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Chapter E5c Numerical age calibration of the Campanian-Maastrichtian succession at Tercis les Bains (landes, france) and in the Bottaccione Gorge (Italy)
The Campanian-Maastrichtian Boundary G. S. Odin (editor) 2001 Elsevier Science B.V.
CHAPTER E5c
Numerical age calibration of the Campanian-Maastrichtian succession at Tercis les Bains (Landes, France) and in the Bottaccione Gorge (Italy) G. S. Odin
Sommaire L'age numerique de la limite Campanien-Maastrichtien definie a Tercis (cote 115,2) peut etre estime par correlation avec le bassin du Western Interior en Amerique du Nord ou les bentonites ont permis de mesurer l'age des biozones d'ammonites regionales. Du fait de I'endemisme tres fort de ces faunes, le seul lien disponible avec Tercis est le niveau de disparition de 1'ammonite Nostoceras hyatti repere par Kennedy et al. (1992) par rapport aux zones regionales. Ces auteurs ont propose, pour ce bio-horizon, un age numerique fonde sur les donnees disponibles a I'epoque. On doit preferer aujourd'hui le meilleur ancrage numerique etabli par Baadsgaard et al. (1993) qui ont mis en oeuvre plusieurs methodes d'investigation pour dater un niveau tres proche du bio-horizon au Canada. Cet ancrage conduit, par une courte extrapolation, a proposer un age de 72,0 ±0,5 Ma pour I'extinction americaine de A^. hyatti, evenement probablement concomitant de la limite d'Etage a Tercis. L'age connu des limites d'Etage du Campanien (83 a 72 Ma) et du Maastrichtien (72 a 65 Ma) permet de calculer des taux de sedimentation moyens apres compaction de 40 m/Ma a Tercis (ainsi qu'il a ete etabh d'apres I'etude des rythmes et epaisseurs dans ce volume) et de 9 m/Ma dans les Gorges du Bottaccione. Dans ce dernier cas, il est demontre ici que le taux de sedimentation est semblable pour les deux Etages aux incertitudes pres ( ± 1 m/Ma). Ceci infirme des estimations
anterieures qui ne semblaient pas verifiees par les observations sedimentologiques. Leur modification est rendue necessaire par la recente mise en evidence d'un hiatus tectonique dans le Maastrichtien de la section italienne et, comme il est ecrit plus loin, par un leger deplacement de la limite d'Etage adoptee anterieurement en Italic. L'utilisation de ces taux de sedimentation permet egalement, en combinaison avec 1'information biostratigraphique (foraminiferes et nannofossiles calcaires), d'estimer I'ampleur des condensations locahsees a Tercis lors de I'etude sedimentologique. Par comparaison avec la succession des Apennins, consideree comme plus regulierement deposee, la condensation la plus nette se situerait dans I'intervalle compris entre les cotes 66.5 et 67.3, qui representerait 0,4 a 0,5 Ma. Une condensation moins importante se situerait vers la cote 45. II n'y aurait pas de lacune ni de condensation decelables au-dessus de la cote 70 dans la section de la grande carriere moderne de Tercis. La combinaison des informations sedimentologiques, biostratigraphiques, magnetostratigraphiques et geochronologiques permet une correlation entre la limite d'Etage definie a Tercis et la succession des Apennins. La limite d'Etage doit se situer entre le sommet de I'anomalie 33 et la disparition de Aspidolithus parens parens et tombe a la cote 320 m avec une incertitude de I'ordre de 3 metres. Cette localisation, anterieure d'un peu moins d'l Ma a celle adoptee auparavant, correspond a la partie moyenne de la magnetozone
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normale 32N2 ou encore a la partie basale de la zone a Gansserina gansseri des Apennins. A la suite de ces considerations, on pent calibrer les sections de Tercis et d'Italic. De la base au sommet, les ages de 77, 76, 75 . . . 70 Ma se placeraient vers les cotes 7, 29, 51, banc a inocerame, 90, limite d'Etage, 140 et 165 a Tercis et, en Italic, tons les 9 metres de part et d'autre de la limite datee de 72 Ma et situee vers 320 m. 1. Introduction Reaching a realistic estimate of the numerical age of the Campanian-Maastrichtian boundary requires considering direct radiometric measurements. Most of the radiometric ages obtained from Upper Cretaceous fossiliferous deposits in the world come from the Western Interior basin of North America. However, the macro-faunal endemism and the absence of pelagic microfauna do not allow to directly correlate these numerical ages with other basins using the biostratigraphic tool only. Other considerations can be taken into account, which is done in this chapter where biostratigraphical (ammonites and planktonic microfossils), sedimentological (mean rates of deposition), and magnetostratigraphical information are combined with geochronological calibration, allowing to estimate the numerical age of the stage boundary newly defined at Tercis. 2. Numerical age of the LO of Nostoceras hyatti in North America Kennedy et al. (1992) used the discovery of a specimen of Nostoceras hyatti to infer a numerical age for the Campanian-Maastrichtian boundary as accepted in northern Germany. The resulting age "71.3-71.4 Ma" has been repeatedly quoted in the literature. However, the proposal suffers from weaknesses essentially (though not only) linked to the lack of accompanying estimate of the uncertainty on this value. 1- The quotation of A^. hyatti is based on a single find of the taxon lying together with common Baculites jenseni in a "ferruginous concretion in the Pierre Shale" (Walsenburg, Colorado); "more than a dozen helices in various states of preservation, the largest consisting of parts of three whorls".
