Early Paleozoic Sedimentation, Magmatism, and Deformation in the Pensacola Mountains, Antarctica: the Signiﬁcance of the Ross Orogeny

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Early Paleozoic sedimentation, magmatism, and deformation in the Pensacola Mountains, Antarctica: The signiﬁcance of the Ross orogeny

Bryan C. Storey British Antarctic Survey, Natural Environment Research Council, High Cross, David I. M. Macdonald* } Madingley Road, Cambridge CB3 OET, United Kingdom Ian W. D. Dalziel Institute for Geophysics, University of Texas, 8701 Mopac Boulevard, Austin, Texas 78759-8345 John L. Isbell Department of Geosciences, University of Wisconsin, Milwaukee, Wisconsin 53201 Ian L. Millar Isotope Geoscience Laboratory, Natural Environment Research Council, Keyworth, Nottingham NG12 5GG, United Kingdom

ABSTRACT exposed Nelson Limestone and by calcrete Proterozoic–Early Cambrian deformation pedogenesis. The Neptune Group is an al- events (Beardmore and Nimrod orogenies; Combined sedimentological, structural, luvial fan complex typical of many syn- and Goodge et al., 1991; Storey et al., 1992). It and geochemical studies of a lower Paleo- post-orogenic red beds. The predominance appears to have been equivalent to the Del- zoic succession within the Pensacola Moun- of nonmarine and shallow marine se- amerian orogeny of eastern Australia (Borg tains, Antarctica, suggest that it probably quences, and the facies and paleocurrent di- and DePaolo, 1991; Flo¨ttmann et al., 1993), formed in a foreland basin setting during rections within the basin, suggest that it which was contemporaneous with closing the Ross-Delamerian orogen, a complex may be more typical of a “piggyback” basin stages of Pan-African suturing events in early Paleozoic convergent margin of Ant- than of a foredeep basin, with the alluvial southern Africa (Cahen et al., 1984) and arctica and Australia. The lower Paleozoic fan complexes derived from advancing equivalent to an unnamed tectonothermal succession lies unconformably on a de- thrust sheets. Growth folds, progressive un- event within the East Antarctic Shield (Stu¨we formed(?) Neoproterozoic sequence (re- conformities, and deformed clasts of under- and Sandiford, 1993). If the recently pro- ferred to here as Sequence 1) and is divided lying strata within basal conglomerates are posed “SWEAT” hypothesis (SouthWest into three unconformity-bounded sequences consistent with active deformation during United States–East AnTarctic; Moores, (Sequences 2–4). The oldest sequence, Se- sedimentation and the proposed tectonic 1991), which suggests that Laurentia and quence 2, comprises Middle–Upper Cam- setting. The presence of variably plunging East Antarctica were joined during Late brian platformal limestone (Nelson Lime- folds, some of which are transected by a Precambrian times, is correct, the conver- stone) and overlying Lower Ordovician slaty cleavage, suggests that deformation gent margin must have developed after sep- silicic volcanic rocks of the Gambacorta was in an oblique-slip setting perhaps due aration of East Antarctica from Laurentia ,Formation (U-Pb zircon age of 501 ؎ 3 Ma). to oblique convergence along this part of the and opening of the Pacific Ocean (Dalziel The volcanic rocks crystallized from a high- Antarctic margin during the Ross-Delame- 1992). temperature anhydrous magma derived rian orogeny. The Ross orogen, together with rocks de- from a lower crustal igneous source and formed during earlier orogenic episodes, may represent magmatism on the inboard INTRODUCTION forms the basement (Stump, 1992) of the side of a magmatic arc now largely absent Transantarctic Mountains, an imposing from this part of the margin. Sequence 3 The Ross orogeny, originally defined by mountain chain that stretches 3500 km from (Wiens Formation), in part conformable Gunn and Warren (1962) as a period of northern Victoria Land to the Pensacola with Sequence 2, represents deposition by early Paleozoic or possibly late Precambrian Mountains (Fig. 1). Strata deformed during unconfined ephemeral streams followed by folding within the Transantarctic Moun- the Ross orogeny are unconformably over- a marine transgressive unit. The base of Se- tains, is traditionally regarded as a major pe- lain by Devonian–Triassic sedimentary rocks quence 4 (Neptune Group) is a major ero- riod of Late Cambrian–Early Ordovician (Beacon Supergroup). The largely unde- sion surface marked by karstification of the compressional deformation and granitic formed Beacon strata rest on a major ero- magmatism (Stump et al., 1986). It is gen- sion surface (Kukri peneplain) and are in- erally interpreted in terms of plate conver- truded by Jurassic continental tholeiites of *Present address: Cambridge Arctic Shelf Pro- gence along the margin of the East Antarctic the Ferrar Supergroup. The Transantarctic gramme, West Building, Gravel Hill, Huntingdon craton (e.g., Borg et al., 1990; Stump, 1995). Mountains were uplifted during Cretaceous Road, Cambridge CB3 ODJ, United Kingdom. The Ross-age folding overprinted Late(?) and Tertiary times (Fitzgerald et al., 1986)

Data Repository item 9630 contains additional material related to this article.

GSA Bulletin; June 1996; v. 108; no. 6; p. 685–707; 18 ﬁgures.

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along the rifted margin of the Precambrian The character of the Ross orogen varies units are enigmatic and controversial. Either craton of East Antarctica, separating the along the length of the Transantarctic they were accreted to this margin during craton from the mostly Phanerozoic crustal Mountains, and it is difﬁcult to correlate subduction (Kleinschmidt and Tessensohn, blocks of West Antarctica (Dalziel and speciﬁc events. In northern Victoria Land, 1987), or they represent allochthonous ter- Elliot, 1982). geologic relationships among fault-bounded ranes introduced by strike-slip faulting (Bradshaw et al., 1985). In the remainder of the Transantarctic Mountains, Cambrian strata have been divided into two discontin- uous belts that trend subparallel to the mountain belt (Rowell and Rees, 1989). The inner marginal cratonic belt (Rowell et al., 1992) contains a thick Lower Cambrian limestone unit (Shackleton Limestone) that is deformed and separated by an angular unconformity from the overlying Douglas Con- glomerate (Rees et al., 1989). The Douglas Conglomerate contains evidence for more than one episode of pre-Devonian deformation (Rowell et al., 1988) and indicates polyphase tectonism in the Cambrian history of the Ross orogen (Rowell et al., 1992). The outer belt contains Middle Cam- brian limestones and thick volcanic and volcaniclastic successions (Byrd Group; Laird, 1981). Rowell and Rees (1989) proposed that the outer belt forms one or more suspect terranes. Rowell et al. (1992) con- cluded, however, that these were not exotic to the Antarctic continent. This paper focuses on the relationships among sedimentation, deformation, and magmatism associated with the Ross orogeny in the Pensacola Mountains. The adjacent Shackleton Range has been considered part of the same geologic province as the Transantarctic Mountains, but the trend of lower Paleozoic structures is perpendicular to those in the Transantarctic Mountains Figure 1. Map of Antarctica illustrating the geographical position of the Pensacola (Buggisch et al., 1990). Dalla Salda et al. Mountains. AR, Argentina Range; FR, Forrestal Range; NR, Neptune Range; PR, Patuxent (1992) and Dalziel et al. (1994) have sug- Range. gested that they represent the continuation of an orogen that extended from Laurentia through South America to Antarctica (Fig. 2). The Pensacola Mountains area is important, therefore, as it lies near the junc- tion of the Delamerian-Ross belt of eastern Australia and the Transantarctic Mountains Figure 2. Cambrian recon- with the proposed Taconic Famatinian- struction (after Dalziel et al., Shackleton belt of Ordovician age. This 1994) showing the Ross- study raises important points about the Delamerian orogen along the depth and rates at which deformational pro- length of the Transantarctic cesses operate. We suggest that many of the Mountains, and a closure of structures had a surface topographic expres- the Iapetus ocean in the Shack- sion that controlled subsequent deposition. leton Range (SR). Cleavage formation appears to have oc- curred relatively quickly at shallow stratigraphic levels. The Ross orogeny can be shown to have had several distinct phases in the central part of the Pensacola Mountains. These phases are probably an expression of

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continuous tectonism in the area during Mountains comprise four main ranges (the formed during three distinct orogenic events Late Cambrian and Ordovician times. Argentina, Forrestal, Neptune and Patuxent (Beardmore, Ross, and Weddell; Schmidt et Ranges) that extend some 450 km, our ap- al., 1965; Schmidt and Ford, 1969; Ford, METHODOLOGY proach had to be focused. The U.S. Geo- 1972). This threefold division has formed logical Survey mapped the area between the basis of all subsequent work in this area This paper is based on field work carried 1962 and 1974 (Ford et al., 1978a, 1978b; (e.g., Weber and Fedorov, 1981; Weber, out in 1987–1988 during the joint United Schmidt et al., 1978). Their work suggested 1982). It is in marked contrast to the single Kingdom–United States West Antarctic that the Precambrian–Permian succession latest Paleozoic–early Mesozoic deforma- Tectonics Project. Because the field season could be divided into three unconformity- tion event documented by most workers (for was limited to 6 weeks, and the Pensacola bound sequences (Fig. 3), which were de- exception see Yoshida, 1983) for the Cam- brian–Permian strata of the nearby Ells- worth Mountains (Fig. 1; Webers et al., 1992). In the light of this discrepancy, our work was planned to evaluate the history of polyphase deformation in the Pensacola Mountains with a view to elucidating the pre-breakup configuration of the Pacific margin of the Gondwana supercontinent (Dalziel et al., 1987; Storey et al., 1988; Grunow et al., 1991). Subsequent to our work, Goldstrand et al. (1994) have documented stratigraphic evidence for the Ross orogeny within the Ellsworth Mountains based on a disconformity within the Cam- brian succession making correlations between the Ellsworth and Pensacola mountains more tenable. The lower Paleozoic part of the succession in the Pensacola Mountains is particularly well developed in the centrally located Neptune Range, where the geological structures all trend approximately north-south (Fig. 4) and are well exposed on a series of east-west–trending ridges. A major fault, re- ferred to here as the Roderick Valley lineament, separates predominantly west-dipping slices within and to the west of the fault zone from folded strata in the central and southern part of the range. The published geologic map (Schmidt et al., 1978) was used to identify key localities of the sequence- bounding unconformities, which were then studied in detail. Strata above and below each unconformity were examined. The U.S. Geological Survey maps augmented by our new field observations were used to con- struct isopach maps for each formation (Fig. 5).

