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5.1. Hastière Formation – HAS

 
Authors: de Dorlodot, 1895; Van Steenwinkel, 1980; Paproth et al., 1983.
 
Description: The formation can be divided into 3 members. The lower member (“Tn1ba”) starts in the DSA with a metre-thick grainstone to rudstone overlain by thin-bedded crinoidal packstones interbedded with shales. The middle member (“Tn1bb”) consists of thick-bedded crinoidal packstones. The upper member (“Tn1bg”) is similar to the lower member, but it is usually more shaly.
 
Stratotype: Abandoned and partly filled quarry (the Demanet quarry in de Dorlodot, 1895) along the road from Hastière-Lavaux to the hamlet of Insemont, in front of the Pont d’Arcole cave (DSA). The section by the railway bridge at Anseremme, south of Dinant, is a parastratotype.
 
Area: This unit can be recognized almost everywhere in the Namur-Dinant Basin.
 
Thickness: In the DSA, the formation has a thickness of about 20 to 35 m, whereas it is only 7.25 m thick in the eastern NSA (Vesder area).
Age: Early Tournaisian (Hastarian). Siphonodella Conodont Zone; Cf1 Foraminifer Zone; RC1a, bRugose Coral Subzones). The thick basal bed of the formation has yielded typical elements of a Devonian fauna, for the most part reworked. Siphonodella duplicata enters 8.5 m above the base of the formation (Van Steenwinkel, 1980). There are rare occurrences of Tournayellina beata and Chernyshinella sp. in the lower and middle members. Septabrunsiina minuta and S. rudis enter in the upper member (Tohogne borehole; Bouckaert et al., 1978). Corals of the RC1 Zone (Conilophyllum priscum, Melanophyllum kremersi) appear at the base of the formation or just above the basal thick bed when it is present. The basal bed itself yields reworked corals from the RC0 Zone. The lower and middle members correspond respectively to the end of the TST and to the HST of the third-order sequence 1 of Hance et al. (2001). The upper member forms the lower part of the TST of sequence 2.