A RT I C L E S
Definition of Late Cretaceous Stage Boundaries in Antarctica Using
Strontium Isotope Stratigraphy
J. M. McArthur, J. A. Crame,1 and M. F. Thirlwall 2
Department of Geological Sciences, University College London, Gower Street,
London WC1E 6BT, United Kingdom
(e-mail: [email protected])
ABSTRACT
87
86
New Sr/ Sr 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
87
Sr/86Sr values allow the stage boundaries of the Turonian/Coniacian, Coniacian/Santonian, Santonian/Campanian,
and Campanian/Maastrichtian, as well as other levels, to be correlated with both the United Kingdom and United
States. These correlations show that current stratigraphic ages in Antarctica are too young by as much as a stage.
Immediate implications of our new ages include the fact that Inoceramus madagascariensis, a useful fossil for regional
austral correlation, is shown to be Turonian (probably Late Turonian) in age; the “Mytiloides” africanus species
complex is exclusively Late Coniacian in age; both Baculites bailyi and Inoceramus cf. expansus have a Late Coniacian/Early Santonian age range; an important heteromorph ammonite assemblage comprising species of Eubostrychoceras, Pseudoxybeloceras, Ainoceras, and Ryugasella is confirmed as ranging from latest Coniacian to very earliest
Campanian. An important new early angiosperm flora is shown to be unequivocally Coniacian in age. Our strontium
isotopic recalibration of ages strengthens the suggestion that inoceramid bivalves became extinct at southern high
latitudes much earlier than they did in the Northern Hemisphere and provides confirmation that, in Antarctica,
belemnites did not persist beyond the Early Maastrichtian.
Introduction
A remarkably complete and extensive Late Cretaceous sedimentary succession more than 3.5 km in
thickness is exposed on the islands of the James
Ross Island group, Antarctica (figs. 1–3; Olivero et
al. 1986; Crame et al. 1991, 1996, and references
therein). This mostly shallow-water, clastic sequence is, in places, very fossiliferous. It offers an
opportunity unrivaled in the Southern Hemisphere
to investigate biotic and environmental changes
during the Late Cretaceous, particularly those leading 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 London, Egham, Surrey TW20 0EX, United Kingdom.
The Late Cretaceous is often regarded primarily
as a time of biotic retractions that culminated in
the spectacular mass extinction at the end of the
Cretaceous, but it was also a time of major evolutionary radiations in both the marine and terrestrial realms when many modern faunas and floras
first became established (see, e.g., Hallam 1994).
Unfortunately, most of our knowledge of these
events comes from the Northern Hemisphere, but
our recalibration, presented here, of the age of the
Antarctic succession provides some key austral
data. For example, a new angiosperm leaf flora from
the Hidden Lake Formation (fig. 3) can now be
dated as entirely Coniacian in age; the taxa present
include members of the Lauraceae, Nothofagaceae,
Annonaceae, and Proteaceae (Hayes 1996). Together with ongoing investigations into palyno-
[The Journal of Geology, 2000, volume 108, p. 623–640] q 2000 by The University of Chicago. All rights reserved. 0022-1376/2000/10806-0001$1.00
623
624
J. M. MCARTHUR ET AL.
morph floras, this assemblage will provide vital information on the structure of Late Cretaceous
temperate rain forests and the radiation of the flowering plants into the Southern Hemisphere (Dettmann and Thomson 1987; D. J. Cantrill, J. E. Francis, and P. Hayes, pers. comm., 1999). Similarly, the
Santa Marta, Snow Hill Island, and López de Bertodano Formations (figs. 1–3) contain early representatives of benthic marine invertebrate groups
that were to flourish globally throughout the Cenozoic era; for example, venerid and tellinid bivalves, and buccinoidean and muricoidean gastropods (Zinsmeister and Macellari 1988; Scasso et al.
1991).
Precise age assignment and stratigraphical correlation are essential for such analysis of palaeoenvironmental and palaeobiological change, so it is
vital to integrate this Antarctic sequence accurately into the standard Northern Hemisphere reference sections. To this end, the principal macroand microfossil groups have proven only partially
successful. Some ammonites can be used for regional correlations, but others, such as the many
representatives of the Kossmaticeratidae, are
largely endemic. Of the microfossil groups that are
present, palynomorphs offer the most potential for
correlation but, at present, do no more than establish biostratigraphical correlations with the Australasian region (see, e.g., Riding et al. 1992).
