Stratigraphy of the

Layered Gabbroic

Dufek Intrusion, Antarctica

GEOLOGICAL SURVEY BULLETIN 1405-D

Work done in cooperation with the 

National Science Foundation

Stratigraphy of the

Layered Gabbroic

Dufek Intrusion, Antarctica

By ARTHUR B. FORD

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1405-D

Description of newly named units in 

a differentiated stratiform mafic 

intrusion in the northern Pensacola 

Mountains of Antarctica

Work done in cooperation with the 

National Science Foundation

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Ford, Arthur Barnes, 1932-

Stratigraphy of the layered gabbroic Dufek intrusion, Antarctica. (Contributions to stratigraphy)

(Geological Survey Bulletin 1405-D)

Bibliography: p. 35-36.

Supt. of Docs. no. I 19.3:1405-D

1. Geology, Stratigraphic-Jurassic. 2. Intrusions (Geology)-Antarctic regions-Pensacola 

Mountains. 3. Gabbro-Antarctic regions-Pensacola Mountains. I. United States National 

Science Foundation. II. Title. HI. Series. IV. Series: United States Geological Survey 

Bulletin 1405-D.

QE75.B9 no. 1405-D [QE681] 557.3'08s [551.V6] 76-608042

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington, D. C. 20402

Stock Number 024-001-02800-0
CONTENTS

Page

Abstract _____________________________________________ Dl 

Introduction __________________________________________ 1

Sedimentary nature of the layered igneous rocks __ _ ______ ______ _ _ 5

General features of the stratigraphy _________ ___ _ ______ ______ 7

Zonal nomenclature _________________________________ 10

Previous nomenclature ____________________________________ 13

Present nomenclature __ _________________________________ 13

Forrestal Gabbro Group_________________________________ 16

Walker Anorthosite_____________________________-- 17

Aughenbaugh Gabbro______________________________ 17

Lower anorthosite member _______ ______________ 21

Neuberg Pyroxenite Member ______ _______________ 21

Frost Pyroxenite Member ________________________ 25

Spear Anorthosite Member ______________________ 26

Saratoga Gabbro ____________________________________ 27

Stephens Anorthosite Member ______________ ________ 30

Lower inclusion member ________________________ 31

Upper inclusion member __________________________ 31

Lexington Granophyre ________________________-_____ 33

Age and correlation__________________________________ 34

References cited ______________________________________ 35

ILLUSTRATIONS

Page

FIGURE 1. Index map of the northern Pensacola Mountains ___ _ __D2

2. Physiographic sketch of north face of Dufek Massif- _____ 4

3. Diagram showing strati graphic variation in mafic index and 

pyroxene composition in the Dufek intrusion ___________ 9

4. Diagram showing stratigraphic distribution of minerals in the Dufek 

intrusion _____________________________ 12

5. Diagram showing stratigraphic nomenclature of the Dufek intrusion 15

6. Photograph of plagioclase cumulate of the Walker Anorthosite __ 18

7. Photograph of Neuberg Peak showing gabbroic cumulates of the 

Aughenbaugh Gabbro ______________________ 19

8. Photograph of well-developed layering in cumulates of upper part of 

the Aughenbaugh Gabbro near Spear Spur ___________ 20

9. Physiographic sketch of western Dufek Massif __________ 22 

10-16. Photographs of:

10. Neuberg Pyroxenite Member and contact of the Walker

Anorthosite with the Aughenbaugh Gabbro on Walker Peak 23

11. Neuberg Pyroxenite Member showing mineral-graded nature of 

unit __________________________________ 24
IV CONTENTS

Page

FIGURES 10-16. Photographs of Continued

12. Mineral-graded pyroxene-rich cumulate layers of the Frost

Pyroxenite Member on Neuberg Peak____________ D25

13. Lower part of Saratoga Gabbro and Stephens Anorthosite

Member at type section on Mount Stephens _________ 28

14. Upper part of Saratoga Gabbro and the overlying Lexington

Granophyre at Sorna Bluff___________________ 29

15. Layered cumulates of upper part of Saratoga Gabbro at Sorna

Bluff _______________.____________ _____--_______________ 30

16. Leucogabbro autoliths in dark cumulates of the upper inclusion

member of the Saratoga Gabbro at Sorna Bluff ___ _ 32

METRIC-ENGLISH EQUIVALENTS

Metric unit

metre (m)

kilometre (km)

square kilometre (km2 )

gram per cubic centimetre (g/cm3)

English equivalent 

3.28 feet 

.62 mile 

.386 square mile 

62.43 pounds per cubic foot
CONTRIBUTIONS TO STRATIGRAPHY

STRATIGRAPHY OF THE

LAYERED GABBROIC DUFEK INTRUSION,

ANTARCTICA

By ARTHUR B. FORD

ABSTRACT

The Dufek intrusion in the northern Pensacola Mountains of Antarctica is a 

differentiated stratiform mafic igneous complex of unusually large areal dimension, 

probably at least 24,000-34,000 km2 , and a thickness estimated to be 8-9 km. It is 

Jurassic in age and presumably correlative with the Ferrar Group of basalt and diabase 

elsewhere in the Transantarctic Mountains. The body consists chiefly of well-layered 

pyroxene gabbro that contains abundant magnetite in higher levels. Minor lithologies of 

layers range from pyroxenite, leucogabbro, and anorthosite to magnetitite. The 

sequence is capped concordantly by a 300-m-thick layer of granophyre. Textures and 

structures indicate that most rocks are cumulates made up of variable amounts of settled 

plagioclase and two pyroxenes, one rich in calcium, in the series augite-ferroaugite, and 

the other poor in calcium, in the series bronzite-inverted pigeonite. Magnetite is a mjaor 

cumulus phase in many rocks. Cumulus apatite occurs at highest levels. The rocks form 

a stratigraphic succession that is similar in many respects to a sequence of sedimentary 

strata.

The Forrestal Gabbro Group consists of all the layered rocks, of which about a 3.5-km 

thickness is exposed. The group is made up of four mappable formations, in ascending 

order the Walker Anorthosite, Aughenbaugh Gabbro, Saratoga Gabbro, and Lexington 

Granophyre. The Walker and Aughenbaugh form a 1.8-km-thick section in Dufek 

Massif, and the Saratoga and Lexington form a 1.7-km-thick section in the Forrestal 

Range.

Major conspicuous layers mapped within the gabbroic units are recognized as 

members. Two of these, the Neuberg Pyroxenite Member and the Frost Pyroxenite 

Member, are key marker units in the Aughenbaugh Gabbro that are widely mappable in 

the area of Dufek Massif. Two other members of the Aughenbaugh are an unnamed 

lower anorthosite member, near the base of the formation, and the Spear Anorthosite 

Member, at the top. Three members are recognized in the Saratoga Gabbro: the Stephens 

Anorthosite Member at the base and two unnamed members characterized by 

conspicuous, rounded, and embayed inclusions of leucogabbro and anorthosite.

INTRODUCTION

Mafic igneous rocks of an unusually large differentiated stratiform 

intrusion make up two of the northern ranges of the Pensacola 

Mountains near the southeastern edge of Ronne Ice Shelf, Antarctica

Dl
D2 CONTRIBUTIONS TO STRATIGRAPHY

(fig. 1). The gabbroic and related rocks are exposed throughout Dufek 

Massif and the Forrestal Range and in many nearby nunataks. They 

form the principal outcrops in the Cordiner Peaks, Davis Valley, and 

Saratoga Table quadrangles of the U.S. Geological Survey's

Davis ValleyN n ^

m^

^Rankine 

Rock

Hannah

\ Walker/^"" YC u XSpearSpur 

\Peak \ Aughenbaugh Peak 

^- ^_ Weuberg Peak

Jaburg ^.___ 

Glacier \

\

Sallee

Snowfield

A 

1P

e

ca? Cordiner 

i\ Peaks

o

90°w

Antarctica

Henderson Bluff

/Camp Spur

/ May '^ Chamt>ers Glacier 

/Valley

Mathis^ SARATOGAS 

SP"L* TABLE

Stephens

Bluff

20 MILES

10 20 KILOMETRES

82 30'

83 00'

83 30'

FIGURE 1. Index map of the northern Pensacola Mountains showing major outcrops 

(stippled), mentioned localities, and southern limit of the Dufek intrusion (heavy 

dashed line). Line F-F' shows the Enchanted Valley fault.
DUFEK INTRUSION, ANTARCTICA D3

l:250,000-scale Antarctica reconnaissance series of topographic maps. 

