Age and composition of Antarctic sub-glacial bedrock reflected by detrital zircons, erratics, and recycled microfossils in the Ellsworth Land–Antarctic Peninsula–Weddell Sea–Dronning Maud Land sector (240°E–0°–015°E)
Graphical abstract
Highlights
► Analysing detrital zircons for U–Pb age, rock type, epsilon Hf, and model age. ► Compiling dated erratics in Antarctica. ► Compiling recycled microfossils from Antarctica.
Introduction
We examine four kinds of evidence for the age and composition of the ~ 98% of Antarctic bedrock (14 × 106 km2 or 9% of the Earth's land area) covered by ice (Fig. 1): (1) detrital minerals in sediment shed from Antarctica (e.g., Belyatsky et al., 2010), including detrital hornblendes (Roy et al., 2007), and new analyses of detrital zircons from strategically located deep-sea (turbiditic) sand (e.g., Veevers and Saeed, 2011); (2) Pb isotope compositions of detrital K-feldspars (Flowerdew et al., 2012), (3) erratics and dropstones that reflect age and composition, and (4) recycled microfossils that reflect age and facies.
Ice carries material plucked from bedrock and unconsolidated material in the Antarctic interior to the coast and offshore where it is deposited as till, and funnelled through canyons in the slope to turbidite fans on the continental rise (O'Brien et al., 2005). Two ice drainages were examined. (1) The sector 240° to 300°E (Fig. 1, drainage F′–II), with the largest outflow of ice, receives material from the ice divide north of the Whitmore Mountains (WM) and the Antarctic Peninsula; and (2) the drainage exiting in the 300° to 015°E (II–AA′) sector in the Weddell Sea and to the east receives material from West Antarctica on one side (II–J″) and from East Antarctica (Coats Land, Dronning Maud Land) on the other (J″–AA′). The present drainage in Antarctica is long-lived, from the initiation at ~ 300 Ma (Carboniferous/Permian) during the uplift of the ancestral Gamburtsev Subglacial Mountains (Veevers, 2011) and at the boundary between East and West Antarctica in the Transantarctic Mountains (TAM) from 125 Ma and 95 Ma in the Cretaceous, and 50–45 Ma in the Eocene (Fitzgerald and Stump, 1997).
Detrital zircons in marine turbidites (DSDP 322, 323, 325, and ODP 694) reflect the age, rock type, εHf, and TDMC of bedrock in the adjacent drainage basin. The samples come from sands with ages from Oligocene/Miocene (23 Ma) to early Pliocene (5 Ma). The sector J″–A′ is covered by the East Antarctic ice sheet, and F′–J″ by the West Antarctic ice sheet (Fig. 1). According to the model of Jamieson et al. (2010), from 34 to 14 Ma, ice sheets oscillated on similar scales to Northern Hemisphere Pleistocene ice sheets, (2) at 14 Ma ice expanded to its maximum offshore extent, and (3) thereafter the East and West Antarctic ice sheets became confined to the continent with relatively minor fluctuations driven by sea level change. Mountainous regions such as the Gamburtsev Subglacial Mountains (GSM) (Bo et al., 2009, Ferraccioli et al., 2011), uplands in Dronning Maud Land, and massifs in West Antarctica have been protected under cold-based ice since 34 Ma, with less than 200 m of erosion, while some peripheral troughs (fjords) were excavated by warm-based ice to depths of several kilometres to provide abundant sediment, as exemplified by the ice-covered fjord of the Aurora Subglacial Basin (ASB, Fig. 1) (Roberts et al., 2011, Young et al., 2011). In the middle (J″) of the sector of interest (240°E–015°E), the Pensacola Basin (PeB, Fig. 1) extends from the Filchner Ice Shelf (FIS) past the South Pole across drainage basins that head in the GSM.
In an allied study, Roy et al. (2007) analysed circum-Antarctic glacimarine sediment for 40Ar–39Ar ages of > 0.150 mm grains of hornblende and Sm–Nd isotopes of bulk < 0.063 mm sediment for TDM model ages and εNd. Their data reflect the adjacent provenance in seven distinct sectors.
Mineral grains in igneous rocks are subject to being eroded, transported, and deposited (first cycle) and then redeposited (second cycle) or inherited in an S-type granite or paragneiss to become separated from their original provenance.
