J.A. Crame

Campanian–Maastrichtian (Cretaceous) stratigraphy of the James Ross Island area, Antarctica

One of the most important outcrops of uppermost Cretaceous (Campanian–Maastrichtian) sedimentary rocks in the southern high latitudes occurs within the James Ross Island group, northeastern Antarctic Peninsula. These rocks comprise a 1500–2000 m thick sequence of predominantly shallow marine clastic sediments that were deposited within a retro-arc basin. They are virtually undeformed and have yielded prolific invertebrate and vertebrate faunas, and a wide range of plant taxa. Campanian–Maastrichtian strata are contained within two component formations of the Upper Cretaceous-lower Tertiary Marambio Group. The lower Santa Marta Formation is approximately 1100m thick and has three constituent members. Ammonites within the Santa Marta Formation indicate an early to late Campanian age assignment. Dinoflagellates suggest that the lower levels of the formation may range into the Santonian Stage. In the northern James Ross Island to Vega Island region there is a conformable transition into the overlying López de Bertodano Formation. Here the basal Cape Lamb Member is unconformably overlain by the Sandwich Bluff Member. Although there are some lithological and fauna1 discrepancies, which are attributed to marked lateral facies changes across the basin, it is believed that the Cape Lamb Member can be correlated with the lower to middle regions of the 1200 m thick sequence of undifferentiated López de Bertodano Formation lithologies exposed on Seymour Island. Molluscan data indicate a late Campanian–Palaeocene age range for this upper formation. Dinoflagellates again suggest that the lowest beds may be slightly older. The correlation presented provides the basis for a formal subdivision of the Campanian and Maastrichtian stages in the southern high latitudes. It has also revealed two major basin shallowing events (in the late Campanian–early Maastrichtian and latest Maastrichtian, respectively) and the existence of a major fault/fault zone across southeastern James Ross Island. The improved temporal framework will aid late Cretaceous palaeoclimatic and palaeobiological studies in Antarctica.

Lithostratigraphy of the Cretaceous Strata of West James Ross Island, Antarctica

As a result of recent field work, a new lithostratigraphic group is defined on the west coast of James Ross Island, Antarctica. Characterised by comparatively coarse-grained sediments, the Gustav Group is 2300 m thick and has an approximate age range of Barremian-Santonian. Four formations are recognised within the group (the Lagrelius Point, Kotick Point, Whisky Bay and Hidden Lake Formations), and of these, the Whisky Bay Formation is further subdivided into six local members. Type sections are erected for each of these stratigraphic units and representative lithologies and faunas described.

Definition of Late Cretaceous Stage Boundaries in Antarctica Using Strontium Isotope Stratigraphy

New 87Sr/86Sr analyses of macrofossils from 13 key marker horizons on James Ross and Vega Islands, Antarctica, allow the integration of the Antarctic Late Cretaceous succession into the standard biostratigraphic zonation schemes of the Northern Hemisphere. The 87Sr/86Sr data enable Late Cretaceous stage boundaries to be physically located with accuracy for the first time in a composite Southern Hemisphere reference section and so make the area one of global importance for documenting Late Cretaceous biotic evolution, particularly radiation and extinction events. The 87Sr/86Sr values allow the stage boundaries of the Turonian/Coniacian, Coniacian/Santonian, Santonian/Campanian, and Campanian/Maastrichtian, as well as other levels, to be correlated with both the United Kingdom and United States. These correlations show that current stratigraphic ages in Antarctica are too young by as much as a stage. Immediate implications of our new ages include the fact that Inoceramus madagascariensis, a useful fossil for regional austral correlation, is shown to be Turonian (probably Late Turonian) in age; the “Mytiloides” africanus species complex is exclusively Late Coniacian in age; both Baculites bailyi and Inoceramus cf. expansus have a Late Coniacian/Early Santonian age range; an important heteromorph ammonite assemblage comprising species of Eubostrychoceras, Pseudoxybeloceras, Ainoceras, and Ryugasella is confirmed as ranging from latest Coniacian to very earliest Campanian. An important new early angiosperm flora is shown to be unequivocally Coniacian in age. Our strontium isotopic recalibration of ages strengthens the suggestion that inoceramid bivalves became extinct at southern high latitudes much earlier than they did in the Northern Hemisphere and provides confirmation that, in Antarctica, belemnites did not persist beyond the Early Maastrichtian.

