R. V. DINGLE1, J. M. McARTHUR2 & P. VROON3
Abstract: Strontium isotope stratigraphy is used to date two interglacial-marine deposits in the Antarctic Peninsula region. On King George Island, interglacial pectinid-rich sediments in the Low Head Member of the Polonez Cove Formation give a strontium isotope stratigraphy age of 29.0^sup 0.7,0.6^ to 29.8^sup +0.8,0.7^ Ma (mid-Oligocene), which, in conjunction with previous K-Ar dating of volcanic rocks, indicates a glacial episode in the Antarctic Peninsula between middle Eocene (42.0 +/- 1.0 Ma) and mid-Oligocene time. In addition, an inter-glacial deposit (Pecten Conglomerate) from tectonically-elevated exposures on Cockburn Island is dated as Pliocene (3.5-5.3 Ma). Published data suggest these latter sediments were deposited under shallow marine conditions, which were warmer than those of present-day Antarctica.
Keywords: Oligocene, Pliocene, Antarctic Peninsula, 67Sr/86Sr, interglacial environments.
Antarctic ice-caps play an important role in regulating the Earth's climate, and establishing their history is crucial to understanding both regional and global Cenozoic climate changes. Although considerable progress has been made in documenting regional glaciations in Antarctica by determining seawater palaeotemperatures (e.g. Zachos et al. 1994) and identifying ice-rafted horizons in deep-water sediments of the Southern Ocean (e.g. Ehrmann & Mackensen 1992), glacial events must be equated ultimately to large-scale ice build-up in specific areas and related to identified tectonic regimes. To achieve this, accurate dating and the establishment of the depositional environments of stratigraphic sequences recording climate changes must be carried out, especially for those deposits containing glacial sediments. To this end, we have applied Sr isotope stratigraphy (for reviews see Veizer 1989; McArthur 1994) to determine ages (timescales of Cande & Kent 1995; Shackleton et al. 1995) of two critical sequences in the Antarctic Peninsula; specifically, an unequivocal Pliocene age for the Pecten Conglomerate on Cockburn Island, and an equally unequivocal mid-Oligocene age for interglacial-marine strata of the Polonez Cove Formation, on King George Island (Figs 1 & 2). The former bears on the question of possible Pliocene interglacials, and the latter on the age of the possible first extensive West Antarctic ice-sheet.
Timing of glaciation of the Antarctic Peninsula
Polish workers (e.g. Birkenmajer 1990) have recognized four glacial episodes in the strata of the South Shetland Islands: Krakow (Eocene, 52-50 Ma); Polonez (Oligocene, 32 30 Ma); Legru (Oligocene, 30-26 Ma) and Melville (Miocene, 2220 Ma) (Fig. 2). The first represents the earliest Cenozoic glacial event so far postulated from Antarctica, possibly contemporaneous with the late Eocene ice-rafted debris from East Antarctica (e.g. Barrett 1989). As it has been suggested that the Polonez episode documents the first extensive glaciation in West Antarctica (Birkenmajer 1990), it is important to determine its age with accuracy.
Elsewhere in the Antarctic Peninsula, pre-Quaternary glacial deposits are poorly known, and limited to the vicinity of James Ross Island (Fig. 2). Possible ice-rafted pebbles have been reported in the upper La Meseta Formation of Seymour Island (late Eocene ?early Oligocene: Gazdzicki et al. 1992), while a Neogene age has been postulated for the glacio-marine Weddell Formation (unconformably overlying the La Meseta Formation on Seymour Island: Zinsmeister & de Vries 1983; Porebski 1995). Further, scattered outcrops of glacio-marine sedimentary rocks of an un-named formation lie between James Ross Island Volcanic Group strata and Cretaceous sedimentary rocks on James Ross Island (M.R.A. Thomson pers. comm. 1994; D. Pirrie pers. comm. 1995). The only relatively well-known sequence occurs on Cockburn Island, where a coarse, pebbly, shelly …