DEPOSITIONAL ENVIRONMENT OF SIRIUS GROUP SEDIMENTS D DEPOSITIONAL ENVIRONMENT OF SIRIUS E GROUP SEDIMENTS, TABLE MOUNTAIN, DRY VALLEYS AREA, ANTARCTICA JAM BY ES R. JAMES R. GOFF1, IAN W. JENNINGS2 AND WARREN W. DICKINSON2 GOF F, IAN 1GeoEnvironmental Consultants, Lyttelton, New Zealand W. 2Research School of Earth Sciences, Victoria University of Wellington, JEN Wellington, New Zealand Goff, J.R., Jennings, I.W. and Dickinson, W.W., 2002: Deposition- In the past few years, Sirius Group deposits have al environment of Sirius Group sediments, Table Mountain, Dry been at the centre of a debate concerning the rela- Valleys area, Antarctica. Geogr. Ann., 84 A (1): 11–24. tive stability of the East Antarctic Ice Sheet (EAIS) ABSTRACT. Outcrops and cores of the Sirius Group sediments since the Miocene (Barrett 1997; Miller and Mabin were studied at Table Mountain, Dry Valleys area, Antarctica. 1998). The debate has largely focussed on the two These sediments form a surficial veneer at least 9.5 m thick. Three opposing views of the ‘stabilist’ approach (March- facies – a gravelly sandstone, a sandstone, and a sandy conglom- erate – are mapped and described from 13 outcrops and three ant et al. 1993; Denton et al. 1993), and the ‘dy- cores. The gravelly sandstone, constituting 13% of all cored ma- namicist’ argument (Webb et al. 1984). Consider- terial, is bimodal with matrix-supported clasts comprising 5–33% able importance has been placed on the presence or of the facies. Fabric analysis indicates that it was deposited pri- lack of Pliocene diatoms in near-surface Sirius marily by lodgment from glacial ice but with minor elements of meltout and flow. The sandstone facies, constituting 77% of all Group deposits and their mode of deposition (Bar- cored material, is a well-sorted, fine- to medium-grained sand, rett et al. 1992; Webb and Harwood 1991; Burkle which commonly has laminated bedding. It is predominantly a and Potter 1996; Barrett 1997). If marine diatoms, glaciofluvial deposit but has some glaciolacustrine elements. The found in the Sirius Group, are attributed to aeolian sandy conglomerate, constituting 10% of all cored material, is a minor facies. It is massive and clast-supported. It was deposited deposition and subsequent recycling (e.g. Stroeven in a high-energy environment suggestive of subglacial meltwater and Prentice 1997), they are unlikely to represent channels. the true age of the deposits. Furthermore, both Sirius Group sediments at Table Mountain are the result of wet- non-marine and marine diatom assemblages found based ice advancing and retreating over waterlain deposits. This is consistent with an advancing ice mass in climatic conditions in the Sirius Group may represent numerous that were warmer than present. The majority of the sediments palaeoenvironmental conditions. were deposited by alpine ice following a similar pathway to the The age of the Sirius Group deposits on Table present-day Ferrar Glacier and as such the depositional environ- Mountain has been estimated by several means. ment is one that concurs with evidence of a stable East Antarctic Ice Sheet approach. At Table Mountain, the predominantly gla- Barrett and Powell (1982) identified three units of ciofluvial and glaciolacustrine facies is inferred to represent a glacigenic sediment: diamictite, conglomerate, and more distal part of the Sirius Group environment than that seen at sandstone. Based upon regional geology, they sug- other outcrops in the Dry Valleys. gested these were probably older than late Mi- ocene. Minimum ages of 2.6–2.9 Ma and 6 Ma were established for exposed surfaces at Table Introduction Mountain using 10Be and 3He/21Ne respectively, The Sirius Group is a series of glacigenic sediments and it is likely that the deposits are indeed much that are found mostly at elevations greater than older (Ivy-Ochs et al. 1995; Bruno et al. 1997). 