Four specimens are illustrated; they are portions of spires (phragmocone) which give no information on the body chamber. It is difficult to compare these specimens from Western Interior with the material from Europe in general, and more precisely with that collected from Tercis where the phragmocone is usually absent but the body chamber present. The phragmocone description might seem different for the material from Western Interior and for that from Tercis. In the American specimens, "the spire is low with a large apical angle" and "most ribs bear small, sharp, transversally elongated tubercles" (Kennedy et al., 1992); the specimens from Tercis show "an apical angle of less than 90°" and "narrow ribs .. . link in pairs to rather coarse tubercles . . . " (Hancock & Kennedy, 1993). 2- Campanian Baculites biozones from Western Interior (figure 1) have been dated geochronologically. A mean duration of about 0.5 Ma can be calculated for these zones. Using this mean duration, interpolated ages can be calculated for any point between the dated levels. However, Kennedy et al. note that the precise origin within the B, jenseni zone of the find at Walsenburg is not known. The interpolation will thus induce an uncertainty of ±0.3 Ma. Similarly, the dated bentonites are not precisely located in their Baculites zones, which adds another uncertainty of about ± 0.3 Ma. The total uncertainty on the interpolation procedure is thus 0.5 Ma or so (without considering the possible differences in duration for individual zones). 3- The radiometric ages quoted by Kennedy & al. (1992) from Western Interior are 70.1 ±0.7 (la) Ma {B. grandis zone, NDS 104) and 73.2 ± 0.7 (la) Ma {B. compressus zone, NDS 105). The mentioned uncertainty is the analytical standard deviation and a more realistic 2a uncertainty should be quoted; it should be added to the uncertainty inherent to the interpolation procedure. Also note that only biotite separates have been dated in these two bentonites and that the old one has a low K content (4.4%), which denotes an alteration following crystallisation; therefore, the reliability of the age provided by the latter is low (in brackets, figure 1). In addition, Kennedy et al. located this date in the B, compressus zone while Odin & Obradovich
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(NDS 105) quoted it from 6 m below the B. compressus zone with an age of 73.1 ±1.4 Ma. More reliable ages (based on K-rich biotite and sanidine) are available from zones dated below; figure 1 gives the weighted mean ages calculated from the multiple biotite dates summarised by Odin & Obradovich (1982: abstracts NDS 104 and 105). 4- Because the key taxon has been found in a single place, its precise location within the full range of A^. hyatti is not firmly established. At Tercis, the range can be assumed to be 20 or 47 metres thick depending on whether the specimens from level 67 are conspecific with A^. hyatti or not. The corresponding deposition duration is 1 or 2 Ma. Therefore, the place of the stage boundary versus the calibrated key fossil will add a new uncertainty which can be as high as ± 1 Ma; in addition, this is correct only if the A^. hyatti range at
Tc Tp •
Measured Ma ages (Ma) 70-
Ammonite zones
70.1 ±1.4
B. grandis
Tercis is contemporaneous with that in Western Interior of USA. Note that it is accepted here (like in Kennedy et al., 1992, for NW Europe) that the evolutionary LO (extinction) of A^. hyatti as documented at Tercis is a good approximation of the chronostratigraphic boundary between the Campanian and the Maastrichtian. As a result, the age estimate at "71.3-71.4" proposed by the previous authors has a combined uncertainty of about ± 2 Ma and thus poorly constrains the LO of A^. hyatti. This estimate can be improved using the cahbration available from Canada. The best and nearest age calibration of the Western Interior series known to us was presented by Baadsgaard et al. (1993). According to these authors, the Snakebite bentonite in Saskatchewan (Canada) is located at the top of the Baculites reesidei zone (figure 1) immediately below the B.
1
Western Interior (Colorado)
Last occurrence N. hyatti
70B. baculus
71-
B. eliasi 7172-
B. jenseni |»72.5
±0A ]i.||li..
72-
7373.7 ±1.0
75-
74.3 ±1.3 73.6 ±1.3
N. hyatti single find ± | r
B. reesidei
B. compressus (73.1 ±1.4)
74-
T
(1)
1 Jeietzkytes nodosus 1 present
B. cuneatus
73-
74-
i
f
D. cheyennense E. jenneyi D. stevensoni D. nebrascense
t 6
B. scotti
Fig. 1. Numerical age of the last occurrence of Nostoceras hyatti (near the Campanian-Maastrichtian boundary) according to radiometric datings in Western Interior of USA and Canada via the presence of ammonite key taxa; B.= Baculites, D.=Didymoceras, E. = Exiteloceras. The mean biozone duration ( - 0 . 5 Ma) is according to the original dating by Obradovich & Cobban (1975; summarised in Odin & Obradovich, 1982) with a scale shown in column Tp. Ages are recalibrated (Tc) using the age at 72.5 ±0.4 (black dot) from Canada (Baadsgaard et al., 1993). The most probable age of the stage boundary (2) results from biostratigraphical relationships; the maximum age (1) is better constrained than the minimum age (3). All ± are intralaboratory analytical uncertainties with a 95% confidence level.
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jenseni zone. Concordant U-Pb zircon ages, a RbSr isochron age, and K-Ar weighted mean ages on biotite and sanidine (irradiation technique, laser heating procedure on ten separates using Fish Canyon tuff sanidine and hornblende MMhb-1 as monitors with ages assumed to be 27.84 and 520.4 Ma respectively) have been obtained. Calculated ages are: a U-Pb concordant zirconcrystal average age at 72.5 ± 0.4 (2a) Ma; a Rb-Sr plagioclase, sanidine, biotite isochron age at 72.5 ± 0.2 (2a) Ma, a sanidine K-Ar weighted mean age at 72.5 ±0.5 (2a) Ma, a biotite K-Ar weighted mean age at 72.6 ± 0.4 (2a) Ma. The consistency is surprisingly good (order of magnitude \%c) considering the fact that decay constants of the naturally instable isotopes used are known with a 10 time-poorer precision (Odin et al., 1999). A pooled age at 72.5 ± 0.4 Ma is suggested for the top of the B. reesidei zone. This age is better than the one used in 1992 which came from the original study by Obradovich & Cobban (1975). Mean biozone durations can be used to interpolate between geochronologically dated levels (Odin, 1992a). The data summarised by Odin & Obradovich (1982) suggest that the mean biozone duration may be around 0.5 Ma (column Tp). In fact, figure 1 shows that the base of the B. jenseni zone is dated and that the only problem is to propose an age for the top of the same zone. We can use the durations suggested by the data of Odin & Obradovich (preliminary scale in column Tp, figure 1) and recalibrate this scale using a better anchor at 72.5 Ma for the top of the B. reesidei zone. This corrected scale is shown in column Tc; note that the previous datings appear younger than the values in this scale but remain consistent when taking into account a realistic analytical uncertainty. Let us consider now the combinations of i- the uncertainty in the location of the specimens of A^. hyatti found inside their range and ii- the location of the specimens inside the B. jenseni zone. If the specimens found in Walsenburg correspond to the top of the range of the taxon and come from the base of the B. jenseni zone, then the Campanian-Maastrichtian boundary is at the base of this zone and its age is 72.5 ±0.4 Ma; this is the maximum age.
