ARTICLES Definition of Late Cretaceous Stage Boundaries In

Home , Barroisiceras, Coniacian, Eubostrychoceras, James Ross Island group, Santonian

ARTICLES Deﬁnition 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 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 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 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.

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 (figs. 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 floras quence is, in places, very fossiliferous. It offers an first 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 flora from Manuscript received November 12, 1999; accepted July 5, the Hidden Lake Formation (fig. 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-

623 624 J. M. MCARTHUR ET AL. morph floras, 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 flow- 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 finer-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 (figs. 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 flourish 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 (figs. 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 (figs. 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 (fig. 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; fig. 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 (figs. 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 (fig. 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 fig. 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 flank of the Antarctic Peninsula (fig. 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, fig. 1). The left inset at the top of the map is expanded as figure 2.BB p Brandy Bay, CL p Cape Lamb,RC p Rum Cove,SMC p St. Martha Cove,TC p Tumbledown Cliffs. mation (fig. 3; Ineson et al. 1986). Some 350–400 into St. Martha Cove; see fig. 2) and typically com- m thick, this unit is characterized by coarse-grained prises massive, very fine to medium-grained sand- sandstones and matrix-supported conglomerates in stones and silty sandstones (fig. 3; Olivero et al. its lower levels, and medium- to fine-grained sand- 1986). It is characterized by a marked increase in stones in its upper ones. Some fine-grained sand- marine benthic fauna and by a rise in the number stones and siltstones within it are intensely bio- of infaunal taxa (Scasso et al. 1991). turbated. The transition into the overlying Santa The uppermost levels of the Santa Marta For- Marta Formation, the basal lithostratigraphic unit mation can be traced across to the base of a 480- within the Marambio Group, is marked by a dis- m-thick sequence exposed on Cape Lamb, Vega Is- tinct change from the rusty brown and greeny land (figs. 2, 3), where they pass conformably brown weathering hues to predominantly gray col- upward into the Cape Lamb Member of the Snow ors. The Santa Marta Formation is some 1200 m Hill Island Formation (Crame et al. 1991; Pirrie et thick in its type area (along line of section running al. 1991). Using SIS, the appearance of the prolific 626 J. M. MCARTHUR ET AL.

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

Gunnarites antarcticus ammonite fauna (ca. 3750 ting tools; the remaining sample was then broken m; fig. 3) within this unit has been dated as earliest into submillimeter-sized fragments. These were Maastrichtian in age (Crame et al. 1999). The top- cleaned by brief immersion in 1.2 M hydrochloric most 111 m of the Cape Lamb section, a sequence acid solution, washed in ultrapure water, and dried of volcaniclastic sandstones and conglomerates in a clean environment. The best-preserved frag- constituting the Sandwich Bluff Member, is as- ments were selected under the binocular micro- signed to the Lo´ pez de Bertodano Formation (figs. scope for analysis. Preservational criteria were de- 2, 3; Pirrie et al. 1997). gree of flakiness, amount of cracking and secondary veining, amount of cementation, color, opacity, and the presence of iron and/or manganese staining. Analytical Methods and Results Chemical Analysis. Data for samples received as Sample Preparation. Belemnite samples 73ap, powders are taken from Pirrie and Marshall (1990). 73am, 73an, 76a5, 76a6, 76b, 109a, 116a, and 122B For other samples, analysis for Ca, Mg, Sr, Na, were received as powders and were analyzed as re- Fe, and Mn was done by inductively coupled ceived. In addition, one aragonitic Nucula and 17 plasma–atomic emission spectrometry (ICP-AES) other calcitic macrofossils (oysters, inoceramids, after dissolution of the sample in 1.8 M acetic acid; belemnites) were analyzed (tables 1, 2). The sam- analysis for Rb was done on the same solutions by ples are from 12 stratigraphic levels; for complete- graphite furnace atomic absorption spectrometry. ness, a thirteenth level is reported from Crame et Results are given in table 1. The precision of the Powdered samples .5%ע al. (1999). From unpowdered samples, visually al- analyses was better than tered portions were removed using diamond cut- were 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 presented herein.CE. p Cenomanian ,SANT. p Santonian . Precise biostratigraphic correlations between localities D.8409 and DJ.685 (southeastern James Ross Island; fig. 1) have yet to be established. Table 1. Isotopic and Elemental Data for Antarctic Fossils Stratigraphic level (m) Section and 87 Sr/86Sr Numerical Ca Mg Sr Na Fe Mn Rb (s.e.) n age (%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm 2 ע sample nos. Sample type Composite Crame Col Mineralogya (mean Vega Island: DJ.83: 3. ע 71.0 17 000004. ע Various 3800 .707736 160–68 James Ross Island: DJ.685: 06. 761 721 1265 831 2412 39.1 4. ע 74.2 4 000005. ע Giant 3300–3600 c, tr d .707646 83 inoceramid D.8409: 01.! 297 163 2421 970 1676 38.9 2 000011. ע Giant ≈3150 c .707554 17 inoceramid 01.! 200 152 2042 970 1394 39.1 3 000009. ע Giant ≈3150 c .707553 14 inoceramid DJ.955: 5. ע 78.7 1 000015. ע Giant ≈3150 .707557 14 inoceramid D.8664: 4 000007. ע 184a2 Nucula 2830 P .707493 01.! 13 73 5019 3510 249 38.4 5. ע 82.6 3 000009. ע 184a1 Nucula 2830 ar, tr c .707493 D.8657: 04. 21 107 3298 1664 897 38.4 4. ע 85.5 3 000009. ע 122B Belemnite 2602 100–103 P .707429 10 210 1523 660 7. ע 85.8 1 000015. ע 116a Belemnite 2594 92.8–95.5 P .707421 9 140 1461 600 5. ע 85.8 2 000011. ע 109a Belemnite 2589 82.5–92.0 P .707422 01.! 1 4 1231 1245 718 39.4 5. ע 85.8 3 000009. ע Belemnite 2565 64.0–65.5 c .707420 95 58 210 1570 600 1 000015. ע 76b Belemnite 2549 49.0–49.5 P .707405 04. 36 115 2774 1560 800 39.4 2 000011. ע 76a7 Belemnite 2548 47.0–49.0 c .707412 17 210 1450 660 2 000011. ע 76a6 Belemnite 2548 47.0–49.0 P .707407 19 210 1760 840 1 000015. ע 76a5 Belemnite 2548 47.0–49.0 P .707406 01.! 3 9 1513 1336 409 39.7 2 000011. ע 76a Belemnite 2548 47.0–49.0 c .707411 16 210 1610 660 2 000011. ע 73an Belemnite 2548 47.0–49.0 P .707409 51 210 1940 840 2 000011. ע 73am Belemnite 2548 47.0–49.0 P .707420 12 210 1720 1020 1 000015. ע 73ap Belemnite 2500 44.3–48.0 P .707415 01.! 4 4 1470 1221 551 38.6 2. ע 86.1 2 000011. ע Belemnite 2500 44.3–48.0 c .707415 73 D.8228: 02. 239 128 5065 1289 4008 38.5 4 000003. ע Inoceramid 2420 c .707385 333 01.! 59 103 5204 1239 3730 38.2 2 000011. ע Inoceramid 2420 c, tr an .707389 331 01.! 38 26 5075 1177 85 38.4 2. ע 87.0 3 000009. ע Inoceramid 2420 c .707379 326 65 79 4372 1201 5215 38.8 4 000012. ע Inoceramid 2360 c .707381 303 01.! 63 103 2780 1275 6850 38.4 3. ע 87.1 4 000002. ע Inoceramid 2360 c, tr d .707381 94 01.! 305 298 858 704 564 39.3 3 000009. ע Oyster 1880 c .707288 114 01.! 71 110 758 701 609 40.5 2. ע 91.0 3 000009. ע Oyster 1880 c .707291 113 01.! 47 221 790 813 403 40.1 3 000009. ע Oyster 1760 c, tr q .707292 82 01.! 95 63 586 589 413 39.1 4. ע 89.8 3 000009. ע Oyster 1760 c .707287 81 ,s.e. of replicate analysis. Numerical ages, and age uncertainties 2ע Note. Mean data only are given for 87Sr/86Sr; full data are in table 2. Uncertainties on 87Sr/86Sr are at 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/86Sr for 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 CRETACEOUS STAGE BOUNDARIES 629

