4.3.2 Ottré Formation - OTT (old: Sm2)

Author: Geukens, 1999.

Description: Purplish slate and silty slate (called “quartzophyllade” in the older literature) subdivided into 3 members.

The Meuville Mbr – (MEU, old: Sm2a), in the north: red or purple slate and "quartzophyllade", purple slate with calcareous nodules, garnet bearing slate. In the north a gradual transition exists between the green "quartzophyllade" of the Lierneux Mbr and the purple "quartzophyllade" of the Meuville Mbr.

The Les Plattes Mbr – (PLA, old: Sm2b), in the north: purplish slate and sandy "quartzophyllade", calcareous levels with iron and manganese nodules. In the south: "quartzophyllade", purple or with purple patches containing levels with interstratified coticule, andalusite, garnet and volcanic rocks. The base is formed by a slaty sandstone, light green, with coarse ottrelite crystals.

The Colanhan Mbr – (COL, old: Sm2c), in the north: purple coloured slate with a few levels of green slate; in the south: purple and green compact layered slates, containing ottrelite.

Stratotype: Not yet defined; type area in the syncline south of Ottré; type area of the Meuville Mbr: outcrops in the Lienne valley north of Chevron; type area of the Les Plattes Mbr: abandoned or active quarries for the “coticule” between Hébronval and Bihain; type area of the Colanhan Mbr: the hill south of Lierneux between the villages of Verleumont and Hébronval and the northern flank of the crest south-west of Vielsalm.

Area: See description.

Thickness: Colanhan Mbr: 50 m; Les Plattes Mbr: 30 m; Meuville Mbr: 70 m; total thickness: 150 m.

Age: Neither micro- nor macrofossils observed; by geometry supposedly between Late Tremadoc (Early Ordovician) and Mid Ordovician.