The Campanian-Maastrichtian Boundary G. S. Odin (editor) 2001 Elsevier Science B.V.

CHAPTER Fl

The Campanian-Maastrichtian boundary: characterisation at Tercis (Landes, SW France)

G. S. Odin

Sommaire

Une description detaillee des niveaux entourant la limite d'Etage est proposee dans ce chapitre en tenant compte de quatre endroits distincts du site geologique de Tercis. Trois limites de bancs sont nettes aux niveaux 110, 114 et 120. La limite d'Etage se situe dans un intervalle de depot continu pres de la cote 115.

Dans la succession principale, quelques niveaux presentent des criteres de condensation discrets. Le plus net est le banc situe entre les cotes 66 et 68, parfois interprete comme une coulee de debris. Aucune lacune significative n'apparait dans I'enre-gistrement des suites de fossiles en correspondance avec les indications sedimentologiques. Un taux moyen de depot de 25 m/Ma a ete calcule sur r ensemble de la succession; les indications locales et les comparaisons avec d'autres successions concourent a confirmer ce taux moyen.

L'interpretation sequentielle de la succession est delicate sur le terrain; elle a ete envisagee d'apres les teneurs et la composition des glauconies ainsi que d'apres les indications fournies par les teneurs en manganese. La succession de Tercis a livre, en outre, un signal magnetique interprete comme le passage de la magnetozone 33 a la magnetozone 32; d'originales indications environnementales sont aussi fournies par la matiere organique. Un eta-lonnage geochronologique sera envisage quand des moyens auront ete trouves pour mettre en oeuvre cette etude sur le geochronometre decouvert.

Le point fort de la succession de Tercis est la variete des signaux biostratigraphiques. Afin de

r exploiter au mieux, une strategic originale a ete mise en oeuvre. On a d'abord tente de caracteriser tous les signaux donnes par 19 groupes de macro-fossiles et 10 groupes de microfossiles. Cette diversite permet d'enrichir la connaissance et d'esperer decouvrir des criteres de correlation nouveaux. On a surtout eu la volonte de qualifier les bio-horizons reperes avec une incertitude effec­tive, estimee d'apres la reproductibilite des etudes realisees independamment par plusieurs experts et en tenant compte de la frequence des fossiles (plus ils sont rares moins leur apparition et leur dispari-tion sont connues avec precision). Les resultats ont conduit a demythifier, au moins pour le geologue non specialiste, certains evenements cle et ont permis d'attribuer a chaque critere biostratigra-phique une valeur pratique essentielle pour I'utilisation d'une section de reference.

Parmi les conclusions de principe on a observe que les macrofossiles en general ne permettent pas des determinations sur le site (contrairement a ce qui est avance usuellement) et que 1'etude au laboratoire, parfois longue, est necessaire; que les microfossiles peuvent etre aussi rares que les macrofossiles; que meme pour des taxons cle cites avec Constance dans la litterature, 1'accord n'est pas fait entre les experts sur les concepts; que la precision des indications publiees dans la litterature est optimiste sur les microfossiles et bien incertaine sur les macrofossiles.

Ces reserves etant faites, meme a Tercis ou les fossiles n'ont pas un etat de preservation ideal, de nombreux bio-horizons ont ete reperes et c'est leur combinaison qui livre des solutions. Des relations

786

temporelles directes entre des taxons n'ayant jamais ete trouves dans une meme succession continue ont ete etablies. C'est vrai pour les ammonites entre elles, pour les ammonites com-parees aux inocerames, pour les macrofossiles compares aux microfossiles, pour des fossiles du Domaine Boreal et du Domaine Tethysien; pour des fossiles d'Europe et d'Amerique du Nord. Pour toutes ces raisons, la succession de Tercis est unique et bien adaptee a son role de section de reference.

L'evolution biologique semble s'accelerer en trois endroits de la succession, autour des cotes 67 puis 97,5 et enfin 115,5. Ces deux dernieres accelerations rassemblent une douzaine d'evene-ments. C'est dans la troisieme que se situe le critere guide choisi par la communaute des stratigraphes (apparition de 1'ammonite P. neubergicus) pour approcher la transition entre les Etages Campanien et Maastrichtien. Le critere d'apparition de ce macrofossile parait, toutefois, peu satisfaisant pour fixer precisement le point stratotypique lui-meme et le correler avec d'autres bassins de depot (rarete, difficulte d'identification, potentiel de diachro-nisme).

Les macrofossiles datent les couches situees au Sud de la Grande Carriere Modeme du Ceno-manien pour I'Unite des argiles claires, du Turo-nien et du Coniacien pour I'Unite Lacave. La grande carriere exploite des niveaux du Campanien superieur au Maastrichtien inferieur. Les affleure-ments entourant le Mur de Bedat laissent supposer la presence de Maastrichtien superieur et de Paleogene inferieur sans interruption nette au droit de la carriere.

Le site geologique de Tercis repond favora-blement a la plupart des criteres recommandes pour abriter un Point Stratotypique Global. On admet que la designation d'une section auxiliaire avec un bon enregistrement magnetostratigraphique (sec­tion italienne du Bottaccione) serait profitable, d'autant qu'il semble aise d'etablir des liens biostratigraphiques et magnetostratigraphiques. Une seconde section auxiliaire (section de Krons-moor en Allemagne) permettrait de relier la "chronologic" des belemnites a laquelle beaucoup d'auteurs sont habitues a la nouvelle definition de la limite d'Etage etablie dans les Landes.

Trois fossiles guide majeurs montrent leur emer­gence et leur extinction dans la succession de Tercis. Le foraminifere Radotruncana calcarata a une zone d'extension totale (cotes 39,0 a 62,5) de duree proche de 1 Ma; son sommet etait autrefois le repere pour situer la limite Campanien-Maas-trichtien dans la Tethys; c'est aujourd'hui du Campanien situe a plus de 2 Ma sous la limite superieure de I'Etage. Le fossile cle est rare mais aise a identifier sans ambigui'te dans un large domaine circum-terrestre. La zone d'extension totale du nannofossile calcaire Quadrum trifidum se situe entre les cotes 52,5 et 134,2 (±2 a 3 metres). La duree de cette zone pent etre estimee entre 3,5 et 4,0 Ma avec un peu plus des 2/3 dans le Campanien et le reste dans le Maastrichtien. Le taxon est raisonnablement aise a reconnaitre et sa distribu­tion paleogeographique est tres large. L'ammonite Nostoceras hyatti a une zone d'extension totale plus difficile a localiser precisement (rarete et identification delicate). Sa duree est de I'ordre de 2 Ma, avec une fin qui coincide presque avec la limite d'Etage. Ce taxon est aussi tres repandu de r Amerique du Nord a la Pologne et du Domaine Boreal a Madagascar.

Deux taxons guide majeurs montrent une lignee evolutive. L'analyse statistique indique, pour le genre de foraminifere benthique Bolivinoides, une population dominante avec 5 pustules sur la demiere loge dans le Maastrichtien et 4 dans le Campanien superieur. Le passage continu se situe dans I'intervalle 107 ±7 a Tercis et, comme dans le Bassin de Mons, sous la transition entre les deux Etages. La seconde lignee est celle du couple Pachydiscus perfidus-P neubergicus. La seconde espece etant le guide favori, I'intervalle de co­existence des deux formes (voire des formes intermediaires) localise I'endroit pres duquel doit se situer la limite Campanien-Maastrichtien.

Enfin, Hoploscaphites pumilus est 1'ammonite symbole de Tercis, la plus abondante dans notre collection. Son interet est paleontologique puis-qu'elle n'a encore ete recoltee que dans peu de regions. Sa distribution encadrant la limite entre les niveaux 94,2 et 123 (voire 1 Ma plus jeune) pourra toutefois servir de lien entre les series d'Amerique du Nord, d'Europe et d'Afrique du Nord.

787

1. Introduction

The tasks of the Maastrichtian Working Group estabhshed in 1991 were to describe the succes­sion, to document the succession with as much stratigraphical tools as possible in order to identify potential correlation features, to compare these features with those known from other sections in order to document the efficiency of these correla­tions and to elaborate a formal Global Standard-stratotype Section and Point (GSSP) for the Campanian-Maastrichtian boundary. The results of the first task are summarised in this chapter.

2. Lithostratigraphical and physico-chemical information

2.1. Lithostratigraphical information

Understanding a lithological succession begins with a precise description; this is not fully an objective exercise, and it is followed by a sub­jective, logically funded, interpretation. For this reason, the precise description of the stage bound­ary interval is done again at the end of the study. The best observations were obtained from floor III in the quarry fronts; they were supplemented on floor IV, floor V, and the E section, which show lithological details differently from a place to another. For example, the bed limits can be interpreted as corresponding to a more or less important depositional feature depending on the observed outcrop; the bed limit at about level 118.2 is almost not visible on floor IV while it is represented by a wall resulting from natural alteration on floor V The main lithological cuts near the stage boundary interval are located at levels 110, 114, and 120 (figure 1). The readability of the cut at level 114 is enhanced by the presence of a slightly clayish bed which has been eroded from the southern (older) side. But the most probable reason why level E 0 (~P 114) forms a wall to the North of the entrance of the quarry is that it had been taken as a limit for quarrying. Most criteria useful for elaboration of the conventional boundary (table 2 below) fall between levels 114 and 117 where no clear cut is present. Even flint nodules are not arranged in course between those levels as they usually are elsewhere in the section.

