New chronostratigraphy of the Cretaceous–Paleogene boundary interval at Bidart (France

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Earth and Planetary Science Letters 224 (2004) 19–32

New chronostratigraphy of the Cretaceous–Paleogene boundary

interval at Bidart (France)

Bruno Galbrun*, Silvia Gardin

Universite Paris VI, CNRS FR 32 ‘‘Centre Parisien de Geologie’’ Case 117, 4 place Jussieu, 75252 Paris cedex 5, France

Received 24 November 2003; received in revised form 24 April 2004; accepted 26 April 2004

Abstract

The Cretaceous–Paleogene boundary section at Bidart (French basque coast) was prevented for a long time to be considered

as a reference section for the K–T boundary due to a suspected hiatus comprising most of Chron C29r.

In our new magneto- and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity

is not revealed in the vector directions but in a small, but consistent, intensity excursion during laboratory demagnetization

trends. Magnetite and maghemite are the main carriers of remanence, with contribution of hematite to the magnetic mineralogy

of the red-brown lowermost Danian and Maastrichtian marly limestones. Magnetite is the main magnetic mineral in the white to

pink Danian limestones. The NRM of marls and marly limestones is characterized by: (1) a present-day field overprint, (2) a

medium-temperature (300–400 jC) reversed component interpreted as a relic of the characteristic remanent magnetization

(ChRM) carried by magnetite grains which are not completely oxidized into maghemite, and (3) a high temperature (up to 590

jC) normal component due to late diagenesis hematite growth. The NRM of Danian limestones is characterized by (1) a

present-day field overprint and (2) a ChRM component of normal or reversed polarity.

The ChRM directions allow a new correlation of the magnetic polarity sequence with the Geomagnetic Polarity Time Scale

(GPTS): Chron C29r corresponds to the Maastrichtian unit and extends up to 3.5 m above the K–T boundary, and Chron C29n

occurs at the top of the section. There is no evidence for a hiatus at or below the K–T boundary, and the section does not extend

up to Chron C28r as previously suggested. These new chronostratigraphic assignments are in good agreement with the previous

biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper.

Our study demonstrates that the section of Bidart can be considered as a key section to study the K–T boundary event(s).

D 2004 Elsevier B.V. All rights reserved.

Keywords: magnetostratigraphy; Chron C29r; K–T boundary; calcareous nannofossils; Bidart; France

1. Introduction

Paleomagnetic interpretation is often an art; and

different people can analyze the same data and reach

0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2004.04.043

* Corresponding author. Fax: +33-1-44273831.

E-mail addresses: [email protected] (B. Galbrun),

[email protected] (S. Gardin).

entirely different conclusions. The Bidart section

(French basque coast) (Fig. 1), one of the most

important Cretaceous–Paleogene boundary section

in Europe, is a good example to illustrate diverging

interpretations of the same data. A long-standing

apparent discrepancy between biostratigraphic and

paleomagnetic data [1] prevented this section to be

considered as a reference one for the K–T boundary

Fig. 1. Location map and lithology of the K–T boundary Bidart

section in SW France, south of town of Biarritz.

B. Galbrun, S. Gardin / Earth and Planetary Science Letters 224 (2004) 19–3220

time interval. Our careful magneto- and biostrati-

graphic study shows that the published normal polar-

ity (interpreted as Chron C30n) and an associated K–

T boundary hiatus is, instead, an expanded record of

the K–T boundary interval with reversed polarity

(Chron C29r).

2. The Bidart section

In the Basque–Cantabric Basin of southwest

France a flysch zone persisted from Albian to Eocene

time allowing the accumulation of very thick forma-

tions of marl and marly limestone with some turbiditic

events [2]. Given the regional tectonics, all sections in

the basin are extremely faulted and discontinuous

[3,4].

The Bidart section crops out along the beach a few

kilometers south of Biarritz. This section, which dis-

plays Campanian and Maastrichtian formations, has

been subject to numerous stratigraphic and paleonto-

logic investigations [5–11], especially for the K–T

boundary interval [1,12–14].

The Bidart section was divided by Clauser [8] into

16 subunits separated by major faults. The K–T

boundary is easily identifiable between the Danian

white to pink massive limestone and the Maastrichtian

red-brown marls and marly limestones (Fig. 1). This

Maastrichtian unit is approximately 8 m thick and is

interrupted by a fault at the base. It consists of red-

brown marls and marly-limestones which become

grey 30 cm below the K–T boundary. This boundary

is marked by an abrupt lithological change: it consists

of a 15-cm-thick clay layer which is green at its base

and passes up to grey-brown and red at the top. The

clay horizon is overlain by red marly limestones,

followed by massive pink-white limestone 0.3 m

above the K–T horizon. A 60-cm-thick slump occurs

1.6 m above the K–T boudary. The Danian unit is

approximatively 20-m thick and is terminated by a

diapir of Triassic salt, but only 4 m of the unit can be

sampled for a magnetostratigraphic study before a

gully which make correlations somewhat difficult.

The magnetostratigraphy of this section was in-

vestigated by Delacotte et al. [1] who recognized

normal and reversed magnetozones and proposed

correlations with the reference section of Gubbio

(Italy). According to these authors, the uppermost

Maastrichtian was assigned to Chron C30n. Thus, a

large hiatus corresponding to the Cretaceous part of

Chron C29r was suspected below the K–T boundary

although the biostratigraphic data did not confirm its

occurrence.

