Continental rift to back-arc basin: Jurassic-Cretaceous stratigraphical and structural evolution of the Larsen Basin, Antarctic Peninsula
Abstract: The Larsen Basin developed in Jurassic times as a result of continental rifting during the early stages of Gondwana break-up. Lower-?Upper Jurassic non-marine sedimentary and volcanic rocks constitute a syn-rift megasequence recording initial largely amagmatic extension and subsequent widespread extension-related silicic volcanism. A succeeding, Kimmeridgian-early Berriasian transgressive megasequence, consisting largely of anoxic-dysoxic hemipelagic mudstones, is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak Jurassic eustatic sea levels served to maximize sediment starvation. The fragmentary record for late BerriasianBarremian times suggests that a ?regressive megasequence may have developed in the earlier part of this period, recording increased sediment yield to the Larsen Basin from the increasingly emergent Antarctic Peninsula arc. Subsequently, strata in the southern, but not the northern, part of the basin underwent relatively intense eastward-verging deformation, possibly during the formation of a retro-arc fold-thrust belt. Where exposed, the lower part of the succeeding Aptian-Eocene megasequence consists of a deep-marine clastic wedge deposited along the fault-bounded western basin margin during a phase of arc uplift and related differential subsidence. Following partial basin inversion in Late Cretaceous times, regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward.
Keywords: Antarctica, South America, Gondwana, Mesozoic, back-arc basins.
The Mesozoic-Cenozoic Larsen Basin is situated on the continental shelf to the east of the northern sector of the Antarctic Peninsula (Fig. 1; Macdonald et al. 1988). It is one of a number of sedimentary basins in the southern South America-Antarctic Peninsula region that formed as a result of Jurassic lithospheric extension during the early stages of Gondwana break-up (Fig. 2), and subsequently developed in a back-arc setting relative to the then continuous AndeanAntarctic Peninsula magmatic arc. In recent years, the potential of the basin-fill succession as a key reference section, and for studies of southern high-latitude palaeobiology and palaeoclimatology (e.g. Ditchfield et al. 1994; Crame et al. 1996), has become increasingly apparent. Although recent ship-based seismic surveys have yielded data on the Cenozoic strata in the northern part of the Larsen Basin (Anderson et al. 1992; Sloan et al. 1995), ice cover limits the scope for marine geophysical work, and surface outcrop studies remain our main basis for understanding its Mesozoic history. The marine basin fill is most extensively exposed in the northern, James Ross Island area (Fig. 1), where it has a Kimmeridgian-Eocene age range (e.g. Whitham & Doyle 1989; Zinsmeister 1982). The fragmentary sedimentary record in the southern Larsen Basin consists of a few relatively small, isolated outcrops in the Kenyon Peninsula area (Fig. 1). In order to build an updated model for basin evolution in Mesozoic times, this paper synthesises earlier studies, including data that have become available since the evolution of all or part of the basin was reviewed by Macdonald et al. (1988), Elliot (1988), and Pirrie et al. (1991), together with new data from the southern Larsen Basin, and comparisons with successions in southern South America and the southern Antarctic Peninsula. The basin fill is divided into a series of megasequences (Fig. 3), used in the sense of Hubbard (1988) as a term for regional-unconformity-bounded depositional packages that represent major, discrete phases of basin evolution. Providing such a framework represents a crucial step towards understanding not only the history of the Larsen Basin, but also the wider interplay between convergent margin tectonics and continental rifting in the Mesozoic of the SW Gondwana region. Because of the localized or fragmentary nature of the stratigraphical record, elements of the model proposed here are speculative (e.g. Early Cretaceous megasequence boundaries; Fig. 3). Nevertheless it is felt that the attempt is worthwhile, not least as a spur to future work.
Basin morphology and sediment thickness
`Larsen Basin’ is used here in the sense suggested by del Valle et al. (1992a), to denote the depositional area including all the upper Mesozoic-lower Cenozoic sedimentary rocks on the continental shelf east of the northern Antarctic Peninsula. Aeromagnetic data show an abrupt transition from highamplitude, short-wavelength anomalies over the Antarctic Peninsula mainland, to low magnetic gradients over the shelf (Renner et al. 1985; Maslanyj et al. 1991). In the James Ross Island and Kenyon Peninsula areas, this boundary broadly coincides with that between the arc massif and the basin fill in the back-arc region, and following the work of Farquharson et aL (1984) in northern Graham Land, Macdonald et al. (1988) used it to define the western limit of the Larsen Basin. They suggested that the quiet magnetic signature might reflect the extent of a continuous sedimentary basin (Fig. 1), with its eastern limit along the edge of the continental shelf. The southern limit was tentatively placed along the 69 degS parallel, although it was suggested that, like the subdued magnetic gradients (Maslanyj et aL 1991), the basin might extend farther south (Macdonald & Butterworth 1990). Subsequently, Jarvis & King (1995) carried out a seismic reflection survey within the predicted basin c. 100 km north of Kenyon Peninsula (Fig. 1). They concluded that the aeromagnetic data were not a good guide to the extent of the basin, which was either locally absent or lay farther to the east.
Aeromagnetic and aerogravity data analysed by LaBrecque & Ghidella (1997) indicate eastward thickening of the sedimentary succession in the Larsen Basin area, with the thickness of nonmagnetic overburden increasing from 3-4 km near the western basin margin, to 4-6 km at the shelf edge. These authors considered a further abrupt increase to a maximum of 10-12 km immediately east of the shelf edge, to represent a major accumulation of sediment along the Jurassic rifted continental margin. The land-based seismic studies of Keller & Diaz (1990) and del Valle et al. (1992b) also indicated that the sedimentary succession thickens to the east, increasing from 4.5 to 6.9 km from NW to SE across James Ross Island, above an acoustic basement they assumed to be of Triassic age. Estimates of exposed stratigraphical thickness in this area are >5 km (Pirrie et al 1991).
