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| Titel |
Stratigraphic implications of trace element and strontium-isotope analyses of Kimmeridgian shell calcite from the Lower Saxony Basin, Germany |
| VerfasserIn |
Fanfan Zuo, Ulrich Heimhofer, Stefan Huck, Jochen Erbacher, Stephane Bodin |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250148955
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| Publikation (Nr.) |
 EGU/EGU2017-13262.pdf |
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| Zusammenfassung |
| Stratigraphic uncertainties due to the lack of open marine marker fossils (e.g. ammonites)
hamper the precise age assignment and stratigraphic correlation of Kimmeridgian strata
found in the Lower Saxony Basin of Northern Germany. Correlation of these deposits with
the Jurassic standard ammonite zonation is still difficult, since the existing ostracod
biostratigraphy is facies-controlled and of only limited stratigraphic precision. In this study, a
chemostratigraphic approach has been chosen and biogenic shell material produced by
brachiopods, oysters and lithiotids is evaluated for its reliability to act as proxy
of the original Jurassic seawater strontium isotope composition. Low-Mg calcite
shells have been collected from three stratigraphic sections accessible in open-cast
quarries located in the Lower Saxony Basin of Northern Germany. In order to identify
diagenetically altered shell calcite, trace element and stable isotope analysis of 227
calcite samples (oysters=101; brachiopods=60; Trichites=52) has been carried out.
The geochemical results reveal that (1) concentration of different trace elements
varies between the different groups of shell-forming organisms, which may be
related to vital effects and (2) high strontium contents, low Mn and Fe contents
and the lack of correlation between these elements indicate near-pristine calcite
shells, and therefore shells are supposed to record the ambient sea water composition
during the Late Jurassic. Strontium-isotope (87Sr/86Sr) analysis of diagenetically
screened samples indicates an Early Kimmeridgian age of the studied deposits,
which is in accordance with ostracod biostratigraphic data. An increasing trend
in 87Sr/86Sr with stratigraphic height fits well with the global strontium-isotope
curve. Besides, similar 87Sr/86Sr ratios derived from different organisms from a
single stratigraphic level highlight the suitability of the shells for strontium-isotope
stratigraphy. Despite the shallow-marine character of the studied deposits, no evidence for
significant riverine influence on the strontium-isotope signature is observed. The new
chemostratigraphic data will provide a more precise age assignment for Kimmeridgian strata
in the Lower Saxony Basin and thus enable the establishment of a solid integrated
stratigraphic scheme that can be used for correlation on both regional and global scale. |
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