Rock clasts or erratics in glacial deposits on land or in the marine margin may provide precise evidence of the age and composition of ice-covered bedrock. Clasts may reflect proximal to distal provenances; for example, glacial clasts of Cambrian archaeocyathan limestone reflect a proximal provenance in the TAM. As seen in Canada, erratics of U–Pb dated granite indicate a provenance 600 km distant (Doornbos et al., 2009).
Recycled microfossils, found offshore in modern sediments, point to sedimentary provenances of Permian and younger ages. They complement evidence from detrital zircons, which reflect terranes overwhelmingly older than 500 Ma in East Antarctica, and less than 500 Ma in West Antarctica.
Provenance from both clasts and grains can be further narrowed by information on the direction of flow (e.g., ice-flow data, cross-bed azimuth of fluvial sandstone) within the drainage basin. As illustrated in the model (Fig. 2), detritus offshore (X′, Y′, and Z′) can be traced back (full arrow) to proximal coastal exposures (X, Y, and Z) and upflow (broken arrow) to potential subglacial provenances (marked with ?) within the drainage basin. Detritus (P′, Q′, and R′) not represented (gaps or exotics) in coastal exposures can be traced upflow to imputed subglacial provenances (P, Q, and R). For example, 550–500 Ma ages in detritus traced to coeval rocks exposed in the proximal TAM could also indicate a distal provenance with this common age in Gondwanaland (Veevers, 2007); whereas ages of ~ 1100 Ma (an age gap in the TAM) would reflect a distal (subglacial) provenance within the drainage basin. Ice-flow patterns in the past (back to the Oligocene) may have differed in detail from the present pattern, so that the provenance would be less precisely defined.
Lateral transport by the westward-flowing coastal current (McGonigal, 2008, p. 37) is least for turbidites, and greatest for fine-grained contourites and ice-rafted debris (grains to blocks). Lateral transport is probably insignificant, at least since the Last Glacial Maximum, as shown by Roy et al.'s (2007) finding that minerals reflect the adjacent provenance in seven distinct sectors of Antarctica. Only if the drainage system and erosion sites changed significantly since the Miocene and Pliocene would the indicated provenance be compromised.
The following abbreviations are used: DML = Dronning Maud Land; GSM = Gamburtsev Subglacial Mountains; MBL = Marie Byrd Land; TAM = Transantarctic Mountains; VSH = Vostok Subglacial Highlands.
In a study of provenance in the Prydz Bay–Marie Byrd Land sector (70°–240° E) of Antarctica, Veevers and Saeed (2011, p. 733) brought together proxies of Antarctic bedrock in the form of (1) detrital zircons analysed for U–Pb age, TDMC, εHf, and rock type, including analyses of Neogene turbidites, (2) erratics that reflect age and composition, and (3) recycled microfossils that reflect age and facies. Each sample was located in its ice-drainage basin for backtracking to the potential provenance. Gaps in age between sample and upslope exposure were specifically attributable to the provenance.
Since then, an extensive geophysical survey of the central third of East Antarctica has revealed the salient geological features (Ferraccioli et al., 2011) (Appendix A), as shown in Fig. 3. The basic structure of East Antarctica is presumed to be a mosaic of Archaean and older Proterozoic cratons wrapped around by younger Proterozoic fold belts, as elsewhere in Gondwanaland (Veevers and Saeed, 2011, p. 714, 719). The first of these to be identified by geophysical sensing is the Adélie Craton (Fig. 3) (Finn et al., 2006). Ferraccioli et al. (2011) mapped bedrock as a complex of cratons — the Gamburtsev, Ruker, Recovery, and South Pole Provinces with stronger lithosphere, and rifted foldbelts — the Lambert, Eastern, and Vostok Rifts, characterised by thinned crust and weaker lithosphere. An inferred Gamburtsev Suture separates the Archaean Ruker Province from the presumed Proterozoic Gamburtsev Province.
The age (and composition) of the provinces can be found from the age of detrital zircons shed from the Gamburtsev region since the earliest Permian (Veevers and Saeed, 2011). Most of the zircons are aged 700–500 Ma (d+), with fewer 1300–900 Ma (c), and fewer still in older groups. The ages are interpreted as showing that the Gamburtsev province comprises a 1300–900 Ma (c) craton surrounded by 700–500 (d+) foldbelts. Glossopteris in place in the northern part of the Lambert Rift and in clasts of red siltstone in moraine in the south indicate a Permian age for the rift fill.