MAJOR SEQUENCES

There are several unconformities in the Neptune Range that can be used with their correlative conformities (cf. Van Wagoner et al., 1988) to define five major sequences Figure 3. Stratigraphy of the Pensacola Mountains, compiled principally from published (Fig. 3). Some of the sequences are con- work by Schmidt et al. (1965, 1978), Schmidt and Ford (1969), and Ford et al. (1978a, formable over much of the outcrop, with 1978b), with additional information from our field mapping during the 1987–1988 austral sequence boundaries only locally .sequence number. unconformable ؍ summer. S

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The existing lithostratigraphic scheme is scheme is retained. The sequences we have the Beardmore orogeny prior to Middle an inconsistent mixture of “formation” and defined serve to link formations into genetic Cambrian times. Storey et al. (1992) sug- rock-based unit names such as “limestone” packages. gested that the Patuxent Formation formed (Schmidt et al., 1978). However, it is not the Sequence 1 consists of siliciclastic turbi- in a Neoproterozoic rift setting, whereas purpose of this paper to redefine stratigraph- dites and mafic lavas and sills of the Patux- Rowell et al. (1992) have suggested that at ic usage, thus the existing lithostratigraphic ent Formation, which was deformed during least some of the marine turbidites within the Patuxent Formation could be Early Cambrian in age, representing deep-water basin or slope deposits on the Cambrian carbonate platform (see below). Sequence 2 is separated by a major angular unconformity from Sequence 1. It comprises the Middle Cambrian Nelson Lime- stone and the overlying felsic volcanic rocks of the Gambacorta Formation. Sequence 3 consists of mudstones, sandstones, and limestones of the Wiens Forma- tion. It was originally thought to be conformable on the Gambacorta Formation (Schmidt et al., 1965), but our mapping has demonstrated an unconformity in the southern Neptune Range at locality 12 (Fig. 5). Sequence 4 rests unconformably on Se- quences 1–3 and consists of probable lower Paleozoic clastic sedimentary rocks of the Neptune Group. Sequence 5 consists of the Devonian Dover Sandstone, glaciogenic sedimentary rocks of the Permian–Carboniferous Gale Mudstone, and Permian sedimentary rocks of the Pecora Formation. The contact between the Dover Sandstone and the underlying Heiser Sandstone of the Neptune Group was originally described as conformable (Schmidt et al., 1965), although Schmidt and Ford (1969) described it as a disconformity representing a break in sedimentation or an interval of moderate erosion. Strata of Sequences 4 and 5 have both been correlated with the Beacon Super- group (Bradshaw and Webers, 1988; Bar- rett, 1991), which is extensively exposed along the length of the Transantarctic Mountains. We agree with this correlation for Sequence 5, but suggest (see below) that Sequence 4 is part of the lower Paleozoic Ross sequence deformed during the Ross orogeny. Sequence 5 and underlying rocks in the Pensacola Mountains were also deformed by a latest Paleozoic–early Mesozo- ic event designated the Weddell orogeny by Ford (1972). This also affected the Paleozoic strata of the Ellsworth Mountains, but not the Beacon Supergroup, and has been widely correlated with the Permian–Triassic Figure 4. Summary geologic map of the Neptune Range based on Schmidt et al. (1978), Gondwanide fold belt of Argentina and with additional information from our field mapping during the 1987–1988 field season. South Africa (Du Toit, 1937).

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LITHOSTRATIGRAPHY lineament, where it reaches its greatest re- magmatic. Data from the Gambacorta For- corded thickness of Ϸ600 m. In the central mation alone do not fit an isochron but give As this paper is primarily concerned and southern Neptune Range, it is absent in a best estimate age of 568 Ϯ 39 Ma, with an with Cambrian and Ordovician strata af- the western area and thickens eastward initial ratio of 0.7052. Our new Rb-Sr anal- fected by the Ross orogeny, only forma- from a well-defined feather edge to Ͼ500 m. yses of 29 whole-rock samples, collected tions from Sequences 2–4 are described. This variation does not involve any facies from six localities, scatter widely and do not Thickness variation and paleocurrents are change and is presumed to be entirely due to further constrain the age of the Gambacorta shown in Figure 5. erosion below the sub–Neptune Group Formation (Table DR11). unconformity. The most reliable age for the Gambacorta Nelson Limestone (Sequence 2) Age. The Nelson Limestone has yielded a Formation is a new U-Pb zircon age of trilobite fauna containing the Amphoton 501 Ϯ 3 Ma (Millar and Storey, 1995). This Facies. The basal part of the Nelson oatesi and Nelsonia schesis faunules. These is consistent with the stratigraphic position Limestone is a clastic unit (0–72 m; Evans et indicate a late Middle Cambrian age of the Gambacorta Formation and is sup- al., 1995) infilling channels and hollows on (Palmer and Gatehouse, 1972) and have ported by initial U-Pb results of Rowell et al. the eroded surface of the previously folded been found as low as 10 m above the basal (1995). It provides the most precise estimate Patuxent Formation. Regionally, the uncon- unconformity (Rowell et al., 1992). Solov’ev of the age of the Gambacorta Formation to formity is relatively flat, although local relief et al. (1984) recognized Upper Cambrian date, which, based on the Harland et al. of at least 10 m is provable in places. There fossils from the southern part of the Nelson (1990) time scale, would be Early Ordovi- has been some reddening of the upper few Limestone. cian in age. It also suggests, on the basis of meters of the Patuxent Formation. The clas- conformable and interbedded relationship tic unit comprises green, gray, and red fine Gambacorta Formation (Sequence 2) between the Gambacorta Formation and pebble conglomerates, granule stone, and underlying Nelson Limestone in Jones Val- medium- to coarse-grained sandstones ar- Facies. The Gambacorta Formation con- ley, that the age of the Nelson Limestone ranged in a crude fining-upward cycle. Vein sists of interbedded gray, green, brown, and may span Middle and Late Cambrian times. quartz pebbles, cleaved phyllite clasts, and reddish brown felsic volcanic rocks. It in- Petrography. The felsites contain varied clasts with a spaced pressure solution cleav- cludes welded ash-flow tuffs, volcanic brec- proportions of embayed quartz, plagioclase, age were derived from the underlying Patux- cias, pyroclastic flows, lava flows, and re- and alkali feldspar phenocrysts in an altered ent Formation. Beds are crudely stratified or worked volcaniclastic sandstones and shales. quartzofeldspathic groundmass. Ferromag- cross-stratified, and wave ripples were found The type locality is at Gambacorta Peak, nesian phenocrysts are mostly altered to at one locality. where the formation could be as much as chlorite, sericite, calcite, epidote, and opaque The overlying limestone comprises ooi- 1500 m thick (Schmidt and Ford, 1969) and oxide aggregates, although primary augite, dal, pisolitic, peloidal, and bioclastic lime- may represent the eruptive center of a vol- hypersthene, and muscovite are preserved in stone with both micritic and sparry matrix. canic complex. Schmidt (1969) defined an a few samples. Pseudomorphs suggest oli- There are occasional lenses and interbeds of elliptical caldera based on a 10 milligal grav- vine was a primary magmatic phase in one calcareous sandstone. There is a crude cy- ity low (Behrendt et al., 1974) bounded by rock. Zoned allanite phenocrysts are a com- clicity, with thin-bedded, bioturbated lime- concentric border faults, which separate the mon accessory phase; zircon, apatite, gar- stone passing up into cross-bedded ooidal crudely bedded Gambacorta Formation net, and opaque oxides are also present. and sandy limestone; cross-bedding can be (sensu stricto) from the massive columnar- Geochemistry. The volcanic rocks of the in sets up to 2 m thick. Some cycles are jointed rocks of the Hawkes rhyodacite, in- Gambacorta Formation are highly fraction- capped by nodular bioclastic limestone; cy- ferred to have been deposited within the ated peraluminous rhyolite (Table DR2; see

cles are 10–30 m thick. caldera. The formation thins rapidly away footnote 1). SiO2 varies from 68%–78%. At the eastern end of Jones Valley, on the from the volcanic center, with an arcuate With few exceptions, due to siliciﬁcation,

western ridge of Wiens Peak (locality 11; feather edge 10–15 km west and northwest TiO2,Al2O3,Fe2O3, CaO, and P2O5 system- Fig. 5), the top 10 m of the Nelson Lime- of the Gambacorta Peak–Mount Hawkes atically decrease with increase in SiO2. Ac- stone contains interbedded lapilli tuff, and caldera. cording to the classiﬁcation of Le Maitre the contact with the overlying Gambacorta Age. Stratigraphic relationships indicate (1989), both sodic and potassic rhyolites ex-

Formation is clearly conformable and gra- that the Gambacorta Formation is late Mid- ist. The sodic rhyolites, with Na2O/K2O ra- dational. Evans et al. (1993) implied (their dle Cambrian or younger in age, and if the tios up to 5.8, are the least fractionated and

Fig. 2c) that there is an unconformity above data of Solov’ev et al. (1984) are correct, have unusually high TiO2,Fe2O3,P2O5, and the Nelson Limestone (although this is not then it is younger than Upper Cambrian. Zr values (up to 503 ppm), whereas the explicitly stated). We did not observe this Attempts to obtain an absolute age for the more fractionated potassic rhyolites are en- relationship and follow Schmidt et al. formation by Rb-Sr dating have not been riched in incompatible elements Y (up to 56 (1965), Schmidt and Ford (1969), and Evans particularly successful. The most widely ppm) and Nb (up to 19 ppm). Some trace et al. (1995) in interpreting the Nelson- quoted age of the Gambacorta Formation is element concentrations show a poor corre-

Gambacorta contact as a conformable 510 Ϯ 35 Ma (initial ratio 0.708; Eastin, lation with SiO2 and may be due to entrain- boundary. 1970). This is based on Rb-Sr whole-rock

Thickness Variation. The Nelson Lime- data and is a combined isochron age for the 1 Gambacorta Formation together with the GSA Data Repository item 9630, Ta- stone is absent from the northern and east- bles DR1–DR5, is available on request from ern nunataks of the Neptune Range. It is Serpan Granite and Gneiss (see below), Documents Secretary, GSA, P.O. Box 9140, Boul- Ͼ500 m thick west of the Roderick Valley which Eastin considered to be comagmatic- der, CO 80301.

