5.2.4 Petite-Commune Formation - PCO (old: Rv4)

Author: Beugnies, 1960.

Description: Thick beds of 5 to 10 m of black quartzite, rich in pyrite, coarse-grained, micaceous, alternating with micaceous coarse-grained pyritic slate, and fine-grained graphitic slate. Some cross-bedding can affect the alternation of quartzite and slate. The quartzite bedding planes are sometimes mammillary in form or covered by mud-cracks. The slate has a mat appearance and frequently contains in the metamorphic zone millimetric chloritoid crystals. In the top of the formation an unnamed member contains black micaceous slate, very soft, with some nodule-like structures, often pyritic, resembling the remainder of the Petite-Commune Fm, but with more slate than quartzite and with roofing slate veins. The latter member was formally called unit Rv5 (Beugnies, 1960; Belanger, 1998 ms).

Stratotype: not yet defined; type area in the outcrops along the Meuse valley east of Laifour.

Area: Rocroi Massif.

Thickness: 500 m (Beugnies, 1960).

Age: The acritarchs studied by Vanguestaine (1978) from a sample located in the lowest part of the formation belong to the Zone 4b, dated as Mid to Late Cambrian. Outcrops in the upper part of the formation (east of the road from Vieux-Moulins-de-Tilhay to Willerzie, along the chemin de la Croix- Scaille) contain an acritarch assemblage of the Upper Cambrian (Roche et al., 1986), named the cf. Zone 5 in Vanguestaine (1986). Another outcrop studied by Ribecai and Vanguestaine (1993) in the Willerzie quarry, also located in the upper part of the formation, contains an acritarch assemblage of the cf. Zone 6 of Vanguestaine (1986) also dated as Late Cambrian.

(M. VANGUESTAINE, I. BELANGER)