To overcome these problems, we have used strontium isotope stratigraphy (SIS) to date and correlate
the sequence (McArthur 1994; Howarth and
McArthur 1997; Veizer et al. 1997). This method
has already enabled us to accurately correlate with
the Northern Hemisphere the base of the Maastrichtian stage in Antarctica (Crame et al. 1999). In
addition, an enhanced Cenozoic chronology of the
northern Antarctic Peninsula region has been established using SIS (Dingle et al. 1997; Dingle and
Lavelle 1998). Reference curves of 87Sr/86Sr against
Northern Hemisphere biostratigraphy for the Late
Cretaceous are available (McArthur et al. 1992,
1993a, 1993b, 1994; McLaughlin et al. 1995; Sugarman et al. 1995), and a time-calibrated 87Sr/86Sr
curve for the period (Howarth and McArthur 1997)
can be used to convert to numerical age the
87
Sr/86Sr values determined for Antarctic fossils.
Lithostratigraphy and Regional Setting
The Upper Cretaceous sedimentary succession of
James Ross Island and Vega Island represents part
of an extensive Late Mesozoic–Early Cenozoic
back-arc basin that formed on the northeastern
flank of the Antarctic Peninsula (fig. 1). These vol-
caniclastic sedimentary rocks constitute part of a
regressive megasequence; the stratigraphically
older Gustav Group is composed of coarser-grained,
submarine fan slope deposits that grade upward
into the finer-grained, shelf-depth deposits of the
Marambio Group (Ineson et al. 1986). The base of
the Gustav Group may be Aptian in age (Riding et
al. 1998), and the top, before this study, was thought
to be Santonian. The Marambio Group, before this
study, was taken to be Santonian to Danian (Ineson
et al. 1986) in age. Details of litho- and biostratigraphical subdivisions within the Gustav and Marambio 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 references therein.
The uppermost levels of the Gustav Group and
lowermost levels of the Marambio Group are well
exposed around the shores of Brandy Bay, northern
James Ross Island (figs. 1, 2). From there, the section continues in a southeasterly direction to the
vicinity of St. Martha Cove and then across to Cape
Lamb, 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 just
north of Cape Gage to Carlsson Bay (fig. 1; Crame
et al. 1991; Pirrie et al. 1997) and repeats the upper
part of this succession on southeastern James Ross
Island, exposing small areas of Marambio Group
sediments at Rabot Point and Carlsson Bay, where
we have collected giant inoceramids from the Santa
Marta Formation.
In a section (D.8228; fig. 2) running along the
southwestern shore of Brandy Bay, the upper Gustav Group comprises two formations: the Whisky
Bay and Hidden Lake Formations (figs. 2, 3). The
former is a complex, highly variable unit characterized by pebble and boulder conglomerates, together with pebbly sandstones; in places there are
marked vertical and lateral facies transitions into
silty mudstones (Ineson et al. 1986). Within the
Whisky Bay Formation, at the 1600-m level in the
combined stratigraphic section, the junction between the Lewis Hill and Brandy Bay members is
marked by a local unconformity (fig. 3; Ineson et
al. 1986; J. A. Crame, pers. obs.). This discontinuity
probably accounts for the absence of Cenomanian
inoceramids and ammonites found in equivalent
strata 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 Brandy
Bay region, the Whisky Bay Formation lithologies
grade up into a distinctive sequence of rusty brown
to greeny brown conglomerates, sandstones, and
siltstones that constitute the Hidden Lake For-
Journal of Geology
L AT E C R E TA C E O U S S TA 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 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
m thick, this unit is characterized by coarse-grained
sandstones and matrix-supported conglomerates in
its lower levels, and medium- to fine-grained sandstones in its upper ones. Some fine-grained sandstones and siltstones within it are intensely bioturbated. The transition into the overlying Santa
Marta Formation, the basal lithostratigraphic unit
within the Marambio Group, is marked by a distinct change from the rusty brown and greeny
brown weathering hues to predominantly gray colors. The Santa Marta Formation is some 1200 m
thick in its type area (along line of section running
into St. Martha Cove; see fig. 2) and typically comprises massive, very fine to medium-grained sandstones and silty sandstones (fig. 3; Olivero et al.
1986). It is characterized by a marked increase in
marine benthic fauna and by a rise in the number
of infaunal taxa (Scasso et al. 1991).
The uppermost levels of the Santa Marta Formation can be traced across to the base of a 480m-thick sequence exposed on Cape Lamb, Vega Island (figs. 2, 3), where they pass conformably
upward into the Cape Lamb Member of the Snow
Hill Island Formation (Crame et al. 1991; Pirrie et
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
m; fig. 3) within this unit has been dated as earliest
Maastrichtian in age (Crame et al. 1999). The topmost 111 m of the Cape Lamb section, a sequence
of volcaniclastic sandstones and conglomerates
constituting the Sandwich Bluff Member, is assigned to the López 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 122B
were received as powders and were analyzed as received. In addition, one aragonitic Nucula and 17
other calcitic macrofossils (oysters, inoceramids,
belemnites) were analyzed (tables 1, 2). The samples are from 12 stratigraphic levels; for completeness, a thirteenth level is reported from Crame et
al. (1999). From unpowdered samples, visually altered portions were removed using diamond cut-
ting tools; the remaining sample was then broken
into submillimeter-sized fragments. These were
cleaned by brief immersion in 1.2 M hydrochloric
acid solution, washed in ultrapure water, and dried
in a clean environment. The best-preserved fragments were selected under the binocular microscope for analysis. Preservational criteria were degree of flakiness, amount of cracking and secondary
veining, amount of cementation, color, opacity, and
the presence of iron and/or manganese staining.