Recently morphometry of coccoliths and geo-

chemical analyses were performed on this section

[15–17], but the controversy between biostrati-

B. Galbrun, S. Gardin / Earth and Planetary Science Letters 224 (2004) 19–32 21

graphic and magnetostratigraphic results remained

unsolved.

3. Biostratigraphy

3.1. Planktonic foraminifera

Premoli-Silva [1] reported that Abatomphalus

mayaroensis, the uppermostMaastrichtian index fossil,

was not encountered in the 8 m of sediments studied.

According to Haslett [9] the 8 m of the studied section

belong to the uppermost Maastrichtian Pseudotextu-

laria deformis zone. This planktonic foraminifer zone

was defined by Keller [18] when A. mayaroensis is

very rare or missing in the uppermost Maastrichtian

sediments, especially in low latitude sections. More

recently Apellaniz et al. [14] recognized the A. mayar-

oensis zone on the (rare) presence of the index species.

The basal 10 cm of the Danian portion of the

section contain only benthic foraminifera (Premoli-

Silva in [1],[14]). According to Haslett [9] and Apel-

laniz et al. [14] this thin interval belongs to the

Guembelitria cretacea zone of Smit [19]. This is

followed by the Parvularugoglobigerina eugubina

zone at about 10 cm above the boundary ([9,14],

Premoli Silva in [1]) and by the Parasubbotina

pseudobulloides zone from 70 cm (Premoli Silva in

[1]). Haslett [9] and Apellaniz et al. [14] found a

younger first occurrence (FO) of P. pseudobulloides at

about 2 m above the boundary.

3.2. Calcareous nannofossils

Perch-Nielsen [1] recorded the upper Maastrichtian

Micula prinsii subzone [20] from the base of the section

she studied (8 m below the Maastrichtian-Danian

boundary). Due to different sampling strategies, the

FO of M. prinsii was reported at different levels

according to the authors who studied this section:

Gorostidi and Lamolda [10] found it at about 7 m

below the boundary; Manivit [8] reported the FO ofM.

prinsii at about 20 m below the boundary. The Danian

sediments belong to the Biantholithus sparsus Zone of

Perch-Nielsen [21] up to the limestone bed just below

the slump and to the Cruciplacolithus primus zone of

Perch-Nielsen [21] from this slump up to the top of the

studied section (Perch-Nielsen in [1], [10,13]).

In our study, splits of samples used for magneto-

stratigraphy were processed to obtain standard smear-

slides for calcareous nannofossils analysis, in order to

establish first order correlations between the nanno-

fossil datums and the magnetic polarity pattern. Cal-

careous nannofossils were investigated on more than

350 fields of view per slide using a light microscope

at 1350� to detect presence/absence of key species.

Assemblages are in general abundant and moderately

preserved.

Our study confirms the occurrence of M. prinsii in

the 8 m of Maastrichtian strata below the K–T

boundary, though its occurrence may be discontinuous

due to preservation because this taxon is dissolution

prone and difficult to confidently identify if poorly

preserved. Nephrolithus frequens is rare but present in

the Maastrichtian samples together with more com-

mon Litraphidites quadratus, Micula murus, and

sparse Ceratholithoides kamptneri. These data are a

further confirmation of the late Maastrichtian age

recorded by previous authors.

The Danian age of the upper part of the section is

also confirmed by our data and a more detailed

succession of calcareous nannofossil events is provid-

ed : the FO (first occurrence) of C. primus (both small

and regular morphotypes; see Appendix A) and

Futuyana petalosa (sample + 1.64), the FO of Cruci-

placolithus intermedius (sample + 2.43), the FO of

Coccolithus pelagicus (sample + 2.49), the FO of

Praeprinsius dimorphosus (sample + 3.30), and the

FO of Cruciplacolithus tenuis (early morphotypes;

sample + 3.59) (Plate 1, Appendix A). This succes-

sion of nannobiohorizons, directly correlated with

magnetochrons in our study, is similar to the nanno-

fossil succession found in the basal Danian in most of

middle to low latitude sections ([21–23] and refer-

ences herein). The presence of a sedimentary slump in

the Danian sequence is biostratigraphically shown to

be associated with a hiatus based on the following

evidence: (1) the FOs of C. primus (both small and

regular morphotypes) and F. petalosa in the same

sample while they normally occur delayed, and (2) the

lack of the «dwarf Biscutum bloom» interval [24],

which normally occurs in most of low latitudes

sections. The marker species of Zone CP2 [25] and

NP3 [26], Chiasmolithus danicus, was not encoun-

tered in the 4 m of Danian strata, either in this study or

in the previous ones.

Plate 1. Key species of calcareous nannofossils. All micrographs are under polarized light. (1) Micula murus, sample �8.05 m. (2) Micula

prinsii (overgrowth specimen), sample �8.05 m. (3) Cruciplacolithus primus < 5Am (small morphotypes), sample + 1.64 m. (4–5)

Cruciplacolithus primus > 5Am, sample + 1.64 m. (6) Cruciplacolithus intermedius, sample +2.43 m. (7) Neobiscutum parvulum, sample + 0.60

m. (8) Futuyania petalosa, sample + 1.64 m. (9) Coccolithus pelagicus, sample + 2.49 m. (10–11) Cruciplacolithus tenuis (early morphotypes),

samples + 3.59 and + 3.70 m. (12) Praeprinsius dimorphosus, sample + 3.30 m.



4. Paleomagnetic analysis

Samples were collected using a portable gasoline

powered drill, and oriented with a magnetic compass.

Average sampling spacing was dependent on the lithol-

ogy and was generally 10–30 cm; the soft Cretaceous

marly limestone was sampled with a larger interval. A

total of 49 samples was collected and analyzed.