Starting in the earliest Permian (~ 300 Ma), the Gamburtsev Province was regionally elevated ground from which streams drained radially (Veevers, 2011). The East Antarctic–Indian rifts initially formed as a result of extension of the crust ~ 250 Ma, followed by transtension ~ 100 Ma (Phillips and Läufer, 2009). Meanwhile, the lower reaches of the radial drainage in Dronning Maud Land (DML), northern Lambert Glacier, and India filled with sediment in the Permian, as dated by fossils found in the resulting sedimentary rock.
In this paper we extend the work of Veevers and Saeed (2011) to the Marie Byrd–Ellsworth Land–Antarctic Peninsula–Weddell Sea–Dronning Maud Land sector (240°E–0°–015°E).
Geological connections of Antarctica with the conjugate continents are shown in Fig. 3, Fig. 4, Fig. 5 (Veevers, 2012). Antarctica adjoined East Africa, such that the Grunehogna Craton continued into the Zimbabwe–Kaapvaal Craton, the Maud belt into Africa on one side and into the Namaqua–Natal belt on the other (marked by magnetic anomalies, Golynsky and Jacobs, 2001), and the orogen into Dronning Maud Land to form the East African–Antarctic Orogen (Fig. 4). The Karoo volcanic rocks continued into DML and Coats Land; the (unrotated) Ellsworth–Whitmore Mountains block, part of the Gondwanide Fold Belt, into the (unrotated) Falkland Islands block; and the silicic volcanics of the Antarctic Peninsula into South America. The Lambert Rift continued into the Mahanadi Rift, and the Pranhita–Godavari Rift was aligned with the Robert Glacier. Geological trends in the Adélie Craton continued into the Gawler Craton in southern Australia. Farther along the Pacific margin, Zealandia connected with West Antarctica.
Golynsky et al. (2000) showed that the major magnetic (Beatty) anomaly of the Namaqua–Natal belt continues into the Falkland Plateau and western DML (Maud belt) (Fig. 4). Likewise, Finn et al. (2006) traced magnetic anomalies the Adélie Craton in Wilkes Land into the Gawler Craton in southern Australia (Fig. 5).
Pb-isotope analyses of K-feldspar from Archaean to Mesozoic igneous and metamorphic rocks across the Weddell Sea region define (as a reflection of protolith age) five distinct basement provinces (Flowerdew et al., 2012) (Fig. 6). The spread of Pb-isotope compositions within and between the provinces is sufficiently large that provenance studies utilising in situ Pb-isotopic measurements of detrital K-feldspar is possible and particularly appropriate for distinguishing sources from West and East Antarctica. A Permian sandstone in the Theron Mountains has its K-feldspar entirely derived from the active continental margin within West Antarctica whereas Permian and Jurassic sandstones from DML and a sand at the base of an ice core at Berkner Island have an East Antarctic source.
The K-feldspar Pb-isotope data (Flowerdew et al., 2012) indicate that the sandstones are derived from local provenances. Exceptions are K-feldspars derived from “a low μ Pb isotopic reservoir” that resembles the crust from the southern Prince Charles Mountains, which is dated 3.2 to 1.6 Ma (Veevers and Saeed, 2008, p. 25), and ~ 2.1 Ga crust indicated by an unradiometric group that lies on an ~ 2.1 Ga Pb–Pb isochron.
Roy et al. (2007) dated > 0.150 mm detrital hornblende grains and determined TDM Nd model ages and εNd values of bulk < 0.063 mm mud in circum-Antarctic sediment (Fig. 7). It is sufficient here to say that the < 350 Ma Ar–Ar ages and < 1.3 Ga TDM model ages of samples west of the Antarctic Peninsula reflect West Antarctica, and the > 440 Ma (with minor 1240–1100 Ma) and > 1.6 Ga TDM ages east of the Antarctic Peninsula reflect East Antarctica. Detailed comparison with bedrock ages will be made in the analysis of the individual sectors.
According to Peters and Hollister (1976, p. 291), “Modal analysis of heavy mineral suites from the four sites of Leg 35 [322, 323, 324, and 325] has revealed pyroxenes and amphiboles to be the dominant species. The mineralogies of sediments from the sites reflect their source rock type, Sites 322 and 325 having a common source of Upper Cretaceous (or possibly younger) intrusives from the Antarctic Peninsula, and Sites 323 and 324 deriving their terrigenous material from older metamorphics and older Middle Jurassic intrusives from the area of the Eights Coast [Fig. 8], and younger volcanics from the Jones Mountains, or Peter I Island.” Peter I Island (Quilty, 2007) (Fig. 1) is built of basalt and trachyandesite lavas less than 330 ka old. Further clues are provided by erratics and ice-rafted debris, described below.
