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Slate deposits of the Armorican Massif - Angers (France)
Within the Armorican Massif the Middle- to Upper-Ordovician slates of Angers as well as the Lower-Carboniferous Kulm slates of Maël-Carhaix are the two important slate deposits. During the Paleozoic the Armorican Massif was the northern part of the northern margin of Gondwana. With the beginning Arenigian a large part of the North- and Central Armorican Massif, including the synclines of Segré and Angers with its roofing slate deposits, was floated. In this shallow-marine environment the 'Grès Armorican Formation' was deposited and contains mainly sandstones with intercalations of slate. During the Arenigian, partly to Llandeilian or Upper-Caradocian, respectively, the transgression reaches its peak and led to the sedimentation of the roofing slate bearing 'Angers Formation'. This sedimentary environment is characterised by an shallow-marine facies within an open shelf. The geographic position of the shelf sediments during the Llanvirnian indicates a coastal sedimentation environment in the northern part and a distal and probably deeper sedimentary environment in the southern direction (Robardet et al. In: Keppie, 1994). Within the Armorican Massif four orogenic phases can be distinguished (Rolet In: Keppie, 1994):
At the end of the Devonian the collision processes had already started (Bretonic Phase). All Lower-Carboniferous units were almost completely transformed to the Armorican Massif and the little remaining 'intramontan' Carboniferous basins had no access to the norther ocean. Two orogenic periods with different types of basins can be separated:
French slate districts. Other: Glimmerschiefer (mica schist).
The Slates of Angers
The primary Middle- to Upper-Ordovician slates in the region of Angers and Segré are situated within the 'basin d'Anjou', with WNW-ESE striking anticlines and synclines. Three slate deposits can be separated and all were mined subsurface:
Angers - Trelazé
The mines of Trelazé are aligned WNW-ESE and thus they copy the syncline of Angers-Trelazé. This syncline consist of two synclines separated by an anticline and the slates are mined within the unit "schistes d'Angers" in two different Niveaus:
Niveau Ardoisier Inférieur, Niveau Ardoisier Intermédiare.
The slates 'Ardoisier Inférieur' show a thickness between 32-98 ft. with a very fine grained sedimentary pattern. At the bottom it is bordered by the 'Ricard'-facies and at the top by the 'Foriace'-facies. Typical for the 'Ricard'-facies is the occurrence of pyritized fossils whereas the 'Foriace'-facie shows knotted pyrites as well as branched pyritizations on the splitting plane. Because of the very fine grained sedimentary pattern and the well developed fracture cleavage plane of the 'Ardoisier Inférieur' this slates can be split to 3 mm (= 0.12 in.) thin shingles. The slates 'Ardoisier Intermédiare' consist of two slate layers, separated by 33 ft. thick sandy unit. The first slate layer 'Veine Nord' shows a relative constant thickness of 131-148 ft. and the second slate layer 'Veine Extrême Nord' is between 33-82 ft. thick with varying fine- and coarse grained parts.
Novant-Segré As in Trelazé the slates 'Ardoisier Inférieur' and 'Ardoisier Intermédiare' occur also in this area but only in the first one slate was mined. The slates are within the WNW-ESE striking syncline of Segré and the fanning fracture cleavage dips 80°-85° to North. Numerous WNW-SSE striking faults cross the syncline and lead to a alternating slate quality (Marty, 1980).
The slates 'Ardoisier Intermédiare' are mined at Noyant and Bel Air and show a thickness of about 262 ft. in the vicinity of Noyant. At the bottom it is limited by a black fossil rich slate containing pyrite nodules. At the top it gradually changes to a more and more sandy and micaceous slate.
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