[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

ARTICLES

Definition of Late Cretaceous Stage Boundaries in Antarctica UsingStrontium 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: j.mcarthur@ucl.ac.uk)

A B S T R A C T

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 schemesof the Northern Hemisphere. The 87Sr/86Sr data enable Late Cretaceous stage boundaries to be physically located withaccuracy for the first time in a composite Southern Hemisphere reference section and so make the area one of globalimportance for documenting Late Cretaceous biotic evolution, particularly radiation and extinction events. The87Sr/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 UnitedStates. 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 regionalaustral correlation, is shown to be Turonian (probably Late Turonian) in age; the “Mytiloides” africanus speciescomplex 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 confirmed as ranging from latest Coniacian to very earliestCampanian. An important new early angiosperm flora is shown to be unequivocally Coniacian in age. Our strontiumisotopic recalibration of ages strengthens the suggestion that inoceramid bivalves became extinct at southern highlatitudes much earlier than they did in the Northern Hemisphere and provides confirmation that, in Antarctica,belemnites did not persist beyond the Early Maastrichtian.

Introduction

A remarkably complete and extensive Late Creta-ceous sedimentary succession more than 3.5 km inthickness is exposed on the islands of the JamesRoss Island group, Antarctica (figs. 1–3; Olivero etal. 1986; Crame et al. 1991, 1996, and referencestherein). This mostly shallow-water, clastic se-quence is, in places, very fossiliferous. It offers anopportunity unrivaled in the Southern Hemisphereto investigate biotic and environmental changesduring the Late Cretaceous, particularly those lead-ing to the KT mass extinction event.

Manuscript received November 12, 1999; accepted July 5,2000.

1 British Antarctic Survey, High Cross, Madingley Road,Cambridge CB3 0ET, United Kingdom.

2 Department of Geology, Royal Holloway University of Lon-don, Egham, Surrey TW20 0EX, United Kingdom.

The Late Cretaceous is often regarded primarilyas a time of biotic retractions that culminated inthe spectacular mass extinction at the end of theCretaceous, but it was also a time of major evo-lutionary radiations in both the marine and terres-trial realms when many modern faunas and florasfirst became established (see, e.g., Hallam 1994).Unfortunately, most of our knowledge of theseevents comes from the Northern Hemisphere, butour recalibration, presented here, of the age of theAntarctic succession provides some key australdata. For example, a new angiosperm leaf flora fromthe Hidden Lake Formation (fig. 3) can now bedated as entirely Coniacian in age; the taxa presentinclude members of the Lauraceae, Nothofagaceae,Annonaceae, and Proteaceae (Hayes 1996). To-gether with ongoing investigations into palyno-

624 J . M . M C A R T H U R E T A L .

morph floras, this assemblage will provide vital in-formation on the structure of Late Cretaceoustemperate rain forests and the radiation of the flow-ering plants into the Southern Hemisphere (Dett-mann and Thomson 1987; D. J. Cantrill, J. E. Fran-cis, and P. Hayes, pers. comm., 1999). Similarly, theSanta Marta, Snow Hill Island, and Lopez de Ber-todano Formations (figs. 1–3) contain early repre-sentatives of benthic marine invertebrate groupsthat were to flourish globally throughout the Ce-nozoic era; for example, venerid and tellinid bi-valves, and buccinoidean and muricoidean gastro-pods (Zinsmeister and Macellari 1988; Scasso et al.1991).

Precise age assignment and stratigraphical cor-relation are essential for such analysis of palaeoen-vironmental and palaeobiological change, so it isvital to integrate this Antarctic sequence accu-rately into the standard Northern Hemisphere ref-erence sections. To this end, the principal macro-and microfossil groups have proven only partiallysuccessful. Some ammonites can be used for re-gional correlations, but others, such as the manyrepresentatives of the Kossmaticeratidae, arelargely endemic. Of the microfossil groups that arepresent, palynomorphs offer the most potential forcorrelation but, at present, do no more than estab-lish biostratigraphical correlations with the Aus-tralasian region (see, e.g., Riding et al. 1992).

To overcome these problems, we have used stron-tium isotope stratigraphy (SIS) to date and correlatethe sequence (McArthur 1994; Howarth andMcArthur 1997; Veizer et al. 1997). This methodhas already enabled us to accurately correlate withthe Northern Hemisphere the base of the Maas-trichtian stage in Antarctica (Crame et al. 1999). Inaddition, an enhanced Cenozoic chronology of thenorthern Antarctic Peninsula region has been es-tablished using SIS (Dingle et al. 1997; Dingle andLavelle 1998). Reference curves of 87Sr/86Sr againstNorthern Hemisphere biostratigraphy for the LateCretaceous are available (McArthur et al. 1992,1993a, 1993b, 1994; McLaughlin et al. 1995; Su-garman et al. 1995), and a time-calibrated 87Sr/86Srcurve for the period (Howarth and McArthur 1997)can be used to convert to numerical age the87Sr/86Sr values determined for Antarctic fossils.

Lithostratigraphy and Regional Setting

The Upper Cretaceous sedimentary succession ofJames Ross Island and Vega Island represents partof an extensive Late Mesozoic–Early Cenozoicback-arc basin that formed on the northeasternflank of the Antarctic Peninsula (fig. 1). These vol-

caniclastic sedimentary rocks constitute part of aregressive megasequence; the stratigraphicallyolder Gustav Group is composed of coarser-grained,submarine fan slope deposits that grade upwardinto the finer-grained, shelf-depth deposits of theMarambio Group (Ineson et al. 1986). The base ofthe Gustav Group may be Aptian in age (Riding etal. 1998), and the top, before this study, was thoughtto be Santonian. The Marambio Group, before thisstudy, was taken to be Santonian to Danian (Inesonet al. 1986) in age. Details of litho- and biostrati-graphical subdivisions within the Gustav and Mar-ambio Groups are contained in Medina and Buatois(1992), Medina et al. (1992), Crame et al. (1996),Pirrie et al. (1997), Riding et al. (1998), and refer-ences therein.

