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3.3 Attert Formation - ATT

Authors: Boulvain et al. (2001).

Description: Greenish or purplish marl and dolomitic marl ("marnolithes"), interbedded with decimetric white dolomitic lenses, some clay, conglomeratic argillaceous sandstone, and occasionally conglomerate with dolomitic cement. In boreholes, gypsum layers.

Stratotype: Access to the N4 road, close to Attert.

Area and thickness: The formation outcrops in the north-eastern part of Belgian Lorraine. Its thickness seems to increase regularly from west to east and from north to south, to reach a maximum of some 50 m in the neighbourhood of Attert. In the central area, at Habay, the formation is only 30 m thick. It disappears then west of Marbehan. In subsurface, in the Latour borehole, the Attert Formation reaches a hundred meters, bearing increasingly conglomeratic facies.

Age: The Attert Formation is dated from Late Triassic. The first miospore-rich bed below the Rhaetian Sandstone shows a Carnian assemblage as described in the "Grès à roseaux" (Biard, 1963; Van der Eem, 1974 in Schuurman, 1977). The vertebrate fauna discovered in the upper Bunte Mergel at Medernach favors a Late Triassic age (?Late Norian), as this assemblage closely resembles those found in the Knollenmergel in Germany (Duffin, 1993; Cuny et al., 1995).

Remarks: In subsurface, in the Campine Basin, the Gruitrode, Bullen, and Bree Members consist of sandstone, conglomerate and siltstone with a continental facies. The "Muschelkalk" and the "Keuper" are characterized by limestone, dolomite and evaporite. Upwards, the Sleen and Aalberg Formations, dated from Rhaetian to Lias, consist of dark clay and marl with some argillaceous limestone levels (Dusar et al., 1987, Wouters & Vandenberghe, 1994).