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3.7 Arlon Formation - ARL

Authors: Dewalque, 1902; Monteyne, 1958; Boulvain et al. (2001).

Description: Grey marl, silty marl, bioturbated argillaceous-silty to sandy limestone. Abundant fauna.

Stratotype: Toernich borehole.

Area and thickness: Belgian Lorraine. The formation has a thickness of about 60 m close to Arlon. More to the west, its thickness decreases very quickly, the formation being represented only by marl beds in the Luxembourg Formation (corresponding successively to Trite, Strassen and Posterie Members). A late western extension of the marly-sandy facies constitutes the Hondelange Member, which overlies the Luxembourg Formation.

Age: In eastern Belgian Lorraine, in the Arlon area, the Arlon Formation extends from the Lower Sinemurian (Arnioceras semicostatum ammonite Zone) to the top of the Lower Pliensbachian (Prodactylioceras davoei Zone) (Maubeuge, 1954; Mergen, 1984). In the western part of the area, the Trite Member is in the Arietites bucklandi Zone, the Strassen Member in the Arnioceras semicostatum Zone and the Posterie Member in the Asteroceras obtusum Zone.

Remarks: All authors have highlighted the difficulty to establish, east of Arlon, in this sandy-argillaceous-limestone unit (Lorraine facies), other subdivisions than biostratigraphic ones. We thus use the name of Arlon Formation here as a global unit and subdivide it into members whenever possible to do so on a lithostratigraphic basis (see intercalations in the Luxembourg Formation)