Transantarctic Mountains

Outcrops of glacigenic sedimentary rocks of the Sirius Group occur as erosional remnants scattered throughout the Transantarctic Mountains as far south as 86°S (Fig. 10.3). The deposits are exposed in two typical settings: (1) as thin erosional remnants at high elevation in palaeovalleys or on flat mountain summits, or (2) as sequences more than 100 m thick along the walls of broad trunk valleys occupied by large outlet glaciers draining the modern EAIS. The main sedimentary facies are massive diamicts, massive boulder gravels, stratified pebbly sands and muds, and laminated sand/mud couplets with dropstones (Fig. 10.3). Diamicts overlie spectacular grooved and striated pavements in some areas, notably at Roberts Massif. In the Dominion Range, bordering Beardmore Glacier, well-preserved Nothofagus leaves, stems and roots, plus mosses and insects are interbedded with the diamict (Francis and Hill, 1996; Ashworth et al., 1997). The diamicts are dominated by subangular to subrounded striated and facetted clasts. The environment of deposition is interpreted as largely subglacial to proglacial, with extensive wet-based, probably polythermal glaciers being the agent of sediment transport, although a marine influence is evident through marine fossil remains in part of the Beardmore Glacier succession (e.g. Mercer, 1972; Mayewski and Goldthwait, 1985; McKelvey et al., 1991; Webb et al., 1996; Stroeven and Prentice, 1997; Wilson et al., 1998, 2002; Hambrey et al., 2003). The lithostratigraphy and depositional setting of the Sirius Group at two of the southernmost locations is summarized in Table 10.1.

A debate was ignited when Webb et al. (1984), based on recycled marine diatoms, linked the Sirius Group to glacial expansion after early Pliocene deglaciation and marine incursion of East Antarctica. In contrast, Denton et al. (1984) attributed the Sirius Group to mid-Miocene overriding of the Transantarctic Mountains based on geomorphological evidence from the Dry Valleys. A dynamic ice-sheet hypothesis and a stable ice-sheet hypothesis developed, which represented contrasting views of Neogene Antarctic climate and glacial dynamics. Pivotal to this debate is our understanding of the uplift and erosional, hence unroofing, history of the Transantarctic Mountains (e.g. Webb et al., 1984; Behrendt and Cooper, 1991; Sugden et al., 1995; Kerr and Huybrechts, 1999). The Transantarctic Mountains are divided into several crustal blocks which vary in size on a range of scales and which probably experienced uplift at different rates and

Figure 10.3: Glacial erosional features and sedimentary characteristics of the Sirius Group, Transantarctic Mountains. (A) Aerial view of sub-Sirius Group glacially abraded surface on Roberts Massif, showing patch of diamict, post-depositional faults, and a superimposed recessional quaternary moraine system. (B) Multiple beds of diamict comprising the > 100 m thick Shackleton Glacier formation at the type locality alongside upper Shackleton Glacier. (C) Large-Scale Glacial Grooves on Jurassic Dolerite, predating deposition of the Sirius Group. (D) Striated dolerite surface with overlying Sirius Group diamict, Roberts Massif. (E) Typical massive diamict facies of the Sirius Group, showing alignment of clasts, interpreted as basal till, Roberts Massif. (F) Laminated silt and thin diamicts with large dropstone, interpreted as an ice-contact lake deposit in the Shackleton Glacier formation, Bennett Platform (redrawn from Hambrey et al., 2003).

Figure 10.3: Glacial erosional features and sedimentary characteristics of the Sirius Group, Transantarctic Mountains. (A) Aerial view of sub-Sirius Group glacially abraded surface on Roberts Massif, showing patch of diamict, post-depositional faults, and a superimposed recessional quaternary moraine system. (B) Multiple beds of diamict comprising the > 100 m thick Shackleton Glacier formation at the type locality alongside upper Shackleton Glacier. (C) Large-Scale Glacial Grooves on Jurassic Dolerite, predating deposition of the Sirius Group. (D) Striated dolerite surface with overlying Sirius Group diamict, Roberts Massif. (E) Typical massive diamict facies of the Sirius Group, showing alignment of clasts, interpreted as basal till, Roberts Massif. (F) Laminated silt and thin diamicts with large dropstone, interpreted as an ice-contact lake deposit in the Shackleton Glacier formation, Bennett Platform (redrawn from Hambrey et al., 2003).

Table 10.1: Representative stratigraphy of the Sirius and Pagodroma Groups with principal lithofacies and interpretation of palaeoenvironment.

Table 10.1: Representative stratigraphy of the Sirius and Pagodroma Groups with principal lithofacies and interpretation of palaeoenvironment.

