3.5 Jamoigne Formation - JAM

Authors: Dumont, 1842; Boulvain et al. (2001).

Description: Bioturbated dark grey marl with decimetre-thick argillaceous-sandy limestone or sandstone beds (increasingly frequent towards the top of the unit). The Warcq Member, capping the formation at the contact with the Luxembourg Formation, consists of light grey argillaceous sand or sandstone. Locally, micaceous sandstone and gravel beds are observed at the base of the Jamoigne Formation. Fauna and bioturbation are quite abundant.

Stratotype: Latour borehole.

Area and thickness: The formation outcrops in Belgian Lorraine, with a rather constant thickness of about 45 m. The thickness gradually increases towards the south to reach 70 m in boreholes close to the French border. In the west, the Jamoigne Formation is transgressive on the Rhaetian (Rossignol Member) and on the Palaeozoic basement (Muno Member) with a more arenaceous and fossiliferous facies.

Age: The Jamoigne Formation is only Hettangian in age in eastern Belgian Lorraine, covering the Psiloceras planorbis, Alsatites liasicus and Schlotheimia angulata ammonite Zones; in the western part, the formation also includes the Warcq Member (Arietites bucklandi Zone) and thus extends into the Lower Sinemurian (Maubeuge, 1954; Mergen, 1984; Guérin-Franiatte & Muller, 1986).

Remarks: This formation was subdivided as various "Assises" by Dumont and later authors on a biostratigraphic basis: namely the Helmsingen or Helmsange, Ansart, Jamoigne ss. and Warcq Assises. Some are not included here because of criteria for their definition; the Member of Warcq is redefined on a purely lithostratigraphic basis.