Table 2. Replicate 87Sr/86Sr Data for Samples 987 and samples, the precision of measurement ,( n p 1) 0.000015ע Sample number 87Sr/86Sr value of replicate analysis (2 s.e.) was better than p ע p ע DJ.83.68–160a 0.000011 (n 2 ), and 0.000009 (n 3 ). Total DJ.685.83 .707651 .707645 .707639 .707647 blanks were !2 ng of Sr; samples contained 110 mg D.8409.17 .707549 .707558 of Sr. Concentrations of Rb were too low to require D.8409.14 .707555 .707553 .707550 corrections for radiogenic Sr. Data are given in table DJ.955.14 .707557 1 as means of replicate analysis. Full data are in D.8664.184a2 .707500 .707498 .707484 .707491 D.8664.184a1 .707494 .707498 .707488 table 2. D.8657.122B .707423 .707432 .707433 D.8657.116a .707421 D.8657.109a .707422 .707422 Discussion D.8657.95 .707418 .707418 .707425 Quality of Sample Preservation. The quality of D.8657.76b .707405 D.8657.76a7 .707411 .707412 preservation of nine belemnites received as pow- D.8657.76a6 .707413 .707401 ders (from the line of section D.8657; fig. 2; tables D.8657.a5 .707406 1, 2) and separate subsamples of several other sam- D.8657.76a .707410 .707412 ples we have analyzed, are discussed in Pirrie and D.8657.73an .707418 .707410 Marshall (1990), who conclude that preservation D.8657.73am .707425 .707414 D.8657.73ap .707415 was good for belemnites and less good for inocer- D.8657.73 .707413 .707418 amids. Our unpowdered samples were visually well D8228.333 .707386 .707389 .707384 .707381 preserved and our inoceramids, after picking, ap- D8228.331 .707391 .707386 pear better preserved than the samples described by D8228.326 .707375 .707381 .707381 those authors. On the basis of XRD, most samples D8228.303 .707381 .707376 .707397 .707368 D8228.94 .707382 .707384 .707379 .707380 were monomineralic, but five (table 1) contained D8228.114 .707288 .707289 .707287 traces of contaminant phases (quartz, dolomite, D8228.113 .707292 .707290 .707292 analcime, calcite). The aragonitic Nucula con- D8228.82 .707292 .707287 .707298 tained about 0.2% calcite, the minimum detectable D8228.81 .707289 .707292 .707279 by XRD, but replicate 87Sr/86Sr analysis gave repro- a See Crame et al. 1999. ducible data so we believe alteration (which pro- ceeds patchily) had not significantly altered the other samples, mineralogy was determined using a 87Sr/86Sr of this sample. Furthermore, contaminant Phillips PW 1710 diffractometer. Instrumental con- calcite would have derived its Sr mostly from pre- ditions were: CuKa radiation generated at 40 kV cursor aragonite and its concentration in secondary and 30 mA; scanning through 5Њ to 50Њ 2v at 0.5Њ calcite is much lower than in primary aragonite, 2v/min, a range that includes the major diffraction thereby lessening any effect of contaminant Sr on peaks of aragonite, calcite, and dolomite. With 87Sr/86Sr. these conditions, the detection limit of our appa- Examination of acid-leached samples of inocer- ratus, determined by standard additions with pure amids by scanning electron microscopy (SEM) (fig. phases, was about 0.2% calcite in aragonite and 4a,4b) showed that trace impurities were deposited 0.5% dolomite in calcite. between individual prisms in a few samples, but Isotopic Analysis. For 87Sr/86Sr analysis, samples individual prisms were easily separated from the of about 20 mg were dissolved in 2.5 M hydro- impurity (fig. 5a,5b) by picking under the micro- chloric acid, and Sr was separated by standard ion- scope. The impurity phases (quartz, analcime, dol- exchange chemistry. Measurements were made omite) in inoceramids contain little Sr, were vol- with a VG 354 mass spectrometer using the multi- umetrically unimportant, were avoided during dynamic routine SrSLL that includes corrections picking, and, for quartz and analcime, are insoluble for isobaric interference from 87Rb (Thirlwall 1991). in acid: the influence of such phases on the Data are presented in table 1 as means of replicate 87Sr/86Sr of picked samples was negligible. Inocer- measurement of 87Sr/86Sr. Data are normalized to a amid prisms were altered and cloudy at their ends, value of 0.1194 for 86Sr/88Sr and adjusted to a value where alteration was localized, but clear in their s.e.,n p 19 ) for middle portions (fig. 5a,5b), and it was the middle 2) 0.0000025 ע of0.7102480 NIST 987, which corresponds to a value of portions that were picked for analysis. Finally, the s.e.,n p 19 ) for EN 1. 87Sr/86Sr values of three giant inoceramids, with 2) 0.0000032 ע 0.7091746 Adjustment was to the cumulative mean of NIST very different preservational states, from one strat- 987 values collected during periods when samples igraphic level and locality at Carlsson Bay in the were analyzed. Based on replicate analysis of NIST southwest of James Ross Island (DJ.955, samples 5, 630 J. M. MCARTHUR ET AL.