The body was named the Dufek intrusion by Ford and Boyd (1968) 

after Dufek Massif (fig. 2), where a major part of its stratigraphy is well 

exposed.

The intrusion was discovered December 9, 1957, during mainly 

geographical and geophysical explorations by oversnow vehicle of 

inner reaches of the Ronne Ice Shelf and adjoining regions. The 

expedition was part of the International Geophysical Year (IGY) 

program at Ellsworth Station (Neuberg and others, 1959). An 

approximately 380-m-thick partial stratigraphic section was measured in the lowest exposed layers of the body during a 6-day visit by the 

IGY party. The similarity of the rocks to those of other stratiform mafic 

complexes was recognized at that time (Aughenbaugh, 1961; Walker, 

1961).

The intrusion was mapped and sampled in detail by the U.S. 

Geological Survey in the austral summer 1965-66. The study was part 

of the final phase in completing geologic mapping of the entire 

Pensacola Mountains (Schmidt and Ford, 1969), a project begun in the 

southernmost range of the mountains in 1962-63. All major outcrops of 

the intrusion were either visited or viewed closely from the air by 

helicopter. This work was carried out from a base camp in the central 

Neptune Range where the party was placed by U.S. Navy LC-130F 

aircraft from McMurdo Station (Huffman and Schmidt, 1966). The 

Cordiner Peaks and the area of exposure of the contact zone, near 

Mount Lechner, were briefly revisited in January 1974 (Ford, 1974; 

Cameron and Ford, 1974).

Geophysical surveys carried out simultaneously with the geologic 

work show that the body extends for great distances under ice sheets 

adjoining Dufek Massif and the Forrestal Range. Geophysical 

anomalies that extend beyond the surveyed area indicate an areal 

distribution of at least 24,000-34,000 km2 (Behrendt and others, 1974).

Except for dike rocks and rocks of the border zone, the intrusion 

consists of generally well layered mafic rocks mostly of gabbroic 

composition. Less common lithologies include leucogabbro, anorthosite, granophyre, and minor pyroxenite and magnetitite. This report 

describes the general stratigraphic features of the layered rocks and 

defines the nomenclature of rock-stratigraphic units used in the 

geologic mapping of the body.

Acknowledgments. This work was carried out under auspices of the 

U.S. Antarctic Research Program of the Office of Polar Programs, 

National Science Foundation (NSF). Most of the logistic support was 

provided by the U.S. Navy Air Development Squadron Six and by NSF. 

UH-1B turbine helicopters, without which this work could not have 

been accomplished in a single field season, were provided by the U.S.
D4 CONTRIBUTIONS TO STRATIGRAPHY
DUFEK INTRUSION, ANTARCTICA D5

Army Aviation Detachment. Skillful piloting in the generally difficult 

terrain was a major factor in the safety and success of the project. 

Coordination, under difficult circumstances, of the field logistics support by Jerry Huffman, NSF, is gratefully acknowledged. The careful 

work of Walter W. Boyd, Jr., who carried out a major part of the 

geologic field study of the Dufek intrusion, is particularly acknowledged. Dwight L. Schmidt and Willis H. Nelson assisted in some of the 

field work during their principal work on sedimentary rocks of the 

region. Laboratory investigations were greatly helped by S. W. Nelson. 

The report benefited from reviews of the manuscript by E. D. Jackson, 

P. D. Rowley, and D. L. Schmidt.

SEDIMENTARY NATURE OF THE 

LAYERED IGNEOUS ROCKS

The Dufek intrusion is characterized by an abundance of conspicuous subhorizontal layering caused by compositional differences of the 

rocks. The layered structure and locally well developed cliff-and-slope 

topography led Aughenbaugh (1961, p. 166) to note the similarity from 

a distance of these igneous rocks with a thick sequence of undisturbed 

sedimentary rocks. Wager and Deer (1939, p. 5), in their original work 

on the Skaergaard intrusion of east Greenland, similarly state that 

"The stratification in the layered series is so marked that, when these 

rocks (of the Skaergaard) were first distantly seen from the deck of the 

ship in 1930, they were mistaken for well bedded red sandstones." The 

resemblance also of rocks of the Stillwater Complex of Montana to 

sedimentary rocks was noted by Jones, Peoples, and Rowland (1960, 

p. 286).

The layered igneous rocks of the Dufek intrusion formed by the 

accumulation of crystals by settling. Rocks of this origin are termed 

"cumulates"; their textures and structures reflect sedimentary 

processes controlled by gravity, hydraulic parameters of the magma, 

and the presence or absence of currents (Jackson, 1971). According to 

Jackson (1967), who follows usage in sedimentary petrology by 

distinguishing between grains and cement, cumulates are divisible 

into two components: (1) the cumulus phases that settled and packed 

together to make the framework of the rock and (2) the postcumulus 

material which is primary material that formed in the place it now 

occupies in the magmatic sediment. The cumulus textures are 

characterized by the alinement of platy minerals, chiefly plagioclase, 

in the plane of layering a feature termed igneous lamination by 

Wager and Deer (1939, p. 37). Jackson (1967, p. 22) distinguishes 

planar and lineate lamination depending upon whether the 

cumulus-mineral parallelism is planar or linear. Noncumulus rocks 

are those such as the granophyre of the Dufek intrusion in which
D6 CONTRIBUTIONS TO STRATIGRAPHY

crystal settling was negligible, as indicated by absence of layering and 

lamination.

Cumulates are named by prefixing names of the settled minerals in 

order of decreasing abundance (Wager and others, 1960, p. 85), in an 

analogous fashion to the naming of sedimentary rocks (Jackson, 1967, 

p. 23). Many rocks of the Dufek intrusion are thus plagioclase-augiteinverted pigeonite-opaque oxide cumulates that may more simply be 

referred to as gabbroic cumulates where proportions of cumulus phases 

are not known or are not otherwise indicated. Anorthositic cumulate 

and pyroxenitic cumulate consist, respectively, of settled plagioclase 

and of settled pyroxene but generally contain minor amounts of other 

phases, including postcumulus minerals. Although most minerals 

occur as either cumulus or postcumulus phases in different rocks of 

this intrusion, quartz and K-feldspar are only postcumulus, and 

apatite is only cumulus in cumulates at a high stratigraphic level.

Layering is the most pronounced primary structure of the Dufek 

intrusion and is analogous to bedding in a sedimentary sequence. 

According to Wager and Brown (1968, p. 5), the term bedding "could 

have been used equally well ibr the sheet-like structures of igneous 

rocks." They opt to use the term layering, however, and to restrict 

bedding for sedimentary rocks. This usage is followed in this report.

