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5.12 Yvoir Formation - YVO

Authors: Conil, 1960; Groessens, 1975; Paproth et al., 1983.

Description: The formation is characterized by dark limestones (mainly packstones) with crinoidal and shelly layers. Chert nodules are abundant throughout. The lower and the upper parts of the formation are partially dolomitized in places. Sandy limestones with thin shaly intercalations and thin layers of brachiopods form a distinct lower member in the Yvoir section, the Hun Member (HUN) which is 13.5 metres-thick (Paproth et al. 1983). In the CSA, the Yvoir Fm is mainly composed of cherty, crinoidal limestones (packstones to grainstones) with numerous corals and brachiopods.

Stratotype: Abandoned quarry behind the station at Yvoir (DSA).

Area: DSA and CSA. The formation is absent elsewhere. In areas close to Waulsortian buildups, the bulk of the Yvoir Formation passes laterally into the Bayard Formation and is thus contemporaneous with the lower part of the buildup succession.

Thickness: The formation is 64 m thick in the stratotype section.

Age: Mainly early Ivorian. The conodont Siphonodella is present in the lowermost part of the formation (Cc1 Conodont Zone, Hastarian). P. c. carina enters a few metres above the base of the formation, indicating an Ivorian age. The first tuberculate endothyrid foraminifera (Spinoendothyra, Tuberendothyra) were recorded from the first few metres of the formation (Cf1gForaminifer Subzone) whereas the Paraendothyra-Granuliferella foraminiferal association (Cf2 Foraminifer Zone) enters in the upper part. The formation lies in the RC3a Coral Subzone, corals being particularly abundant and diversified in the Condroz area. Most of them belong to species or genera that are widespread in Eurasia. The Yvoir Fm corresponds to the main part of the TST of the third-order sequence 3 (Hance et al., 2001).