This is taken as a criterion of continuous deposition in the interval where the stage boundary transition is proposed.

A major goal of the lithological study is to document the continuous character of the deposi­tion and the continuity of the geological record. In

The stage boundary interval on floor P III at Tercis

level

120 _

118 -

116 _

114 -

112 -

110 -

108 -

C::>foCr) Q^ ot:>%o

--r_^_.^_-r

0 Q ' Q fl " 1 0

Q<^<1 O 0 (bo b (

MM \>\t\i \i\

-

° - . " .

0 0 0 •a

0 0 0 0 o

— — _ _ J D '0

0 o Q o o

»<'<? ^ - o " « o - 0

u ^ •^ D <3

0 O 0 O 0 0

z < 1= X O OC H CO < <

z < z < Q.

< o

abundant flint nodules, zoned

^^ slightly clayish

-<- clear cut, ochreous clay below (P III, PIV)

flint nodules, zoned, some large

- ^ more clayish

M- small pycnodonts

^ wet level (P IV and E)

scattered small flint nodules sometimes elongated

, ^ clear cut, open on P III (wall on E at E 0)

•^- some small flint nodules

< ^ some small flint nodules

M- large flint nodules

- ^ clear cut, ochre some small flint nodules

M- clear cut, (PIV), § some clay below (E -5) ^

6 CO

O

Fig. 1. Detail of tlie boundary interval in floor P III; informa­tion from other outcrops has been added for better characterisation of the succession.

788

the main section (level 0 to level 160), the sedimentological information allows to identify a few beds where the presence of some glaucony and phosphate might have documented a break in the record. However, the mineralogical analysis (chap. Bla) has shown that the most obviously "con­densed" bed (the inoceramid-rich glauconitic phosphatic bed at level 66.5-67.3) contains neither evolved glauconitic minerals nor a large content of green grains. Phosphatised clasts, in turn, are rare and incompletely transformed (plate I). Therefore, no geologically important gap could be identified in the main section. The suspectedly condensed beds are all located in the upper Campanian portion of the series.

The conclusion reached from the sedimento­logical approach is confirmed by biostratigraphic considerations. The latter indicate that no break in the record is documented by the biological evolu­tion. This is attested by the continuous records of several fossil groups: the dinocysts, the calcareous nannofossils, and the planktonic foraminifera.

The most important turnover of the calcareous nannofossil record (an interval where FOs and LOs are "condensed") has been identified by Gardin & Monechi (chap. C3c) between levels 84.0 and 86.0 or between 86 and 90 according to Melinte & Odin (chap. C3d). These authors quote about 10 bio-horizons of local significance at a place where no sedimentological break has been identified. There are possibly two contemporaneous dinocyst signals (reentry of Cerodinium diebelii at level 86.9 observed by E. Antonescu; LO of Xenascus cer-atioides observed by P. Schi0ler above 86.9 also. In terms of planktonic foraminifera there is a single bio-horizon (LO of Globotruncana obliqua observed by J. Ion) which falls in this interval. The calcareous nannofossil fossil break, if any, curi­ously corresponds to the disappearance of measurable magnetic signals in the rock. The two features: turnover of the calcareous nannofossils, and magnetic properties of the sediment could result from a minute environmental change, the lithological nature of which has not yet been identified; this change could have included a decrease in some iron-bearing mineral which previously carried the palaeomagnetic signature in the sediment.

Planktonic foraminiferal information essentially evidences arrivals of fauna at five levels (at 11.4 and 13.7; at 39.0 to 41.1; at 76.0 to 77.2; at 97.2; and at 117.8) according to Ion & Odin (chap. C5c). The first, fourth and fifth arrivals correspond to environmental or sample improvements; the second and third ones are not abrupt. The youngest arrival is the most important for our purpose with the FOs of two key taxa (Contusotruncana contusa and Globotruncanita stuarti but the latter is also quoted significantly below by Arz & Molina (down to level 95.7, chap. C5b) and is known to occur locally down to the calcarata zone in the Tethyan Realm. According to J. Ion, two LOs occur nearby: that of Globotruncanitella tercensis immediately below and that of Globotruncana rosetta-Globotruncanita insignis immediately above. The distribution by Arz & Molina essentially confirms the third arrival (between levels 73.9 and 79.4 with FOs of several species of the genus Rugoglobigerina) and the fifth arrival (at level 116.8 with essentially the FOs of Rugoglobigerina scotti (an early form) and Gubler-ina acuta). It is reasonable to conclude that there is no abnormal break in the distribution of the planktonic foraminifera which detects no deposi-tional gap long enough to disturb the continuous evolutionary trend of this fossil group.

A comparison of the succession studied at Tercis with the one deposited in the Apennines (figure 2 in chap. E5c) allowed to propose estimates of the duration of the condensed beds. The Apennines succession represents deposition in a quiet oceanic basin supposed to accumulate deposits constantly and regularly; it comprises a series of common chronologically significant bio- and magneto-hori­zons allowing correlation. The result of the comparison is that the suspectedly most condensed portions around levels 45 and 67 would correspond to a depositional rate two to four times slower than in the rest of the succession (figure 3 in chap. Blc).

The mean rate of deposition following compac­tion has been deduced from the sedimentary thickness, the information on possible gaps of deposition, rhythmic features, stage duration, and comparison with other sections. In this volume, the mean rate of deposition is concluded to be 25 m/Ma in the major portion of the succession (chap. Blc).

789

This estimate is not more precise than a few tens percent.

A twice as quick mean rate is theoretically possible and would correspond to the fact that the 1 m rhythm observed in the field would result from the influence of the 20 ka precession period of the earth orbit. Consistency with the bio- and magneto-stratigraphically constrained intervals would be achieved postulating two conditions at least: i- that deposits are lacking, which would represent 3 out of the 6 Ma of deposition recorded between levels 30 and 160, and ii- that at least three condensed intervals are present at three different places in the succession: between levels 40 and 60, between levels 65 and 80, and between levels 90 and 114. If this is accepted, at least one condensed interval would represent significantly more than 1 Ma deposition; such a phenomenon is not evidenced by any sedimentological or biostratigraphical feature and the corresponding hypothesis thus appears unrealistic.

The concept of mean depositional rate must be carefully treated; the estimated mean rate suggests that every metre of sediment roughly represents the record of 40 ka of deposition (or 30 or 50), especially where rhythmic features are obvious. Interpreting the deposition of 1 cm of limestone as representing 400 years would be another question; the particular cm of succession might as well represent an instantaneous deposition or a break of 10 ka. In addition, the problem of biological/ sedimentological reworking of the sediment in situ may locally complicate the application of this "mean rate". However, there are few indications of such disturbances at Tercis; some worm burrows have been observed but they are generally parallel with the bedding plane, and no decimetre vertical disturbance is known.

The eustatic sea-level changes have not been recorded by obvious lithological features in the succession at Tercis and the sequence interpretation is extremely difficult in the field. The single skilful field-approach known to us (a common field trip with Jan Hardenbol) has been little demonstrative. In absence of obvious feature in the succession, the applied procedure was to consider the published "chart" and to try to fit it to the succession via the biostratigraphic information. The experimental

interpretation of Odin & Amorosi (chap. Blc) uses the discrete presence of various contents and qualities of glaucony. The resulting scheme includes four, possibly five systems tracts which are valid for the Aturian Basin only and would need to be confirmed in other portions of this same basin. The chemostratigraphical approach (evolution of the Mn content in the limy fraction of the sediment) leads to poorly identified changes which probably stand for sea-level fluctuations approximately con­sistent with the proposal derived from glaucony.

2.2. Physico-chemical information

The physico-chemical tools were applied to the succession at Tercis with a contrasting success which is not fully dependent on the quality of the succession. Several series of samples from Tercis were analysed for trace elements. One may suspect that the information obtained from the many analyses undertaken is not convincing enough about the chronological efficiency of that tool since expert interpretation of the data came out very difficult to obtain.

Stable isotope analyses of the sediments of the succession have also been undertaken and mean­ingful results obtained but no report could be prepared for this volume.

Strontium isotopes were analysed on biologic phosphate. Unfortunately, the strontium isotopic composition of the analysed fish scales does not record the isotopic composition of the contempora­neous sea-water and, therefore, the Sr isotope stratigraphical tool is not efficient on this material in the succession at Tercis.