Figure 4. SEM photographs of inoceramid samples D.8409.14 (a) and DJ.8228.303 (b), showing trace impurities cementing 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: the vealed the compact layering and surface ornamen- well-preserved sample, DJ.955.14, has an 87Sr/86Sr tation on individual layers that we interpret to be table 1) indistinguishable characteristic of pristine oyster carbonate. Optical ; 0.000015 ע 0.707557) from that of giant inoceramids from northern James examination of oysters showed flaky, translucent calcite (fig. 5c,5d) with only local Fe or Mn staining ; 0.000011 ע Ross Island (locality D.8409;0.707554 table 1). Such agreement would be unlikely unless that was avoided during sampling. these samples were unaltered and also from the Concentrations of Fe and Mn exceeded 305 ppm same stratigraphic level. Sample DJ.955.8 was only in sample 83 (cf. Veizer 1983; Veizer et al. rather cemented and altered and its 87Sr/86Sr is 1992; McArthur 1994; table 1). There is a weak 0.707548, while sample DJ.955.5 was solidly ce- correlation between Fe and Mn in the samples (fig. mented so that prisms had completely fused and 6) but no relationship between 87Sr/86Sr and either were not visible as individuals; its 87Sr/86Sr value is Fe or Mn (fig. 6). We attribute the measured con- 0.707532. Thus, severe alteration has lowered centrations of Fe and Mn not to the occurrence of 87Sr/86Sr by no more than 0.000025, a value only a structural Fe and Mn in altered carbonate, but to little more than analytical uncertainty; a lowering the presence of surface coatings of Fe/Mn oxyhy- of 87Sr/86Sr would be expected on alteration since droxides on crystallites. the clastic sediments of the area are largely of man- Within each stratigraphic level, different samples tle affinity in this back-arc environment of the Ant- have indistinguishable 87Sr/86Sr values (table 1, cf. arctic Peninsula. DJ.8409, samples 14 and 17 with DJ.955, sample Examination of oysters with SEM (fig. 4c,4d) re- 14; in section D.8228, cf. sample 81 with sample Journal of Geology L A T E CRETACEOUS STAGE BOUNDARIES 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 samples D.8228.82 (c) and D.8228.113 (d). All views are at #25 magnification.

82, sample 113 with sample 114, sample 94 with because stratigraphic separation of many closely sample 303, sample 326 with samples 331 and 333, spaced samples was small, and represented a period etc.) thereby attesting to good preservation. Finally, of time so small, that evolutionary differences be- samples have 87Sr/86Sr values that are consistent tween levels in marine 87Sr/86Sr would have been with their stratigraphical order. The above facts undetectable with our measurements. For strati- suggest that the samples are well preserved and re- graphic levels with more than three measurements tain their original 87Sr/86Sr values, so we accept our of 87Sr/86Sr (table 1), mean values of 87Sr/86Sr and 2 87Sr/86Sr data as recording the 87Sr/86Sr values of Late s.e. of the means were used to derive numerical Cretaceous seawater. ages and uncertainty limits on the ages. Where Numerical Ages. These have been determined fewer than three values were available, uncertain- using version 3:10/99 of the LOWESS look-up table ties were taken to be twice the standard error of ,0.000015ע of Howarth and McArthur (1997); these authors the cumulative mean for NIST 987 of -forn p 1 , 2, and 3, re ,0.000009ע or ,0.000011ע give a full description of the LOWESS method and table, table derivation, and estimation of uncer- spectively. The uncertainty on the ages combines tainty. The relevant part of the 87Sr/86Sr curve, de- the uncertainty inherent in the reference curve of rived from version 3:10/99, is shown in ﬁgure 7. In Howarth and McArthur (1997, version 3:10/99) and table 1, we have calculated numerical ages for sam- the uncertainty (2 s.e.) in the mean 87Sr/86Sr values ples bundled into stratigraphic levels rather than of each stratigraphic level (table 1). provide a separate age for each sample. We do so Stratigraphic Correlations. Numerical ages suffer 632 J. M. MCARTHUR ET AL.