4.1. Rock magnetism

A set of rock magnetic experiments was conducted

to characterise and identify the magnetic mineralogy of

the two main lithologies. We first analyzed the acqui-

sition of IRM (Isothermal Remanent Magnetization)

up to 1.6 T and its subsequent thermal demagnetiza-

tion. Following the procedure described by Lowrie

[27], magnetic fields of 1.6, 0.4 and 0.12 T were

successively applied to each of the three perpendicular

directions prior to thermal demagnetization (Fig. 2).

For the white to pink Danian limestone, 90% of the

maximum IRM is acquired below 0.2 T and saturation

was achieved at 0.4 T (Fig. 2A). Thermal demagneti-

zation shows that the low field (0.12 T) component is

dominant and decreases up to a maximum unblocking

temperature of approximately 580 jC indicating mag-

Fig. 2. IRM acquisition of two typical samples (A, C) and thermal demagne

field strenghs (B, D) (stratigraphic levels refer to K/T boundary level as 0

netite (Fig. 2B). The harder components, less than 5%

of the total IRM, decrease regularly up to temperature

of 300–350 jC and suggest the presence of a Fe-

sulphide (pyrrhotite or greigite).

The acquisition of IRM for the Maastrichtian marly

limestones shows also an initial steep rise until approx-

imatively 0.2 T (80%) and saturation do not complete-

ly occur in applied field up to 1.6 T (Fig. 2C). The low

coercivity component corresponds to a maximum

unblocking temperature of approximately 580 jCwhich indicates magnetite (Fig. 2D). However, the

decreasing intensity curve of this component displays

an intermediate unblocking temperature of 200–250

jC which probably corresponds to the presence of

maghemite [28]. The presence of maghemite is possi-

bly the result of low-temperature oxidation of magne-

tite, likely explained by intense weathering of these

beach outcrops. The relative contribution of these two

low coercivity components to the IRM is difficult to

estimate. The high coercivity component shows an

unblocking temperature of 100 jC indicating the

presence of goethite. Unblocking temperatures greater

than 600 jC for the medium blocking field component

may indicate low coercivity hematite.

Hysteresis loops in fields up to 0.8 T were estab-

lished for some samples. A typical Maastrichtian marly

tization of IRM acquired in three orthogonal directions with different

m.).

Table 1

Fitted IRM component parameters for a typical Maastrichtian marly

limestone

Sample Component SIRM

(A/m)

B1/2

(mT)

Range

(mT)

Contribution

(%)

–4.3 m 1 0.18 21 16.2–27 11

2 1.46 66 49–112 87

3 0.04 372 310–447 2

The fitting procedure used was that of Kruiver et al. [30].


limestone shows a strong paramagnetic signal caused

by clay minerals (Fig. 3a). After subtracting this

paramagnetic component, the ferrimagnetic compo-

nent appears clearly (Fig. 3b): it is mostly due to low

coercivity minerals, confirming the previous IRM

analysis. Given the weakness of the magnetization in

Danian limestones no reliable hysteresis loop could be

established from these samples.

To estimate the relative contribution of the indi-

vidual magnetic minerals to the bulk IRM, especially

the red-brown marly limestone, we tried to decompose

the IRM acquisition curves into their magnetic com-

ponents [29]. We used the IRM fitting program of

Kruiver et al. [30]. Each magnetic component can be

described by: (1) its saturation IRM (SIRM) and thus

its contribution to the bulk IRM curve; (2) B1/2, the

applied field at which half of the SIRM is reached;

Fig. 3. Hysteresis loop up to 0.8 T of a typical Maastrichtian marly

limestone (a), and ferrimagnetic hysteresis loop after subtracting the

paramagnetic contribution (b).

and (3) DP, the dispersion parameter of the cumulative

log-normal distribution. The procedure requires inter-

active analysis towards the final model fit by optimis-

ing the IRM components parameters to minimize the

residuals between the measured data and fitted curves.

Different solutions with two or three components

were tested. The three-component fits are significantly

better, as statistically tested, than the two component

fits; they are also more in accordance with the

previous results of IRM thermal demagnetization

curves. The three-component fit of a typical sample

(Table 1) reveal a high coercivity component (mean

372 mT), as well as the differentiation of two separate

low coercivity components (means 21 and 66 mT).

The high coercivity magnetic component corresponds

most probably to hematite, and perhaps to goethite, as

suggested from the thermal demagnetization curves of

the IRM medium (0.4 T) and high field (1.6 T)

components on Fig. 2D. No distinction can be made

between hematite and goethite because of their coer-

civity overlap. The two low coercivity components

are in accordance with the results suggested by the

thermal demagnetization curves of the IRM 0.12 T

field component (Fig. 2D), in which two components

were discriminated by their unblocking temperatures.

Component 1 of lower coercivity (mean 21 mT) is

probably magnetite with an unblocking temperature of

approximately 580 jC (Fig. 2D), and component 2

(mean 66 mT) corresponds probably to magnetite

partially oxidized to maghemite with intermediate

unblocking temperature of 200–250 jC.

