4.2.8. Souverain-Pré Formation - SVP
Authors: Mourlon, 1875; Dreesen et al., 1985.
Description: The most representative rocks of the Souverain-Pré Fm (former name: Souverain-Pré “macigno”) are metric-thick layers of nodular limestones with shaly to silty cement. The nodules are often aligned along the stratification and dissolved, giving rise to cellular alignements, very characteristic of the formation. Bioclastic character (crinoids, brachiopods, bryozoans) is often marked, especially in the limestone nodules. Decimetric beds of calcareous sandstones are also interbedded in the limestones. The Souverain-Pré Fm is interpreted as a “back-reef” facies of crinoidal mud mounds named the Baelen Mbr and described here-after.
In the Dinant Synclinorium, the Souverain-Pré Fm passes laterally, in a more inshore direction, to the Ciney Fm. At the bottom of the Souverain-Pré Fm, limestone and thin sandstone beds alternate, forming the transition to the underlying Esneux Fm. The transition zone between the Souverain-Pré and the lateral or overlying Ciney Fm is gradual and characterized by the occurrence of more sandy facies with discrete sedimentary structures.
In the Vesdre Nappe, the Souverain-Pré Fm passes laterally to the east to the Baelen Mbr (former name: Crinoidic Red Marble of Baelen). This is a reef mud mound forming a heterogenous limestone complex. The marble s.s. consists of relatively pure fine-grained limestones, occurring in massive beds, rubefied, and frequently interbedded with clearer coarse-grained limestones. Crinoids, stromatactis and slumps are abundant. The marble is restricted to the central part of the complex and is surrounded by sandy or argillaceous, nodular limestones, very rich in crinoids, and micaceous, carbonate sandstones.
Stratotype: Ourthe valley, railway trench, Souverain-Pré railway station.
Area: Dinant Synclinorium, Vesdre Nappe, Theux Window.
Thickness: variable : 50 to 120 m in the Dinant area; 80 to 140 m in the Achêne-Leignon area; 120 to 180 m in the Silenrieux area; 80 to 100 m in the Vesdre Nappe. Baelen Mbr: 20 to 150 m.
Age: The Souverain-Pré Fm is a diachronous formation of Middle Famennian age (Fa2ab). At Blaimont, the formation corresponds to the Scaphignathus velifer conodont zone; at Houyet and in the Vesdre nappe, it covers the P. marginifera zone and the lower part of the Sc. velifer zone. In the Achêne-Leignon area, the formation starts with the P. marginifera zone and covers the base of the Palmatolepis rugosa trachytera zone. The Baelen Mbr is dated of the Middle Famennian (Fa2a), late P. marginifera zone.