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| Titel |
Reconstruction of paleoenvironments by analyzing spatial shell orientation |
| VerfasserIn |
Susanne Lukeneder, Alexander Lukeneder, Gerhard W. Weber, Ulrike Exner |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078816
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| Zusammenfassung |
| Fossils, especially their mass-occurrences, can be exploited as useful source of information
about the depositional conditions. Particularly abundant fossils with elongated shape such as
belemnites are useful indicators to draw conclusions about influencing factors (e.g.
paleocurrents) of paleoenvironments. Orthocone cephalopods, gastropods, bivalves,
foraminifers, vertebrate bones and others have been used so far in field-based spatial
orientation studies (Flügel 2004). Normal coiled (planispiral) cephalopods can also provide
such depositional information. A new method for reconstructing spatial shell orientation in
3D is presented here. A roughly 225 million-year-old (Carnian, Triassic) monospecific
mass-occurrence of the ammonoid Kasimlarceltites krystyni from the Taurus Mountains in
Turkey (project FWF P22109-B17; Lukeneder et al. 2012), embedded in limestone, is used
for this pilot study.
The most obvious method for digitization of the ammonoids, μ-computed tomography (CT),
was not successful in this case due to the lack of density differences between the ammonoids
(i.e. secondary calcite shells) and the embedding source rock (carbonate). Therefore we had
to come back to the classic method of grinding, which, despite its invasive character, cannot
always be disregarded, particularly if digital recording methods are not applicable
and samples are large enough to sacrifice parts. A 150x170x140 mm block of the
ammonoid bearing limestone bed has been grinded to 70 slices, with a distance of 2mm
between each slice. By using a semi-automatic region growing algorithm of the 3D
visualization software Amira, the ammonoids were segmented, and a 3D model of this
mass-occurrence reconstructed. We used landmarks as well as trigonometric and
vector-based calculations to compute the diameters and the spatial orientation of each
ammonoid. For the diameters, the longest distance (longitudinal axis) of each shell
(landmark a & b) and the orthogonal distance from this cord to one side of the
shell (transverse axis) was measured (landmark s & c). Spatial orientation was
characterized by dip and dip direction of the longitudinal axis, as well as by strike and
azimuth of a plane defined by both axes. The exact spatial orientation data was
determined for a sample of 699 ammonoids within the bed and statistically analyzed. The
results provide a hint on the geodynamic processes (paleocurrents), depositional
conditions (allochthonous or autochthonous) and other general information about the
ancient environment. The method can be adapted for other mass-occurring fossils and
thus represents a good template for studies of topographical paleoenvironmental
factors.
References:
Flügel, E. 2004. Microfacies of carbonate rocks. Analysis, Interpretation and Application.
Springer, Berlin Heidelberg New York, p.182.
Lukeneder S., Lukeneder A., Harzhauser M., Islamoglu Y., Krystyn L., Lein R. 2012. A
delayed carbonate factory breakdown during the Tethyan-wide Carnian Pluvial Episode along
the Cimmerian terranes (Taurus, Turkey). Facies 58: 279-296. |
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