Chemical Analysis. Data for samples received as
powders are taken from Pirrie and Marshall (1990).
For other samples, analysis for Ca, Mg, Sr, Na,
Fe, and Mn was done by inductively coupled
plasma–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 by
graphite furnace atomic absorption spectrometry.
Results are given in table 1. The precision of the
analyses was better than 55%. Powdered samples
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.
Section and
sample nos.
Isotopic and Elemental Data for Antarctic Fossils
Stratigraphic level (m)
Sample type
Composite Crame Col
Vega Island:
DJ.83:
68–160
Various
3800
James Ross Island:
DJ.685:
83
Giant
3300–3600
inoceramid
D.8409:
17
Giant
≈3150
inoceramid
14
Giant
≈3150
inoceramid
DJ.955:
14
Giant
≈3150
inoceramid
D.8664:
184a2
Nucula
2830
184a1
Nucula
2830
D.8657:
122B
Belemnite
2602
116a
Belemnite
2594
109a
Belemnite
2589
95
Belemnite
2565
76b
Belemnite
2549
76a7
Belemnite
2548
76a6
Belemnite
2548
76a5
Belemnite
2548
76a
Belemnite
2548
73an
Belemnite
2548
73am
Belemnite
2548
73ap
Belemnite
2500
73
Belemnite
2500
D.8228:
333
Inoceramid
2420
331
Inoceramid
2420
326
Inoceramid
2420
303
Inoceramid
2360
94
Inoceramid
2360
114
Oyster
1880
113
Oyster
1880
82
Oyster
1760
81
Oyster
1760
100–103
92.8–95.5
82.5–92.0
64.0–65.5
49.0–49.5
47.0–49.0
47.0–49.0
47.0–49.0
47.0–49.0
47.0–49.0
47.0–49.0
44.3–48.0
44.3–48.0
Mineralogya
87
Sr/86Sr
(mean 5 2 s.e.)
n
Numerical Ca
Mg
Sr
Na
Fe
Mn
Rb
age
(%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)
.707736 5 .000004 17
71.0 5 .3
c, tr d
.707646 5 .000005
4
74.2 5 .4
c
.707554 5 .000011
c
39.1 2412
831
1265
721
761
.06
2
38.9 1676
970
2421
163
297
!.01
.707553 5 .000009
3
39.1 1394
970
2042
152
200
!.01
.707557 5 .000015
1
78.7 5 .5
P
ar, tr c
.707493 5 .000007
.707493 5 .000009
4
3
82.6 5 .5
P
P
P
c
P
c
P
P
c
P
P
P
c
.707429
.707421
.707422
.707420
.707405
.707412
.707407
.707406
.707411
.707409
.707420
.707415
.707415
5
5
5
5
5
5
5
5
5
5
5
5
5
.000009
.000015
.000011
.000009
.000015
.000011
.000011
.000015
.000011
.000011
.000011
.000015
.000011
3
1
2
3
1
2
2
1
2
2
2
1
2
c
c, tr an
c
c
c, tr d
c
c
c, tr q
c
.707385
.707389
.707379
.707381
.707381
.707288
.707291
.707292
.707287
5
5
5
5
5
5
5
5
5
.000003
.000011
.000009
.000012
.000002
.000009
.000009
.000009
.000009
4
2
3
4
4
3
3
3
3
85.5
85.8
85.8
85.8
5
5
5
5
.4
.7
.5
.5
38.4
249
3510
5019
73
13
!.01
38.4
1664
1523
1461
1245
1570
1560
1450
1760
1336
1610
1940
1720
1221
3298
1470
107
210
140
4
210
115
210
210
9
210
210
210
4
21
10
9
1
58
36
17
19
3
16
51
12
4
.04
38.6
897
660
600
718
600
800
660
840
409
660
840
1020
551
38.5
38.2
38.4
38.8
38.4
39.3
40.5
40.1
39.1
4008
3730
85
5215
6850
564
609
403
413
1289
1239
1177
1201
1275
704
701
813
589
5065
5204
5075
4372
2780
858
758
790
586
128
103
26
79
103
298
110
221
63
239
59
38
65
63
305
71
47
95
39.4
39.4
39.7
86.1 5 .2
87.0 5 .2
87.1 5 .3
91.0 5 .2
89.8 5 .4
1231
2774
1513
!.01
.04
!.01
!.01
.02
!.01
!.01
!.01
!.01
!.01
!.01
!.01
Note. Mean data only are given for Sr/ Sr; full data are in table 2. Uncertainties on Sr/ Sr 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/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.
