Magnetostratigraphy and biostratigraphy of Cretaceous-Tertiary continental deposits, Ager Basin, Province of Lerida, Spain

Palaeogeography, Palaeoclimatology, Palaeoecology, 102 (1993): 41-52 Elsevier Science Publishers B.V., Amsterdam

41

Magnetostratigraphy and biostratigraphy of Cretaceous-Tertiary continental deposits, Ager Basin, Province of Lerida, Spain B. G a l b r u n a, M . F e i s t b, F. C o l o m b o c, R. R o c c h i a d a n d Y. T a m b a r e a u e aUniversitO Paris vI, DOpartement de Gdologie Sddimentaire, URA-CNRS 1315, 4 place Jussieu, 75252 Paris cedex 5, France bUniversitd Montpellier II, Institut des Sciences de l'Evolution, URA-CNRS 327, Place E. Bataillon, CP 062, 34095 Montpellier Cedex 2, France cUniversitat de Barcelona, Departament de Geologia Dinglmica Geofisica i Paleontologia, Zona Universithria de Pedralbes, 08028 Barcelona, Spain dCentre des Faibles Radioactivitds, Laboratoire rnixte CEA-CNRS, Avenue de la Terrasse, 91198 G(f-sur- Yvette Cedex, France eUniversitd Paul Sabatier, Laboratoire de Gkologie Structurale et Tectonophysique, URA-CNRS 1405, 38 rue des Trentesix Ponts, 31400 Toulouse, France (Received September 4, 1992; revised and accepted December 14, 1992)

ABSTRACT Galbrun, B., Feist, M., Colombo, F., Rocchia, R. and Tambareau, Y., 1993. Magnetostratigraphy and biostratigraphy of Cretaceous-Tertiary continental deposits, Ager Basin, Province of Lerida, Spain. Palaeogeogr., Palaeoclimatol., Palaeoecol., 102: 41-52. At Fontllonga (Northeast Spain, Province of Lerida) a nearly continuous outcrop covers the stratigraphic interval from the Maastrichtian to the Thanetian in continental deposits. Magnetostratigraphy of the section was determined from 72 samples. The magnetic polarity sequence was satisfactorily correlated with chrons 32R to 26R (top of Lower Maastrichtian to Lower Thanetian). Chron 29R, in which the K/T boundary occurs, is unambiguously recognizable considering the biostratigraphic data (charophyte, ostracod, palynomorph). The K/T boundary occurs in the uppermost part of the Microchara cristata subzone, which includes Late Cretaceous and Early Palaeocene species. This subzone is interpreted as an interval of crisis, following the Late Maastrichtian extinctions and preceeding the Palaeocene radiation. No Ir-rich horizon has been yet found in this section. The stratigraphic position of the last dinosaur remains, found in the chron 31N interval, leaves open the question of a causal link between the extinction of Cretaceous reptiles and the K/T event.

Introduction The b o u n d a r y between the Cretaceous and the Tertiary is one o f the most exciting boundaries for the evolution o f life. A catastrophic event apparently occurred at that time which p r o d u c e d one o f the most i m p o r t a n t and sudden crises ever experienced by life on Earth. In marine deposits, an a n o m a l o u s l y high iridium a b u n d a n c e has been reported all a r o u n d the Earth in exact coincidence with the sharp decrease o f the carbonate content resulting f r o m the planktonic crisis (Alvarez et al., 1982). The o v e r a b u n d a n c e o f Ir is n o w k n o w n to 0031-0182/'93/$05.00

be associated, in m a n y sites, with Ni-rich spinels (Robin et al., 1991) and shocked minerals (Bohor et al., 1984). The origin o f these m a j o r geochemical and mineralogical features is still controversial but the most widely accepted opinion is that they are due to a cosmic collision at the close o f the Cretaceous. The discovery o f soot in the strata immediately overlying the b o u n d a r y (Wolbach et al., 1985) has been interpreted as the result o f a worldwide wildfire. This shows that the K / T catastrophe, which destroyed a significant fraction o f Cretaceous living species in the oceans, also profoundly affected life on land.