from the uncertainties associated with placing such ages onto biostratigraphic schemes. Correlation with 87Sr/86Sr, however, bypasses such problems by directly comparing 87Sr/86Sr values in different se- quences. In figures 8–10, we show where in the biostratigraphic schemes for the Northern Hemi- sphere (McArthur et al. 1993a, 1993b, 1994) the 87Sr/86Sr values for our Antarctic specimens occur. Correlations with the biostratigraphic zonation of the English Chalk are shown in figures 8 and 9. In figure 8, 87Sr/86Sr values through the Chalk, as proven in a borehole at Trunch, Norfolk, United Kingdom (data from McArthur et al. 1993a, 1993b), are plotted against depth in the borehole. In this section, a hard ground disturbs the 87Sr/86Sr curve at the Turonian/Coniacian boundary, but minor ex- trapolation allows isotopic correlation into the uppermost Turonian. The detailed zonation of the sequence is given in figure 9, together with the levels to which Antarctic 87Sr/86Sr ratios correlate on the basis of the 87Sr/86Sr curve in figure 8. The lowest Antarctic level (samples 81, 82, 113, 114) correlates with the upper part of the upper Turonian macrofossil zone of Sternotaxis planus and, in the nannofossil zonation of Burnett (1990, revised from Sis- singh 1977), at a level between the upper part of CC12 and the lower part of CC13. Samples 94, 303, 326, 331, and 333 are Late Coniacian in age and correlate with the lower part of the macrofossil zone of Micraster coranguinum and the nannofossil zone topmost CC14/bottommost CC15. Samples 73, 73ap, 73ap, 73am, 76a, 76a5, 75a6,76a7, 76b, 95, 109a, 116a, and 122B spread across 20 m of sequence from the topmost Coniacian to the lower Santonian and are all within both CC15 and the macrofossil zone of M. coranguinum. Sample 184a correlates with the Lower Campanian macrofossil zone of Offaster pilula and the nannofossil zone CC17; 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 lower part of CC20; sample 83 is Late Campanian in age and correlates with the midrange of the Belemni- tella mucronata zone and the top of CC20. Samples 68–160 are basal Maastrichtian in age, fall in the upper Belemnella lanceolata zone, and are discussed in Crame et al. (1999). Correlations with the standard ammonoid zonation for the U.S. Western Interior using the U.S. Western Interior 87Sr/86Sr standard curve (McArthur et al. 1994) are plotted in figure 10, where 87Sr/86Sr for zones are plotted in the middle of the zonal Figure 6. Covariance of Mn, Fe, and 87Sr/86Sr in Ant- ranges, which are all taken to be of equal duration. arctic macrofossils. Sample DJ.685.83 is off scale on all As the 87Sr/86Sr calibration is based on a zonal plots and is not shown. scheme, it is less precise than it would be if based Journal of Geology L A T E CRETACEOUS STAGE BOUNDARIES 633

Figure 7. Reference curve of Howarth and McArthur (1997, version 3:10/99) for the interval 65–95 Ma. Confidence intervals 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, and U.S. reference curves for the period Turonian as is the case for Europe. In addition, the 87Sr/86Sr to middle Santonian (the curve for the Late San- of several zones (e.g., Clioscaphites saxitonianus) tonian and Campanian is better defined; McArthur must be derived by interpolation, making correla- et al. 1993a, 1993b). For more precise correlations, tion with such levels less accurate than would be more data are required for this interval for both the case were more data available. Samples 81, 82, Antarctica and Europe. This is particularly true for 113, and 114 fall within the latest Turonian zones the zone of C. saxitonianus for which the lack of of Scaphites whitfieldi and Prionocyclus quadra- 87Sr/86Sr data results in the 87Sr/86Sr calibration tus. Samples 94, 303, 326, 331, and 333 fall within curve (fig. 10) perhaps giving ages that are too young the Late Coniacian zone of Scaphites depressus, by one zone. and samples 73, 73ap, 73ap, 73am, 76a, 76a5, Stratigraphic Interpretation. At approximately 75a6,76a7, 76b, 95, 109a, 116a, and 122B correlate the 1350-m level in the combined section a dis- with the C. saxitonianus zone of the basal Santon- tinctive fauna that includes the inoceramid bivalve ian. Sample 184a is placed in the zonal range upper Birostrina concentrica (Parkinson) indicates a Late Scaphites hippocrepis III/lower Baculites sp. Albian age (fig. 3; Ineson et al. 1986). Immediately smooth; samples D.8409.14, D.8409.17, and above the 1600-m unconformity a second inocer- DJ.955.14 correlate with the junction of the zones amid, Inoceramus madagascariensis Heinz, is lo- of Baculites sp. smooth and Baculites sp. weakly cally abundant, suggesting a Late Turonian/Early ribbed; sample 83 falls in the Exitaloceras jennyi Coniacian age (Heinz 1933; Crame 1983) for this zone and samples 68–160 (Crame et al. 1999) fall level, although Crampton (1996) suggests that its in the uppermost Baculites eliasi zone, which is occurrence in the Barroisiceras onilakyense am- the lowermost Maastrichtian macrofossil zone. monite zone in Madagascar is more compatible These attempts to correlate with Northern with a middle Coniacian age. The unequivocal Late Hemisphere schemes are somewhat compromised Turonian age SIS ages of samples 81, 82, 113, and by the scarcity of data used to compile the European 114 in both Europe and North America (figs. 8–10) 634 J. M. MCARTHUR ET AL.

Figure 8. Strontium isotope correlation of Antarctic levels with the 87Sr/86Sr profile of the English Chalk of the United 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 73 and 76 represent samples 73a, 73ap, 73am, 73an, 76a5, 76a6, 76a7, and 76b. show that this might be the youngest possible age representatives of a complex of Coniacian inocer- for this species in Antarctica (fig. 3). amid taxa that seem to have their closest links with Unfortunately, the presence of the unconformity European forms such as Inoceramus (Inoceramus) at about 1600 m in the section means that there is inaequivalvis Schlu¨ ter and Inoceramus (Inocera- no direct evidence of the age of the base of the I. mus) koegleri Andert. As these are both Lower Con- madagascariensis zone in Antarctica. It does not iacian taxa in Europe, this seems a suitable place occur in palaeontologically well-dated Late Ceno- at which to set the Turonian/Coniacian boundary. manian strata in the Tumbledown Cliffs/Rum This first complex grades into a second that in- Cove area (fig. 1; Ineson et al. 1986), and thus it cludes the widely interpreted species “Mytiloides” may be entirely Turonian (?Late Turonian) in age africanus (Heinz). This taxon has been given both (fig. 3). Such an age is of considerable stratigraphical Coniacian and Santonian ages in Madagascar (Sor- significance, for this taxon is widespread in the nay 1964), but it is Early Coniacian in Europe Southern Hemisphere, occurring in Patagonia, (Herm et al. 1979; Walaszczyk 1992). It is also ap- Madagascar, and New Zealand (Crampton 1996) as parent that some forms within this second complex well as Antarctica. As the range of I. madagascar- resemble the highly variable Inoceramus australis iensis in New Zealand is mostly coincident with Woods from the Piripauan Stage (uppermost Con- that of the Teratan Stage, it seems that the age of iacian/middle Santonian) of New Zealand (see, e.g., this local chronostratigraphic unit must be revised Crampton 1996, plate 9F). The late Middle/Late from middle Coniacian to Turonian (?Late Turon- Coniacian 87Sr/86Sr ages for samples 94, 303, 326, ian). This in turn has implications for the age ranges 331, and 333 (figs. 8–10) indicate that almost the of other inoceramid taxa and stage boundaries in entire “M.” africanus species complex is Coniacian New Zealand (Crampton 1996, fig. 26). in age (fig. 3). The same conclusion can be applied Just beneath the junction between the Whisky to a co-occurring large, erect inoceramid resem- Bay and Hidden Lake Formations (1930-m level of bling Inoceramus expansus Baily from South Africa fig. 3; Herm et al. 1979; Crame 1983) occur the first that was thought to be no older than Santonian Journal of Geology L A T E CRETACEOUS STAGE BOUNDARIES 635