The uppermost levels of the Gustav Group andlowermost levels of the Marambio Group are wellexposed around the shores of Brandy Bay, northernJames Ross Island (figs. 1, 2). From there, the sec-tion continues in a southeasterly direction to thevicinity of St. Martha Cove and then across to CapeLamb, Vega Island (figs. 1, 2; Olivero et al. 1986;Crame et al. 1991; Pirrie et al. 1991). A major ENE/WSW-trending thrust fault (or faults) runs from justnorth of Cape Gage to Carlsson Bay (fig. 1; Crameet al. 1991; Pirrie et al. 1997) and repeats the upperpart of this succession on southeastern James RossIsland, exposing small areas of Marambio Groupsediments at Rabot Point and Carlsson Bay, wherewe have collected giant inoceramids from the SantaMarta Formation.

In a section (D.8228; fig. 2) running along thesouthwestern shore of Brandy Bay, the upper Gus-tav Group comprises two formations: the WhiskyBay and Hidden Lake Formations (figs. 2, 3). Theformer is a complex, highly variable unit charac-terized by pebble and boulder conglomerates, to-gether with pebbly sandstones; in places there aremarked vertical and lateral facies transitions intosilty mudstones (Ineson et al. 1986). Within theWhisky Bay Formation, at the 1600-m level in thecombined stratigraphic section, the junction be-tween the Lewis Hill and Brandy Bay members ismarked by a local unconformity (fig. 3; Ineson etal. 1986; J. A. Crame, pers. obs.). This discontinuityprobably accounts for the absence of Cenomanianinoceramids and ammonites found in equivalentstrata in the Tumbledown Cliffs–Rum Cove region(TC and RC in fig. 1; Ineson et al. 1986).

At approximately the 1900-m level in the BrandyBay region, the Whisky Bay Formation lithologiesgrade up into a distinctive sequence of rusty brownto greeny brown conglomerates, sandstones, andsiltstones that constitute the Hidden Lake For-

Journal of Geology L A T E C R E T A C E O U S S T A G E B O U N D A R I E S 625

Figure 1. Map showing the location of the James Ross Island group, Antarctic Peninsula. Based in part on Crameand Luther (1997, fig. 1). The left inset at the top of the map is expanded as figure 2. Bay,BB p Brandy CL p CapeLamb, Cove, Martha Cove, Cliffs.RC p Rum SMC p St. TC p Tumbledown

mation (fig. 3; Ineson et al. 1986). Some 350–400m thick, this unit is characterized by coarse-grainedsandstones and matrix-supported conglomerates inits lower levels, and medium- to fine-grained sand-stones in its upper ones. Some fine-grained sand-stones and siltstones within it are intensely bio-turbated. The transition into the overlying SantaMarta Formation, the basal lithostratigraphic unitwithin the Marambio Group, is marked by a dis-tinct change from the rusty brown and greenybrown weathering hues to predominantly gray col-ors. The Santa Marta Formation is some 1200 mthick in its type area (along line of section running

into St. Martha Cove; see fig. 2) and typically com-prises massive, very fine to medium-grained sand-stones and silty sandstones (fig. 3; Olivero et al.1986). It is characterized by a marked increase inmarine benthic fauna and by a rise in the numberof infaunal taxa (Scasso et al. 1991).

The uppermost levels of the Santa Marta For-mation can be traced across to the base of a 480-m-thick sequence exposed on Cape Lamb, Vega Is-land (figs. 2, 3), where they pass conformablyupward into the Cape Lamb Member of the SnowHill Island Formation (Crame et al. 1991; Pirrie etal. 1991). Using SIS, the appearance of the prolific

626 J . M . M C A R T H U R E T A L .

Figure 2. Sketch map of the geology at the Brandy Bay/St. Martha Cove/Cape Lamb region, northern James RossIsland group, showing positions of key stratigraphic sections. Group, Group.G.G. p Gustav M.G. p Marambio

Gunnarites antarcticus ammonite fauna (ca. 3750m; fig. 3) within this unit has been dated as earliestMaastrichtian in age (Crame et al. 1999). The top-most 111 m of the Cape Lamb section, a sequenceof volcaniclastic sandstones and conglomeratesconstituting the Sandwich Bluff Member, is as-signed to the Lopez de Bertodano Formation (figs.2, 3; Pirrie et al. 1997).

Analytical Methods and Results

Sample Preparation. Belemnite samples 73ap,73am, 73an, 76a5, 76a6, 76b, 109a, 116a, and 122Bwere received as powders and were analyzed as re-ceived. In addition, one aragonitic Nucula and 17other calcitic macrofossils (oysters, inoceramids,belemnites) were analyzed (tables 1, 2). The sam-ples are from 12 stratigraphic levels; for complete-ness, a thirteenth level is reported from Crame etal. (1999). From unpowdered samples, visually al-tered portions were removed using diamond cut-

ting tools; the remaining sample was then brokeninto submillimeter-sized fragments. These werecleaned by brief immersion in 1.2 M hydrochloricacid solution, washed in ultrapure water, and driedin a clean environment. The best-preserved frag-ments were selected under the binocular micro-scope for analysis. Preservational criteria were de-gree of flakiness, amount of cracking and secondaryveining, amount of cementation, color, opacity, andthe presence of iron and/or manganese staining.

Chemical Analysis. Data for samples received aspowders are taken from Pirrie and Marshall (1990).For other samples, analysis for Ca, Mg, Sr, Na,Fe, and Mn was done by inductively coupledplasma–atomic emission spectrometry (ICP-AES)after dissolution of the sample in 1.8 M acetic acid;analysis for Rb was done on the same solutions bygraphite furnace atomic absorption spectrometry.Results are given in table 1. The precision of theanalyses was better than 55%. Powdered sampleswere too small to be subject to XRD analysis. For

Figure 3. Stratigraphical correlations between northern James Ross Island and Vega Island, positions of samples,and the ranges of some key fossil taxa. Vertical scale (100-m intervals on left) is a composite one for the entire region(Crame et al. 1996). Based on Crame and Luther (1997, fig. 2) with stage boundaries revised from data presentedherein. , . Precise biostratigraphic correlations between localities D.8409 andCE. p Cenomanian SANT. p SantonianDJ.685 (southeastern James Ross Island; fig. 1) have yet to be established.