© 1993 Elsevier Science Publishers B.V. All rights reserved

42

In continental deposits, the K/T boundary has not been so widely documented. A complete record of the K/T transition is found only in North American sections where Ir (Orth et al., 1981), shocked quartz (Bohor et al., 1984) are always found in coincidence with a fern spike which indicates that the plant cover was seriously damaged and, at least temporarily, drastically reduced. At other continental places in the world, in southern Europe and China, the boundary has not been precisely identified yet. The present situation is that the reaction of land env.ironment to the K/T event is poorly known and based mainly on observations on the North American continent. Therefore, it is of prime importance to find additional locations on other continents in order to evaluate the geographical variations of the effects of the K/T event on non-marine environments and determine to what extent the evolution of organisms (charophytes, palynofloras, molluscs, ostracods, etc.) is related to this event. For that reason, we have undertaken a systematic investigation of the numerous non-marine K/T sites in the western Mediterranean area. The general characteristic of all the studied sections is that the transitional beds from Maastrichtian to Paleocene are nearly barren of fossils. This interval, supposed to contain the boundary, is traditionally defined to lie between the last "in situ" dinosaur remains and some welldated early Tertiary marine intercalations (Babinot and Durand, 1980; Babinot et al., 1983; Buffetaut and Le Loeuff, 1991). Generally, this represents a sedimentary interval of a few tens or hundred meters. Magnetostratigraphy is potentially a good tool to correlate non-marine sediments. Such studies were carried out in the Arc Basin (Southeast France) but the results are yet ambiguous (Hansen et al., 1989; Krumsiek and Hahn, 1989; Westphal and Durand, 1990; Galbrun et al., 1991). In addition, all these sections are often vertically discontinuous and show strong lithologic lateral variations, so that, up to now, the position of the K/T boundary has remained uncertain. The Fontllonga section, in the Ager Basin (Province of Lerida, northeastern Spain) has revealed an uncommon diversity and abundance of plant microfossils (Feist and Colombo, 1983; M6dus et al., 1988) and a good potential for magnetostrat-

B. G A L B R U N ET AL.

igraphy. This section offers a reasonable possibility for defining the K/T boundary on the basis of palaeontological, geophysical and geochemical data. Geological setting and lithology of the Fontllonga section The Fontllonga section is situated in the southern part of the Ager Basin, along the road from Tremp to Balaguer, near the village of Fontllonga (base of the section: x = 0°50'27 '', y = 41°59'54"; topographic map no. 328: "Artesa de Segre") (Fig. 1). The Ager Basin is part of the pre-Pyrenean marginal mountains, which were built up from the rn 210 200 100 t80 170

I

160

Mi~gontl

150

0

6

-- ~

140

Ionga

10 krn I r

'

JL-,

-~,TfiTrem~

130 120 110 100

~ntl Iong~

90 80

COLOUR

~

70

~

Grey

0

! km

"~,~ Baleguer

~ Red ~ Brown

~

~ Verlegated [ - ~ 20-

~

Limestones Sandy I Im6stones Silty ~.!aystones Sandstonesend conglomeretes

'bOO-

Fig. 1. Geographiclocation of, and simplifiedlithologic succession for, the Fontllonga section.

MAGNETOSTRATIGRAPHY AND BIOSTRATIGRAPHY OF CRETACEOUS-TERTIARY CONTINENTAL DEPOSITS. AGER BASIN, SPAIN

north during the Pyrenean orogeny. Its base comprises marine sediments ranging from Triassic to Late Cretaceous in age. These basal sequences are capped disconformably with the non-marine Tremp Formation which includes the Fontllonga succession (Mey et al., 1968). The Ager Basin sequence ends with the Ilerdian marine limestones with alveolinas. The Tremp Formation includes nearly 720 m of non-marine sediments in the Fontllonga area. The succession is represented as follows, in ascending order: Unit 1

Lower limestones, overlying unconformably the imperfectly dated Maastrichtian marine Bona Formation. This unit is 80 m thick. Its represents shallow lake deposits and has yielded an assemblage of ostracods, molluscs, palynofloras and charophytes characteristic of the Upper Maastrichtian (Feist and Colombo, 1983; Masriera and Ullastre, 1983; Madus et al., 1988) Unit 2

Lower terrigenous sequence, conformably lying on (1). This sequence is 200 m thick, and includes red silty claystones with locally well developed sandstone banks. Thin calcareous horizons are locally developed. This unit is supposed to contain the Cretaceous-Tertiary boundary (M6dus et al., 1988). Unit 3

Vallcebre Limestone Formation (Sole Sugranes, 1970), conformably lying on Unit 2. It is 75 m thick and comprises lacustrine facies alternating with some palustrine beds. Karstifications are locally well developed. This unit is of Paleocene age. Unit 4

Upper terrigenous sequence. This sequence has a stratigraphic thickness of 380 m, and consists of red silty claystones alternating with some sandstone beds. Evaporitic facies are abundant in the

43

upper part of the sequence. This unit has yielded an Upper Thanetian charophyte assemblage (Feist and Colombo, 1983). The Tremp Formation is overlain by the Ilerdian marine limestone of the Cadi Formation. The section studied here is 230 m thick and extends from the top of Unit 1 to the base of Unit 3 (Fig. 1).