Late Cretaceous stratigraphy and sedimentology of Cape Lamb, Vega Island, Antarctica

Abstract An important new section for the latest Cretaceous marine sedimentary record is described from Cape Lamb, Vega Island, Antarctica. Some 480 m thick, it has been divided into three lithostratigraphic units of member status. Both micro- and macrofossil evidence indicate that members A and B are of late Campanian to early and mid-Maastrichtian age; the latter unit is unconformably overlain by the late Maastrichtian Member C. Definition of the lithostratigraphic units will aid local correlations within the James Ross Island area. Partial correlation with the major Cretaceous-Tertiary boundary reference section on Seymour Island has been achieved using both ammonite and dinoflagellate cyst taxa. It is apparent that there are major lithofacies differences between the two localities, which are thought to reflect a proximal to distal depth gradient into the depositional basin. The macrofauna has been grouped into two broad associations: a lower one based on the ammonite Gunnarites and an upper one based on Maorites . It is hoped that this sequence may form the basis of a formal kossmaticeratid ammonite zonation of the latest Cretaceous of the southern Gondwana margins. The sedimentology of members A and B reflects deposition in a shelf setting and records a transgressive-regressive pulse. Member C was deposited in shelf, shoreface and marginal marine environments, and shows evidence of both coeval arc volcanism and localized basin uplift.

Cretaceous-Tertiary high-latitude palaeoenvironments: James Ross Basin, Antarctica

High-latitude settings are sensitive to climatically driven palaeoenvironmental change and the resultant biotic response. Climate change through the peak interval of Cretaceous warmth, Late Cretaceous cooling, onset and expansion of the Antarctic ice sheet, and subsequently the variability of Neogene glaciation, are all recorded within the sedimentary and volcanic successions exposed within the James Ross Basin, Antarctica. This site provides the longest onshore record of Cretaceous–Tertiary sedimentary and volcanic rocks in Antarctica and is a key reference section for Cretaceous–Tertiary global change. The sedimentary succession is richly fossiliferous, yielding diverse invertebrate, vertebrate and plant fossil assemblages, allowing the reconstruction of both terrestrial and marine systems. The papers within this volume provide an overview of recent advances in the understanding of palaeoenvironmental change spanning the mid-Cretaceous to the Neogene of the James Ross Basin and related biotic change, and will be of interest to many working on Cretaceous and Tertiary palaeoenvironmental change.

Miocene glaciomarine sedimentation in the northern Antarctic Peninsula region: the stratigraphy and sedimentology of the Hobbs Glacier Formation, James Ross Island

The onshore record of Cenozoic glaciation in the Antarctic Peninsula region is limited to a number of isolated localities on Alexander Island, the South Shetland Islands and in the James Ross Island area. In the James Ross Island area, Late Cretaceous sedimentary rocks are unconformably overlain by a unit of diamictites and tuffs, which occur at the base of the James Ross Island Volcanic Group. These rocks are here defined as the Hobbs Glacier Formation, and on the basis of palynological studies are assigned to a Miocene (?late Miocene) age. The diamictites are interpreted as representing glaciomarine sedimentation close to the grounding line of either a floating ice shelf or a grounded tidewater glacier in a marine basin. Provenance studies indicate that the glacier was flowing from the Antarctic Peninsula towards the southeast. Volcanic tuffs conformably overlie the diamictites and are interpreted as representing deposition in a periglacial delta front setting in either a marine or non-marine basin, away from direct glacial influence. The Hobbs Glacier Formation and overlying James Ross Island Volcanic Group help to enhance our understanding of the Neogene glacial chronology of West Antarctica.

