Aux confins du Lot, de l'Aveyron et du Tarn et Garonne, sur le causse de Limogne se trouvent les "trous à phosphate", gouffres à ciel ouvert de plus de 25 mètres de profondeur.
Exploitations minières de la fin du 19ème siècle, ils ont suscité une véritable frénésie étendue à l'échelle européenne.
Aujourd'hui, ce sont des paléontologues qui exploitent ces gisements contenant des fossiles uniques, permettant de restituer l'évolution de l'ère tertiaire sur plus de 32 millions d'années.
Visite guidée : Durée : 45 min. en plein air. Prévoir un pull (température moyenne de 15° dans le gouffre).
Accès libre : Espace Animaux du passé, Sentier du temps, Sentier paléontologique, Bac de fouille pour les enfants.
Espace pique-nique, buvette et glaces.
Visite à faire en famille
Ouverture 2019 :
Du 1er au avril au 30 juin : visite à 15h00 et 16h30
Du 1er juillet au 31 août : en continu de 10h00 à 17h00 (une visite chaque début d'heure)
Du 1er septembre au 04 novembre : visite à 15h00 et 16h30
Tarifs 2019 :
Adulte : 8€ - Enfant (6 à 14 ans ) : 5€ - Gratuit jusqu'à 5 ans
On the borders of the Lot, the Aveyron and of Tarn et Garonne, on the Causse (limestone plateau) of Limogne, one finds the ‘phosphate pits’, chasms open to the sky and more than 25 metres (80 feet) deep.
Mined at the end of the 19th century, they gave rise to a frenzy of activity on a Europe-wide scale.
Nowadays, it is the palaeontologists who explore these deposits which contain unique fossils allowing them to reconstruct evolution during the first Tertiary age over a period of more than 32 million years.
A long history
It all starts in the Secondary Age during the mid-Jurassic period; that is to say 160 million years ago. At the time Quercy forms part of a vast coastal area on which is deposited a carbonate-rich sludge. Slowly these soft deposits are transformed into a hard rock: limestone. This forms the structure of the ‘causses’ (limestone plateaus) and lends the area its characteristic appearance.
The sea then recedes a first time at the end of the Jurassic period; 35 million years ago. The region is then subjected to tectonic upheaval and to erosion. The result is a glazed surface on which, during the upper Cretaceous period (starting less than 100 million years ago), the sea returns and on its bed are deposited sandy limestone sediments.
During the transition between the Secondary and Tertiary periods (62 million years ago), at which time the last dinosaurs died out, the sea retreats for the last time. A general lowering of ocean levels associated with a local uplifting of the surface creates a gradient favourable to the infiltration of water through the fissures in the limestone.
Thus, throughout Quercy at the dawn of the first Tertiary Period, a series of vast galleries is formed several hundred metres under the surface level at this time. At the same time, the Cretaceous sandy limestone is eroded; the carbonates dissolve and are carried away by strong currents while the insoluble residues accumulate. The finer sands, more easily moved by the water, get into the cracks and block off the underlying caves. The heavier clays, being more difficult to put into suspension, settle on the bottom. A thick layer is formed at the heart of which, because of the tropical climate, the process of laterisation (conversion of soft less-soluble deposits into hard rock by the infiltration of silica) takes place. This results in concentrations of less soluble minerals notably concretions of iron oxide and of phosphate nodules.
Eventually the lowering of the surface cuts off the top of the ancient cavity. When the conditions are favourable sand is drawn down deeper into the rocky mass and the cavity becomes open to the sky. Under the effect of gravity, it is rapidly filled with phosphate-rich clays together with the remains of organisms living in the area, which themselves constitute a second source of phosphate.
This action of draining followed by rapid refilling takes place in a random fashion. Thus each phosphatière has its own filling, which occurred in just a short interval of time, so that two adjacent phosphatières can contain deposits dating from several million years apart. All depends on the moment the sand departs: the oldest deposits date from 50 million years ago and the most recent from only 18 million with intermediate ones between these two extremes.