. H. Nelson and was continued in the Cordiner 

Peaks in January 1974 by Ford. 

This report briefly outlines geologic relations and 

reviews work done in the area of the map sheet 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 1A. 
A 

EXPLANATION 

Bedrock exposure 

""""' Escarpment 

Hachures on downslope side 

Direction of ice flow 

B 

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

B, Location of Pensacola Mountains in Antarctica. 

The principal rock exposures in the mapped 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 generally small and isolated nunataks in the southern part of 

the map area. Relations between sedimentary formations are inferred to be the same as in the Neptune 

Range, about 40 km south of Dufek Massif, 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 rock-stratigraphic 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 

"';jl, 

2 

Williams ( 1969). The summary below is drawn largely 

from these sources. 

SEDIMENTARY ROCKS 

The sedimentary units exposed in this area belong 

chiefly to the youngest of three depositional sequences known to the south in the Neptune Range. 

In the Neptune Range the sequences 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 a few small nunataks 

near the southwest corner of the map area but 

probably are much more extensive under the cover 

of ice. Correlatives of the second sequence are not 

exposed but could occur beneath the ice cover. 

Rocks of the third sequence form the principal 

nunataks south 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 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 Davis Valley and Cordiner Peaks quadrangles. 

In the Schmidt Hills quadrangle, the Patuxent unconformably underlies Cambrian limestone. 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 mapped area, consists 

of a fossiliferous 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 preDevonian angular unconformities that are known to 

exist below the Neptune Group in the central and 

southern parts of the Neptune Range. 

The third sequence, which is at least 4,000 m thick 

in the Neptune Range, consists in ascending order of 

the Neptune Group, Dover Sandstone, Gale Mudstone, 

and the Pecora Formation. The Neith Conglomerate, a 

coarse, poorly sorted unit at the base of the Neptune 

Group, is exposed in the map area only on small 

nunataks of the Spanley Rocks. Although its upper 

contact is not exposed, the unit 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 Ridg~ Conglomerate, in the type area of the Neptune Group. 

The Neith may be as much as 1 ,000 m thick in the 

Schmidt Hills quadrangle. The Dover Sandstone 

which overlies the Neptune Group, is disconfor~ably overlain by the Gale Mudstone, a diamictite of probable continental glacial origin (Schmidt and Williams 

1969; Schmidt and Friedman, 1974 ). The contact ' 

between the Gale and Dover is well exposed on Rosser 

Ridge. The Pecora Formation is not exposed in the 

map area. Although its contact with the underlying 

Gale is not exposed in the Pensacola Mountains 

the two formations appear to be concordant. R~re diagnostic fossils indicate a Devonian age for the 

Dover and a Late Permian age for the Pecora (Schopf, 

3 

1968). Depositional environments ranged 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 high-angle 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 (500-530 m.y.) orogenies, 

which were 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 having 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 (Graddock, 1972; Dalziel and 

Elliott, 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 

Cordiner Peaks area, the folds are open and upright. 

The rocks are moderately folded about gently plunging or horizontal axes trending about N. 1 O-l5°E. The 

large anticline at the west end of Rosser Ridge has an 

amplitude of at least 400-500 m and a half wavelength of about 4-5 km. Coarse well-developed axial-
plane cleavage in the Gale Mudstone suggests that · 

folds are overturned a few degrees westward (fig. 2). 

Joint patterns show crudely developed orthogonal 

symmetry geometrically related to major folds. 

•• 0 

• 0 0 

Cbo OR 0 0 0 0 • .. • Average • •• • o8 cleavage • • • • 0 

•• =· • • 

•• • 

• • • 

• • • • •• 

Figure 2 - Lower-hemisphere stereographic projection of 

poles to cleavage (circles), joints (dots), and basalt 

dikes (squares) in the Cordiner Peaks. X's mark 

average cleavage and dike poles. After Ford (1974) 

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. 

MAFIC DIKES 

Tholeiitic basalt and diabase dikes are locally 

abundant in the eastern and central parts of Rosser 

Ridge where they cut the Dover Sandstone and Gale 

Mudstone. Their general orientation of about 

N. 45°E. in strike and steep southeastward dip shows 

little apparent relation to the dominant joint and 

cleavage fracture systems (fig. 2). The dikes range in 

thickness from a few centimeters to about 6m. Dikes· 

thicker than about 1m generally have diabasic interiors. Devitrified glassy selvages are common. The 

dikes are nonporphyritic and appear homogeneous 

except for grain-size and textural variations related to 

distance from contacts. The rocks are commonly 

slightly altered, as indicated by presence of minor 

amounts of calcite and other secondary minerals 

after plagioclase and pyroxene. 

