resulted from the subduction of the Phoenix Plate until the early Tertiary, and, after ..... Pedersen Nunatak sequence tends to support the younger age assigned ...
Geological Society of America Memoir 169 1988
Tectonic setting and evolution of the James Ross Basin, northern Antarctic Peninsula David H. Elliot Byrd Polar Research Center and Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 ABSTRACT
INTRODUCTION
The upper Mesozoic to lower Cenozoic sequence in the region of James Ross Island is the only exposed marine succession of that age in Antarctica. The sequence makes up part of the fill of the James Ross Basin and includes: (1) an upper Jurassic mudstone-tuff sequence, the Nordenskjold Formation; (2) a Lower to Upper Cretaceous conglomerate-sandstone-mudstone-tuff assemblage, the Gustav Group and equivalents; (3) an Upper Cretaceous to Paleocene poorly consolidated sand, silt, mud and tuff sequence, the Marambio Group and equivalents; and (4) an Eocene sequence of weakly consolidated, nonvolcanic fine sands and silts—the La Meseta Formation. Sedimentary facies include proximal submarine fans, shelf settings, and deltaic environments. Sea-floor anomaly data from the Pacific Ocean suggest that development of Upper Mesozoic to Cenozoic fore-arc, magmatic arc, and back-arc terrains of the Peninsula resulted from the subduction of the Phoenix Plate until the early Tertiary, and, after reorganization of spreading centers in Late Cretaceous time, subduction of the Aluk Plate. Strata in the James Ross Island region constitute the sedimentary and volcanic fill of an ensialic back-arc basin developed on the Weddell Sea flank of the Antarctic Peninsula. Broad correlations can be made between the strata and evolution of the James Ross Basin, the tectonic and magmatic history of the peninsula, and plate subduction.
has been amplified and refined. Suarez (1976) interpreted the Upper Jurassic and Cretaceous rocks of the southern Antarctic Peninsula in terms of three tectono-stratigraphic units that represent fore-arc, magmatic arc, and back-arc terrains associated with Pacific crust subduction. Cande and others (1982) presented seafloor magnetic data for the southeastern Pacific Ocean. Barker (1982) described the ridge crest-trench collision that occurred along the western margin of the Peninsula and also discussed the Mesozoic plate tectonic history. The evolution of the fore-arc, magmatic arc, and back-arc terrains of late Mesozoic to early Cenozoic age in the peninsula region was described in a previous paper (Elliot, 1983). This chapter reviews the stratigraphy, tectonic setting, and evolution of the back-arc basin in the James Ross Island region, and relates it to the subduction history of the Antarctic Peninsula.
Exploration by the Swedish South Polar Expedition at the beginning of this century led to the discovery of fossiliferous rocks of Cretaceous and Tertiary age on James Ross Island and adjacent islands (Figs. 1, 2), and to the establishment of the basic geological framework for the northern Antarctic Peninsula (Andersson, 1906; Nordenskjold, 1905). The two structural zones recognized were a cordilleran belt of folded strata, which is unconformably overlain by undeformed sedimentary and volcanic beds of Mesozoic age and intruded by plutons; and a platform to the east, which consists of only slightly deformed upper Mesozoic and Cenozoic strata overlain by upper Tertiary basalts. Systematic survey of the Antarctic Peninsula began with the establishment of the Falkland Islands Dependencies Survey, now the British Antarctic Survey. Knowledge of the geology increased rapidly (Adie, 1964,1972). The first plate tectonic interpretation REGIONAL SETTING of the geologic evolution of the peninsula was presented by Dalziel and Elliot (1971, 1973), and since then the simple model of An extensive sequence of largely undeformed marine rocks, Pacific crust subduction beneath the western continental margin ranging in age from Late Jurassic to early Tertiary, crops out
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Figure 1. Location map for the Antarctic Peninsula, with present geotectonic setting and Andean lithotectonic units superimposed (data for South America and South Georgia from Dalziel et al., 1974, and Storey et al., 1977).
James Ross Basin tectonic setting
Figure 2. Location and simplified geologic map for the northern Antarctic Peninsula.