4.2. Natural remanent magnetization (NRM) analysis

The NRM of all samples analyzed was measured

with a three-axis RS-O1 LETI/CEA cryogenic mag-

netometer. Each sample was measured in eight ori-

entations, using four readings at each orientation for


weak samples. The NRM intensities display a large

variation depending on lithology. The red-brown

Maastrichtian marly limestone has an average inten-

sity of 15 mA/m; the NRM intensity drops to 0.2–0.4

mA/m in a 15-cm-thick grey bed 30 cm below the K–

T boundary, and then increases again until the bound-

ary. The lowermost Danian marly limestone shows the

Fig. 4. Susceptibility changes during thermal demagnetization of

two typical white Danian limestones (A) and three red-brown

Maastrichtian marly limestones (B).

highest NRM intensities values of the section, up to

35 mA/m. The NRM decreases in the white to pink

Danian limestone to 0.4 mA/m, with the exception of

the slump which displays NRM intensity values quite

similar to the Maastrichtian marly limestone. Usually

slumps are avoided by paleomagnetists, but the Dan-

ian slump of the K–T boundary Bidart section was

previously studied by Delacotte et al. [1] who defined

a normal polarity for this unit and correlated it to

Chron C29n. We studied it also to test this chrono-

stratigraphic assignment.

To determine characteristic remanent magnetization

directions and to establish the magnetostratigraphy of

the section, we thermally demagnetised one sample

from each stratigraphic level. Since heating often

induces chemical changes, we have monitored the

intial susceptibility Xin during thermal demagnetiza-

tion for a set of samples of each lithology (Fig. 4). For

the Danian white to pink limestone the Xin does not

display any variation up to 400 jC (Fig. 4A). Suscep-

tibility strongly increases in the range 400–500 jCand then decreases at temperature higher than 500 jC.Such increasing trends have been attributed to oxida-

tion of pyrite and formation of magnetite [31]. The

variations of magnetic susceptibility during heating of

Maastrichtian brown marly limestones are different

(Fig. 4B). Xin decreases up to 300 jC, remains stable,

increases from 400 to 500 jC, and finally decreases

again slowly. The decay of susceptibility up to 300 jCcould indicate the presence of maghemite as suspected

from IRM analysis. The increasing of Xin at 400 jCcan be interpreted as the transformation of pyrite to

magnetite. Despite this increasing of Xin in all lithol-

ogies, no creation of large amount of single domain

magnetite and induced large viscous remanent mag-

netization were observed. Thus the thermal demagne-

tization can be used without any problem as an

effective magnetic cleaning for all the samples.

Thermal demagnetization was performed up to 450

jC with 50 jC steps, and up to 590 jC with steps of

25–30 jC. A multicomponent behaviour of the NRM

is clear in the majority of the samples (Figs. 5 and 6),

especially in the red-brown marly limestone. The

directions of the components of magnetization were

analyzed by a least-squares fit of a line through

selected data points [32].

Representative thermal demagnetization diagrams

of the Maastrichtian and lowermost Danian samples


show up to four components (Fig. 5): (1) a viscous

component, removed at 100 jC, (2) a low-temperature

(LT) component ranging 100–300 jC, with a present

day field direction (declination 6j and inclination

60j), (3) a reversed polarity medium-temperature

(MT) component, removed between 300 and 400

jC, and (4) a normal polarity high temperature (HT)

component ranging from 400–450 to 560–590 jC.The MT and HT components are antiparallel. In the

levels displaying the highest NRM intensities—i.e.,

lowermost Danian marly limestones and some levels

in the last meter of Maastrichtian—only the LT and

HT components are visible, the MT component is

missing or forms a cluster (Fig. 5, sample + 0.38 m).

This type of behaviour is also seen in samples from

the Danian slump, 1.6 m above the K–T boundary

(Fig. 6, sample + 2.03 m).

The Danian white to pink limestone shows a more

common behaviour (Fig. 6, samples + 0.93, + 2.22

and + 3.69m) : (1) a very soft component is removed at

100 jC, (2) a normal polarity, low temperature (LT)

component removed between 100 and 250/300 jC, and(3) a normal (sample + 3.69 m) or reversed (samples

+ 0.93 and + 2.22 m) high temperature (HT) compo-

nent removed between 300/350 and 500/590 jC.

5. Discussion

The results of the magnetic mineralogy and NRM

analysis require to discuss the origin of each magnetic

component and to assess which one is the character-

istic remanent magnetization (ChRM) representing the

Earth’s magnetic field at the time of deposition.

For the Maastrichtian and lowermost Danian red-

brown marls and marly limestones (Fig. 5), the LT

component is a recently acquired overprint, contribut-

ing most of the NRM. It likely corresponds to the low

coercivity (0.12 T) component with an unblocking

temperature of 250–350 jC (Fig. 2D), and to compo-

nent 2 of the IRM analysis (Table 1). This component is

Fig. 5. Orthogonal demagnetization plots of typical samples of

Maastrichtian and lower Danian red marls and marly limestones

(stratigraphic levels � 8.05, � 5.05 and � 0.05 m below the K–T

boundary, and + 0.38 m above the boundary). Open and closed

circles represent projections on the vertical and horizontal planes,

respectively.

Fig. 6. Orthogonal demagnetization plots of typical samples of

Danian white to pink limestones (stratigraphics levels + 0.93,

+ 2.22 and + 3.69 m above the K–T boundary), and of the Danian

slump ( + 2.03 m). Open and closed circles represent projections

on the vertical and horizontal planes, respectively.


most probably carried by maghemite due to weathering

and seems more prominent approximatively 1 m below

and above the K–T boundary, an interval of soft marls

which is more susceptible to weathering.

The MT component, of reversed polarity, is defined

only between 300 and 400 jC; we are confident that

this component represents a relic of the primary

magnetization. It is probably carried by magnetite

grains which are not completely oxidized into maghe-

mite. Magnetite appears on the thermal demagnetiza-

tion curves of the IRM 0.12 T field component (Fig.

