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2.3.8 Huet Formation - HUE

Version: revised October 2011, J. Verniers and A. Herbosch

Authors: after the abandoned quarry of Huet, north of hamlet of Fauquez, western side of the Sennette valley. These fossiliferous rocks are known since Dumont (1848) and previously described as « grauwacke fossilifère de Fauquez » (Leriche, 1920). Informally mentioned by Van Grootel et al. (1997), and described in Verniers et al. (2001). Formally described in Verniers et al. (2005).

Description: Greenish to grey mudstone, siltstone and fine sandstone, poorly sorted, with characteristic orange-yellow alveoli of decalcified fossil fragments. Heterogeneous sub-metrical beds, with a faint fining upward granulometry, a slightly undulating base and sometimes vague oblique stratification. Some levels are rich in (often decalcified) macrofossils (bryozoans, brachiopods, corals, cephalopods, crinoids, cystoids and trilobites; Mailleux, 1926) that indicate a shallow shelf environment of formation. These fossils are fragmented and concentrated mostly at the base of the beds. All the features mentioned point to a tempestite deposit above storm wave base (Vanmeirhaeghe et al., 2005). This postulated rather shallow position of this unit between the covering and underlaying units, both estimated to be deep marine, could possibly be explained by a eustatic drop in sea-level postulated at this time period (Vanmeirhaeghe, 2006). The base of the formation is not observed due to presence of faults, the upper limit is quite abrupt with the Fauquez Fm.

Stratotype: Sennette valley, in the Fauquez area, along the abandoned railway section and in the abandoned Huet quarry, 230 to 300 m north of the railway bridge in Fauquez (Verniers et al., 2005, fig. 2 and 7).

Area: Senne basin: Sennette, Coeurq, Senne and Thisnes (Nivelles) valleys and Orneau valley; possibly in boreholes of Western Flanders (Legrand, 1968).

Thickness: Sennette valley: minimum 60 m (Van Grootel et al., 1997; Verniers et al., 2005).

Age: The brachiopods suggest a Caradoc or Ashgill age for the formation (Maillieux, 1926a). The cystoids from the Sennette and the Orneau valley studied by Regnell (1951) indicate an (upper) Caradoc age. The trilobites from the Orneau valley at the Lefèvre quarry, studied by Maillieux (1926a) indicated a (mid) Caradoc age. A new collection of trilobites in Grand-Manil studied by Richter and Richter (1951), point to an (early) Ashgill age. The location of the new collection is unclear and one cannot exclude that it was sampled from the covering unit, the Madot Fm. The chitinozoan assemblages containing abundantly Lagenochitina baltica, L. prussica, Belonechitina robusta and Tanuchitina bergstroemi indicate undoubtedly a late Caradoc; from which a latest Oandu (early Cheneyan, late mid Caradoc) as the oldest possible age for the Huet Fm is concluded in relation to the Baltoscandian stages (Van Grootel et al., 1997; Van Grootel & Verniers, 1998 ms). Vanmeirhaeghe et al. (2005) and Vanmeirhaeghe (2006), in their restudy of the chitinozoans, placed the formation in the F. spinifera Zone inferred by the accompanying species. In Baltoscandia, it has a late Oandu – early Vormsi (mid Katian) age, whereas on British Avalonia, Vandenbroucke (2005) showed that F. spinifera Zone occurs from the upper Onnian (uppermost Caradoc, middle Katian) to the lower Cautleyan (lower Ashgill, middle Katian). Hence, they deduce a late Onnian (latest Caradoc, mid Katian) age for the Huet Formation.

Remarks: Synonym: "Formation de la grauwacke fossilifère de Fauquez" (Leriche, 1920). In the older litterature authors might have confused the macrofossil levels in the Madot Formation with the ones in the Huet Formation.