Table 1. Isotopic and Elemental Data for Antarctic Fossils

Section andsample nos. Sample type

Stratigraphic level (m)

Mineralogya

87Sr/86Sr(mean 5 2 s.e.) n

Numericalage

Ca(%)

Mg(ppm)

Sr(ppm)

Na(ppm)

Fe(ppm)

Mn(ppm)

Rb(ppm)Composite Crame Col

Vega Island:DJ.83:

68–160 Various 3800 .707736 5 .000004 17 71.0 5 .3James Ross Island:

DJ.685:83 Giant

inoceramid3300–3600 c, tr d .707646 5 .000005 4 74.2 5 .4 39.1 2412 831 1265 721 761 .06

D.8409:17 Giant

inoceramid≈3150 c .707554 5 .000011 2 38.9 1676 970 2421 163 297 !.01

14 Giantinoceramid

≈3150 c .707553 5 .000009 3 39.1 1394 970 2042 152 200 !.01

DJ.955:14 Giant

inoceramid≈3150 .707557 5 .000015 1 78.7 5 .5

D.8664:184a2 Nucula 2830 P .707493 5 .000007 4184a1 Nucula 2830 ar, tr c .707493 5 .000009 3 82.6 5 .5 38.4 249 3510 5019 73 13 !.01

D.8657:122B Belemnite 2602 100–103 P .707429 5 .000009 3 85.5 5 .4 38.4 897 1664 3298 107 21 .04116a Belemnite 2594 92.8–95.5 P .707421 5 .000015 1 85.8 5 .7 660 1523 210 10109a Belemnite 2589 82.5–92.0 P .707422 5 .000011 2 85.8 5 .5 600 1461 140 995 Belemnite 2565 64.0–65.5 c .707420 5 .000009 3 85.8 5 .5 39.4 718 1245 1231 4 1 !.0176b Belemnite 2549 49.0–49.5 P .707405 5 .000015 1 600 1570 210 5876a7 Belemnite 2548 47.0–49.0 c .707412 5 .000011 2 39.4 800 1560 2774 115 36 .0476a6 Belemnite 2548 47.0–49.0 P .707407 5 .000011 2 660 1450 210 1776a5 Belemnite 2548 47.0–49.0 P .707406 5 .000015 1 840 1760 210 1976a Belemnite 2548 47.0–49.0 c .707411 5 .000011 2 39.7 409 1336 1513 9 3 !.0173an Belemnite 2548 47.0–49.0 P .707409 5 .000011 2 660 1610 210 1673am Belemnite 2548 47.0–49.0 P .707420 5 .000011 2 840 1940 210 5173ap Belemnite 2500 44.3–48.0 P .707415 5 .000015 1 1020 1720 210 1273 Belemnite 2500 44.3–48.0 c .707415 5 .000011 2 86.1 5 .2 38.6 551 1221 1470 4 4 !.01

D.8228:333 Inoceramid 2420 c .707385 5 .000003 4 38.5 4008 1289 5065 128 239 .02331 Inoceramid 2420 c, tr an .707389 5 .000011 2 38.2 3730 1239 5204 103 59 !.01326 Inoceramid 2420 c .707379 5 .000009 3 87.0 5 .2 38.4 85 1177 5075 26 38 !.01303 Inoceramid 2360 c .707381 5 .000012 4 38.8 5215 1201 4372 79 6594 Inoceramid 2360 c, tr d .707381 5 .000002 4 87.1 5 .3 38.4 6850 1275 2780 103 63 !.01114 Oyster 1880 c .707288 5 .000009 3 39.3 564 704 858 298 305 !.01113 Oyster 1880 c .707291 5 .000009 3 91.0 5 .2 40.5 609 701 758 110 71 !.0182 Oyster 1760 c, tr q .707292 5 .000009 3 40.1 403 813 790 221 47 !.0181 Oyster 1760 c .707287 5 .000009 3 89.8 5 .4 39.1 413 589 586 63 95 !.01

Note. Mean data only are given for 87Sr/86Sr; full data are in table 2. Uncertainties on 87Sr/86Sr are at 52 s.e. of replicate analysis. Numerical ages, and age uncertainties,are computed from bundled data from each stratigraphic level, not data for each sample. Data for DJ.83, Vega Island, are from Crame et al. 1999. Values of 87Sr/86Srfor stage boundaries and numerical ages of same (Obradovich 1993) are Maas/Camp, 0.707729, 71.3 Ma; Camp/Sant, 0.707476, 83.5 Ma; Sant/Con, 0.707406, 86.3 Ma;Con/Tur, 0.707315, 88.7 Ma. The Late Turonian minimum in 87Sr/86Sr is 0.707290.a , , , , , , .P p powdered tr p trace an p analcime q p quartz d p dolomite c p calcite ar p aragonite

Journal of Geology L A T E C R E T A C E O U S S T A G E B O U N D A R I E S 629

Table 2. Replicate 87Sr/86Sr Data for Samples

Sample number 87Sr/86Sr value of replicate analysis

DJ.83.68–160a

DJ.685.83 .707651 .707645 .707639 .707647D.8409.17 .707549 .707558D.8409.14 .707555 .707553 .707550DJ.955.14 .707557D.8664.184a2 .707500 .707498 .707484 .707491D.8664.184a1 .707494 .707498 .707488D.8657.122B .707423 .707432 .707433D.8657.116a .707421D.8657.109a .707422 .707422D.8657.95 .707418 .707418 .707425D.8657.76b .707405D.8657.76a7 .707411 .707412D.8657.76a6 .707413 .707401D.8657.a5 .707406D.8657.76a .707410 .707412D.8657.73an .707418 .707410D.8657.73am .707425 .707414D.8657.73ap .707415D.8657.73 .707413 .707418D8228.333 .707386 .707389 .707384 .707381D8228.331 .707391 .707386D8228.326 .707375 .707381 .707381D8228.303 .707381 .707376 .707397 .707368D8228.94 .707382 .707384 .707379 .707380D8228.114 .707288 .707289 .707287D8228.113 .707292 .707290 .707292D8228.82 .707292 .707287 .707298D8228.81 .707289 .707292 .707279a See Crame et al. 1999.