2D) with a maximum unblocking temperature of 580

jC, and is probably the component 1 on IRM compo-

nents analysis (B1/2 = 20 mT) (Table 1). The HT

component of normal polarity, ranging from 400–

450 to 560–590 jC, is possibly due to authigenic

pigmentary hematite as evident from the red-brown

colour of the marly limestones. This hematite compo-

nent corresponds to the component 3 of the IRM

analysis component (Table 1). From a relative large

spectrum of grain sizes, it results in a large spectrum of

unblocking temperature and field coercivity [33] that

seems to be the case here. Such hematite growth could

occur early in the diagenetic evolution and continue for

several hundreds of thousand years after the deposition

of the host sediment. In the case of the Bidart section,

the uppermost Maastichtian and lowermost Danian

marl and marly limestone were deposited during a

period of reversed polarity (C29r). The growth of

pigmentary hematite, which occured in late diagenesis,

recorded a normal polarity corresponding to the field

of a subsequent normal period, probably C29n.

Only the LT and HT components are present in

some levels around the K/T boundary, which were

more subject to weathering (Fig. 5, sample + 0.38 m),

and in the Danian slump (Fig. 6, sample + 2.03 m).

Likely the slump stayed longer time in an unconsol-

idated state than the massive Danian limestones and

so it may have acquired the secondary normal polarity

overprint due to hematite. The white to pink Danian

limestone is more easy to interpret (Fig. 6). The

normal polarity LT component, ranging from 100 to

250/300 jC, is again an overprint due to weathering,

while we interpret the normal or reversed HT com-

ponent, removed between 300/350 and 590 jC, to be

the primary component.

Thus, the directions of MT component from Maas-

trichtian and lowermost Danian marls and marly


limestones and of HT component from white to pink

Danian limestones are the ChRM directions and are

used to define the magnetic polarity sequence of the

section (Fig. 7). Not all the samples studied could give

a reliable ChRM, but the mean directions computed

on those we are confident are ‘‘quite antiparallel’’

Fig. 7. Lithology of the K–T boundary section at Bidart, calcareous n

directions (declination, inclination), polarity sequence (black/white is norm

of Delacotte et al. [1]. ChRM directions (Middle temperature component

and high temperature component in the Danian limestone) are plotted as b

Maastrichtian marly limestones, the lower Danian limestones and the Dan

(Table 2). On these two populations we applied the

reversal test of McFadden and McElhinny [34]: the

critical angle cc is 21.16j and the observed angle cobetween the two mean polarities is 5.4j. The reversaltest is thus positive and classified as class B according

to [34]. This supports the primary origin of these

annofossil biostratigraphic events, NRM intensity, paleomagnetic

al/reversed polarity) and Chron assignment, previous interpretation

in the Maastrichtian marly limestones and lower Danian limestones

lack circles; the high temperature normal overprint directions in the

ian slump are plotted as open circles.

Table 2

Means characteristic remanent magnetization directions. (a) Normal

polarity samples at the top of the section excluding those from the

Danian slump, (b) all reversed polarity samples, (c) reversed samples

with reliable ChRM directions

N D I k a95

Normal (a) 4 14.9 46 41.3 14.5

Reversed (b) 34 204.4 � 51 6.6 10.4

Reversed (c) 27 197.0 � 51 16.8 7


ChRM directions. The established magnetic polarity

sequence is characterized by a reversed magnetozone

which includes the entire Maastrichtian part of the

section and up to 3.5 m above the K–T boundary, and

a normal magnetozone at the top of the section (Fig.

7). Considering the reliable biostratigraphic data and

the position of the K–T boundary it is possible to

correlate the proposed magnetic polarity sequence to

the GPTS: the reversed magnetozone corresponds to

Chron C29r and the subsequent normal magnetozone

belongs to Chron C29n.

This interpretation differs substantially from the

one provided by Delacotte et al. [1]. These authors

recognized a normal polarity for most of the Maas-

trichtian unit of the section and assigned it to Chron

C30n (Fig. 7). Thus, a long hiatus corresponding to

the Cretaceous part of Chron C29r was suspected

below the K–T boundary. Such a long hiatus is not

supported by the biostratigraphic data: the 8 m studied

below the K/T boundary belong to the uppermost

Fig. 8. Summary of paleomagnetic and biostratigraphic data across the late

[23,35–40]).

Maastrichtian planktonic foraminifera zone, the P.

deformis Zone [9]. The calcareous nannofossil M.

prinsii occurs within the 8 m studied, while its FO

is defined near the Chrons C29r–C30n boundary [35]

(Fig. 8). The present section can thus be attributed to

Chron C29r, though we cannot completely rule out the

hypothesis that this reversed interval does not repre-

sent the entire late Cretaceous part of the Chron. The

absence of the planktonic foraminifer Plummerita

hantkeninoides (Premoli-Silva in [1]) [9], its FO

defining the uppermost Maastrichtian CF1 Zone of

Keller [18] and within the M. prinsii subzone, might

indicate a missing uppermost Maastrichtian. However

the planktonic foraminifera assemblages of this sec-

tion are scarce and poorly preserved (Premoli-Silva in

[1],[9]) and thus do not represent the best means of

identifying a missing/absent uppermost Maastrichtian

interval.

In the lowermost Danian, Delacotte et al. [1]

defined some levels of reversed polarity as Chron

C29r (Fig. 7). They assigned a normal polarity for

the Danian slump and correlated it with Chron C29n.