If the specimens found in Walsenburg correspond to the top of the range of the taxon and come from the top of the B. jenseni zone, then the Campanian-Maastrichtian boundary is at the top of this zone and its age is about 72.0 Ma. In the most unfavourable circumstance, the specimens found in Walsenburg correspond to the base of the range of the taxon and come from the top of the B. jenseni zone; then the CampanianMaastrichtian boundary is 1 to 2 Ma younger than 72 Ma. In agreement with Kennedy et al. (1992) it is reasonable to favour the second situation: the specimens found are near the top of the range of A^. hyatti. This is because in Poland, A^. hyatti mostly occurs in the last third of the range of the ammonite Jeletzkytes nodosus. In the Western Interior, /. nodosus is also present and its range covers all the biozones from the B. compressus to the B. jenseni (Baadsgaard et al., 1993, note that this taxon has not been found from the B. jenseni zone in Canada). It can be postulated that the same relationship exists in the USA and Poland and thus the last occurrence of A^. hyatti would lie near the top of the B. jenseni zone (dotted area 2 in figure 1). If all the above hypotheses are correct, the LO of A^. hyatti is 72.0 Ma old with an uncertainty of less than 1 Ma. Considering the well-known ammonite record observed at Tercis, there is a strong probability for the LO of A^. hyatti to be sub-contemporaneous with the Campanian-Maastrichtian stage boundary. The resulting age is in agreement with previous estimates at about 72 Ma (partly from glaucony ages, Odin & Odin, 1990) and 72.0 ±0.5 (Odin, 1994). 3. Timing of upper Campanian-lower Maastrichtian horizons in European sections The correlation between the sections at Tercis, in the Bottaccione Gorge, and in Western Interior has been discussed by Lewy & Odin (this volume, chap. B2d). Five key horizons are available for the connection of the two European sections. They are numbered 1 to 5 (horizontal arrows 2, 3, and 4 in figure 2). The numerical scale shown in the column North America is directly derived from radiometric
779 dates obtained from the regional biozonal scheme in Western Interior. The connection between the American scale and the two European series is done via the Tercis section at a single point, which is the top of the A^. hyatti range.
Depositional mean rates are an independent way to establish a continuous correlation between the two European sections. Depositional rate and the sedimentary breaks are discussed above for the succession at Tercis (figure 3 in chap. Blc): there is
Fig. 2. Geochronological calibration of the upper Campanian-lower Maastrichtian European sedimentary succession. The magneto-biostratigraphic correlation between Tercis and the Bottaccione is after Lewy & Odin (chap. B2d); it is consistent with the mean accumulation rates of compacted sediments in the two areas when the Tercis section is locally expanded (hatched) where slightly condensed deposition has been documented by glaucony content (chap. Bla). The connection to the American series is through the LO of A^. hyatti as shown in figure 1. The approximate relative durations of the Western Interior ammonite biozones are those documented by the radio-isotopic ages quoted by Hicks et al. (1999 for black circles); for the black square, see figure 1. The calibrated American occurrence of Radotruncana calcarata (75.2 Ma) is from Obradovich et al. (1990).
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probably no geologically significant break of deposition in the succession at Tercis and the depositional rate is rather constant, each metre of sediment generally representing 40 ka of deposition (25 m/Ma). However, some beds represent slower deposition; the two main ones are at about level 45 and at bed 66.5-67.3; others might be present at the base of the section (level -1 and level +6) (beds shown hashed infigure2) and possibly around level 84 and level 98. In the Apennines, PremoH Silva & Sliter (1994) suggested that the Campanian beds were regularly deposited at a rate of 9.3 m/Ma and the Maastrichtian ones at a rate of 6.4 m/Ma. Let us consider the rate of 9.3 m/Ma for the Campanian interval between 225 m and 327 m; the stage was deposited between 83 Ma and 72 Ma (the preferred ages of the base and the top of the Campanian stage, respectively Odin, 1994); the mean rate of deposition is thus: 102/11 = 9.3 m/Ma. The uncertainty on this mean rate includes the one on the location of the boundaries (about 5% of the thickness) and that on the duration of the stage (about 5% of the duration); this means that the rate is known with a 10% precision and can be realistically written 9 ± 1 m/Ma. Let us similarly consider the above-quoted rate of 6.4 m/Ma for the Maastrichtian interval between 327 m and 372 m which covers the whole Maastrichtian between 72 Ma and 65 Ma; the mean rate is: 45/7 = 6.4 m/Ma. Taking uncertainties into consideration, this arithmetic value must be turned into a more realistic estimate of 6.5 ± 1 m/Ma. Concerning the Maastrichtian interval, Gardin, del Planta et al. (this volume, chap. E4) quote Chauris et al. who have shown that about 10 m of Maastrichtian deposits were missing in the upper portion of the Maastrichtian stage due to a fault not known from the previous authors. The Maastrichtian arithmetic mean of the depositional rate thus becomes: (45 + 10 m)/7 Ma = 7.9 m/Ma. Gardin, del Planta et al. also suggest that this is a minimum due to a probable sedimentary break at the top of the Maastrichtian as documented by the lack of deposition of the Micula prinsii-hcaring beds (which might represent several 0.1 Ma) which are usually deposited elsewhere in the Tethys (Odin, Desreumaux et al., chap. E5b). In addition, the Campanian-Maastrichtian boundary as defined
