Slate deposits of Maël-Carhaix [France]

Roofing slates in the area of the basin of Châteaulin-Carhaix are mined since the 17th and 18th century and the slate district can be divided in two areas:

• northern area: the group of Maël-Carhaix, Locarn, Trébrivan, Plounévézel

• southern area: several sites of lesser importance at Moustoir

The Lower-Carboniferous in the area of Maël-Carhaix consist of conglomerates and dark-grey phyllitic slates of Lower-Viséan age. This slates are overlaid by a greywacke-slate series of the Upper-Viséan to Lowest-Namurian. The roofing slates of Maël-Carhaix lie within the overlaying series 'Formation des schistes de Châteaulin' which is characterised by an alternating greywacke-slate series.
The 'Châteaulin basin' opened due to dextral movements along the North-Armorican Shear Zone (NASZ) and the mechanism of opening is thought to be in the form of a lateral ramp graben. Within the 'Châteaulin basin' olistolites and olistostromes occur caused by gravitative tectonics during the Tournaisian.

During the Upper-Vise and Lower-Namurian the basin was filled and new orogenic processes from Namurian and Westphalian followed. This gave rise to a closing and upwarping of the basin due to the reactivation of the dextral wrench faults. It was related with the folding of the Dinantian units in E-W striking folds, N-vergent thrusts as well as epi- to anchimetamorphic metamorphism.
This folding led to the origin of a fanning fracture cleavage S1 which was partly overprinted by a crenulation foliation S2.

According to the different fabric development and the mineralogical composition suitable and unsuitable roofing slates can be distinguished (see table).