Paléokarsts et brèches d'effondrement dans le Frasnien moyen des environs de Visé. Leur influence dans la paléogéographie dinantienne
Résumé
La formation au Famennien d'un brachyanticlinal dans la région de Visé, produisit l'exondation des terrains frasniens. Les calcaires du Frasnien moyen furent alors karstifiés dans les zones les plus élevées du brachyanticlinal, et bréchifiés en raison de la dissolution d'évaporites sous-jacentes, dans les zones plus basses. Au cours du Dinantien, le relief frasnien fut à six reprises envahi par la mer, ce qui produisit sa destruction partielle et son enfouissement progressif par les sédiments.
Abstract
Devonian deposits in the Visé area have been affected by updoming during Famennian times. This is suggested by the rapid variations in the nature, thickness and relative ages of the overlying Dinantian rocks, and by the pre-Carboniferous tensional joints occurring in the F2 carbonates. Middle Frasnian breccias are interpreted as solution-collapse breccias, thus suggesting the possible (former) occurrence of evaporites in the lower part of F2. Middle Frasnian solution-collapse breccias have also been observed in the N' quarry at Richelle, where they pass laterally into non-brecciated karstified F2 carbonates similar to those found in the Souvré Valley. The frequent occurrence is noted of phosphatic (fluorapatite) crusts lining the walls of the paleokarst cavities. In the area of maximum uplift, karstification of the Middle Frasnian carbonates under vadose water conditions occurred, whereas in the less updomed parts, brecciation took place, due to solution-collapse under saturated water conditions.
The Dinantian period is marked by six important marine transgressions in the Visé area, which took place during "Tn1b", "Tn2a", "Tn3c", "V1", "V3a" or "V3bα "and "upper V3c". They progressively destroyed and drowned with sediments the Devonian relief.