In contrast, the analysis of organic matter proved informative (Baudin et al, chap. B2c). The few analysed specimens represent continental input, the origin of which (forest fire-burnt plant remains) has been clarified.

In summary, the geochemical characterisation of the succession at Tercis failed to provide correlative information both because the tectonical history of the deposits has reset the geochemical equilibria (Sr isotopes) and because the approach needs to be applied with more attention than done up to now.

The magnetostratigraphic approach has not been applied without problem. The information obtained

790

comprises the presence of a portion of a long normal episode with no limit at the bottom and the transition to a reverse episode to the top of the former episode. Below and above, the measured magnetic intensity was too low (lower than 10'^ A/ m) for a meaningful record to be identified. This information obtained several years ago in the quarried portion of the geological site could only be correctly interpreted in the light of the biostrati-graphical data gathered in this volume (Lewy & Odin, chap. B2d). More recent measurements undertaken on samples from the B section indicate that this section contains sediments which could document the palaeomagnetic record of the upper portion of the Maastrichtian stage.

Taking the above-quoted reversal between levels 80 and 81 as an anchor and combining depositional mean rates and diagnostic bio-horizons allowed to correlate the quarried portions of the succession at Tercis with the contemporaneous record in the Apennines. The quarried portion of the geological site at Tercis would comprise the whole magneto-zone 32 and portions of magnetozone 33N below and magnetozone 31R above. An uncertainty of ± 0.2 to 0.3 Ma can be proposed for the correlation (chap. E5c). This uncertainty might be seen as too large; however, let us recall that the Campanian-Maastrichtian boundary was still recently located at levels now known to differ by more than 2 Ma depending on the bio-horizon selected as the key marker.

3. Biostratigraphy

3.1. Approach

An original methodological approach was set up. Two factors had to be considered with particular caution: i- the possibiHty of bias due to the investigation and ii- the possibility of bias due to the record, both leading to uncertainties which had to be documented experimentally. An estimate of the uncertainty about the investigation had to be undertaken by comparison of results obtained by several experts for the most important fossil groups; the reproducibility of the results is the key for that estimate. Locally, the estimate of the

uncertainty about the record (influence of local factors) had to be approached by comparison of results obtained on several fossil groups; the consistency between the different fossil records is the key for that estimate of uncertainty. The knowledge of the succession at Tercis and the definition of the stage boundary which is concluded from that knowledge will both have to take into consideration the actual constraints provided by the diversity in the results, and benefit by this diver­sity.

This theoretical consideration is emphasised here because recendy accepted reference sections are essentially characterised with a single fossil group. Commonly, the key fossil group is selected before the future reference section is selected, which eliminates potentially good sections. However, the major difficulty in the definition of a stage boundary using the Global Standard-stratotype Section and Point procedure is to find a good section and not to select a key marker. A good section is primarily the one with diversified strati-graphical tools and continuous deposition. In other words, the field should be considered a major criterion for selection of the key correlation tool. However, the reverse is in use and the resulting slow progress in the definition of many stage boundaries is repeatedly pointed out by the staffs of the Commission on Stratigraphy and of the Inter­national Union of Geological Sciences.

Finally, the question of uncertainty is underesti­mated; the only uncertainty taken into account is usually the sampling interval, which commonly leads to locate bio-horizons with an unjustified precision. The causes of uncertainty have been discussed in different chapters. Let us recall that all studies are equally weighted because the pragmatic studies can be quick, others are patient and obstinate, but all aim at concluding to a chronos-tratigraphical age; the definition of that age is our very subject. For microfossils collected with a precision of ±10 cm the resulting biostrati-graphical information has sometimes an uncertainty of ± several metres; this apparently poorer precision is more realistic than the precision given in individual reports.

The question is partly different for the record obtained from macrofossils. When different opin-

791

ions are given for the interpretation of a series of fossils, the uncertainty for the corresponding bio-horizon can be considered as the interval between the youngest and the oldest identification of the taxon. When a single study is available, the scarceness of those fossils should lead to give a definition of bio-horizons in the form "FO at or lower than level X" and "LO at or higher than level Y". The information in paragraph 3.3 below is to be considered in the light of these principles.

3.2. Diversity of the fossils

Nineteen macrofossil groups have been distin­guished in the taxa collected from the geological site at Tercis. Their presence and abundance is presented in chapter Bid. An account on the diversity of the taxa is proposed in table 1 which gives a comparison with the fauna at Riigen, where a rich fauna and flora of the Boreal Realm has been studied for 150 years. The Riigen section is

Plate I. 1. Pachydiscus sp., the largest invertebrate fossils from Tercis (top Hontarede Unit). 2. Partly phosphatised fossils from the inoceramid-rich bed (II 66.4), this unique piece with three Baculites remains (scale 2/3) is diagnostic of a condensed bed; phosphatisation is less intense in the living chamber than in the more densely coloured phragmocone; the mineralisation is incomplete everywhere suggesting a moderate condensation. 3. Three views (scale 1/1) of the recently collected brachiopod Rhynchonella baugasii from DSO -19.

792

restricted to the lower Maastrichtian and the depth of deposition similar to the Tercis one (150-250 m, personal communication: Reich, 2000; Reich & Frenzel, 2000).

The major difference in the macrofossil content is the abundance of bryozoans at Riigen. Bryozoans are present at Tercis but they have not yet been studied. Bryozoans apart, the Tercis succession still appears half as much diversified in macrofossils than the succession at Riigen. The macrofossil groups richer in the island of Riigen than at Tercis comprise sponges, macro-arthropods, and ichno-fossils. However, sponges are diversified at Tercis (chap. Bid) but their study remains to be under­taken. An enrichment of the Tercis fauna could also be obtained from a systematic collection and study of gasteropods, bivalves, and nautiloids. The ammonite fauna is well-documented at Tercis where the largest specimens are heavy pachydiscids (plate I) which can still be found today.

Ten fossil groups have been distinguished among the microfossils collected in the geological site at Tercis. Their presence and abundance (chap. Bid) is also documented by diversity of the taxa (table 1). Comparison with the Boreal Rugen fauna reviewed by Reich & Frenzel (1999; 2000) shows more siliceous microfossils, benthic foraminifera, ostracod and holothurian remains at Riigen but more planktonic foraminifera, calcareous nanno-fossils, dinoflagellates, and pollen of angiosperms at Tercis. As for siliceous fossils, however, rare dissolved and fragmented remains of diatoms are probably present in the limy phase of the sediment (personal communication, L. Jacobsen, VI-2000). According to B. Andreu (personal communication, 11-1999) the ostracod faunal list of Tercis could also be supplemented. Ossicles of echinoderms are present in the sediments at Tercis; most of them are still to be identified and the list of ophiuroids and holothurians will increase when additional study is undertaken. Finally, investigation of micro-brachio-pods has not yet been extensively done and this fossil group could also enrich the faunal list.

The planktonic microfossils are equally diversi­fied at Tercis and in the section of the Apennines near Gubbio. Near Gubbio, Premoli Silva & Shter (1994) list about 115 taxa of planktonic for­aminifera in the interval of time equivalent to the

Campanian-Maastrichtian outcrop of Tercis; Gar-din, del Planta et al. (this volume, chap. E4) list about 105 taxa of calcareous nannofossils in the same succession. At Tercis 125 and 150 taxa have been listed for the foraminifera (chap. C5d) and nannofossils (chap. C3e) respectively.

In summary, the planktonic microfossils of the succession at Tercis are as diversified as in the Apennines. It is not claimed that the fossils are similarly abundant but only that, with enough time spent, it is possible to document a similarly wide and stratigraphically meaningful spectrum of

Table 1. Numbers of genera and species identified in the samples collected from the geological site at Tercis; comparison with the outcrops of the Island of Rugen (Baltic sea, NE Germany). Information from Tercis is valid at the date of June 1999; numbers will increase with additional studies.

Microfossils Algae unicellular ? Pithonellids Pollen incertae sedis Spores Pteridophyta Pollen Gynnnospermae Pollen Angiospermae Diatonfis, Radiolarians Dinoflagellates/Acritarchs Calcareous nannofossils Benthic foraminifera Planktonic foraminifera Crustaceans Ostracods Echinoderms Ophiuroids

Holothurians

TOTAL

Macrofossils Plant remains Sponges Corals Brachiopods Bryozoans "Worms" Scaphopods Gasteropods Bivalves Inoceramids

Others Cephalopods Ammonites

Nautiloids Coleoids

Crustaceans Decapods Cirripeds

Echinoderms Asteroids Echinoids Crinoids

Vertebrates Sharks Other fishes Reptilians

Ichnofossils +others

TOTAL

* after Reich M. & Frenzel P., 19£

Tercis (Vi-1999)

genera

few 01 02 06 06 55 00 82 69 40 27 31 12 03

331

nd nd nd 11 nd nd

-23 08 18 25 nd 00 nd 01 08 23 04 07 03 02

-133

9; ** after

species

nd 02 02 13 08 >120 00 145 >150 >70 125 53 20 04

708

nd many few 21 nd nd

-25 20 >29 >47 nd 00 few 01 16 45 07 08 >3 02

-224

Reich M.,

Rugen chalk *

genera

nd 4

----7 39 31 90 10 64 18 22

285

nd 13 05 20 86 13 01

-01 32 07 01 02 02 05 10 22 09 >4 >4 >2 >19

268

1999.

species

nd 6

>20 68 62 234 18 137 37 43 **

625

nd 21 06 35 244 18 02

-04 61 17 04 >5 >5 13 19 33 20 >9 >6 >2 >32

556

793

organisms. This is in conclusive contrast with claims spread during recent years against the quality of the Tercis succession. The macrofossil assemblage at Tercis is much richer than that of oceanic deposits and as diversified as in the chalk facies. In this respect, the geological site at Tercis is a well-suited succession for a global reference.