Layers in the Dufek body range in thickness from a few millimetres 

to many tens of metres. Terminology of layers in this report generally 

follows Jackson (1967, p. 22). Some layers, termed "mineral graded," 

show gradual stratigraphic change in proportions of cumulus 

minerals; others, termed "isomodal," are characterized by uniform 

proportions of cumulus minerals. Mineral-graded layers are much 

more common than isomodal layers in the Dufek intrusion. A few 

conspicuous mineral-graded layers of pyroxenitic cumulate are 

traceable with little change in thickness for tens of kilometres along 

strike lines in Dufek Massif. Many layers have sharply defined bases 

and gradational tops, thus resembling graded bedding of sedimentary 

rocks. Trough layering (Ford, 1970, fig. 6), similar to that described in 

the Skaergaard intrusion (Wager and Deer, 1939, p. 45), occurs at 

several stratigraphic levels and is analogous to cut-and-fill channels in 

water-deposited sediments. The truncation of thin layering in 

cumulates beneath the troughs clearly indicates that magma-current 

erosion was involved in the origin of the trough-layered structures in 

the Dufek intrusion.

Although magma currents were operative, there is little evidence for 

them in the fabric of the rocks. Most rocks of the intrusion show 

well-developed planar lamination, but lineate lamination was not 

observed. Lack of linear fabric indicates that the lamination was 

imparted by crystal settling after cessation of lateral current flow. The 

currents were probably of convective origin, as described in detail by
DUFEK INTRUSION, ANTARCTICA D7

Hess (1960, p. 134) and by Wager and Brown (1968, p. 210) for other 

layered intrusions. Like turbidity currents in aqueous environments, 

they were bottom-flowing currents laden with suspended crystals, and 

in places they scoured the floor beneath them. At times the currents 

were canalized, eroding deep channels, as indicated by the occurrence 

of trough layering at several levels in the layered sequence.

Although all major mineral-graded layers (as well as size-graded 

layers, if they are found in future study) are probably related to 

turbidity like current activity, other types of layering may have a 

different origin. Cumulus textures in rocks of thick stratigraphic 

sections (fig. 7) that are free of major mineral-graded layers indicate 

that crystals continued to settle during lengthy quiescent intervals. 

Fine-scale mineral-graded layering on the order of a few millimetres to 

about 1 cm in thickness, however, is generally present in these rocks. 

The fine-scale layering, which is similar to that described as "rhythmic 

layering" by Wager and Deer (1939, p. 41), is believed to have formed 

by the differential settling mechanism of Coats (1936).

Stratification in the Dufek intrusion thus formed, crystal on crystal 

and layer on layer, in a manner similar to stratification in sedimentary 

rocks; as noted by Jackson (1971, p. 145), "the law of superposition is as 

rigorously obeyed as it is in sedimentary rocks." Minor disconformities 

can be recognized where scour channels cut layering in underlying 

cumulates. Diastemlike breaks, representing intervals of nondeposition, may be present, but none have been recognized in the measured 

sections. The disconformities possibly mark bases of cyclic units such 

as those recognized in other layered intrusions (Jackson, 1971, p. 150). 

The cyclic units, according to Jackson, are regularly repetitive 

sequences of layers formed from repetitive influxes of small batches of 

magma, perhaps brought in by convection currents. Compositional 

changes in pyroxene strongly suggest the presence of cyclic units in the 

column (Himmelberg and Ford, 1976), but the stratigraphy of these 

units has not been studied in detail.

Separation of crystals from the originally basaltic Dufek magma led 

to progressive chemical changes, including notably an increase in the 

ratio of iron to magnesium and an increase in silica and alkalies in late 

stages. Granophyric residue of granodioritic composition (Ford, 1970) 

was eventually produced. The granophyre forms a layer, probably 

about 300 m thick, that concordantly caps the layered mafic sequence. 

It is noncumulus and apparently formed essentially in place, perhaps 

analogously to evaporites of the final desiccation stage in a sedimentary cycle.

GENERAL FEATURES OF THE STRATIGRAPHY

Owing to their stratigraphic characteristics, the layered igneous 

rocks were studied largely by using techniques as in the study of a
D8 CONTRIBUTIONS TO STRATIGRAPHY

stratified sedimentary sequence. Ten partial stratigraphic sections 

were measured in the area of Dufek Massif, and 11 in the Forrestal 

Range. 1 Most sections are intercorrelated within each range by using 

one or more key marker layers or horizons. The composite Dufek 

Massif section is about 1,800 m thick, and the composite Forrestal 

Range section is about 1,700 m thick. No markers in Dufek Massif can 

be correlated with the Forrestal Range. Chemical and mineralogic 

differences and geometry of the layered structure indicate that the two 

composite sections constitute different stratigraphic parts of one 

intrusion the Dufek Massif section being a lower part and the 

Forrestal Range section an upper part.

Rocks of Dufek Massif form a gentle homocline dipping 5°-10° 

southeastward beneath Sallee Snowfield. Projecting the average dip 

southeastward from Dufek Massif suggests that a 2-3-km-thick 

section of unexposed rock lies beneath the ice cover of Sallee Snowfield. 

The approximate chemical and mineralogic characteristics of this 

inferred hidden middle section, here called the "Sallee Snowfield 

section," can be estimated by projecting trends from the exposed 

sections (figs. 3, 4).

A second hidden section, here called simply the "basal section," is 

inferred to lie beneath the lowest exposed rock layers of Dufek Massif. 

Reconnaissance geophysical surveys by Behrendt, Henderson, Meister, and Rambo (1974, p. 14) suggest that this section is 1.8-3.5 km 

thick. Part of the basal section presumably consists of early ultramafic 

cumulates similar to those of other major layered intrusions (Jackson, 

1961; Wager and Brown, 1968). Comparison of plagioclase composition 

and of pyroxene occurrence and composition between the Dufek and 

other layered complexes, such as the Stillwater and Bushveld, 

suggests that mafic cumulates on the order of at least 1 km in thickness 

may lie between the lowest exposed Dufek layers and the possible 

ultramafic cumulates.

The roof of the intrusion is presumably eroded. The missing section, 

here called the "eroded section," probably included an upper part of the 

granophyre layer as well as mafic rock similar to that of the upper 

border group of the Skaergaard intrusion described by Wager and 

Brown (1968, p. 17). The thickness of the eroded section is unknown 

but by comparison with the Skaergaard is inferred to be less than 1 km.

Though differing in detail, the igneous stratigraphy shows a 

differentiation trend broadly similar to those of other major layered 

intrusions. The Dufek Massif section consists of early, but not earliest, 

differentiates, and the Forrestal Range section consists of late 

differentiates, including granophyre. The unexposed basal and Sallee

'The sections measured in Dufek Massif were designated D-A to D-J, and those in the Forrestal Range F-A to F-K. 

These designations are used in stratigraphic descriptions in this report.
DUFEK INTRUSION, ANTARCTICA D9

Snowfield sections are inferred to contain, respectively, earliest and 

middle-stage differentiates. The composition of bulk rocks and 

minerals shows progressive changes with stratigraphic height within 

each of the exposed sections. The most conspicuous changes are in the 

relative amounts of iron and magnesium, as indicated by the mafic 

index of bulk rocks (fig. 3A) and by Fe/Mg ratios in pyroxenes (fig. 3B). 

The offsets in figure 3 between trends in each of the exposed sections 

suggest that similar changes occur in the intervening Sallee Snowfield 

section. Projection of trend lines downward beneath the Dufek Massif 

section implies downward decrease in the Fe/Mg ratio, but the 

magnitude is unknown.

Owing to variable amounts of several chief minerals in the rocks, 

including postcumulus minerals, the interpretation of bulk-rock data 

in terms of fractionation history is equivocal. Compositional variation 

of individual minerals is more significant (Wager and Brown, 1968, p. 

150). Preliminary optical studies show that cumulus plagioclase 

ranges from Ango-85 near *ne base of the Dufek Massif section to about 

An5Q near the top of the Forrestal Range section (Ford and Boyd, 1968). 

