2.3.7 Cimetière de Grand-Manil Formation
Version: revised February 2017, A. Herbosch and J. Verniers
Authors: This lithostratigraphic unit was first described by Michot (1980) as « …schistes compact verts affleurant au N du Moulin de Grand-Manil ». He grouped this unit with the overlying shelly slate (now Huet Formation) in the Moulin Formation. More recently, Delcambre & Pingot (2002 Fig. 15) grouped within the Bornival Formation two very poorly outcropping units: the « Unité de la Chapelle Sainte Adèle: schistes silteux verts, argileux, assez homogènes et micacés » and the « Unité du Cimetière de Grand-Manil: siltites gréseuses compactes, riches en pyrite ». Herbosch (2005) correlated the « green slate » from Grand-Manil with other « green slate » that crops out near Rebecq in the Senne valley under the new name Hospice de Rebecq Formation. The study of the magnetic properties of the Brabant slates (Debacker et al., 2010) strongly suggests that the latter correlation was erroneous and that the green rocks of Rebecq belong to the Asquempont Member of the Oisquercq Formation. After new investigations, Debacker et al. (2011) concluded that the “green slate” of Grand-Manil is indeed different from the green slate of Rebecq and proposed the name « Cimetière de Grand-Manil Formation » for this new formation.
Description: The formation shows faintly stratified greenish-grey to often olive-green slate (mudstone to siltstone). Slates with a more silty grainsize and pluricentimetric sequences of planar, oblique and convolute laminations are frequently observed and are interpreted as very distal turbidites of Bouma type. In outcrop, weathered and oxidized nodules parallel to the bedding are also frequently observed (Debacker et al., 2011 Plate 1 photos D to H).
Stratotype: No stratotype could be designated but discontinuous small outcrops occur around Grand-Manil, in the Orneau valley. The best outcrops are along a path to the NW corner of the cemetery (Herbosch, 2005 Fig. 4 outcrop number 3, 5, 6; Herbosch & Verniers, 2014 Fig. 7 outcrop A).
Area: The formation poorly crops out in the Orneau valley and estimated to occur also in the Sennette valley near the location of the ruined castle of Fauquez (Debacker et al., 2011).
Thickness: Its thickness is estimated at about 100 m. The upper and lower contacts have not yet been observed.
Age: No macrofossils and no microfossil have yet been investigated. Based on its geometrical position between the Cheneyan (lower Katian) Bornival Formation and the lower Onnian (middle Katian) Huet Formation , this formation should be placed in the lower half of the Streffordian, mid Katian Stage (Herbosch & Verniers, 2014 Figs. 2, 3). A biostratigraphical hiatus already recognized at this level by Verniers et al. (2001) was confirmed by Vanmeirhaeghe (2006), who emphasized the strongly differing chitinozoan assemblages of the Huet Formation compared to those in the Ittre and Bornival formations.