87
86
87
86
Journal of Geology
Table 2.
Replicate
Sample number
DJ.83.68–160a
DJ.685.83
D.8409.17
D.8409.14
DJ.955.14
D.8664.184a2
D.8664.184a1
D.8657.122B
D.8657.116a
D.8657.109a
D.8657.95
D.8657.76b
D.8657.76a7
D.8657.76a6
D.8657.a5
D.8657.76a
D.8657.73an
D.8657.73am
D.8657.73ap
D.8657.73
D8228.333
D8228.331
D8228.326
D8228.303
D8228.94
D8228.114
D8228.113
D8228.82
D8228.81
a
L AT E C R E TA C E O U S S TA G E B O U N D A R I E S
87
Sr/86Sr Data for Samples
87
86
Sr/ Sr value of replicate analysis
.707651
.707549
.707555
.707557
.707500
.707494
.707423
.707421
.707422
.707418
.707405
.707411
.707413
.707406
.707410
.707418
.707425
.707415
.707413
.707386
.707391
.707375
.707381
.707382
.707288
.707292
.707292
.707289
.707645
.707558
.707553
.707639
.707550
.707498
.707498
.707432
.707484
.707488
.707433
.707422
.707418
.707425
.707647
.707491
987 and samples, the precision of measurement
(2 s.e.) was better than 50.000015 (n p 1),
50.000011 (n p 2), and 50.000009 (n p 3). Total
blanks were !2 ng of Sr; samples contained 110 mg
of Sr. Concentrations of Rb were too low to require
corrections for radiogenic Sr. Data are given in table
1 as means of replicate analysis. Full data are in
table 2.
Discussion
.707412
.707401
.707412
.707410
.707414
.707418
.707389
.707386
.707381
.707376
.707384
.707289
.707290
.707287
.707292
629
.707384
.707381
.707397
.707379
.707287
.707292
.707298
.707279
.707381
.707368
.707380
See Crame et al. 1999.
other samples, mineralogy was determined using a
Phillips PW 1710 diffractometer. Instrumental conditions were: CuKa radiation generated at 40 kV
and 30 mA; scanning through 57 to 507 2v at 0.57
2v/min, a range that includes the major diffraction
peaks of aragonite, calcite, and dolomite. With
these conditions, the detection limit of our apparatus, determined by standard additions with pure
phases, was about 0.2% calcite in aragonite and
0.5% dolomite in calcite.
Isotopic Analysis. For 87Sr/86Sr analysis, samples
of about 20 mg were dissolved in 2.5 M hydrochloric acid, and Sr was separated by standard ionexchange chemistry. Measurements were made
with a VG 354 mass spectrometer using the multidynamic routine SrSLL that includes corrections
for isobaric interference from 87Rb (Thirlwall 1991).
Data are presented in table 1 as means of replicate
measurement of 87Sr/86Sr. Data are normalized to a
value of 0.1194 for 86Sr/88Sr and adjusted to a value
of 0.7102480 5 0.0000025 (2 s.e., n p 19) for
NIST 987, which corresponds to a value of
0.7091746 5 0.0000032 (2 s.e., n p 19) for EN 1.
Adjustment was to the cumulative mean of NIST
987 values collected during periods when samples
were analyzed. Based on replicate analysis of NIST
Quality of Sample Preservation.
The quality of
preservation of nine belemnites received as powders (from the line of section D.8657; fig. 2; tables
1, 2) and separate subsamples of several other samples we have analyzed, are discussed in Pirrie and
Marshall (1990), who conclude that preservation
was good for belemnites and less good for inoceramids. Our unpowdered samples were visually well
preserved and our inoceramids, after picking, appear better preserved than the samples described by
those authors. On the basis of XRD, most samples
were monomineralic, but five (table 1) contained
traces of contaminant phases (quartz, dolomite,
analcime, calcite). The aragonitic Nucula contained about 0.2% calcite, the minimum detectable
by XRD, but replicate 87Sr/86Sr analysis gave reproducible data so we believe alteration (which proceeds patchily) had not significantly altered the
87
Sr/86Sr of this sample. Furthermore, contaminant
calcite would have derived its Sr mostly from precursor aragonite and its concentration in secondary
calcite is much lower than in primary aragonite,
thereby lessening any effect of contaminant Sr on
87
Sr/86Sr.
Examination of acid-leached samples of inoceramids by scanning electron microscopy (SEM) (fig.