other samples, mineralogy was determined using aPhillips PW 1710 diffractometer. Instrumental con-ditions were: CuKa radiation generated at 40 kVand 30 mA; scanning through 57 to 507 2v at 0.572v/min, a range that includes the major diffractionpeaks of aragonite, calcite, and dolomite. Withthese conditions, the detection limit of our appa-ratus, determined by standard additions with purephases, was about 0.2% calcite in aragonite and0.5% dolomite in calcite.

Isotopic Analysis. For 87Sr/86Sr analysis, samplesof about 20 mg were dissolved in 2.5 M hydro-chloric acid, and Sr was separated by standard ion-exchange chemistry. Measurements were madewith a VG 354 mass spectrometer using the multi-dynamic routine SrSLL that includes correctionsfor isobaric interference from 87Rb (Thirlwall 1991).Data are presented in table 1 as means of replicatemeasurement of 87Sr/86Sr. Data are normalized to avalue of 0.1194 for 86Sr/88Sr and adjusted to a valueof (2 s.e., ) for0.7102480 5 0.0000025 n p 19NIST 987, which corresponds to a value of

(2 s.e., ) for EN 1.0.7091746 5 0.0000032 n p 19Adjustment was to the cumulative mean of NIST987 values collected during periods when sampleswere analyzed. Based on replicate analysis of NIST

987 and samples, the precision of measurement(2 s.e.) was better than 50.000015 ( ),n p 150.000011 ( ), and 50.000009 ( ). Totaln p 2 n p 3blanks were !2 ng of Sr; samples contained 110 mgof Sr. Concentrations of Rb were too low to requirecorrections for radiogenic Sr. Data are given in table1 as means of replicate analysis. Full data are intable 2.

Discussion

Quality of Sample Preservation. The quality ofpreservation of nine belemnites received as pow-ders (from the line of section D.8657; fig. 2; tables1, 2) and separate subsamples of several other sam-ples we have analyzed, are discussed in Pirrie andMarshall (1990), who conclude that preservationwas good for belemnites and less good for inocer-amids. Our unpowdered samples were visually wellpreserved and our inoceramids, after picking, ap-pear better preserved than the samples described bythose authors. On the basis of XRD, most sampleswere monomineralic, but five (table 1) containedtraces of contaminant phases (quartz, dolomite,analcime, calcite). The aragonitic Nucula con-tained about 0.2% calcite, the minimum detectableby XRD, but replicate 87Sr/86Sr analysis gave repro-ducible data so we believe alteration (which pro-ceeds patchily) had not significantly altered the87Sr/86Sr of this sample. Furthermore, contaminantcalcite would have derived its Sr mostly from pre-cursor aragonite and its concentration in secondarycalcite is much lower than in primary aragonite,thereby lessening any effect of contaminant Sr on87Sr/86Sr.

Examination of acid-leached samples of inocer-amids by scanning electron microscopy (SEM) (fig.4a, 4b) showed that trace impurities were depositedbetween individual prisms in a few samples, butindividual prisms were easily separated from theimpurity (fig. 5a, 5b) by picking under the micro-scope. The impurity phases (quartz, analcime, dol-omite) in inoceramids contain little Sr, were vol-umetrically unimportant, were avoided duringpicking, and, for quartz and analcime, are insolublein acid: the influence of such phases on the87Sr/86Sr of picked samples was negligible. Inocer-amid prisms were altered and cloudy at their ends,where alteration was localized, but clear in theirmiddle portions (fig. 5a, 5b), and it was the middleportions that were picked for analysis. Finally, the87Sr/86Sr values of three giant inoceramids, withvery different preservational states, from one strat-igraphic level and locality at Carlsson Bay in thesouthwest of James Ross Island (DJ.955, samples 5,

630 J . M . M C A R T H U R E T A L .

Figure 4. SEM photographs of inoceramid samples D.8409.14 (a) and DJ.8228.303 (b), showing trace impuritiescementing prisms in the latter, and oyster samples D.8228.82 (c) and D.8228.113 (d), showing layering in shell calcite.Fields of view are 460 mm (a), 930 mm (b), 64 mm (c), and 93 mm (d).

8, and 14; figs. 1, 3), showed little difference: thewell-preserved sample, DJ.955.14, has an 87Sr/86Sr( ; table 1) indistinguishable0.707557 5 0.000015from that of giant inoceramids from northern JamesRoss Island (locality D.8409; ;0.707554 5 0.000011table 1). Such agreement would be unlikely unlessthese samples were unaltered and also from thesame stratigraphic level. Sample DJ.955.8 wasrather cemented and altered and its 87Sr/86Sr is0.707548, while sample DJ.955.5 was solidly ce-mented so that prisms had completely fused andwere not visible as individuals; its 87Sr/86Sr value is0.707532. Thus, severe alteration has lowered87Sr/86Sr by no more than 0.000025, a value only alittle more than analytical uncertainty; a loweringof 87Sr/86Sr would be expected on alteration sincethe clastic sediments of the area are largely of man-tle affinity in this back-arc environment of the Ant-arctic Peninsula.

Examination of oysters with SEM (fig. 4c, 4d) re-

vealed the compact layering and surface ornamen-tation on individual layers that we interpret to becharacteristic of pristine oyster carbonate. Opticalexamination of oysters showed flaky, translucentcalcite (fig. 5c, 5d) with only local Fe or Mn stainingthat was avoided during sampling.