Magnetic stratigraphy Ninety-five cores were collected using a portable gasoline drill and oriented with a magnetic compass. The density of sampling depended of the quality of outcrops. Standard cores of 2.5 cm in diameter and 2.25 cm length were cut in the laboratory. Natural remanent magnetization (NRM) was measured with a three axis RS-01 (LETI/CEA) cryogenic magnetometer at the D6partement de G6ologie S6dimentaire of Universit6 Paris VI. The background noise level of this instrument enabled samples as weakly magnetized as 2 x 10- ~ A/m to be measured with good precision. Within the section there was important variation in initial NRM intensities depending of the various lithologies. White lacustrine limestones at the top of the section have weak NRM intensities, averaging 3.5 x 10 s A/m. NRM intensities of mainly red sandstones, claystones and silty claystones were slightly higher, averaging 4.8 x 10 4 A/m, the full range of variation was between 10-4-2.2 x 10 -3 A/re. Grey limestones at the base of the section were also weakly magnetized, of the order of 1.5-3 × 10 -5 A/re. Most NRM directions, before bedding correction, were near the present earth magnetic field direction (Fig. 2). This secondary magnetic overprint could be easily removed by magnetic cleaning (Lowrie and Heller, 1982). Thermal demagnetization was used as a routine technique in most samples to isolate primary remanence components. Some specimens were studied by mixed treatment (thermal demagnetization and subsequent alternating field, AF, demagnetization) which can be very effective for old sedimentary rocks (Galbrun et al., 1988). AF demagnetization was not used alone given the frequent occurrence of minerals displaying high coercivity spectrums, such as goethite, in continental sediments (Galbrun et al., 1991).

B. GALBRUN

44

ET AL.

weak NRM intensity. Thus only 72 palaeodirections could be computed (Fig. 4). These palaeodirections form two roughly antipoda1 clusters of normal and reversed polarity with some specimens exhibiting intermediate directions. Plotted in regard of the lithologic column, the palaeodirections allow to propose a magnetic polarity sequence (Fig. 5). This sequence does not cover the whole section due to some gaps in the outcrops and some limestone beds being too weakly magnetized. The polarity sequence is characterized in the lower part by a predominant normal polarity and in the upper part by a predominant reversed polarity. Some polarity zones are defined by single samples, they are represented in the polarity column by partial bars. 180

Fig. 2. Stereographic projection of natural remanent magnetization directions of all samples studied. Most of these directions, prior to any magnetic cleaning, are of positive inclination and suggest a secondary magnetization due to the present earth magnetic field. Solid (open) circles represent projection of the lower (upper) hemisphere.

The results of demagnetization are represented on orthogonal vector plots (Fig. 3). The samples treated in 50°C steps up to 450°C are characterized by simple behaviour. An initial unstable component was removed at 200-250°C (Figs. 3a,b). Above this temperature the characteristic remanent magnetization direction was defined by an approximately straight line to the origin of the vector diagram. Ten samples were treated by mixed cleaning, i.e., 300°C heating and subsequent AF demagnetization. This technique was also effective but the final end points were somewhat scattered (Fig. 3c), possibly due to the acquisition of anhysteretic remanent magnetization components in the Schonstedt demagnetizer. Thus the mixed treatment was not used as a routine technique. The directions of characteristic remanent magnetization of single samples were determined by least squares analysis to vectors in the region of linear decay towards the origin of the vector plot (Kirschvink, 1980). These directions are of normal (Fig. 3a) or reversed polarity (Fig. 3b). Some samples, mainly lacustrine limestones, could not be measured after demagnetization because of their

Palaeontological data Charophytes

These plant microfossils have proved to be useful for zonation and correlation of non-marine sediments, especially for the Cretaceous and Palaeogene. However the Upper Cretaceous-Lower Palaeocene interval has not yet been subdivided precisely and an undivided interval corresponding to a time interval of nearly 5 m.y. was intercalated between the Upper Maastrichtian Septorella ultima zone (Grambast, 1971, 1974) and the Upper DanoMontian Dughiella bacillaris subzone (Riveline, 1986). The Fontllonga succession, which includes abundant and diverse charophyte floras near the K/T transition, offers the possibility to define a more complete zonal scheme, with three additional subdivisions. Charophytes are present in lacustrine calcareous sediments all through the Fontllonga section but are missing in sandstones and red, silty claystones of fluvial origin. The successive associations allow the identification of five local biozones, based on the intervals between the lowest occurrences of stratigraphically significant species (Fig. 6): -Local Zone 1: Lower Microchara cristata Zone. Interval corresponding to the co-occurrence of M. cristata and Peckichara cancellata. By comparison with the Aix Basin (SE France), where the Upper Cretaceous charophytes zones were first

MAGNETOSTRATIGRAPHYAND BIOSTRATIGRAPHYOF CRETACEOUS TERTIARYCONTINENTALDEPOSITS, AGER BASIN,SPAIN

NRM=8"~xlO'4A/m

100"t2:

45

NIN

Q

~

+++\- - 3 2 30000"0" C Up/East

N/N

~\

Up/East

".

137.60 m

NRM= \

4.a~+o-'A/m\

\

N/N

"-T,<4 5roT

t~"nT~ 20rnT~

~(~15~

30tnT Up/East Fig. 3, Orthogonal projections of thermal (A,B) demagnetization and mixed (C) cleaning (thermal and subsequent alternating field demagnetization) for three typical samples. Open (solid) circles are projections in the horizontal (vertical) plane. See complementary explanations in text.

46

B. G A L B R U N

0

/o.'..+

.

°e

/

\

. ."

I 2,o

. . . .