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Figure 5. Summary diagrams illustrating thickness variations and paleocurrents for every formation between the Nelson Limestone and the Heiser Sandstone, based on 17 key localities in the Neptune Range. Thickness data are derived in part from the U.S. Geological Survey map of Schmidt et al. (1978) and in part by the authors. There was a remarkable agreement between results derived from the two techniques. All paleocurrent data come from the authors. XB, cross bedding; TXB, trough cross bedding; RC, ripple crest. The number of readings and vector means are also given. on 27 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/6/685/3382482/i0016-7606-108-6-685.pdf elgclSceyo mrc ultn ue1996 June Bulletin, America of Society Geological

Figure 5. (Continued). 691 on 27 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/6/685/3382482/i0016-7606-108-6-685.pdf 9 elgclSceyo mrc ultn ue1996 June Bulletin, America of Society Geological 692

Figure 5. (Continued). SIGNIFICANCE OF THE ROSS OROGENY, ANTARCTICA

the Gambacorta Formation was most likely of igneous origin, whereas the source for the Thiel Mountains porphyry was sedimentary.

Serpan Gneiss

At Serpan Peak, 20–40 km to the north of the Gambacorta Peak–Mount Hawkes caldera, Schmidt and Ford (1969) mapped another largely concealed igneous center. The only exposed part of this igneous center is the Serpan Gneiss, a gray and pink foli- ated hornblende-biotite granodiorite and biotite granite that crops out at Serpan Peak. It is believed to be the border phase of the “Median Granite,” which was inferred by Schmidt and Ford (1969) to underlie the Median Snowfield on the basis of locally abundant granitic erratics and on a negative gravity anomaly of at least 30 milligals Figure 6. Chondrite normalized rare earth element (Table DR2; see footnote 1) diagram (Behrendt et al., 1974). Our new Rb-Sr data for a felsite from the Gambacorta Formation (R.4720.10), granite from the Serpan Gneiss from the Serpan Gneiss do not further con- (R.4728.7), and porphyry from the Thiel Mountains (V6-8A). strain the existing age of 555 Ϯ 26 Ma (initial ratio 0.7065; Eastin, 1970), only con- firming a likely Cambrian–Ordovician age ment of accessory phases such as zircon and tion of mafic granulites in the source region (Table DR1; see footnote 1). allanite, a feature of highly fractionated and that the felsites may have been pro- The main granitoid at Serpan Peak is a rhyolites. The rhyolites are light rare earth duced by melting previously underplated biotite-granite with accessory titanite, zir- element–enriched with chondrite normal- igneous rocks. This is consistent with the con, apatite, and allanite. The granite is in-

ized LaN/YbN ratios of up to 22.7 (Ta- ideas of Eby (1992), who has related all A- tensely sheared and altered with extensive ble DR3; see footnote 1). type granites to an I-type (igneous) source. crystal plastic deformation and grain-size re- The peraluminous nature of the very frac- Geochemical comparisons between the duction. In the most sheared samples, the tionated high-Si rhyolites of the Gam- Gambacorta Formation and the contempo- ferromagnesian minerals are replaced by bacorta Formation may indicate the impor- raneous Thiel Mountains porphyry (Rb-Sr epidote, chlorite, and mica assemblages; tance of crustal fusion during their isochron age 502 Ϯ 5 Ma, initial 87Sr/86Sr feldspar crystals are altered to sericite and formation. If this is the case, the presence of ratio of 0.714; Pankhurst et al., 1988), which fractured by brittle dislocations; quartz hypersthene and augite, characteristic of lies 300 km southeast of the Neptune Range forms ribbon structures and has recrystal- charnockitic assemblages, indicates crystal- within the Transantarctic Mountains, indi- lized to ﬁne-grained aggregates. A marginal, lization from a high-temperature anhydrous cate similar major and trace element con- more maﬁc phase of the main granitoid con- melt, with the solubility of zircon in high- centrations (Fig. 6). Both contain hyper- tains hornblende and biotite and abundant temperature melts accounting for the high- sthene and augite phenocrysts. The main coarse-grained titanite. Zr values (Watson and Harrison, 1983). difference between the two suites is that the The sheared granitoid of the Serpan

Concentrations of high ﬁeld strength ele- Thiel Mountains porphyry is more peralu- Gneiss has a SiO2 range of 70%–76% (Ta- ments (Zr, Nb, and Y), although not partic- minous than the Gambacorta Formation ble DR5; see footnote 1) and for the most ularly diagnostic, are above those normally and contains the aluminum-rich phase part has major, trace, and rare earth ele- expected for crustally derived rocks and ap- cordierite, which is typical of S-type granit- ment characteristics similar to the silicic vol- proach values typical of anorogenic grani- oid rocks. This difference is also reﬂected in canic rocks of the Gambacorta Formation.

toids (within plate granitoids of Pearce et the isotopic compositions; although they The main exception to this is that the Na2O/ ε 87 al., 1984) termed A-type by Loiselle and both have similar Nd value, the initial Sr/ K2O ratios are Ͻ1, and the Y and Zr values Wones (1979) and White (1979). Loisell and 86Sr ratios are signiﬁcantly different, with of the granite are consistently lower than Wones considered A-type magmas to be de- the Thiel Mountains porphyry having the those of the volcanic rocks. The marginal

rived either by partial melting of relatively higher initial ratios (for comparison, see Ta- maﬁc phase has 57% SiO2 with high Ti anhydrous (probably residual) granulite fa- bles DR1 and DR4 [refer to footnote 1 for (1.32%) and P2O5 values (0.49%). cies rocks during basaltic underplating, or by availability] and Sr isotopic data for Thiel Petrogenetic relationships between the direct fractionation of basaltic magmas. The Mountains porphyry in Pankhurst et al., Gambacorta Formation and the Serpan isotopic characteristics of the Gambacorta 1988). Both require crustal melting during Gneiss are uncertain, and our data do not Formation are inconsistent with a direct the Cambrian, involving a middle Late Pro- permit a rigorous comparison. There is, mantle origin, and melting of a felsic source terozoic source (Td model ages 1350–1100 however, an important petrographic distinc- cannot account for the high Y and Nb val- Ma). The immediate source for the melts tion between the two units; the felsic vol- ues. These values suggest a major propor- that ultimately crystallized to form felsites of canic rocks contain augite and hypersthene

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ervation, but he tentatively identiﬁed them as Notiobolus sp.; this genus is of Cambrian or Early Ordovician age.

Neptune Group (Sequence 4)

Sequence Boundary. The base of the red beds of the Neptune Group is a widespread Figure 7. The Wiens Forma- unconformity with considerable overstep. tion unconformably overlain by At different localities the Neptune Group conglomerates of the Neptune rests on the Wiens (Fig. 7), Nelson, Gam- Group. The pale white lenses bacorta and Patuxent (Fig. 8) Formations. are thick oolitic limestones The contact with the Patuxent Formation is partly covered in scree within always angular (Fig. 8); however, the contact the Wiens Formation. They are with other formations is locally paracon- of variable thickness and up to formable. At a local scale, the base of the 6 m thick. The exposed section group cuts out tens of meters of strata. illustrated in the photograph is Where the Neptune Group rests on the Ϸ50 m thick. The photograph green member of the Wiens Formation, the is of the southeast end of Elliott upper few centimeters to 2.5 m of the Wiens Ridge and was taken facing Formation has been reddened. Paleosphe- northwest from Wiens Peak. roidal weathering was found at the top of the Gambacorta Formation at a single locality near Brown Ridge. The most spectacular evidence for an emergent unconformity comes from the Nelson Limestone, which has a very well- developed paleokarst in at least two localities. At the east end of Jones Valley, the Neptune Group truncates an open fold in the Nelson Limestone. The limestone is cut by dikes and pipes, which are filled with red as the main ferromagnesian phases, whereas is parallel-laminated and intercalated with and green sandstone and geopetal spar. The the Serpan Gneiss contains hornblende, bi- minor units of lapilli tuff. There are up to source of the sandstone is unclear but most otite, and titanite, a mineralogy typical of four interbedded units of white oolitic lime- closely resembles overlying parts of the I-type granitoids as defined by White and stones, which are also nodular or sandy in Neptune Group such as the Elliott Sand- Chappell (1977). places. Limestone units are highly varied. stone or Elbow Formation. There is clear On Wiens Peak, the thickest is 6 m (Fig. 7), evidence of downward introduction of this Wiens Formation (Sequence 3) while in Gambacorta Valley, 3.5 km south, clastic material with intermittent flow. The the thickest is 27 m. The limestones dis- top of the Nelson and base of the Neptune Facies. The Wiens Formation consists of play a variety of styles and scales of Group have been affected by formation of a red and green sandstone, siltstone, and cross-bedding. series of calcrete paleosols. Fuller descrip- mudstone, with subordinate limestone and Thickness Variation. The Wiens Forma- tions of these paleosols will be published minor conglomerate. The clastic rocks are tion is absent from the northern and eastern elsewhere. The age of karstification is poorly volcaniclastic, representing erosion and re- nunataks. It is present at every locality west constrained but was clearly prior to deposi- working of the Gambacorta Formation. of the Roderick Valley lineament, where it tion of the Neptune Group. Our only observations come from the reaches its greatest thickness, almost At Nelson Peak and adjacent areas, there southern Neptune Range, where the forma- 1100 m. In the central and southern Nep- are major developments of the following tion consists of two members. The lower red tune Range, it is only present in the extreme two types of breccia, which are thought to member consists of thin-bedded red silt- south, almost coincident with the outcrop of represent deeper levels of the paleokarst: stones and very fine-grained sandstones with the Gambacorta Formation, where it is up Type A consists of angular blocks of lime- locally derived conglomerate at the base of to 250 m thick. stone up to 10 m across, with a matrix of red the formation. Small symmetrical ripples, Age. Several small inarticulate brachio- mudstone (Fig. 9). These form large, irreg- mud cracks, and raindrop impressions are pods were recovered from the Wiens Peak ular units, approximately parallel to bed- common. Coarser, cross-bedded sandstone area and are the first body fossils to be col- ding, with indeterminate margins. Lime- beds with scoured bases and associated lected from this formation. Samples were stone blocks are similar to the overlying mud-flake breccias are interbedded. There sent to A. J. Rowell (University of Kansas), unbrecciated limestones. This type of brec- are no large-scale channels. The upper who informed us that they have the features cia probably represents collapse of major green member consists of splintery, green, of early Paleozoic Lingulellinae. Generic as- caves, perhaps during deformation. very fine-grained, tuffaceous sandstone; this signment is debatable because of their pres- Type B consists of subvertical pipes of