4a, 4b) showed that trace impurities were deposited
between individual prisms in a few samples, but
individual prisms were easily separated from the
impurity (fig. 5a, 5b) by picking under the microscope. The impurity phases (quartz, analcime, dolomite) in inoceramids contain little Sr, were volumetrically unimportant, were avoided during
picking, and, for quartz and analcime, are insoluble
in acid: the influence of such phases on the
87
Sr/86Sr of picked samples was negligible. Inoceramid prisms were altered and cloudy at their ends,
where alteration was localized, but clear in their
middle portions (fig. 5a, 5b), and it was the middle
portions that were picked for analysis. Finally, the
87
Sr/86Sr values of three giant inoceramids, with
very different preservational states, from one stratigraphic level and locality at Carlsson Bay in the
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
well-preserved sample, DJ.955.14, has an 87Sr/86Sr
(0.707557 5 0.000015; table 1) indistinguishable
from that of giant inoceramids from northern James
Ross Island (locality D.8409; 0.707554 5 0.000011;
table 1). Such agreement would be unlikely unless
these samples were unaltered and also from the
same stratigraphic level. Sample DJ.955.8 was
rather cemented and altered and its 87Sr/86Sr is
0.707548, while sample DJ.955.5 was solidly cemented so that prisms had completely fused and
were not visible as individuals; its 87Sr/86Sr value is
0.707532. Thus, severe alteration has lowered
87
Sr/86Sr by no more than 0.000025, a value only a
little more than analytical uncertainty; a lowering
of 87Sr/86Sr would be expected on alteration since
the clastic sediments of the area are largely of mantle affinity in this back-arc environment of the Antarctic Peninsula.
Examination of oysters with SEM (fig. 4c, 4d) re-
vealed the compact layering and surface ornamentation on individual layers that we interpret to be
characteristic of pristine oyster carbonate. Optical
examination of oysters showed flaky, translucent
calcite (fig. 5c, 5d) with only local Fe or Mn staining
that was avoided during sampling.
Concentrations of Fe and Mn exceeded 305 ppm
only in sample 83 (cf. Veizer 1983; Veizer et al.
1992; McArthur 1994; table 1). There is a weak
correlation between Fe and Mn in the samples (fig.
6) but no relationship between 87Sr/86Sr and either
Fe or Mn (fig. 6). We attribute the measured concentrations of Fe and Mn not to the occurrence of
structural Fe and Mn in altered carbonate, but to
the presence of surface coatings of Fe/Mn oxyhydroxides on crystallites.
Within each stratigraphic level, different samples
have indistinguishable 87Sr/86Sr values (table 1, cf.
DJ.8409, samples 14 and 17 with DJ.955, sample
14; in section D.8228, cf. sample 81 with sample
Journal of Geology
L AT E C R E TA C E O U S S TA 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 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
sample 303, sample 326 with samples 331 and 333,
etc.) thereby attesting to good preservation. Finally,
samples have 87Sr/86Sr values that are consistent
with their stratigraphical order. The above facts
suggest that the samples are well preserved and retain their original 87Sr/86Sr values, so we accept our
87
Sr/86Sr data as recording the 87Sr/86Sr values of Late
Cretaceous seawater.
Numerical Ages.
These have been determined
using version 3:10/99 of the LOWESS look-up table
of Howarth and McArthur (1997); these authors
give a full description of the LOWESS method and
table, table derivation, and estimation of uncertainty. The relevant part of the 87Sr/86Sr curve, derived from version 3:10/99, is shown in figure 7. In
table 1, we have calculated numerical ages for samples bundled into stratigraphic levels rather than
provide a separate age for each sample. We do so
because stratigraphic separation of many closely
spaced samples was small, and represented a period
of time so small, that evolutionary differences between levels in marine 87Sr/86Sr would have been
undetectable with our measurements. For stratigraphic levels with more than three measurements
of 87Sr/86Sr (table 1), mean values of 87Sr/86Sr and 2
s.e. of the means were used to derive numerical
ages and uncertainty limits on the ages. Where
fewer than three values were available, uncertainties were taken to be twice the standard error of
the cumulative mean for NIST 987 of 50.000015,
50.000011, or 50.000009, for n p 1, 2, and 3, respectively. The uncertainty on the ages combines
the uncertainty inherent in the reference curve of
Howarth and McArthur (1997, version 3:10/99) and
the uncertainty (2 s.e.) in the mean 87Sr/86Sr values
of each stratigraphic level (table 1).
Stratigraphic Correlations. Numerical ages suffer
632
J. M. MCARTHUR ET AL.
Figure 6. Covariance of Mn, Fe, and 87Sr/86Sr in Antarctic macrofossils. Sample DJ.685.83 is off scale on all
plots and is not shown.