The next polarity intervals were thus assigned to

Chrons 28r and 28n. However, the calcareous nanno-

fossil events found in this investigation and in previous

studies are more indicative of an earliest Danian age

and correlate well with the upper part of Chron C29r

and Chron C29n for the following reasons: (1) assemb-

lages are dominated by abundant Thoracosphaera spp.

and F. petalosa with few Cruciplacolithus species and

Maastrichtian–early Danian considered in this work (data are from


Cretaceous species still present, these assemblages are

typical of earliest Danian [21]; (2) the FOs of C.

pelagicus and P. dimorphosus were recorded within

the lowest Paleocene NP2 and CP1 zones of Martini

[26] and Okada and Bukry [25] by previous authors

([23] and references herein); (3) early morphotypes of

C. tenuis were observed from sample + 3.59, which is

indicative of a normal polarity interval (interpreted

here as C29n), in agreement with the findings of

Berggren et al. [23]; (4) the calcareous nannofossil

C. danicus, whose FO lies in the uppermost part of

Chron C29n [23], was not encountered. Thus, the

interpretation of the normal interval at the top of the

studied section and the slump to Chrons C28n and

C29n respectively is highly unlikely.

Delacotte et al. [1] interpreted their paleomagnetic

data from the marl and marly limestone using the

directions of the high temperature (HT) component as

the primary directions. They have not detected the

medium temperature (MT) component, mainly be-

cause they used a spinner magnetometer which did

not allow precise measurements.

6. Conclusion

Rock magnetic analysis, careful thermal demagne-

tization and analysis of NRM components allow us to

propose a new magnetostratigraphic interpretation of

the K–T boundary Bidart section (France).

Magnetite, maghemite, and hematite are the car-

riers of remanence in the red-brown lowermost Dan-

ian and Maastrichtian marly limestones. The NRM of

these strata is dominated by a double normal overprint

due to, (1) a low-temperature (100–300 jC) present-day field overprint probably resulting from weather-

ing and its resultant maghemitisation and, (2) a high

temperature (up to 590 jC) normal component caused

by late diagenesis hematite growth. The characteristic

remanent magnetization (ChRM) carried by magnetite

grains which are not completely oxidized into maghe-

mite appears only as a relic reversed component of

medium temperature ranging from 300 to 400 jC.Magnetite is the main magnetic mineral in the

white to pink Danian limestones in which the NRM

is characterized by a secondary present day field

overprint and a ChRM component of normal or

reversed polarity.

A new magnetic polarity sequence is established,

and we propose a new correlation with the GPTS. The

Maastrichtian unit and the limestone beds up to 3.5 m

above the K–T boundary are of reversed polarity and

correspond to Chron C29r. A normal magnetozone

occurring at the top of the section is correlated with

Chron C29n.

These chronostratigraphic assignments are well in

accordance with the previously established biostrati-

graphic data and the new calcareous nannofossil data

presented in this paper. The calcareous nannofossil M.

prinsii, whose FO is defined around the boundary

between Chrons C29r and C30n occurs in the entire

8 m below the K–T boundary. This allows the

confident correlation of the Maastrichtian sediments

with Chron C29r. The succession of calcareous nan-

nofossil FOs above the K–T boundary (FO of C.

primus, C. intermedius, C. pelagicus, P. dimorphosus

and early morphotypes of C. tenuis), indicates an

earliest Danian age correlating well with Chrons

C29r and C29n. On the contrary, C. danicus, whose

FO lies at the base of Chron C28r [23], was not

encountered either in this or in the previous studies.

From the magnetostratigraphic and biostratigraphic

point of view, there is no evidence for a hiatus at

the K–T boundary section at Bidart. This section is

thus suitable to carry out high-resolution studies of

the paleoenvironmental changes across the K–T

boundary.

Acknowledgements

We are grateful to our collegues Xixi Zhao

(University of Santa Cruz) and Mark Leckie (Univer-

sity of Massachussets) for their suggestions and

careful review of an early draft of the paper. Jim

Ogg (Purdue University) is warmly thanked for

comments and opening paragraph, we are grateful to

Cor Langereis (Utrecht University) for his very

detailed and constructive review. The rock magnetism

analysis were performed at the Institut de Physique du

Globe de Paris, we thank Maxime Legoff for

assistance. Thanks to Pauline Kruiver (Utrecht

University) who helped us by mail to use her IRM

fitting program while we were on the ODP Leg 210 in

the Atlantic Ocean. We thank Eric Robin and Robert

Rocchia for assistance in the field. [VC]


Appendix A. Taxonomy of calcareous nannofossil

species cited in the text

� Ceratolithoides kamptneri Bramlette and Martini,

1964� Coccolithus pelagicus (Wallich, 1871), Schiller,

1930� Cruciplacolithus intermedius van Heck and Prins,

1987

Remarks: In this study we considered as C. inter-

medius a Cruciplacolithus having a length of more

than7Amanda thick, robust cross in the central area.

� Cruciplacolithus primus Perch-Nielsen, 1977

Remarks: The first C. primus to occur in basal

Danian are 3–4 Am in length (early morphotypes).

‘‘Regular’’ morphotypes are 5–7 Am in length.

Both morphotypes display a thin, weakly birefrin-

gent cross in the central area.

� Cruciplacolithus tenuis (Stradner 1961), Hay and

Mohler in Hay et al., 1967

Remarks: InthisworkC.tenuis isintendedastheform

having «feet» (calcite blocks) at the end of the central

cross (shown by Romein, 1979). The specimens

shown in Plate 1 (10,11) having incoming, small,

calcite blocks visible when the specimen is rotated at

45j,areconsideredasearlymorphotypesofC.tenuis.