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same stratigraphic order, and in some localities there are intimate interbeddings of associations through stratigraphic thicknesses of tens to hundreds of meters. However, the Schmidt and Ford stratigraphy is acceptable as a first approximation and is used here pending a major revision of the Neptune Group. The group displays a crude, albeit irregular, fining-upward trend. The Heiser Sandstone always forms the top of the group. Conglomerates. Schmidt and Ford (1969) and Schmidt et al. (1978) recognized two separate conglomerate units, based on clast type, and we are including the Rhyolite Breccia Member as a third conglomerate unit (see above). The Brown Ridge Con- glomerate occurs in the central Neptune Range and west of the Roderick Valley lineament, with clasts of Gambacorta Forma- Figure 8. Angular unconformity between the overlying Neptune Group and the under- tion and Nelson Limestone; the Neith Con- lying Patuxent Formation at Brown Ridge on the north side of Nelson Peak in the Neptune glomerate occurs in the northern nunataks, Range. The Patuxent Formation is tightly folded about upright folds. The exposed section with clasts of Patuxent Formation metasedi- illustrated in the photograph is Ϸ100 m thick. The photograph is of the western side of mentary rocks. The Rhyolite Breccia Mem- Brown Ridge and was taken facing east. ber is restricted to the Wiens Peak area and is made exclusively of clasts derived from the Gambacorta Formation. All of the conglomerate units consist of breccia with clearly defined margins. These erate, made exclusively of clasts derived unbedded or thick-bedded clast-supported are up to 50 m in diameter and extend 100– from the Gambacorta Formation. cobble conglomerate with a matrix of coarse 200 m vertically. Limestone blocks are an- Our field observations indicate that the to very coarse sandstone (Fig. 10). Cobbles gular, poorly sorted, and randomly oriented, four formations recognized by Schmidt and are well rounded, with evidence of weath- but are of more varied facies than Type A. Ford (1969) are represented by four distinct ering prior to deposition. Beds are massive The matrix between the blocks is finely lam- facies associations in the Neptune Group. (disorganized) or display bedding-parallel inated calc-tufa (“flowstone”). These brec- The associations do not always occur in the imbrication. At some localities, 1- to 3-m- cias represent development and infilling of major solution pipes. On Nelson Peak, the upper part of a Type B pipe contains some blocks of Gambacorta Formation. Facies. The Neptune Group is red sandstone and conglomerate that varies in thickness from 600 to Ͼ2000 m. Its outcrop is restricted to the Neptune Range and the southern part of the Forestal Range and Dufek Massif. Schmidt and Ford (1969) recognized the following fourfold division: Brown Ridge Conglomerate, Elliott Sandstone, Elbow Formation, and Heiser Sandstone. They mapped the Elliott Sandstone overlapping the conglomerates and resting directly on the unconformity, and suggested northward overlap of the Elliott Sandstone by the El- bow Formation. Schmidt et al. (1978) also recognized a Rhyolite Breccia Member within the Elliott Sandstone, which was thought to be a pyroclastic flow deposit with welded textures. However, we saw no sign of Figure 9. Limestone breccia with angular fragments of pale limestone encased in dark welded texture in this unit and conclude that red mudstone; this is part of a major cave-collapse deposit Ϸ2 km west of Nelson Peak. Note it is a variant of the Brown Ridge Conglom- person for scale.

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thick conglomerate beds are capped by Elliott Sandstone. This unit comprises Valley area, this change is stratigraphic, with structureless or cross-bedded sandstone coarse-grained, red sandstone and pebbly a decreasing percentage of volcanic clasts beds and lenses. Conglomerate bed bases sandstone, with minor interbeds of red mud- up-section. Sandstone beds are 40–150 cm are only locally erosional; protruding clasts stone (Fig. 11). Lithology varies from lithic thick, with erosive bases. Beds are generally are common on bed tops. (volcanic) arenite to arkose. In the Jones cross-bedded or planar-laminated; both planar and trough cross-stratification are found, commonly with planar sets passing down-paleocurrent to trough sets. Elbow Formation. This formation is similar to the Elliott Sandstone in being dominantly red sandstone with subordinate mudstone (Fig. 12). Sandstone is well sorted, cross-bedded, fine and very fine grained, generally in beds 0.05–1 m thick. Beds tend to be formed of a single set of planar-tabular cross-stratified units. Set expansion into scours is common, with associated mud- flake breccia units. Many sets exhibit mud- draped reactivation surfaces; some terminate abruptly, passing into ripple cross-laminated sandstone or structureless mudstone. The interbedded fine facies comprise 20%–40% of the association and are mainly structureless or faintly laminated siltstone or silty mudstone. There are minor amounts of pure claystone. Wave ripples and mud cracks are Figure 10. Clast-supported cobble conglomerate of the Brown Ridge Conglomerate of the common. Neptune Group 3 km west of Wiens Peak. Note the pale limestone clasts derived from the Heiser Sandstone. This unit consists of four facies (Fig. 13). Facies A, mottled red Nelson Limestone. The hammer handle is Ϸ65 cm long. and green very fine-grained sandstone, forms a spectrum from interlaminated sandstone with minor burrows to thoroughly bioturbated sandstone with no original sedimentary structures. Facies B is low-angle laminated fine- to medium-grained sandstone including a variety of onlap and down- lap relationships, internal scours, wave ripples, and primary current lineation. Facies C is cross-bedded fine- to medium-grained sandstone, commonly with reactivation surfaces and small-scale syn-sedimentary deformation. Facies D is flat-laminated fine- to Figure 11. Cross-bedded medium-grained sandstone. Facies B–D are sandstone of the Elliott Sand- all manifestations of large complex bar stone on a ridge 5 km west of forms, which may be bioturbated. Mount Hawkes at the head of Thickness Variation and Paleocurrents. Jones Valley. The hammer han- The group as a whole thickens westward dle is Ϸ35 cm long. from 600 to Ͼ2200 m. Isopachs are arcuate, convex to the west, with the thinnest part of the group in the south-central Neptune Range. The thickness variation seems to be unaffected by the Roderick Valley lineament, although the isopachs run approximately parallel to the structure. The dominant transport directions are north, south, and west, although there are considerable variations both between and within individ- ual units. The conglomerate units are arranged in east-west–trending belts. Both the Rhyolite

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Breccia Member and the Neith Conglomer- ate have approximately west-directed paleocurrents. The Brown Ridge Conglomer- ate thickens northward from a well-defined, east-west–trending feather edge; isopachs also trend east-west. The thickest development both of this unit (Ͼ700 m) and the whole outcrop of the Neith Conglomerate lies west of the Roderick Valley lineament, suggesting that the Roderick Valley lineament may have been active during deposition of the basal conglomerates. Large areas of the central and southern Neptune Range and of the southern area west of the Rod- erick Valley lineament have no conglomerate at the base of the Neptune Group. These are also the areas where the whole group is thinnest. The Elliott Sandstone is absent from the north and east, with a probable northwest- Figure 12. Coarse cross-bedded sandstone (pale gray) interbedded with flat-laminated southeast–trending feather edge. The El- fine sandstone and siltstone (darker shades) of the Elbow Formation from the northwest bow Formation is thin or absent where the side of Jones Valley. Note the drapes of dark siltstone on the foresets of the prominent bed Elliott Sandstone is thickest, and is thickest in the middle of the photograph, marking successive reactivation events; note also final in the north and east. Isopachs seem to be independent of the Roderick Valley linea- abandonment of this bar form at left side near the ice ax. The ice ax, for scale, is Ϸ70 cm long. ment. In the south, paleocurrents are consistently northerly (localities 11 and 12; Fig. 5) through the entire 1000 m thickness of the unit. In the south-central area (locality 10; Fig. 5), paleocurrents are mainly west directed, with a significant minor northerly mode. In the north-central area (locality 14; Fig. 5), basal paleocurrents are southerly, switching abruptly to northerly. The Elbow Formation is thickest west of the Roderick Valley lineament and thinnest in the south, where the Elliott Sandstone is thickest. The limited paleocurrent data show dominantly Figure 13. Well-bedded southerly transport, with minor east- and Heiser Sandstone from the west-directed modes. Both wave and current southern side of Jones Valley, ripple crests are oriented north-south, sug- showing the four facies dis- gesting east-west wind directions and low cussed in the text: A, mottled stage flow at right angles to the main trans- red and green very fine-grained port defined by the cross-bedding. sandstone; B, low-angle lami- The Heiser Sandstone is the only forma- nated fine- to medium-grained tion of the Neptune Group found through- sandstone; C, cross-bedded out the outcrop of the group. It varies in fine- to medium-grained sand- thickness from 140–500 m, with the thickest stone; and D, flat-lying lami- strata in the southwest. Paleocurrents indi- nated fine- to medium-grained cate southerly or southwest transport. Rip- sandstone. The hammer han- ple crests are oriented north-south, with a dle is Ϸ35 cm long. pattern virtually identical to the Elbow Formation. Age. No body fossils or age-significant trace fossils have been found in the Neptune Group, nor are there any contemporaneous igneous rocks that can be dated. The highly oxidized sedimentary rocks suggest that mi- crofossils would not be preserved, although no investigations have been made to date.