Concentrations of Fe and Mn exceeded 305 ppmonly in sample 83 (cf. Veizer 1983; Veizer et al.1992; McArthur 1994; table 1). There is a weakcorrelation between Fe and Mn in the samples (fig.6) but no relationship between 87Sr/86Sr and eitherFe or Mn (fig. 6). We attribute the measured con-centrations of Fe and Mn not to the occurrence ofstructural Fe and Mn in altered carbonate, but tothe presence of surface coatings of Fe/Mn oxyhy-droxides on crystallites.

Within each stratigraphic level, different sampleshave indistinguishable 87Sr/86Sr values (table 1, cf.DJ.8409, samples 14 and 17 with DJ.955, sample14; in section D.8228, cf. sample 81 with sample

Journal of Geology L A T E C R E T A C E O U S S T A G E B O U N D A R I E S 631

Figure 5. Samples in figure 4 shown in plane-transmitted light in the form prepared for picking under the microscope.Clear inoceramid prisms of samples D.8409.14 (a) and DJ.8228.303 (b) and translucent flakes of oyster samplesD.8228.82 (c) and D.8228.113 (d). All views are at #25 magnification.

82, sample 113 with sample 114, sample 94 withsample 303, sample 326 with samples 331 and 333,etc.) thereby attesting to good preservation. Finally,samples have 87Sr/86Sr values that are consistentwith their stratigraphical order. The above factssuggest that the samples are well preserved and re-tain their original 87Sr/86Sr values, so we accept our87Sr/86Sr data as recording the 87Sr/86Sr values of LateCretaceous seawater.

Numerical Ages. These have been determinedusing version 3:10/99 of the LOWESS look-up tableof Howarth and McArthur (1997); these authorsgive a full description of the LOWESS method andtable, table derivation, and estimation of uncer-tainty. The relevant part of the 87Sr/86Sr curve, de-rived from version 3:10/99, is shown in figure 7. Intable 1, we have calculated numerical ages for sam-ples bundled into stratigraphic levels rather thanprovide a separate age for each sample. We do so

because stratigraphic separation of many closelyspaced samples was small, and represented a periodof time so small, that evolutionary differences be-tween levels in marine 87Sr/86Sr would have beenundetectable with our measurements. For strati-graphic levels with more than three measurementsof 87Sr/86Sr (table 1), mean values of 87Sr/86Sr and 2s.e. of the means were used to derive numericalages and uncertainty limits on the ages. Wherefewer than three values were available, uncertain-ties were taken to be twice the standard error ofthe cumulative mean for NIST 987 of 50.000015,50.000011, or 50.000009, for , 2, and 3, re-n p 1spectively. The uncertainty on the ages combinesthe uncertainty inherent in the reference curve ofHowarth and McArthur (1997, version 3:10/99) andthe uncertainty (2 s.e.) in the mean 87Sr/86Sr valuesof each stratigraphic level (table 1).

Stratigraphic Correlations. Numerical ages suffer

632 J . M . M C A R T H U R E T A L .

Figure 6. Covariance of Mn, Fe, and 87Sr/86Sr in Ant-arctic macrofossils. Sample DJ.685.83 is off scale on allplots and is not shown.

from the uncertainties associated with placing suchages onto biostratigraphic schemes. Correlationwith 87Sr/86Sr, however, bypasses such problems bydirectly comparing 87Sr/86Sr values in different se-quences. In figures 8–10, we show where in thebiostratigraphic schemes for the Northern Hemi-sphere (McArthur et al. 1993a, 1993b, 1994) the87Sr/86Sr values for our Antarctic specimens occur.

Correlations with the biostratigraphic zonationof the English Chalk are shown in figures 8 and 9.In figure 8, 87Sr/86Sr values through the Chalk, asproven in a borehole at Trunch, Norfolk, UnitedKingdom (data from McArthur et al. 1993a, 1993b),are plotted against depth in the borehole. In thissection, a hard ground disturbs the 87Sr/86Sr curveat the Turonian/Coniacian boundary, but minor ex-trapolation allows isotopic correlation into the up-permost Turonian. The detailed zonation of the se-quence is given in figure 9, together with the levelsto which Antarctic 87Sr/86Sr ratios correlate on thebasis of the 87Sr/86Sr curve in figure 8. The lowestAntarctic level (samples 81, 82, 113, 114) correlateswith the upper part of the upper Turonian macro-fossil zone of Sternotaxis planus and, in the nan-nofossil zonation of Burnett (1990, revised from Sis-singh 1977), at a level between the upper part ofCC12 and the lower part of CC13. Samples 94, 303,326, 331, and 333 are Late Coniacian in age andcorrelate with the lower part of the macrofossilzone of Micraster coranguinum and the nannofossilzone topmost CC14/bottommost CC15. Samples73, 73ap, 73ap, 73am, 76a, 76a5, 75a6,76a7, 76b,95, 109a, 116a, and 122B spread across 20 m of se-quence from the topmost Coniacian to the lowerSantonian and are all within both CC15 and themacrofossil zone of M. coranguinum. Sample 184acorrelates with the Lower Campanian macrofossilzone of Offaster pilula and the nannofossil zoneCC17; Campanian samples D.8409.14, D.8409.17,and DJ.955.14 correlate with the top of the Goni-oteuthis quadrata macrofossil zone and the lowerpart of CC20; sample 83 is Late Campanian in ageand correlates with the midrange of the Belemni-tella mucronata zone and the top of CC20. Samples68–160 are basal Maastrichtian in age, fall in theupper Belemnella lanceolata zone, and are dis-cussed in Crame et al. (1999).