\ 0• . ""

INCLINATION

" •

1° \

1 4 +°

oO o

o

o

°o

VooO

/ /

y

° °:oOo° iBO

Fig. 4. Stereographicplot of characteristicdirectionsof magnetization for the Fontllonga section. Directions are corrected for dip of strata. Solid (open) circles represent projection of the lower (upper) hemisphere. defined (Grambast, 1971), this interval corresponds to the Lower Rognac Limestone Formation ("Calcaire de la Gare de Rognac") and to the lower part of the Upper Rognac Limestone Formation ("Calcaire de Rognac" s. str.). These formations, on which magnetbstratigraphic data were obtained, are attributed to the Maastrichtian (Westphal and Durand, 1990; Galbrun et al., 1991). --Local Zone 2: Middle Microchara cristata Zone. Interval including the co-occurrence of M. cristata and Septorella ultima. Level 62.60 m, which corresponds to the upper limit of this zone, contains the last Clavatoraceae, represented by S. ultima and S. brachycera. It is important that the last dinosaur eggshell fragments were also found in this bed. The present assemblage, including S. ultima is attributed to the Upper Maastrichtian by reference to the "Marnes d'Auzas" of the Prepyrenees, dated by foraminifera (Massieux et al., 1979) and by direct correlation with palynoflora (Mrdus et al., 1988). --Local Zone 3: Lower Upper Microchara cristata Subzone. Interval between the uppermost occurrence of S. ultima and the lowest occurrence of Peckichara sp. 1. Following the extinctions of P. sertulata, as well of the Clavatoraceae family

ET AL.

and dinosaurs, Local Zone 3 contrasts with the preceeding one by a drastic impoverishment in the number of taxa and of specimens. One of the few species appearing during this interval in the Fontllonga section, Amblyochara sp. A, is represented in the same stratigraphic position, in the latest Maastrichtian "Marnes d'Auzas", in the northern pre-Pyrenees (Lepicard et al., 1985) as well as in the Montgai section, in the Ribagorzana Valley, Tremp Basin (sample CF.FNMGI, Universit6 de Montpellier). --Local Zone 4: Upper Upper Microchara cristata Subzone. Interval including the association of M. cristata and Peckichara sp. 1. The latter species appears as a specific marker of the uppermost Maastrichtian. Thus it also occurs 50 m above the Rognac Limestone in the Aix Basin, France, (sample CF.2911, University of Montpellier), in levels immediately overlying the beds containing the last dinosaur eggshell fragments. Maedleriella aft. michelina, which appears with P. sp. 1 in level 109.70 m is interpreted as an ancestor of the Thanetian species. --Local Zone 5: Peckichara toscarensis Zone. This interval, between the lowest occurrences of Peckichara toscarensis and Dughiella bacillaris, differs from the two preceding ones by a Tertiary aspect of the charophyte assemblage, which is similar to the Danian-Montian ones reported from the southern Pre-Pyrenees (northeast Spain), at the base of the Vallcebre limestone (Feist and Colombo, 1983). Dughiella wanghuangensis, described from the Paleocene of southern China (Huang, 1988), corresponds to the primitive form of Dughiella bacillaris reported from localities of the north Pyrenean Danian-Montian (Massieux et al., 1989). The upper limit of Zone 5 is defined by the lowest occurrence of Dughiella bacillaris. This species extends from the Danian-Montian to the Upper Thanetian. The presence of the genus Platychara and Maedleriella sp. A, never reported from the Thanetian, and the absence of markers of this age at stratigraphic level 175.30 m, indicates that this level is of Danian-Montian age.

Ostracods and molluscs The ostracods and molluscs recovered from the continental sequence of the Fontllonga section are

MAGNETOSTRATIGRAPHY AND BIOSTRATIGRAPHY OF CRETACEOUS TERTIARY CONTINENTAL DEPOSITS, AGER BASIN, SPAIN

47

POLARITY DECLINATION 270

INCLINATION

INO~UAL

270 *gO

-90 ~11~VERSED

Y •

200-

~

1 I

I I

175-

I

1150-

125-

100-

7

7~,-

rSO-

I I

f 2~.

Fig. 5. Characteristic directions of magnetization (declination, inclination) plotted on the stratigraphic column and polarity interpretation (black is normal polarity, white in reversed, partial bars are single sample polarity intervals).

B.GALBRUNETAL.

48

i

26R~J~

CHAROPHYTE

CHAROPHYTESPECIES

AGE

LOCALZONES

m -

Z

.4 ¸

200-

~.b~

Ma

-,= ,.,

.

~

. ~ t ~ .~,--

~ ,, .,: .8 ,,, g o .,= .,= ~ ~

z

PECKICHARA TOSCARENSIS ZONE

O

4 UPPER UPPER MICROCHARA CRISTATA SUBZONE

Z

3 LOWER UPPER MICROCHARA

I 0

CRISTATA SUBZONE Fr

DINOSAUREGGSHEL

2 MIDDLE

°u1

¢°111

MICROCHARA CRISTAIA ZONE

31R

cC

32N

I

26. I I I I

LOWER MICROCHARA CRISTATA

Lfl O.. 0..