D.8657, at Crame Col (figs. 2, 3), give 87Sr/86Sr ratios that correlate with the basal Santonian C. saxitonianus zone of the U.S. Western Interior and the upper M. coranguinum zone of NW Europe (figs. 9, 10), so we place the Coniacian-Santonian boundary at a level just beneath the first occurrence of these belemnites, around the 150-m level in Crame Col and 2500 m in the combined section (fig. 3). As a result of this placement, a distinctive group of heteromorph ammonites based on the genera Eubos- trychoceras, Pseudoxybeloceras, Ainoceras, and Ryugasella (fig. 3) that, because of their affinities with taxa from Japan, Madagascar, and the Pacific, were originally taken to indicate an Early Cam- panian age (Olivero 1988), must instead be no younger than earliest Campanian and are seen mainly to be Santonian or even latest Coniacian in age, as inferred by Olivero (1992), as some of them range 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 locality through its correlation with the upper S. hippocrepis III/lower Baculites sp. smooth zone in the U.S. Western Interior and O. pilula zone in NW Europe (figs. 8, 9). A slightly younger latest Early Campanian age is assigned to the two giant inoceramid bivalve samples from locality D.8409 and one from locality DJ.955.14 from the same stratigraphic level in Carlsson 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) indicates a yet younger Campanian age. Precise biostratigraphic correlations have yet to be made between Rabot Point, Carlsson Bay, and northern Figure 9. Strontium isotope correlation of Antarctic James Ross Island (Crame and Luther 1997) but our samples with the biostratigraphy of the Cretaceous Eng- 87Sr/86Sr data indicate that locality DJ.865 should lish Chalk (McArthur et al. 1993a, 1993b). Macrofossil biostratigraphy and stage boundaries from Wood et al. occur around the 3300–3600-level on our compos- (1994). Nannofossil zonation from Burnett (1990). Sam- ite section (fig. 3) and that locality DJ.955 is strati- ple label 14 includes DJ.955.14 and D.8409.14. Sample graphically at the same levels as locality D.8409. labels 73 and 76 represent samples 73a, 73ap, 73am, 73an, In this study, the Santonian/Campanian boundary 76a5, 76a6, 76a7, and 76b. is placed slightly beneath the level of D.8664 (sample 184a; fig. 3) and the Campanian/Maastrichtian boundary is placed in the lowermost levels of the (Crame 1983). Even the first occurrences of the dis- Snow Hill Island Formation on Vega Island (Crame tinctive heteromorph ammonite Baculites bailyi et al. 1999). Woods in the lowermost levels of the Santa Marta Palaeobiological and Palaeoenvironmental Implica- Formation must now be taken to represent a Late tions. Early extinction patterns for inoceramid bi- Coniacian rather than Early Santonian age (sensu valves and dimitobelid belemnites in the James Crame et al. 1991). One immediate and important Ross Basin have already been established (Crame implication of these new findings is that the Hid- et al. 1996; Zinsmeister and Feldmann 1996). The den Lake Formation is entirely Coniacian in age last inoceramids, of the giant species A. rabotensis, (fig. 3; see below). are found well below the KT boundary in strata The dimitobelid belemnites from locality believed, before this study, to be mid to Late Cam- 636 J. M. MCARTHUR ET AL.

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 of variation of 87Sr/86Sr through the zones. 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. panian in age (Crame et al. 1996, fig. 2). Our patterns, it is necessary to plot the stratigraphical 87Sr/86Sr ages from localities D.8409 and DJ.955 ranges of other key taxa through the Late Creta- show that these giant inoceramids are, in the Eu- ceous of Antarctic. Ammonites will be one partic- ropean sense, Early Campanian in age, while those ularly important group to study in this respect; al- from locality DJ.865.83 are mid Late Campanian in though they are present right up to the KT age. These ages confirm the disparity in age be- boundary at most key localities in the world, they tween last occurrences in the Northern and South- had probably been in decline since the mid to early ern Hemispheres. Late Cretaceous (Kennedy 1989). Ward and Signor Similarly, before this study, the last dimitobelid (1983) have documented a steady decline in num- belemnites in the Crame Col/Brandy Bay region bers of genera from the Coniacian to the Maastrich- were thought to be Early to mid Campanian (Crame tian stage. Zinsmeister and Feldmann (1996) re- et al. 1996, fig. 2). Nevertheless, the new strontium corded a reduction in numbers of ammonite species ages from localities D.8657 and D.8664 indicate from about 45 in the lower Santa Marta Formation that this age range should in fact be Early Santon- (Late Santonian/Early Campanian in this study) to ian/Early Campanian and that there is an even 15 in the Late Maastrichtian of Seymour Island, greater time gap between these occurrences and a with just five at the KT boundary (see also Zins- single, later Maastrichtian (?mid Maastrichtian) ho- meister 1998). However, our inventory of Antarctic rizon of belemnites recorded on Seymour Island, ammonites is still incomplete, and an important which is some 625 m beneath the KT boundary Coniacian fauna from the uppermost Whisky Bay, (Doyle and Zinsmeister 1988). Hidden Lake, and lower Santa Marta Formations In order to understand what controls extinction (fig. 3) is in the process of being described (M. R. Journal of Geology L A T E CRETACEOUS STAGE BOUNDARIES 637