at Tercis would correlate to about level 320 m and not 327 m as suggested by Premoli Silva and Sliter (1994). Wefinallyreach the arithmetic mean rate of [(372-320)+ 10]/7 = 8.9 m/Ma, which is a minimum for the Maastrichtian stage. The Campanian arithmetic mean rate must also be recalculated in the light of the new location of the stage boundary at 320 m; it becomes (320-225)/ll = 8.6 m/Ma. Taking into account the uncertainty on these arithmetic values, the simple calculations above demonstrate that there is no significant difference in the depositional rate of the Campanian or the Maastrichtian stages in the Apennines succession. Considering the nearly constant mean rate of deposition accepted in the Apennines and sub constant at Tercis, figure 2 is drawn with an assumed similar time scale, each Ma corresponding to 9 m in the Bottaccione section and to 25 m in the section at Tercis. The correspondence has been improved considering that the Bottaccione section had a constant rate and that the section at Tercis included some portions with slower deposition (therefore, a smaller thickness corresponds to 1 Ma); thus hatched portions have been added to Tercis to bring biostratigraphical correlation lines 2 and 3 at the same relative level. The result of this exercise is that i- key horizons 4 and 5 do not need improvement; ii- about 0.4 Ma seems to be missing between levels 60 and 80 at Tercis (between key horizons 4 and 3); and also iiiabout 0.1 to 0. 2 Ma missing between levels 40 and 60. If the FO of Rucinolithus magnus is synchronous in the two successions, it would also seem necessary to introduce about 0.2 Ma between levels 10 and 40 at Tercis, but the sedimentological approach has only detected a slightly condensed deposition a few metres below. These conclusions are correct if there is no local decrease (or increase) in the depositional rate in the corresponding interval in the Apennines. 4. Location of the Campanian-Maastrichtian transition in the Bottaccione Gorge The point indirectly dated at 72.0 ± 0.5 Ma at Tercis (the Campanian-Maastrichtian boundary defined at Tercis at level 115.2) can be correlated to the
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Bottaccione Gorge section considering that depositions above and below the boundary have proportional thickness. The bracketing key horizons 4 and 5 shown in figure 2 are parallel and horizontal, it is thus suggested to draw the same arrow at the level of the boundary, which leads to a correspondence between the boundary at Tercis and the middle of the normal magnetozone 32N2 between 319 and 320 m in the Bottaccione Gorge. This correlation differs from the one suggested by Premoli Silva and Sliter (1994) by 7 metres, an interval which represents 0.5-1 Ma deposition. Another (rough and less constrained) way for correlation is to use the total range of Q. trifidum evidenced in the two sections. At Tercis the stage boundary is located in the upper portion of the range of Q. trifidum at about 70 to 80% of the total duration; in the Bottaccione Gorge section, 75% of the duration of the total range of Q. trifidum corresponds to about 320 m and this perfectly agrees with the former estimate. The uncertainty in the proposed correlation for the Campanian-Maastrichtian boundary comprises i- the uncertainty in the location of the bio-horizons in the Bottaccione section, ii- the corresponding ones at Tercis, iii- the uncertainty about the contemporaneous character of the bio-horizons recorded at Tercis and in the Apennines, and iv- the uncertainty about the constancy of the depositional rate in the interval where interpolation is undertaken. Concerning point i, sampling was made at 1 metre intervals in the Bottaccione section, so that each bio-horizon is located at ±0.5 to 1 metre, that is about ±0.1 Ma. At Tercis, the overall uncertainty in the location of the bio-horizons is known from the comparison of several studies by different experts; it is well constrained (for key correlation 5) at ± 2 to 3 metres, which represents ±0.1 Ma. The uncertainty about the contemporaneous character of the biostratigraphic events in the two sections cannot be estimated with confidence. Three events (LO of A. parous constrictus, LO of Q. trifidum and LO of Q. gothicum) are combined to infer the younger key correlation 5 and this combination corresponds to a 4th bio-horizon which is the LO of A. parous parcus. At Tercis, the maximum interval between the LO of Q. gothicum and the LO of A. p.
constrictus is about 30 metres, which represents about 1 Ma of deposition. In the Apennines, the same interval is 3 (± 1) m thick and thus represents 0.3 Ma. This difference suggests that some biohorizons are reproduced with a precision poorer than about 0.4 Ma. In contrast, the uncertainty in the location of the magnetostratigraphic signal (if correctly identified) is negligible because a palaeomagnetic reversal is contemporaneous everywhere in our planet (± 10 to 20 ka: the possible duration of the reversal -i- the time needed for recording in the sediment). Finally, the interpolation procedure uncertainty is difficult to estimate. However, the general constancy of the sedimentological character of the two involved sections and, thus, their quality as series accepting application of such an interpolation procedure suggest that this uncertainty is small. In particular, the constant period of the rhythmic deposition at Tercis is good evidence of constancy. In short, the total uncertainty can be (approximately and optimistically) estimated by adding ±0.1 to ± 0.2 Ma for the location of the biohorizons and ±0.1 to ±0.2 Ma for the interpolation procedure. It is thus suggested that the CampanianMaastrichtian boundary as defined at Tercis can be correlated with 320 m in the middle of the normal magnetozone identified as 32 N2 in the Bottaccione section with a precision probably better than ± 3 metres. 5. Conclusion The numerical age of the stage boundary defined at Tercis between levels 115 and 116 can be estimated by biostratigraphical correlation with the Western Interior sedimentary basin of North America. There, a number of ammonite biozones have been dated using the geochronological approach. The key correlation is done via the presence of A^. hyatti as previously proposed by Kennedy et al. (1992) but using a more suitable numerical age provided by Baadsgaard et al. (1993). The resulting age falls in the 72.0 ± 0.5 Ma interval. The mean depositional rates of the successions at Tercis and in the Bottaccione Gorge can plausibly be calculated considering their sedimentologically founded constancy. The mean rates of deposition