3.3. Biostratigraphical progress

3.3.1. Analytical uncertainty in biostratigraphy at Tercis

Macrofossils are considered to be ideal markers for the biostratigraphical approach in the field while microfossils are claimed to be ideal when cores or other small samples are used for study. It is intended here to temperate these claims and support that only combining both can provide the appro­priate situation for establishing a reference section, a reference level, and a reference point.

Macrofossils do not often allow to document the age of a succession in the field. A first problem has been illustrated in many geological excursions: macrofauna is usually much too rare for instanta­neous dating of a succession. How many geological excursions failed to found THE diagnostic fossil? In addition, even when a long-term collection is possible, designating a precise boundary often remains difficult because the presence of macro­fossils is seldom continuous. The second problem is the fact that either few taxa can be identified without doubt or/and that few experts can identify these taxa in the field. This has been repeatedly documented in the Campanian-Maastrichtian suc­cession at Tercis: many ammonites and most inoceramids collected during this study could not be definitely identified in the field. This is true for specimens of the genera Pachydiscus, Nostoceras, and Hoploscaphites i.e. all ammonites usually considered to be diagnostic for the boundary. Experts who are requested to deliver identification commonly require to look at the fossils in their laboratory where they need to compare with their reference material (pictures and casts). In addition, it is often necessary to have a series of specimens for biometric investigation (ammonites and belem-nites). The field identification is only a theoretical possibility. However, the reader can imagine the

happiness of a non expert when he feels himself able to identify some taxa; this is a realistic possibility with the inoceramid genus Trochocer-amus or with the ammonite species Diplomoceras cylindraceum . . . as long as nobody demonstrates the reverse. Therefore, field identification is not impossible for a few taxa. This strongly encourages to raise the small number of recognisable taxa to the rank of key taxa and to diminish the weight of traditional forms difficult to identify.

Steady collection has led to gather more than 2600 specimens of macrofossils from Tercis. The most abundant macrofossil group, the echinoids, with the succession of forms of the genus Micra-ster, is the single one which can illustrate a biostratigraphical evolution with easy-to-identify specimens observed in the rock during a field trip. This evolution is apparently not used in other places and its significance is possibly local. Still among echinoids, the genus Echinocorys is com­mon; it was pointed out by Hancock et al. (1993) as a useful source for local subdivision of the series and interbasinal correlation toward the Boreal Domain and the British chalk (Southern England). However, we were unable to recognise the 14 species suggested by N. Peake in 1993; Echi­nocorys are difficult to identify; environment influences the test morphology (species are variable in shape) and homeomorphs can be found at different levels. In summary, stratigraphy with the genus Echinocorys can be a source of confusion instead of a means of correlation.

The inoceramids is the second macrofossil group in abundance. Their identification is not possible in the field but for a few experts in the world and the specimens submitted to A. Dhondt where often qualified as "cf." or "aff." at Tercis. The only easily recognisable taxon is the genus Trochoceramus and Walaszczyk et al. (1996) proposed to consider its FO as a key horizon located near the stage boundary. It has been shown in this volume that its FO is older; the typical ornamentation of the genus would, however, encourage an increase in interest for this taxon.

Ammonites have shown their limits during our studies. The difficulty encountered when identify­ing most taxa, their distribution restricted to a few levels in many sections (although this is not the

794

case at Tercis), and the small number of specimens which can be hoped for in many outcrops are reasons why, in the author's opinion, one of these molluscs cannot be used as the single key fossil to locate the Campanian-Maastrichtian reference level.

However, the evolutionary change from Pachy-discus perfidus to Pachydiscus neubergicus is documented in the Tercis succession (appendix 2 in chap. D4g). If other experts agree about this point, it could reinforce the significance of the FO of P. neubergicus at Tercis because this local event would document a real evolutionary change and thus be a meaningful key for correlation, which is not the case for the FO alone often observed to occur later than the known inception. The transi­tional forms are located between about level 115 and level 118. The definition of the boundary level itself needs a more precise and more easily documented criterion. D. cylindraceum is another key taxon; Kennedy & Summesberger (1986) listed 56 references illustrating this taxon. It would be profitable to devote more interest to this easy-to-recognise taxon.

The experience of the Campanian-Maastrichtian boundary at Tercis has also made restrictions in the admitted principle that microfossils are able to continuously document the age of a succession. The first problem is the fact that the definition of many taxa is less widely accepted than a non expert is inclined to believe it. This is true for the microfossil groups best documented in this volume (dinocysts, calcareous nannofossils, foraminifera); accepted conventions on the definition of a number of taxa still remain to be agreed on as lamentably noted in many chapters above. This is already known but the remark should not be considered fully trivial if one remembers that most published microfossil study reports suggest boundaries of occurrences with a precision of a few 10 ka (the sampling interval). It is shown in this volume that identification by different experts can lead to boundaries differing by 10 times larger intervals. The case of the key marker Globotruncanita elevata can be recalled as an extreme situation (chap. C5d). For some experts, the taxon disappears at the base of the quarried succession at Tercis; for others, it is a diagnostic form, the last occurrence of

which is located in the boundary interval; a last expert sees typical forms at the bottom of the dark-flint-bearing Les Vignes subunit far above the stage boundary.

The second problem with microfossils is that a taxon can be difficult to find due to the fact that it is rare or that alteration leads to hide specific morphological characters. The scarcity of some species is documented at Tercis with the case of Radotruncana calcarata, for which only two con­vincing thin sections were agreed on by several experts following the first series of investigation. The taxon was not more common than the ammon­ites D. cylindraceum or P neubergicus. Repeated thin section preparations led to draw a precise range for the foraminifera with limits agreed within about ± 1 metre. The problem of preservation added to the scarcity is illustrated with the case of the LO of Eiffellithus eximius or of the pair LO Reinhard-tites anthophorus - FO R. levis. Stratigraphical uncertainties equivalent to ± 0.2/0.5 Ma are docu­mented for these bio-horizons (chap. C3e) Other bio-horizons are reproduced with an uncertainty of about ± 50 ka.

The third problem with the microfossil control is a fundamental one: the historical chronostrati-graphical boundaries are defined from internal platform or from coastal fades where planktonic microfossils are absent. This makes the definition in terms of microfossils highly dependent on potentially imprecise correlations between differ­ing depositional environments. At Tercis, it exists a case where the problems of identification, fre­quency, and stage definition can be excluded: it is the LO of the planktonic foraminifera Rado­truncana calcarata. Although a verification by an expert is always necessary, this taxon can be identified by a non expert; its range is restricted enough for it to be a precise key marker even when rare, and it has long been used as the guide fossil for the Campanian-Maastrichtian boundary. In our opinion and in agreement with the 50 years old claim by Bartenstein (1948), this foraminifera has a role to play in the stratigraphy of the Campanian.

3.3.2. Connection between fossil groups at Tercis

The succession at Tercis places in our hands the record of diversified fossil groups which are not

795

commonly found together for different reasons, including the palaeogeographical and the environ­mental endemisms. Figure 2 summarises the information obtained from the quarried portion of the geological site at Tercis.

Concerning the macrofossils, the continuous record across the stage boundary allows to docu­ment for the first time clear relationships between different species of ammonites including Pachy-discus perfidus and P. neubergicus, Hoploscaphites pumilus and H. cf? constrictus, Diplomoceras cylindraceum, Pseudokossmaticeras tercense and Psk. brandti. Correlation between inoceramids and ammonites at Tercis has led to clarify the location of the FO of Trochoceramus with regard to ammonite bio-horizons as already quoted above.

Concerning the microfossils, there are few outcrops where planktonic foraminifera, calcareous nannofossils, and dinocyst bio-horizons can be directly tied. Although some more progress is desirable in the detail of the location of the bio-horizons for dinocysts, it is particularly interesting to have an example of the location of the subcon-temporaneous FOs of Cerodinium diebelii and Microdinium carpentieriae and LO of the genus Odontochitina at a level which can be dated about 1.5 to 2 Ma before the stage boundary. In addition, at least four LOs, those of Corradinisphaeridium horridum, Raetiaedinium truncigerum (-R. evitti-gratium), Samlandia mayii and S. carnarvonensis seem to bracket the stage boundary (figure 2).

