ARTICLES De®nition of Late Cretaceous Stage Boundaries in Antarctica Using Strontium Isotope Stratigraphy J. M. McArthur, J. A. Crame,1 and M. F. Thirlwall2 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom (e-mail: [email protected]) ABSTRACT 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 ®rst 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 ªMytiloidesº africanus species complex is exclusively Late Coniacian in age; both Baculites bailyi and Inoceramus cf. expansus have a Late Con- iacian/Early Santonian age range; an important heteromorph ammonite assemblage comprising species of Eubostry- choceras, Pseudoxybeloceras, Ainoceras, and Ryugasella is con®rmed as ranging from latest Coniacian to very earliest Campanian. An important new early angiosperm ¯ora 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 con®rmation that, in Antarctica, belemnites did not persist beyond the Early Maastrichtian. Introduction A remarkably complete and extensive Late Creta- The Late Cretaceous is often regarded primarily ceous sedimentary succession more than 3.5 km in as a time of biotic retractions that culminated in thickness is exposed on the islands of the James the spectacular mass extinction at the end of the Ross Island group, Antarctica (®gs. 1±3; Olivero et Cretaceous, but it was also a time of major evo- al. 1986; Crame et al. 1991, 1996, and references lutionary radiations in both the marine and terres- therein). This mostly shallow-water, clastic se- trial realms when many modern faunas and ¯oras quence is, in places, very fossiliferous. It offers an ®rst became established (see, e.g., Hallam 1994). opportunity unrivaled in the Southern Hemisphere Unfortunately, most of our knowledge of these to investigate biotic and environmental changes events comes from the Northern Hemisphere, but during the Late Cretaceous, particularly those lead- our recalibration, presented here, of the age of the ing to the KT mass extinction event. Antarctic succession provides some key austral data. For example, a new angiosperm leaf ¯ora from Manuscript received November 12, 1999; accepted July 5, the Hidden Lake Formation (®g. 3) can now be 2000. dated as entirely Coniacian in age; the taxa present 1 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, United Kingdom. include members of the Lauraceae, Nothofagaceae, 2 Department of Geology, Royal Holloway University of Lon- Annonaceae, and Proteaceae (Hayes 1996). To- don, Egham, Surrey TW20 0EX, United Kingdom. gether with ongoing investigations into palyno- [The Journal of Geology, 2000, volume 108, p. 623±640] q 2000 by The University of Chicago. All rights reserved. 0022-1376/2000/10806-0001$1.00 623 624 J. M. MCARTHUR ET AL. morph ¯oras, this assemblage will provide vital in- caniclastic sedimentary rocks constitute part of a formation on the structure of Late Cretaceous regressive megasequence; the stratigraphically temperate rain forests and the radiation of the ¯ow- older Gustav Group is composed of coarser-grained, ering plants into the Southern Hemisphere (Dett- submarine fan slope deposits that grade upward mann and Thomson 1987; D. J. Cantrill, J. E. Fran- into the ®ner-grained, shelf-depth deposits of the cis, and P. Hayes, pers. comm., 1999). Similarly, the Marambio Group (Ineson et al. 1986). The base of Santa Marta, Snow Hill Island, and Lo pez de Ber- the Gustav Group may be Aptian in age (Riding et todano Formations (®gs. 1±3) contain early repre- al. 1998), and the top, before this study, was thought sentatives of benthic marine invertebrate groups to be Santonian. The Marambio Group, before this that were to ¯ourish globally throughout the Ce- study, was taken to be Santonian to Danian (Ineson nozoic era; for example, venerid and tellinid bi- et al. 1986) in age. Details of litho- and biostrati- valves, and buccinoidean and muricoidean gastro- graphical subdivisions within the Gustav and Mar- pods (Zinsmeister and Macellari 1988; Scasso et al. ambio Groups are contained in Medina and Buatois 1991). (1992), Medina et al. (1992), Crame et al. (1996), Precise age assignment and stratigraphical cor- Pirrie et al. (1997), Riding et al. (1998), and refer- relation are essential for such analysis of palaeoen- ences therein. vironmental and palaeobiological change, so it is The uppermost levels of the Gustav Group and vital to integrate