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appear to be constant in the Campanian and the Maastrichtian portions of the succession in the two areas. It is higher at Tercis (40 m/Ma) than in the Apennines (9 m/Ma); the uncertainty on the latter is 10% of the estimate. Using these rate estimates in the two sections, the selected key microfossil biohorizons appear to be entirely contemporaneous if one assumes that some portions at Tercis are slightly condensed compared to the Bottaccione Gorge. The sedimentological information suggests that one condensed portion can be located in the interval 66-68 and the other at about level 45. No sedimentary break longer than about 0.1 Ma would be present above level 70 in the main section at Tercis. Consistent sedimentological, biostratigraphical, magnetostratigraphical, and geochronological data allow to propose continuous correlation of the
Campanian-Maastrichtian boundary interval at Tercis with the auxiliary section in the Bottaccione Gorge. There, the stage boundary falls at metre level 320 with a ±3 metres uncertainty. This corresponds to the basal part of the Italian Gansserina gansseri zone and mid portion of the magnetozone of normal polarity 32 N2. Acknowledgements Correlating different sections, difficult as it is, has been considerably helped by e-mail exchanges with Zeev Lewy and oral discussions with Silvia Gardin over the past few months. M. A. Lamaurelle is thanked for improvement of the previous version of this chapter and relevant discussion. (Prepared: January 2000; revised: May 2000)
CHAPTER E5c
Numerical age calibration of the Campanian-Maastrichtian succession at Tercis les Bains (Landes, France) and in the Bottaccione Gorge (Italy) G. S. Odin
Sommaire L'age numerique de la limite Campanien-Maastrichtien definie a Tercis (cote 115,2) peut etre estime par correlation avec le bassin du Western Interior en Amerique du Nord ou les bentonites ont permis de mesurer l'age des biozones d'ammonites regionales. Du fait de I'endemisme tres fort de ces faunes, le seul lien disponible avec Tercis est le niveau de disparition de 1'ammonite Nostoceras hyatti repere par Kennedy et al. (1992) par rapport aux zones regionales. Ces auteurs ont propose, pour ce bio-horizon, un age numerique fonde sur les donnees disponibles a I'epoque. On doit preferer aujourd'hui le meilleur ancrage numerique etabli par Baadsgaard et al. (1993) qui ont mis en oeuvre plusieurs methodes d'investigation pour dater un niveau tres proche du bio-horizon au Canada. Cet ancrage conduit, par une courte extrapolation, a proposer un age de 72,0 ±0,5 Ma pour I'extinction americaine de A^. hyatti, evenement probablement concomitant de la limite d'Etage a Tercis. L'age connu des limites d'Etage du Campanien (83 a 72 Ma) et du Maastrichtien (72 a 65 Ma) permet de calculer des taux de sedimentation moyens apres compaction de 40 m/Ma a Tercis (ainsi qu'il a ete etabh d'apres I'etude des rythmes et epaisseurs dans ce volume) et de 9 m/Ma dans les Gorges du Bottaccione. Dans ce dernier cas, il est demontre ici que le taux de sedimentation est semblable pour les deux Etages aux incertitudes pres ( ± 1 m/Ma). Ceci infirme des estimations
anterieures qui ne semblaient pas verifiees par les observations sedimentologiques. Leur modification est rendue necessaire par la recente mise en evidence d'un hiatus tectonique dans le Maastrichtien de la section italienne et, comme il est ecrit plus loin, par un leger deplacement de la limite d'Etage adoptee anterieurement en Italic. L'utilisation de ces taux de sedimentation permet egalement, en combinaison avec 1'information biostratigraphique (foraminiferes et nannofossiles calcaires), d'estimer I'ampleur des condensations locahsees a Tercis lors de I'etude sedimentologique. Par comparaison avec la succession des Apennins, consideree comme plus regulierement deposee, la condensation la plus nette se situerait dans I'intervalle compris entre les cotes 66.5 et 67.3, qui representerait 0,4 a 0,5 Ma. Une condensation moins importante se situerait vers la cote 45. II n'y aurait pas de lacune ni de condensation decelables au-dessus de la cote 70 dans la section de la grande carriere moderne de Tercis. La combinaison des informations sedimentologiques, biostratigraphiques, magnetostratigraphiques et geochronologiques permet une correlation entre la limite d'Etage definie a Tercis et la succession des Apennins. La limite d'Etage doit se situer entre le sommet de I'anomalie 33 et la disparition de Aspidolithus parens parens et tombe a la cote 320 m avec une incertitude de I'ordre de 3 metres. Cette localisation, anterieure d'un peu moins d'l Ma a celle adoptee auparavant, correspond a la partie moyenne de la magnetozone
776
normale 32N2 ou encore a la partie basale de la zone a Gansserina gansseri des Apennins. A la suite de ces considerations, on pent calibrer les sections de Tercis et d'Italic. De la base au sommet, les ages de 77, 76, 75 . . . 70 Ma se placeraient vers les cotes 7, 29, 51, banc a inocerame, 90, limite d'Etage, 140 et 165 a Tercis et, en Italic, tons les 9 metres de part et d'autre de la limite datee de 72 Ma et situee vers 320 m. 1. Introduction Reaching a realistic estimate of the numerical age of the Campanian-Maastrichtian boundary requires considering direct radiometric measurements. Most of the radiometric ages obtained from Upper Cretaceous fossiliferous deposits in the world come from the Western Interior basin of North America. However, the macro-faunal endemism and the absence of pelagic microfauna do not allow to directly correlate these numerical ages with other basins using the biostratigraphic tool only. Other considerations can be taken into account, which is done in this chapter where biostratigraphical (ammonites and planktonic microfossils), sedimentological (mean rates of deposition), and magnetostratigraphical information are combined with geochronological calibration, allowing to estimate the numerical age of the stage boundary newly defined at Tercis. 2. Numerical age of the LO of Nostoceras hyatti in North America Kennedy et al. (1992) used the discovery of a specimen of Nostoceras hyatti to infer a numerical age for the Campanian-Maastrichtian boundary as accepted in northern Germany. The resulting age "71.3-71.4 Ma" has been repeatedly quoted in the literature. However, the proposal suffers from weaknesses essentially (though not only) linked to the lack of accompanying estimate of the uncertainty on this value. 1- The quotation of A^. hyatti is based on a single find of the taxon lying together with common Baculites jenseni in a "ferruginous concretion in the Pierre Shale" (Walsenburg, Colorado); "more than a dozen helices in various states of preservation, the largest consisting of parts of three whorls".