Correlations with the standard ammonoid zo-nation for the U.S. Western Interior using the U.S.Western Interior 87Sr/86Sr standard curve (McArthuret al. 1994) are plotted in figure 10, where 87Sr/86Srfor zones are plotted in the middle of the zonalranges, which are all taken to be of equal duration.As the 87Sr/86Sr calibration is based on a zonalscheme, it is less precise than it would be if based

Journal of Geology L A T E C R E T A C E O U S S T A G E B O U N D A R I E S 633

Figure 7. Reference curve of Howarth and McArthur (1997, version 3:10/99) for the interval 65–95 Ma. Confidenceintervals of the mean line are drawn at 95% confidence interval. Arrows show correlative ages of levels in Antarctica.Sample label 14 includes DJ.955.14 and D.8409.14. Sample labels 73 and 76 represent samples 73a, 73ap, 73am, 73an,76a5, 76a6, 76a7, and 76b.

on a continuous profile through a rock sequence,as is the case for Europe. In addition, the 87Sr/86Srof several zones (e.g., Clioscaphites saxitonianus)must be derived by interpolation, making correla-tion with such levels less accurate than would bethe case were more data available. Samples 81, 82,113, and 114 fall within the latest Turonian zonesof Scaphites whitfieldi and Prionocyclus quadra-tus. Samples 94, 303, 326, 331, and 333 fall withinthe Late Coniacian zone of Scaphites depressus,and samples 73, 73ap, 73ap, 73am, 76a, 76a5,75a6,76a7, 76b, 95, 109a, 116a, and 122B correlatewith the C. saxitonianus zone of the basal Santon-ian. Sample 184a is placed in the zonal range upperScaphites hippocrepis III/lower Baculites sp.smooth; samples D.8409.14, D.8409.17, andDJ.955.14 correlate with the junction of the zonesof Baculites sp. smooth and Baculites sp. weaklyribbed; sample 83 falls in the Exitaloceras jennyizone and samples 68–160 (Crame et al. 1999) fallin the uppermost Baculites eliasi zone, which isthe lowermost Maastrichtian macrofossil zone.

These attempts to correlate with NorthernHemisphere schemes are somewhat compromisedby the scarcity of data used to compile the European

and U.S. reference curves for the period Turonianto middle Santonian (the curve for the Late San-tonian and Campanian is better defined; McArthuret al. 1993a, 1993b). For more precise correlations,more data are required for this interval for bothAntarctica and Europe. This is particularly true forthe zone of C. saxitonianus for which the lack of87Sr/86Sr data results in the 87Sr/86Sr calibrationcurve (fig. 10) perhaps giving ages that are too youngby one zone.

Stratigraphic Interpretation. At approximatelythe 1350-m level in the combined section a dis-tinctive fauna that includes the inoceramid bivalveBirostrina concentrica (Parkinson) indicates a LateAlbian age (fig. 3; Ineson et al. 1986). Immediatelyabove the 1600-m unconformity a second inocer-amid, Inoceramus madagascariensis Heinz, is lo-cally abundant, suggesting a Late Turonian/EarlyConiacian age (Heinz 1933; Crame 1983) for thislevel, although Crampton (1996) suggests that itsoccurrence in the Barroisiceras onilakyense am-monite zone in Madagascar is more compatiblewith a middle Coniacian age. The unequivocal LateTuronian age SIS ages of samples 81, 82, 113, and114 in both Europe and North America (figs. 8–10)

634 J . M . M C A R T H U R E T A L .

Figure 8. Strontium isotope correlation of Antarctic levels with the 87Sr/86Sr profile of the English Chalk of theUnited Kingdom. Open circles are individual data of McArthur et al. (1993b). Solid line is visual fit to the data.Arrows show correlative levels in Antarctica. Sample label 14 includes DJ.955.14 and D.8409.14. Sample labels 73and 76 represent samples 73a, 73ap, 73am, 73an, 76a5, 76a6, 76a7, and 76b.

show that this might be the youngest possible agefor this species in Antarctica (fig. 3).

Unfortunately, the presence of the unconformityat about 1600 m in the section means that there isno direct evidence of the age of the base of the I.madagascariensis zone in Antarctica. It does notoccur in palaeontologically well-dated Late Ceno-manian strata in the Tumbledown Cliffs/RumCove area (fig. 1; Ineson et al. 1986), and thus itmay be entirely Turonian (?Late Turonian) in age(fig. 3). Such an age is of considerable stratigraphicalsignificance, for this taxon is widespread in theSouthern Hemisphere, occurring in Patagonia,Madagascar, and New Zealand (Crampton 1996) aswell as Antarctica. As the range of I. madagascar-iensis in New Zealand is mostly coincident withthat of the Teratan Stage, it seems that the age ofthis local chronostratigraphic unit must be revisedfrom middle Coniacian to Turonian (?Late Turon-ian). This in turn has implications for the age rangesof other inoceramid taxa and stage boundaries inNew Zealand (Crampton 1996, fig. 26).

Just beneath the junction between the WhiskyBay and Hidden Lake Formations (1930-m level offig. 3; Herm et al. 1979; Crame 1983) occur the first

representatives of a complex of Coniacian inocer-amid taxa that seem to have their closest links withEuropean forms such as Inoceramus (Inoceramus)inaequivalvis Schluter and Inoceramus (Inocera-mus) koegleri Andert. As these are both Lower Con-iacian taxa in Europe, this seems a suitable placeat which to set the Turonian/Coniacian boundary.This first complex grades into a second that in-cludes the widely interpreted species “Mytiloides”africanus (Heinz). This taxon has been given bothConiacian and Santonian ages in Madagascar (Sor-nay 1964), but it is Early Coniacian in Europe(Herm et al. 1979; Walaszczyk 1992). It is also ap-parent that some forms within this second complexresemble the highly variable Inoceramus australisWoods from the Piripauan Stage (uppermost Con-iacian/middle Santonian) of New Zealand (see, e.g.,Crampton 1996, plate 9F). The late Middle/LateConiacian 87Sr/86Sr ages for samples 94, 303, 326,331, and 333 (figs. 8–10) indicate that almost theentire “M.” africanus species complex is Coniacianin age (fig. 3). The same conclusion can be appliedto a co-occurring large, erect inoceramid resem-bling Inoceramus expansus Baily from South Africathat was thought to be no older than Santonian

Journal of Geology L A T E C R E T A C E O U S S T A G E B O U N D A R I E S 635

Figure 9. Strontium isotope correlation of Antarcticsamples with the biostratigraphy of the Cretaceous Eng-lish Chalk (McArthur et al. 1993a, 1993b). Macrofossilbiostratigraphy and stage boundaries from Wood et al.(1994). Nannofossil zonation from Burnett (1990). Sam-ple label 14 includes DJ.955.14 and D.8409.14. Samplelabels 73 and 76 represent samples 73a, 73ap, 73am, 73an,76a5, 76a6, 76a7, and 76b.

