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.
Antarctic late Cenozoic pectinid-bearing sedimentary strata are chiefly confined to localities in the northern part of the Antarctic Peninsula, in the McMurdo Sound area, and Marine Plain, East Antarctica. Ages of these deposits range from Oligocene to Holocene. Chlamys-like scallops, which are absent from today's Southern Ocean, thrived in Antarctic waters during both glacial and interglacial episodes, but disappeared during the Late Pliocene. Their extinction is believed to result from the combined effects of increased carbonate solubility, habitat loss and limitations in food availability, associated with major cooling.
Reservoir quality of the Fulmar Formation in the UK Central Graben depends on the original distribution of facies as well as subsequent modification by partly facies-dependant diagenesis. Numerous sequence stratigraphical analyses of the Humber Group have been published recently, each of which has elements which are convincing geologically. Nevertheless, it is our experience that facies and reservoir prediction within the Humber Group remain difficult. Sequence stratigraphical interpretations in the Group are clearly non-unique due to major uncertainties in the data. (1) Biostratigraphical uncertainties include species concepts, recognition of agediagnostic taxa in thermally altered floras, validity of repeated micropaleontological events and equivocal paleoenvironmental information. (2) Sedimentological uncertainties are due to the gross facies similarity of the Fulmar Formation over a large geographic area and throughout the Upper Jurassic, which make the detection of potential maximum flooding surfaces and especially sequence boundaries difficult even in many cored wells. Furthermore strong syndepositional subsidence variations due to both salt movement and tectonics, lead to aliased sampling of the subsurface in wells, which are mostly situated on highs. (3) Seismic correlation difficulties are due to persistent seismic imaging problems below the Late Cimmerian ('X') Unconformity. (4) The depositional and sequence stratigraphical models for Humber Group deposition need to be set in the context of a transgressively skewed, back-stepping system with strong local subsidence control. The overall transgression is thought to be due to the combination of sea-level rise, widening and deepening of the basin due to tectonics, and reduced sediment supply through time. In this situation existing models have to be applied with even more care than normal if they are not to be misleading (e.g. concepts of forced regressions). These difficulties are clarified in the context of a Kimmeridgian time slice through the Humber Group. Multiple options exist in well log correlations. As a result palaeogeo-g~aphic reconstructions of the time-slice have an error margin of + 15 km, which has serious implications on the validity of reservoir quality predictions. Recently published sequence stratigraphical analyses of the Humber Group include both Vail-type schemes (e.g. Donovan et al. 1993) and Galloway-type schemes (e.g. Partington et al. 1993a). The framework presented in this paper (Fig. 1) combines elements from both approaches. There is general agreement on the age of maximum flooding surfaces between Shell nomenclature and Partington et al. (1993a), although we have not been able to recognize all their surfaces in our datasets. Sequence boundaries indicated by Donovan et al. (1993) are all recognized by ourselves. Donovan et al. have tied their sequence boundaries to the global sea-level chart of Haq et al. (1988). However biostratigraphical datings indicate that at least some sequence boundaries found in the Humber Group have no ...
Boreholes in the large, extinct Antarctic Neogene pectinid Zygochlamys anderssoni suggest that this scallop was preyed upon by a large predatory muricid gastropod i possibly a Trophon species). The holes occur in mature individuals, which contrasts with the situation in modern Zygochlamys delicatula from New Zealand, where gastropod predation is apparently restricted to juveniles only. This difference is ascribed to dissimilarities in the lifestyles of these scallops; whereas the former was probably byssally attached throughout ontogeny, adults of the latter become tree living after an initial period of byssal attachment. During the late Pliocene, a change towards higher motility in Chlamys-like pectinids of the Southern Ocean may have caused the loss of an important food source for the larger muricids.
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