Depositional and structural history Early Jurassic-Kimmeridgian
Stratigraphical record. Although the rocks underlying the marine Larsen Basin sedimentary fill are largely unexposed, outcrops along the western basin margin suggest they may be similar to coeval successions in southern South America. There, following the development of a regional angular unconformity, Upper Triassic-Lower Jurassic sedimentary rocks were deposited in areally restricted, largely non-marine, NNWtrending, extensional fault-bounded basins (Fig. 2a; Uliana & Biddle 1987; Tankard et al. 1995). Extension was accompanied by magmatism, which became increasing widespread, reaching a maximum in mid-Jurassic times, by which time bimodal, but largely silicic, ignimbrite-dominated volcanic successions were being deposited across much of southern South America (e.g. Gust et al. 1985; Uliana & Biddle 1987). On the western margin of the Larsen Basin, mechanical basement consists of the largely low-grade metasedimentary rocks of the ?uppermost Carboniferous-Triassic Trinity Peninsula Group (Smellie & Millar 1995) to the north of Jason Peninsula, and highergrade metamorphic rocks that have yielded ages as old as Silurian (Milne & Millar 1989) farther south (Fig. 1). Conglomeratic, basement-derived, non-marine Lower Jurassic strata of the Botany Bay Group, deposited in relatively small fault-bounded basins, unconformably overlie the Trinity Peninsula Group in the area to the NE of Sobral Peninsula (Fig. 1; Farquharson 1984; Rees 1993). A 600 rn thick succession of generally finer grained, probably lacustrine strata, thought to be of similar age, occupies a similar stratigraphical position in the area inland from Cape Disappointment (Fig. 1; Fleet 1968). The Lower Jurassic strata are conformably overlain by a largely subaerial, mid-Jurassic volcanic succession consisting mainly of rhyolitic ignimbrites (Fleet 1968; Riley & Leat 1999). These rocks are extensively exposed in the Cape Disappointment area (maximum observed thickness 1000 m) and on Jason Peninsula (exposed thickness c. 450 m, base not exposed) (Fig. 1; Pankhurst et al. 1998), and crop out less extensively in scattered areas to the north and south. Radiometric ages, which span a broad Mid-Late Jurassic range, were summarized by Riley & Leat (1999), who suggested that emplacement took place during a relatively short period in Mid-Jurassic times (Fig. 3), with younger ages recording a metamorphic resetting event. Although their eastward extent is unknown, comparison with southern South America (e.g. Magallanes Basin: Fig. 2) suggests that similar volcanic rocks may underlie much of the marine Larsen Basin fill.
Basin evolution. The formation of restricted rift basins and the widespread volcanism that followed across the PatagoniaAntarctic Peninsula region are thought to record lithospheric extension and crustal anatexis related to the early stages of Gondwana break-up (e.g. Gust et al. 1985; Storey et al 1992). Although an intra-arc setting has generally been assumed for the Botany Bay Group basins (e.g. Pirrie & Crame 1995), it seems likely that, as in southernmost South America (e.g. Urien et al 1995), similar basins would have developed across a wide area inboard of the Gondwanan margin in latest Triassic-Early Jurassic times (Fig. 2a). This initial extension was followed by voluminous ignimbrite-dominated volcanism in mid-Jurassic times, forming part of a silicic large igneous province with an Early Jurassic-earliest Cretaceous age range that extends across much of the southern South AmericaAntarctic Peninsula region (Fig. 2a; Pankhurst et al 1998; Riley & Leat 1999). Extension culminated in the formation of oceanic crust. In southernmost South America, this took place close to the palaeo-Pacific continental margin, along the western edge of the Magallanes Basin, forming the Rocas Verdes marginal basin (Fig. 2b), which had begun to open by 150 Ma (Mukasa & Dalziel 1996). Farther south, sea-floor spreading took place in the Weddell Sea, to the east of the Larsen Basin and, although magnetic anomalies older than C34 (83 Ma) have not been identified with certainty (Livermore & Hunter 1996), is generally thought to have begun by Late Jurassic times (e.g. Storey et aL 1996).
The silicic volcanic rocks (Tobifera Formation) that underlie the Magallanes Basin sedimentary succession are non-marine (Natland et al. 1974), except along the western basin margin. There, early Late Jurassic silicic eruptions and radiolarian-rich mud deposition in a relatively deep-marine setting preceded and overlapped with formation of oceanic crust in the marginal basin (Fig. 4a; Wilson 1991; Hanson & Wilson 1991). Only the westernmost part of the silicic volcanic succession thought to underlie the Larsen Basin area is exposed. However, it is suggested that in mid-Jurassic times the region consisted of an extensive, extension-related subaerial volcanic province passing into a deep-marine trough, as in the Magallanes Basin area, but here towards the east along the incipient Weddell Sea continental margin (Fig. 5a). In southernmost South America, normal faulting continued through mid-Jurassic times, but essentially ended with cessation of the largely silicic volcanism, and the formation of oceanic crust in the marginal basin (Gust et al 1985). The volcanic rocks thicken into numerous graben and half-graben, typically with the fault on the eastern side showing greatest displacement, and are locally absent over basement highs (Gust et aL 1985; Biddle et al 1986). Sub-basin-bounding normal faults are steeply dipping, and some have listric profiles at depth (Gust et al 1985). The Larsen Basin area is likely to have been structurally similar, but dominated by down-to-east extensional faults (Fig. 5a). Based on comparisons with southernmost South America, the Lower Jurassic sedimentary rocks and the mid Jurassic volcanic successions are interpreted as comprising a syn-rift megasequence (Fig. 3). As in the Magallanes Basin area (Biddle et al 1986; Wilson 1991), these rocks appear to be intimately linked to the early stages of basin formation and can be considered part of the basin-fill succession, rather than as basement.
It is widely thought that Late Triassic-Jurassic regional extension in SW Gondwana was contemporaneous with, and perhaps causally related to (Gust et al. 1985; Storey et al. 1992) subduction and magmatic arc development along the palaeoPacific continental margin (e.g. Pankhurst 1990; Leat et al 1995). However, the nature of the arc remains uncertain, and in southern South America its presence has been questioned (e.g. Wilson 1991). Moreover, Storey & Kyle (1997) have recently suggested that, although plate-boundary forces may have played an important role, an active mantle mechanism (megaplume) was the main force driving SW Gondwana extension and initial break-up. In the Antarctic Peninsula, an early Jurassic gap in intrusive activity was followed by widespread mid-Jurassic (178-159 Ma) silicic plutonism in eastern Graham Land (Leaf et al. 1995). The plutons show ‘S-like’ mineralogies and relatively high ^sup 87^ Sr/”Sr, and are considered to contain large components of upper crustal melt (Leaf et al. 1995, 1997). They may represent a subduction-influenced continental-margin zone of the contiguous extensional volcanic province to the east, rather than a discrete magmatic arc. Their intrusion was followed by a further, Late Jurassic, gap in plutonic activity (c. 159-142 Ma: Leaf et al. 1997).