Electron-microprobe studies show that pyroxene forms two coexisting 

cumulus phases through most of both sections. Both phases, one rich in

9 1 

o

i= 0

1 1 1

c 

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8

IT

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' ; 8 "~ . £ Lexmgton jjog o Granophyre ^ A

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60 80

MAFIC INDEX 

 Mafic rocks 

o Felsic rocks

100 0.2 0.4 0.6 0.8

B CATION RATIO Fe/(Fe + Mg) 

A Ca-rich pyroxene 

 Ca-poor pyroxene

FIGURE 3. Variation (A) of mafic index of bulk rocks [(FeO+Fe2O3 )xlOO/(FeO+ 

FeaOg+MgO)] and (B) of cation ratio Fe/(Fe+Mg) in pyroxenes with height in the 

Dufek intrusion. A, after Ford (1970); B, after Himmelberg and Ford (1976).
D10 CONTRIBUTIONS TO STRATIGRAPHY

calcium in the series augite-ferroaugite, and the other poor in calcium 

in the series bronzite-inverted pigeonite, show a general upward 

increase in the ratio Fe/(Fe+Mg), as shown in figure 2B. Calcium rich 

pyroxene ranges from Ca36.4Mg48. 7Fei4.9 to Ca3o.oMg23.5Fe46.5, and 

calcium poor pyroxene ranges from Ca3.5Mg69.iFe27.4 to Can.4 

Mg34.oFe54 6 (Himmelberg and Ford, 1976).

Iron enrichment of the magma during early fractionation led to later 

crystallization of opaque iron-bearing oxides. Only a preliminary 

study has been made of the mineralogy of opaque oxides (Himmelberg 

and Ford, 1975); for brevity, they are henceforth referred to simply as 

magnetite, a mineral that appears to be the predominant among 

several phases of iron- and titanium-bearing oxides. Magnetite first 

appears as a cumulus phase at about 1,600 m above base of the Dufek 

Massif section. It occurs in minor amounts from there to the top of the 

section and increases in amount upward to become a major component 

in many rocks of the Forrestal Range section. Upward increase in 

modal magnetite is reflected in a general upward increase in rock 

densities, which average about 2.95 g/cm3 in the lower section and 

about 3.05 in the upper one (Ford and Nelson, 1972). Stratigraphic 

difference in modal magnetite is also reflected in wide variations in 

aeromagnetic intensities over the region (Behrendt and others, 1974).

ZONAL NOMENCLATURE

The term "zone" has been used in a variety of ways in studies of 

layered intrusions of the Dufek type. Jackson (1967, p. 22) defines 

zone as "an informal, mappable, rock-stratigraphic unit in a layered 

intrusion, characterized by lithologic homogeneity or distinctive 

lithologic features." He obviously considers this usage as parallel to 

the use of "formation" in sedimentary rocks, and he further defines 

"member" as an informal subdivision of a zone. Jones, Peoples, and 

Rowland (1960) subdivide the Stillwater Complex into four such zones 

on the basis of their mappability, and their subdivisions are retained in 

more recent work (Jackson, 1961; Page and Nokleberg, 1974). Hess 

(1960) recognizes different subdivisions, in part, and employs 

capitalized terminology such as "Ultramafic Zone" and "Upper Gabbro 

Zone." Hess's "Anorthosite Zone" is further subdivided into several 

"Anorthosite subzones." His named units are not formal rockstratigraphic units, however, in the absence of designated type 

sections and geographic features after which they should be named 

(American Commission on Stratigraphic Nomenclature, 1970, Art. 

13). In his study of the Duke Island ultramafic complex of southeastern 

Alaska, Irvine (1974, p. 5) uses the term zone "in only an informal way 

for physically or geometrically distinct units of rock* * *", without 

necessarily a Stratigraphic connotation.
DUFEK INTRUSION, ANTARCTICA D11

Others use the term "zone" in a different sense, analogous to 

biostratigraphic zones in sedimentary rocks. For subdividing cumulate sequences, Wager and Brown (1968, p. 30) point out the analogy 

between cumulus mineral phases in layered intrusions and fossils in 

sedimentary sequences. The appearance and disappearance of mineral 

phases, and their changing compositions, are suggestive of an 

evolutionary sequence of a fossil assemblage. The mineral assemblages define a relative age sequence in a cumulate pile related 

ultimately to progressive cooling of the magma. As in sediments, the 

sequence may be disturbed by factors both internal and external to the 

system, such as erosion by convection currents, influxes of new 

magma, and loss of material by volcanism. Wager and Brown (1968, p. 

30) subdivide igneous sequences of layered intrusions on the basis of 

mineral zones and subzones that they define as being "the thickness of 

layered rocks developed while a specific cumulus mineral or 

assemblage of cumulus minerals was forming." Mappability is not 

inherent in their definition. They do, however, use zones named 

"lower," "middle," and "upper" as informal units in mapping the 

Skaergaard intrusion. Morse (1969) uses units termed "Upper Zone" 

and "Lower Zone" in subdividing and mapping the Kiglapait intrusion 

of Labrador.

The layered rocks of the Dufek intrusion were originally subdivided 

into informal mappable rock-stratigraphic units termed "zones" (Ford 

and Boyd, 1968), following usage of that term by workers on the 

Stillwater Complex. That scheme is herein abandoned, and the units 

are given formal names of group, formation, and member rank. The 

term "zone" is retained but used in the sense of Wager and Brown 

(1968, p. 30). Zones here recognized are mineral zones based on the 

appearance and disappearance of cumulus minerals in the sequence. 

Although mappability is not required for recognition of this type of 

zone, mineral-zone boundaries may correspond to boundaries of 

rock-stratigraphic map units, as they apparently do in the Kiglapait 

(Morse, 1969) and Skaergaard (Wager and Brown, 1968) intrusions.

Mineral-zone subdivisions of other layered intrusions (as the 

Skaergaard) have been given names such as "lower," "middle," and 

"upper." However, stratigraphic comparison between intrusions is 

obscured by use of different definitions for these terms. Whereas the 

"upper zone" of the Skaergaard is defined by the lowest occurrence of 

iron-rich cumulus olivine and that of the Bushveld by appearance of 

cumulus magnetite (Wager and Brown, 1968), the "Upper Zone" of the 

Kiglapait intrusion is defined by the lowest occurrence of cumulus 

clinopyroxene (Morse, 1969).

From the analogy made earlier of cumulus-mineral assemblages to 

fossil assemblages in sedimentary rocks, zones (and subzones) of
D12 CONTRIBUTIONS TO STRATIGRAPHY

layered intrusions might better be named after characteristic cumulus 

minerals in a similar fashion to the naming of biostratigraphic zones 

after characteristic fossils in the assemblage (American Commission 

on Stratigraphic Nomenclature, 1970, Art. 24). The naming of mineral 

zones after key cumulus minerals also follows the standard practice of 

naming metamorphic zones after an index mineral (Turner, 1968, 

p. 25).

Preliminary petrographic studies indicate that cumulus minerals of 

the Dufek intrusion have a stratigraphic distribution as shown in 

figure 4. Names of major cumulus minerals may be used for naming

Quartz K-feldspar Magnetite 

. Ca-poor Ca-rich Apatite 

Ohvme pyroxene pyroxene M i i

Lexington Granophyre I

Sallee Snowfield section (hidden)

FIGURE 4. Stratigraphic distribution of minerals in the Dufek intrusion. Cumulus 

minerals shown by solid line; postcumulus minerals and, in granophyre, noncumulus minerals shown by dashed line. After Himmelberg and Ford (1976).
DUFEK INTRUSION, ANTARCTICA D13

zones, and names of minor minerals for naming subzones. Although 

zonal terminology is not formalized in this report, an example is figure 

4, which shows that the "magnetite zone" extends from about 1.6 km 

in the Dufek Massif section, probably continuing through the concealed Sallee Snowfield section, to the top of the mafic rocks in the 

Forrestal Range section. The "apatite subzone" makes up the highest 

part of the zone. Postcumulus minerals are not considered in the zonal 

nomenclature.

