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Age and environment of Miocene–Pliocene glaciomarine deposits, James Ross Island, Antarctica

H. A. JONKERS^*,*, J. M. LIRIO, R. A. DEL VALLE and S. P. KELLEY

^* British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
Instituto Antártico Argentino, Cerrito 1248, Buenos Aires 1010, Argentina
Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK

^* Author for correspondence; present address: Dommerswijk 10, 7782 PA De Krim, The Netherlands

Knowledge of the late Miocene–Pliocene climate of West Antarctica, recorded by sedimentary units within the James Ross Island Volcanic Group, is still fragmentary. Late Miocene glaciomarine deposits at the base of the group in eastern James Ross Island (Hobbs Glacier Formation) and Late Pliocene (3 Ma) interglacial strata at its local top on Cockburn Island (Cockburn Island Formation) have been studied extensively, but other Neogene sedimentary rocks on James Ross Island have thus far not been considered in great detail. Here, we document two further occurrences of glaciomarine strata, included in an expanded Hobbs Glacier Formation, which demonstrate the stratigraphic complexity of the James Ross Island Volcanic Group: reworked diamictites intercalated within the volcanic sequence at Fiordo Belén, northern James Ross Island, are dated by ⁴⁰Ar/³⁹Ar and ⁸⁷Sr/⁸⁶Sr at c. 7 Ma (Late Miocene), but massive diamictites which underlie volcanic rocks near Cape Gage, on eastern James Ross Island, yielded an Ar–Ar age of < 3.1 Ma (Late Pliocene). These age assignments are confirmed by benthic foraminiferal index species of the genus Ammoelphidiella. The geological setting and Cassidulina-dominated foraminiferal biofacies of the rocks at Fiordo Belén suggest deposition in water depths of 150–200 m. The periglacial deposits and waterlain tills at Cape Gage were deposited at shallower depths (< 100 m), as indicated by an abundance of the pectinid bivalve ‘Zygochlamys’ anderssoni and the epibiotic foram Cibicides lobatulus. Macrofaunal and foraminiferal biofacies of glaciomarine and interglacial deposits share many similarities, which suggests that temperature is not the dominant factor in the distribution of late Neogene Antarctic biota. Approximately 10 m.y. of Miocene–Pliocene climatic record is preserved within the rock sequence of the James Ross Island Volcanic Group. Prevailing glacial conditions were punctuated by interglacial conditions around 3 Ma.

Key Words: Antarctica • stratigraphy • Ar/Ar • micropalaeontology • palaeoclimatology

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