Stratigraphical record. The only rocks of this age known from the Larsen Basin are frost-shattered sandstones exposed at Cape Framnes on Jason Peninsula, and the mudstonedominated Nordenskjold Formation, which is exposed at scattered locations along the eastern margin of the Antarctic Peninsula farther to the north (Fig. 1). The Cape Framnes sandstones contain a Kimmeridgian-early Tithonian, relatively shallow marine molluscan macrofauna (Riley et al. 1997). They almost certainly directly overlie the subaerially erupted (Smellie 1991) silicic volcanic rocks of Jason Peninsula (no contact is exposed; Riley et al. 1997), although Riley & Leat (1999, fig. 3) imply that there may be a major time gap between the two successions (Fig. 3). Neither the base nor the top of the Nordenskjold Formation, which consists mainly of radiolarian-rich mudstone (Farquharson 1983a), is exposed. Most of its constituent strata are assigned to the ?Kimmeridgian-Tithonian Longing Member and the overlying, Tithonian-Berriasian Ameghino Member. The former consists of unbioturbated mudstones with minor thin, silicic (phenocrysts are mainly quartz and zoned andesine; Farquharson 1983a) tuff beds; in the latter, bioturbation is common and tuffs are thicker bedded and more abundant (Whitham & Doyle 1989). A third unit, the Larsen Member, consisting of unbioturbated mudstones with abundant tuffs and thin sandstone beds, is of probable Berriasian age, but its relationship to the Ameghino Member is uncertain (Whitham & Doyle 1989). The two main members are considered to record a change from largely anoxic, basinal deposition, to largely dysoxic, slope sedimentation (Doyle & Whitham 1991; Whitham 1993). The Ameghino Member shows abundant syndepositional deformation related to creep-dominated downslope movement (Whitham 1993), and evidence of progressively increasing mean dissolved oxygen levels (Doyle & Whitham 1991). Macdonald et al (1988) suggested a maximum possible thickness of 800 m for the Nordenskjold Formation. The thickness of the Cape Framnes beds is unknown.
South of the Larsen Basin, the Latady Formation of the SE Antarctic Peninsula (Fig. 1) is in large part time-equivalent to the Cape Framnes-Nordenskjold succession (Fig. 3). This formation has an estimated thickness of at least several kilometres (neither top nor bottom observed), and is dominated by paralic and sandstone-rich shallow-marine facies, which show a progressive northeastward decrease in mean sandstone:mudstone ratios and conglomerate clast-size (Laudon et al. 1983). Mid-Jurassic faunas are found in the SW part of the formation (Behrendt Mountains: Fig. 1), where the oldest known strata contain a nearshore, Bajocian molluscan assemblage (guilty 1983). Elsewhere, all described Latady faunas are of Kimmeridgian-Tithonian age, though much of the succession is barren of fossils (Laudon et aL 1983). The Latady Formation is structurally complex and its internal stratigraphy remains poorly understood. Nevertheless, it appears broadly to young towards the Weddell Sea (Rowley & Williams 1982), away from an interfingering contact with largely silicic volcanic rocks of the Mount Poster Formation (Rowley et al. 1982), which may be analogous to the syn-rift volcanic succession in the northern Antarctic Peninsula (Pankhurst et al. 1998). Except in the SW, the younging trend is accompanied by a change from non-marine facies to the marginal-marine/ near-shore shelf facies that dominate the formation, and finally to open-marine, outer-shelf black shales of latest Tithonian age (Laudon et al. 1983).
Basin evolution. The Late Jurassic palaeogeography of the northern Antarctic Peninsula region has been the focus of a series of studies in the last two decades. In all models, the Nordenskjold Formation, together with comparable mudstone successions of similar age in South Georgia, the South Shetland Islands and southern South America, and at DSDP/ ODP sites in the South Atlantic and eastern Weddell Sea, is considered to represent deposition in an anoxic epicontinental sea covering the incipient South Atlantic-Weddell Sea region (e.g. Farquharson 1983b; Doyle & Whitham 1991; Pirrie & Crame 1995). All models have placed a magmatic arc along the present spine of the Antarctic Peninsula. Farquharson (1982, 1983b) suggested that the arc was largely submarine, but later workers (e.g. Rees 1993) thought that it formed a more continuous landmass. Central to this argument has been the progressive revision of the age of the non-marine Botany Bay Group, from Early Cretaceous (Farquharson 1984), to no younger than Late Jurassic (Millar et al. 1990), and most recently, to Early Jurassic (Rees 1993). Farquharson (1983a) suggested that anoxia was caused by a combination of upwelling of unrestricted westerly palaeo-Pacific currents near the outboard margin of the regional basin, resulting in an expanded oxygen minimum zone (OMZ), and the existence of sills in basinal areas farther from the Pacific margin. Later workers favoured a barred basin model (e.g. Macdonald et al 1988; Doyle & Whitham 1991). In order to account for the presence of, and facies changes in, Kimmeridgian-Tithonian anoxic-dysoxic mudstones on the Pacific margin of the arc (Anchorage Formation, Byers Peninsula: Fig. 1), Pirrie & Crame (1995) proposed a combination of the two models. They suggested that an initially largely submerged arc became progressively more emergent from late Kimmeridgian times onward, and that this resulted in uplift of the Anchorage Formation site above the upwelling-related OMZ, but by forming a barrier to circulation, prolonged anoxia in the back-arc region into Berriasian times.
The present paper aims to tie the earlier work into a broader model for basin evolution. In the Magallanes Basin, cessation of large-scale silicic volcanism and normal faulting was followed by regional thermal subsidence (Gust et aL 1985) and the spread of oxygen-poor marine waters eastward across the subaerial volcanic province (Fig. 4b; Wilson 1991). It is suggested that a westward, but otherwise similar transgression took place in the Larsen Basin. On lithological grounds, Riley et al. (1997) regarded the Cape Framnes sandstones as a northward extension of the Latady Formation. However, it may be better to view them as basal strata of the marine Larsen Basin succession, stratigraphically equivalent to the shallow-marine-fluvial Springhill Formation of the Magallanes Basin, which forms a transgressive sandstone sheet separated from underlying Tobifera Formation silicic volcanic rocks by a sequence boundary dated at 151 Ma (late Kimmeridgian) by Biddle et al. (1986).
The Nordenskjold Formation is considered to record deposition in a deepened basin following the initial transgression represented by the Cape Frames beds. With its intercalated tuff beds it is lithologically similar to the mudstone-rich ‘distal’ facies associations of the deepmarine, western part of the Tobifera Formation (Hanson & Wilson 1991). This indicates that unlike in the Magallanes Basin (Biddle et al. 1986) and the craton-ward part of the Rocas Verdes Basin (Wilson 1991), silicic pyroclastic input continued into Berriasian times. However, it is clear that the Nordenskjold tuffs, which form generally thin beds within hemipelagic mudstones, for which rates of deposition must have been relatively slow, represent a marked decrease in the intensity and/or proximity of volcanism compared to the underlying ignimbrite succession. Whether they record latestage volcanism in the Jurassic extensional province (see Storey & Alabaster 1991), or activity in the Antarctic Peninsula arc, is uncertain. Whitham (1993) found depositional depths for the Nordenskjold Formation difficult to assess beyond setting a minimum limit of 50-100 m. Based on depth to a hypothetical OMZ, he tentatively suggested depths of 500-1000 m for the Longing Member, and
Comparison with the Magallanes Basin suggests that the Cape Framnes beds and Nordenskjold Formation represent a post-rift transgressive megasequence (Fig. 5b). It is difficult to ascertain the precise timing of the rift-sag transition in the Larsen Basin from available data, and some normal faulting may have continued. However, Nordenskjold facies are consistent with the relative tectonic quiescence one might expect during a thermal relaxation phase following active extension. In the northern Antarctic Peninsula region, the Nordenskjold strata appear to record the peak transgression following Gondwana break-up. Maximum retreat of depositional systems coincided with a phase of relative magmatic quiescence in the Antarctic Peninsula arc, and hemipelagic settling dominated deposition in sediment-starved basinal areas. The presence of plant debris and a limited terrigenous clastic content in the Nordenskjold Formation indicate that some land areas existed in the region. However, arguments for an extensive landmass based on the Early Jurassic age for the Botany Bay Group (Rees 1993), appear untenable, as those strata represent an earlier, pre-transgression stage of basin evolution. It is probable that, as suggested by Farquharson (1983b), the Antarctic Peninsula was the site of a discontinuous archipelago in Late Jurassic times (Fig. 5b), which became increasingly emergent as arc construction/uplift progressed during Berriasian times (see below). This is consistent with models for anoxia suggested by Farquharson (1983a) and Pirrie & Crame (1995); while its main cause is likely to have been rift-topography/transgression-related stagnation, upwelling could have played a role along the palaeo-Pacific margin, and in the later stages of Nordenskjold deposition, an emerging arc may have formed a barrier to circulation.