A. Thomson and P. Bengston, pers. comm., 1999). still, a group of nine samples from five close levels It may be that there is an even longer-term pattern correlates with the upper M. coranguinum zone of to the decline of ammonites in Antarctica, a pattern Europe and the basal Santonian Clioscaphites sax- similar to that seen in other regions. itonianus zone in the U.S. Western Interior. Eleven The richest Antarctic Late Cretaceous inocer- samples from four higher levels are Campanian to amid bivalve fauna is based on two species com- Maastrichtian in age. plexes that this study shows are almost entirely 2. The 87Sr/86Sr data show that the uppermost Coniacian in age (fig. 3). This finding fits a global Whisky Bay Formation is no younger than earliest pattern of marked expansion of inoceramids in the Coniacian in age, with most of it being assigned to Middle Turonian–Coniacian, followed by a gradual the Turonian. The succeeding Hidden Lake For- decline into the Late Maastrichtian (Pergament mation is entirely Coniacian in age, and the bound- 1978; Sornay 1981; Dhondt 1992). The congruity of ary between the respective Gustav and Marambio these worldwide patterns between ammonites and groups must now be set at Late Coniacian. The inoceramids suggests that we may be looking at a Coniacian/Santonian boundary may be as high as single, underlying cause, and an obvious one to con- the 150-m level in the Crame Col section, and the sider is climate change. For example, Huber (1998) Santonian stage may be represented by no more has highlighted the similarity of Late Cretaceous than approximately 225 m of strata. The Santa palaeotemperature curves between Arctic and Ant- Marta Formation is Late Coniacian/latest Campan- arctic regions. In the former, fossil leaf physiog- nomy suggests a temperature maximum during Tu- ian in age and the Campanian/Maastrichtian ronian/Coniacian time; in the latter, oxygen boundary is placed in the lowermost levels of the isotope palaeotemperature analysis of deep sea Snow Hill Island Formation on Vega Island. cores suggests a maximum temperature during Tu- 3. Inoceramus madagascariensis, an important ronian/Santonian time. There is increasing evi- fossil for regional correlations, should now be re- dence to suggest that the Campanian and Maas- garded as Turonian (and probably Late Turonian) in trichtian stages represent a pronounced phase of age. Similarly, certain members of the “Mytiloides” global cooling (Zinsmeister and Feldmann 1996; africanus species complex have considerable re- Huber 1998). gional stratigraphical potential, and they can be Stable isotope palaeothermometry has also been taken to be almost exclusively Late Coniacian in attempted through the James Ross Island group suc- Antarctica. Inoceramus cf. expansus has a Late cession and a pronounced Turonian/Coniacian Coniacian/Early Santonian age range. An impor- peak in values established (Ditchfield et al. 1994). tant group of Antarctic heteromorph ammonites, This study now needs to be refined and extended, centered on the genera Eubostrychoceras, Pseu- for the values obtained have been plotted in terms doxybeloceras, Ainoceras, and Ryugasella,is of either major lithostratigraphical subdivisions or shown to be no younger than earliest Campanian standard Cretaceous stages (Ditchfield et al. 1994; in age, with members of this group ranging down Dingle and Lavelle 1998). Individual values can to the uppermost Coniacian. now be replotted on the composite stratigraphic 4. The new age calibration of the Antarctic Late section and tied in much more precisely to the ver- Cretaceous succession has important implications tical ranges of key fossil taxa. for both extinction and radiation events. It provides further evidence that the inoceramid bivalves be- Conclusions came extinct extremely early in Antarctica and that the dimitobelid belemnites did not persist in Ant- 1. Strontium isotope analysis of macrofossils from arctica beyond the Early Maastrichtian, becoming Antarctica shows that the oldest group of samples correlate definitively with the Upper Turonian extinct earlier in Antarctica than in the Northern Sternotaxis planus zone of Norfolk, United King- Hemisphere. Thus, between the Northern and dom, and approximately the junction between the Southern Hemispheres there is a disparity in the Scaphites whitfieldi and Prionocyclus quadratus age of some extinction events. zones (Upper Turonian) in the U.S. Western Inte- 5. A major radiation of Antarctic angiosperm taxa rior. A younger group of five samples, from two is dated as Coniacian in age. The potential now close stratigraphic levels, correlate with the up- exists to date the origination and radiation of a permost Coniacian Scaphites depressus zone of the number of other modern plant and animal groups U.S. Western Interior and the lower Micraster cor- through the latest Cretaceous in the southern high anguinum zone of the Chalk of NW Europe. Higher latitudes. 638 J. M. MCARTHUR ET AL.

ACKNOWLEDGMENTS ments, A. Osborn for assistance with elemental The isotopic measurements were made by J.M.M. analysis, and British Antarctic Survey colleagues who assisted with ﬁeldwork in the James Ross Is- in the Radiogenic Isotope Laboratory at Royal Hol- land group. We are particularly grateful to D. Pirrie loway University of London; the laboratory is sup- (Camborne School of Mines) for providing the sam- ported, in part, by the University of London as an ples from localities D.8657 and D.8664 and useful intercollegiate facility. Elemental analysis was discussion. We thank K. Macleod, B. Zinsmeister, done in the ICP-AES Laboratory at Royal Holloway and an anonymous referee for constructive reviews University of London; the laboratory is supported, that improved the ﬁnal article. This is a contri- in part, by the Natural Environment Research bution to International Geological Correlation Pro- Council (NERC) as a central facility, and we thank gramme project 381, “South Atlantic Mesozoic the director, J. N. Walsh, for its use. We thank G. Correlations,” and was supported by NERC grant Ingram for assisting with the isotopic measure- GR3/AFI2/38.