According to Barker et al. (1988, p. 458), at ODP 694 in the Weddell Sea, the lithic grains are dominated by sedimentary rocks with a few volcanic and plutonic rocks. The most likely source areas are the southern Antarctic Peninsula [Palmer Land] and the catchment area of the Ronne Ice Shelf [Ellsworth–Pensacola Mountains]. The occurrence of hornblende and fresh plagioclase in the sands suggests a source area with some volcanic rocks. As detailed below, ice-rafted dropstones in Plio–Pleistocene sediment are identified as a pyroxene–amphibole gneiss, and in the Miocene sediment as biotite hornfels, sillimanite schist, amphibole–biotite gneiss, and pyroxene tholeiite. The lithology of volcanic and metamorphosed sedimentary rocks suggests the Antarctic Peninsula as the source area.
Clay-mineral assemblages in surface sediments in the Amundsen Sea (240°E to 265°E, G′–H sector) indicate various provenances (Ehrmann et al., 2011). “The most striking feature in the present-day clay mineral distribution is the high concentration of kaolinite, which is mainly supplied by the Thwaites Glacier system [255°E] and indicates the presence of hitherto unknown kaolinite-bearing sedimentary strata in the hinterland, probably in the Byrd Subglacial Basin …. Smectite originates from the erosion of volcanic rocks in Ellsworth Land [265°E] and western Marie Byrd Land [230°E]”.
Because of the central position of the GSM in East Antarctica and indeed Eastern Gondwanaland, drainage since the Permian to the present day has been outwards. In the Permian, Africa, in a distal downslope position from the proximal DML, received detritus from the GSM provenance (Veevers and Saeed, 2007). From this long-lasting configuration, Antarctica itself is the exclusive potential provenance of sediment in the region.
Section snippets
Provenance studies of detrital zircons in Neogene turbidites shed from Antarctica
Ice carries material plucked from Antarctic bedrock and unconsolidated material to the coast and offshore to be deposited as till, and funnelled through canyons in the slope to turbidite fans on the continental rise (e.g., O'Brien et al., 2005). Unlike contourites, which are prone to be displaced laterally by bottom currents, or ice-rafted debris (IRD), the sediment in turbidites is carried directly from bedrock in the source drainage basin via the continental slope to the abyssal plain. For
Erratics, dropstones, and recycled macrofossils
Subglacial Antarctica is further revealed by studies of erratics, dropstones, and recycled macrofossils in and around Antarctica. The areas on either side of the Antarctic Peninsula contain abundant plutonic and metamorphic clasts that indicate basement provenances, and clasts of Archaeocyathan limestone and reddish-brown sandstone that indicate sedimentary provenances.
Recycled microfossils
Recycled microfossils indicate younger provenances.
Drainage basins
The six kinds of evidence — from ice-rafted pebbles (labelled p1, etc.), detrital zircons (z1) and hornblendes (h1), Pb isotopes (Pb1), erratics (e1), and microfossils (m1) — are now brought together for each major drainage region shown in Fig. 21.
Discussion and conclusion
In Antarctica, since at least the earliest Permian, detritus has been dispersed northward from the central uplift of the GSM–VSH; lesser amounts of detritus have been shed from the convergent Pacific margin throughout the Palaeozoic and Mesozoic. This pattern of dispersal was modified in the Cretaceous and Cenozoic by uplift of the TAM.
Studies of detritus — pebbles, detrital zircons and hornblendes — Pb isotopes of K-feldspars, erratics, and microfossils in the 240°E–0°–015°E sector of
Acknowledgements
We thank M.J. Flowerdew, P.E. O'Brien, and E. M. Truswell for help with references, A.V. Golynsky and P.E. O'Brien for bedrock elevation data, and M.J. Flowerdew for supplying Excel files of analyses of detrital zircons from West Antarctica. Samples of DSDP and ODP cores were provided from the repositories of the IODP. We are indebted to M.J. Flowerdew, G.L. Leitchenkov, and P.G. Quilty for their insightful reviews. This study is supported by ARC DP0344841 and grants from Macquarie University.
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