The dikes are not deformed, and thus they postdate the Triassic orogeny in this area. In the entire 

Pensacola Mountains, basalt dikes known to be of 

4 

Triassic or younger age occur only on Rosser Ridge, 

one of the nunataks of Paleozoic country rock 

nearest to the Dufek intrusion. (Basaltic dikes are also 

known in the Argentina Range but are dated only as 

Cambrian or younger, and theoleiitic basalt and diabase sills occur in the Pecora Escarpment where they 

intrude the Permian Pecora Formation.) The geographic distribution of the dikes suggests that they 

are related to the Dufek intrusion (Ford, 1974); therefore, the dikes are tentatively assigned a Jurassic age. 

If the dikes were emplace~ at the same time as the 

Dufek, their slight alteration can be interpreted as 

being a contact effect during slow cooling of the 

Dufek magma. 

THE DUFEK INTRUSION 

The first work on the Dufek intrusion by A ughenbaugh (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 area south of this 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 augite-ferroaugite, 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, 197 5, 

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 

giv~n formal rock-stratigraphic names using standard 

noncumulate igneous rock nomenclature. The cumulate terminology of Jackson ( 1967, 1971 ), however, 

is used 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 intrusion are discussed by Ford ( 1976) 

and are shown in figure 3. 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 "Forrestal 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 ( 197 4) 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 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 Forrestal 

Range. 

5 

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 mineralgraded 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 below. 

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 3. Cumulates of 

gabbroic composition generally consist of subequal 

amounts of plagioclase and pyroxene. Magnetite is a 

chief modal variant occurring 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 proxene 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 

constjtuents 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 so_ 85 

in anorthositic cumulates near the base of the Dufek 

Massif section to about An45-55 in gabbroic rocks 

and granophyre near the top of the Forrestal Range 

section (Ford and Boyd, 1968). Electron microprobe 

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 3 - Rock-stratigrilphic 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 Ford (1976) and Himmelberg and Ford (1976) 

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40 60 80 100 0.2 0.4 0.6 0.8 

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

• Mafic rocks • Ca-rich pyroxene 

o Felsic rocks • Ca-poor pyroxene 

Figure 4 - Stratigraphic variations in chemical composition in the 

Dufek intrusion. A, Mafic index, (FeO + Fe 03 ) x 1 00/(FeO + 

Fe2 03 + MgO), of whole rocks (after Ford, 197t)). B, Cation ratio, 

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

Ford, 1976). 

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 Fe 46.5, and that 

Ca-poor cumulus pyroxenes in the series bronziteinverted pigeonite range from Ca 3.5 Mg 69.1 Fe 2 7.4 

to Ca 11.4 Mg34.0 Fes4.6 (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 bulkrock compositions (fig. 4 ). Although major-element 

contents of the oxide minerals show no regular 

variation with height, the minor elements vanadium 

and aluminum in ilmeno-magnetites generally decrease with height and thus are useful indicators of 

fractionation trends during the stage of oxide-mineral 

crystallization (Himmelberg and Ford, 1977). 

FELSIC DIKES 

Dikes of aplite, felsic pegmatite, and less common 

alaskite, leucogabbro, and gabbroic pegmatite occur 

sporadically throughout the Dufek intrusion and at 

many stratigraphic levels in the layered sequence. 

Zoning occurs locally. Dikes are more abundant in 

the Forrestal Range than in Dufek Massif. They are 

mostly vertical to subvertical and in places pinch out 

downward. Thicknesses of different dikes range from 

a few millimeters to several meters. Compositions of 

dikes seem to show changes related to stratigraphy 

and compositional changes of cumulate-host rocks, 

which suggests that they formed by the filling of 

fractures with late intercumulus liquids from local 

7 

sources at various stages during consolidation of 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. 