1600 m in over 40 locations throughout the There are few comprehensive lithostratigraphic Transantarctic Mountains (TAM) (Mercer 1972; studies of Sirius Group deposits in the Dry Valleys Mayewski 1975; Barrett and Powell 1982; area and those that are available relate either to McKelvey et al. 1991). They have been generally provenance (Faure and Taylor 1981; Taylor and described as a compact glacial drift that uncon- Faure 1983) or to a broad-brush interpretation of formably covers pre-Tertiary rocks (Mercer 1972). glacial geology (Mayewski 1975; Barrett and Pow- Geografiska Annaler · 84 A (2002) · 1 11 JAMES R. GOFF, IAN W. JENNINGS AND WARREN W. DICKINSON Fig. 1. (A). Study area: Table Mountain, Transantarctic Mountains, Antarctica (tg = Tedrow Glacier, eg = Emmanuel Glacier). (B). Geology of Table Mountain: detail shows locations of Sirius Group deposits and study sites (transect A–B is shown in Fig. 5, mm f = mass movement feature). Parallel cores were taken from sites TM-1 (1, 1B, 1C) and TM-7 (7A, 7B). ell 1982). The most comprehensive work on the been relatively stable during warm periods in the glacial geology of the Sirius Group was reported by Pliocene (Stroeven and Prentice 1997). Stroeven and Prentice (1997) who addressed the is- At Table Mountain the Sirius Group forms a ve- sue of alpine versus ice sheet glaciation, an issue neer of sediment, deposited on a gentle slope that which is linked to the stabilist–dynamicist debate. formed the bottom of the ancestral Ferrar Valley. Stroeven and Prentice (1997) refute the dynamicist Today the Sirius Group at Table Mountain is theory that the EAIS was much reduced in area and perched about 1000 m above the present surface of volume during the Pliocene by producing an alpine the Ferrar Glacier, at an elevation of 2000 m. In this ice interpretation for Sirius Group deposits at paper we investigate the depositional environment Mount Fleming. They suggest that the Sirius Group of the Sirius Group at Table Mountain using surface deposits at Mount Fleming were not laid down by exposures and cores. expansion of the EAIS, which is assumed to have 12 Geografiska Annaler · 84 A (2002) · 1 DEPOSITIONAL ENVIRONMENT OF SIRIUS GROUP SEDIMENTS (A) (B) (C) Fig. 2. (A) Site 11.1. Sharp and un- dulated lower contact between grav- elly sandstone facies and Terra Cot- ta Siltstone (hammer for scale). (B) Sandstone outcrop at site 6.34 (per- son for scale). (C) Sandy conglom- erate at site 6.28 (notebook for scale). Physical setting Mountain is early Palaeozoic granite which is un- Table Mountain is located on the southern side of conformably overlain by Devonian sediments of the Ferrar Glacier and is bounded to the southwest the Beacon Supergroup comprising the continental by the Tedrow Glacier, and to the east by the Em- sequence of Windy Gully Sandstone, Terra Cotta manuel Glacier (Fig. 1A). Basement rock at Table Siltstone, and New Mountain Sandstone (Fig. 1B). Geografiska Annaler · 84 A (2002) · 1 13 JAMES R. GOFF, IAN W. JENNINGS AND WARREN W. DICKINSON GRAVEL Conglomerate 80% SANDY CONGLOMERATE FACIES TM-1C Sandy conglomerate TM-6 TM-7B OUTCROPS McMurdo Ice Shelf 30% Taylor Glacier CIROS-1 Gravelly sandstone GRAVELLY SANDSTONE FACIES 5% 1% Sandstone Fig. 3. Ternary plot of lithostrati- 9:1 1:1 1:9 graphic facies including core and MUD SANDSTONE SAND FACIES outcrop samples. Table 1a. Fabric data from major outcrops: summary of field data and directional information Lithology: (no./no. striae top/no. striae base/no. keels) No. Eigenvector Mean Site clasts (degrees) S1 Roundness Sphericity a-axis D SS Q QA G 1.0 54 217 0.75 0.55 0.59 17.8 29/0/0/9 16/8/8/4 4/1/1/0 5/0/0/0 0 TM-1a 51 290 0.46 0.57 0.63 11.6 21/0/2/0 16/5/9/3 4/0/1/1 10/1/1/2 0 TM-1c 52 272 0.58 0.58 0.64 29.2 37/0/0/0 8/1/1/1 3/0/0/0 4/0/0/0 0 TM-1e 51 279 0.69 0.58 0.63 29.8 29/6/7/5 8/2/3/2 3/0/2/2 11/2/4/4 0 J2 51 290 0.60 0.57 0.62 20.6 36/1/1/14 9/3/4/3 3/0/0/0 3/1/1/0 0 J3 51 270 0.52 0.58 0.57 34.6 39/0/0/3 8/6/6/6 0 4/0/0/1 0 TM-6a 51 282 0.59 0.58 0.56 20.5 34/6/7/4 13/4/5/1 2/0/0/0 2/0/0/0 0 TM-6b 51 282 0.91 0.59 0.58 25.5 23/5/5/5 20/8/7/6 7/4/0/0 1/1/0/0 0 TM-6c 51 280 0.90 0.59 0.53 21.2 27/0/4/8 14/0/4/7 6/0/2/0 4/0/1/2 0 TM-6d 51 280 0.90 0.58 0.55 21.2 26/0/0/9 14/0/0/4 6/0/0/0 5/0/0/0 0 TM-7a 51 284 0.80 0.59 0.56 18.8 37/8/11/9 9/3/3/4 3/0/0/1 1/0/0/0 1/0/0/0 TM-7b 51 285 0.89 0.59 0.56 15.8 41/13/17/6 5/4/3/1 3/1/1/0 2/0/0/0 0 J6 51 281 0.79 0.58 0.55 17.0 32/13/15/13 12/6/6/6 1/0/0/0 2/0/2/1 4/2/3/0 6.34 51 277 0.73 0.60 0.54 13.8 44/8/8/7 1/1/1/0 2/0/0/0 4/1/1/2 0 11.1a 51 259 0.94 0.56 0.56 14.2 6/0/0/2 45/0/0/4 0 0 0 11.1b 51 281 0.80 0.59 0.54 12.2 39/17/18/3 8/4/5/0 3/3/3/1 1/1/1/0 0 12.17 51 280 0.74 0.59 0.55 19.8 32/15/15/6 7/3/3/2 4/2/2/0 3/2/2/2 5/1/1/1 J11 43 221 0.75 0.52 0.57 18.2 0 0 2/0/0/0 41/1/4/3 0 J12 43 233 0.77 0.52 0.64 16.9 0 17/0/4/2 3/0/0/1 23/0/0/1 0 Refer to Fig.