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from Dundee Island and Active Sound in the northeast to Cape Fairweather and Cape Marsh in the southwest (Fig. 2). The sequence, which includes the Upper Cretaceous and Paleogene strata exposed on Seymour Island, constitutes the sedimentary fill of the James Ross Basin and records an important part of the geologic history of the Antarctic Peninsula. East- to southeast-directed subduction beneath the Antarctic Peninsula is strongly suggested by the asymmetry displayed in the compositional trends of the upper Mesozoic to Cenozoic magmatic arc rocks (Saunders et al., 1982; Weaver et al., 1982), and supports the inferences that can be drawn from Cenozoic seafloor magnetic data for the southeastern Pacific Ocean (Barker, 1982; Cande et al., 1982; Fig. 3). Thus, the peninsula region is regarded as an active plate margin with plate convergence located along its western boundary. Three major lithotectonic units are recognized in the Antarctic Peninsula and can be related in a general manner to plate interaction. Along the length of the peninsula, substantial thicknesses of volcanic rocks with sparse sedimentary intercalations record the onset of magmatism. Volcanism began in the mid-Jurassic in eastern Ellsworth Land (Quilty, 1977), extended to the whole of the peninsula by latest Jurassic time, and continued to the midTertiary. Emplacement of plutonic bodies accompanied the extensive volcanic activity. Magmatic activity shifted first to the southeast in Palmer Land during the early Cretaceous, but the subsequent trend was a gradual migration to the north and west across the peninsula, and by the mid-Tertiary was confined to the northwestern flank of the peninsula and the South Shetland Islands (Elliot, 1983; Smellie et al., 1984). Early stages in magmatic arc development in the northern Antarctic Peninsula are marked by volcanic rocks interbedded with, or overlying, sequences of alluvial fan deposits and associated plant-bearing beds—the Botany Bay Group of Farquharson (1984). Contemporaneous volcanism is recorded at Hope Bay (Elliot and Gracanin, 1983), the Longing Gap area, and Camp Hill (Farquharson, 1982b), but volcanic rocks are conspicuously absent from the conglomerate clasts in the basal parts of these sequences and the sequences on Joinville Island and the South Orkney Islands (Elliot and Wells, 1982; Elliot et al., 1977). Volcanic strata are not known to occur between the pre-Jurassic basement and the basal conglomerate beds. The age of this welldefined lithotectonic unit is uncertain, although it is probably in the range of latest Jurassic to Early Cretaceous (Farquharson, 1984). Evidence for a magmatic arc along the Antarctic Peninsula is also found in the adjacent sedimentary basins. Fore-arc basin deposits are preserved on the western side of the peninsula, principally on Alexander Island and the South Shetland Islands. The shallow marine sedimentary rocks on Alexander Island, consisting of a thick sequence of conglomerates, sandstones, and mudstones, range in age from Late Jurassic to middle Cretaceous (Aptian-Albian). Lavas and pyroclastic rocks are interbedded in the Jurassic part (Elliott, 1974), and airfall detritus is abundant in the Cretaceous beds (Home and Thomson, 1972). Most of the fore-arc sediments are marine;
however, the presence of fossil tree trunks in growth position at a number of horizons (Jefferson, 1982) confirms nonmarine deposition for the upper part of the sequence. Although unequivocal evidence for the provenance of the sequence is lacking (Thomson, 1982a), the bulk of the sediments were probably derived from an active volcanic arc to the east along the axis of the southern Antarctic Peninsula (Elliott, 1974). In the South Shetland Islands (Fig. 2), the deposits are only locally preserved. On Livingston Island, Upper Jurassic and lowermost Cretaceous marine shales and sandstones with a detrital volcanic component pass up into subaerial volcanic rocks (Smellie et al., 1980, 1984), possibly as young as Barremian (Askin, 1983). A thin nonmarine sequence that includes tuffs and lavas is present on an adjacent headland, President Head, on Snow Island (D. H. Elliot, unpublished data; Smellie et al., 1980, 1984). The sequence is Valanginian to Barremian in age (Askin, 1983). Upper Jurassic volcaniclastic rocks are also present on Low Island (Smellie, 1979) and have been assigned an Oxfordian and possibly younger age (Thomson, 1982b). Two rather different successions deposited in back-arc environments occur on the east side of the magmatic arc. A volcanogenic sequence of conglomerates, sandstones, and shales crops out in an arc from eastern Ellsworth Land to the Black Coast (Singleton, 1980; Laudon et al., 1983). This marine