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| Dalle à ammonites | |
|---|---|
| Stratigraphic range:Sinemurian | |
 Dalle à ammonites inDigne-les-Bains | |
| Type | Fossil site |
| Thickness | 20 cm |
| Lithology | |
| Primary | Grey limestone |
| Location | |
| Coordinates | 44°7′10″N6°14′3″E / 44.11944°N 6.23417°E /44.11944; 6.23417 |
| Region |  Provence-Alpes-Côte d'Azur |
| Country | France |
| Type section | |
| Named for | Ammonite |
  Dalle à ammonites (France) | |
TheDalle à ammonites (orDalle aux ammonites) atDigne-les-Bains (Alpes-de-Haute-Provence) is a remarkable natural site consisting of a rock strata bearing a large number of fossilizedammonites. This site is located approximately 1.5 kilometres (0.93 mi) south ofDigne-les-Bains fromBarles road. It is part of theGéologique de Haute-Provence National Nature Reserve.
Theslab, inclined at 60°, is made of grey limestone. It bears around 1,500ammonites, 90% of which belonging to the speciesCoroniceras multicostatum, dating from theSinemurian (Early Jurassic). Those ammonites can attain a diameter of 70 centimetres (28 in).
Nautiloids,belemnites,Pecten seashells and otherbivalves can also be found in the deposit.
The deposit thickness is estimated to be 20 centimetres (7.9 in), deposited over a 100,000-year period.[1]
As well known as it is, the slab wasn't known until recently :Alcide d'Orbigny, who prospected the area in the middle of the 19th century, doesn't mention it. Geologists studying theichthyosaur remains near the site only mentions numerous species ofammonites and othermolluscs.
The dalle is thought to have been partially unearthed for the first time in June 1941, during road works on what is now the D 900A. In 1979, before the establishment of the Géologique de Haute-Provence National Nature Reserve, earthworks began to uncover the slab and unearthed 600 ammonites over 160 square metres (1,700 sq ft).
Until 1994 only 200 square metres (2,200 sq ft) were visible. After this date, the Géologique de Haute-Provence National Nature Reserve took the task of clearing an additional 150 square metres (1,600 sq ft) to attain a total area of 350 square metres (3,800 sq ft).[citation needed]
The 160 square metres (1,700 sq ft) already unearthed in 1992 of the slab were molded in 1992 by the geological reserve for the city ofKamaishi in Japan (who desired to buy the original). The mold was realized in 30 parts, which were sent and reassembled in Japan.
An almost horizontalAmmonoidea slab, from theTriassic or theJurassic, is located belowFressac bridge over theConturby river inGard.
The Dalle à ammonites was created during theAlpine orogeny. The Dignes-les-Bains area is located in the FrenchPrealps, in theDauphinois geological facies. It bears the traces of the oceanisation preceding the orogeny beginning.
TheAlps were erected in several phases, after asubduction happened in thepaleo-oceanTethys. The Dalle à ammonites, as well as other sites in the area, are witnesses of the first step of this process, and of what was life in this ocean before the apparition of the mountains. They bring paleontological context for the understanding of paleoenvironments and period conditions.
The fossils aspect helps to understand thesedimentation process. The good conservation of the shells give informations over the marine dynamism responsible for the deposit conditions: a hectic environment, like a beach or a coast, were the waves actions were strongly felt, with a tendency to break the shells, leading to fragmentary deposits, settled asfaluns orlumachella. The majority of the fossils on the slab are still almost complete, which lead to the hypothesis of a milder environment, deeper, and better preserved. Moreover, this deposit was effected over thecarbonate compensation depth, and hence at an average depth.
Thetapophenosis analysis, as well as the position of the shells, lead to the conclusion that the Dalle à ammonites was an accumulation facies: a large number of dead animals were carried by weak marine currents, towards their deposit site. Traces ofbioturbation can be attributed to thebenthic fauna, such as scavenger worms, who thrived in this biotope.