Four specimens are illustrated; they are portions of spires (phragmocone) which give no information on the body chamber. It is difficult to compare these specimens from Western Interior with the material from Europe in general, and more precisely with that collected from Tercis where the phragmocone is usually absent but the body chamber present. The phragmocone description might seem different for the material from Western Interior and for that from Tercis. In the American specimens, "the spire is low with a large apical angle" and "most ribs bear small, sharp, transversally elongated tubercles" (Kennedy et al., 1992); the specimens from Tercis show "an apical angle of less than 90°" and "narrow ribs .. . link in pairs to rather coarse tubercles . . . " (Hancock & Kennedy, 1993). 2- Campanian Baculites biozones from Western Interior (figure 1) have been dated geochronologically. A mean duration of about 0.5 Ma can be calculated for these zones. Using this mean duration, interpolated ages can be calculated for any point between the dated levels. However, Kennedy et al. note that the precise origin within the B, jenseni zone of the find at Walsenburg is not known. The interpolation will thus induce an uncertainty of ±0.3 Ma. Similarly, the dated bentonites are not precisely located in their Baculites zones, which adds another uncertainty of about ± 0.3 Ma. The total uncertainty on the interpolation procedure is thus 0.5 Ma or so (without considering the possible differences in duration for individual zones). 3- The radiometric ages quoted by Kennedy & al. (1992) from Western Interior are 70.1 ±0.7 (la) Ma {B. grandis zone, NDS 104) and 73.2 ± 0.7 (la) Ma {B. compressus zone, NDS 105). The mentioned uncertainty is the analytical standard deviation and a more realistic 2a uncertainty should be quoted; it should be added to the uncertainty inherent to the interpolation procedure. Also note that only biotite separates have been dated in these two bentonites and that the old one has a low K content (4.4%), which denotes an alteration following crystallisation; therefore, the reliability of the age provided by the latter is low (in brackets, figure 1). In addition, Kennedy et al. located this date in the B, compressus zone while Odin & Obradovich
777
(NDS 105) quoted it from 6 m below the B. compressus zone with an age of 73.1 ±1.4 Ma. More reliable ages (based on K-rich biotite and sanidine) are available from zones dated below; figure 1 gives the weighted mean ages calculated from the multiple biotite dates summarised by Odin & Obradovich (1982: abstracts NDS 104 and 105). 4- Because the key taxon has been found in a single place, its precise location within the full range of A^. hyatti is not firmly established. At Tercis, the range can be assumed to be 20 or 47 metres thick depending on whether the specimens from level 67 are conspecific with A^. hyatti or not. The corresponding deposition duration is 1 or 2 Ma. Therefore, the place of the stage boundary versus the calibrated key fossil will add a new uncertainty which can be as high as ± 1 Ma; in addition, this is correct only if the A^. hyatti range at
Tc Tp •
Measured Ma ages (Ma) 70-
Ammonite zones
70.1 ±1.4
B. grandis
Tercis is contemporaneous with that in Western Interior of USA. Note that it is accepted here (like in Kennedy et al., 1992, for NW Europe) that the evolutionary LO (extinction) of A^. hyatti as documented at Tercis is a good approximation of the chronostratigraphic boundary between the Campanian and the Maastrichtian. As a result, the age estimate at "71.3-71.4" proposed by the previous authors has a combined uncertainty of about ± 2 Ma and thus poorly constrains the LO of A^. hyatti. This estimate can be improved using the cahbration available from Canada. The best and nearest age calibration of the Western Interior series known to us was presented by Baadsgaard et al. (1993). According to these authors, the Snakebite bentonite in Saskatchewan (Canada) is located at the top of the Baculites reesidei zone (figure 1) immediately below the B.
1
Western Interior (Colorado)
Last occurrence N. hyatti
70B. baculus
71-
B. eliasi 7172-
B. jenseni |»72.5
±0A ]i.||li..
72-
7373.7 ±1.0
75-
74.3 ±1.3 73.6 ±1.3
N. hyatti single find ± | r
B. reesidei
B. compressus (73.1 ±1.4)
74-
T
(1)
1 Jeietzkytes nodosus 1 present
B. cuneatus
73-
74-
i
f
D. cheyennense E. jenneyi D. stevensoni D. nebrascense
t 6
B. scotti
Fig. 1. Numerical age of the last occurrence of Nostoceras hyatti (near the Campanian-Maastrichtian boundary) according to radiometric datings in Western Interior of USA and Canada via the presence of ammonite key taxa; B.= Baculites, D.=Didymoceras, E. = Exiteloceras. The mean biozone duration ( - 0 . 5 Ma) is according to the original dating by Obradovich & Cobban (1975; summarised in Odin & Obradovich, 1982) with a scale shown in column Tp. Ages are recalibrated (Tc) using the age at 72.5 ±0.4 (black dot) from Canada (Baadsgaard et al., 1993). The most probable age of the stage boundary (2) results from biostratigraphical relationships; the maximum age (1) is better constrained than the minimum age (3). All ± are intralaboratory analytical uncertainties with a 95% confidence level.
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jenseni zone. Concordant U-Pb zircon ages, a RbSr isochron age, and K-Ar weighted mean ages on biotite and sanidine (irradiation technique, laser heating procedure on ten separates using Fish Canyon tuff sanidine and hornblende MMhb-1 as monitors with ages assumed to be 27.84 and 520.4 Ma respectively) have been obtained. Calculated ages are: a U-Pb concordant zirconcrystal average age at 72.5 ± 0.4 (2a) Ma; a Rb-Sr plagioclase, sanidine, biotite isochron age at 72.5 ± 0.2 (2a) Ma, a sanidine K-Ar weighted mean age at 72.5 ±0.5 (2a) Ma, a biotite K-Ar weighted mean age at 72.6 ± 0.4 (2a) Ma. The consistency is surprisingly good (order of magnitude \%c) considering the fact that decay constants of the naturally instable isotopes used are known with a 10 time-poorer precision (Odin et al., 1999). A pooled age at 72.5 ± 0.4 Ma is suggested for the top of the B. reesidei zone. This age is better than the one used in 1992 which came from the original study by Obradovich & Cobban (1975). Mean biozone durations can be used to interpolate between geochronologically dated levels (Odin, 1992a). The data summarised by Odin & Obradovich (1982) suggest that the mean biozone duration may be around 0.5 Ma (column Tp). In fact, figure 1 shows that the base of the B. jenseni zone is dated and that the only problem is to propose an age for the top of the same zone. We can use the durations suggested by the data of Odin & Obradovich (preliminary scale in column Tp, figure 1) and recalibrate this scale using a better anchor at 72.5 Ma for the top of the B. reesidei zone. This corrected scale is shown in column Tc; note that the previous datings appear younger than the values in this scale but remain consistent when taking into account a realistic analytical uncertainty. Let us consider now the combinations of i- the uncertainty in the location of the specimens of A^. hyatti found inside their range and ii- the location of the specimens inside the B. jenseni zone. If the specimens found in Walsenburg correspond to the top of the range of the taxon and come from the base of the B. jenseni zone, then the Campanian-Maastrichtian boundary is at the base of this zone and its age is 72.5 ±0.4 Ma; this is the maximum age.