sequence is interbedded with and overlain by intermediate to silicic extrusive rocks: it is clearly related to an active magmatic arc. Invertebrate faunas indicate a Middle to Late Jurassic age for the sedimentary succession (Quilty, 1977, 1983; Rowley and Williams, 1982; Thomson, 1983). These volcanogenic sequences show considerable deformation and are cut by plutonic complexes. In eastern Ellsworth Land and the Black Coast, the deformation was latest Jurassic to earliest Cretaceous in age (Quilty, 1977; Singleton, 1980), whereas on the Lassiter Coast it was pre-middle Cretaceous (Rowley and Williams, 1982). The other succession, located at the northern end of the Peninsula, constitutes the exposed strata of the James Ross Basin. This basin contains a thick sequence of Upper Jurassic to lower Tertiary clastic sediments and tuff. In fact it is just a small part of an extensive region of late Mesozoic and Cenozoic sedimentation located on the continental shelf east of the peninsula. Details of the James Ross succession are described below. Although Mesozoic sea-floor anomalies are absent from the southeastern Pacific Ocean, reconstructions strongly suggest that the Late Jurassic and Cretaceous magmatic and tectonic history of the peninsula is related to subduction of oceanic crust, the Phoenix Plate, which formed at the same time as M anomalies in the Western Pacific (Larson, 1977; Barker, 1982). Reorganization of spreading centers occurred toward the end of the Cretaceous (Fig. 4). Spreading between New Zealand and West Antarctica was initiated at about 84 Ma (anomaly 34), but the developing Antarctic and Bellingshausen Plates (Stock and Molnar, 1987) extended no farther northeastward (Larson et al., 1979) than the Tharp Fracture Zone of the Eltanin Fracture Zone system. To the northeast, spreading on the preexisting Pacific-Phoenix Ridge con-
James Ross Basin tectonic setting
Figure 3. The Antarctic Peninsula and southeastern Pacific Ocean. Sea-floor magnetic anomaly data from Barker (1982), Cande et al. (1982), and Cande (personal communication, 1987). Seafloordivided into regions based on spreading ridge at which oceanic crust was generated (from Cande et al., 1982; Stock and Molnar, 1987): A = Pacific-Antarctic/Bellingshausen; B = Aluk-Antarctic/Bellingshausen; G = Pacific-Phoenix (Aluk); D = Pacific-Farallon; E = Antarctic-Farallon; F = Antarctic-Nazca; G = Spreading on a now-extinct ridge east of the Shackleton Fracture Zone.
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tinued. A new ridge system, the Bellingshausen-Aluk Ridge (referred to as the Antarctic-Aluk Ridge prior to recognition of the Bellingshausen Plate), was initiated between the Tharp and Heezen Fracture Zones by 66 Ma (anomaly 29). The BellingshausenPacific and Bellingshausen-Aluk Ridges propagated northeastward, replacing the Pacific-Phoenix Ridge. The Bellingshausen Plate was locked onto the Antarctic Plate at about 43 Ma (anomaly 18). The Aluk Plate, formed by spreading on the Bellingshausen-
Aluk Ridge, and after 43 Ma, the Antarctic-Aluk Ridge, was subducted beneath the Antarctic Peninsula. Subduction, however, was shut off as the Bellingshausen/ Antarctic-Aluk Ridge collided with the trench located along the western margin of the Antarctic Peninsula (Barker, 1982). In a northeasterly direction, progressively younger sea floor lies adjacent to the peninsula and suggests that ridge crest-trench collision, in segments defined by fracture zones, started at about 50
James Ross Basin tectonic setting
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taceous strata to the northeast and southwest indicate the probable minimum extent of the basin, but the relations to the principal exposures are uncertain. Seismic data suggest its northeastward extension is to be found on the platform south of the South Orkney Islands (Farquharson et al., 1984; Harrington et al., 1972). Aeromagnetic data indicate thick sedimentary sequences are present under the Larsen Ice Shelf as far south as the base of the peninsula. Estimates of stratigraphic thickness have been made only for the James Ross Island region; del Valle et al. (1983) suggest 6.4 km, and Ineson et al. (1986) suggest 4.8 km, excluding the Tertiary Cross Valley and La Meseta Formations on Seymour Island. Although the Nordenskjold Formation (Farquharson, 1982a), the Ameghino Formation of Medina and Ramos (1980), does not represent deposition in a deepening basin, it is included here because it probably forms the basal sequence above the basement that lies east of the peninsula. The Upper Jurassic (Kimmeridgian-Tithonian) Nordenskjold Formation crops out at isolated localities between Cape Fairweather and Joinville Island (Fig. 2). It consists of a succession of alternating