Although the initial transgression took place earlier in the SE Antarctic Peninsula, at least in the Behrendt Mountains area, the Latady Formation and the Cape FramnesNordenskjold succession appear to represent part of the same transgressive trend. However, the rate and calibre of sediment influx were higher in the southern region, resulting in a thicker, generally more sandstone-rich succession. The source area is likely to have been a substantial Palmer Land landmass to the west (Laudon et al. 1983). The dominance of paralic and shallow-marine facies may reflect a more even balance between sediment accumulation rate and base-level rise. The northeastward decrease in sandstone:mudstone ratios and conglomerate clast-size suggests the possibility of an eventual northward transition to hemipelagic Nordenskjold facies. Although there appears to be no evidence for anoxia in the Latady Formation, this may simply reflect the lack of detailed sedimentological and palaeoecological data. Alternatively, the heavier coarse elastic influx may have destroyed stratification in the water mass. Uppermost Tithonian black shales record peak transgression and relative sediment starvation on the outer shelf during the last stages of Latady deposition.
Stratigraphical record. No strata known to be younger than latest Tithonian are exposed on the eastern margin of the Antarctic Peninsula south of the Larsen Basin. In the northern Larsen Basin, there is a gap in the sedimentary record between the Nordenskjold and Pedersen formations (Fig. 3). The latter is of earliest Aptian age on Sobral Peninsula, where it is in tectonic contact with upper Tithonian-lower Berriasian Ameghino Member strata. However, the problematic outcrop at Pedersen Nunatak may be older (see below).
In the southern Larsen Basin, strata of possible Hauterivian-Barren-ian age are exposed to the south of Kenyon Peninsula, at Crabeater Point and in a number of smaller outcrops to the SE (Fig. 6). These strata form the youngest part of a sedimentary and volcanic succession that lies within and to the NE of a complex thrust zone that separates it from the plutonic and metamorphic rocks of the arc massif to the SW (Fraser & Grimley 1972; Fig. 6). Stratigraphical relationships in the area are generally uncertain because of faulting and limited exposure. The older part of the succession consists of largely silicic volcanic rocks and unfossiliferous sedimentary strata. Fraser & Grimley (1972) divided the latter into a non-volcaniclastic, quartzose succession, which they tentatively correlated with the Trinity Peninsula Group, and a volcaniclastic succession, for which they suggested a possible Jurassic age (Fig. 3). Neither the base nor the top of the Crabeater Point succession is exposed. At Crabeater Point (Fig. 7), it has an exposed thickness of c. 370 m and consists mainly of intensely bioturbated, black, cleaved mudstone, in which poorly preserved radiolarians and foraminifera are commonly present, but rarely abundant. These strata contain a sparse, ?Early Cretaceous invertebrate macrofauna (Thomson 1967; Taylor et al. 1979, p. 53), including inoceramid bivalves which Crame (1985) tentatively assigned to a species with an Aptian-Albian range, possibly extending down into the Barremian. A possible HauterivianBarremian age has since been suggested for inoceramids collected by the author (J.A. Crame pers. comm. 1998), based on similarities to specimens from the Spartan Glacier Formation of Alexander Island (Fig. 1; Crame & Howlett 1988). Much of the succession consists of lithologically uniform mudstone, but some intervals include paler-coloured sandstone horizons (e.g. Fig. 7e). These generally range from millimetre-scale siltstonevery fine sandstone laminae, which are commonly laterally impersistent due to bioturbation, to sharp-based, normally graded beds of fine-very fine sandstone up to 2 cm thick. Rare thicker beds (to 80 cm) are fine or medium (rarely mediumcoarse) grained, normally graded, and typically otherwise structureless or vaguely parallel-stratified, although some show Bouma sequences. They may form metre-scale amalgamated packets (Fig. 7e), but generally occur as isolated, relatively parallel-sided, sharp-based units. Evidence for syndepositional sediment instability includes a low-angle (c. 5′) truncation surface in the mudstones, which is interpreted as a draped slide scar, and a solitary sandstone dyke. Compositionally, sandstones from Crabeater Point fall into three main types. Most appear to consist either of redeposited silicic tephra (altered shards, euhedral-subhedral plagioclase, minor quartz and biotite), or of material derived from a complex source terrain (plagioclase, quartz, varied volcanic and metamorphic lithic grains, K-feldspar). A rarer third type consists mainly of texturally uniform, microlitic-lathwork volcanic lithic grains, suggesting a subaerial, basaltic-intermediate, neovolcanic provenance (see Critelli & Ingersoll 1995). Strata correlated with those exposed at Crabeater Point on lithological, petrographic and structural grounds (Thomson 1967; Fraser & Grimley 1972) crop out on the south side of Poseidon Pass, and in cliff-faces on the south side of the Cape Keeler ridge and 6 km NE of Crabeater Point (Fig. 6). The mudstonedominated strata in Poseidon Pass are lithologically similar to those at Crabeater Point, but the other outcrops include thick, apparently massive intervals of brown-weathering sandstone, as well as black mudstones with minor sandstone beds. Possible synsedimentary slide deposits, consisting of large blocks in chaotic orientation, have been observed on the Cape Keeler ridge (D.LM. Macdonald unpublished report 1985). Magmatism and deformation. The most intense episode of pluton intrusion in the Antarctic Peninsula started at c. 142 Ma, and began to wane at c. 100 Ma (Leat et al. 1995). The Early Cretaceous plutons are mainly intermediate-silicic, I-type granitoids with relatively low “Sr/”Sr (Leat et al. 1995). In the north, magmatism was concentrated in western Graham Land and the South Shetland Islands (Leat et al. 1995). In Palmer Land, the locus of magmatism appears to have broadened from west to east during Early Cretaceous times (Storey et al. 1996).
