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5.32. Onoz Formation – ONO

Author: Hance, this paper.

Description: The formation overlies the Namur Dolostones and includes 2 members, from base to top:

- Parc Member (PRC): Alternating dark limestones (bioclastic packstones to grainstones), dolomitic limestones and dolomites. Bed thickness varies widely. The limestones have yielded a poorly diversified fauna of rugose corals, brachiopods and vermetid gastropods. Fenestral limestones occur in the upper third of the member.

- Leurquin Member (LEU): Dark, thin- to thick-bedded limestones with a remarkable development of algal facies (Ortonella and Solenopora oncoids) including also beds of oolitic grainstones and some breccias.

Stratotype: Two abandoned quarries on the left bank of the R. Orneau at Onoz (NSA).

Area: The formation extends across the Orneau and Ligne valleys on the northern border of the NSA. Eastwards, it passes into the Terwagne Formation and into the Neffe Formation.

Thickness: Parc Member, at least 37 m; Leurquin Member, more than 45 m.

Age: Moliniacian (early Visean). Dorlodotia briarti occurs in the lowermost part and indicates the RC5 Zone. Cf4b(?) – dForaminifer Subzones.