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Thus the group could be of any age from A 200-m-thick limestone unit is also crop. It is marked by major erosion of the Early Ordovician to Middle Devonian, al- found at Des Roches Nunataks in the south- underlying rocks, karstification of the lime- though relationships between sedimenta- west Patuxent Range. This unit has not been stone, and calcrete pedogenesis. A similar tion and deformation discussed below sug- paleontologically dated and is not in strati- variety of contact types has been described gest that it is likely to be early Paleozoic, graphic contact with any other unit. How- by Crowder (1990) from the unconformity possibly Ordovician. The interpretation of a ever, from its simple deformation style, its between the Pennsylvanian limestones of disconformity with the overlying Dover attitude relative to the deformed rocks of the Lisburne Group and the clastic rocks of Sandstone (Schmidt and Ford, 1969) implies the Patuxent Formation, and the fact that it the Sadlerochit Group (Permian) in Alaska; that there may be a time gap here. We are in is cut by sedimentary dikes filled with red he attributed the different contact types to agreement with this interpretation. sandstone, we assume that it is equivalent to different positions on a boundary with high the Nelson Limestone. The presence of this topographic relief. The time of formation of INTERPRETATION OF limestone unit in the Patuxent Range sug- SB4 is equivocal, but tilting, uplift, and STRATIGRAPHY gests limestone deposition covered an area karstification may have begun during the of at least 7500 km2 (Neptune Range) and time of eruption of the Gambacorta Forma- Sequence 2: Nelson Limestone and possibly as much as 11 500 km2 (Neptune tion. There was clearly syn-depositional tec- Gambacorta Formation and Patuxent Ranges) during Middle Cam- tonism during deposition of Sequence 3, and brian times. This area was undergoing gen- much of the Nelson Limestone was eroded The lower sequence boundary (SB2) is a tle subsidence for at least 40–20 m.y. Shal- at this time. The SB3/SB4 couplet could rep- major regional erosional unconformity. It is low marine carbonate systems of this size resent as much as 40 m.y. of subaerial ero- overlain by a thin clastic fluvial unit with are usually formed on epeiric platforms or sion and weathering in parts of the area. wave ripples and then by a thick limestone ramps on passive continental margins (see, The conglomerate units were deposited representing a marine transgression. The fa- e.g., Markello and Read, 1981; Sellwood, by sheetfloods in east-west–trending valleys. cies, recently described by Evans et al. 1986; Osleger and Read, 1993). This period The source area was nearby, and clasts had (1995), are consistent with deposition in a of tectonic quiescence in the Pensacola probably undergone some stream transport. high-energy, relatively shallow-water setting Mountains was brought to a close, at least The Elliott Sandstone and Elbow Formation as a series of grainstone shoals with inter- locally, by the onset of magmatism in Early were deposited by intermittent, unconfined shoal micrite and wackestone facies. Similar Ordovician times, as represented by the flows producing a series of bars or sand facies have been described from the modern Gambacorta Formation and Serpan Gneiss. waves. Standing water remained long Bahama Banks (Ball, 1967) and the Upper Its tectonic significance is discussed below. enough to allow wind ripples to be gener- Jurassic Smackover Formation of Texas ated. Taken together, the three units prob- (Wilson, 1975). Osleger and Read (1993) Sequence 3: Wiens Formation ably represent different areas of a major al- have published a major study on correlation luvial fan complex with a coarse inner fan of the Upper Cambrian in North America, When the lower sequence boundary area (conglomerates) interdigitating with and all of the Nelson Limestone facies are (SB3) is exposed, it is conformable on the outer fan (Elliott Sandstone) and fan fringe similar to rocks that they assign to shallow underlying Gambacorta Formation. Over (Elbow Formation) deposits laid down by a subtidal and peritidal associations. much of the rest of the area, SB3 is coinci- series of ephemeral streams. The whole area Correlation of limestone units within the dent with SB4; it is impossible to say was inundated by a gradual marine trans- Pensacola Mountains is uncertain. The Ar- whether this is due to nondeposition of the gression (Heiser Sandstone). The Neptune gentina and Neptune Ranges contain car- Wiens Formation or to erosion associated Group as a whole is typical of many post- bonate units with platformal facies, but of with the base of Sequence 4. Only in the orogenic red bed formations (e.g., Woodrow different ages (Early and Middle Cambrian, valley west of Gambacorta Peak (Loca- and Sevon, 1985). respectively). Rowell et al. (1992) used these lity 12; Fig. 5) is the boundary clearly un- age differences to propose that a carbonate conformable. There it dissects preexisting STRUCTURAL HISTORY shelf-clastic fan system grew across the area volcanic topography that may have been un- as a series of distinct oceanward systems. dergoing syn-depositional uplift. There was Structural cross sections together with an However, the lack of proven Lower Cam- major westerly downthrow on the Roderick interpretation of the structural evolution of brian clastic units within either the Patuxent Valley lineament (probably Ͼ1 km) at or the Pensacola Mountains have been pre- or Neptune Ranges (i.e., outboard of the before the time of formation of the se- sented by Schmidt et al. (1965, 1978), Lower Cambrian Limestone in the Argen- quence boundary. Schmidt and Ford (1969), Ford (1972), We- tina Range) does not support this model; The basal red member was deposited by ber and Fedorov (1981), and Weber (1982). the new Ordovician date for the Gam- unconfined flows in ephemeral streams. The One of the intriguing aspects of the struc- bacorta Formation of 501 Ϯ 3 Ma (Millar change to green sandstones corresponded to tural evolution of the mountain range is that and Storey, 1995) suggests that deposition of a marine transgression, and the upper mem- deformation structures associated with the Nelson Limestone may have extended ber was deposited in a shallow marine set- three orogenic events, the late Precambrian into the Late Cambrian, a conclusion sup- ting with carbonate shoals. or Early Cambrian Beardmore orogeny, the ported by the paleontological work of Cambrian–Ordovician Ross orogeny, and Solov’ev et al. (1984). This implies that the Sequence 4: Neptune Group the Permian–Triassic Weddell orogeny, are margin was more stable through Cambrian broadly coplanar. This creates a major prob- time than the Rowell et al. (1992) model The lower sequence boundary (SB4) is lem in recognizing the effects of the different implies. coincident with SB3 over much of the out- orogenic events in the older sequences. In

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Planar fabrics associated with the folding are not uniformly developed throughout the sedimentary succession. Coarse-grained units generally contain a spaced pressure solution cleavage, whereas argillaceous units contain a single penetrative slaty fabric. Cleavage is generally steeply dipping, trending north- northeast–south-southwest (Fig. 15); cleavage poles define a cluster on an equal-area projection. The variation in dip is due either to cleavage refraction or possibly to the variable effect of the Weddell folding on preexisting Ross structures. Bedding-cleavage intersection lineations are variably plunging, similar to the fold hinges. The intersection lineations and fold hinges form a north-south girdle of points on an equal-area projection (Fig. 15). Zones of more intense cleavage development are present along the boundaries of the different sedimentary formations. This phenomenon is most likely a response to competency contrasts among the different formations. In some cases, contacts are sheared and faulted. Bedding-cleavage intersection lineations and fold hinges in these high-strain zones in most cases plunge moderately, in- dicating oblique-slip movement. For example, on the western side of Wiens Peak, a ductile shear zone separates the lower and upper members of the Wiens Formation. Strata from the upper member have been truncated by the shear. The geometry of the minor folds within the shear zone and shear indicators suggest the lower member (hang- ing wall flat) was thrust obliquely over the Figure 14. Equal-area, lower hemisphere projections of bedding planes from the Nelson upper member (footwall ramp). Limestone, Gambacorta Formation, Wiens Formation, and Neptune Group in the Neptune Cleavage-fold relationships are not al- Range. ways straightforward, and in some cases cleavage transects the folds. At locality R.4730 (Fig. 16), the prominent valley on fact, the only record of these events is in the basis of structural geometry alone. The main the western side of Gambacorta Peak, cleav- decrease in intensity of deformation from effect of the Weddell orogeny was most age is downward-facing on the steep limb of the older to the younger sequences, the likely to tighten preexisting Ross structures a large-scale anticline; the cleavage tran- presence of unconformities between the ma- rather than create new ones. sects the profile plane of the fold in a clock- jor stratigraphic sequences, and randomly Throughout the Neptune Range, the sed- wise sense (negative, according to notation oriented cleaved clasts from underlying se- imentary strata of Sequences 2–4 trend of Johnson, 1991), and bedding-cleavage in- quences in the basal conglomerates of over- north-south, are moderately to steeply intersection lineations are steep. The relation- lying sequences. clined, and folded about large-scale open to ship between fold and cleavage develop- The following description of the struc- tight folds with a wavelength of 2–3 km. ment is uncertain. They may have formed in tures associated with the Ross orogeny is Poles to bedding define a broad girdle of the same progressive deformational epi- based on detailed studies of a number of points (Fig. 14) approximating a great circle sode, or cleavage may be a Weddell struc- key localities in the Neptune Range. The (azimuth, 276Њ; dip, 84ЊN). There is some ture that overprinted a preexisting Ross structures affecting each of the sedimen- variation in the orientation of the great cir- fold. tary units are plotted separately on stereo- cles defined by structures in each of the dif- The granitic protolith (believed to be the grams (Figs. 14 and 15). It is recognized that ferent formations (Fig. 14). Second-order Median Granite) of the Serpan Gneiss was some of these structures may relate to the asymmetric folds are variably plunging to deformed by thin mylonitic shear zones to younger Weddell orogeny. The coplanarity north and south (Fig. 15). The mean hinge form the Serpan Gneiss. Hornblende and of structures makes it impossible to differ- line lineation azimuth is 196Њ with a mean biotite are aligned and partly replaced by entiate between the two orogenies on the plunge of 74Њ. secondary phases, quartz has recrystallized

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volved a component of strike slip. Al- though the overall sense of displacement is unknown, local shear fabrics within the Serpan Gneiss indicate sinistral movement. Schmidt et al. (1978) mapped the Roderick Valley lineament as an oblique dextral strike-slip fault, which, as suggested here, may have had a controlling inﬂuence on sedimentation patterns during Cambrian–Ordovician times. Dextral motion may have been the primary movement sense during Ross deformation.