ZONE

Fig. 6. Magnetostratigraphy and charophyte biostratigraphy in the Fontllonga section : interpreted magnetic polarity column and correlation with the magnetic polarity time scale of Haq et al. (1987), lithologic column, distribution of charophytes and proposed biozones.

MAGNETOSTRATIGRAPHY AND BIOSTRATIGRAPHY OF CRETACEOUS-TERTIARY CONTINENTAL DEPOSITS, AGER BASIN, SPAIN

very rare and thus provided little biostratigraphic information. A few horizons had ostracod assemblages, which enabled correlation with other sections and areas where the ostracods are more abundant. A typical but poor Late Maastrichtian fauna has been recovered at levels 7.80 m and 32.70 m with the gastropod Pyrgulifera sp. and the ostracod Virgatocypris sp. 2 Babinot. These taxa characterize the last horizons with dinosaurs and Septorella (Charophyta) in SE France (Provence, Languedoc). The same horizons are characterized only by this ostracod in the Northern Pyrenees (Petites Pyr~n6es, Corbi~res). These taxa are associated with a polymorph ostracod assemblage assigned to Frambocythere gr. tumiensis Helmdach, showing both male and female adult specimens. Another poor assemblage of F. gr. tumiensis composed essentially of larval specimens and without any males occurs in level 71.20 m, associated with the gastropod Islamia gr. indecisa (Cossmann). Commonly in the Pyrenees, among the ostracods, the widespread species F. tumiensis persists from the Maastrichtian through the Palaeocene. Two populations are well defined. The earliest one (Maastrichtian age) shows a strong intraspecific polymorphism (3 sub-species described) and an obvious sexual dimorphism as in levels 7.80 m and 32.70 m in Fontllonga section. The following well known assemblage assigned to Frambocythere tumiensis ludi Tambareau, dated to the Late Danian (or "Continental Montian") in Belgium and in the Pyrenees (Tambareau, 1984; Lepicard et al., 1985), differs from the previous one by a change in the outline of the carapaces, less strong polymorphism and the lack of male specimens suggesting a parthenogenetic reproduction. These differences might be related with deterioration of the environment, for example climatic cooling. Throughout the Pyrenees, the specimens of Frambocythere (most often at a larval stage as in level 71.20 m of the Fontllonga section) found in the interval between the two well defined populations are too rare to allow us to place them inside the morphological variability field of one of the various morphotypes, or sub-species, or to evaluate their level in the evolutionary lineage of the genus.

49

Similar observations can be applied to the gastropod Islamia gr. indecisa occurring both in Maastrichtian and in Palaeocene. Thus the observations on the Fontllonga section are in agreement with those published for the northern Pyrenees (Gruas-Cavagnetto et al., 1992). In this area the K/T boundary does not seem to be situated immediately above the "Marnes d'Auzas" Formation where a high spore percentage is noticed in the palynofloras distribution (M6dus et al., 1988) and where the last appearance of various characteristic Late Maastrichtian faunal and floral taxa occurs: dinosaur remains, gastropods such as the genus Bauxia Caziot or Rognacia abbreviata Matheron, large and ornate ostracods Parancandona occitanica Babinot and Tambareau and Virgatocypris aft. sp. 2 Babinot, charophyte genus Septorella (Massieux et al., 1979). Another Cretaceous assemblage has been identified 40 m above the previous association, at the top of "Marnes d'Auzas" Formation, in lagoonal levels announcing the Danian transgression. This Cretaceous assemblage contains Porochara oblonga, Peckichara caudata, Strobilochara sp., Lamprothamnium sp. in the western part of the Petites Pyr6n~es and, in the eastern one, Maedleriella sp. A., Microchara cf. cristata, Peckichara sp. 1 Feist. This last association occurs in Fontllonga level 109.70 m, and thus 47 m above the last dinosaur egg-shells and Septorella assemblages.

Pollen and spores Palynological data have already been published and will not be discussed here (M6dus et al., 1988). Two samples gave useful biostratigraphic results; the first at level 116.40 m contains an Upper Maastrichtian palynoflora, and the second at 142.50 m contains a Danian pollen assemblage (Fig. 6). Iridium

Iridium concentrations were measured by Instrumental Neutron Activation Analysis (INAA) of whole rock samples. Samples were irradiated for a few hours in the 2× 1014 n c m -2 s -1 neutron bean of the Osiris reactor at the Pierre Siie Labora-

50

tory in Saclay (France). Ir was counted with a 7-7 spectrometer detecting the 316-468 keV 7-ray coincidence resulting from the decay of 192Ir. The instrument permits the detection of concentrations as low as 50-100 pg/g. Samples were systematically collected at all discontinuities or major lithological changes which have been identified between levels 110 m and 200 m. The sampled interval is significantly wider than the extent of,the 29R zone because the sampling was carried out before paleomagnetic data obtained. 35 samples were analyzed according to the method described above. No significant concentration higher than 100 pg/g has been found in the section. Discussion