In Early Cretaceous times, the Latady and Mount Poster formations were strongly folded during a phase of tectonism termed the Palmer Land deformational event by Kellogg & Rowley (1989). The horizontal to gently plunging fold axes parallel the curved axis of the southern Antarctic Peninsula. Folds are commonly asymmetric, and verge to the south or SE. Many show chevron geometry. A penetrative axial-planar cleavage is well developed in finer grained facies. Several N- and NW-dipping thrust faults have been identified in the Latady Formation (Rowley & Williams 1982), and more may be present. The age of the deformation is not closely constrained. It post-dated deposition of the youngest Latady strata in late Tithonian times (c. 145 Ma; Fig. 3), and predated the intrusion into the fold belt of plutons that have yielded K-Ar and Rb-Sr ages ranging from 122 to 96 Ma (Pankhurst & Rowley 1991). The Nordenskjold Formation appears to have been little affected by the Palmer Land deformational event. Syndepositional and early postdepositional deformation (including layer-parallel extensional and compressional structures in carbonate concretions) in the Ameghino Member can be attributed to slope-related sediment instability (Whitham 1993). The more intense, later deformation seen in Nordenskjold strata on Sobral Peninsula, which has been interpreted in terms of NW-directed thrusting, extensional faulting and transpressive strike-slip (Whitham & Storey 1989; Vaughan & Storey 1997), affects adjacent strata now known to be of early Aptian age to an equal extent, and is likely to be related to Late Cretaceous basin inversion (see below). The tectonically formed pressure-solution seams which Whitham (1993) reported from most Nordenskjold outcrops, including glide blocks within Albian strata on James Ross Island (Ineson 1985; see below), seem to be the only evidence for limited, possibly pre-Aptian compression.
The strata at Crabeater Point are deformed into a series of NE-verging asymmetric folds (Fig. 7a-b), elements of which were first noted by Thomson (1967) and D. 1. M. Macdonald (unpublished field report 1985). Some of the folds have chevron geometry, and there is a well-developed, penetrative, broadly axial planar cleavage. Measured bedding/cleavage intersection lineations and a best-fit girdle on rSo (Fig. 7c) indicate that the NW-SE-striking fold axes plunge gently to the WNW. Orientations of strata in other exposures of the Crabeater Point succession are consistent with similar folding. The age of this deformation, which may be related to the SW-dipping thrust zone to the south and SW of Kenyon Peninsula, is again poorly constrained. The virtually undeformed Santonian strata exposed on Table Nunatak, 50 km to the NE (Hathway et al. 1998), provide a younger age limit. Basin evolution. The plutonic record (Leat et al. 1995, 1997) indicates that the beginning of this period represents the onset, or renewal, of large-scale Antarctic Peninsula arc magmatism following the Late Jurassic quiescent phase. Suggested mechanisms for producing the Palmer Land deformational event include a range of crustal-block collision/accretion events (Kellogg & Rowley 1989; Grunow et al. 1991; A.P.M. Vaughan pers. comm. 1998) and the westward subduction of Weddell Sea lithosphere (Grunow 1993). Kellogg & Rowley (1989) favoured a model involving gravitational-spreading of a thin-skinned fold-thrust belt off crust that had been thickened and isostatically uplifted as a result of arc magmatism. Storey et al. (1996) attributed the deformation to a compressional phase between 150 and 140 Ma, during the latter part of the gap in Antarctic Peninsula plutonism. However, continuing Latady and Nordenskjbld deposition during this period (Tithonian-early Berriasian; Fig. 3) suggests that this time range may be too early. Deposition of those strata was accompanied, or closely followed, by the initial part of the Early Cretaceous plutonic phase (c. 140 Ma), which seems to have been dominated by syn-magmatic crustal extension (Vaughan et al. 1997), suggesting a later date for any compressional episode.
Storey et al. (1996) suggested that deformation of Larsen Basin strata in the Kenyon Peninsula area might be related to a mid to Late Cretaceous (c. 113-80 Ma) compressional phase. However, although this event resulted in partial basin inversion in the northern Larsen Basin (see below), the deformation it produced differs in intensity and polarity from that seen at Crabeater Point. The similarity in deformation style between the Latady and Crabeater Point strata, with fold axes parallel to the curved trend of the Antarctic Peninsula and vergence towards the Weddell Sea, suggests that they may have been deformed during the same episode. If this is so, the fossil age from Crabeater Point, together with the age of 122 Ma (Barremian-Aptian boundary; Fig. 3) for the oldest postdeformation plutons, would indicate a Hauterivian-Barremian age for the Palmer Land deformation. The lack of substantial pre-Aptian deformation in the Nordenskjold Formation is likely to be significant. Development of a retro-arc fold-thrust belt may have been a function of the greater thickness of the Latady sediment pile compared to its northern equivalent. Alternatively, the stress regime may have been markedly different in the two regions.
Sandstones and siltstones in accessible parts of the Crabeater Point succession are interpreted as turbidites. Sandstone petrography suggests derivation from active arc volcanoes and their plutonic/metamorphic basement. It is difficult to identify depositional processes for the pervasively bioturbated mudstones, but deposition from lowconcentration turbidity currents with a subordinate component of hemipelagic settling seems likely. The presence of a slide scar and possible synsedimentary slide deposits suggest slope-related sediment instability. Available data suggest that the Crabeater Point strata represent sub-storm-wave-base deposition on a mud-dominated slope apron punctuated by sand-rich submarine fans.
This late Berriasian-Barremian period is the least wellunderstood phase of basin evolution. The Crabeater Point strata may form part of a broadly regressive succession reflecting increasing elastic input from the rising magmatic arc. However, the nature of the boundary with the older transgressive megasequence is uncertain. A possible unconformity between the Nordenskjold-equivalent Anchorage Formation and the succeeding, regressive, Berriasian-Valanginian marine intra-arc succession (President Beaches and Chester Cone formations) on Byers Peninsula in the South Shetland Islands (Figs 1 & 3; Hathway & Lomas 1998) may represent a time-equivalent surface. In the model suggested here, the Palmer Land deformation event would have produced an unconformity at the top of the proposed Crabeater Point megasequence. If the age suggested for the deformation by Storey et al. (1996) is accepted, any resulting unconformity would pre-date the Crabeater Point strata, which might then form an older part of the succeeding Lower Cretaceous-Eocene megasequence (see below).