If the specimens found in Walsenburg correspond to the top of the range of the taxon and come from the top of the B. jenseni zone, then the Campanian-Maastrichtian boundary is at the top of this zone and its age is about 72.0 Ma. In the most unfavourable circumstance, the specimens found in Walsenburg correspond to the base of the range of the taxon and come from the top of the B. jenseni zone; then the CampanianMaastrichtian boundary is 1 to 2 Ma younger than 72 Ma. In agreement with Kennedy et al. (1992) it is reasonable to favour the second situation: the specimens found are near the top of the range of A^. hyatti. This is because in Poland, A^. hyatti mostly occurs in the last third of the range of the ammonite Jeletzkytes nodosus. In the Western Interior, /. nodosus is also present and its range covers all the biozones from the B. compressus to the B. jenseni (Baadsgaard et al., 1993, note that this taxon has not been found from the B. jenseni zone in Canada). It can be postulated that the same relationship exists in the USA and Poland and thus the last occurrence of A^. hyatti would lie near the top of the B. jenseni zone (dotted area 2 in figure 1). If all the above hypotheses are correct, the LO of A^. hyatti is 72.0 Ma old with an uncertainty of less than 1 Ma. Considering the well-known ammonite record observed at Tercis, there is a strong probability for the LO of A^. hyatti to be sub-contemporaneous with the Campanian-Maastrichtian stage boundary. The resulting age is in agreement with previous estimates at about 72 Ma (partly from glaucony ages, Odin & Odin, 1990) and 72.0 ±0.5 (Odin, 1994). 3. Timing of upper Campanian-lower Maastrichtian horizons in European sections The correlation between the sections at Tercis, in the Bottaccione Gorge, and in Western Interior has been discussed by Lewy & Odin (this volume, chap. B2d). Five key horizons are available for the connection of the two European sections. They are numbered 1 to 5 (horizontal arrows 2, 3, and 4 in figure 2). The numerical scale shown in the column North America is directly derived from radiometric
779 dates obtained from the regional biozonal scheme in Western Interior. The connection between the American scale and the two European series is done via the Tercis section at a single point, which is the top of the A^. hyatti range.
Depositional mean rates are an independent way to establish a continuous correlation between the two European sections. Depositional rate and the sedimentary breaks are discussed above for the succession at Tercis (figure 3 in chap. Blc): there is
Fig. 2. Geochronological calibration of the upper Campanian-lower Maastrichtian European sedimentary succession. The magneto-biostratigraphic correlation between Tercis and the Bottaccione is after Lewy & Odin (chap. B2d); it is consistent with the mean accumulation rates of compacted sediments in the two areas when the Tercis section is locally expanded (hatched) where slightly condensed deposition has been documented by glaucony content (chap. Bla). The connection to the American series is through the LO of A^. hyatti as shown in figure 1. The approximate relative durations of the Western Interior ammonite biozones are those documented by the radio-isotopic ages quoted by Hicks et al. (1999 for black circles); for the black square, see figure 1. The calibrated American occurrence of Radotruncana calcarata (75.2 Ma) is from Obradovich et al. (1990).
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probably no geologically significant break of deposition in the succession at Tercis and the depositional rate is rather constant, each metre of sediment generally representing 40 ka of deposition (25 m/Ma). However, some beds represent slower deposition; the two main ones are at about level 45 and at bed 66.5-67.3; others might be present at the base of the section (level -1 and level +6) (beds shown hashed infigure2) and possibly around level 84 and level 98. In the Apennines, PremoH Silva & Sliter (1994) suggested that the Campanian beds were regularly deposited at a rate of 9.3 m/Ma and the Maastrichtian ones at a rate of 6.4 m/Ma. Let us consider the rate of 9.3 m/Ma for the Campanian interval between 225 m and 327 m; the stage was deposited between 83 Ma and 72 Ma (the preferred ages of the base and the top of the Campanian stage, respectively Odin, 1994); the mean rate of deposition is thus: 102/11 = 9.3 m/Ma. The uncertainty on this mean rate includes the one on the location of the boundaries (about 5% of the thickness) and that on the duration of the stage (about 5% of the duration); this means that the rate is known with a 10% precision and can be realistically written 9 ± 1 m/Ma. Let us similarly consider the above-quoted rate of 6.4 m/Ma for the Maastrichtian interval between 327 m and 372 m which covers the whole Maastrichtian between 72 Ma and 65 Ma; the mean rate is: 45/7 = 6.4 m/Ma. Taking uncertainties into consideration, this arithmetic value must be turned into a more realistic estimate of 6.5 ± 1 m/Ma. Concerning the Maastrichtian interval, Gardin, del Planta et al. (this volume, chap. E4) quote Chauris et al. who have shown that about 10 m of Maastrichtian deposits were missing in the upper portion of the Maastrichtian stage due to a fault not known from the previous authors. The Maastrichtian arithmetic mean of the depositional rate thus becomes: (45 + 10 m)/7 Ma = 7.9 m/Ma. Gardin, del Planta et al. also suggest that this is a minimum due to a probable sedimentary break at the top of the Maastrichtian as documented by the lack of deposition of the Micula prinsii-hcaring beds (which might represent several 0.1 Ma) which are usually deposited elsewhere in the Tethys (Odin, Desreumaux et al., chap. E5b). In addition, the Campanian-Maastrichtian boundary as defined
at Tercis would correlate to about level 320 m and not 327 m as suggested by Premoli Silva and Sliter (1994). Wefinallyreach the arithmetic mean rate of [(372-320)+ 10]/7 = 8.9 m/Ma, which is a minimum for the Maastrichtian stage. The Campanian arithmetic mean rate must also be recalculated in the light of the new location of the stage boundary at 320 m; it becomes (320-225)/ll = 8.6 m/Ma. Taking into account the uncertainty on these arithmetic values, the simple calculations above demonstrate that there is no significant difference in the depositional rate of the Campanian or the Maastrichtian stages in the Apennines succession. Considering the nearly constant mean rate of deposition accepted in the Apennines and sub constant at Tercis, figure 2 is drawn with an assumed similar time scale, each Ma corresponding to 9 m in the Bottaccione section and to 25 m in the section at Tercis. The correspondence has been improved considering that the Bottaccione section had a constant rate and that the section at Tercis included some portions with slower deposition (therefore, a smaller thickness corresponds to 1 Ma); thus hatched portions have been added to Tercis to bring biostratigraphical correlation lines 2 and 3 at the same relative level. The result of this exercise is that i- key horizons 4 and 5 do not need improvement; ii- about 0.4 Ma seems to be missing between levels 60 and 80 at Tercis (between key horizons 4 and 3); and also iiiabout 0.1 to 0. 2 Ma missing between levels 40 and 60. If the FO of Rucinolithus magnus is synchronous in the two successions, it would also seem necessary to introduce about 0.2 Ma between levels 10 and 40 at Tercis, but the sedimentological approach has only detected a slightly condensed deposition a few metres below. These conclusions are correct if there is no local decrease (or increase) in the depositional rate in the corresponding interval in the Apennines. 4. Location of the Campanian-Maastrichtian transition in the Bottaccione Gorge The point indirectly dated at 72.0 ± 0.5 Ma at Tercis (the Campanian-Maastrichtian boundary defined at Tercis at level 115.2) can be correlated to the