The magnetic polarity sequence of the Upper Cretaceous and Lower Tertiary is now well established. This result was obtained by numerous magnetostratigraphic studies of marine sedimentary sections from Italy (Alvarez et al., 1977; Channell and Medizza, 1981) and from DSDP/ODP cores (Chave, 1984; Petersen et al., 1984; Tauxe et al., 1984; Galbrun, 1992). In particular it is well known that the K/T boundary occurs within chron 29R but its exact position is not yet well established, because of non-uniform sedimentation during the chron in the studied sections. However, some standard magnetic polarity sequences correlated with the marine biozonations were proposed (Haq et al., 1987) and these are used here as reference time scales. Considering the biostratigraphic data and the relative thickness of magnetic polarity zones established on the Fontllonga section, some correlations can be proposed with the standard magnetic sequence (Fig. 6). The most easily recognizable "fingerprint" in the Fontllonga polarity sequence is the succession ofchrons 29-31. The main normal polarity sequence from levels 52-120 m corresponds to chron 31N and chron 30. The short and well-defined reversed zone observed between levels 102 m and 106 m must therefore correspond to chron 30R. The boundary between chrons 31N and 31R is impossible to define because of a gap in the outcrop. However, chron 3IN is very thick in comparison with its duration in the standard

B. GALBRUN ET AL.

sequence, due to the high sedimentation rate in this part of the section which is represented by silty claystones and some sandstone beds. The boundary between chrons 31R and 32 is located at level 33 m. The other outcrop gap at the base of the section makes uncertain the correlation with the reference sequence; however, the reversed polarity zone occuring in the limestones at the base of the section can reasonably be attributed to the upper subchron 32R. Chron 29R, in which the K/T boundary occurs, is unambiguously determined. It extends from levels 120 m to 145 m and thus should be 25 m thick, but unfortunately its exact thickness cannot be measured because of a small fault in silty claystones between two sandstone sequences. It is also impossible to place the position of the boundary between chrons 29 and 28 which occurs in a gap of outcrop. The proposed correlations agree well with biostratigraphic data. The last representatives of the Clavatoraceae charophyte family and of dinosaur eggshells occur at level 62.60 m, which is part of the thick normal polarity zone referred to chron 31N. The charophyte Peckichara sp. l, which seems to signify the latest Maastrichtian, occurs at level 109.70 m which is of normal polarity and was referred to chron 30N. A Maastrichtian palynoflora was found at level 116.40 m, and a Danian at level 142.50 m. These two associations occur in a normal polarity level corresponding to chron 30N, and at the top of a reversed polarity zone refered to chron 29R respectively. The first well known Palaeocene charophyte species occur at level 146.80 m of normal polarity which is referred to chron 29N of Danian age. The upper part of the magnetic polarity sequence established on the Fontllonga section is very discontinuous due to a gap in the outcrop and the weak NRM intensity in the lacustrine limestone of the Vallcebre Formation. Therefore, chron 27 was not recognized, occurring probably in the gap below the Vallcebre Formation limestone. Palaeodirections were computed from four samples from this formation; three of them are of reversed polarity, and might correspond to the long chron 26R mainly of Thanetian age. In conclusion the palaeomagnetic study of the

MAGNETOSTRATIGRAPHY AND BIOSTRATIGRAPHY OF CRETACEOUS-TERTIARY CONTINENTAL DEPOSITS, AGER BASIN. SPAIN

continental deposits of Fontllonga section provides a clear magnetostratigraphic zonation. This result coupled with biostratigraphic data (charophytes, ostracods, palynomorphs) allows correlations with the standard magnetic polarity sequence. The studied section extends from chrons 32R to 26R, or from top of Lower Maastrichtian to Lower Thanetian. The most recognizable sequence is the succession of chrons 28R to 31N. The chron 29R, in which the K / T boundary occurs, is unambiguously recognizable: such a good result on a continental sequence has only been obtained in N o r t h America. On the Arc Basin (Southeast France) the position of chron 29R is not so well defined (Westphal and Durand, 1990; Galbrun, 1991). The lack of any anomalously high Ir concentration in the section of Fontllonga is intriguing but not surprising. In any case, it cannot be considered as an argument against the worldwide character of the K / T event. Considerations about the sedimentation conditions and the quality of the outcrop can quite easily account for our present impossibility to precisely define the K / T boundary. First the 29R zone is entirely contained in a sequence of fluvial deposits which are known to be essentially reworked and discontinuous. Therefore, the probability to find the non-reworked horizon containing the K / T boundary markers (Ir, Ni-rich spinels and shocked minerals) in the Fontllonga section is weak. We cannot hope to get the cosmic fallout layer at the first attempt: multiple sections must be explored. Second, the 29R zone at Fontllonga is, unfortunately, disturbed by a small fault which prevents a continuous exploration of the uncertainty zone. This local event does not mask the merits of K / T sections in the Ager Basin. On the contrary, the quality of paleontological and paleomagnetic data obtained is the best argument to extend the search for the Ir-rich layer to other sections in the Fontllonga area. It is interesting to point out that the local dinosaur and Clavatoraceae family extinctions at level 62.60 m would have occurred about two millions years before the beginning of the Tertiary. These extinctions would thus be independent of the postulated K / T boundary event. They could perhaps be due to the climatic deterioration resulting from successive coolings during the Campanian

51

and Maastrichtian. This difference in stratigraphic positions between the last dinosaur eggshells and the probable K / T boundary was also suggested in Southern France.