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 sequences. In figures 8–10, we show where in the
biostratigraphic schemes for the Northern Hemisphere (McArthur et al. 1993a, 1993b, 1994) the
87
Sr/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 extrapolation 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 Sissingh 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 Gonioteuthis quadrata macrofossil zone and the lower
part of CC20; sample 83 is Late Campanian in age
and correlates with the midrange of the Belemnitella 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
ranges, which are all taken to be of equal duration.
As the 87Sr/86Sr calibration is based on a zonal
scheme, it is less precise than it would be if based
Journal of Geology
L AT E C R E TA C E O U S S TA 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. 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,
as is the case for Europe. In addition, the 87Sr/86Sr
of several zones (e.g., Clioscaphites saxitonianus)
must be derived by interpolation, making correlation with such levels less accurate than would be
the case were more data available. Samples 81, 82,
113, and 114 fall within the latest Turonian zones
of Scaphites whitfieldi and Prionocyclus quadratus. Samples 94, 303, 326, 331, and 333 fall within
the Late Coniacian zone of Scaphites depressus,
and samples 73, 73ap, 73ap, 73am, 76a, 76a5,
75a6,76a7, 76b, 95, 109a, 116a, and 122B correlate
with the C. saxitonianus zone of the basal Santonian. Sample 184a is placed in the zonal range upper
Scaphites hippocrepis III/lower Baculites sp.
smooth; samples D.8409.14, D.8409.17, and
DJ.955.14 correlate with the junction of the zones
of Baculites sp. smooth and Baculites sp. weakly
ribbed; sample 83 falls in the Exitaloceras jennyi
zone and samples 68–160 (Crame et al. 1999) fall
in the uppermost Baculites eliasi zone, which is
the lowermost Maastrichtian macrofossil zone.
These attempts to correlate with Northern
Hemisphere schemes are somewhat compromised
by the scarcity of data used to compile the European
and U.S. reference curves for the period Turonian
to middle Santonian (the curve for the Late Santonian and Campanian is better defined; McArthur
et al. 1993a, 1993b). For more precise correlations,
more data are required for this interval for both
Antarctica and Europe. This is particularly true for
the zone of C. saxitonianus for which the lack of
87
Sr/86Sr data results in the 87Sr/86Sr calibration
curve (fig. 10) perhaps giving ages that are too young
by one zone.
Stratigraphic Interpretation.
At approximately
the 1350-m level in the combined section a distinctive fauna that includes the inoceramid bivalve
Birostrina concentrica (Parkinson) indicates a Late
Albian age (fig. 3; Ineson et al. 1986). Immediately
above the 1600-m unconformity a second inoceramid, Inoceramus madagascariensis Heinz, is locally abundant, suggesting a Late Turonian/Early
Coniacian age (Heinz 1933; Crame 1983) for this
level, although Crampton (1996) suggests that its
occurrence in the Barroisiceras onilakyense ammonite zone in Madagascar is more compatible
with a middle Coniacian age. The unequivocal Late
Turonian age SIS ages of samples 81, 82, 113, and
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
for this species in Antarctica (fig. 3).
Unfortunately, the presence of the unconformity
at about 1600 m in the section means that there is
no direct evidence of the age of the base of the I.
madagascariensis zone in Antarctica. It does not
occur in palaeontologically well-dated Late Cenomanian strata in the Tumbledown Cliffs/Rum
Cove area (fig. 1; Ineson et al. 1986), and thus it
may be entirely Turonian (?Late Turonian) in age
(fig. 3). Such an age is of considerable stratigraphical
significance, for this taxon is widespread in the
Southern Hemisphere, occurring in Patagonia,
Madagascar, and New Zealand (Crampton 1996) as
well as Antarctica. As the range of I. madagascariensis in New Zealand is mostly coincident with
that of the Teratan Stage, it seems that the age of
this local chronostratigraphic unit must be revised
from middle Coniacian to Turonian (?Late Turonian). This in turn has implications for the age ranges
of other inoceramid taxa and stage boundaries in
New Zealand (Crampton 1996, fig. 26).
Just beneath the junction between the Whisky
Bay and Hidden Lake Formations (1930-m level of
fig. 3; Herm et al. 1979; Crame 1983) occur the first
representatives of a complex of Coniacian inoceramid taxa that seem to have their closest links with
European forms such as Inoceramus (Inoceramus)
inaequivalvis Schlüter and Inoceramus (Inoceramus) koegleri Andert. As these are both Lower Coniacian taxa in Europe, this seems a suitable place
at which to set the Turonian/Coniacian boundary.