The seismic profile obtained by Jarvis & King (1995) north of Kenyon Peninsula (Fig. 1), showed non-reflective rocks with high seismic velocities (4.7-5.5 km s -‘). This area may be underlain by volcanic rocks of the Jurassic syn-rift succession, or by deformed sedimentary strata, perhaps laterally equivalent to the Crabeater Point succession, in which seismic velocities might be expected to approach those recorded from the Latady Formation (average 5.2 km s -‘: A.C. Bell pers. comm. 1997). In either case, the profile would lie within the Larsen Basin, and the seismic data would not invalidate Macdonald et al.’s (1988) contention that the passive magnetic signature reflected the extent of the basin.
Stratigraphical record The early part of this period is represented by the Pedersen Formation and the lower Gustav Group (Fig. 3). The latter is extensively exposed on western James Ross Island (Fig. 1), where the strata are assigned to the Lagrelius Point, Kotick Point, and Whisky Bay formations, which have a combined thickness of c. 2500 m (Ineson et al. 1986). The basal part of this succession is of early Aptian age (Riding et al. 1998), and although the biostratigraphy remains poorly known (Ineson et al. 1986), a Coniacian age seems likely for its upper part (e.g. Pirrie et al. 1991). The Pedersen Formation (del Valle & Fourcade 1986; a formal revision of the status of this unit is in preparation) is exposed only on southern Sobral Peninsula (750-1000 m thick) and on Pedersen Nunatak (142 m) (Fig. 8). At the former location, ^sup 40^Ar/”Ar ages (Hathway & Lomas 1998, p. 63) and palynological studies (J. B. Riding unpublished reports 1998) indicate an earliest Aptian age. The lithologically similar strata at Pedersen Nunatak have yielded conflicting age determinations (Thomson & Farquharson 1984; Elliot 1988), although the presence of ammonite fragments of possible Hauterivian age suggests they may be older than the Sobral Peninsula beds.
The Pedersen Formation and lower Gustav Group consist of conglomerate-dominated strata, interpreted as gravelly submarine fan deposits; and mudstone- and sandstone-dominated slope apron deposits (Farquharson et al. 1984; Ineson 1989; Pirrie et al. 1991). Ineson (1989) found depositional depths difficult to quantify, but suggested a range of 300-1000 m. Clast composition and palaeocurrent data indicate a source area to the NW consisting of active arc volcanoes, older volcanic rocks, and the underlying Trinity Peninsula Group (Farquharson 1982; Ineson 1989; Pirrie 1991). The base of the succession is not exposed. On Sobral Peninsula, an apparent lack of radiolarians, as well as the absence of intercalated tuffs (Whitham & Doyle 1989), distinguishes Pedersen Formation pelites from the Nordenskjold Formation mudstones with which they are in fault contact (Fig. 8e). Allochthonous blocks of Nordenskjold Formation strata up to 800 rn across are present in the lower, mid-Albian part of the Whisky Bay Formation (Ineson 1985, 1989).
The ?Coniacian-Santonian Hidden Lake Formation (Ineson et al. 1986), the uppermost unit of the Gustav Group, crops out on NW James Ross Island (Fig. 9a), where it is locally unconformable on the Whisky Bay Formation. It ranges from 300 to >400 rn thick and is thought to represent shelf deposition at depths of >200 m (Pirrie et al. 1991). Its largely neovolcanic-andesitic provenance is considered to record a major phase of coeval arc volcanism (Pirrie 1991). Younger Larsen Basin strata, extensively exposed in the James Ross Island area, are assigned to the Santonian-Palaeocene Marambio Group, which rests gradationally on the Hidden Lake Formation (Ineson et al. 1986), and the Palaeocene-Eocene Seymour Island Group. They constitute a shallow-marine shelf-deltaic succession with an exposed thickness of c. 3250 m, in which a series of unconformity-bounded regressivetransgressive cycles have been identified (Pirrie et al. 1991). Farther south, shallow-marine successions time-equivalent to Marambio Group strata are exposed on Table Nunatak (late Santonian; Hathway et al. 1998) and Cape Marsh (Campanian; del Valle & Medina 1985) (Fig. 1). Basal Marambio Group sandstones have a largely palaeovolcanic provenance, reflecting decreased arc volcanic activity, and the intensity of volcanism appears to have decreased further during deposition of the rest of the group (Pirrie 1991).
Structural geology. The degree of deformation shown by the Aptian-Eocene succession broadly decreases up-section and away from the western margin of the basin. To the NW of Mount Lombard on Sobral Peninsula, Pedersen Formation mudstones and sandstones and adjacent Nordenskj8ld Formation strata (Fig. 8e) are deformed into 100-500 ra scale, NW-verging asymmetric folds, and cut by numerous high- and low-angle faults (Whitham & Doyle 1989). 7 km to the NE, Nordenskjbld Formation strata form a kilometre-scale anticline with a NNE-trending axis and axial-planar cleavage. This deformation has been interpreted in terms of NW-directed thrusting, extensional faulting and transpressive strike-slip (Whitham & Storey 1989; Vaughan & Storey 1997). The Pedersen Formation strata on Sobral Peninsula form an open, gently SW-plunging syncline (Fig. 8b, d). On the western limb of this fold, mudstones at the base of the succession are underlain by a SE-dipping thrust zone including a wedge of highly tectonized conglomerate, beneath which are steeply NW-dipping conglomerates (Elliot 1966; del Valle et al. 1991). Pedersen Formation strata on Pedersen Nunatak form two shallow synclines with NE-trending axes, separated by a SSE-dipping high angle reverse fault (Fig. 8c; Elliot 1966; del Valle et al. 1991).
Gustav Group strata on western James Ross Island form a NE-trending monoclinal syncline (Fig. 9) within which dips decrease progressively up-section to the SE (from subvertical in the Lagrelius Point Formation). Noting constant vitrinite reflectance values in the lower Gustav Group, Whitham & Marshall (1988) showed that those strata had undergone progressive syndepositional southeastward tilting. This seems to have largely ceased before deposition of the Hidden Lake Formation, which dips at c. 12* to the SE (Fig. 9). The Marambio and Seymour Island groups generally dip to the SE at 5-7 (Pirrie et al. 1991). The strata exposed on the James Ross Island archipelago show little deformation apart from the marginal tilting. Rare large-scale normal faults are thought to be related to the emplacement of Neogene volcanic rocks (Pirrie et al. 1991). The strata exposed on Table Nunatak and Cape Marsh also dip gently towards the Weddell Sea, and show a similar lack of deformation. Seismic reflection data from NW James Ross Island (Fig. 1; Keller & Diaz 1990) show maximum dips of 15 to the SE in the subsurface, indicating that the steep dips are confined to a zone close to the basin margin. Together with a seismic profile obtained from SE James Ross Island by del Valle et al. (1992b), these data show a sedimentary succession gently dipping and thickening to the SE. Correlations suggested for the three sedimentary packets identified by Keller & Diaz (1990) and del Valle et al. (1992b), cannot be tied to outcrop data and must be viewed as speculative. Nevertheless, the presence of an angular unconformity at the top of the basal packet (R3), which del Valle et al. (1992b) correlated with the Nordenskjold Formation, is noteworthy.