this Antarctic sequence accu- lowermost levels of the Marambio Group are well rately into the standard Northern Hemisphere ref- exposed around the shores of Brandy Bay, northern erence sections. To this end, the principal macro- James Ross Island (®gs. 1, 2). From there, the sec- and microfossil groups have proven only partially tion continues in a southeasterly direction to the successful. Some ammonites can be used for re- vicinity of St. Martha Cove and then across to Cape gional correlations, but others, such as the many Lamb, Vega Island (®gs. 1, 2; Olivero et al. 1986; representatives of the Kossmaticeratidae, are Crame et al. 1991; Pirrie et al. 1991). A major ENE/ largely endemic. Of the microfossil groups that are WSW-trending thrust fault (or faults) runs from just present, palynomorphs offer the most potential for north of Cape Gage to Carlsson Bay (®g. 1; Crame correlation but, at present, do no more than estab- et al. 1991; Pirrie et al. 1997) and repeats the upper lish biostratigraphical correlations with the Aus- part of this succession on southeastern James Ross tralasian region (see, e.g., Riding et al. 1992). Island, exposing small areas of Marambio Group To overcome these problems, we have used stron- sediments at Rabot Point and Carlsson Bay, where tium isotope stratigraphy (SIS) to date and correlate we have collected giant inoceramids from the Santa the sequence (McArthur 1994; Howarth and Marta Formation. McArthur 1997; Veizer et al. 1997). This method In a section (D.8228; ®g. 2) running along the has already enabled us to accurately correlate with southwestern shore of Brandy Bay, the upper Gus- the Northern Hemisphere the base of the Maas- tav Group comprises two formations: the Whisky trichtian stage in Antarctica (Crame et al. 1999). In Bay and Hidden Lake Formations (®gs. 2, 3). The addition, an enhanced Cenozoic chronology of the former is a complex, highly variable unit charac- northern Antarctic Peninsula region has been es- terized by pebble and boulder conglomerates, to- tablished using SIS (Dingle et al. 1997; Dingle and gether with pebbly sandstones; in places there are Lavelle 1998). Reference curves of 87Sr/86Sr against marked vertical and lateral facies transitions into Northern Hemisphere biostratigraphy for the Late silty mudstones (Ineson et al. 1986). Within the Cretaceous are available (McArthur et al. 1992, Whisky Bay Formation, at the 1600-m level in the 1993a, 1993b, 1994; McLaughlin et al. 1995; Su- combined stratigraphic section, the junction be- garman et al. 1995), and a time-calibrated 87Sr/86Sr tween the Lewis Hill and Brandy Bay members is curve for the period (Howarth and McArthur 1997) marked by a local unconformity (®g. 3; Ineson et can be used to convert to numerical age the al. 1986; J. A. Crame, pers. obs.). This discontinuity 87Sr/86Sr values determined for Antarctic fossils. probably accounts for the absence of Cenomanian inoceramids and ammonites found in equivalent strata in the Tumbledown Cliffs±Rum Cove region Lithostratigraphy and Regional Setting (TC and RC in ®g. 1; Ineson et al. 1986). The Upper Cretaceous sedimentary succession of At approximately the 1900-m level in the Brandy James Ross Island and Vega Island represents part Bay region, the Whisky Bay Formation lithologies of an extensive Late Mesozoic±Early Cenozoic grade up into a distinctive sequence of rusty brown back-arc basin that formed on the northeastern to greeny brown conglomerates, sandstones, and ¯ank of the Antarctic Peninsula (®g. 1). These vol- siltstones that constitute the Hidden Lake For- Journal of Geology L A T E CRETACEOUS STAGE BOUNDARIES 625 Figure 1. Map showing the location of the James Ross Island group, Antarctic Peninsula. Based in part on Crame and Luther (1997, ®g. 1). The left inset at the top of the map is expanded as ®gure 2.BB p Brandy Bay, CL p Cape Lamb,RC p Rum Cove,SMC p St. Martha Cove,TC p Tumbledown Cliffs. mation (®g. 3; Ineson et al. 1986). Some 350±400 into St. Martha Cove; see ®g. 2) and typically com- m thick, this unit is characterized by coarse-grained prises massive, very ®ne to medium-grained sand- sandstones and matrix-supported conglomerates in stones and silty sandstones (®g. 3; Olivero et al. its lower levels, and medium- to ®ne-grained sand- 1986). It is characterized by a marked increase in stones in its upper ones.