Publication: Ammonoid succession at the Bajocian-Bathonian transition in the Bas Auran area, Digne district, South-East France.
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2008
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Università degli studi di Milano. Dipartimento di Scienze della Terra "A. Desio"
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Abstract
The uppermost Bajocian to lowermost Bathonian
ammonoid succession has been studied in the Bas Auran area in view
of choosing one of its sections as the Global Stratotype Section and
Point (G.S.S.P.) of the Bathonian Stage. The Bas Auran ammonite
assemblages display exceptional values of record quality. The stratigraphic
distribution of 629 specimens referred to 63 species and 35
genera, collected during the last forty years from three sections (Ravin
du Bès, Ravin d'Auran, Ravin des Robines), is plotted and analyzed,
bed by bed. Over 85 stratigraphic levels, through 9 m in thickness,
have been studied at the top of the "Marno-calcaires à Cancellophycus"
formation, ranging from the latest Bajocian (Parkinsoni Zone,
Bomfordi Subzone) to the earliest Bathonian (Zigzag Zone, Convergens
Subzone) and the base of the Macrescens Subzone. Ammonoid
assemblages are composed of Northwest European and Mediterranean
elements, associated with Sub-Mediterranean ones, allowing chronocorrelation
between the Northwest European Convergens Subzone
and the Sub-Mediterranean Parvum Subzone. The basal boundary
of the Zigzag Zone and of the Bathonian Stage is placed at the base
of Sturani's bed 23 in the Ravin du Bes section and is identified by the
first occurrence of Gonolkites convergens, and coincides with the loest
occurrence of Morphoceras parvum. Features of the ammonoid
succession indicate relatively homogeneous and good record quality,
gradual biostratigraphic change and high degree of taxonomic similarity
between the Bomfordi and Convergens subzones. Palaeontological
criteria also indicate relatively high values of palaeontological
and stratigraphic completeness at the base of levels RB070-RB071
(= level 23 in Sturani 1967) which corresponds to the Bajocian/Bathonian
boundary. The Ravin du Bès Section, with forty-six successive
ammonoid fossil-assemblages of the Convergens Subzone belonging
to three biohorizons through five metres of thickness, shows maximum
values of biostratigraphic and biochronostratigraphic completeness,
being one of the most complete in the world. Therefore, the
Ravin du Bès Section should be chosen as the Global Stratotype Section and Point for the base of the Bathonian Stage. Two biostratigraphic
logs summarize the ammonite content from the uppermost
Bomfordi to basal Macrescens subzones. Three plates illustrate the
most characteristic ammonoid taxa at the Bajocian-Bathonian passage
in the Bas Auran succession.