Basin evolution. The Pedersen Formation and lower Gustav Group represent a substantial increase in sediment supply relative to the Nordenskj6ld Formation, indicating that largescale uplift of the arc massif had taken place by earliest Aptian times. An abrupt change in aeromagnetic signature between the arc massif and the sedimentary basin, and the deposition of a thick, in large part coarse-grained, deep-marine, marginal clastic wedge indicate that the basin margin in the James Ross Island area was actively fault-controlled from at least Aptian to early Coniacian times (Fig. 5c; Farquharson et al. 1984; Ineson 1989). Differential subsidence across this margin is thought to have prevented significant progradation of shallowmarine facies during this period (Ineson 1989). The lower Gustav Group strata on James Ross Island appear to form a syntectonic cumulative wedge system (see Riba 1976; Anad6n et al. 1986), with a single progressive unconformity resulting from tilting about a subhorizontal rotation axis related to uplift of the Antarctic Peninsula massif (Fig. 9b-c). The locally unconformable base of the Hidden Lake Formation can be interpreted as a syntectonic angular unconformity recording decelerating differential subsidence, but the extent of the resulting rotative onlap (see Riba 1976) is uncertain. In a speculative, reconstructed cross-section, Whitham & Marshall (1988) showed extensive onlap of Hidden Lake Formation strata over the erosionally truncated older wedge system (Fig. 9b). If it was originally present, the proximal part of this onlapping succession has since been removed by erosion. Subsequently, basin subsidence decreased enough to be outpaced by the also reduced sediment input (Pirrie et al 1991), resulting in basinward progradation of shallow-marine facies in the James Ross Island area (Fig. 5d), and on available evidence throughout the Larsen Basin (e.g. Hathway et al. 1998). Within the overall regressive trend, relative base-level changes produced a series of sequence boundaries, some of which have been tentatively correlated with lowstands on the Haq et al (1987) global sea-level curve (Pirrie et al 1991).
The Pedersen Formation has undergone substantial postdepositional deformation and its relationship to the syntectonic wedge system farther NE is uncertain. The deformation, which resulted in NW-directed thrusting and related folding, represents a phase of basin inversion that can only be dated as younger than earliest Aptian. It is not known whether the thrusts and reverse faults represent reactivated extensional basement structures or formed entirely during basin inversion. Inversion appears to have been transpressive (e.g. Vaughan & Storey 1997), but it is uncertain whether it was caused by strike-slip-dominated tectonics or compression resolved obliquely against the basin margin. It seems probable that this deformation is related to the ?Coniacian deceleration and/or cessation of differential subsidence during deposition of the Hidden Lake Formation, which Macdonald et al. (1988) and Pirrie et aL (1991) attributed to inversion-related tectonic uplift.
The nature and age of the unexposed base of the Lower Cretaceous-Eocene succession (Pedersen Formation to Seymour Island Group), which forms a regressive megasequence (Pirrie et al 1991), is uncertain. On Byers Peninsula (Fig. 1), uplift above sea-level and the development of a Valanginian-Aptian hiatus was followed by intra-arc extension and the deposition of largely pyroclastic, non-marine strata in earliest Aptian times (c. 120 Ma; Hathway 1997). The latter succession is similar in age to the Pedersen Formation on Sobral Peninsula (Fig. 3), and it too may have been deposited during an extensional phase following a period of possibly compressional arc uplift and regional unconformity development, possibly related to the Palmer Land deformational event. However, confirmation of a Hauterivian age for the Pedersen Nunatak strata would extend the age range of the Pedersen Formation downward (Fig. 3). Nordenskjold Formation megaclasts in the Gustav Group have yielded vitrinite reflectance values similar to those of the enclosing strata, indicating that Nordenskjold strata on the basin margin did not undergo significant burial prior to their redeposition in Albian times (Whitham & Storey 1989). This would suggest that arc uplift began soon after deposition of the youngest Nordenskjold strata in Berriasian times. Storey et al. (1996) suggested that Early Cretaceous arc extension was followed by a mid-Late Cretaceous (c. 113-80 Ma) compressional phase. The proposed Coniacian age (c. 88 Ma; Fig. 3) for partial inversion of the northern Larsen Basin, would fall within the latter part of this episode, which also coincided with an Andean compressional phase that resulted in closure of the Rocas Verdes marginal basin (Fig. 4c-d) in Late Cretaceous times (92-79 Ma; Olivero & Martinioni 1996).
Synthesis and discussion
Three megasequences can be confidently identified in the Larsen Basin, and the existence of a fourth is tentatively suggested (Fig. 3). The base of the oldest is marked by a ?latest Triassic-Early Jurassic unconformity developed on the Trinity Peninsula Group and older cratonic rocks, which represents the onset of continental rifting. All exposed rocks of the succeeding syn-rift megasequence are non-marine, although marine strata may have been deposited farther east, nearer the incipient Weddell Sea continental margin. Lower Jurassic alluvial fan conglomerates (Botany Bay Group) and lacustrine mudstones at the base of the syn-rift succession were deposited in areally restricted, fault-bounded basins developed during an initial phase of largely amagmatic extension. These strata are overlain by a widespread Middle-?Upper Jurassic volcanic succession consisting mainly of silicic ignimbrites, which represents part of a large igneous province extending into southern South America (Pankhurst et al. 1998). This voluminous volcanism is thought to be the result of crustal anatexis during the later stages of active extension.
The apparent superposition of Upper Jurassic shallow marine sandstones on the subaerial ignimbrites at Cape Framnes is considered to mark the upper boundary of the non-marine succession. By analogy with the Magallanes Basin, this trangression is considered to mark a syn-rift-post-rift transition, which probably coincided with the onset of seafloor spreading in the Weddell Sea. The overlying transgressive megasequence includes the basal Cape Framnes sandstones, and the Nordenskjold Formation, which represents sedimentstarved hemipelagic deposition on the by then largely drowned continental margin. Its deposition coincided with the latter part of a gap in Antarctic Peninsula plutonism, and represents a marked decrease in the intensity of nearby volcanism relative to the syn-rift ignimbrite succession. The Latady Formation of the SE Antarctic Peninsula appears to form part of the same transgressive trend. However, it is thicker, and is dominated by relatively sandstone-rich non- and shallow-marine facies, indicating that a southern, Palmer Land province, including a substantial landmass with significant relief, had become differentiated from a largely submerged province to the north by Kimmeridgian times (see Thomson et al. 1983).
