2.1.1. Hainaut Group - HAI
Authors: Robaszynski (1975) and herein.
Description: Clays, sands and conglomerates deposited irregularly, in lenticular "heaps" along and above each other, forming successions, which are complex and variable from one site to the next. They correspond to continental deposits in which neither carbonate rocks nor glauconite have ever been found.
The white,grey, black and occasionally red clays were used formerly to produce refractory tiles. For their kaolinite content they are still used today in the cement industry. They always contain high quantities ofquartzosesilt and, locally, lignified wood, often pyritized.
The sands and conglomerates are well represented at the top of the clay levels at Hautrage. They form an extensive torrential delta at Thieu (former St-Pierre quarry).
Several formations may be recognised, ignoring the order of deposition:
- Sainte-Barbe Clays Formation (SBA): grey, sandy, lignitic and highly fossiliferous clays, filling a sinkhole.
- Hautrage Clays Formation (HAU): silto-quartzose,grey, white, reddish, black clays with intercalations of sandy,siltyand lignito-pyritic beds.
- Baudour Clays Formation (BAD):grey siltyand variegated clays.
- Saint-Pierre Gravel Formation (SPR): coarse- and fine-grained white sands, gravel, eroded pebbles, conglomerates, silty beds, torrential and deltaic deposits with cross-bedded stratification.
Locally, only continental faunas and floras have been collected from fossiliferous deposits.
Stratotype: In the Haine River basin, and for each of the formations:
- Sainte-Barbe Fm.: in the former Ste-Barbe pit of Bernissart where the iguanodontid dinosaurs were discovered;
- Hautrage Fm.: in the Hautrage clay pits;
- Baudour Fm.: in the Baudour clay pits;
- St.-Pierre Fm.: in the former Château St.-Pierre quarry at Thieu and in the foundation pits of the sluices at the Strépy-Bracquegnies ships-lift.
Area: Three types of deposits:
- a circa 40 km long "band" on the northern slope of the Mons Basin, from Hautrage to Thieu: clays, sand and gravel;
- the sinkholes (on top of thePalaeozoiclimestones, filled in by the Wealden facies), e.g. at Bernissart;
- dissolution pockets in thePalaeozoiclimestone bedrock (in the Tournaisis, in Entre Sambre-et-Meuse, .....).
Thickness: Several tens of m; a 150 m deep borehole at Hautrage did not reach the base of the continental clays.
Age: Late Jurassic ("Purbeckian") to Early Cretaceous (Aptian) and possibly even extending into Early Cenomanian (white sands in the lift foundation pit at Strépy?) as based on floral and faunal content: flora: Cycadites, Cedrus, Pinus, ferns, conifers (Elatides, Pityostrobus), gymnosperm pollen grains (no angiosperms), pteridophyte spores; fauna: especially at the famous Bernissart site where several hundreds of specimens were collected, among which: dinosaurs: Iguanodon bernissartensis, I. mantelli; crocodiles: Bernissartia fagesi, Goniopholis simus; turtles: Chitracephalus dumoni; numerous fishes: Oligopleurus, Lepidotes, Amiopsis, Pholidophorus.....; also bivalvemolluscs: Unio, Cyrena etc.
Remarks: - The "Groupe(continental) du Hainaut" was proposed by Robaszynski in Marlière & Robaszynski (1975, p. 4) to replace the "Groupe continental infra-crétacé" as suggested by the Mesozoic Commission who adapted the names to international standards. It equates with "Wealdien" or "faciès wealdiens" of authors.
- The name has been used with the rank of Formation by Doremus (1997).
- References: J. Cornet (1923); Conseil Géologique de Belgique (1929); Marlière (1946, 1954); Robaszynski (in Marlière & Robaszynski, 1975).
Legend : Danube-Bouchon clay pit in Hautrage, type section for the Hautrage Clay Formation (plate 1 in Gomez et al., 2008. Palaeoflora from the Wealden facies strata of Belgium – mega- and mesofossiles of Hautrage. Memoirs of the Geological Survey of Belgium 55, p. 56).