PREVIOUS NOMENCLATURE

Previous nomenclature of the Dufek intrusion by Aughenbaugh 

(1961), Walker (1961), and Ford and Boyd (1968) was informal. 

Aughenbaugh, who did not map his units, subdivided the rocks of his 

partial stratigraphic section into a "normal and leucocratic series" 

(Aughenbaugh, 1961, p. 160). The characteristic rock of his normal 

series is described, on field appearance, as a "medium-grained diorite 

which grades locally into gabbro, anorthosite, or a gneissic-looking 

rock." Owing to different usage of petrographic terminology and to the 

location of his stations only on an uncontrolled sketch map, the 

correlation of his units with those of Ford and Boyd (1968, p. 221) and 

those of this report is uncertain but is believed to be as shown in figure 

5. Aughenbaugh's leucocratic series and the anorthosite zone of Ford 

and Boyd (1968) are probably equivalents. Aughenbaugh shows a 

6-m-thick layer of brown melagabbro that is probably equivalent to 

pyroxenitic cumulate of the lower brown zone of Ford's and Boyd.

Ford and Boyd (1968) subdivided the layered rocks into numerous 

mappable rock-stratigraphic units that were called zones (fig. 5). The 

term "zone" is discontinued in this sense for reasons discussed earlier. 

Their zones were subunits of larger divisions that they called series, a 

term that has been used for other layered intrusions, but that is a 

misusage of the term according to the Code of Stratigraphic 

Nomenclature (American Commission on Stratigraphic Nomenclature, 1970, Art. 9f).

PRESENT NOMENCLATURE

Continued and more detailed study of the intrusion and the 

preparation of geologic maps now require a formal rock-stratigraphic 

nomenclature to be established. This nomenclature is compared with 

previous nomenclature in figure 5. Whereas cumulate terminology is 

generally used for rock description in this report, the formal names 

herein defined use common igneous-rock names of the characteristic 

lithology of the unit. Cumulate names, though generally preferable to 

describe rocks such as these, would require lengthy compound terms
D14 CONTRIBUTIONS TO STRATIGRAPHY

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D16 CONTRIBUTIONS TO STRATIGRAPHY

such as "plagioclase-in verted pigeonite-augite-magnetite cumulate," 

which should be avoided in naming formal rock-stratigraphic units 

(American Commission on Stratigraphic Nomenclature, 1970, Art. 

lOc). Rock names used in this report therefore are of two types based on 

(1) total mode and (2) types and proportions of cumulus minerals. As 

previously stated, a common igneous-rock name may be combined with 

the term "cumulate" for referring to rocks whose settled phases are not 

specifically indicated.

FORRESTAL GABBRO GROUP

The new name Forrestal Gabbro Group is here applied to all the 

layered rocks of the Dufek intrusion. The name is taken from the 

Forrestal Range, a lengthy mountain mass containing nearly half the 

exposed stratigraphy of the group. Its type locality is in the southern 

part of the range. As its name implies, the group consists chiefly of 

rocks of gabbroic composition. It includes rocks of wide range in 

total-mode composition, from pyroxenite and magnetitite to felsic 

rocks such as leucogabbro, anorthosite, and granophyre.

Most rocks are well-laminated pyroxene-plagioclase cumulates, and 

many in the upper part of the group also contain a large proportion of 

cumulus magnetite. Single-mineral cumulates or cumulates with only 

minor amounts of a second cumulus phase form thin layers, lenses, and 

tongues at many levels in the layered sequence of mostly gabbroic 

cumulates. The thin units include pyroxene cumulate (pyroxenite), 

magnetite cumulate (magnetitite), and plagiociase cumulate (anorthosite), which also makes up a major unit of formation rank at the 

lowest exposed part of the group. The group includes a noncumulus 

layer of granophyre that forms the top of the layered sequence.

In addition to pyroxene and plagioclase, which occur through the 

entire stratigraphy of the group, and magnetite, which is abundant in 

the upper part, the cumulus mineralogy includes minor amounts of 

iron-rich olivine and apatite in high Stratigraphic levels, as shown in 

figure 4. Quartz and K-feldspar are minor postcumulus minerals in 

higher gabbroic cumulates and are major constituents of the 

granophyre. Amphibole and mica are locally present, chiefly in the 

uppermost part, and probably formed by alteration or as postcumulus 

phases.

Neither the base nor top of the Forrestal Gabbro Group is exposed. 

The group is believed to be on the order of 8-9 km thick, which includes 

the measured thicknesses of the Dufek Massif and Forrestal Range 

sections and the estimated thicknesses of the concealed basal section 

and Sallee Snowfield section. The top of the group is probably eroded, 

as discussed earlier.

The Forrestal Gabbro Group is subdivided into four formations as
DUFEK INTRUSION, ANTARCTICA D17

shown in figure 5. Two of the formations are partly subdivided into 

members, some formal and others informal. All formally named units 

were mapped at a scale of 1:40,000 in the 1965-66 fieldwork.

WALKER ANORTHOSITE

Layered anorthositic cumulates form a thick and conspicuously 

light colored unit on lower slopes above Enchanted Valley at the west 

end of Dufek Massif. This unit is here named the Walker Anorthosite 

after Walker Peak. The type section (D-C) of the formation is on the 

western spur of Walker Peak (figs. 9, 10) where a thickness of about 

230 m is exposed. The base of the formation is not exposed, and the unit 

presumably constitutes an upper part of the hidden basal section of the 

intrusion. It is unknown whether this or some unit of mafic rock lies on 

the inferred basal ultramafic rocks. The top of the formation is exposed 

over a distance of about 8 km on the north side of the western Dufek 

Massif and for about 5 km on the south side.

The Walker Anorthosite is cut off on the northwest by the Enchanted 

Valley fault, a near-vertical fault set composed of three individual 

faults mapped locally on the north side of Dufek Massif (fig. 1). Rocks 

from higher in the section are successively downdropped to the north 

along at least the two southern of the three faults and probably also 

along the northern one. Toward the southeast, the Walker presumably 

extends beneath Sallee Snowfield.

The formation is predominantly composed of medium-grained 

plagioclase cumulates. Plagioclase, generally poorly zoned, is mostly 

in the compositional range Any^gs- Except locally, layering is less 

conspicuous than in other parts of the Dufek Massif and Forrestal 

Range sections. Layering is generally indistinct and gradational and 

only pronounced where cumulus pyroxenes are present (fig. 6). Most 

rocks are well laminated. They are white to cream or light gray owing 

to their high feldspar content, and they commonly have a spotted 

appearance because of the presence of coarse poikilitic postcumulus 

pyroxene crystals, as seen in figure 6.

AUGHENBAUGH GABBRO

A sequence of mostly mafic cumulates overlies the Walker 

Anorthosite and makes up the main part of Dufek Massif and most 

nearby nunataks. This unit is here named the Aughenbaugh Gabbro 

after Aughenbaugh Peak, where, though poorly accessible, it is well 

exposed on cliff walls and spurs. It is a widespread formation that 

extends for a distance of nearly 50 km from Walker Peak, its most 

westerly exposures, to Rankine Rock, the most northeasterly nunatak 

of the Dufek Massif area. Section D-D, measured on the main
D18 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 6. Plagioclase cumulate of the Walker Anorthosite at the base of the main 

northwest spur of Neuberg Peak. Shows typical spotted appearance of the 

postcumulus pyroxene. Indistinct gradational layers about 40 cm above hammer 

contain cumulus pyroxene. Hammer 42 cm long.

northwest spur of Neuberg Peak (fig. 7), is the type section of the 

Aughenbaugh. This section is more complete and more accessible than 

that on Aughenbaugh Peak, about 1.6 km to the east. The type section 

contains the lowest occurring 775-m thickness of the formation, the 

total minimum thickness of which is about 1,600 m. The upper part is 

eroded at Neuberg Peak. Stratigraphically higher rocks are exposed on 

the southern side of Dufek Massif owing to the southeastward dip of 

the layering. Thus, a 760-m-thick section, D-J, measured on Spear 

Spur (fig. 8) is here designated a reference section for the upper part of 

the formation.
DUFEK INTRUSION, ANTARCTICA D19

FIGURE 7. Neuberg Peak (right-center skyline) showing gabbroic cumulates in upper 

part of type section D-D of the Aughenbaugh Gabbro on spur leading to foreground. 