(Crame 1983). Even the first occurrences of the dis-tinctive heteromorph ammonite Baculites bailyiWoods in the lowermost levels of the Santa MartaFormation must now be taken to represent a LateConiacian rather than Early Santonian age (sensuCrame et al. 1991). One immediate and importantimplication of these new findings is that the Hid-den Lake Formation is entirely Coniacian in age(fig. 3; see below).

The dimitobelid belemnites from locality

D.8657, at Crame Col (figs. 2, 3), give 87Sr/86Sr ratiosthat correlate with the basal Santonian C. saxiton-ianus zone of the U.S. Western Interior and theupper M. coranguinum zone of NW Europe (figs. 9,10), so we place the Coniacian-Santonian boundaryat a level just beneath the first occurrence of thesebelemnites, around the 150-m level in Crame Coland 2500 m in the combined section (fig. 3). As aresult of this placement, a distinctive group of het-eromorph ammonites based on the genera Eubos-trychoceras, Pseudoxybeloceras, Ainoceras, andRyugasella (fig. 3) that, because of their affinitieswith taxa from Japan, Madagascar, and the Pacific,were originally taken to indicate an Early Cam-panian age (Olivero 1988), must instead be noyounger than earliest Campanian and are seenmainly to be Santonian or even latest Coniacian inage, as inferred by Olivero (1992), as some of themrange as low as approximately the 2450-m level (fig.3).

The nuculid bivalve from locality D.8664 (figs.2, 3) indicates an Early Campanian age for this lo-cality through its correlation with the upper S. hip-pocrepis III/lower Baculites sp. smooth zone in theU.S. Western Interior and O. pilula zone in NWEurope (figs. 8, 9).

A slightly younger latest Early Campanian age isassigned to the two giant inoceramid bivalve sam-ples from locality D.8409 and one from localityDJ.955.14 from the same stratigraphic level inCarlsson Bay (figs. 2, 3, 7, and 8). A further speci-men of Antarcticeramus rabotensis Crame and Lu-ther from Rabot Point (locality DJ.685; fig. 1) in-dicates a yet younger Campanian age. Precisebiostratigraphic correlations have yet to be madebetween Rabot Point, Carlsson Bay, and northernJames Ross Island (Crame and Luther 1997) but our87Sr/86Sr data indicate that locality DJ.865 shouldoccur around the 3300–3600-level on our compos-ite section (fig. 3) and that locality DJ.955 is strati-graphically at the same levels as locality D.8409.In this study, the Santonian/Campanian boundaryis placed slightly beneath the level of D.8664 (sam-ple 184a; fig. 3) and the Campanian/Maastrichtianboundary is placed in the lowermost levels of theSnow Hill Island Formation on Vega Island (Crameet al. 1999).

Palaeobiological and Palaeoenvironmental Implica-tions. Early extinction patterns for inoceramid bi-valves and dimitobelid belemnites in the JamesRoss Basin have already been established (Crameet al. 1996; Zinsmeister and Feldmann 1996). Thelast inoceramids, of the giant species A. rabotensis,are found well below the KT boundary in stratabelieved, before this study, to be mid to Late Cam-

636 J . M . M C A R T H U R E T A L .

Figure 10. Strontium isotope correlations of Antarctic samples with the biostratigraphy of the U.S. Western Interior.Open circles are samples of McArthur et al. (1994), which are joined by the straight lines to give a best estimate ofvariation of 87Sr/86Sr through the zones. Sample label 14 includes DJ.955.14 and D.8409.14. Sample labels 73 and 76represent samples 73a, 73ap, 73am, 73an, 76a5, 76a6, 76a7, and 76b.

panian in age (Crame et al. 1996, fig. 2). Our87Sr/86Sr ages from localities D.8409 and DJ.955show that these giant inoceramids are, in the Eu-ropean sense, Early Campanian in age, while thosefrom locality DJ.865.83 are mid Late Campanian inage. These ages confirm the disparity in age be-tween last occurrences in the Northern and South-ern Hemispheres.

Similarly, before this study, the last dimitobelidbelemnites in the Crame Col/Brandy Bay regionwere thought to be Early to mid Campanian (Crameet al. 1996, fig. 2). Nevertheless, the new strontiumages from localities D.8657 and D.8664 indicatethat this age range should in fact be Early Santon-ian/Early Campanian and that there is an evengreater time gap between these occurrences and asingle, later Maastrichtian (?mid Maastrichtian) ho-rizon of belemnites recorded on Seymour Island,which is some 625 m beneath the KT boundary(Doyle and Zinsmeister 1988).

In order to understand what controls extinction

patterns, it is necessary to plot the stratigraphicalranges of other key taxa through the Late Creta-ceous of Antarctic. Ammonites will be one partic-ularly important group to study in this respect; al-though they are present right up to the KTboundary at most key localities in the world, theyhad probably been in decline since the mid to earlyLate Cretaceous (Kennedy 1989). Ward and Signor(1983) have documented a steady decline in num-bers of genera from the Coniacian to the Maastrich-tian stage. Zinsmeister and Feldmann (1996) re-corded a reduction in numbers of ammonite speciesfrom about 45 in the lower Santa Marta Formation(Late Santonian/Early Campanian in this study) to15 in the Late Maastrichtian of Seymour Island,with just five at the KT boundary (see also Zins-meister 1998). However, our inventory of Antarcticammonites is still incomplete, and an importantConiacian fauna from the uppermost Whisky Bay,Hidden Lake, and lower Santa Marta Formations(fig. 3) is in the process of being described (M. R.