This first complex grades into a second that includes the widely interpreted species “Mytiloides”
africanus (Heinz). This taxon has been given both
Coniacian and Santonian ages in Madagascar (Sornay 1964), but it is Early Coniacian in Europe
(Herm et al. 1979; Walaszczyk 1992). It is also apparent that some forms within this second complex
resemble the highly variable Inoceramus australis
Woods from the Piripauan Stage (uppermost Coniacian/middle Santonian) of New Zealand (see, e.g.,
Crampton 1996, plate 9F). The late Middle/Late
Coniacian 87Sr/86Sr ages for samples 94, 303, 326,
331, and 333 (figs. 8–10) indicate that almost the
entire “M.” africanus species complex is Coniacian
in age (fig. 3). The same conclusion can be applied
to a co-occurring large, erect inoceramid resembling Inoceramus expansus Baily from South Africa
that was thought to be no older than Santonian
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L AT E C R E TA C E O U S S TA G E B O U N D A R I E S
Figure 9. Strontium isotope correlation of Antarctic
samples with the biostratigraphy of the Cretaceous English Chalk (McArthur et al. 1993a, 1993b). Macrofossil
biostratigraphy and stage boundaries from Wood et al.
(1994). Nannofossil zonation from Burnett (1990). 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.
(Crame 1983). Even the first occurrences of the distinctive heteromorph ammonite Baculites bailyi
Woods in the lowermost levels of the Santa Marta
Formation must now be taken to represent a Late
Coniacian rather than Early Santonian age (sensu
Crame et al. 1991). One immediate and important
implication of these new findings is that the Hidden Lake Formation is entirely Coniacian in age
(fig. 3; see below).
The dimitobelid belemnites from locality
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 Eubostrychoceras, 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 Campanian 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 specimen of Antarcticeramus rabotensis Crame and Luther 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
James Ross Island (Crame and Luther 1997) but our
87
Sr/86Sr data indicate that locality DJ.865 should
occur around the 3300–3600-level on our composite section (fig. 3) and that locality DJ.955 is stratigraphically at the same levels as locality D.8409.
In this study, the Santonian/Campanian boundary
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
Snow Hill Island Formation on Vega Island (Crame
et al. 1999).
Palaeobiological and Palaeoenvironmental Implications. Early extinction patterns for inoceramid bi-
valves and dimitobelid belemnites in the James
Ross Basin have already been established (Crame
et al. 1996; Zinsmeister and Feldmann 1996). The
last inoceramids, of the giant species A. rabotensis,
are found well below the KT boundary in strata
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
87
Sr/86Sr ages from localities D.8409 and DJ.955
show that these giant inoceramids are, in the European sense, Early Campanian in age, while those
from locality DJ.865.83 are mid Late Campanian in
age. These ages confirm the disparity in age between last occurrences in the Northern and Southern Hemispheres.
Similarly, before this study, the last dimitobelid
belemnites in the Crame Col/Brandy Bay region
were thought to be Early to mid Campanian (Crame
et al. 1996, fig. 2). Nevertheless, the new strontium
ages from localities D.8657 and D.8664 indicate
that this age range should in fact be Early Santonian/Early Campanian and that there is an even
greater time gap between these occurrences and a
single, later Maastrichtian (?mid Maastrichtian) horizon 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 stratigraphical
ranges of other key taxa through the Late Cretaceous of Antarctic. Ammonites will be one particularly important group to study in this respect; although they are present right up to the KT
boundary at most key localities in the world, they
had probably been in decline since the mid to early
Late Cretaceous (Kennedy 1989). Ward and Signor
(1983) have documented a steady decline in numbers of genera from the Coniacian to the Maastrichtian stage. Zinsmeister and Feldmann (1996) recorded a reduction in numbers of ammonite species
from about 45 in the lower Santa Marta Formation
(Late Santonian/Early Campanian in this study) to
15 in the Late Maastrichtian of Seymour Island,
with just five at the KT boundary (see also Zinsmeister 1998). However, our inventory of Antarctic
ammonites is still incomplete, and an important
Coniacian fauna from the uppermost Whisky Bay,
Hidden Lake, and lower Santa Marta Formations
(fig. 3) is in the process of being described (M. R.
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L AT E C R E TA C E O U S S TA G E B O U N D A R I E S
A. Thomson and P. Bengston, pers. comm., 1999).
It may be that there is an even longer-term pattern
to the decline of ammonites in Antarctica, a pattern
similar to that seen in other regions.
The richest Antarctic Late Cretaceous inoceramid bivalve fauna is based on two species complexes that this study shows are almost entirely
Coniacian in age (fig. 3). This finding fits a global
pattern of marked expansion of inoceramids in the
Middle Turonian–Coniacian, followed by a gradual
decline into the Late Maastrichtian (Pergament
1978; Sornay 1981; Dhondt 1992). The congruity of
these worldwide patterns between ammonites and
inoceramids suggests that we may be looking at a
single, underlying cause, and an obvious one to consider is climate change. For example, Huber (1998)
has highlighted the similarity of Late Cretaceous
palaeotemperature curves between Arctic and Antarctic regions. In the former, fossil leaf physiognomy suggests a temperature maximum during Turonian/Coniacian time; in the latter, oxygen
isotope palaeotemperature analysis of deep sea
cores suggests a maximum temperature during Turonian/Santonian time. There is increasing evidence to suggest that the Campanian and Maastrichtian stages represent a pronounced phase of
global cooling (Zinsmeister and Feldmann 1996;
Huber 1998).