Layering poorly developed in this part of section. About 520 m of section exposed from 

foreground saddle (near helicopter) to Frost Pyroxenite Member (dark layers marked 

FPM).

The base of the Aughenbaugh Gabbro is exposed at the base of the 

type section where the formation sharply and concordantly overlies 

the Walker Anorthosite, as it also does at Walker Peak (fig. 10). The 

top is not exposed; the highest rocks visible, at the top of reference 

section D-J, have an eroded surface. The Aughenbaugh presumably 

extends under cover of ice to the southeast where it forms a lower part 

of the Sallee Snowfield section. On the north, the formation is faulted 

and downdropped, along with the Walker Anorthosite, by the 

Enchanted Valley fault.

The Aughenbaugh Gabbro consists predominantly of light- to 

medium-gray medium-grained plagioclase-augite-inverted pigeonite 

cumulate, with minor amounts of cumulus magnetite in the uppermost
D20 CONTRIBUTIONS TO STRATIGRAPHY

200 m of the formation. Plagioclase is generally in the compositional 

range Ango-TO- Most rocks are well laminated and show millimetreand centimetre-scale mineral-graded layering. A 600-m-thick lower 

middle part of the formation (fig. 7), between the two conspicuous 

pyroxenitic units, is generally free of major layers. Sections of the 

formation above (fig. 8) and below this fairly homogeneous appearing 

midsection contain conspicuous mineral-graded and rare isomodal 

layers one to tens of metres thick.

Although many of the major layered units might be recognized as 

subdivisions of the formation, only four that are especially conspicuous 

are designated as members (fig. 5). Two of these, given formal names, 

are dark mineral-graded units of pyroxenitic cumulate that have been

FIGURE 8. Spear Spur (right center) showing reference section D-J and the 

well-developed layering in upper cumulates of the Aughenbaugh Gabbro above the 

Frost Pyroxenite Member (FPM). Arrows mark trough-layered tongue-shaped body of 

pyroxenitic cumulate about 30 m below the Frost. About 670 m of section exposed in 

cliffs above the Frost.
DUFEK INTRUSION, ANTARCTICA D21

mapped over large distances in Dufek Massif (fig. 9). They are key 

markers used in correlating measured sections in the region. The other 

two, one formally and the other informally named, are anorthositic 

cumulates that are of more local distribution.

LOWER ANORTHOSITE MEMBER

A conspicuous layer of white to cream plagioclase cumulate, about 

12 m thick, near the base of the Aughenbaugh Gabbro is designated by 

the informal name "lower anorthosite member." It lies, in the type 

section of the Aughenbaugh, about 40 m above the formation base. The 

unit is an isomodal layer with sharply defined basal and top contacts. 

The rocks are similar in appearance to plagioclase cumulates of the 

Walker Anorthosite. As in the Walker, they are characterized by the 

presence of coarse poikilitic postcumulus pyroxenes that give them a 

spotted appearance. The extent of the unit is unknown, as it is mostly 

covered by snow and ice. It is not present at some localities (fig. 10) 

where a similar stratigraphic interval is exposed, which suggests that 

it has a limited lateral distribution compared with other major layers.

NEUBERG PYROXENITE MEMBER

A conspicuous dark layer, about 5 m thick, of pyroxene-rich 

cumulate lies about 200 m above the base of the Aughenbaugh Gabbro 

in the western part of Dufek Massif (figs. 9,10). This unit is here named 

the Neuberg Pyroxenite Member after Neuberg Peak, on the main 

northwest spur of which it is well exposed. Its type section is at an 

elevation of about 1,075 m in the type section D-D of the 

Aughenbaugh.

The Neuberg Pyroxenite Member is a mineral-graded layer showing 

a sharp basal contact and gradational upper contact with adjacent 

plagioclase-rich gabbroic cumulates (fig. 11). The Neuberg is a 

feldspar-bearing pyroxenitic cumulate, as are some of the upper 

pyroxenites of the Bushveld intrustion (Wager and Brown, 1968, p. 

365). Postcumulus and cumulus plagioclase are minor constituents, in 

general, and cumulus plagioclase gradually increases in amount 

upward in the member.

The Neuberg is characterized by the presence of unusually coarse 

clusters of pyroxene grains showing uniform crystallographic orientation. These clusters, as much as 5-7 cm across, are visible in the field by 

their cleavage reflectance. Much of the pyroxene crystallized as 

pigeonite, which settled and later inverted to orthopyroxene (Himmelberg and Ford, 1976). Textural relations indicate that the individual 

settled grains initially had random orientation and that the 

development of uniform orientation in large clusters was a post-
D22 CONTRIBUTIONS TO STRATIGRAPHY

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DUFEK INTRUSION, ANTARCTICA D23

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FIGURE 10. Aerial view south of Walker Peak showing the light-colored layered 

cumulates of the Walker Anorthosite about 200 m below the thin dark layer of the 

Neuberg Pyroxenite Member (NPM). Compare figures 9 and 11. Type section D-C of 

the Walker on right skyline spur beyond photograph. Arrow marks contact of the 

Walker (WA) with the Aughenbaugh Gabbro (AG).

cumulus process. The clusters are similar to those described by von 

Gruenewaldt (1970, p. 70), who interprets the preferred orientation as 

having formed during inversion as a result of directed pressure by the
D24 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 11. Mineral-graded Neuberg Pyroxenite Member at its type section showing 

sharp basal contact. Gradational upper contact marked by arrow.

overlying crystal mass. This unusual texture is characteristic of the 

pyroxenitic cumulates of the Aughenbaugh Gabbro, and it is also 

present, though rarer and less conspicuous, in pyroxene-rich gabbroic 

cumulates of the formation.

The Neuberg Pyroxenite Member is exposed at a nearly constant 

thickness for distances of about 11 km on both the north and south 

sides of the west end of Dufek Massif. A thin dark pyroxene-rich 

cumulate layer near the top of Hanna Peak is believed to correlate with 

the Neuberg. However, it is at a considerably lower elevation on this 

nunatak than in Dufek Massif, probably as a result of downdropping 

along the Enchanted Valley fault. At both its northern and southern 

terminations of exposure, the Neuberg passes beneath snow- and 

ice-covered talus aprons.
DUFEK INTRUSION, ANTARCTICA 

FROST PYROXENITE MEMBER

D25

A second conspicuously dark pyroxenitic unit occurs about 800 m 

above the base of the Aughenbaugh Gabbro. The unit is well exposed, 

though not easily accessible, near the top of Neuberg Peak (fig. 12) and

FPM

FIGURE 12. Aerial view east of Neuberg Peak showing mineral-graded pyroxene-rich 

cumulate layers of the Frost Pyroxenite Member (FPM) in the upper cliffs. Upper part 

of type section D-D of the Aughenbaugh Gabbro on left-center spur.
D26 CONTRIBUTIONS TO STRATIGRAPHY

in higher parts of the main north face of Dufek Massif. Its elevation 

gradually decreases eastward, owing to dip, and reaches the level of 

the lower ice plateau near Davis Valley at the east end of the massif 

where the unit is easily accessible. This unit is here named the Frost 

Pyroxenite Member for Frost Spur, its type locality, where it is part of a 

section (D-H) measured in the upper Aughenbaugh.