A significant progress in our knowledge of the biostratigraphy across the stage boundary provided by the succession at Tercis is the connection between macro- and microfossil ranges. The rela­tionship between the ranges of the ammonites A . {Bostrychoceras) polyplocum, and Nostoceras (N.) hyatti on the one hand and the planktonic for­aminifera Rd. calcarata on the other hand is documented for the first time. The co-occurrence interval of A . {B.) polyplocum and Rd. calcarata is 6.2 metres-thick. This represents 0.2 to 0.3 Ma of deposition or a maximum of up to 0.4 Ma if one assumes that the beds around level 45 represent a condensed deposition. There is no co-occurrence between the ranges of A . {N.) hyatti and Rd. calcarata, the former being entirely younger than the latter. If one agrees that the whole A . hyatti

range is below the FO of Belemnella lanceolata in the Boreal area, then the A . hyatti biozone almost represents the vertical difference between the former location of the Campanian-Maastrichtian boundary in the Boreal Domain (at the level of the FO of B. lanceolata) and its location in the Tethyan Domain (at the level of the LO of Rd. calcarata).

3.3.3. Biostratigraphical cuts in the succession at Tercis

There is no acute biostratigraphical cut in the succession; however, increases in the rate of change of the fossils occur at three places which can be centred at levels 67, 100, and 115.

The change around level 67 comprises the decrease in the genus Micraster which becomes definitely rare compared to the genus Echinocorys and this suggests a deepening of the sea floor. The major faunal change concerns the ammonites with five species apparently first occurring and five others occurring only in the slightly phosphatic inoceramid-rich bed. The absence of ammonites below and the richness of that particular bed explain the frequency of apparent bio-horizons and indicate that they are not all evolutionary events. The consistent echinoid bio-horizons LO of Micra­ster corcolumbarium and FO of M. aturicus at level 68 ± 1 heighten the ammonitic signal. Among the microfossils, the FOs of two dinocysts {Cerodinium diebelii and Microdinium carpentieriae) are impre­cisely located at level 70 ± 6. The key level LO of the foraminifera Rd. calcarata could be associated with the former changes although located below at level 62.5. Level 67 is the less appropriate for the location of a chronostratigraphical boundary at Tercis compared to the next two levels documented by a comprehensive series of bio-horizons.

The change centred at level 100 is first litho-logical with the appearance of flint nodules and a glaucony content which becomes definitely very low. A dozen of bio-horizons could contribute to document a unit boundary near this level (table 2). They concern ammonites and inoceramids, dino-flagellates cysts, planktonic and benthic fora­minifera. The four ammonite bio-horizons and the inoceramid one are gathered in a short interval of deposition at level 99 ±5. The appearance of the

796

IHONTA D'AVEZAC UNIT LES VIGNES UNIT

_L HbUbp grey-^ glauconyw glaucony-rich gglaucg glauconyo . «. . b 9 Imstw poorlmsto limestone 01 pooro rich to poor S '^^®

dark flint

N. (B.) polyplocum

T. pulcherrimus

Nostoceras hyatti + allied ,;, Pachydiscus neubergicus

I I'Diplomoceras cylindraceum Psk. brandti

Pachydiscus perfidus h - ^ ^ tercense • in^ipscaphites cf. ?constrictus

Lithological Units

Level

Geochemlcal sequences

Glaucony Abundance

Ammonite zones

and biohorizons

simpsoni corcolumbarium aturlcus

I conica ^conoideai

brongniarti

amaudi

MICRASTER

ECHINOCORYS

Echinoid bed intervals

Genus Trochoceramus Inoceramids

Meonia semiglobularis Cretirhynchia retracta Brachiopodes

ro CO

E? masureae f—• r Cerodinium diebelii i R. truncigerum n ^ I ' I Microdinium carpentieriae |

Genus Odontochitina

Dinocyst bed intervals

and biohorizons

C. horridum Samlandia mayii & S. caman/onensis

N)

Q. sissinghi

NC19 CC20 CC21

_! w

I Quadrum trifidum \ — j -

Nannoconids Quadrum pothicum Aspidolithus parous constrictus

NC20 NC21 CC22 CC23A

Calcareous nannofossil bed intervals

and biohorizons

Roth 78 Sissingh 77

zones

62.5

G. ventricosa / G. rugosa [Rd. calcarata| Globotruncanella havanensis | Rg. scotti /Ct. contusa

Marginotruncana marginata & M. undulata

Rd. calcarata — •

Heterohelix glabrans

Gl. havanensis

I G. falsostuarti

hexacamerata

FO Rg. rotundata

Rugoglobigerina scotti

iContusotruncana contusa

Planktonic foraminiferal

zones and

biohorizons

Bolivinoides iaevigatus

dominant 3 lobes Bolivinoides \ 4 lobes

Gavelinella clementiana.

Dominant 5 lobes Bolivinoides

^ Epistominella alata (reticulate)

Benthic foraminiferal biohorizons

Time in 0.1 Ma

Age estimate Ma

C A M P A N I A N M A A S T R I C H T I A N Stage

797

Fig. 2. Summarised stratigraphical data obtained from the Campanian-Maastrichtian quarried portion of the geological site at Tercis. Maximum abundance of glaucony 3% (level 45). Dinocyst and calcareous nannofossil bed intervals (1, 2, 3, 4) are defined by the bio-horizons shown beside the corresponding column. The uncertainty for individual bio-horizons is not shown for purpose of clarity; it can be found in the previous chapters. Ammonites N. (B.): Nostoceras (Bostrychoceras); Ps.: Pseudoxybeloceras spp. ; Psk.: Pseudokossmaticeras; T.: Trachyscaphites; Dinocysts C : Corradinisphaeridium; E.?: Exochosphaeridium; R.: Raetiaedinium; Foraminifera Rd.: Radotruncana

genus Trochoceramus can be seen in the field. The data by Ion & Odin (chap. C5c) could suggest the presence of 6 FOs at level 97.2 or so but most of them seem to be connected to the availability of easy to disaggregate samples which provide an abundant fauna. The single planktonic forami-niferal bio-horizon shown in table 2 is slightly older than the central point of the change; this FO of Globotruncanita stuarti was also quoted by Ward & Orr (1997) from level 90 but the species is only quoted from much higher in the study by Ion & Odin (chap. C5c), which appears younger than usual. In Tunisia, the taxon co-occurs with Rd. calcarata (Robaszynski et al., 1998) and the old finds at Tercis are supported.

The calcareous nannofossil event LO of nanno-conids is not contemporaneous with the main change but is close and it was unanimously observed at the same place by all experts. The benthic foraminiferal record shows a change in a narrow interval with the three FOs of Epistominella alata (reticulate form) and Colettes reticulosus (possibly the same taxon as the former), and of Gavelinopsis aff. ventricosus. These bio-horizons usually occur in "Maastrichtian" deposits; their location in uppermost Campanian beds at Tercis suggests that these FOs have an evolutionary significance and could be correlated from a basin to another. Still among the benthic foraminifera, the bio-horizon corresponding to the FO of Bolivi-

Table 2. Lists of bio-horizons suggesting an increase in the evolutionary changes around level 100 and around level 115 in the succession at Tercis.

Biohorizons centered near level 100 (mean level -97.5) Dinoflagellate LO Chatangiella'} robusta 102.8 ±7.2

LO Xenascus ceratioides 91.1 ±4.5 LO Palaeohystrichophora infusorioides 191.1 (or below) LO genus Subtilisphaera 97.5 ±0.5

Nannofossils LO nannoconids 90.5 ±1.5 Planktonic foraminifera (FO Gt. stuarti < 92.5 ±3.1) Benthic foraminifera (FO Bolivinoides 5 lobes > 4 107.4 ±7.4)

FOs Colettes reticulosus /Epistominella alata (retic.) 104 ±6 / 96.4 ±0.{ FO Gavelinopsis aff. ventricosus 99.6 ±2.4

(also restricted occurrence of dominant Bolivinoides with 4 lobes: -98) Inoceramids FO genus Trochoceramus < 97.7 Echinoids LO Micraster aturicus > 94.3 Ammonites LO Nostoceras helicinum > 102.5

LO Nostoceras hyatti 11 (Kuchler) > 103.5 FO Hoploscaphites pumilus < 94.2 FO intermediate form P. perfidus-neubergicus < 97.5

Biohorizons centered near level 115 (mean level =115.5) Dinoflagellate hO Corradinisphaeridium horridum 112.4 ±2.4

LO Raetiaedinium truncigerum (~ R. evittigratium) 118.6 ±3.8 FO Alterbidinium minus 118.6 ±3.8

Planktonic foraminifera ¥0 Contusotruncana contusa 116.5 ±0.3 FO Rugoglobigerina scotti 116.2 ±0.5

Benthic foraminifera (FO Bolivinoides 5 lobes > 4 107.4 ±7.4) LO Gavelinella clementiana 115.5 ±0.7

Ammonites LO Nostoceras (N.) approximans > 113 LO Nostoceras hyatti & allied forms (A . sp. 2) > 114.1 FO Hoploscaphites cflconstrictus < 116.8 FO Pachydiscus neubergicus < 116.1 FO Didymoceras cylindraceum < 111 ±3 LO Pachydiscus perfidus + intermediate form

perfidus-neubergicus > 117.5

798

noides with sutures dominantly showing 5 lobes instead of 4 for specimens found in the previous deposits is imprecisely located at level 107 ±7; it documents the group of changes near level 100 better than the one near level 115 since in the Mons Basin (Belgium), Robaszynski & Christensen (1989) support the opinion that the form with 5 lobes is already common in the Campanian Craie de Spiennes.

