GEOLOGICA BELGICA (2007) 10/1-2: 47-67 SEDIMENTOLOGY AND MAGNETIC SUSCEPTIBILITY OF THE COUVIN FORMATION (EIFELIAN, SOUTH WESTERN BELGIUM): CARBONATE PLATFORM INITIATION IN A HOSTILE WORLD Cédric MABILLE & Frédéric BOULVAIN (8 figures and 2 plates) Pétrologie sédimentaire, Département de Géologie, Bat. B20, Université de Liège, Sart-Tilman, B-4000 Liège E-mail : [email protected], [email protected] ABSTRACT. The Eifelian of Belgium is mainly characterised by a mixed siliciclastic-carbonate sedimentation on a ramp profile. In this context, the Couvin Formation is the more important and remarkable exception. It represents a carbonate platform initiation in a hostile environment. This work is mainly based on the stratotype, corresponding to two stratigraphic sections located in Couvin, along the southern border of the Dinant Synclinorium. These sections are the Eau Noire and Falaise de l’Abîme sections. Unfortunately, they are discontinuous. To allow a better understanding of the sedimentary dynamics, the data are complemented by a shorter but continuous section located in Villers-la-Tour (3.5 km West of Chimay). Petrographic study leads to the definition of 14 microfacies which are integrated in a palaeogeographical model. It corresponds to a platform setting where the reef complex is mainly constituted by an accumulation of crinoids, stromatoporoids and tabulate corals. The microfacies evolution is interpreted in terms of bathymetrical variations. It shows a general shallowing-upward trend encompassing the vertical succession of fore-reef settings, reef development, back-reef and then lagoon environment. This interpretation is supported by trends in mean magnetic susceptibility data, providing a better understanding of the sedimentary dynamics. Moreover, these data show positive correlation with concentrations of detritic minerals, but an inverse relationship with well washed skeletal limestones. The comparison of the three studied sections leads to considerations concerning the lateral variability in the Couvin Formation indicating more agitated conditions in the Abîme Member in Villers-le-Tour section. KEYWORDS: Eifelian, Couvin Formation, type area, carbonate platform initiation. 1. Introduction (Gosselet, 1888). Then, Mailleux introduced a new way to name the devonian formations (Mailleux, 1910). This Placing the Couvin Formation in its geological context is nomenclature (see Fig. 2) was slightly modified by various important to understand the interest of the present work. authors, and by Mailleux himself. Following the key of In fact, at the Eifelian-Givetian boundary, a large carbonate the Belgian geological map, the term “Cobm” was platform developed throughout northern Europe (Fig. introduced to refer to the basal limestone of the Couvinian 1A). Studying the Eifelian is consequently interesting to stage with stromatoporoids and corals (Heliolites porosa, understand this important palaeoenvironmental and Favosites polymorpha) (Mailleux, 1912). Then, “Co2b” ecological event. was proposed for the same lithostratigraphic interval This work details the sedimentology and the magnetic (Mailleux & Demanet, 1929) and later replaced by “Co2a susceptibility data on the stratotype of the Couvin + Co2b” (Bultynck, 1970), “Co2a” referring to Euryspirifer Formation cropping out at the southern flank of the Dinant intermedius zone and “Co2b” to the stromatoporoid Synclinorium between Macon to the west and Dion to the limestone. This author recognized three biostromal units east of the Meuse River (Fig. 1B). The thickness of this separated by subsidence phases. The term “Calcaire de formation is estimated to about 380m between Couvin Couvin” was reintroduced by Bultynck & Godefroid and Nismes but rapidly decreases to the east. (1974) and by Tsien (1974). The Couvin Formation name In this context, the Couvin Formation is a case study was finally officialised by Bultyncket al., (1991) and two because it represents a first start of the carbonate factory members were defined (in stratigraphical order): the in a mixed siliciclastic dominated environment (Fig. 2). Foulerie Member and the Abîme Member. 1.1. Short history 1.2. Location of sections The first definition of the “Calcaire de Couvin” was made The three studied sections are located in the Chimay- by Gosselet (1860) and refers to the most important mass Couvin area, southwestern Belgium (Fig. 3). The first one, of Eifelian limestone cropping out from Couplevoie to the Eau Noire section, crops out along the west side of the Nismes. This limestone was thought to be a fringing reef Eau Noire River. The starting point of sampling and 48 CÉDRICMABILLE&FRÉDÉRICBOULVAIN A Fenno S ca ndian High L o n d o n - B M ra old b unab a ian n High t H ig h Normannian High Lig Landmasses eri ra an C ille or ord Carbonate platform dill C era gian Vos Other depositional o - ern environments Arv BRABANT MASSIF B M IU OR LIN NC SY UR AM N IF SS A M T 50 Km M O IU EL OR V IN A CL ST YN T S AN DIN e s u e M 20 Km D Fault Carboniferous M Ch N C & Famennian Middle Devonian & Frasnian