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| Titel |
High-resolution stratigraphy and faunal associations in the Upper Sinemurian organic-rich deposits of the western Iberian margin (Lusitanian Basin, Portugal) |
| VerfasserIn |
Luís V. Duarte, María J. Comas-Rengifo, Ricardo Paredes, M. Cristina Cabral, Ricardo L. Silva, Ana C. Azerêdo |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053262
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| Zusammenfassung |
| The Upper Sinemurian of the Lusitanian Basin (Portugal) is represented by marl-limestone
alternations with black shales. These organic-rich deposits are particularly well developed in
the western part of the basin, corresponding to the Polvoeira Member of the Água de
Madeiros Formation (Duarte et al., 2010). Based on a high-resolution stratigraphical study of
this unit in the reference sections of S. Pedro de Moel region, a multidisciplinary study
involving sedimentology, geochemistry (total organic carbon: TOC) and invertebrate
fauna (ammonites, brachiopods, bivalves and ostracods) analysis is presented in this
work.
The approximately 42 m thick organic-rich studied succession is dated from the
Oxynotum to the top of Raricostatum ammonite zones. Black shales occur in the whole
succession being associated with more argillaceous sediments in the upper part of the unit.
The TOC, analyzed in 129 horizons, shows a large variation, reaching the maximum value of
22% at the top of the Oxynotum Zone. More than 60% of the studied samples present TOC
content above 3%. In these cases, lithofacies correspond to dark-grey marls, sometimes
exhibiting a marked microlamination.
Despite the apparent uniform distribution of the organic-rich facies across the section, the
fauna record shows a marked differentiation. In black fissile organic-rich shales, TOC is
generally above 6%, and benthonic macrofauna and ostracods are absent. However, the lower
part of the succession (Oxynotum Zone), clearly more calcareous, contains a large abundance
and diversity of brachiopods (e.g. Lobothyris, Zeilleria, Cincta, Piarorhynchia,
Tetrarhynchia, Spiriferina), infaunal (Pholadomya) and epifaunal bivalves (dominated by
Gryphaea), and ostracods in a few rare levels (e.g. Ogmoconchella, Polycope, Cytherelloidea,
Cardobairdia). With the exception of some rhynchonellids (e.g. Piarorhynchia,
Tetrarhynchia) and a few sporadic disarticulated bivalves, these assemblages basically
disappear in the middle-upper part of the unit (Raricostatum Zone), being replaced by
different species of Oxytomidae and Pectinidae and by nektonic fauna, such as ammonites
and belemnites. Actually, Oxynoticeratids and Echioceratids are particularly abundant in
some horizons and allow the recognition of the four standard subzones of the Raricostatum
Zone.
Our data suggest that the occurrence of these organic-rich deposits is linked with cyclic
oxygen depletion at the sea floor. All the sedimentary arguments, including the lateral
correlation with other sections, confirm the Aplanatum Subzone as the time interval
correlative with the deepest sedimentation episode in the whole Sinemurian of the Lusitanian
Basin.
Duarte, L.V., Silva, R.L., Oliveira, L.C.V., Comas-Rengifo, M.J. & Silva, F. (2010) –
Organic-rich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal:
Total Organic Carbon and relation to transgressive-regressive facies cycles. Geologica Acta 8
(3), 325-340. |
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