thin-bedded, radiolarian-rich mudstones and airfall tuffs. Lower Cretaceous rocks identified at Sobral Peninsula consist of a thick sequence of coarse clastic marine strata with a large volcanic component. They have been assigned a late HauterivianBarremian age (Farquharson, 1982a). At Pedersen Nunatak, to the southwest, the marine conglomeratic sequence lacks an airfall volcanic component. The age of these beds is uncertain. According to Medina et al. (1981), it is Early Cretaceous, but Farquharson (1983a) assigned the beds a Late Cretaceous (Turonian-Maastrichtian) age. The strata at Sobral Peninsula and Pedersen Nunatak have been placed in the Pedersen Formation (see del Valle and Fourcade, 1986). The Cape Sobral beds are lateral equivalents of the lower part of the sequence on James Ross Island, although possibly extending further back in time, and are part of the same tectono-stratigraphic unit. An isolated outcrop of conglomerate, sandstone, shale, and tuff on Tabarin Peninsula (Scasso et al., 1986) also belongs to this unit. The stratigraphic scheme proposed by Ineson et al. (1986) for the Cretaceous sequence on James Ross Island area is followed here. The Gustav Group, which is restricted to the northwest part of the island, forms the lower part. It is predominantly a conglomerate and sandstone succession, though finer grained beds are important locally. The oldest rocks, the Lagrelius Point Formation, are a sequence of pebble and cobble conglomerates with minor interbedded sandstones. They are in contact only with the Upper Cenozoic James Ross Island Volcanic Group (Nelson, 1975). The Kotick Point Formation consists of thin-bedded, finegrained sandstones and silty mudstones or clays, with interbedded breccia and conglomerate intervals that include clasts up to several meters across. Slide blocks of Nordenskjold Formation lithology are an important component of the formation. The STRATIGRAPHY OF THE JAMES ROSS BASIN succeeding Whiskey Bay Formation shows great lateral facies The principal exposures of strata forming the sequence that variation; locally, however, the succession has been broken out fills the basin occur on James Ross Island or in its vicinity (Fig. 2; into members. Pebble and boulder conglomerates, breccias, pebTable 1). Scattered outcrops of Upper Jurassic and Lower Cre- bly sandstones, sandstones, and mudstones are all present. The Ma and continued to about 4 Ma. A remnant of the Aluk Plate, the Drake Plate of Barker (1982), forms the sea floor off the South Shetland Islands (Fig. 1). Subduction in that sector slowed markedly about 4 m.y. ago, and shortly thereafter, opening of the Bransfield Trough—a young marginal basin—was initiated. The Phoenix Plate was subducted during the Mesozoic; the rate of convergence was high, possibly >10 cm/yr, but the orientation of convergence is unknown. The late Cretaceous and Cenozoic history is more complicated because of the development of the Pacific-Antarctic, Pacific-Bellingshausen (anomaly 32 to anomaly 18 time), and Bellingshausen-Aluk (anomaly 29 to anomaly 18 time) and Antarctic-Aluk (10 and possibly >15 cm/yr, for those segments associated with Pacific-Phoenix spreading. After about 50 Ma, the rates of convergence dropped to less than about 5 cm/yr (Barker, 1982). The eastern margin of the Antarctic Peninsula continental crust abuts the Weddell Sea floor (Fig. 5); unfortunately, details of the continent-ocean boundary are poorly known because of perennial pack ice and icebergs. Published bathymétrie charts (e.g., LaBrecque, 1986) suggest the presence of a thick sedimentary prism at the foot of the continental slope. Sea-floor magnetic anomaly data indicate that at least part of the eastern Weddell Sea floor is Mesozoic in age, but exactly how old remains uncertain. The age of the oldest anomaly is 155 to 160 m.y. (M25 or M29, LaBrecque [1986] and British Antarctic Survey [1985], respectively). Even older crust is present between the oldest anomaly and the Explora-Andenes Escarpment (Kristoffersen and Haugland, 1986). The depth of the Weddell Sea floor is consistent with a Mesozoic age. It seems likely that the Antarctic Peninsula margin is of long standing. The margin may be a passive margin whose origin lies in the early stages in the dispersal of Gondwanaland and in the initial rifting between Africa-South America and Antarctica-India-Australia. On the other hand, the margin might be a transform boundary now relatively deeply buried by sediment derived from the magmatic arc, sited on the Peninsula, that was active for much of Jurassic to late Tertiary time. The lack of a pronounced magnetic signature on the few tracks crossing the margin (La Brecque, 1986) adds to the uncertainties.
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