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 of this 

area 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, Buckeye 

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 

GRAVITY 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 Davis Valley 

and part of the Cordiner Peaks quadrangles should 

be used in conjunction with the aeromagnetic intensity and simple Bouguer gravity maps of Behrendt 

and others ( 1973), which are also of 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 upon 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 

gjcm3, 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, and an assumed density contrast of 0. 27 gfcm3, the basal unexposed part of the 

8 

..J 

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 map area, anomalies are highest - about 

1 ,000 gammas - over Mount Malville in the Forrestal 

Range. Maximum anomalies are lower - about 400 

gammas - in the northeast end of Dufek Massif near 

Davis Valley and are still lower toward the west end of 

the massif. The high magnetic anomalies, accordingly, 

are clearly associated with the magnetite-bearing to 

magnetite-rich gabbroic cumulates of the upper part of 

the intrusion. 

The aeromagnetic and gravity data indicate that the 

Dufek intrusion extends for great distances northeastward and northward beneath the sheet of continental 

ice. Aeromagnetic anomalies extend to the ·edge of the 

surveyed region, and thus the full extent of the body is 

unknown. The magnetic survey indicates that the intrusion is at least about 24,000 km2 in area, and gravity 

data suggest that it has an additional area of 10,000 

km2, according to Behrendt and others (1974). 

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 ma,y 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. 

REFERENCES CITED 

Aughenbaugh, N. B., 1961, Preliminary report on the 

geology of the Dufek Massif: Internatl. Geophys. 

Year World Data Center A, Glaciology, Glac. Rept. 

4, p. 155-193. 

Barrett, P. J., Grindley, G. W., and Webb, P. N., 1972, 

The Beacon Supergroup of east Antarctica, in Adie, 

R. J., ed., Antarctic geology and geophysics: Oslo, 

Universitetsforlaget, p. 319-332. 

Beck, M. E., Jr., 1972, Palaeomagnetism and magnetic 

polarity zones in the Jurassic Dufek intrusion, Pensacola Mountains, Antarctica: Royal Astron. Soc. 

Geophys. Jour., v. 28, p. 49-63. 

Beck, M. E., Jr., Ford, A. B., and Boyd, W. W., Jr., 1968, 

Paleomagnetism of a stratiform mafic intrusion in the 

Pensacola Mountains, Antarctica: Nature, v. 217, 

p. 534-535. 

Beck, M. E., Jr., and Griffin, N. L., 1971, Magnetic 

intensities in a differentiated gabbroic body, the 
.... 

Dufek intrusion, Pensacola Mountains, Antarctica, 

in Geological Survey research 1971: U.S. Geol. 

Survey Prof. Paper 750-B, p. Bll7-Bl21. 

Beck, M. E., Jr., and Schmidt, D. L., 1971, Reset 

direction of remanent magnetization for Upper 

Cambrian rhyodacitic welded tuff, Pensacola 

Mountains, Antarctica, in Geological Survey research 

1971: U.S. Geol. Survey Prof. Paper 750-C, p. 

Cl74-Cl78. 

Behrendt, J. C., Henderson, J. R., Meister, Laurent, and 

Rambo, W. L., 1974, Geophysical investigations of 

the Pensacola Mountains and adjacent glacierized 

areas of Antarctica: U.S. Geol. Survey Prof. Paper 

844, 28 p. 

Behrendt, J. C., Rambo, W. L., Henderson, J. R., 

Wanous, R. E., and Meister, Laurent, 1973, Simple 

bouguer gravity and aeromagnetic maps of the Davis 

Valley quadrangle and part of the Cordiner Peaks 

quadrangle and vicinity, Antarctica: U.S. Geol. 

Survey Geophys. lnv. Map GP-887. 

Cameron, R. E., and Ford, A. B., 1974, Baseline analyses 

of soils from Mount Lechner and Cordiner Peaks, 

Pensacola Mountains, Antarctica: Antarctic Jour. 

U.S., v. 9, no. 4, p. 116-119. 

Clarkson, P. D., 1972, Geology of the Shackleton 

Range: a preliminary report: British Antarctic 

Survey Bull. 31, p. 1-15. 

Craddock, Campbell, 1972, Antarctic tectonics, in Adie, 

R. J., ed., Antarctic geology and geophysics: Oslo, 

Universitetsforlaget, p. 449-455. 