Journal of Geology L A T E C R E T A C E O U S S T A G E B O U N D A R I E S 637

A. Thomson and P. Bengston, pers. comm., 1999).It may be that there is an even longer-term patternto the decline of ammonites in Antarctica, a patternsimilar to that seen in other regions.

The richest Antarctic Late Cretaceous inocer-amid bivalve fauna is based on two species com-plexes that this study shows are almost entirelyConiacian in age (fig. 3). This finding fits a globalpattern of marked expansion of inoceramids in theMiddle Turonian–Coniacian, followed by a gradualdecline into the Late Maastrichtian (Pergament1978; Sornay 1981; Dhondt 1992). The congruity ofthese worldwide patterns between ammonites andinoceramids suggests that we may be looking at asingle, underlying cause, and an obvious one to con-sider is climate change. For example, Huber (1998)has highlighted the similarity of Late Cretaceouspalaeotemperature curves between Arctic and Ant-arctic regions. In the former, fossil leaf physiog-nomy suggests a temperature maximum during Tu-ronian/Coniacian time; in the latter, oxygenisotope palaeotemperature analysis of deep seacores suggests a maximum temperature during Tu-ronian/Santonian time. There is increasing evi-dence to suggest that the Campanian and Maas-trichtian stages represent a pronounced phase ofglobal cooling (Zinsmeister and Feldmann 1996;Huber 1998).

Stable isotope palaeothermometry has also beenattempted through the James Ross Island group suc-cession and a pronounced Turonian/Coniacianpeak in values established (Ditchfield et al. 1994).This study now needs to be refined and extended,for the values obtained have been plotted in termsof either major lithostratigraphical subdivisions orstandard Cretaceous stages (Ditchfield et al. 1994;Dingle and Lavelle 1998). Individual values cannow be replotted on the composite stratigraphicsection and tied in much more precisely to the ver-tical ranges of key fossil taxa.

Conclusions

1. Strontium isotope analysis of macrofossils fromAntarctica shows that the oldest group of samplescorrelate definitively with the Upper TuronianSternotaxis planus zone of Norfolk, United King-dom, and approximately the junction between theScaphites whitfieldi and Prionocyclus quadratuszones (Upper Turonian) in the U.S. Western Inte-rior. A younger group of five samples, from twoclose stratigraphic levels, correlate with the up-permost Coniacian Scaphites depressus zone of theU.S. Western Interior and the lower Micraster cor-anguinum zone of the Chalk of NW Europe. Higher

still, a group of nine samples from five close levelscorrelates with the upper M. coranguinum zone ofEurope and the basal Santonian Clioscaphites sax-itonianus zone in the U.S. Western Interior. Elevensamples from four higher levels are Campanian toMaastrichtian in age.

2. The 87Sr/86Sr data show that the uppermostWhisky Bay Formation is no younger than earliestConiacian in age, with most of it being assigned tothe Turonian. The succeeding Hidden Lake For-mation is entirely Coniacian in age, and the bound-ary between the respective Gustav and Marambiogroups must now be set at Late Coniacian. TheConiacian/Santonian boundary may be as high asthe 150-m level in the Crame Col section, and theSantonian stage may be represented by no morethan approximately 225 m of strata. The SantaMarta Formation is Late Coniacian/latest Campan-ian in age and the Campanian/Maastrichtianboundary is placed in the lowermost levels of theSnow Hill Island Formation on Vega Island.

3. Inoceramus madagascariensis, an importantfossil for regional correlations, should now be re-garded as Turonian (and probably Late Turonian) inage. Similarly, certain members of the “Mytiloides”africanus species complex have considerable re-gional stratigraphical potential, and they can betaken to be almost exclusively Late Coniacian inAntarctica. Inoceramus cf. expansus has a LateConiacian/Early Santonian age range. An impor-tant group of Antarctic heteromorph ammonites,centered on the genera Eubostrychoceras, Pseu-doxybeloceras, Ainoceras, and Ryugasella, isshown to be no younger than earliest Campanianin age, with members of this group ranging downto the uppermost Coniacian.

4. The new age calibration of the Antarctic LateCretaceous succession has important implicationsfor both extinction and radiation events. It providesfurther evidence that the inoceramid bivalves be-came extinct extremely early in Antarctica and thatthe dimitobelid belemnites did not persist in Ant-arctica beyond the Early Maastrichtian, becomingextinct earlier in Antarctica than in the NorthernHemisphere. Thus, between the Northern andSouthern Hemispheres there is a disparity in theage of some extinction events.

5. A major radiation of Antarctic angiosperm taxais dated as Coniacian in age. The potential nowexists to date the origination and radiation of anumber of other modern plant and animal groupsthrough the latest Cretaceous in the southern highlatitudes.

638 J . M . M C A R T H U R E T A L .

A C K N O W L E D G M E N T S

The isotopic measurements were made by J.M.M.in the Radiogenic Isotope Laboratory at Royal Hol-loway University of London; the laboratory is sup-ported, in part, by the University of London as anintercollegiate facility. Elemental analysis wasdone in the ICP-AES Laboratory at Royal HollowayUniversity of London; the laboratory is supported,in part, by the Natural Environment ResearchCouncil (NERC) as a central facility, and we thankthe director, J. N. Walsh, for its use. We thank G.Ingram for assisting with the isotopic measure-

ments, A. Osborn for assistance with elementalanalysis, and British Antarctic Survey colleagueswho assisted with fieldwork in the James Ross Is-land group. We are particularly grateful to D. Pirrie(Camborne School of Mines) for providing the sam-ples from localities D.8657 and D.8664 and usefuldiscussion. We thank K. Macleod, B. Zinsmeister,and an anonymous referee for constructive reviewsthat improved the final article. This is a contri-bution to International Geological Correlation Pro-gramme project 381, “South Atlantic MesozoicCorrelations,” and was supported by NERC grantGR3/AFI2/38.

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