Group

Formations

Principal facies

Paleoenvironment

Shackleton Glacier Beardmore Glacier

Bennett Platform Meyer Desert (Pliocene)

Massive and weakly stratified diamict,

Subglacial; some

Shackleton Glacier Cloudmaker

(Pliocene-?Miocene)

massive boulder gravel, stratified sandstone, breccia; sand/ mud laminate with dropstones at Shackleton Glacier. Well preserved Nothofagus flora at Beardmore Glacier. All facies well

supraglacial; proglacial glaciofluvial; ice contact lake

Sirius

indurated

Eroded remnants as

Massive and stratified diamictite; sandy

Subglacial with extensive

erratics

breccia/conglomerate; partially silicified wood fragments; well lithified

mass-movement and fluvial reworking; well-developed flora

Shackleton erosion surface/dominion erosion surface

Group

Formations

Principal facies

Paleoenvironment

Amery Oasis Fisher Massif

Bardin Bluffs

Pago-droma

(Pliocene-?Pleistocene)

Battye Glacier Fisher Bench (mid-Miocene) (mid-Miocene)

Mount Johnston (?01igocene-early

Massive boulder gravel; massive diamict; gravel; laminate with dropstones. Rich diatom microflora. All weakly to well indurated

Ice-proximal fjordal, grounding-line fan complexes predominant, to distal glaciomarine

Miocene)

Ages of formations are given where determined using diatom biostratigraphy. No correlation is implied in the Sirius Group. Data from Webb et al. (1996) for the Sirius Group at Beardmore Glacier; Hambrey et al. (2003) for the Sirius Group at Shackleton Glacier; Hambrey and McKelvey (2000a) for the Pagodroma Group.

Miocene)

Ages of formations are given where determined using diatom biostratigraphy. No correlation is implied in the Sirius Group. Data from Webb et al. (1996) for the Sirius Group at Beardmore Glacier; Hambrey et al. (2003) for the Sirius Group at Shackleton Glacier; Hambrey and McKelvey (2000a) for the Pagodroma Group.

during different periods, principally in the late Cretaceous and early Tertiary (prior to c. 40 Ma; Stump and Fitzgerald, 1992; Fitzgerald, 1994; Bussetti et al., 1999; van der Wateren et al., 1999). The uplift histories of adjacent blocks might also be different. The younger uplift and erosional history is only relatively well known for the Dry Valleys block, as a result of detailed detrital studies by the Cape Roberts Project (CRP). Those studies showed that at the deepest levels cored, equivalent to c. 34 Ma, the kilometre-thick Kirkpatrick Basalts (Jurassic) had already been almost completely removed and the sand modes were dominantly derived from sandstones of the Victoria Group, then the Taylor Group (Devonian-Triassic Beacon Supergroup; Smellie, 2000a,b, 2001a,b, unpublished; Talarico et al., 2000; Sandroni and Talarico, 2001). By c. 33Ma, the 2km-thick Beacon Supergroup had been cut through to expose outcrops of basement rock (early Palaeozoic and Precambrian granitoids and metamorphic rocks), which then began to contribute significant detritus. Between that time and c. 29 Ma, tectonic stability and, presumably, little uplift-related erosion are inferred from the essentially unchanging detrital modes. Further changes in the detrital modes between 24 and 16 Ma suggest another phase of instability and possibly uplift/unroofing, after which the record is not preserved. Although the detrital record is interpreted here in terms of simple uplift and unroofing, an alternative explanation is that the changes observed might reflect varied phases of climate-related erosion independent of uplift (e.g. enhanced downcutting by glaciers; cf. Kerr and Huybrechts, 1999). This is an ambiguity that has yet to be resolved.

According to the dynamic ice-sheet hypothesis, the diatom assemblages incorporated in the Sirius Group record periods when East Antarctic basins were ice-free and became inundated by the sea (Harwood, 1986; Harwood and Webb, 1998). The time intervals of which diatoms are lacking record the stages when either ice was covering the inland basins or the floors of the basins were exposed. According to stabilists, however, the basic assumption derived from ice-sheet modelling is that deglaciation requires considerable climatic warming (Huybrechts, 1993). A substantial body of internally consistent evidence (including ash and cosmogenic dating) for a pre-middle Miocene landscape, subsequently unmodified by ice, has been published in numerous papers (e.g. Marchant et al., 1993a,b,c, 1996; Sugden et al., 1995; Sugden, 1996; Sugden and Denton, 2004). For example, isotopic ages of > 14 Ma for the in situ, unweathered, ash deposits from the higher elevated regions of the Dry Valleys (Marchant et al., 1993b) are in disagreement with the concept of warming causing major deglaciation in the early-mid-Pliocene. The diatom evidence for a Pliocene age of the Sirius Group has been disputed (Barrett, 1996; Gersonde et al., 1997; Stroeven et al., 1998a,b).

Field studies and studies of aerial photographs provided evidence of rejuvenation of faulting during and after deposition of the Sirius Group (Hambrey et al., 2003). Moreover, based on morphostratigraphic constraints and provenance analyses, several authors pointed out that the Sirius Group comprises deposits of multiple glaciations within different wet-based icesheet drainage systems, which operated during consecutive stages of glacial denudation in the Transantarctic Mountains (e.g. Mercer, 1968, 1972; Brady and McKelvey, 1979, 1983; McKelvey et al., 1991; Stroeven, 1997; Van der Wateren et al., 1999; Passchier, 2001, 2004; Hambrey et al., 2003). Unfortunately, the absolute ages of the individual formations within the Sirius Group remain inconclusive and lithostratigraphic correlations are hampered by the complex tectonic framework of the Transantarctic Mountains. Given the considerable linear geographical spread of the Sirius Group over a distance of c. 1,500 km and spanning nearly 10° of latitude, the grouping of these strata under a single group name has exacerbated the arguments concerning the age. Resolution of the age question remains a key challenge for Antarctic geologists, but it is likely that, although the Sirius Group may contain disputed Pliocene or Miocene elements, it could also extend back to Oligocene time in view of the presence of glaciomarine sediments of this age offshore (Barrett, 1996; Hambrey et al., 2002; Francis et al., 2008).

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