Although the Late Jurassic transgression cannot be dated beyond a broad Kimmeridgian-Tithonian range, it may have coincided with the late Kimmeridgian-early Tithonian flooding event seen in basins throughout southern South America (e.g. the Andean Basin, Fig. 3) (Hallam 1991; Ardill et al. 1998). In central and northern Chile and Argentina, this relative sea-level rise followed a late Oxfordian regression that resulted in widespread lowstand evaporite deposition (e.g. Hallam 1991; Legarreta & Uliana 1996), and a regional tectonic control is proposed (e.g. Ardill et al. 1998). In the Larsen and Magallanes basins, where transgression followed the cessation of large-scale silicic volcanism, a regional control (i.e. basin subsidence) is also likely. Nevertheless, coincidence with the peak Jurassic eustatic sea-level stand in late Kimmeridgian-early Tithonian times (Fig. 3; Haq et al. 1987; Hallam 1988), must have increased the extent of the transgression (e.g. Hallam et al. 1986). The presence of Middle Jurassic strata in the southern part of the Latady Formation, suggesting earlier rifting in that area, is consistent with propagation of a spreading ridge system from the Gondwana interior northward through the Weddell Sea region into the Rocas Verdes basin in Mid-Late Jurassic times, as proposed by Grunow et al (1991), Dalziel (1992) and Mukasa & Dalziel (1996).
The top of the post-rift megasequence is not exposed, and basin evolution during the succeeding late BerriasianBarremian period is poorly understood. The onset of intense plutonism in the Antarctic Peninsula in Berriasian times may represent either the beginning, or the renewal of large-scale arc magmatism, following the preceding quiescent phase. The intra-arc Byers Peninsula succession (Fig. 3) shows evidence for increased sediment supply and regression in BerriasianValanginian times (Hathway & Lomas 1998). This may record falling eustatic sea level (Fig. 3; Crame et al. 1993) and/or sustained arc uplift, either driven by intrusion emplacement (see Leat et al. 1997), or related to the early stages of a collisional orogeny that would culminate in the Palmer Land deformational event (A.P.M. Vaughan pers. comm. 1998). It is tentatively suggested that two megasequence boundaries, neither of which is exposed, may have developed in the Larsen Basin in late Berriasian-Barremian times. The first, perhaps produced by an early phase of arc uplift, may mark the base of a ?regressive megasequence recording increased sediment yield from the rising volcanic arc/orogen, represented by the slope mudstones and possible submarine fan sandstones of the ?Hauterivian-Barremian Crabeater Point succession. It is uncertain whether strata time-equivalent to this succession are present in the northern Larsen Basin, although the Pedersen Nunatak beds are possible candidates. The second megasequence boundary, perhaps related to the Palmer Land deformational event, may form the base of the regressive Lower Cretaceous-Eocene succession exposed in the northern Larsen Basin. In that area, differential subsidence during a phase of arc uplift accompanied by intra- and possibly back-arc extension, resulted in deposition of a deep-marine, syntectonic clastic wedge close to the western basin margin in Aptian?Coniacian times. Following partial basin inversion in ?Coniacian times, reduced basinal subsidence rates allowed accommodation to be filled and regression to take place.
Eustatic curves (Haq et al. 1987; Sahagian et al. 1996) show a long term sea-level rise from mid-Valanginian to early Barremian times, followed by a period of relative stability until early Albian times, and then a steady rise to a TuronianConiacian maximum (Fig. 3). Little can be inferred from the fragmentary Berriasian-Aptian Larsen Basin record beyond a generalized regression related to arc uplift/construction. Nevertheless, this would suggest that, as in southern South America (Hallam 1991), regional tectonics overprinted the eustatic signal during this period. The Hauterivian-Barremian age proposed for the Crabeater Point strata suggests that their deformation may have coincided with the onset of a sustained regressive phase in the Andean (Fig. 3; Hallam et al. 1986) and Magallanes (Macellari 1988) basins in early Barremian times. No evidence for relative sea-level change has yet been identified in the lower Gustav Group, and the main control on its deposition is thought to have been the tectonic/magmatic evolution of the adjacent arc (Pirrie et al. 1991). Late Cretaceous shallowing of the basin took place in ConiacianSantonian times, when global sea levels were relatively stable at a level slightly below the Turonian Mesozoic-Cenozoic maximum (Fig. 3; Haq et al 1987), and is considered to be related to partial tectonic inversion. Base-level changes driven by short-term eustatic variations are likely to be easiest to identify in the succeeding, Santonian and younger succession, deposited on a relatively stable shelf (Pirrie et aL 1991). Based on the limited available outcrop data, and compari”ons with basin elsewhere in SW Gondwana, the Mesozoic succession in the Larsen Basin can be divided into a series of megasequences, separated by boundaries that record the principal periods of change in basin geometry. As in the coeval southern South American basins (e.g. Hallam 1991), regional tectonics and associated igneous activity were the dominant controls on deposition. Lower-?Upper Jurassic non-marine sedimentary and volcanic rocks constitute a syn-rift megasequence recording initial amagmatic extension and subsequent widespread extension-related silicic volcanism resulting from megaplume-driven continental rifting in the early stages of Gondwana break-up. Cessation of large-scale volcanism was followed by marine transgression and the deposition of a Kimmeridgian-lower Berriasian post-rift megasequence dominated by hemipelagic mudstones. This period, during which flooding-related sediment starvation coincided with an apparent lull in Antarctic Peninsula magmatism, appears to represent the transition from the continental rifting phase to a regime dominated by magmatic arc development and subduction-related tectonism focused along the continental margin. The fragmentary rock record for late BerriasianBarremian times suggests that a ?regressive megasequence may have developed in the earlier part of this period as a result of increased sediment yield from the increasingly emergent Antarctic Peninsula arc. Subsequently, strata in the southern, but not the northern, Larsen Basin underwent relatively intense deformation with vergence towards the Weddell Sea. This deformation may be related to the Palmer Land deformational event, which affected Jurassic strata in SE Palmer Land. Whether this represents a collisional orogen or a retroarc fold-thrust belt developed as a result of magmatic thickening of are crust is uncertain. Fossil ages from the southern Larsen Basin suggest that the culminating deformation took place in ?Hauterivian-Barremian times, at the younger end of the permitted age range. Where exposed, the lower part of the succeeding Aptian-Eocene megasequence consists of an aggradational deep-marine clastic wedge deposited along the faultbounded western basin margin, during a phase of arc uplift and extension. Reduced basin margin tectonism and the basinward progradation of shallow marine facies followed partial basin inversion in Late Cretaceous (?Coniacian) times. Thanks go to P. Thompson for assistance during the 1995-96 Larsen Basin field season, and to the British Antarctic Survey Air Unit and Rothera base personnel for logistic support. J. A. Crame is thanked for faunal determinations, and, together with A. P. M. Vaughan and M. R. A. Thomson, for comments on an early version of the manuscript. Constructive reviews of the paper by J. R. Ineson and D. 1. M. Macdonald, and careful editorial work by D. Pirrie are gratefully acknowledged.
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Received 1 September 1998; revised typescript accepted 7 June 1999. Scientific editing by Duncan Pirrie.
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