� Futuyania petalosa (Ellis and Lohmann, 1973),

Varol 1989� Lithraphidites quadratus Bramlette and Martini,

1964� Micula murus (Martini, 1971), Bukry, 1973� Micula prinsii Perch-Nielsen, 1979a� Nephrolithus frequens Gorka, 1957� Neobiscutum (Romein, 1979) Varol 1989� Praeprinsius dimorphosus (Perch-Nielsen 1969),

Varol and Jakubowsky, 1979� Thoracosphaera Kamptner, 1967, spp.

References

[1] O. Delacotte, M. Renard, C. Laj, K. Perch-Nielsen, I. Premoli-

Silva, S. Clauser, Magnetostratigraphie et biostratigraphie du

passage Cretace-Tertiaire de la coupe de Bidart (Pyrenees

Atlantiques), Geologie de la France 3 (1985) 243–254.

[2] B. Mathey, Les flyschs Cretace superieur des Pyrenees bas-

ques, age, anatomie, origine du materiel, milieu de depot et

relations avec l’ouverture du Golfe de Gascogne, These Univ.

de Bourgogne Dijon 1986, p. 403.

[3] P. Razin, Evolution tecto-sedimentaire alpine des Pyrenees

basques a l’Ouest de la transformante de Pamplona (Province

du Labourd), These Univ. Bordeaux III 1989, p. 464.

[4] L. Lecallonnec, S.Gardin,B.Galbrun,M.Renard, J.P.Bellier, P.

Razin,Donnees nouvelles sur leCampanien superieur de la serie

de la Baie de Loya (Pays-Basque, France): biostratigraphie et

magnetostratigraphie, C. R. Acad. Sci. 325 (1997) 351–357.

[5] P.D. Ward, Maastrichtian ammonite and inoceramid ranges

from bay of Biscay Cretaceous-Tertiary boundary sections,

in: M.A. Lamolda, E.G. Kaufman, O.H. Walliser (Eds.), Palae-

ontology and Evolution; Extinction events, Revista Espanola

de Paleontologia, no. extraordinario, 1988, pp. 119–126.

[6] B.K. Nelson, G.K. MacLeod, P.D. Ward, Rapid change in

strontium isotopic composition of sea water before the Creta-

ceous/Tertiary boundary, Nature 351 (1991) 644–647.

[7] P.D. Ward, W. Kennedy, Maastrichtian ammonites from the

Biscay region (France, Spain), J. Paleontol. Mem. 34

(1993).

[8] S. Clauser, Etudes stratigraphiques du Campanien et du Maas-

trichtien de l’Europe occidentale, Cote Basque, Charentes

(France), Limbourg (Pays-Bas), Doc.-BRGM 235 (1994) 243.

[9] S.K. Haslett, Planktonic foraminiferal biostratigraphy and

palaeoceanography of the Cretaceous–Tertiary boundary sec-

tion at Bidart, south–west France, Cretac. Res. 15 (1994)

179–192.

[10] A. Gorostidi, M. Lamolda, La nanoflora calcarea y el transito

KT de la seccion de Bidart (SW de Francia), Rev. Esp. Pale-

ontol. (1995) 153–168.

[11] H.B. Vonhof, J. Smit, High-resolution late Maastrichtian–early

Danian oceanic 87Sr/86Sr record : implications for Cretaceous–

Tertiary boundary events, Geology 25 (1997) 347–350.

[12] P.Bonte,O.Delacotte,M.Renard,C.Laj,D.Boclet,C.Jehano,R.

Rocchia, An iridium rich layer at the C/T boundary in the Bidart

section (SWFrance), Geophys. Res. Lett. 11 (1984) 473–476.

[13] C. Seyve, Nannofossil biostratigraphy of the Cretaceous–

Tertiary boundary in the French Basque Country, Bull. Cent.

Rech. Explor. Prod. Elf-Aquitaine 14 (1990) 553–572.

[14] E. Apellaniz, J.I. Baceta, G. Bernaola-Bilbao, K. Nunez-Betelu,

X. Orue-Etxebarria, A. Payros, V. Pujaltes, E. Robin, R. Roc-

chia, Analysis of uppermost Cretaceous– lowermost Tertiary

hemipelagic successions in the Basque Country (western Pyre-

nees): evidence for a sudden extinction of more than half

planktic foraminifer species at the K/T boundary, Bull. Soc.

Geol. Fr. 168 (1997) 783–793.

[15] F. Minoletti, Mise au point d’un protocole de separation des

assemblages de nannofossiles calcaires, apport a la micropa-

leontologie et a la geochimie des producteurs carbonates pela-

giques. These de l’Universite Paris VI 2002, p. 238.

[16] F. Minoletti, S. Gardin, E. Nicot, M. Renard, Mise au point

d’un protocole experimental de separation granulometrique

d’assemblages de nannofossiles calcaires: applications paleo-


ecologiques et geochimiques, Bull. Soc. Geol. Fr. 172 (2001)

437–446.

[17] N. Thibault, F. Minoletti, S. Gardin, M. Renard, Morphometrie

de nannofossiles calcaires au passage Cretace-Paleocene des

coupes de Bidart (France) et d’Elles (Tunisie). Comparaison

avec les isotopes stables du carbone et de l’oxygene, Bull. Soc.

Geol. France 175 (2004) (in press).

[18] G. Keller, Extinction, survivorship and evolution of planktic

foraminifera across the Cretaceou/Tertiary boundary at El Kef,

Tunisia, Mar. Micropaleontol. 13 (1988) 239–263.