Dalziel, I. W. D., and Elliot, D. H., 1973, The Scotia 

Arc and Antarctic margin, in Nairn, A. E. M., and 

Stehli, F. G., ed., The ocean basins and margins, v. 1: 

New York, Plenum, p. 171-246. 

Eastin, Rene, and Faure, Gunter, 1972, Geochronology 

of the basement rocks of the Pensacola Mountains, 

Antarctica (abs.): Geol. Soc. America Abs. with 

Programs, v. 4, no. 7, p. 496. 

Elliot, D. H., 1975, Tectonics of Antarctica: a review: 

Amer. Jour. Science, v. 275-A, p. 45-106. 

Ford, A. B., 1968, Origin of microfractures and joints 

in the Dufek intrusion, Antarctica (abs.): Geol. 

Soc. America Spec. Paper 115, p. 325. 

1970, Development of the layered series and 

~ping granophyre of the Dufek intrusion of Antarctica, in Visser, D. J. L., and Von Gruenewaldt, 

Gerhard, ed., Symposium on the Bushveld Igneous 

Complex and other layered intrusions: Geol. Soc. 

South Africa, Spec. Pub. No. 1, P. 492-510. 

___ 1972, The Weddell orogeny- latest Permian 

to early Mesozoic deformation at the Weddell Sea 

margin of the Transantarctic Mountains, in Adi~, 

R. J., ed., Antarctic geology and geophysics: Oslo, 

Universitetsforlaget, p. 419-425. 

1974, Basalt dikes of the Cordiner Peaks, 

Pensacola Mountains, Antarctica--satellitic bodies 

of the Dufek intrusion?: Antarctic Jour. U.S., 

v. 9, no. 4, p. 149-l 53. 

___ 1976, Stratigraphy of the layered Dufek 

intrusion of Antarctica: U.S. Geol. Survey Bull. 

9 

1405-D, p. Dl-D36. 

Ford, A. B., and Boyd, W. W., Jr., 1968, The Dufek 

intrusion, a major stratiform gabbroic body in the 

Pensacola Mountains, Antarctica: Internat. Geol. 

Cong., 23d, Proc., v. 2, p. 213-228. 

Ford, A. B., and Nelson, S. W., 1972, Density of the 

stratiform Dufek intrusion, Pensacola Mountains, 

Antarctica: Antarctic Jour. U.S., v. 7, no. 5, 

p. 147-149. 

Ford, A. B., Schmidt, D. L., Boyd, W. W., Jr., and 

Nelson, W. H., 1978, Geologic map of the Saratoga 

Table quadrangle,Pensacola Mountains, Antarctica: 

U. S. Geol. Survey Antarctic Geol. Map 

A-9, scale 1:250,000. 

Grapes, R. H., and Reid, D. L., 1971, Rhythmic 

layering and multiple intrusion in the Ferrar 

Dolerite of south Victoria Land, Antarctica: 

New Zealand Jour. Geology and Geophysics, 

v. 14, p. 600-604. 

Griffin, N. L., 1969, Paleomagnetic properties of the 

Dufek intrusion, Pensacola Mountains, Antarctica: 

California Univ., Riverside, M.S. thesis, 93 p. 

Grindley, G. W., 1963, The geology of the Queen 

Alexandra Range, Beardmore Glacier, Ross Dependency, Antarctica; with notes on the correlation of Gondwana sequences: New Zealand Jour. 

Geology and Geophysics, v. 6, p. 307-34 7. 

Gunn, B. M. 1963, Layered intrusions in the Ferrar 

Dolerites, Antarctica: Mineral. Soc. America Spec. 

Paper 1, p. 124-133. 

Himmelberg, G. R., and Ford, A. B., 1973, Pyroxene 

compositional trends in the Dufek intrusion, 

Pensacola Mountains, Antarctica: Antarctic Jour. 

U.S., V. 8, p. 260-263. 

___ 1975, Petrologic studies of the Dufek intrusion: iron-titanium oxides: Antarctic Jour. U.S., 

v. lO,no. S,p. 241-244. 

___ 1976, Pyroxenes of the Dufek intrusion, 

Antarctica: Jour. Petrology, v. 17, pt. 2, p. 219-

243. 

___ 1977, Iron-titanium oxides of the Dufek intrusion, 

Antarctica: Am. Mineralogist, v. 62, p. 623-633. 

Huffman, J. W., and Schmidt, D. L., 1966, Pensacola 

Mountains project: Antarctic Jour. U.S., v. 1, no. 