RELATIONSHIP BETWEEN SEDIMENTATION AND DEFORMATION

Most of our work in the Neptune Range strongly supports the mapping and interpre- tations of Schmidt et al. (1978). The crucial new evidence, which reveals the presence of an unconformity and a deformational event between the Gambacorta and Wiens formations, comes from the north side of an unnamed valley 8 km west of Gambacorta Peak (locality R.4730; locality 12 in Fig. 5). The new observations are recorded in detail Figure 15. Equal-area, lower hemisphere below. projections of slaty cleavage (S1), folds (F1) and bedding-cleavage intersection lineations Key Locality (L1) from the Paleozoic strata in the Neptune Range. At locality R.4730, a stratigraphic sequence of Gambacorta Formation (Fig. 16), Wiens Formation, and Neptune Group is well exposed in an anticline-syncline fold pair with wavelength Ϸ800 m. At the base of the sequence, an eroded anticline of the Gambacorta Formation is unconformably overlain by the Wiens For- mation (Fig. 16A). There is pronounced relief on the anticlinal structure where canyons, up to 30 m deep and 30 m wide, dissect the hinge zone. The Wiens Formation laps to finer-grained aggregates and elongate monly trend parallel to bedding, although onto, and varies in thickness across, the an- ribbon textures, and feldspar has been de- some are at a high angle. Although displace- ticlinal structure. There are randomly ori- formed by internal shear bands. Lineations ment indicators were not usually observed, entated clasts of cleaved Gambacorta For- on shear planes indicate transcurrent mo- both dip-slip and strike-slip motions oc- mation within the basal strata of the Wiens tion, and associated shear fabrics record curred. Syn-sedimentary normal faults with Formation (Fig. 17). These relationships movement in a sinistral sense. up to 1 m displacement were observed in the suggest that the Gambacorta Formation was In the sedimentary rocks, cleavage is de- Neptune Group on the south side of Jones deformed and that the observed relief ex- fined by new white mica minerals, the align- Valley. isted prior to deposition of the Wiens ment of preexisting detrital phyllosilicates, Formation. and development of pressure solution seams. Interpretation Five stratigraphic sections were measured In sandstones, detrital grains are internally across the Wiens Formation–Neptune deformed, cleavage is commonly anasto- The presence of both moderate- and Group unconformity (Fig. 16C). The top of mosing, and in some cases two cleavages are steep-dipping folds of Sequences 2–4 the Wiens Formation consists of green silty developed at a high angle to each other. within the Neptune Range, together with mudstone with interstratified oolitic lime- Brittle faults are present throughout the the cleavage transecting the folds in the stones. There is a band of reddened mud- succession and are generally concentrated valley section west of Gambacorta Peak, stone up to 2.5 m thick just below the un- along formation boundaries. Faults com- suggest that deformation may have in- conformity. On the western flank of the

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present anticline, an oolitic grainstone lime- ozoic sequences in the Pensacola Mountains 1983). An active orogenic belt exerts a load stone thins from2matsection 1 to 0.2 m at were actively deforming during sedimenta- on the underlying lithosphere by the em- section 2, and pinches out before section 3. tion, there are few absolute constraints on placement of thrust sheets, and the litho- Immediately overlying this bed are2mof the tectonic setting of this basin. The basin sphere responds by downward flexure. Sub- interbedded oolitic limestone and mudstone evolved through deposition of a platform sidence beneath the load passes laterally (couplets are 0.06–0.2 m thick). The uncon- carbonate sequence in Middle Cambrian into a foredeep basin. As the orogen formity itself cuts out strata on both sides of times, a period of voluminous silicic volcanic evolves, foredeep sedimentary sequences the syncline in opposite senses. Taken to- activity in Early Ordovician times, and dep- may be deformed and incorporated into the gether, these data suggest that the anticline osition of a thick nonmarine alluvial fan orogenic load. Although this view of fore- was present as a topographic high and that complex that was interrupted by two shallow land basins helps to explain some of the there was an open syncline, slightly wider marine transgressions. Paleocurrents were stratigraphic relationships in the Pensacola than the present syncline, found in Wiens predominantly axial, trending north-south Mountains during early Paleozoic times, the strata prior to deposition of the Neptune along the length of the orogen and westerly, predominance of nonmarine and shallow Group. directed away from the volcanic centers marine sequences, the distribution of the fa- Within the overlying Neptune Group, (Fig. 18). cies, the westerly directed (i.e., convergent most of the sandstones are arkoses or li- In the rest of the Transantarctic Moun- margin-directed) paleocurrents, and the tharenites, as elsewhere in the Elliott Sand- tains, the Ross orogen is considered a typical relationship between sedimentation and stone. However, only within the confines of convergent margin related to subduction of deformation may be more typical of a “pig- the exposed syncline, a quartzose sandstone proto–Pacific Ocean crust beneath the East gyback” basin than a foredeep basin. Piggy- containing scattered quartz pebbles and a Antarctic margin. In the absence of subduc- back is a class of foreland basin, introduced thin quartz pebble conglomerate occurs di- tion accretion complexes, the most com- by Ori and Friend (1984), to describe sedi- rectly above the unconformity. This quart- pelling evidence is the compressional de- mentary basins formed within and carried zose unit is thickest (10 m) near the trough formation (Goodge et al., 1993) and the upon active thrust sheets (e.g., Po basin of the syncline and wedges out on the crest interpretation of the paired I- and S-type complex in northern Italy and the Pyrenean of the adjacent anticlines. Above the quartz magmatic belts of the Granite Harbour In- Ebro basin complex of northern Spain). sandstone unit, there is a 1.5- to 20-m-thick trusive Complex as a subduction-related Strata of the Ebro basin, like those of the transitional unit containing equal amounts batholith emplaced ca. 500 Ma (Borg et al., Pensacola Mountains, comprise alluvial fan of rhyolite and quartz clasts; rhyolite in- 1990). Comparisons between Sr and Nd iso- complexes, progressive unconformities, and creases in abundance upward. In the over- topic characteristics of the Gambacorta syn-sedimentary synclines related to succes- lying units, rhyolite clasts are dominant, and Formation and the time-equivalent Granite sive movements during the later phases of quartz pebbles are rare. Within the quart- Harbour Intrusives in the central Transant- floor thrust movements (Anadoń et al., zose unit, paleocurrent orientations are arctic Mountains indicate that the Gam- 1985). In the case of the Pensacola Moun- toward the present southeast. This contrasts bacorta Formation has isotopic similarities tains, the Neptune Group would have been with paleocurrents toward the present north to some of the granitic rocks in both the derived from the outer thrust belt and trans- or northwest in the overlying units contain- Beardmore Glacier area (Borg et al., 1990) ported westward into the basin. Uplift on ing rhyolitic clasts. The presence of the and the Horlick Mountains (Borg and active thrusts, represented perhaps by the quartz sandstone-conglomerate confined to DePaolo, 1994). Granitoids in the Beard- zones of high strain described above, may ε the syncline suggests that the unit was de- more Glacier area have low but varied Nd have exposed the underlying Patuxent For- posited within a structurally produced de- values (Ϫ1.55 to Ϫ8.18), whereas granitoids mation, permitting deposition of Sequence 4 pression that was developing at the time of in the Horlick Mountains (Ϫ0.49 to Ϫ2.77) on Sequence 1 and development of growth deposition of the Neptune Group. have values just outside the range of the structures at locality R.4730. The structural There are, therefore, four lines of evi- Gambacorta Formation (Ϫ2.8 to Ϫ4.5). data suggest oblique-slip movement, which dence that suggest the area was actively de- Borg et al. (1990) have interpreted the iso- may also have had a fundamental control on forming during deposition: (1) Clasts of the topic signatures as resulting from syn-sub- basin development in the Pensacola Moun- underlying sequence, showing incipient duction melting of heterogeneous Precam- tains. We are not suggesting that the basin cleavage development, are found in the brian lower crust with model ages (TDM) of formed as a strike-slip basin, because sedi- basal conglomerate of the overlying se- 1.8–1.3 Ga. In this model, silicic volcanic mentary facies and thicknesses are not typ- quence (Fig. 17). (2) An anticline syncline rocks of the Gambacorta Formation would ical of basins formed in this way (see, e.g., pair formed progressively. (3) Distinctive represent part of the inboard edge of the papers in Ballance and Reading, 1980), but conglomerate and sandstone facies of the magmatic arc, and the Wiens Formation and that the deformation of the foreland basin Neptune Group are confined to the trough Neptune Group, part of a foreland basin se- involved a component of transcurrent of the syncline (Fig. 16). (4) Paleocurrent quence developed on a Middle Cambrian motion. orientations within the coarse facies deviate platform carbonate sequence (Nelson Lime- In a foredeep model, the westerly di- 180Њ from the regional flow of overlying stone). If this were the case, then the main rected paleocurrents could conceivably be strata within the Neptune Group. part of the magmatic arc and fore-arc basin derived from a forebulge on the cratonic sequences must have been tectonically re- margin. However, forebulge sedimentation TECTONIC SETTING moved from this part of the margin. generally results in thin conglomerate layers The relationship between orogenic belts (Armin, 1987), and the very large volume of Although sedimentological and structural and foreland basins are well explained by the Neptune Group makes it unlikely that it considerations suggest that the lower Pale- elastic beam theory (Karner and Watts, was derived from a forebulge. Furthermore,