781
Bottaccione Gorge section considering that depositions above and below the boundary have proportional thickness. The bracketing key horizons 4 and 5 shown in figure 2 are parallel and horizontal, it is thus suggested to draw the same arrow at the level of the boundary, which leads to a correspondence between the boundary at Tercis and the middle of the normal magnetozone 32N2 between 319 and 320 m in the Bottaccione Gorge. This correlation differs from the one suggested by Premoli Silva and Sliter (1994) by 7 metres, an interval which represents 0.5-1 Ma deposition. Another (rough and less constrained) way for correlation is to use the total range of Q. trifidum evidenced in the two sections. At Tercis the stage boundary is located in the upper portion of the range of Q. trifidum at about 70 to 80% of the total duration; in the Bottaccione Gorge section, 75% of the duration of the total range of Q. trifidum corresponds to about 320 m and this perfectly agrees with the former estimate. The uncertainty in the proposed correlation for the Campanian-Maastrichtian boundary comprises i- the uncertainty in the location of the bio-horizons in the Bottaccione section, ii- the corresponding ones at Tercis, iii- the uncertainty about the contemporaneous character of the bio-horizons recorded at Tercis and in the Apennines, and iv- the uncertainty about the constancy of the depositional rate in the interval where interpolation is undertaken. Concerning point i, sampling was made at 1 metre intervals in the Bottaccione section, so that each bio-horizon is located at ±0.5 to 1 metre, that is about ±0.1 Ma. At Tercis, the overall uncertainty in the location of the bio-horizons is known from the comparison of several studies by different experts; it is well constrained (for key correlation 5) at ± 2 to 3 metres, which represents ±0.1 Ma. The uncertainty about the contemporaneous character of the biostratigraphic events in the two sections cannot be estimated with confidence. Three events (LO of A. parous constrictus, LO of Q. trifidum and LO of Q. gothicum) are combined to infer the younger key correlation 5 and this combination corresponds to a 4th bio-horizon which is the LO of A. parous parcus. At Tercis, the maximum interval between the LO of Q. gothicum and the LO of A. p.
constrictus is about 30 metres, which represents about 1 Ma of deposition. In the Apennines, the same interval is 3 (± 1) m thick and thus represents 0.3 Ma. This difference suggests that some biohorizons are reproduced with a precision poorer than about 0.4 Ma. In contrast, the uncertainty in the location of the magnetostratigraphic signal (if correctly identified) is negligible because a palaeomagnetic reversal is contemporaneous everywhere in our planet (± 10 to 20 ka: the possible duration of the reversal -i- the time needed for recording in the sediment). Finally, the interpolation procedure uncertainty is difficult to estimate. However, the general constancy of the sedimentological character of the two involved sections and, thus, their quality as series accepting application of such an interpolation procedure suggest that this uncertainty is small. In particular, the constant period of the rhythmic deposition at Tercis is good evidence of constancy. In short, the total uncertainty can be (approximately and optimistically) estimated by adding ±0.1 to ± 0.2 Ma for the location of the biohorizons and ±0.1 to ±0.2 Ma for the interpolation procedure. It is thus suggested that the CampanianMaastrichtian boundary as defined at Tercis can be correlated with 320 m in the middle of the normal magnetozone identified as 32 N2 in the Bottaccione section with a precision probably better than ± 3 metres. 5. Conclusion The numerical age of the stage boundary defined at Tercis between levels 115 and 116 can be estimated by biostratigraphical correlation with the Western Interior sedimentary basin of North America. There, a number of ammonite biozones have been dated using the geochronological approach. The key correlation is done via the presence of A^. hyatti as previously proposed by Kennedy et al. (1992) but using a more suitable numerical age provided by Baadsgaard et al. (1993). The resulting age falls in the 72.0 ± 0.5 Ma interval. The mean depositional rates of the successions at Tercis and in the Bottaccione Gorge can plausibly be calculated considering their sedimentologically founded constancy. The mean rates of deposition
782
appear to be constant in the Campanian and the Maastrichtian portions of the succession in the two areas. It is higher at Tercis (40 m/Ma) than in the Apennines (9 m/Ma); the uncertainty on the latter is 10% of the estimate. Using these rate estimates in the two sections, the selected key microfossil biohorizons appear to be entirely contemporaneous if one assumes that some portions at Tercis are slightly condensed compared to the Bottaccione Gorge. The sedimentological information suggests that one condensed portion can be located in the interval 66-68 and the other at about level 45. No sedimentary break longer than about 0.1 Ma would be present above level 70 in the main section at Tercis. Consistent sedimentological, biostratigraphical, magnetostratigraphical, and geochronological data allow to propose continuous correlation of the
Campanian-Maastrichtian boundary interval at Tercis with the auxiliary section in the Bottaccione Gorge. There, the stage boundary falls at metre level 320 with a ±3 metres uncertainty. This corresponds to the basal part of the Italian Gansserina gansseri zone and mid portion of the magnetozone of normal polarity 32 N2. Acknowledgements Correlating different sections, difficult as it is, has been considerably helped by e-mail exchanges with Zeev Lewy and oral discussions with Silvia Gardin over the past few months. M. A. Lamaurelle is thanked for improvement of the previous version of this chapter and relevant discussion. (Prepared: January 2000; revised: May 2000)