Abstract:
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 Bel?n,
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 Belen 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. 