There is no lithological change around level 115 at Tercis but at least twelve bio-horizons are condensed in the interval 111-119. The ammonite fauna shows 6 or 7 bio-horizons; dinoflagellate cysts, benthic and planktonic foraminifera are also concerned with 2 or 3 bio-horizons each. The bio-horizons are not exactly contemporaneous and the differences of level are clear enough to document a progressive, though rapid, change. Many bio-horizons have a correlation potential with other depositional basins. Among the macrofossils, the historical interest of the ammonites is large but the practical interest of some of them is overestimated. This is the case of the FO of Hoploscaphites constrictus which is difficult to document but is repeatedly quoted "3.5-5.0 m above" that of Belemnella lanceolata at Kronsmoor (Kennedy, 1986b; Kennedy & Summesberger, 1986; Kennedy et a l , 1992; Hancock & Kennedy, 1993; Ward & Kennedy, 1993; Kennedy & Christensen, 1997 . . . ) . In spite of search by different members of the Working Group, specimens from Kronsmoor remained invisible or unavailable either as a cast or as a figure for comparison to the material from Tercis.

Brachiopods could also document this interval with the FO of Cretirhynchia retracta which is located at level 116.9 (with some cf. specimens below; chap. Dl). This fossil group would need additional study on more material.

Among microfossils, dinocysts are extremely meaningful and promising; they significantly evolve near level 115 with three bio-horizons listed. Additional ones could be considered, such as the contemporaneous LOs of Samlandia mayii and S. carnarvonensis identified by several experts above level 122.4. Other bio-horizons, such as the first common occurrence of Alterbidinium sp. cf. A. acutulum at 112.4 ±2.4, followed by the FO of

Alterbidinium minus-\-Alterbidinium sp. at 118.6 ± 3.8 and by the FO of A. varium, seem to be of local interest only.

The interest of the planktonic foraminiferal bio-horizons is high, with the FO of what is called Contusotruncana contusa by three experts in this volume. The Rugoglobigerina scotti bio-horizon is considered important by Arz & Molina (chap. C5b) who were alone to identify this form at Tercis. This bio-horizon has been observed as contemporaneous with the FO of Pachydiscus neubergicus in Spain according to the same authors, which is consistent with the observation at Tercis. Two other for­aminiferal FOs (Gublerina acuta and Contuso­truncana walfischensis) are also suspected by Arz & Molina between levels 115.7 and 116.8 (chap. C5b). Finally, Ion & Odin (chap. C5c) quote two LOs occurring nearby: that of Globotruncanitella tercensis (between levels 116.2 and 117.8) and that of the intermediate form Globotruncana rosettaJ Globotruncanitella insignis (above level 119.9).

4. Chronostratigraphy

4.1. Age of the deposits below the quarry

The geological site of Tercis comprises outcrops of different ages. Macrofossil-bearing deposits observed to the South of the quarry are strati-graphically older than those in the quarry. According to Vigneaux (1975, p. 196) "Le Ceno-manien est constitue par un calcaire blanc a Rudistes (Sphaerulites, Caprotina) Alveolina creta-cea et Milioles" (the Cenomanian comprises a rudist-bearing white limestone with Sphaerulites . . .) on the northern flank of the Tercis diapir in contact with the Campanian-Maastrichtian. Our observations led to different conclusions.

Microfossils are uncommon in these sediments but M. Bilotte (personal communication, III-1998) observed a probable Campanian fauna in the glauconitic limestones of the Hontarede Unit, comprising Nummofallotia cretacea (Coniacian?-Santonian to Maastrichtian) and Goupillaudina sp. (younger than the Santonian). A quick collection of macrofossils led to the information shown in figure 3. The rudist-bearing rocks (Radiolitidae) of the Lacave Unit are Turonian to lower Campanian and

799

the underlying oyster-bearing Pale Clay Unit is Cenomanian. The ossicles of asteroids collected from levels -16 and -18 are the same as those of the so called Campanien 2 of Charente ( = for-aminiferal zone C III: Neumann & Odin, this volume, chap. El). This might be in contradiction with the suggested Coniacian age for the brachio-pod R. baugasii recently collected at level -19 (plate I) except if the hypothesis of a sedimentary break between levels 18 and 19 is considered.

trichtian age because the LO of Aspidolithus parcus constrictus is not so far from the top of the quarry; the two taxa Tranolithus orionatus and Reinhard-tites levis are still present at level 174 and typical forms of the taxon Lithraphidites quadratus are not yet present. This interval of time is shown in figure 4 in the profile called P (synthetic section of the five floors).

4.3. Age of the deposits of the B outcrop

4.2. Age of the quarried succession

The basal levels of the quarried succession contain Echinocorys fonticola and Micraster coranguinum, which points to a late Campanian age. This is confirmed by the oldest ammonites from the base of the d'Avezac Unit including Hoplitoplacenti-ceras dolbergense and the small heteromorphs of the genus Pseudoxybeloceras. The topmost levels of the quarried succession are of early Maas-

Boulanger & Lezaud (1965) documented the Maas-trichtian-Danian boundary interval which shows the "disparition des Globotruncana . . . progres-sivement remplacees par les Globigerines a test mince et Globorotalia du groupe compressa'' (members of the genus Globotruncana disappear and are replaced by globigerinids with thin shells and Globorotalia gr. compressa). The authors also indicate that a portion of the upper Maastrichtian

< o

Age off the pre-quarry units

P. 01,

•10H

-10

•3(W

FO Bostrychoceras polyplocum (upper Campanian)

Ceratolithoides aculeus present (upper Campanian)

|-^ Metopaster chilipora (mid Campanian) ^_ Rhynchonella baugasii {Con\ac\an)

h Phymosoma regulars an6 Globator ovulum (Turanian Coniacian) Periaster conicus (lower Turonian)

Neithea aequicostata (pre-Turonian) I-*- ? Polydiadema sp. (pre-Turonian)

Mecaster cenomanensis (Cenomanian)

p. CI. = Pale Clay Unit

Fig. 3. Key biostratigraphical information on the outcrops stratigraphically located below (to the South of) the Modern Grande Carriere.

Ma

60

70

80

90

100_

Stage

DANIAN

MAASTRICHTIAN

CAMPANIAN

SANTONIAN CONIACIAN TURONIAN

CENOMANIAN

ALBIAN pp

Profile

Fig. 4. Schematic chronostratigraphical interpretation of the outcrops observed in the geological site at Tercis. B: outcrop comprising the Bedat Wall; E: outcrop to the NW entrance of the quarry; P: main section (5 floors of the eastern end of the quarry); PQ: prequarry outcrops South of the South Wall; PC: isolated outcrop for the oyster-bearing Pale Clay; S: synthetic section.

800

deposits has been eroded to the East of the quarry. The age of the Bedat succession immediately North of the quarry has been preUminarily investigated by J. Ion using planktonic foraminifera. The single find of Abathomphalus mayaroensis (thin section) about 25 metres below the Bedat Wall suggests that the corresponding upper Maastrichtian biozone begins at least at this level. Several samples contain the diagnostic species Kassabianafalsocalcarata at the top of the biozone. The Bedat Wall itself and the overlying levels contain diagnostic Palaeogene globigerinids. From these data, it can be concluded that the Maastrichtian deposits are probably com­plete to the North of the quarry and that the succession could be continuous across the Meso-zoic-Cenozoic boundary.

Echinoids are not rare in this succession; the genus Echinocorys has been observed below the Bedat Wall and several Palaeogene genera (Echino-conus, Isopneustes) are present above. Ammonites are represented by some spiral forms including Pachydiscus jacquoti and P. armenicus, and many pieces of the heteromorph genus Glyptoxoceras. The youngest ammonite remains (pieces of Bacu-lites) were collected in 1998 about 15 metres below the Bedat Wall; they are more than 0.6 Ma older than the top Maastrichtian boundary.

