4.2.1. Famenne Group - FAM
Authors: d’Omalius d’Halloy, 1839; Thorez & Dreesen, 1986.
Description: The Famenne Grp is essentially made up of green (sometimes violet) shales or pelites alternating with more silty beds. Siltstone beds are increasing towards the top. Locally, the group is rich in carbonate nodules (of some cm long) and in lenticular sandy or calcareous-sandy beds (of some cm thick) containing crinoids and brachiopods accumulations (among which, rhynchonellids). Fossils are often decalcified. The Famenne Grp contains one or several oolitic hematite beds hypothetically attributed to debris-flows and characterizing a fluxo-turbiditic mechanism.
Former name : Famenne shales or schists, subdivided into Senzeille “assise” in the lower part and Mariembourg “assise” in the upper part; also named Famenne Fm in many recent works.
Stratotype: see Senzeille and Mariembourg Formations.
Area: Namur and Dinant Synclinoria. The Famenne Grp pass laterally to the Hodimont Fm in the Vesdre Nappe.
Thickness: 0 to < 50 m at the northern side of the Namur Synclinorium; 50 to 60 m at the southern side of the de Namur Synclinorium and at the northern side of the Dinant Synclinorium; 260 m in the Silenrieux-Walcourt area; 400 m in the Achêne-Leignon area (central part of the Dinant Synclinorium); maximum development on the southern side of the Dinant Synclinorium.
Age: Lower Famennian (Fa1a and Fa1b-c units, corresponding respectively to the old Senzeille and Mariembourg “assises”); Palmatolepis triangularis and Palmatolepis crepida crepida conodont biozones.
Remark: To consider Mariembourg and Senzeille as lithological units is questionable as for the limit between them is essentially based on paleontological markers or on a discrete (condensed) oolitic ironstone bed. Indeed, up to now (in 2001), in the various sheets of the new geological map of Wallonia, the distinction between them has not been made. The authors refer to the Famenne Fm, but the terminology of Famenne Grp, used by Thorez & Dreesen (1986), is preferred.