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the magnitude of the flexural uplift on a cratonic basin margin would be greater on thin, rather than thick, elastic lithosphere, and (in contrast to the broad open basin in the Pensacola Mountains) basins formed on thin elastic lithosphere tend to be deep and narrow. The early stages of foreland basin development are generally a consequence of loading on initially stretched lithosphere, which in some cases represents highly attenuated passive margin sequences. Such an early passive margin history is common elsewhere: for example, the Carboniferous Arkoma basin in the south-central United States (Houseknecht, 1986), or the molasse basins of western Switzerland (Homewood et al., 1986). In the same way, the Nelson Limestone may represent a passive margin sequence formed on continental lithosphere attenuated during thermal subsidence following rifting. Subduction in the Transant- arctic Mountains was most likely active during Early Cambrian times in at least the central part of the mountain range (Goodge et al., 1993). If this was also the case within the Pensacola Mountains, then the Nelson Limestone is more likely to be part of a foreland platform than a passive margin sequence. One puzzling aspect of the foreland basin model for the Pensacola Mountains is the occurrence of silicic volcanic activity. With the exception of the southern Andes, primary volcanic deposits are generally absent from foreland basins (see papers in Allen et al., 1986). Thick Jurassic silicic volcanic rocks are present within the Magallanes basin in Patagonia, but they are related to an earlier tectonic regime that preceded and accompanied formation of a marginal basin floored by an ophiolite sequence (Biddle et Figure 16. (A) Locality R.4730 west of Gambacorta Peak. The Gambacorta Formation al., 1986). Similar extensional basins have (pale unit at base of cliffs) is unconformably overlain by the Wiens Formation on the west not to date been described from the limb of an anticline. Note the thinning of the Wiens Formation toward the crest of the Transantarctic Mountains margin, although anticline (extreme right of photograph) and deep canyons along the boundary. The exposed it is possible that the volcanic rocks of the section is Ϸ500 m thick. The unconformities between the Gambacorta and Wiens Forma- Gambacorta Formation, together with si- tions, and between the Wiens Formation and Neptune Group, are shown by arrows. The licic and mafic volcanic rocks in the Schmidt photograph is taken facing northward. (B) Growth fold of the Wiens Formation, uncon- and Williams Hills (now also known to be formably overlying Neptune Group at locality R.4730. The exposed section is Ϸ500 m thick. Early Ordovician in age), may represent a Note thickening of white quartz-rich sandstone unit at base of Neptune Group toward core back-arc basin along this margin (Millar and of syncline. Storey, 1995). The Gambacorta Formation may not, as has been considered above, be related to ac- age (see review by Stu¨we and Sandiford, diford (1993) have related this thermal tive margin tectonics, but could be part of a 1993). Some of the Cambrian granitoids event to substantial basaltic underplating of regional tectonothermal event that affected within the East Antarctic Shield are or- asthenosphere-derived melts during a pe- the whole of the East Antarctic Shield, re- thopyroxene-bearing (charnockitic rocks) riod of lithospheric thinning. This could sulting in pervasive resetting of isotopic sig- and have relatively high Y, Nb, and Zr val- have resulted in uplift of the crust and par- natures, emplacement of Cambrian grani- ues (Sheraton and Black, 1988) similar to tial melting of the lower crust to form the toids, and formation of shear zones of this the Gambacorta Formation. Stu¨we and San- range of igneous rocks within the shield, in-

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Figure 16. (Continued). (C) Sketch of west- east cross section and measured sections of the Paleozoic Wiens Formation and part of the Elliott Sandstone of the Neptune Group at locality R.4730, incorporating Figures 16A and 16B. Quartz arenite unit is stippled. Note the change in thickness of the strata and the facies variation toward the core of the syncline. X is the location of Figure 17.

cluding the comparable Gambacorta For- Antarctic Shield. Discriminating between erupted (501 Ϯ 3 Ma) close to the Cam- mation and Thiel Mountains Porphyry. This these models is not possible as far as the brian–Ordovician boundary. The Cambrian model does not preclude an active margin igneous rocks are concerned because both and Ordovician strata rest unconformably origin for these rocks, and Stuwe and San- models result in melting of lower crustal as- on deformed rocks of the Patuxent Forma- diford also predicted increased activity semblages, but on balance of evidence we tion. The latter were deformed during the along the continental margins at this time. favor an arc-foreland basin model. Beardmore orogeny, an event designated by In conclusion, the presence of volcanic Grindley and McDougall (1969) to encom- rocks in the Pensacola Mountains between THE ROSS OROGEN: A LATE pass all late Precambrian tectonic events in the Middle Cambrian limestone and the NEOPROTEROZOIC–EARLY the Transantarctic Mountains. The Patuxent overlying alluvial fan complexes may repre- PALEOZOIC CONVERGENT MARGIN Formation is one of several graywacke-shale sent either local variations in the kinematics sequences of the Beardmore Group, includ- of plate interactions, subduction geome- Deformation of the postulated foreland ing the Goldie, Duncan, and La Gorce For- tries, and a temporary migration of subduc- basin sequence in the Pensacola Mountains mations (Stump et al., 1986). In the Nimrod tion magmatism into the foreland region, or is Early Ordovician or younger, because it Glacier area, the Goldie Formation is un- else be part of a larger tectonothermal re- affected both the Middle Cambrian Nelson conformably overlain by (Laird et al., 1971; gime that affected the whole of the East Limestone and volcanic rocks that were Stump, 1995) and in faulted contact with

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be due to primary variations in the orientation of the margin resulting in zones of both normal and oblique subduction (e.g., Pensa- cola Mountains). This makes the assump- tion that there have not been any major post-Ross rotations within the orogen. Continuation of the Ross orogen from the Pensacola Mountains northward toward the Weddell Sea and Africa is uncertain. The absence of early Paleozoic deformation in the Ellsworth Mountains that lie “outboard” of the Pensacola Mountains with respect to the East Antarctic craton has puzzled geologists for many years. The Cambrian–Per- mian succession of the Ellsworth Mountains is generally described as being devoid of major unconformities and deformed only by the latest Paleozoic–early Mesozoic Gond- wanide folding (see Webers et al., 1992). However, one unconformable relationship Figure 17. Unconformity between Gambacorta and Wiens formations, showing cleaved within the Ellsworth Mountains sequence clast of Gambacorta Formation (white arrow), which has been incorporated in the Wiens has been claimed (Yoshida, 1983), and Formation after cleavage formation. The photograph was taken at locality R.4730 west of Goldstrand et al. (1994) have documented Gambacorta Peak and is marked as an X on Figure 16C. stratigraphic evidence for the Ross orogeny. Hence, given the coplanar nature of the cleavages in the sequences of the Pensacola (Rowell et al., 1986) the Early Cambrian ic event, but part of a complex history of Mountains described here, we are not con- Shackleton Limestone, constraining defor- convergent margin tectonics active since at vinced that the strata of the Ellsworths have mation to Early Cambrian or older in age, least Late Proterozoic times. This is in been sufﬁciently well studied to rule out a whereas in the Pensacola Mountains the agreement with the conclusions of Goodge widespread early Paleozoic deformational Patuxent Formation is unconformably over- et al. (1993) and most recent investigators event or events. Moreover, the Ellsworth- lain by Middle Cambrian Nelson Limestone for the rest of the orogen. Additional de- Whitmore crustal block appears to be al- constraining deformation to Middle Cam- tailed stratigraphic, geochemical, and geo- lochthonous (Schopf, 1969; Watts and brian or older in age. chronological studies are needed to further Brammall, 1981; Dalziel and Elliot, 1982) The view of the Beardmore orogeny as a unravel the complex history of tectonics and and is generally restored to a position be- separate Neoproterozoic event from the to make correlations along the length of the tween the Pensacola Mountains and the Ross orogeny has been challenged (Rowell Ross orogen and within the Ellsworth Cape Mountains of southern Africa. In this et al., 1992) by using information from the Mountains. conﬁguration, the absence in the Ellsworth central and Weddell Sea sectors of the Structural and stratigraphic studies (see Mountains of the early Paleozoic deforma- Transantarctic Mountains to postulate early summary by Goodge et al., 1993) within the tion found along the East Antarctic–Austra- Middle Cambrian deformation, which may Ross orogen indicate both orogen-normal lian margin from the Pensacola Mountains have included the Beardmore event. This (Laird et al., 1971; Gibson and Wright, 1985; to eastern Australia would not be so sur- preceded the main Late Cambrian–Early Kleinschmidt and Tessensohn, 1987; Flott- prising, but it would still be enigmatic. Ordovician period of contraction and mag- mann and Kleinschmidt, 1991) and oblique In the Shackleton Range, Ϸ150 km north- matism assigned to the Ross orogeny. Fur- displacements (Weaver et al., 1984; Brad- northeast of the Pensacola Mountains, there thermore, new U-Pb age dates of 541–521 shaw et al., 1985; Rowell and Rees, 1989). is an early Paleozoic fold belt trending at Ma, from igneous and metamorphic rocks Detailed kinematic studies of high-grade right angles to the Transantarctic Moun- deformed during the Nimrod orogeny in the metamorphic rocks by Goodge et al. (1991) tains and the Ross orogen. The structure of central Transantarctic Mountains (Goodge in the central Transantarctic Mountains the lower Paleozoic sequence in the Shack- et al., 1993), and a date of 551 Ϯ 4 Ma, from demonstrate orogen-parallel displacement leton Range is dominated by south-vergent unfoliated quartz syenite in the Skelton Gla- in a ductile shear zone, which these authors nappes that were formed during the Ordo- cier area of the Transantarctic Mountains have related to oblique subduction during vician (Buggisch et al., 1990). Dalla Salda et (Rowell et al., 1993), demonstrate latest Pro- Late Proterozoic and Early Cambrian times al. (1992) have recently suggested that the terozoic and Early Cambrian magmatism (Goodge et al., 1993). This interpretation is Ordovician Taconic orogen of North Amer- and deformation in those areas (based on consistent with our structural data from the ica may have continued through the Fama- the age of the Precambrian-Cambrian Pensacola Mountains, which indicate defor- tinian belt that is in the basement of the boundary at ca. 540 Ma; Compston et al., mation in an oblique-slip setting. Within the Andean Cordillera, into the early Paleozoic 1992). We have shown that the Ross orog- framework of the Ross orogen as an active fold belt of the Shackleton Range (Fig. 2). eny in the Pensacola Mountains should not plate boundary, the variation in kinematic According to this hypothesis, the “Gond- be viewed as a single deformational orogen- history in different parts of the orogen may wanian” Iapetus ocean opened between

