Toward the astronomical calibration of the Cretaceous : the Maastricthian (Late Cretaceous) and Cretaceous-Paleogene boundary
Vers une calibration astronomique du Crétacé : le cas du Maastrichtien (Crétacé supérieur) et de la limite Crétacé-Paléogène
Résumé
The Cretaceous time scale is the combination of biostratigraphic, magnetostratigraphic data, and of geomagnetic polarity timescale based on the magnetic anomaly patterns, and constrained by rare radioisotopic dating. The cyclostratigraphic approach, based on the analyse of the sedimentary cycles and the study of their relationship with the orbital parameter evolution, has been applied here in order to re ne the Upper Cretaceous time scale by building an astronomical timescale. Signal processing tools (spectral analysis, amplitude spectrograms) used during the cyclostratigraphic analysis have been tested on the Contessa and Bottaccionne reference sections of Gubbio (Umbria Marche, Italy), and the calcareous marl alternations of Bidart (Basque country, France). The cyclostratigraphic study aiming at the astronomical calibration of the Maastrichtian has been performed on sedimentary series possessing a good recovery of the upper Cretaceous and a good bio and magnetostratigraphic frame : series from ODP leg 122 (Indian Ocean), leg 207 (Equatorial Atlantic) and leg 208 (South Atlantic), and DSDP leg 74 (South Atlantic). The Maastricthian formations on these sites are constituted by calcareous marl and nannofossils ooze. Analyses of the recent ODP sites have been performed on magnetic susceptibility and lightness variations. Sudies of the older ODP and DSDP sites have been performed on variations of the sediments colour, expressed in grey level and obtained from the cores photographs. Cyclostratigraphic studies allowed the detection of the orbital control of the sedimentation on every sedimentary series, with the record of the precession, obliquity, and 100 and 405 kyr eccentricity variations. Variations of the sedimentation rate have been identi ed on each site. Cyclostratigraphic analysis of Bidart section has highlitghted an exceptionnal record of the precession. Study of the Contessa section and hole 762C on an intervalle covering the C31r magnetochron allowed the estimation of this magnetochron duration to 2.08 0.03 Ma. The orbital tuning of every sedimentary series from ODP and DSDP sites has then been performed using the La0X astronomical solution, and based on the 405 kyr eccentricity cycles. The association of the sedimentary series possessing the best record of the orbital control has allowed the creation of a detailed cyclostratigraphic frame. Each 100 and 405 kyr cycles within this frame has been numbered according to its position relative to the Cretaceous- Paleogene boundary (K-PgB). This cyclostratigraphic frame is thus a relative time scale covering about 8 Myr, from the upper Campanian to the K-PgB. The duration of each magnetochron, from the C32r2r to the C29r has then been estimation with an accuracy of 0.03 Ma. This study provide thus the rst constraints on the Mesozoïc oceanic accretion rates based on the orbital parameter record in sedimentary series. The astronomical calibration of the tuned records has been performed using the new astronomical solution La10. Due to the chaotic motions of the planets in the Solar System, accurate estimations of the 405 kyr eccentricity amplitude variations can not be calculated. Therefore, these modulations can not be used to anchor the sedimentary series on the astronomical solution. Two time scales, shifted of a 405 kr eccentricity cycle, have thus been created, taking into account the radioisotopic dating available for the K-PgB. Two sets of ages, estimated with a good accuracy, are proposed for each geological event identi ed in the sedimentary series (bio-events, magnetochron boudaries . . .). The age of the Cretaceous-Paleogene boundary has been estimated to 65.59 0.01 Ma and 66 0.01 Ma. This last age is closer to the estimates obtained by the most recent recalibrations of the 40Ar=39Ar standards. This estimate is the rst dating of the K-PgB obtained thanks to the astronomical calibration of maastrichtian sedimentary series.
L'approche cyclostratigraphique, qui repose sur l'analyse des cyclicités sédimentaires et l'étude de leurs relations avec les évolutions des paramètres orbitaux de la Terre, a été employée ici dans le but de construire une échelle de temps astronomique pour le Maastrichtien. Plusieurs sites ont été étudiés : les forages ODP des legs 122 (marge Nord ouest australienne), 207 (Atlantique équatorial), et 208 (Atlantique sud), le leg DSDP 74X et les coupes à terre situées à Gubbio (Italie) et Bidart (Pays Basque). Les analyses ont portées sur les variations de la susceptibilité magnétique, de la réflectance et des niveaux de gris. L'étude cyclostratigraphique a permis de détecter un contrôle orbital de la sédimentation sur l'ensemble des séries étudiées, avec l'enregistrement de cyclicités correspondants aux variations de l'excentricité à 100 et 405 ka, de l'obliquité et de la précession. Un calage temporel de l'ensemble des séries provenant des forages DSDP et ODP a ensuite été effectué à l'aide de la solution astronomique La0x, sur la base de l'identification des cycles d'excentricité à 405 ka. Ce cadre ainsi créé constitue une échelle de temps relative couvrant 8 Millions d'années, depuis le Campanien supérieur jusqu'à la limite K-Pg. La durée de chaque magnétochrone, du C32r2r au C29r, a ainsi pu être estimée avec une précision de l'ordre de 0.03 Ma. La calibration astronomique des séries calées en temps a été effectuée à l'aide de la nouvelle solution astronomique La10.Deux séries d'âges sont proposées pour les événements géologiques reconnus dans les séries sédimentaires. L'âge de la limite K-Pg a été estimé à ~65.59 Ma ou ~66 Ma.