The Frost Pyroxenite Member can be seen on Neuberg Peak to 

consist of at least six thin mineral-graded layers, three of which have 

pyroxenitic cumulate bases and the others pyroxene-rich gabbroic 

cumulate bases. The dark pyroxene-rich bases have sharp contacts 

with underlying gabbroic cumulates. Upper contacts are gradational 

owing to gradual upward increase in amount of cumulus plagioclase. 

The sequence of layers totals about 43 m in thickness. Two of the 

pyroxene cumulate layers, each between 1 and 2 m thick, occur near 

the base of the Frost, and the third, about 4 m thick, marks the top of 

the member. Together, the three dark-brown pyroxenitic layers form a 

conspicuous unit (fig. 12) that is traceable with little variation in 

thickness for a distance of about 35 km along nearly the entire length 

of Dufek Massif. At the type locality and at Spear Spur (fig. 8), 

trough-layered cumulates of cut-and-fill channels occur about 30 m 

below the Frost.

SPEAR ANORTHOSITE MEMBER

A sequence of three layers of cream to light-gray anorthositic 

cumulates that are interlayered with medium- to dark-gray gabbroic 

cumulates lies within the upper part of the Aughenbaugh Gabbro. This 

well-layered sequence is here named the Spear Anorthosite Member 

after Spear Spur, above which lies its type section below the summit 

plateau of Dufek Massif. Elsewhere, in most places it has been 

removed by erosion. Its type section is at the top of the reference section 

(D-J) for the upper Aughenbaugh (fig. 8).

The Spear Anorthosite Member is about 105 m thick in its type 

section. Its base is at the sharp lower contact of the lowest layer of 

plagioclase cumulate; its top in the section is the erosional upper 

surface of the highest layer of plagioclase cumulate. From lowest to 

highest, the three plagioclase cumulate layers are about 46 m, about 

12 m, and at least 5 m thick. They are separated by layers of 

pyroxene-plagioclase-magnetite cumulate, a lower one about 18 m 

thick and an upper one about 24 m thick. The plagioclase cumulates 

contain minor amounts of postcumulus pyroxene and magnetite. The 

coarse postcumulus pyroxenes give the anorthositic cumulates a 

spotted appearance similar to that of the lower anorthosite member 

and the Walker Anorthosite.
DUFEK INTRUSION, ANTARCTICA D27 

SARATOGA GABBRO

The name Saratoga Gabbro is here given to the generally 

well-layered mostly gabbroic cumulates that are exposed throughout 

the Forrestal Range. Most of the range is covered by snow and ice and 

thus exposures of the formation are generally small and widely 

scattered. The unit probably underlies most of the ice-covered 

Saratoga Table, for which it is named. The formation is chiefly exposed 

in west-facing escarpments of the Saratoga and Lexington Tables. 

Smaller exposures are seen in many nunataks on and near these two 

mesalike tablelands as well as in the widely scattered nunataks at the 

north end of the range. The unit thus extends for a distance of at least 

100 km in a northeasterly direction and about 30 km in a southeasterly 

direction. Geophysical evidence (Behrendt and others, 1974) suggests 

that it continues, beneath ice, for at least another 100 km northeastward beyond the Forrestal Range.

Correlation of the 11 partial stratigraphic sections measured in the 

Forrestal Range is more uncertain than correlation of sections in 

Dufek Massif because layers are less continuous, conspicuous markers 

are scarcer, and outcrop areas are smaller and more isolated by snow 

and ice. On the basis of tentative correlation of these sections, the total 

exposed thickness of the Saratoga Gabbro is about 1,400 m. The partial 

section (F-B) measured on the western spur of Mount Stephens (fig. 13) 

is designated the type section of the formation. The type section 

contains about a 610-m thickness of well-layered rocks, chiefly 

pyroxene-plagioclase-magnetite cumulates locally with interlayers of 

plagioclase cumulates and minor thin magnetite cumulates. The 

upper part of the Saratoga is not present in the type section. Higher 

layers are exposed to the east owing to a gentle southeastward dip of 

layering. Accordingly, three partial sections, from lowest to highest, 

F-E, F-F, and F-G, measured at Sorna Bluff (fig. 14) are designated as 

reference sections to illustrate stratigraphic features in the upper 730 

m of the formation. Rocks of the reference sections are generally 

similar in lithology to those of the type section. A snow-covered 

interval estimated to be 50-60 m thick separates the type section and 

the lowest reference section.

The base of the Saratoga Gabbro is not exposed. The formation 

presumably makes up at least a higher part of the ice-covered Sallee 

Snowfield section of the intrusion. The top is well exposed in summit 

areas of Sorna Bluff, Mount Lechner, and Mount Zirzow where the unit 

is sharply and apparently concordantly overlain by granophyre.

The Saratoga differs from the Aughenbaugh Gabbro in several 

important respects. Although it is generally more conspicuously 

layered (fig. 15), its layers have far less lateral continuity. Layers
D28 CONTRIBUTIONS TO STRATIGRAPHY

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DUFEK INTRUSION, ANTARCTICA D29

FIGURE 14. Sorna Bluff, view east, showing reference sections, F-E, F-F, and F-G, for 

upper part of Saratoga Gabbro. Upper inclusion member (UIM) of the Saratoga lies 

about 107 m below contact, marked by arrow, of the Saratoga with the overlying 

Lexington Granophyre (LG). Photograph by D. L. Schmidt.

thicken and thin in relatively short distances. Mafic cumulates of the 

Saratoga are considerably richer in magnetite than those of the 

Aughenbaugh. Owing to a high magnetite content, rocks generally are 

darker, commonly a dark brownish gray, than those of the 

Aughenbaugh. Magnetite locally forms the sole or the greatly 

predominant cumulus phase in thin layers or lenses of magnetite 

cumulate (magnetitite) in the basal parts of some mineral-graded 

layers. Pyroxenes in both the calcium-rich and calcium-poor series are 

higher in Fe/(Fe+Mg) ratios than those in the Aughenbaugh 

(Himmelberg and Ford, 1976), and plagioclase compositions, generally
D30 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 15. Spur at west end of Sorna Bluff, view west, showing conspicuous layering 

typical of upper part of Saratoga Gabbro. Spur is reference section F-E. Compare 

figure 14.

in the range An50_6o, are more sodic than those in the Aughenbaugh. 

Apatite and rare fayalite, which are unknown in the Aughenbaugh, 

occur as cumulus phases in the upper part of the Saratoga Gabbro. 

Three of the more prominent mappable units of the Saratoga are 

designated below as members.

STEPHENS ANORTHOSITE MEMBER

A conspicuously layered sequence of alternating white to light-gray 

plagioclase-rich cumulates and dark-gray gabbroic cumulates makes 

up the lower 300 m of the Saratoga Gabbro. The unit is here named the 

Stephens Anorthosite Member for Mount Stephens. The type section,
DUFEK INTRUSION, ANTARCTICA D31

in the lower part of the type section F-B of the Saratoga, is on the lower 

end of the main western spur of Mount Stephens (fig. 13). The member 

consists of four conspicuou.s light-colored mineral-graded layers of 

mainly plagioclase cumulate that are interlayered with dark-gray 

pyroxene-plagioclase-magnetite cumulate. The plagioclase cumulate 

layers are 7-15 m thick. The layers have sharp bases, and the tops are 

gradational owing to a gradually upward increasing amount of 

cumulus pyroxene. Trough-layered structure is locally present. The 

base of the member is not exposed; the top is the gradational upper 

contact of the highest plagioclase cumulate layer. The member is 

exposed only in the vicinity of Mount Stephens, to the north and south 

of which it passes under adjoining ice sheets.

