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The Ammonite Slab of Digne – les – Bains

FRANCE

View of the exceptional concentration of ammonites with in situ specimens of Coroniceras multicostatum. The largest are up to 70 cm in diameter. (©Haute-Provence Global Geopark).

Geological Period

Lower Jurassic, Sinemurian

Main geological interest

Paleontology
Stratigraphy and sedimentology

Location

Haute-Provence Geopark, France.
44°07’10.0″N, 6°14’03.0″E

View of the exceptional concentration of ammonites with in situ specimens of Coroniceras multicostatum. The largest are up to 70 cm in diameter. (©Haute-Provence Global Geopark).

World famous and outstanding accumulation of fossils from a Lower Jurassic marine environment.

The site is a spectacular concentration of fossil ammonites, and offers the best exposure for the Coroniceras multicostatum biohorizon (Lower Sinemurian, top of Bucklandi Zone) in Europe allowing for new palaeontological analyses. Thus, this site constitutes an international reference for this stratigraphic level and is emblematic of the creation of UNESCO Global Geoparks. This designation coincided with the “International Declaration of the Rights of the Memory of the Earth” written in Digne-les-Bains in 1991. These scientific, aesthetic and historical aspects contribute to the worldwide reputation of this site.

Drone view showing the site facilities: visitors’ path, mediation area, footbridge accessible to people with reduced mobility, and observation platform. (©Haute-Provence Global Geopark).

  • Geological description

The slab belongs to the Subalpine Basin marly-limestone Lower Jurassic sequence and is part of the Nappe de Digne, a top-tothe-South allochthonous unit. The Lower Jurassic strata record the early history of the Southeast Basin in the Alpine Tethys Ocean during its rifting.

The slab (ca. 320 m2) bears more than 1,550 cephalopods (some nautilus and many ammonites), up to 70 cm in diameter, and a benthic fauna composed of bivalves and crinoids (Corna et al., 1990). One of the distinctive features is that the ammonites are 99% represented by a single species (Coroniceras multicostatum). It characterises a well-known stratigraphic biohorizon in Western Europe, but the Digne slab is the only well-exposed outcrop presenting such a concentration of fossils and constitutes an international reference for this stratigraphic level (Dommergues and Meister, 1991; Bert and Pagès, 2021). No preferential orientation indicates that no dominant current guided the shells deposition.

Field surveys, completed by a drill hole, confirmed the extension of the fossiliferous level under the surface, over several thousand square metres. The site is in the French Geoheritage Inventory, the National Geological Nature Reserve of Haute-Provence (Martini, 1979) and the Haute-Provence UNESCO Global Geopark (Neige et al., 2021).

  • Scientific research and tradition

This world-renowned site exposes a reliable biostratigraphic marker for establishing correlations with the faunas of the Mediterranean Tethys. It is the first place where intraspecific variability of a large Lower Jurassic ammonite species was studied using 3D-Scan. The site hosts more than 20,000 visitors a year.

Stratigraphic log of the Isnards section (Corna et al., 1990) including the ammonite slab (top of bed 50). Details of the ammonites and bivalves distribution.

  • Reference

Bert, D. and Pagès, J.-S. (2021) ‘Overview of the Les Isnards Ammonite Slab Geoheritage Site (Digne-les-Bains, Southeastern France)’, Geoconservation Research, 4(2), pp. 330–337. Available at: https://doi.org/10.30486/gcr.2021.1917851.1074.

Corna, M. et al. (1990) ‘Quelques points remarquables dans le Sinémurien des Alpes de Provence (France) : précisions biostratigraphiques et paléontologiques’, Géologie Méditerranéenne, 17(1), pp. 3–37. Available at: https://doi.org/10.3406/geolm.1990.1432.

Dommergues, J.-L. and Guiomar, M. (2011) ‘La ” Dalle à ammonites de Digne ” (Réserve Naturelle Géologique de Haute-Provence, France). Étude d’un site fossilifère d’importance patrimoniale.’, Revue de Paléobiologie, 30(1), pp. 261–293.

Dommergues, J.-L. and Meister, C. (1991) ‘Area of mixed marine faunas between two major paleogeographical realms, exemplified by the Early Jurassic (Late Sinemurian and Pliensbachian) ammonites in the Alps’, Palaeogeography, Palaeoclimatology, Palaeoecology, 86(3), pp. 265–282. Available at: https://doi.org/10.1016/0031-0182(91)90085-6.

Martini, G. (1979) Aménagement d’une Réserve géologique dans les Alpes de Haute-Provence. Master degree. Université de Provence.

Neige, P., Dommergues, J.-L. and Guiomar, M. (2021) ‘La dalle aux ammonites de Digne-les-Bains : un patrimoine d’âge Sinémurien de réputation mondiale’, in Stratotype Sinémurien. Neige, P., Dommergues, J.-L. Muséum National d’Histoire Naturelle, pp. 270–275.

  • Author(s)

Sylvie CRASQUIN
Sorbonne Université-MNHN-CNRS

Myette GUIOMAR
Haute-Provence National Nature Reserve

Guy MARTINI
Global Geopark Network

Jean-Simon PAGÈS
Haute-Provence UNESCO Global Geopark

Nicolas CHARLES
PhD – BRGM / French Geological Survey

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