4, p. 123-124. 

Jackson, E. D., 1967, Ultramafic cumulates in the 

Stillwater, Great Dyke, and Bushveld intrusions, 

in Wyllie, P. J ., ed., Ultramafic and related rocks: 

New York, John Wiley and Sons, p. 20-38. 

___ 1971, The origin of ultramafic rocks by cumulus processes: Fortschr. Mineralogie, v. 48, 

p. 128-174. 

Juckes, L. M., 1972, The geology of north-eastern 

Heimefrontfjella, Dronning Maud Land: British 

Antarctic Survey, Sci. Report 65,44 p. 

Kovacs, Austin, and Abele, Gunars, 1974, Blue ice 

runway site survey, Pensacola Mountains: Antarctic 

Jour. U.S., v. 9, p. 175-177. 

Neuburg, H. A. C., Thiel, Edward, Walker, P. T., 

Behrendt, J. C., Aughenbaugh, N. B., 1959, The 
Filchner Ice Shelf: Assoc. Am. Geographers 

Annals, v. 49, no. 2, p. 110-119. 

Palmer, A. R., and Gatehouse, C. G., 1972, Early and 

Middle Cambrian trilobites from Antarctica: U.S. 

Geol. Survey Prof. Paper 456-D, 37 p. 

Schmidt, D. L., Dover, J. H., Ford, A. B., and Brown, 

R. D., Jr., 1964, Geology of the Patuxent Mountains 

in Adie, R. J., ed., Antarctic geology: New York, 

John Wiley and Sons, p. 276-283. 

Schmidt, D. L., and Ford, A. B., 1969, Geologic map, 

with accompanying text, of the Pensacola and 

Thiel Mountains; sheet 5, Geologic maps of 

Antarctica: New York, Am. Geog. Soc., scale 

1: 1 ,000,000. 

Schmidt, D. L., and Friedman, Irving, 1974, Continental deposition of Antarctic tillite indicated by 

carbon and oxygen isotopes: U.S. Geol. Survey 

Jour. Research, v. 2, no. 6, p. 711-715. 

Schmidt, D. L., and Williams, P. L., 1969, Continental glaciation of late Paleozoic age, Pensacola 

Mountains, Antarctica, in Amos, A. J., ed., 

Gondwana stratigraphy, lUGS Symposium, Buenos 

Aires, October, 1967: UNESCO Earth Sci. Ser., 

v. 2, Paris, p. 617-649. 

Schmidt, D. L., Williams, P. L., and Nelson, W. H., 

1978, Geologic map of the Schmidt Hills quadrangle 

and part of the Gambacorta Peak quadrangle, 

10 

Pensacola Mountains, Antarctica: U.S. Geol. Survey Antarctic Geol. Map A-8, scale 1 :250,000. 

Schmidt, D. L., Williams, P. L., Nelson, W. H., and 

Ege, J. R., 1965, Upper Precambrian and Paleozoic 

stratigraphy and structure of the Neptune Range, 

Antarctica, in Geological Survey research 1 965: 

U.S. Geol. Survey Prof. Paper 525-D, p. Dll2-Dll9. 

Schopf, J. M., 1968, Studies in Antarctic paleobotany: Antarctic Jour. U.S., v. 3, no. 5, p. 176-177. 

___ 1969, Ellsworth Mountains; position in west 

Antarctica due to sea-floor spreading: Science, 

V. 164, p. 63-66. 

Stephenson, P. J., 1966, Geology, 1. Theron Mountains, Shackleton Range and Whichaway Nunataks: 

Trans-Antarctic Expedition, 1955-1958, Sci. 

Repts., no. 8, 79 p. 

Wager, L. R., and Brown, G. M., 1968, Layered 

igneous rocks: Edinburgh, Oliver and Boyd, 558 p. 

Walker, P. T., 1961, Study of some rocks and minerals 

from the Dufek Massif, Antarctica: Internatl. 

Geophys. Year World Data Center A, Glaciology, 

Glac. Report 4, p. 195-213. 

Williams, P. L., 1969, Petrology of upper Precambrian 

and Paleozoic sandstones in the Pensacola Mountains, Antarctica: Jour. Sed. Petrology, v. 39, 

no. 4, p. 1455-1465.