The succession of the middle part of the “Marno-calcaires à Cancellophycus” Formation, typical of the French Subalpine Basin and cropping out in the area south of Digne, and its ammonite assemblages have been revised with the double aim of providing an updated biostratigraphic scheme for the succession of uppermost Humphriesianum Zone (Blagdeni Subzone) to lower Niortense Zone (Banksii and Polygyralis subzones), and of describing the evolutionary trend of the dimorphic couple Caumontisphinctes-Infraparkinsonia which is fundamental for defining the GSSP of the late Bajocian Substage. Three sections (Ravin du Feston, RF, Les Dourbes, LD, Ravin de la Coueste, RC) were sampled in 2011 and the newly collected material was added to the stock of specimens already present in the collection of the University of Torino. Field and laboratory data concur to indicate that the ammonite composite moulds belong to a single taphorecord and are classified as resedimeted elements that, before burial, were repeatedly transferred to the bottom before burial by currents flowing down from platform sectors of the Subalpine Basin. Taxonomic details are provided for each biostratigraphic unit allowing the subdivision of the Blagdeni Subzone in three biohorizons (Dubium, Coronatum and Festonensis) and the subdivision of the Banksii Subzone in two biohorizons (Diniensis and Nodatus). The FO of pertinent index-species is defined for each subzone and biohorizon. Selected taxa with [M] and [m] counterparts are described in details with Infraparkinsonia pierae n. sp. Results concern different topics: shell architecture and morphology can be considered as valid parameters to support the origin of Caumontisphinctes (C. garnieri) from Phaulostephanus; the records of scattered specimens of Phaulostephanus, Subcollina, Patrulia and Torrensia reflect discontinuous connections and two-ways dispersion, correlated to the early-Bajocian sea-level rise, between the East Pacific Subrealm and the Tethyan Realm, via Central Atlantic. Therefore, the Ravin de la Coueste is quite suitable to be proposed as the late Bajocian GSSP, at bed RC-281.
Ammonite assemblages and biostratigraphy at the Lower to Upper Bajocian boundary in the Digne area (SE France). Implications for the definition of the Late Bajocian GSSP
PAVIA, Giulio;ZUNINO, MARTA
2012-01-01
Abstract
The succession of the middle part of the “Marno-calcaires à Cancellophycus” Formation, typical of the French Subalpine Basin and cropping out in the area south of Digne, and its ammonite assemblages have been revised with the double aim of providing an updated biostratigraphic scheme for the succession of uppermost Humphriesianum Zone (Blagdeni Subzone) to lower Niortense Zone (Banksii and Polygyralis subzones), and of describing the evolutionary trend of the dimorphic couple Caumontisphinctes-Infraparkinsonia which is fundamental for defining the GSSP of the late Bajocian Substage. Three sections (Ravin du Feston, RF, Les Dourbes, LD, Ravin de la Coueste, RC) were sampled in 2011 and the newly collected material was added to the stock of specimens already present in the collection of the University of Torino. Field and laboratory data concur to indicate that the ammonite composite moulds belong to a single taphorecord and are classified as resedimeted elements that, before burial, were repeatedly transferred to the bottom before burial by currents flowing down from platform sectors of the Subalpine Basin. Taxonomic details are provided for each biostratigraphic unit allowing the subdivision of the Blagdeni Subzone in three biohorizons (Dubium, Coronatum and Festonensis) and the subdivision of the Banksii Subzone in two biohorizons (Diniensis and Nodatus). The FO of pertinent index-species is defined for each subzone and biohorizon. Selected taxa with [M] and [m] counterparts are described in details with Infraparkinsonia pierae n. sp. Results concern different topics: shell architecture and morphology can be considered as valid parameters to support the origin of Caumontisphinctes (C. garnieri) from Phaulostephanus; the records of scattered specimens of Phaulostephanus, Subcollina, Patrulia and Torrensia reflect discontinuous connections and two-ways dispersion, correlated to the early-Bajocian sea-level rise, between the East Pacific Subrealm and the Tethyan Realm, via Central Atlantic. Therefore, the Ravin de la Coueste is quite suitable to be proposed as the late Bajocian GSSP, at bed RC-281.
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