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Figure 18. Block diagrams (view toward the north) illustrating the paleoenvironmental and structural development of the Pensacola Mountains during formation of (a) Middle Cambrian Nelson Limestone, (b) Wiens Formation, (c) conglomerate units of the Neptune Group, (d) Elliott-Elbow Formations of the Neptune Group, and (e) Heiser Sandstone of the Neptune Group; (f) is a sketch structural .sequence number ؍ section across the Neptune Range after the Ross folding event. S

the Appalachian and proto-Andean mar- Mountains region may have occupied a the Pensacola Mountains is equivocal, but gins of Laurentia and Gondwana at the position comparable to that of southeast- on balance it probably formed in a foreland end of the Precambrian and may have con- ern Asia today, namely at the intersection basin. The succession can be divided into tinued into the East Antarctic sector of the between an actively subducting Paciﬁc three unconformity-bounded sequences (Se- Gondwana craton near the head of the margin (analogous to our interpretation of quences 2–4 within this paper). Sequence 2 present Weddell Sea, with the “southern the Pensacola Mountains), and a colli- is a carbonate platform overlain by silicic cone” of Laurentia remaining close to the sional orogen such as that along the north- volcanic rocks, Sequence 3, ephemeral Ellsworth and Pensacola mountains well ern edge of the Australian plate (analo- stream deposits and a marine transgressive into the Cambrian (Dalziel et al., 1994). gous to the Shackleton Range). unit, and Sequence 4, an alluvial fan complex. As subduction-related magmatism and de- (2) The Early Ordovician U-Pb zircon age formation appear to have been well estab- CONCLUSIONS of 501 Ϯ 3 Ma for the silicic volcanic rocks lished along the Transantarctic margin of of the Gambacorta Formation within Se- the Gondwana craton by the earliest Cam- (1) The tectonic setting of the lower Pa- quence 2 suggests that formation of the un- brian (Goodge et al., 1993), the Pensacola leozoic succession in the Neptune Range of derlying and interbedded Nelson Limestone

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may have extended from Middle to Late J. K. T., and Christie-Blick, N., eds., Strike-slip deformation, Fitzgerald, P. G., Sandiford, M., Barrett, P. J., and Gleadow, basin formation and sedimentation: Society of Economic A. J. W., 1986, Asymmetric extension in the Transantarctic Cambrian times. Paleontologists and Mineralogists Special Publication 37, Mountains and Ross embayment: Earth and Planetary Sci- p. 303–318. ence Letters, v. 81, p. 67–78. (3) Although the volcanic rocks of the Armin, R. A., 1987, Sedimentology and tectonic significance of Flo¨ttmann, T., and Kleinschmidt, G., 1991, Opposite thrust sys- Gambacorta Formation have geochemical Wolfcampian (Lower Permian) conglomerates in the Pe- tems in northern Victoria Land, Antarctica: Imprints of dregosa basin: Southeastern Arizona, southwestern New Gondwana’s Paleozoic accretion: Geology, v. 19, p. 45–47. similarities to contemporaneous within- Mexico and northern Mexico: Geological Society of Amer- Flo¨ttmann, T., Kleinschmidt, G., and Funk, T., 1993, Thrust pat- ica Bulletin, v. 99, p. 42–65. terns of the Ross/Delamerian orogens in northern Victoria plate charnockitic rocks in East Antarctica, Ball, M. M., 1967, Carbonate sand bodies of Florida and the Ba- Land (Antarctica) and south eastern Australia and their they most likely represent the inboard edge hamas: Journal of Sedimentary Petrology, v. 37, p. 556–591. implications for Gondwana reconstructions, in Findlay, Ballance, P. F., and Reading, H. G., editors, 1980, Sedimentation R. H., Unrug, R., Banks, M. R., and Veevers, J. J., eds., of a magmatic arc that encroached onto a in oblique-slip mobile zones: International Association of Gondwana eight: Assembly, evolution and dispersal: Rot- Sedimentologists Special Publication 4, 265 p. terdam, Netherlands, Balkema, p. 131–140. foreland carbonate platform. Barrett, P. J., 1991, The Devonian to Triassic Beacon supergroup Ford, A. B., 1972, Weddell orogeny—Latest Permian to early Mes- (4) Growth folds, progressive unconfor- of the Transantarctic Mountains and correlatives in other ozoic deformation at the Weddell Sea margin of the parts of Antarctica, in Tingey, T. J., ed., The geology of Transantarctic Mountains, in Adie, R. J., ed., Antarctic ge- mities, and cleavage relationships suggest Antarctica: Oxford, United Kingdom, Oxford Monographs ology and geophysics: Oslo, Norway, Universitetforlaget on Geology and Geophysics 17, p. 120–152. Oslo, p. 419–425. deformation was synchronous with sedimen- Behrendt, J. C., Henderson, J. R., Laurent, M., and Rambo, W. L., Ford, A. B., Schmidt, D. C., Boyd, W. W., Jr., and Nelson, W. H., tation within at least part of the lower Pa- 1974, Geophysical investigations of the Pensacola Moun- 1978a, Geologic map of the Saratoga Table quadrangle, tains and adjacent glacierized areas of Antarctica: U.S. Ge- Pensacola Mountains, Antarctica: U.S. Geological Survey leozoic succession, and that the Neptune ological Survey Professional Paper 844, 28 p. Antarctic Geology Map A-9, scale 1:250 000. Biddle, K. T., Uliana, M. A., Mitchum, R. M., Jr., Fitzgerald, Ford, A. B., Schmidt, D. C., and Boyd, W. W., Jr., 1978b, Geologic Group is part of that succession. M. G., and Wright, R. C., 1986, The stratigraphic and struc- map of the Davis Valley quadrangle and part of the (5) The predominance of nonmarine al- tural evolution of the central and eastern Magallanes basin, Cordiner Peak quadrangle, Pensacola Mountains, Antarc- southern South America, in Allen, P. A., and Homewood, tica: U.S. Geological Survey Antarctic Geology Map A-10, luvial fan complexes and shallow marine P., eds., Foreland basins: International Association of Sedi- scale 1:250 000. mentologists Special Publication 8, p. 41–61. Gibson, G. M., and Wright, T. O., 1985, The importance of thrust within Sequences 3 and 4 suggests a “piggy- Borg, S. G., and DePaolo, D. J., 1991, A tectonic model of the faulting in the tectonic development of northern Victoria back” rather than a foreland foredeep basin Antarctic Gondwana margin with implications for south- Land, Antarctica: Nature, v. 315, p. 480–483. eastern Australia: Isotopic and geochemical evidence: Tec- Goldstrand, P. M., Fitzgerald, P. G., Redfield, T. F., Stump, E., setting for the Pensacola Mountains during tonophysics, v. 196, p. 339–358. and Hobbs, C., 1994, Stratigraphic evidence for the Ross Borg, S. G., and DePaolo, D. J., 1994, Laurentia, Australia, and orogeny in the Ellsworth Mountains, West Antarctica: Im- early Paleozoic times. Antarctica as a Late Proterozoic supercontinent: Con- plications for the evolution of the paleo-Pacific margin of (6) The rocks show some structural fea- straints form isotopic mapping: Geology, v. 22, p. 307–310. Gondwana: Geology, v. 22, p. 427–430. Borg, S. G., DePaolo, D. J., and Smith, B. M., 1990, Isotopic Goodge, J. W., Borg, S. G., Smith, B. K., and Bennett, V. C., 1991, tures that suggest deformation in a strike- structure and tectonics of the central Transantarctic Moun- Tectonic significance of Proterozoic ductile shortening and tains: Journal of Geophysical Research, v. 95, p. 6647–6667. translation along the Antarctic margin of Gondwana: Earth slip setting, perhaps due to oblique conver- Bradshaw, M. A., and Webers, G. F., 1988, The Devonian rocks of and Planetary Science Letters, v. 102, p. 58–70. Antarctica, in McMillan, N. J., Embray, A. S., and Glass, gence along this part of the Antarctic Goodge, J. W., Walker, N. W., and Hansen, V. C., 1993, Neoprot- D. J., eds., Calgary, Alberta, Canadian Association of Pe- erozoic–Cambrian basement-involved orogenesis within the margin during the early Paleozoic Ross troleum Geologists, p. 238–241. Antarctic margin of Gondwana: Geology, v. 21, p. 37–40. Bradshaw, J. D., Weaver, S. D., and Laird, M. G., 1985, Suspect Grindley, G. W., and McDougall, I., 1969, Geology of the Shack- terranes in north Victoria Land Antarctica, in Howell, D. C., orogeny. leton Coast, in Bushnell, V. C., and Craddock, C., eds., Ant- Jones, D. L., Cox, A., and Nur, A., eds., Proceedings, cir- arctic map folio series: New York, American Geographical cum-Pacific terrane conference: Stanford, California, Stan- Society Antarctic Folio 12. ford University Press, p. 36–39. Grunow, A. M., Kent, D. V., and Dalziel, I. W. D., 1991, New ACKNOWLEDGMENTS Buggisch, W., Kleinschmidt, G., Kruezer, H., and Krumm, S., paleomagnetic data from Thurston Island: Implications for 1990, Stratigraphy, metamorphism and nappe-tectonics in the tectonics of West Antarctica and the Weddell Sea: Jour- the Shackleton Range (Antarctica): Geoda¨tische und Geo- nal of Geophysical Research, v. 96, p. 17935–17954. This paper forms part of a joint British physikalische Vero¨ffentlichungen, v. 15, p. 64–86. Cahen, L., Snelling, N. J., Delhal, J., and Vail, J. 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