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| Titel |
Is these a link between eustatic variations, platform drowning, oceanic anoxic events, and ammonite faunal turnovers ? Case study of the Aptian sediments along the northern Tethyan margin |
| VerfasserIn |
Antoine Pictet, Karl Föllmi, Jorge Spangenberg |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250100418
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| Publikation (Nr.) |
 EGU/EGU2014-16382.pdf |
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| Zusammenfassung |
| The early Aptian witnessed an important episode of paleoenvironmental change, which has
been linked to major marine volcanic activity related to the formation of the Ontong-Java
large igneous province (e.g., Larson and Erba, 1999). This phase culminated in the formation
of hemipelagic and pelagic organic-rich sediments, whereas profound changes are
also observed in shallow-water settings, with the step-by-step disappearance of the
northern Tethyan platform. Results show that the northern Tethyan platform has
passed through three major crises in its evolution during the early Aptian. A first
one started with an emersion phase, marked by a subaerial karstified discontinuity
reported from the middle early Aptian (Deshayesites forbesi or early D. deshayesi
zone). This is directly followed by the drowning of the Urgonian platform along the
northern Tethyan margin, preceding the Selli Episode. The period following this
drowning phase coincides with the negative and the following positive excursions in the
δ13C records and went along with the deposition of the so-called Lower Grünten
Member, which is the result of heterozoan carbonate production and characterized by
increased detrital input. Ammonite fauna witnessed an important diversification
of hemipelagic forms, especially inside the heteromorph Ancyloceratacea. This
radiation is probably linked to the expansion of hemipelagic facies, one of the main
habitats of ammonites. A second phase, reported from the late early Aptian (late D.
deshayesi zone), started with a small drowning event, marked by a firmground and by a
phosphatic enrichment. This stratigraphical layer also corresponds to the establishment
of the anoxic Apparein level. Above, the Upper Grünten Member continues with
heterozoan carbonate production or with glauconitic condensed sediments. The
corresponding δ13C record is a the onset of a long-term decrease. The ammonite
fauna is marked by a first turnover with the disappearance of Deshayesites, and the
appearance of Dufrenoyia, Eotetragonites, Aconoceras, Colombiceras, and other genera
(Daupin, 2002; Dutour, 2005). A third drowning phase, reported from the latest early
Aptian (late D. furcata zone), occurred with the establishment of a second emersion
phase, characterised by a significant karstified discontinuity. Just above, organic-rich
sediments were deposited. This third phase is marked by the complete disappearance
of platforms in favour of marl deposits, and coincides with the emergence of the
anoxic “Niveau Noir level” which is considered as a document of an important
phase of widespread dys- to anaerobic conditions within the Selli Episode (Föllmi,
2012). The ammonite fauna is marked by a second turnover with the replacement of
Dufrenoyia, Cheloniceras, and Pseudohaploceras, by Epicheloniceras, Zucherella,
Gabbioceras, and other genera (Daupin, 2002; Dutour, 2005). In conclusion, the combined
effect of sea-level change and subsequent oceanic anoxia as the consequence of
climatic change is a key element in the platform crises and associated important faunal
turnover.
References
DAUPHIN L. (2002) Litho-, bio-, et chronostratigraphie comparées dans le bassin
vocontien à l’Aptien. Phd Univ. des sciences et technologies – Lille I. DUTOUR, Y. (2005).-
Biostratigraphie, évolution et renouvellements des ammonites de l’Aptien supérieur
(Gargasien) du bassin vocontien (Sud-Est de la France).- Thèse Université Claude
Bernard-Lyon 1, 302 p. (inedit) FÖLLMI, K.B. (2012).- Early Cretaceous life, climate and
anoxia.- Cretaceous Research (2012), doi:10.1016/ j.cretres.2011.12.005 LARSON, R.L.,
ERBA, E., 1999. Onset of the mid-Cretaceous greenhouse in the BarremianeAptian: igneous
events and the biological, sedimentary, and geochemical responses. Paleoceanography 14,
663-678. |
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