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| Titel |
Timing of Early Aptian demise of northern Tethyan carbonate platforms - chemostratigraphic versus biostratigraphic evidence |
| VerfasserIn |
Stefan Huck, Adrian Immenhauser, Ulrich Heimhofer, Niels Rameil |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034732
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| Zusammenfassung |
| A lively controversy still exists between different authors dealing with the timing of northern
Tethyan platform drowning and the Early Aptian oceanic anoxic event (OAE 1a). To the
present day, there is no consensus if the OAE 1a black shales must be attributed
to the Deshayesites weissi or the Deshayesites deshayesi zone (see discussion in
Moreno-Bedmar et al., 2009). OAE 1a black shale deposition has been traditionally
attributed to the Deshayesites weissi zone (Gradstein et al., 2004). Despite this
disagreement about the biostratigraphic timing, several authors postulate a relation
between biotic perturbations and environmental changes linked to OAE 1a, e. g. the
disappearance of coral-rudist reefs related with the demise of the northern Tethyan Urgonian
platforms in the Helvetic Alps (Weissert et al., 1998; Föllmi et al., 2008). In the
central and southern Tethyan realm (Istria, Oman), OAE 1a is likely expressed as
the transient mass occurrence of microencrusters (Lithocodium-Bacinella) and the
coeval demise of the characteristic mid-Cretaceous framework-builders (rudists,
corals). Chemostratigraphic data indicate that these microbial blooms coincide
with the Deshayesites weissi zone (Huck et al., 2010, Rameil et al, 2010). These
observations raise the question whether northern Tethyan platform drowning is coeval to
microbial bloom periods in the central and southern Tethys? The analysis of all
available literature and unpublished evidence demonstrates that well constrained age
data are surprisingly scarce and controversial. The goal of the present research
project is to compile a chemostratigraphic framework for the northern Tethyan
platform drowning (Haute-Savoie, SE France) in order to shed light on the temporal
constraints of platform drowning versus pelagic black shale deposition versus microbial
blooms.
Two Barremian to Aptian shoalwater sections (Cluses section, Grande Forclaz section) in
the Subalpine Chains were investigated applying chemostratigraphy (carbon, strontium)
and detailed sedimentological analysis. The lower part of the studied interval of
both sections comprises limestones rich in rudist bivalves and intercalated oncoidal
beds (including Lithocodium-Bacinella). The upper part consists of open to slightly
protected lagoonal limestones (peloidal-foraminiferal grainstones) alternating with
Orbitolina-rich intervals (Lower Orbitolina limestones?). Rudist shells are well
preserved and relatively common. In the uppermost part, pulsed shedding of silt-sized
siliciclastics is recorded and both sections are finally truncated and capped by the helvetic
Garschella Formation, represented by siliciclastic glauconite-rich sedimentary rocks
alternating with more argillaceous intervals. In essence, this stratigraphic succession is
typical for the drowned Lower Aptian platforms observed along the northern Tethyan
margin. Due to the lack of ammonites and a debated biostratigraphic control based on
orbitolinids, a high-resolution chemostratigraphic framework (carbon and strontium)
for the studied sections is established. Carbon-isotope chemostratigraphy is based
on carbonate bulk samples. The obtained ages derived by 86Sr/87Sr isotope ratios
(Strontium Isotope Stratigraphy, SIS) from screened rudists’ low-Mg calcite are used to
calibrate the carbon isotope stratigraphy as well as to pinpoint the timing of platform
drowning.
With respect to the timescale of Gradstein et al. (2004) preliminary chemostratigraphic
data from Urgonian shoalwater sections in SE France indicate that the platforms at
the northern margin of the Tethys Ocean drowned at the onset of OAE 1a black
shale interval (Deshayesites weissi zone, close to the Aptian/Barremian boundary).
This points to a near-coeval nature of pelagic black shale deposition, Oman and
Istrian Lithocodium-Bacinella facies and platform demise in the northern Tethyan
realm.
References
Föllmi, K.B. (2008) A synchronous, middle Early Aptian age for the demise of the
Helvetic Urgonian platform related to the unfolding oceanic anoxic event 1a (« Selli event »).
Revue de Paléobiologie, 27, 461-468.
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Huck, S., Rameil, N., Korbar, T., Heimhofer, U., Wieczorek, T.D. and Immenhauser, A.
(2010) Latitudinally different responses of Tethyan shoal-water carbonate systems to the
Early Aptian oceanic anoxic event (OAE 1a). Sedimentology (pending acceptance of revised
version).
Moreno-Bedmar, J.A., Company, M., Bover-Arnal, T., Salas, R., Delanoy, G., Martinez,
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carbon isotope stratigraphy and platform drowning events: a possible link? Palaeogeography,
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