Acknowledgements This paper is the contribution no. 527 to Programme C N R S - I N S U - - " D y n a m i q u e et Bilan de la Terre (Changement de l'environnement global dans le pass6)". Many thanks to J. Villatte for her investigation of molluscs from Fontllonga, and for her helpful comments on the stratigraphy. We are grateful to D.H. Tarling and M. Westphal for useful comments.

References Alvarez, W., Alvarez, L.W., Asaro, F. and Michel, H.V., 1982. Current status of the impact theory for the terminal Cretaceous extinctions. Geol. Soc. Am. Spec. Pap., 190: 305-315. Alvarez, W., Arthur, M.A., Fisher, A.G., Lowrie, W., Napoleone, G., Premoli Silva, I. and Roggenthen, W.M., 1977. Upper Cretaceous-Paleocene magnetic stratigraphy at Gubbio, Italy. V. Type section for the Late Cretaceous-Paleocene geomagneticreversal time scale. Geol. Soc. Am. Bull., 88: 367-389. Babinot, J.F. and Durand, J.P., 1980. Valdonnien, Fuv61ien, B6gudien, Rognacien, Vitrollien. In: C. Cavelier and L Roger (Editors), Les ~tages fran~ais et leurs stratotypes. M~m. Bur. Rech. G6ol. Min., 109: 171-192. Babinot, J.F., Freytet, P. et al., 1983. Le S6nonien sup&ieur continental de la France m~ridionale et de l'Espagne septentrionale: 6tat des connaissances biostratigraphiques. G6ol. M6dit., 10: 245-268. Bohor, B.F., Foord, E.E., Modreski, P.J. and Triplehorn, D.M., 1984. Mineralogical evidence for an impact event at the Cretaceous-Tertiary boundary. Science, 224: 867-869. Buffetaut, E. and Le Loeuff, J., 1991. Late Cretaceous dinosaur faunas of Europe: some correlation problems. Cretaceous Res., 12: 159-176. Channell, J.E.T. and Medizza, F., 1981. Upper Cretaceous and Paleogene magnetic stratigraphy and biostratigraphy from the Venetian (Southern Alps). Earth Planet. Sci. Lett., 55: 419-432. Chave, A.D., 1984. Lower Paleocene-Upper Cretaceous magnetostratigraphy, Sites 525, 527, 528, and 529, Deep Sea Drilling Project Leg 74. In: T.C. Moore Jr., P.D. Rabinowitz et al,, Init. Rep. DSDP, 74: 525-531. Feist, M. and Colombo, F., 1983. La limite Cr~tac6-Tertiaire dans le nord-est de l'Espagne du point de vue des charophytes. G6ol. M6dit., 10: 303-325. Feist, M. and Freytet, P., 1983. Cons6quences stratigraphiques