Villers- Couvin Lower Devonian la-Tour SERPONT MASSIF ROCROI MASSIF Caledonian Figure 1: A: Palaeogeographic setting at the Eifelian (390 Ma), after Ziegler (1982) and McKerrow & Scotese (1990) showing the large carbonate platform which develops throughout northern Europe and overcomes the mixed siliciclastic-carbonate ramp. B: Geological setting and location of the studied sections at the southern flank of the Dinant Synclinorium. C: Couplevoie, M: Macon, Ch: Chimay, N: Nismes, D: Dion. SEDIMENTOLOGYANDMAGNETICSUSCEPTIBILITYOFTHECOUVINFORMATION(EIFELIAN,S.W.BELGIUM) 49 description is located behind the swimming pool in the 1.3. Methods Foulerie locality. The base of the section (Pl. 1A) Bed-to-bed description and sampling were carried out in corresponds to the base of the Couvin Formation as 2002 and 2003 for the Eau Noire and the Abîme sections. defined as the first succession of thick-bedded limestones The description of Villers-la-Tour section is adapted from overlying the last thick bed of calcareous shales of the the work of Marc Bertrand (Bertrand, 1990; Bertrand et Eau Noire Formation (Bultynck et al., 1991). Although al., 1993). From the samples, 930 thin sections were the first 80 and last 50 metres of section are more or less prepared. The textural classification used to characterize continuous (Pl. 1B), the middle interval of 160 metres is the microfacies follows mainly the textural-structural poorly exposed. So, even if the studied interval has an principles introduced by Dunham (1962) and Embry & approximate thickness of 285 metres, only 135 metres of Klovan (1972) but also Folk (1959). The description of outcrop are really present. The section ends behind the stromatoporoids is based on morphological classification houses in the Rochettes Street (Pl. 1C). by Kershaw (1998). The terms used are branching, laminar, domical and bulbous. The term massive (Tucker & Wright, 1990) is used for both domical and bulbous The second section corresponds to the cliff above the forms when the difference cannot be made (fragments or “Trou de l’Abîme” cavern, on the east side of the Eau thin sections). The term “coverstone” characterises Noire River (Pl. 1D). This section has a thickness of 33 microfacies where laminar organisms cover mud and metres. The stratigraphic gap between the two sections bioclastic debris (Tsien, 1984). can be approximately estimated to 85 metres after Bultynck (1970) and from the geological map (Marion & This led to the definition of 14 microfacies, followed Barchy, 1999). In the present work, the total thickness of by the construction of a sedimentological model, and by the Couvin Formation is estimated to 400 m. This value is the plotting of microfacies curves. These microfacies are to be compared with the classical estimation of 380 metres compared to those defined for the limestone strata near the in Couvin (Bultynck & Dejonghe, 2001). The difference Eifelian-Givetian boundary in Belgium and France can easily be explained by gaps in the Eau Noire section, (Mabille & Boulvain, in press; Préat & Kasimi, 1995) and presence of faults, various estimations made for gaps to Standard Microfacies (Wilson, 1975). between the two sections, etc. The bulk magnetic susceptibility for every sample was measured using a KLY-3 Kappabridge. The magnetic susceptibility response was measured three times and The third section is located 3.5 kilometres West of these values were averaged. Resolution of 0.01g was Chimay, in an abandoned quarry in Villers-la-Tour (Pl. sufficient for determining the sample weight. These 1E). This section shows a 55 m-thick continuous operations allow the calculation of the mass-calibrated succession of strata. magnetic susceptibility of each sample. Conodont Gosselet Mailleux Stage Zones Formation Generalized lithostratigraphic section 1888 1910 After BULTYNCK P. & DEJONGHE L., 2001 Nismes N Fr2 FRASNIAN falsiovalis A I Frasnian N S Fromelennes A Fr1 ansatus R F Figure 2: Generalized rhenanus/ lithostratigraphic section varcus N Mont d’Haurs of Middle Devonian A I Gv1b GIVETIAN Givetian T formations at the southern E timorensis V I Terres d’Haurs border of the Dinant G Synclinorium, after Trois-Fontaines Gv1a hemiansatus Bultynck & Dejonghe Co2d Hanonet (2001). The studied ensiensis La Lomme Sandstones interval corresponds to kockelianus the Couvin Formation. It Co2c is located under the australis Jemelle boundary between the N Eifelian (ramp-related A I sedimentation) and the N EIFELIAN I V Givetian (carbonate Eifelian U O costatus p l a t f o r m - r e l a t e d C Studied interval sedimentation). The Co2a + Couvin Couvin Formation Co2b partitus represents a first start of Co1b+Co1c Eau Noire the carbonate factory in a patulus 100m mixed siliciclastic Coa1 Saint-Joseph EMSIAN dominated environment. Em2b serotinus Hierges See Fig. 5 for legend. 50 CÉDRICMABILLE&FRÉDÉRICBOULVAIN Couvin Chimay 2 1 3 Modern alluvium Famenne Group Frasnes Group Givet Group Hanonet Fm. Jemelle Fm. N A Couvin Fm. 2 Emsian 1km Praguian and Lochkovian 3 Fault B Modern alluvium Road Jemelle Fm.