Stable isotope palaeothermometry has also been
attempted through the James Ross Island group succession and a pronounced Turonian/Coniacian
peak in values established (Ditchfield et al. 1994).
This study now needs to be refined and extended,
for the values obtained have been plotted in terms
of either major lithostratigraphical subdivisions or
standard Cretaceous stages (Ditchfield et al. 1994;
Dingle and Lavelle 1998). Individual values can
now be replotted on the composite stratigraphic
section and tied in much more precisely to the vertical ranges of key fossil taxa.
Conclusions
1. Strontium isotope analysis of macrofossils from
Antarctica shows that the oldest group of samples
correlate definitively with the Upper Turonian
Sternotaxis planus zone of Norfolk, United Kingdom, and approximately the junction between the
Scaphites whitfieldi and Prionocyclus quadratus
zones (Upper Turonian) in the U.S. Western Interior. A younger group of five samples, from two
close stratigraphic levels, correlate with the uppermost Coniacian Scaphites depressus zone of the
U.S. Western Interior and the lower Micraster coranguinum zone of the Chalk of NW Europe. Higher
637
still, a group of nine samples from five close levels
correlates with the upper M. coranguinum zone of
Europe and the basal Santonian Clioscaphites saxitonianus zone in the U.S. Western Interior. Eleven
samples from four higher levels are Campanian to
Maastrichtian in age.
2. The 87Sr/86Sr data show that the uppermost
Whisky Bay Formation is no younger than earliest
Coniacian in age, with most of it being assigned to
the Turonian. The succeeding Hidden Lake Formation is entirely Coniacian in age, and the boundary between the respective Gustav and Marambio
groups must now be set at Late Coniacian. The
Coniacian/Santonian boundary may be as high as
the 150-m level in the Crame Col section, and the
Santonian stage may be represented by no more
than approximately 225 m of strata. The Santa
Marta Formation is Late Coniacian/latest Campanian in age and the Campanian/Maastrichtian
boundary is placed in the lowermost levels of the
Snow Hill Island Formation on Vega Island.
3. Inoceramus madagascariensis, an important
fossil for regional correlations, should now be regarded as Turonian (and probably Late Turonian) in
age. Similarly, certain members of the “Mytiloides”
africanus species complex have considerable regional stratigraphical potential, and they can be
taken to be almost exclusively Late Coniacian in
Antarctica. Inoceramus cf. expansus has a Late
Coniacian/Early Santonian age range. An important group of Antarctic heteromorph ammonites,
centered on the genera Eubostrychoceras, Pseudoxybeloceras, Ainoceras, and Ryugasella, is
shown to be no younger than earliest Campanian
in age, with members of this group ranging down
to the uppermost Coniacian.
4. The new age calibration of the Antarctic Late
Cretaceous succession has important implications
for both extinction and radiation events. It provides
further evidence that the inoceramid bivalves became extinct extremely early in Antarctica and that
the dimitobelid belemnites did not persist in Antarctica beyond the Early Maastrichtian, becoming
extinct earlier in Antarctica than in the Northern
Hemisphere. Thus, between the Northern and
Southern Hemispheres there is a disparity in the
age of some extinction events.
5. A major radiation of Antarctic angiosperm taxa
is dated as Coniacian in age. The potential now
exists to date the origination and radiation of a
number of other modern plant and animal groups
through the latest Cretaceous in the southern high
latitudes.
638
J. M. MCARTHUR ET AL.
ACKNOWLEDGMENTS
The isotopic measurements were made by J.M.M.
in the Radiogenic Isotope Laboratory at Royal Holloway University of London; the laboratory is supported, in part, by the University of London as an
intercollegiate facility. Elemental analysis was
done in the ICP-AES Laboratory at Royal Holloway
University of London; the laboratory is supported,
in part, by the Natural Environment Research
Council (NERC) as a central facility, and we thank
the director, J. N. Walsh, for its use. We thank G.
Ingram for assisting with the isotopic measure-
ments, A. Osborn for assistance with elemental
analysis, and British Antarctic Survey colleagues
who assisted with fieldwork in the James Ross Island group. We are particularly grateful to D. Pirrie
(Camborne School of Mines) for providing the samples from localities D.8657 and D.8664 and useful
discussion. We thank K. Macleod, B. Zinsmeister,
and an anonymous referee for constructive reviews
that improved the final article. This is a contribution to International Geological Correlation Programme project 381, “South Atlantic Mesozoic
Correlations,” and was supported by NERC grant
GR3/AFI2/38.
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