LOWER INCLUSION MEMBER

Two units of dark gabbroic cumulates are characterized by abundant 

rounded and embayed pillowy masses, 1-3 m across, of medium- to 

coarse-grained rock that is of noticeably lighter color than the 

surrounding rock (fig. 16). These inclusions are chiefly quartz-bearing 

leucogabbro with minor amounts of anorthosite and granophyre. They 

are locally so abundant that the layers have the appearance of a 

breccia. They are nonlaminated, indicating that they did not form by 

cumulus processes similar to those that formed the gabbroic cumulate 

host rock. Textures of the inclusions generally range from 

hypidiomorphic to xenomorphic granular. The inclusions are believed 

to be autoliths rafted in laterally by magma currents (Ford and Boyd, 

1968, p. 224). The source of the autoliths may have been near the roof of 

the magma chamber where quartzofeldspathic materials were 

accumulating or crystallizing in late-stage differentiated magma. 

Their origin is believed to be similar to that of plagioclase "rafts" in the 

Kiglapait intrusion described by Morse (1969).

The lower of the two units is here designated by the informal name 

"lower inclusion member." Its base lies at a stratigraphic height of 

about 518 m in the type section of the Saratoga Gabbro and at an 

elevation of about 2,020 m on the western spur of Mount Stephens. The 

member is about 46 m thick in this section. It is exposed at several 

places near the top of the western escarpment of Saratoga Table and 

makes up the rim of the table above Mathis Spur. The member extends 

for a distance of about 10 km north of Mount Stephens; it was not seen 

to the south or east where the escarpment is mostly covered by ice. The 

upper and lower contacts at Mathis Spur and Mount Stephens are 

indistinct.

UPPER INCLUSION MEMBER

The upper of the two inclusion-bearing units, here informally
D32 CONTRIBUTIONS TO STRATIGRAPHY

designated the upper inclusion member, is best exposed at Sorna Bluff 

(fig. 16) in the upper middle part of reference section F-G of the 

Saratoga Gabbro. It is also exposed locally near the rim of Lexington 

Table at Mount Lechner and near May Valley. Similar anorthosite and

FIGURE 16. Leucogabbro autoliths in dark cumulates of the upper inclusion member of 

the Saratoga Gabbro at Sorna Bluff. See location in figure 14. Lexington Granophyre 

(LG) lies about 100 m above the member. Ice axe, right center, is 90 cm long.
DUFEK INTRUSION, ANTARCTICA D33

leucogabbro inclusions occur in dark gabbroic cumulates at Henderson 

Bluff, but in the absence of definitive markers, the section (F-K) 

measured at this isolated nunatak cannot be correlated with other 

sections of the Saratoga Gabbro. The upper inclusion member makes 

up a stratigraphic interval from about 1,195 m to about 1,295 m above 

the base of the Saratoga, in reference section F-G at Sorna Bluff. The 

lower and upper contacts, which generally are indistinct, are 

arbitrarily placed at the base of the lowest and top of the highest 

inclusion at each locality. On the basis of vertical distribution of 

inclusions, the thickness of the member ranges from about 100 m at 

Sorna Bluff to about 64 m at Camp Spur and to about 107 m at Mount 

Lechner.

LEXINGTON GRANOPHYRE

A layer of light-gray to cream granophyre makes up the highest 

formation of the Forrestal Gabbro Group. The unit, here named the 

Lexington Granophyre, is locally well exposed at a number of places on 

escarpments of Lexington Table, for which it is named, and on the 

north rim of Saratoga Table (fig. 14). It probably underlies much of the 

ice cover of Lexington Table. The thickest measured section of the 

Lexington is at Mount Zirzow. This section (F-J), about 238 m thick, is 

designated the type section of the formation. An additional 50-60 m is 

believed to underlie the ice cover of nearby highlands, and so the total 

thickness present is probably about 300 m. The unit is also well 

exposed at Mount Lechner and Sorna Bluff where sections respectively 

about 204 m and 116 m thick were measured. At these and all other 

localities where it was seen, the basal contact with underlying 

gabbroic rock is sharp and apparently concordant. An upper part of the 

Lexington and the inferred upper border rocks and roof rocks of the 

intrusion presumably were removed by erosion.

The rocks are characterized by fine-grained xenomorphic-granular 

textures showing mosaics and irregular myrmekitic intergrowths of 

quartz and K-feldspar. Micrographic textures are locally present. The 

rocks are commonly microporphyritic and contain anhedral to 

subhedral microphenocrysts of zoned andesine and of pale green 

commonly zoned augite. The pyroxene and its alteration products of 

green amphibole and mica generally make up 1-5 percent of the mode. 

Apatite needles are ubiquitous but less than 1 modal percent.

The granophyre formed at the end stage in differentiation of the 

Dufek magma (Ford, 1970). The rocks are uniform in appearance. They 

are apparently structureless and show no evidence of crystal settling. 

The layer is free of inclusions that might suggest an origin by 

contamination with silicic country rocks. It is volumetrically small in 

comparison to the rest of the igneous body; in fact, assuming that only a
D34 CONTRIBUTIONS TO STRATIGRAPHY

negligible part was eroded and that relative thicknesses of layers 

represent relative volumes, the granophyric differentiates make up 

only about 3-4 percent of the entire 8-9-km-thick body.

AGE AND CORRELATION

From field evidence the age of the Dufek intrusion can only be placed 

as Permian or younger. The mostly ice covered intrusive contact crops 

out in a single small area on the end of the west spur of Mount Lechner 

where recrystallized Dover Sandstone (Devonian) abuts fine-grained 

gabbro of a high-angle marginal border of the body. Elsewhere, on 

nunataks near the Saratoga Table, rocks as young as the Pecora 

Formation (Late Permian) lie in the thermal aureole.

Beck's (1972) south paleomagnetic pole position for the intrusion, at 

lat 56.5° S, long 168° W, is near but statistically distinct from the 

Antarctic Ferrar Dolerites' (Jurassic) composite pole. He concludes 

that the Dufek intrusion and Ferrar Dolerites probably should not be 

ascribed to a single brief pulse of magmatic activity. Although the 

slight difference in pole positions suggests a slight age difference 

between the Dufek and the Ferrar, the difference in pole locations 

might also be accounted for by differential rotations of blocks along 

faults such as the Enchanted Valley fault that is known to displace 

rocks of the Dufek intrusion. The paleomagnetic evidence, accordingly, is suggestive only of approximate age equivalence between the 

Dufek and the Ferrar Dolerites.

Radiometric dating supports the general correlation of the Forrestal 

Gabbro Group with the Ferrar Group, which includes the Ferrar 

Dolerites (Grindley, 1963, p. 339-340) and the Kirkpatrick Basalt 

(Elliot, 1970, p. 305). Potassium-argon dating of three mineral 

separates of plagioclase from the Dufek intrusion yielded an average 

age of 168.0±3.4 m.y. (million years) (R. W. Kistler, written commun., 

1969), an age that is within the 163-179-m.y. range reported by Elliot 

(1970) for five basalts and one diabase within the Ferrar Group in the 

central Transantarctic Mountains. All three Dufek plagioclase ages, 

considered to represent a Jurassic intrusive event, are within the 

experimental error of the reported average. The Jurassic magmatism 

in the Transantarctic Mountains probably occurred in a number of 

pulses not presently discriminated by radiometric dating. Although 

most lie within a Middle Jurassic time span, reported ages of 

Ferrar-equivalent rocks in the entire Transantarctic Mountains range 

from 191 m.y. to 147 m.y. (Elliot, 1970). The apparent culmination of 

activity in the Middle Jurassic may have resulted in intrusive 

emplacement on a vast scale at one place, as in the northern Pensacola 

Mountains, and in outpourings of major flood basalts as the 

Kirkpatrick at another.
DUFEK INTRUSION, ANTARCTICA D35 

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#GPO 690- 613-1976