[19] J. Smit, Extinction and evolution of planktonic foraminifera

after a major impact at the Cretaceous–Tertiary boundary, in:

L.T. Silver, P.H. Schultz (Eds.), Geological implication of

impacts of large asteroids and comets on the Earth, Spec.

Pap.-Geol. Soc. Am., vol. 190, 1982, pp. 329–352.

[20] K. Perch-Nielsen, Mesozoic calcareous nannofossils, in:

H.M. Bolli, J.B. Saunders, K. Perch-Nielsen (Eds.), Plankton

stratigraphy, Cambrige University Press, Cambrige, 1985,

pp. 427–554.

[21] K. Perch-Nielsen, Nouvelles observations sur les nannofos-

siles calcaires a la limite Cretace-Tertiaire pres de El Kef,

Cah. Micropaleontol. 3 (1981) 25–36.

[22] S. Gardin, Late Maastrichtian to early Danian calcareous nan-

nofossils at Elles. A tale of 1 million years across the K/T

boundary, in: J. Remane, T. Adatte (Eds.), The K/T Boundary

in Tunisia, Palaeogeogr. Palaeoclimatol. Palaeoecol. Special

Issue, vol. 178, 2002, pp. 211–231.

[23] W.A. Berggren, M.-P. Aubry, M. van Fossen, D.V. Kent,

R.D. Norris, F. Quillevere, Integrated Paleocene calcareous

plankton magnetobiochronology and stable isotope stratigra-

phy: DSDP Site 384 (NW Atlantic Ocean), Palaeogeogr.

Palaeoclimatol. Palaeoecol. 159 (2000) 1–51.

[24] S. Gardin, S. Monechi, Paleoecological change in middle to

low-latitude calcareous nannoplankton at the Cretaceous/Ter-

tiary boundary, Bull. Soc. Geol. Fr. 169 (1998) 709–723.

[25] H. Okada, D. Bukry, Supplementary modification and intro-

duction of code numbers to the low latitude coccolith biostrati-

graphic zonation (Bukry, 1973; 1975), Mar. Micropaleontol. 5

(1980) 321–325.

[26] E. Martini, Standard Tertiary and Quaternary calcareous nan-

noplankton zonation, in: A. Farinaci (Ed.), Proc. II Plankt.

Conf. Roma, vol. 2, 1971, pp. 739–785.

[27] W. Lowrie, Identification of ferromagnetic minerals in a rock

by coercivity and unblocking temperature properties, Geo-

phys. Res. Lett. 17 (1990) 159–162.

[28] J.O. Pascual, J.M. Pares, C.G. Langereis, J.D.A. Zijderveld,

Magnetostratigraphy and rock magnetism of the Ilerdian stra-

totype at Tremp, Spain, Phys. Earth Planet. Inter. 74 (1992)

139–157.

[29] D.J. Robertson, D.E. France, Discrimination of remanence-

carrying minerals in mixtures, using isothermal remanent

magnetisation acquisition curves, Phys. Earth Planet. Inter.

82 (1994) 223–234.

[30] P.P. Kruiver, M.J. Dekkers, D. Heslop, Quantification of mag-

netic coercivity components by the analysis of acquisition

curves of isothermal remanent magnetization, Earth Planet.

Sci. Lett. 189 (2001) 269–276.

[31] W. Lowrie, F. Heller, Magnetic properties of marine lime-

stones, Rev. Geophys. Space Phys. 20 (1982) 171–192.

[32] J.L. Kirschvink, The least-squares line and plane and the anal-

ysis of palaeomagnetic data, Geophys. J. R. Astron. Soc. 62

(1980) 699–718.

[33] J.E.T. Channell, R. Freeman, F. Heller, W. Lowrie, Timing of

diagenetic haematite growth in red pelagic limestones from

Gubbio (Italy), Earth Planet. Sci. Lett. 58 (1982) 189–201.

[34] P.L.McFadden,M.W.McElhinny, Classification of the reversal

test in paleomagnetism, Geophys. J. Int. 103 (1990) 725–729.

[35] Shipboard Scientific Party, Explanatory notes. In: B.E.

Tucholke, J.-C. Sibuet, A. Klaus, et. al. (Eds.), Proc. ODP

Init. Repts. 210, http://www-odp.tamu.edu/publications/ (in

press).

[36] W.A. Berggren, D.V. Kent, C.C. Swisher III, M.P. Aubry,

A revised Cenozoic geochronology and chronostratigraphy,

in: W. Berggren, D.V. Kent, M.P. Aubry, J. Hardenbol

(Eds.), Geochronology, Time Scales and Global Stratigraphic

Correlation, Soc. Sediment. Geol., Spec. Publ., vol. 54, 1995,

pp. 129–212.

[37] J.A. Burnett, Upper Cretaceous, in: P.R. Bown (Ed.), Calcar-

eous Nannofossil Biostratigraphy, Kluwer, London, 1998,

pp. 132–199.

[38] G. Keller, L. Li, N. MacLeod, The Cretaceous –Tertiary

boundary stratotype section at El kef, Tunisia: how cata-

strophic was the mass extinction? Palaeogeogr. Palaeoclima-

tol. Paleoecol. 119 (1995) 221–254.

[39] L. Li, G. Keller, Maastrichtian climate, productivity and fau-

nal turnovers in planktic foraminifera in South Atlantic DSDP

Site 525A and 21, Mar. Micropaleontol. 33 (1998) 55–86.

[40] W. Sissingh, Biostratigraphy of Cretaceous calcareous nanno-

plankton, Geol. Mijnb. 56 (1977) 37–65.

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