REFERENCES CITED

Burnett, J. A. 1990. A new nannofossil zonation scheme review. Palaeogeogr. Palaeoclimatol. Palaeoecol. 141: for the Boreal Campanian. Int. Nannoplankton Assoc. 215–232. Newsl. 12(3):67–70. Ditchfield, P. W.; Marshall, J. D.; and Pirrie, D. 1994. High Crame, J. A. 1983. Cretaceous inoceramid bivalves from latitude palaeotemperature variation: new data from Antarctica. In Oliver, R. L.; James, P. R.; and Jago, J. the Tithonian to Eocene of James Ross Island, Ant- B., eds. Antarctic earth science. Canberra, Australian arctica. Palaeogeogr. Palaeoclimatol. Palaeoecol. 107: Academy of Science; and Cambridge, Cambridge Uni- 79–101. versity Press, p. 298–302. Doyle, P., and Zinsmeister, W. J. 1988. The new dimi- Crame, J. A.; Lomas, S. A.; Pirrie, D.; and Luther, A. 1996. tobelid belemnite from the Upper Cretaceous of Sey- Late Cretaceous extinction patterns in Antarctica. J. mour Island, Antarctica. In Feldmann, R. M., and Geol. Soc. Lond. 153:503–506. Woodburne, M. O., eds. Geology and paleontology of Crame, J. A., and Luther, A. 1997. The last inoceramid Seymour Island, Antarctic Peninsula. Geol. Soc. Am. bivalves in Antarctica. Cretac. Res. 18:179–195. Mem. 169:285–290. Crame, J. A.; McArthur, J. M.; Pirrie, D.; and Riding, J. Hallam, A. 1994. An outline of Phanerozoic biogeogra- B. 1999. Strontium isotope correlation of the basal phy. Oxford, Oxford University Press. Maastrichtian Stage in Antarctica to the European and Hayes, P. 1996. Late Cretaceous angiosperm leaf floras US biostratigraphic schemes. J. Geol. Soc. Lond. 156: of the Antarctic Peninsula. Palaeontol. Newsl. 32:16. 957–964. Heinz, R. 1933. Inoceramen von Madagaskar und ihre Crame, J. A.; Pirrie, D.; Riding, J. B.; and Thomson, M. Bedeutung fu¨ r die Kreide-Stratigraphie. Z. Dtsch. R. A. 1991. Campanian-Maastrichtian (Cretaceous) Geol. Ges. 85:241–259. stratigraphy of the James Ross Island area, Antarctica. Herm, D.; Kauffman, E. G.; and Weidman, J. 1979. The J. Geol. Soc. Lond. 148:1125–1140. age and depositional environment of the “Gosau”- Crampton, J. S. 1996. Inoceramid bivalves from the Late Group (Coniacian-Santonian), Bradenburg/Tirol, Aus- Cretaceous of New Zealand. Institute of Geological tria. Mitt. Bayer. Staatssamml. Palaeontol. Hist. Geol. and Nuclear Sciences Monograph 14, 188 p. Dettmann, M. E., and Thomson, M. R. A. 1987. Creta- 19:27–92. ceous palynomorphs from the James Ross Island area, Howarth, R. J., and McArthur, J. M. 1997. Statistics for Antarctica—a pilot study. Br. Antarct. Surv. Bull. 77: strontium isotope stratigraphy: a robust LOWESS fit 13–59. to the marine strontium isotope curve for the period Dhondt, A. V. 1992. Cretaceous inoceramid biogeogra- 0 to 206 Ma, with look-up table for the derivation of phy: a review. Palaeogeogr. Palaeoclimatol. Palaeoe- numerical age. J. Geol. 105:441–456. col. 92:217–232. Huber, B. T. 1998. Tropical paradise at the Cretaceous Dingle, R.; McArthur, J. M.; and Vroon, P. 1997. Oligo- poles? Science 282:2199–2200. cene and Pliocene interglacial events in the Antarctic Ineson, J. R.; Crame, J. A.; and Thomson, M. R. A. 1986. Peninsula dated using Sr isotope stratigraphy. J. Geol. Lithostratigraphy of the Cretaceous strata of west Soc. Lond. 154:257–264. James Ross Island, Antarctica. Cretac. Res. 7:141–159. Dingle, R. V., and Lavelle, M. 1998. Late Creta- Kennedy, W. J. 1989. Thoughts on the evolution and ex- ceous–Cenozoic climatic variations of the northern tinction of Cretaceous ammonites. Proc. Geol. Assoc. Antarctic Peninsula: new geochemical evidence and 100:251–279. Journal of Geology L A T E CRETACEOUS STAGE BOUNDARIES 639