Strontium isotope correlation of the basal Maastrichtian Stage in Antarctica to the European and US biostratigraphic schemes

New 87Sr/86Sr dating allows the correlation of a marker horizon within the prolific Late Cretaceous Gunnarites antarcticus faunal assemblage of the Cape Lamb Member, Snow Hill Island Formation, Vega Island, Antarctica with reference sections in Europe and the USA. This horizon is between 81.5 and 96.5 m above the base of the G antarcticus assemblage. Replicate analysis of six macrofossils from within it yielded a mean value for 87Sr/86Sr of 0.707 735 9 ± 0.000 004 3 (± 2 s.e., n=17). This ratio in turn yields a numerical age of 71.0 ± 0.2 Ma when compared to the standard 87Sr/s6Sr reference curve for the latest Cretaceous, for which the Campanian-Maastrichtian boundary is placed at 71.3 ± 0.5 Ma. The value of 0.707 735 9 ± 0.000 004 3 correlates to a level in the Belemnella lanceolata belemnite Zone of the Chalk of northwestern Germany that is 2.5 ± 5 m beneath the base of the overlying B. pseudobtusa Zone (and 7.5 m above the Campanian-Maastrichtian belemnite boundary), and to an interval within the ammonite zonation of the US Western Interior that spans the early Maastrichtian Baculites baculus and B. eliasi zones, but with a most likely level within the B. eliasi Zone. An earliest Maastrichtian age is thereby determined for the Antarctic horizon, and indeed for the entire G. antarcticus assemblage. Gunnarites, sensu lato, is an important ammonite marker for the base of the Maastrichtian throughout the southern high-latitude regions, and the associated large heteromorph ammonite Diplomoceras may comprise a macrofossil link back to the Maastrichtian type sections.

Lithostratigraphy of Upper Jurassic-Lower Cretaceous strata of eastern Alexander Island, Antarctica

Abstract As a result of recent field work, the 4000m thick Kimmeridgian-Albian Fossil Bluff Formation of eastern Alexander Island is elevated to Group status (Fossil Bluff Group). Four formations are recognised within the group (which in ascending order are the Ablation Point, Himalia Ridge, Spartan Glacier and Pluto Glacier Formations), characterised by slumped strata, conglomerates, mudstones and sandstones, respectively. Type sections are selected, and representative lithologies and faunas described for each of these stratigraphic units. This succession constitutes probably the best exposed sequence through the Jurassic-Cretaceous boundary at a southern high palaeolatitude.

Late Mesozoic bipolar bivalve faunas

Bipolar bivalve genera probably existed through the greater part of late Mesozoic (i.e. late Jurassic-late Cretaceous) time. Of the various theories put forward to account for their presence, those based on some form of global climatic zonation seem most appropriate. Although equatorial-polar temperature gradients were substantially less in late Mesozoic time than at the present day, high latitude regions were subjected to temperate (or even cool-temperate) climatic regimes. Macro-benthonic marine faunas may have been more susceptible to differentiation in response to these climatic patterns than nektonic ones. This is particularly so in the late Mesozoic strata of the Southern Hemisphere.

Stratigraphy and regional significance of the Upper Jurassic-Lower Cretaceous Byers Group, Livingston Island, Antarctica

The Byers Group, exposed on Byers Peninsula, western Livingston Island, Antarctica, comprises a mudstone dominated sequence at least 1 km thick which accumulated in a marginal fore-arc environment. The basal, 105 m thick Anchorage Formation consists of radiolarian mudstones and tuff-rich interbeds of Kimmeridgian-Tithonian age; it correlates with Upper Jurassic organic-rich mudstone units throughout the proto-South Atlantic region. The succeeding 244 m thick Devils Point Formation marks the first major pulse of coarse volcaniclastic material into the basin. It is in turn followed by the extensive President Beaches Formation, comprising several hundred metres of finely laminated mudstones with at least two major sandstone intercalations. Molluscan and dinoflagellate cyst taxa indicate a Berriasian age and comparatively nearshore depositional environment for this unit. An unconformity of late Berriasian or early Valanginian age separates the three lowest formations from the Chester Cone Formation. The fine-grained Sealer Hill Member at the base of the latter is dated as Valanginian, and grades up into several hundred metres of pebbly sandstones and pebble-granule conglomerates. These mark the second major volcaniclastic pulse and may be of Hauterivian or even younger age. Definition of this major new group will facilitate more precise Upper Jurassic-Lower Cretaceous stratigraphical correlations within the southern South America-Scotia arc-Antarctic Peninsula region. It will also aid our understanding of the critical palaeogeographical transition in the northern Antarctic Peninsula from anoxic basin to active magmatic arc.