52

de la r6partition des charophytes dans le Campanien et le Maastrichtien du Languedoc. G6ol. M6dit., 10: 291-301. Galbrun, B., 1992. Magnetostratigraphy of Upper Cretaceous and Lower Tertiary sediments, Sites 761 and 762, Exmouth Plateau, northwest Australia. In: U. von Rad, B.U. Haq et al., Proc. ODP, Sci. Res., 122: 699-716. Galbrun, B., Gabilly, J. and Rasplus, L., 1988. Magnetostratigraphy of the Toarcian stratotype, Thouars and Airvault sections (Deux-S6vres, France). Earth Planet. Sci. Lett., 87: 453-462. Galbrun, B., Rasplus, L. and Durand, J.P., 1991. La limite Cr&ac6-Tertiaire dans le domaine provencal: 6tude magn6tostratigraphique du passage Rognacien-Vitrollien ~i l'Ouest du synclinal de l'Arc. C. R. Acad. Sci., Paris, 312: 1467-1473. Grambast, L., 1971. Remarques phylog6n6tiques et biochronologiques sur les Septorella du Cr&ac6 terminal de Provence et les Charophytes associ6es. Pal6obiol. Cont., 2: 1-38. Grambast, L., 1974. Les derni6res Clavatorackes (charophytes) et la zonation du Maestrichtien continental, In: 2nd RAST, Nancy, R6sum6. G6ol. Soc. Fr., p. 196. Gruas-Cavagnetto, C., Tambareau, Y. and Villatte, J., 1992. D&ouverte de pollens, Dinoflagell6s et Foraminif+res dans le Danien des Petites Pyr6n6es: implications sur la position de la limite Cr&ac6/Tertiaire. G~obios M6m. Sp6c., 14: 19-28. Hansen, H.J., Gwozdz, R. and Rasmussen, K.L., 1989. The continental Cretaceous-Tertiary boundary in the Aix-enProvence region, South France. A preliminary paleomagnetic study. Cab. R6s. G6ol. Haute-Provence, 1: 43-50. Haq, B.U., Hardenbol, J. and Vail, P.R., 1987. The chronology of fluctuating sea level since the Triassic. Science, 235: 1156-1167. Huang, R.J., 1988. Charophytes of Nanxiong Basin, Guangdong and its Cretaceous-Tertiary boundary. Acta Palaeontoe Sin., 27 (4): 457-474. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. R. Astron. Soc., 62: 699-718. Krumsiek, K. and Hahn, G.G., 1989. Magnetostratigraphy near the Cretaceous-Tertiary boundary at Aix-en-Provence (southern France). Cah. R6s. G~ol. Haute-Provence, I: 38-42. Lepicard, B., Bilotte, M., Massieux, M., Tambareau, Y. and ViUatte, J., 1985. Faunes et flores au passage Cr&ac& Tertiaire en faci6s continental dans les Petites Pyr6n6es (zones sous-pyr6n6enne). G6obios, 18: 787-800. Lowrie, W. and Heller, F., 1982. Magnetic properties of marine limestones. Rev. Geophys. Space Phys., 20: 171-192. Masriera, A. and Ullastre, J., 1983. Essai de synth6se stratigraphique des couches continentales de la fin du Cr6tac6 des

B. GALBRUN ET AL.

Pyr6n6es Catalanes (NE de l'Espagne). G6ol. M6dit., 10: 283-290. Massieux, M., Tambareau, Y. and Villatte, J., 1979. D&ouverte de Septorella brachycera et de Septorella ultima Grambast (Charophytes, Clavatorac6es) dans le Maastrichtien sup&ieur des Petites Pyr6n6es, cons6quences stratigraphiques. G6obios, 12: 899-905. Massieux, M., Tambareau, Y. and Villatte, J., 1989. Nouveaux gisements ~ Charophytes du Dano-Montien Nord-Pyr6n6en. Rev. Micropal6ontol., 32: 140-150. M6dus, J., Feist, M., Rocchia, R., Batten, D.J., Boclet, D., Colombo, F., Tambareau, Y. and Villatte, J., 1988. Prospects for recognition of the palynological Cretaceous-Tertiary boundary and an iridium anomaly in nonmarine facies of the eastern Spanish Pyrenees: a preliminary report. Newsl. Stratigr., 18 (3): 123-138. Mey, P.W.H., Nagtegaal, P.J.C., Roberti, K.J. and Hartevelt, J.J.A., 1968. Lithostratigraphic subdivision of post-hercynian deposits in the south central Pyrenees, Spain. Leidse Geol. Meded., 41: 221-228. Orth, C.J., Gilmore, J.S., Knight, J.D., Pillmore, C.L., Tschudy, R.H. and Fassett, J.E., 1981. An iridium abundance anomaly at the palynological Cretaceous-Tertiary boundary in Northern New-Mexico. Science, 214: 1341-1343. Petersen, N.P., Heller F. and Lowrie, W., 1984. Magnetostratigraphy of the Cretaceous/Tertiary geological boundary. In: K.J. Hsii, J.L. LaBrecque et al., Init. Rept. DSDP, 73: 657-661. Riveline, J., 1986. Les Charophytes du Pal6og~ne et du Mioc6ne inf6rieur d'Europe Occidentale. Cah. Pal6ontol., 227 pp. Robin, E., Boclet, D., Bont6, P., Froget, L., J6hanno, C. and Rocchia, R., 1991. The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at E1 Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122). Earth Planet. Sci. Lett., 108: 181-190. Sole Sugranes, L., 1970. Estudio geologico del Pirineo entre los rios Segre y Llobregat. Thesis. Univ. Barcelona. Tambareau, Y., 1984. Les Ostracodes du "Montien continental" de Hainin, Hainault, Belgique. Rev. Micropal6ontol., 27: 144-156. Tauxe, L~, Tucker, P., Petersen, N.P. and LaBrecque, J.L., 1984. Magnetostratigraphy of Leg 73 sediments. In: K.J. Hsii, J.L. LaBrecque et al., Init. Rept. DSDP, 73: 609-622. Westphal, M. and Durand, J.P., 1990. Magn6tostratigraphie des s6ries continentales fluvio-lacustres du Cr6tac6 sup6rieur dans le synclinal de l'Arc (r6gion d'Aix-en-Provence, France). Bull. Soc. G6ol. Fr., 6, 4: 609-620. Wolbach, W.S., Lewis, R.S. and Anders, E., 1985. Cretaceous extinctions: evidence for wildfires and scarch for meteoritic material. Science, 230: 167-170.