McArthur, J. M. 1994. Recent trends in Sr isotope stra- ogy of the Admiralty Sound region, James Ross Basin, tigraphy. Terra Nova 6:331–358. Antarctica. Cretac. Res. 18:109–137. McArthur, J. M.; Kennedy, W. J.; Chen, M.; Thirlwall, M. Pirrie, D.; Crame, J. A.; and Riding, J. B. 1991. Late Cre- F.; and Gale, A. S. 1994. Strontium isotope stratigra- taceous stratigraphy and sedimentology of Cape phy for Late Cretaceous time: direct numerical cali- Lamb, Vega Island, Antarctica. Cretac. Res. 12: bration of the Sr isotope curve based on the US West- 227–258. ern Interior. Palaeogeogr. Palaeoclimatol. Palaeoecol. Pirrie, D., and Marshall, J. D. 1990. Diagenesis of Ino- 108:95–119. ceramus and Late Cretaceous palaeoenvironmental McArthur, J. M.; Kennedy, W. J.; Gale, A. S.; Thirlwall, geochemistry: a case study from James Ross Island, M. F.; Chen, M.; Burnett, J.; and Hancock, J. M. 1992. Antarctica. Palaios 5:336–345. Strontium isotope stratigraphy in the Late Creta- Riding, J. B.; Crame, J. A.; Dettmann, M. E.; and Cantrill, ceous: intercontinental correlation of the Campanian/ D. J. 1998. The age of the base of the Gustav Group Maastrichtian boundary. Terra Nova 4:385–393. in the James Ross Basin, Antarctica. Cretac. Res. 19: McArthur, J. M.; Thirlwall, M. F.; Chen, M.; Gale, A. S.; 87–105. and Kennedy, W. J. 1993a. Strontium isotope stratig- Riding, J. B.; Keating, J. M.; Snape, M. G.; Newnham, S.; raphy in the Late Cretaceous: numerical calibration and Pirrie, D. 1992. Preliminary Jurassic and Creta- of the Sr isotope curve and intercontinental correla- ceous dinoflagellate cyst stratigraphy of the James tion for the Campanian. Paleoceanography 8:859–873. Ross Island area, Antarctic Peninsula. Newsl. Stratigr. McArthur, J. M.; Thirlwall, M. F.; Gale, A. S.; Kennedy, 26:19–39. W. J.; Burnett, J. A.; Mattey, D.; and Lord, A. R. 1993b. Scasso, R. A.; Olivero, E. B.; and Buatois, L. A. 1991. Strontium isotope stratigraphy of the Late Cretaceous: Lithofacies, biofacies, and ichnoassemblage evolution a new curve, based on the English Chalk. In Hailwood, of a shallow submarine volcaniclastic fan-shelf dep- E. A., and Kidd, R. B., eds. High resolution stratigra- ositional system (Upper Cretaceous, James Ross Is- phy. Geol. Soc. Lond. Spec. Publ. 70:195–209. land, Antarctica). J. S. Am. Earth Sci. 4:239–260. McLaughlin, O. M.; McArthur, J. M.; Thirlwall, M. F.; Sissingh, W. 1977. Biostratigraphy of Cretaceous nan- Howarth, R.; Burnett, J.; Gale, A. S.; and Kennedy, W. noplankton. Geol. Mijnb. 56:3765. J. 1995. Sr isotope evolution of Maastrichtian sea- Sornay, J. 1964. Sur quelques nouvelles espe`ces water, determined from the chalk of Hemmoor, NW d’Inoce´rames du Seńonien de Madagascar. Ann. Pa- Germany. Terra Nova 7:491–499. leontol. Invertebr. 50:167–179. Medina, F. A.; Buatois, L.; and Lopez Angriman, A. 1992. ———. 1981. Inoce´rames. Cretac. Res. 2:417–425. Estratigrafia del Grupo Gustav en la isla James Ross, Sugarman, P. J.; Miller, K. G.; Bukry, D.; and Feigenson, Antartica. In Rinaldi, C. A., ed. Geologia de la isla M. D. 1995. Uppermost Campanian-Maastrichtian James Ross. Buenos Aires, Instituto Antarctico Ar- strontium isotopic, biostratigraphic, and sequence gentino, p. 167–192. stratigraphic framework of the new Jersey Coastal Medina, F. A., and Buatois, L. A. 1992. Bioestratigrafia Plain. Geol. Soc. Am. Bull. 107:19–37. del Aptiano-Campiano (Cretacico Superior) en la isla Thirlwall, M. F. 1991. Long-term reproducibility of James Ross. In Rinaldi, C. A., ed. Geologia de la isla multicollector Sr and Nd isotope ratio analysis. Chem. James Ross. Buenos Aires, Instituto Antarctico Ar- Geol. (Isotope Geosci. Section) 94:85–104. gentino, p. 37–46. Veizer, J. 1983. Trace elements and isotopes in sedimen- Obradovich, J. D. 1993. A Cretaceous timescale. In W. tary carbonates. In Reeder, R. J., ed. Carbonates: min- G. E. Caldwell, ed. Evolution of the Western Interior eralogy and chemistry. Rev. Mineral. 11:265–299. Foreland Basin. Geol. Assoc. Can. Spec. Pap. 39: Veizer, J.; Clayton, R. N.; and Hinton, R. W. 1992. Geo- 379–396. chemistry of Precambrian carbonates. IV. Early Pa- .Ga) seawater. Geochim 0.25 ע Olivero, E. B. 1988. Early Campanian heteromorph am- laeoproterozoic (2.25 monites from James Ross Island, Antarctica. Natl. Cosmochim. Acta 56:875–885. Geogr. Res. 4:259–271. Veizer, J.; Deiter, B.; Diener, A.; Ebneth, S.; Podlaha, O. ———. 1992. Asociaciones de ammonites de la Forma- G.; Bruckschen, P.; Jasper, T.; et al. 1997. Strontium cion Santa Marta (Cretacico tardio), isla James Ross, isotope stratigraphy: potential resolution and event Antartica. In Rinaldi, C. A., ed. Geologia de la isla correlation. Palaeogeogr. Palaeoclimatol. Palaeoecol. James Ross. Buenos Aires, Instituto Antarctico Ar- 132:65–77. gentino, p.47–76. Walaszczyk, I. 1992. Turonian through Santonian depos- Olivero, E. B.; Scasso, R. A.; and Rinaldi, C. A. 1986. its of the central Polish Uplands; their facies devel- Revision of the Marambio Group, James Ross Is- opment, inoceramid palaeontology and stratigraphy. land, Antarctica. Contrib. Inst. Antarct. Argent. Acta Geol. Pol. 42:1–122. 331:1–28. Ward, P. D., and Signor, P. W., III. 1983. Evolutionary Pergament, M. A. 1978. Upper Cretaceous stratigraphy tempo in Jurassic and Cretaceous ammonites. Palaeo- and inocerams of the Northern Hemisphere. Tr. Geol. biology 9:183–198. Inst. Akad. Nauk SSSR 322, 215 p. (In Russian.) Wood, C. J.; Morter, A. A.; and Gallois, R. W. 1994. Upper Pirrie, D.; Crame, J. A.; Lomas, S. A.; and Riding, J. B. Cretaceous stratigraphy of the Trunch Borehole (TG 1997. Late Cretaceous lithostratigraphy and palynol- 23 SE 8). In Arthurton, R. S.; Booth, S. J.; Morigi, 640 J. M. MCARTHUR ET AL.

A. N.; Abbott, M. A. W.; and Wood, C. J. Geology of a harbinger of global biotic catastrophe? In MacLeod, the country around Great Yarmouth. Memoir for N., and Keller, G., eds. Cretaceous-Tertiary mass ex- 1 : 50,000 geological sheet 162 (England and Wales). tinctions: biotic and environmental changes. New Her Majesty’s Stationery Ofﬁce, London. York, Norton, p. 303–325. Zinsmeister, W. J. 1998. Discovery of ﬁsh mortality Zinsmeister, W. J., and Macellari, C. A. 1988. Bivalvia horizon at the K-T boundary on Seymour Island: re- (Mollusca) from Seymour Island, Antarctic Penin- evaluation of events at the end of the Cretaceous. J. sula. In Feldmann, R. M., and Woodburne, M. O., Paleontol. 72:556–571. eds. Geology and paleontology of Seymour Island, Zinsmeister, W. J., and Feldmann, R. M. 1996. Late Cre- Antarctica Peninsula. Geol. Soc. Am. Mem. 169: taceous faunal changes in the high southern latitudes: 253–284.