The Palaeogene back-arc deposits exposed on Seymour Island and Cockburn Island comprise more than 1,000 m of shallow marine to coastal fossiliferous clastic sedimentary rocks mainly of Palaeocene and Eocene age (Elliot, 1988; Sadler, 1988; Marenssi et al., 1998). The Maastrichtian-Danian sequence forms a simple ~N-S homocline dipping gently to the east while the Late Palaeocene and the Eocene sediments fill incised valleys trending NW-SE.
The Early Eocene to latest Late Eocene La Meseta Formation (Elliot and Trautman, 1982) is an unconformity-bounded unit (La Meseta Alloformation of Marenssi et al., 1996, 1998) of maximum composite thickness of 720 m, which fills a 7 km wide valley cut down into older strata after the regional uplift and tilting of the Palaeocene and Cretaceous older beds.
The La Meseta Formation comprises mostly poorly consolidated siliciclastic fine-grained sediments deposited in deltaic, estuarine and shallow marine environments as part of a tectonically controlled incised valley system (Marenssi, 1995; Marenssi et al., 2002). The richly fossiliferous Eocene sediments have yielded the only fossils of land mammals in the whole Antarctic continent (Reguero et al., 2002) along with fossil wood, fossil leaves, a rare flower, plus marine vertebrates (including giant penguins) and invertebrates (Stilwell and Zinsmeister, 1992; Gandolfo et al., 1998a,b,c; Francis et al., 2004; Tambussi et al., 2006).
Overall, evidence from fossils, sediments and geochemistry from Seymour Island indicates generally warm and ice-free conditions during the earliest part of the Eocene but followed by gradual cooling. Dingle et al. (1998), based on chemical analysis (chemical index of alteration, CIA) and clay mineralogy, recorded a climatic deterioration from warm, non-seasonally wet conditions during the early Middle Eocene (smectite-dominated clay assemblage and CIA values <0.7 to >0.6) to a latest Eocene cold, frost-prone and relatively dry regime (illite-dominated clay association and CIA
values <0.6). Gazdzicki et al. (1992) showed a 6 m decrease in d13C values in biogenic carbonates and Tatur et al. (2006) recorded an increase in the Cd/Ca ratio in bivalve shells from the upper part of the La Meseta Formation. They interpreted these results as a change from stratified to vigorously mixed oceanic conditions related to the cryosphere development in the Southern Ocean by the end of the Eocene. Unpublished stable isotope (81SO and 813C) measurements (Feldmann and Marenssi) obtained from molluscan shells of the La Meseta Formation show a period of warmer seawater temperatures during 51-47 Ma and a drop of 1.5°C during 35-34 Ma. Further, Ivany et al. (2008), also undertaking isotopic analysis of bivalves from the La Meseta Formation, propose the possibility of winter sea-ice formation in this region during the late Middle Eocene (c. 37 Ma). They suggest mean ocean temperatures have approached freezing at this time by assuming a sea water 818O value of —3m as predicted for the east Antarctic Peninsula by Huber et al. (2003) in their early Eocene climate model. This information is consistent with ice-rafted debris reported for the upper part of the La Meseta Formation (Doktor et al., 1988) and palaeoclimatic evidence of a severe climatic deterioration towards the end of the Eocene. It is possible that by the end of the Eocene, limited ice, perhaps as valley glaciers, was already present in the area.
By the end of the Eocene, it is possible that an ice sheet extended over much of the peninsula, although average Seymour Island Shelf temperatures did not reach below zero (Ivany et al., 2008). Ivany et al. (2006) reported 5-6 m thick glacial deposits that conformably overlie the typical marine sandstones of the La Meseta Formation but are beneath the glacial diamictites of the younger Weddell Sea Formation. Based on dinocyst stratigraphy and strontium isotopes, these authors suggested an age of 33.57-34.78 Ma for these glaciomarine deposits (supported by marine Sr dates from Dingle and Lavelle, 1998; Dingle et al., 1998; Dutton et al., 2002), at or very close to the E/O boundary. There is therefore a small but intriguing window into the early stages of the icehouse world in the James Ross Basin.
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