The preliminarily investigated B section out­crops extend on both sides of the Maastrichtian-Danian boundary; they are shown (B) in figure 4; they are not connected to the quarried succession and cannot help to document the Campanian-Maastrichtian succession but the observed fossil content would encourage to undertake further study after cleaning of the overgrown lacking portions with the aim of documenting the whole Maas­trichtian stage continuously with micro- and macrofossils.

The synthetic succession documented in the geological site at Tercis is combined in the leftmost column (S) of figure 4.

5. Stratigraphical interest of the geological site at Tercis

The section at Tercis has been considered of high stratigraphical interest following proposal during

the Brussels meeting in 1995 (Odin, 1995; Odin, compiler, 1996b). The studies have shown that the more the section was known, the more useful it appeared for global correlation. This usefulness results from its easy access, its content in diversi­fied fossil groups (while the physico-chemical tools appear less promising), and its continuous deposi­tion widely bracketing the stage boundary.

5.1. Suitability of the geological site at Tercis for location of a reference section

Recommendations for the definition of a Global Standard-stratotype Section and Point (GSSP) were made by the International Commission on Stratig­raphy (Cowie et al., 1986; Remane et a l , 1996). Based on several applications of these recom­mendations, a set of ten rules was compiled (Odin, 1992b).

- 1. The GSSP must be accessible without restric­tion of political or philosophical nature, excessive effort or expenses. This is guaranteed by the local authorities at Tercis; not only the geological site is accessible but it has been established following request of the undersigned MWG leader to be used as a scientific reference. However, there is contra­diction between accessibility and preservation. Accessibility should thus be understood with the restriction that permission for visit must be obtained. Permission will be delivered if this is the interest of the geological site; the material collected from the site must remain available for visitors to come, i.e. must remain in the local collection if worthwhile to enrich the knowledge and patrimony of the site. - 2. A permanent artificial marker should fix the GSSP in the outcrop to allow its easy identification and the section should be preserved for long. The marker for the GSSP will be fixed in floor IV of the quarry at the appropriate level when the complete procedure of agreement comes to an end. In addition to this future marker, the whole succession has been marked since 1992. These marks are planned to be further improved. The long-term preservation is guaranteed by the local authorities. Besides, the life of the site will be enhanced by the

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planned creation of a geological station with some facilities available for educational or research activity. - 3. The GSSP should be selected within a marine, if possible pelagic, depositional environment in order to facilitate correlations with a large palaeo-geographic domain. The succession at Tercis is marine enough for planktonic microfossils to be present but it is not deep oceanic, which allows deposition of a variety of benthic, sub-benthic, and nektonic macrofossils. - 4. The GSSP has to be chosen within a section with continuous deposition across the boundary; the sedimentation rate should be sufficiently high (5 to 50 m per Ma) in order to register environmental variations in detail; the thickness of the section below and above the GSSP should be sufficiently large in order to record the eustatic cycles preceding and following the boundary (i.e. deposits corresponding to 1-2 Ma on each side of the boundary). The continuous character of the deposi­tion at Tercis is discussed in several chapters of this volume; the record at the stage transition can reasonably be accepted as continuous. The Cam-panian side of the boundary is more documented than the Maastrichtian one (figure 2) but nearly 2 Ma of deposition are present and have been studied above the stage transition. More could be studied in the future following the planned arrangement of the geological site. - 5. The section should be free of structural complications and strong diagenetic alterations and redeposition; tectonic activity may change the original succession of the strata, whereas strong diagenetic changes may destroy or alter the magnetic, geochemical, and geochronological records; undisturbed deposition without excessive bioturbation, redeposition or reworking is also needed. At Tercis, there is no difficult geometric problem with tectonics. There are geochemical problems (isotopic reequilibration) probably con­nected with the exceedingly strong diapiric tectonism. Bioturbation or trace of reworking have not been evidenced at the level of the stage transition. - 6. The absence of fades change across the boundary should reduce the presence of deposi­tional hiati (see 4.) and insure the representativity

of the variations of the stratigraphic tools (see 7, 8, 9). At Tercis, the homogeneous character of the deposition is such that it is difficult to identify changes and, for example, to undertake a sequence stratigraphic interpretation in the field. - 7. A good palaeontological record should enable bio stratigraphic correlations based on several groups of fossils. This point is the most important for correlation and it is particularly well docu­mented at Tercis with microfossils and macrofossils. The excellence of the palaeonto­logical record is also due to the palaeogeographical location of the site at the northernmost boundary of the Tethyan Domain (possible N-S exchanges), which allows Tethyan fossils to be interlayered with Boreal fossils, and at the westernmost extrem­ity of Europe, which allowed American species to reach the European domain. - 8. Applicability of magneto stratigraphy. This is important for the Campanian-Maastrichtian transi­tion where an easy-to-recognise record exists. At Tercis, a single magnetozone boundary has been identified in a portion of the succession. It is proposed in this volume to refer to an auxiliary section for this tool; the planktonic fossil connec­tion allows to locate the magnetic record of the Apennines in parallel with the section at Tercis (figure 2) with a good probability. - 9. Applicability of chemostratigraphy. Chemos-tratigraphic signals have an excellent oceanic correlation potential; some of them are virtually instantaneous (e. g. "iridium anomaly" at the top of the Maastrichtian). For the Campanian-Maastrich­tian transition, there are no major changes. At Tercis this stratigraphical tool has proved partly inefficient but could be considered via another auxiliary section such as Kronsmoor (Germany), where the Sr isotope record was claimed to be rehable (Mc Arthur et al., 1992; 1994). - 10. Applicability of geochronology. The surprising discovery of a promising geochronometer within the total range zone of Rd. calcarata (chap. Bla) has opened interesting prospects.

Many established GSSP are based on a single biostratigraphic change and the suitability of the succession at Tercis appears thus better as a whole.

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5.2. Previous criteria for locating the boundary and the record at Tercis

In 1984, the preliminary proposals for the Campan-ian-Maastrichtian boundary (Birkelund et al., 1984) mentioned six taxa, the ranges of which were useful for correlation of the boundary:

1- evolutionary appearance of the ammonite Hoploscaphites constrictus (J. Sowerby, 1817) type specimen at the British Museum of National History, London; 2- evolutionary appearance of the ammonite Pachy-discus (P.) neubergicus (von Hauer, 1858); type specimen at the Geologische Bundesanstalt, Vienna, Austria (Ward & Kennedy, 1993, Fig. 28); 3- evolutionary appearance of the belemnite Belem-nella lanceolata (Schlotheim, 1813); 4- evolutionary appearance of the foraminifera Globotruncana falsostuarti Sigal, 1952; 5- extinction of the foraminifera Radotruncana calcarata (Cushman, 1927); 6- extinction of the calcareous nannofossil Quad-rum trifidum (Stradner in Stradner & Papp., 1961) Hattner & Wise, 1980.

Since the 1984 restatement, 3 extra bio-horizons have been suggested, discussed, or used: 7- appearance of Nostoceras hyatti Stephenson, 1941 (suggested by Atabekian, 1995); 8- extinction of Nostoceras hyatti suggested by Madeleine Neumann (unpublished circular to the Subcommission); 9- extinction of the calcareous nannofossil Aspido-lithus parcus constrictus (Hattner, Wind & Wise, 1980) Perch-Nielsen, 1984.

Finally, the possible use of a physico-chemical criterion has been favourably considered (Brussels Meeting, 1995). The chronological succession of the nine bio-horizons above may vary from one place to another but the main problem is that they have never been found all together (nor a majority of them) in a single section, so that most previous chronological comparisons had to be indirect. The record in Tercis comprises eight of them which have been documented in sequence by at least two independent experts. This makes this succession the most complete known to date. Only criterion 3

lacks in the geological site and the depositional basin. The presence of criterion 1 is also debatable (as it is probably in most other basal Maastrichtian outcrops). In summary, the section at Tercis is the best documented presently to unify previous uses.

It is not claimed that THE correct chronological order of the nine bio-horizons is documented at Tercis but, only, that there is a direct relationship with an estimate of the time interval between them. Criterion n°9 (LO Aspidolithus parcus constrictus) is the most recent bio-horizon (according to data from Tercis, the LO of A. constrictus would lie in beds deposited about 1.8 Ma after the boundary interval). Criterion n°6 (LO Quadrum trifidum) is older than the former by about 1 Ma.

Four criteria are very near one from the other: n°l (FO H. constrictus) & n°2 (FO P. neubergicus) might be contemporaneous and subcontempora-neous with n°3 (FO Belemnella lanceolata), while n°8 (extinction of Nostoceras hyatti) is considered immediately older than n°2 by most authors as documented at Tercis, or showing a short co­occurrence with P. neubergicus (in N Spain).

