3.1.8 Fosses Formation - FOS

Authors: Gosselet, 1873; Malaise, 1900; Michot, 1954; Martin, 1969a; Tourneur et al., 1993.

Description: From top to bottom green and dark green fine sandy shale, with trilobites and brachiopods; greywacke, calcareous shale with locally some limestone beds rich in brachiopods (Michot, 1934).

Composed of two members: a lower Bois de Presles Member – (BDP): clayey limestone, calcareous shale, fossiliferous with brachiopods, cystoids and trilobites (Martin, 1969a after Michot, 1927, 1934); an alternation of thin-bedded muddy limestone and calcareous shale; calcareous shale with thin limestone layers in the upper part of the member; rather rich in brachiopods, crinoids, trilobites, bryozoans, corals, echinoderm debris, cystoids, molluscs and algae; the lowest beds contain a coarse crinoidal limestone above a basal conglomerate (Cocriamont conglomerate) (Tourneur et al., 1993).

An upper Faulx-les-Tombes Member – (FLT): “schistes mouchetés” of Lassine (unpublished in Martin, 1969); green sandy shale with on breaking surfaces blackish elliptic or fusiform spots and a rare macrofauna level (Martin, 1969a after Michot, 1927, 1934); bioturbated calcareous shale (Tourneur et al., 1993).

Stratotype: 1 km east of the city of Fosses, in the northern bank of the Fuette and Rosière rivers (Malaise, 1873); also between the city of Fosses and the village Sart-Eustache.

Area: Condroz inlier, also in the Puagne area.

Thickness: 100-110 m (Michot, 1957); upper member: 50 m, lower member: about 50 m (Tourneur et al., 1993).

Age: Based on trilobites, brachiopods etc., considered to be Caradoc or Caradoc and Ashgill (see Martin, 1969a); the cystoids studied by Regnell (1951) indicated the Ashgill. A restudy of the brachiopods and trilobites indicated the pre-Hirnantian Ashgill (Sheehan, 1987; Lespérance & Sheehan, 1987).

Remarks: Synonym: Assise de Fosse; Schistes de Fosse; Horizon de Fosse; Grauwacke de Fosse.

(J. VERNIERS)