1.2.5. Mirwart Formation - MIR

 

Authors: Hebert, 1855; Stainier, 1994b.

 

Description: The Mirwart Formation corresponds to the former Anor Formation.  However, the lack of a good section in the Anor area has justified the abandon of the Anor name.  Furthermore, the name "Mirwart" was already used in the legends of the geological maps at the scale 1:40,000 (published between 1900 and 1909) to describe the upper part of the formation.

At the bottom, the Mirwart Formation begins with greyish green shalesoverlying the quartzites of the St-Hubert Formation.  At the top, green quartzites underly the carbonate and fossiliferous beds of the Villé Formation.  In between, there are occurrences of thick lenticular masses of green, light blauw, greenish blauw, sometimes white or cream quartzites and sandstones, interbedded in green grey or black shales and siltstones.  Sandstones often contain pebbles of black shale.  Towards the top, the black shales contain plants remains.  In the 100 uppermost meters, the sandstone layers may also contain rare shells.

 

Stratotypes: Mirwart, sections along the Lhomme river and the Namur-Arlon railway track.  Other good section for the upper part at Villé, near La Roche-en-Ardenne.

 

Area:S and E of the Dinant Synclinorium; Halleux Anticline.

 

Thickness: 300 m at Couvin, 450 m in the Meuse valley, 600 m at Mirwart, 600 m on the eastern side of the Dinant Synclinorium, > 700 m N at Halleux, 1000 m W of Dochamps.

 

Age: Lochkovian-Pragian.  On the base of miospore zonation, diachronous character from W to E.  At Mirwart, top of the Z, E, Po and W Subzones, BZ and PoW Oppel Miospore Biozones (= Lochkovian-Pragian border); at Couvin, in the Pèrnelle valley : PaaSubzone of the PoW Oppel Miospore Biozone (= Pragian).  Former Early Siegenian (Sg1) age.