Criterion n°4 (FO of Globotruncana falsostuarti) is older than the above group by about 1.5 Ma; the stratigraphical relationship with regard to n°7 (FO A . hyatti) varies from place to place; at Tercis, n°4 (level 75.0 ± 1.0) is younger than n°7 by about 0.3 Ma, but Ward & Kennedy (1993) quoted n°7 younger than n°4 in the Basque country.

Criterion n°5 (LO of Rd. calcarata) is the oldest bio-horizon which would lie in beds deposited slightly more than 2.5 Ma before the stage boundary. The previous main criterion for locating the Campanian-Maastrichtian boundary in the Tethyan Realm is still supported by some authors with several good reasons including the ease of identification, the wide palaeogeographic distribu­tion (figure 4 in chap. F2), and the fact that only a few tens of grams of sediment are required for that purpose in oceanic basins (which is only valid locally).

In summary, the two criteria n°3 and n° 5 were commonly used in the past. They were never observed in direct relationship in any known succession. This is still the case but indirect relationship was documented further by the Work­ing Group in this volume.

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5,3, Selected keys for characterisation of the succession and of the stage boundary

There are three major key fossils in the succession at Tercis with both their FO and LO documented in the section. The first key fossil is Radotruncana calcarata. The TRZ duration of about 1 Ma at Tercis is consistent with what is documented from the Apennines succession. The fossil is easy to identify although commonly rare among the micro-fossil fauna.

The second key fossil is Quadrum trifidunr, its TRZ brackets the stage boundary between level 52.5 and level 134.2 ( ± 2 to 3 metres, which represents about 100 ka). The duration of this TRZ can be estimated to be 3.5^.0 Ma; slightly more than 2/3 of the biozone is below the stage boundary interval. The taxon is reasonably easy to recognise without taxonomical ambiguity.

The third key fossil is Nostoceras hyatti with a TRZ more difficult to localise with precision due to the two facts that it is never abundant and identification is tricky. The TRZ is probably 2 Ma long but the top of the TRZ interestingly almost corresponds to the stage boundary.

Two other fossils have a good potential for correlation because a transition within an evolu­tionary lineage is documented in the succession. The first one is the benthic foraminiferal genus Bolivinoides, the typical lower Maastrichtian form of which is dominantly characterised by a suture with 5 lobes instead of 4 in upper Campanian levels. The location of the transition is not yet precisely documented because the number of specimens counted is statistically too low in Campanian samples (figure 1 in chap. C4c). The second lineage is the one between the two forms Pachydiscus perfidus and P, neubergicus. The FO of the latter species has been favoured as the guide-event to approach the stage boundary so that the domain of transition between the two forms almost coincides with the transition between the Campan­ian and the Maastrichtian stages.

Finally, Hoploscaphites pumilus is a fossil of palaeontological interest. It is characterised by its small size ("pumilus" means dwarf); 1/4 of the collected ammonites pertain to this taxon at Tercis and this is the largest collection available in the

world. The FO (level 94.2) and LO (possibly near level 123 for some experts but probably 1 Ma younger for others) bracket the stage boundary. The correlation potential of this taxon is good toward North America; it could also become significant in Europe and North Africa (Robaszynski et al., 1998) in a near future.

6. Conclusion

The aim of estabhshing a reference section for the Campanian-Maastrichtian at Tercis has been achieved thanks to the accumulation of a consider­able amount of information by a large group of experts. This information could not have been gathered without an impressive effort. But the effort has been widely rewarded by the variety of data obtained and summarised in this chapter.

The succession is now well characterised for lithostratigraphical and biostratigraphical tools. The physico-chemical tools are generally moder­ately to poorly applicable. The geochronological approach remains to be applied on promising material.

In addition to the characterisation of the section itself, the information gathered from Tercis allowed to solve stratigraphical problems. Among the problems, one is methodological: it has been proposed in this volume to estimate the uncertainty in the biostratigraphical exercise by considering different interpretations of the same record. A conclusion is that there may be large differences in the quality of the signals delivered by different taxa, some being precise and reproducible, others being less reproducible. This urges to moderate the weight to give to single markers, in particular as means for correlation of the stage boundary or as single key fossils for location of the conventional stage boundary. The latter will have to take advantage of the diversity of fossils for a better documented boundary to be defined and made easy to correlate.

Another stratigraphical problem was the rela­tionship between different stratigraphical tools and, especially, between the macrofossil and the micro-fossil evolutionary changes. Essentially known for its macrofossil content, the succession at Tercis can

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be used as a key section for microfossils allowing mutual relationships to be established for the first time. Other relationships established at Tercis for the first time are those between Boreal and Tethyan successions. Finally, the palaeogeographical loca­tion of the geological site to the western end of Europe allows to establish relationships between North American and European successions.

Because some stratigraphical tools are not fully efficient at Tercis, auxiliary successions are con­sidered. In this aim, correlations with other depositional basins have been established and their precision estimated. This is the subject of the next chapter where a magnetostratigraphically charac­terised series will be considered; a Boreal succession will also be presented for possible integration of the belemnite stratigraphical tool.

Acknowledgements

The information used in this chapter results from the work of the whole Maastrichtian Working Group, each member of which is deeply thanked; the ideas and opinions quoted above are under the unique responsibility of the author. L. Jacobsen was kind enough to search for siliceous microfossils in a test specimen; M. Reich is thanked for providing unpublished information about the Riigen fossils. Laboratory pictures are by C. Abrial; the scale in picture 1 is kindly provided by O. Giliane. F. Robaszynski added the light of his own experience on a previous version of this chapter. M. A. Lamaurelle is thanked for her patience and effort in improving the clarity of this chapter.

(Prepared: May 2000; revised: July 2000)

Notes added in proofs

Key fossils by G. S. Odin In addition to the 3 key fossils having their TRZ documented at

Tercis (N. hyatti, G. calcarata, Q. trifidum) there is also a fourth one N. (B.) polyplocum which has been found by several experts at Tercis and is present in both Tethyan (El Kef) and Boreal Realms.

The oldest H. constrictus by M. Machalski The oldest specimen from Kronsmoor will be described soon (see note added in chap. D4g)

Inoceramids from Riigen by I. Walaszczyk The number of inoceramid genera counted for the Lower Maas­trichtian of Riigen is underestimated. Inoceramids, belonging to at least three genera, were actually published from Riigen by Wolansky (1932) and a single specimen representing one other genus is housed in the collections of the Natural History Museum in London. The problem itself is the proper taxonomic inter­pretation of these forms.

The list with provisional determinations of the Riigen genera is as follows:

1. Cataceramus sp. {Cataceramus?subcircularis)\ Wolansky (1932, pi. 5, fig. 1);

2. Platyceramus sp. or a new, very similar genus: Wolansky (1932, pi. 4, fig. 4); I have seen numerous fragments of this type during my visit on the island. Seitz (1970, p. 137) referred the specimen illustrated by Wolansky (1932) to Platyceramus;

3. Probably a new genus as represented by the specimen illustrated by Wolansky (1932, pi. 5, fig. 7);

4. Trochoceramus sp., the specimen from the Maastrichtian of Riigen in the Museum of Natural History in London. Seitz, (1970) and Nestler (1982), referred to the genus Trochoceramus the specimen illustrated by Wolansky (1932, pi. 5, fig. 3) referred originally to Inoceramus (Aristoceramus) bohmi Heinz, but later referred by Heinz (1933) to the new species Inoceramus rugiae Heinz, 1933. The Trochoceramus affinity of this specimen, and of Heinz' /. rugiae is, however, not convincing.

The genus-level comparison of inoceramid faunas between differ­ent regions, particularly in the Campanian and Maastrichtian, may be misleading, as concepts of the Campanian and Maastrichtian genera are still variably interpreted by particular workers. The other problem in such a comparison is the sample size. The number of specimens on which the number of inoceramid genera from the Maastrichtian of Riigen is estimated is extremely low. It is actually less than 10 specimens! Thus the 4 genera represented in such a small sample is really a high value. The sample size of the collection from Riigen is too small to allow a rigorous statistical treatment but counted ratio between the number of specimens to the number of genera in it suggests a diversified, at the genus level, inoceramid fauna in the Maastrichtian of Riigen.

References Heinz R., 1933. Inoceramen von Madagascar und ihre Bedeutung fiir die Kreide-Stratigraphie. Zeits. Deutsch. Geolog. Gesell., Berlin, 85 (4): 241-259. Nestler H., 1982 (2nd edition) Die Fossihern der RUgener Schreibkreide. Die Neue Brehmn-Bticherei. Leipzig, 1-102. Seitz O., 1970. Ober einige Inoceramen aus der Oberen Kreide. 2. Die Muntigler Inoceramenfauna und ihre Verbreitung im Ober-Campan und Maastricht. Beih. Geolog. Jahrb., Hannover, 86: 105-171. Wolansky D., 1932. Die Cephalopoden und Lamellibranchiaten der Ober-Kreide Pommerns. Abhandl. geologisch-palaeontologischen Institut der Universitat Greifswald, Greifswald, 9: 1-72.