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 6 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. 685 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/6/685/3382482/i0016-7606-108-6-685.pdf by guest on 27 September 2021 STOREY ET AL. 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 un- conformity from the overlying Douglas Con- glomerate (Rees et al., 1989). The Douglas Conglomerate contains evidence for more than one episode of pre-Devonian deforma- tion (Rowell et al., 1988) and indicates polyphase tectonism in the Cambrian his- tory of the Ross orogen (Rowell et al., 1992). The outer belt contains Middle Cam- brian limestones and thick volcanic and vol- caniclastic successions (Byrd Group; Laird, 1981). Rowell and Rees (1989) proposed that the outer belt forms one or more sus- pect 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 orog- eny in the Pensacola Mountains. The adja- cent 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 strati- graphic 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 686 Geological Society of America Bulletin, June 1996 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/6/685/3382482/i0016-7606-108-6-685.pdf by guest on 27 September 2021 SIGNIFICANCE OF THE ROSS OROGENY, ANTARCTICA 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