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2.3.9. Fauquez Formation – FAU (revised october 2011, A. Herbosch and J. Verniers)

 

Authors: From the locality of Fauquez, Sennette valley. Informally known in the litterature since Malaise (1873). This formation has been described as « schistes noirs de Fauquez » or «grauwacke fossilifère de Fauquez » by Maillieux (1926). Used as « Formation de Fauquez » by Herbosch et al. (1991, fig. 6) and more formally by Van Grootel et al. (1997).

Description: Centimetric to subcentimetric alternation of black slate and dark grey silty slate: lamination often present, are marked by millimetric to sub-millimetric pyrite crystals. From about 4 m above the base abundant graptolites are found. The rhythmicity, enhanced by the numerous pyrite levels, and the sequence of sedimentary structure (particularly well visible in borehole core; Herbosch et al., 1991) point to a low-density turbiditic interpretation (Stow model) and a deep marine environment of deposition. The dark colour and the conservation of the fine sedimentary structure indicate anoxic conditions. The lower boundary is a rapid transition, with a distinct change in granulometry, colour and strata thickness changing from decimetric in the Huet Fm. to cm or mm in the Fauquez Fm. The upper boundary with the Madot Fm. is unobserved because by fault.

Stratotype: Section in the sunken road « rue de Fauquez », at Fauquez, east side of the Sennette valley, municipality of Ittre. See detailed log in Verniers et al. (2005, fig. 4).

Area: Dender valley (described as "Unité I", in the Lessines borehole by Herbosch et al., 1991, 2005), Senne and Sennette valley. Not present in the Orneau valley, as the result of faulting (Herbosch, 2005).

Thickness: >35 m in the Sennette valley (Van Grootel & Verniers, 1998 ms). >58 m in the Lessines borehole, Dender valley (Herbosch et al., 1991).

Age: The graptolites in the Dender valley and Sennette valley indicate a clingani or linearis biozone (Elles in Maillieux, 1926, 1930). The restudy by Bulman (1950) indicated the clingani biozone for the Dender valley and the linearis biozone for the Sennette valley. Maletz and Servais (1998) could not observe the nominal species of the biozone(s) but concluded to an assemblage possibly belonging to the Pleurograptus linearis and/or upper part of the Dicranograptus clingani Biozone, corresponding to the middle part of the Katian (Ogg et al., 2008).

Earlier chitinozoan studies (Van Grootel, 1995; Van Grootel et al., 1998; Samuelsson and Verniers, 2000) mentionned the presence of Lagenochitina baltica, L. prussica, Belonechitina robusta and Tanuchitina bergstroemi indicating a late Vormsi to early Pirgu in terms of Baltoscandian stages, which correspond to a middle to late Katian for Ogg et al. (2008 fig. 5.5). Restudy of samples from Fauquez and Lessines borehole (Vanmeirhaeghe et al., 2005; Vanmeirhaeghe, 2006) shows that the formation show the Cyathochitina reticulifera subzone of the Fungochitina spinifera Zone which has a lower mid Katian age on Baltoscandia zonation (Rhakvere to early Vormsi stage) and a mid Katian on Britain zonation (late Onnian to earliest Cautleian stage). This confirms the former dating by graptolites. The Fauquez Formation can be placed in the mid Katian and more precisely in Time Slices Ka2 to Ka3 (Ogg et al., 20008 fig. 5.4 and 5.5).