Bulletin de la Société belge de Géologie T. 95 fasc. 4 pp. 257-280 Bruxelles 1986 Bulletin van de Belgische Vereniging voor Geologie V. 95 dee l 4 blz. 257-280 Brussel 1986 CONODONT SUCCESSION ACROSS THE TOURNAISIAN-VISJ;:AN BOUNDARY BEDS AT SALET, BELGIUM by Zdzislaw BELKA (*) and Eric GROESSENS (**) KEY WORDS Conodonta, Tournaisian, Visean, Boundary, Salet, Dinant Synclinorium, I l'HRODIJCT I OM, The taxonorny of the conodont This study offers a rev1s1on of genera as SiphonodeZZa, Gnathodus, PseudopoZyg­ the conodont fauna recovered frorn the nathus, ScaZiognathus, Mestognathus , that ex­ section called "Route de Salet" which is hibi t a great importance for the Lower exposed along the Molignée Valley, in a Carboniferous stratigraphy have recently small quarry and it continues along the been revised by SANDBERG et aZ. (1978), road to Salet (Text-figs 1-2). The pre­ LANE, SANBERG, ZIEGLER (1980), Helka cise location and accessibility of this (1983) and LANE & ZIEGLER (1983). New exposure is given by CONIL & GROESSENS concept of several species as well as (1974 : 24), GROESSENS (1975 : 45) and descriptions of new taxa stirnulated us to HANCE (1985). It is a classical and one re-investigate the conodon~ occurring in of the rnost important sections for the the classical sections of the Dinant Syn­ Lower Carboniferous stratigraphy. As a cl inoriurn. The purpose of this study is parastratotype section for the Lower also to atternpt an application, in the Viséan (CONIL, 1967) it attracted the Dinantian type area, of the prelirninary interest of many workers being the area standard conodont zonation proposed by of an intense search for microfossils LANE, SANDBERG & ZIEGLER (1980) for the (see HANCE, 1985). Several foraminiferal post- SiphonodeZZa interval. taxa (cf. CONIL & LYS, 1964) have been 0 5 10km Fig. 1 - General location map. Patterned area shows extent of the Carboniferous deposits in the Din~nt Synclinorium. ( *) Institute of Geology, University of Warsaw, Al. Zwirki i Wigury 93, 02-089 Warszawa (Poland). ( **) Geological Survey of Belgium, 13 rue Jenner, B-1040 Brussels (Belgium). 257 described from this exposure for the first grains. The Leffe facies extends land­ time and it is also a type locality for wards for many kilometers away from the two stratigraphically important conodont buildups (LEES & CONIL, 1980; LEES, 1982) species, DoUymae bouckaerti GROESSENS and and it always separates the latter from Eotaphrus buUyneki ( GROESSENS). The holoty­ the Molignée faciès. types of these conodonts are reillustrat­ ed- herein. The Calcaire de la Molignée forms the upper unit sampled for cono­ The section at Salet (Text- donts in the "Route de Salet" section. fig. 2), moreover, is a type section for The limestones are wackestones and lime two lithostratigraphical units of the mudstones with numerous shaly interbeds. classic Dinantian of Belgium, the Calcaire The composition of allochems is similar Noir de la Malignée (GROESSENS, 1975) and to that-of the Leffe facies, but from the the Calcaire de Salet (GROESSENS, CONIL & 'Bed 87 the increase of foraminifers in LEES, 1976). The base of the Moliniacian, place of bryozoans is noted to occur .. i.e. the stage of the Viséan, locally dis­ According to HANCE (1985 : 182), the lower tinguished in Belgium, was also defined part of the Calcaire de la Molignée at here at the Bed 52 (PAPROTH et al., 1983 : Salet reflects regressive conditions, 188) . whereas the Viséan transgression is indi­ cated much later by the first appearance The investigated conodont fauna of the foraminiferal fauna dominated by includes the raaterial collected and ori­ the Archaediscidae in the Bed 215. ginally described by GROESSENS (1971, 1975) and also conodonts obtained during additional recent sampling that produced CONODONT DISTRIBUTION, some new conodont occurrences. The sam­ ples were collected from the lower por­ Very closely sampling of long tion of the sequence only up to the Bed intervals in the "Route de Salet" section 285 where there occur slump structures provided substantial account of conodont at the top of the Calcaire de la Molignée succession in the late Tournaisian and (Text-fig. 2). early Viséan strata. Although the observ­ ed occurrences of the majority of cono­ dont species (Table 1) agree with their LITHOLOGY, recognized stratigraphie ranges (cf. LANE, SANDBERG & ZIEGLER, 1980), some species, At Salet, a 320 meter thick however, demonstrate new occurrences that section of the Lower Carboniferous carbo­ appear to be very significant for the nates is exposed, but only 130 m were phylogeny of these species and for the covered by this study. The investigated Lower Carboniferous conodont stratigraphy. portion of the section consists of three iithological units (Text-fig, 2) that The most remarkable is the first constitute proximal lateral equivalents appearance of Gnathodus texanus , a zonal of the Waulsortian facies (cf, LEES, marker of the texanus Zone. The lower 1984). range of this species was- known to start simultaneously with the extinction of The lowest unit is developed as Scaliognathus anchoraUs (LANE, SANDBERG & crinoidal and bryozoan packstones that are ZIEGLER, 1980 : Tab. 2; LANE & ZIEGLER, massive or interbedded with thin shales in 1983). In contrast, G, texanus occurs at the lower part of this unit. These sedi­ Salet just below the appearance of ments pass laterally into the Bayard fa­ Scaliognathus anchoralis europensis (Text-fig. cies considered to be located in the 3). Similary to that, the ranges of both neighborhood of the Waulsortian buildups. species are recently noted to overlap in the Dinantian rocks of southwestern Ire­ Overlying is the Leffe facies, land (THORNBURY, 1985) and VARKER & which comprises well-bedded wackestones SEVASTOPULO (1985 : Tab. 6) indicated and packstones that contain various allo­ that for the Lower Carboniferous conodonts chems and first of all peloids, bioclasts in Britain and Ireland the range of of echinoderms and bryozoans, and coated Gnathodus texanus extends from the middle s N , Mohgnee. 1 1 0 w Ol 0 0 - 0 10 2om Bioul 22 Bioul 8 ~ 1 Fig. 2 - The lower part of the "Route de Salet" section sampled for conodonts. Black triangle indicates the position of the Tournaisian-Viséan boundary. Bed numeration after Overlau et Conil, 1965. 258 part of the anchoraZis Zone. The record of conodont zones in the Tn3a-V1b inter­ of G. texanus at Salet does not appear to val has also been outlined at that time. be a freak of nature. It is well docu­ These zones were incorporated later by mented in our fauna by transitional forms AUSTIN (1973) into his proposa! of the (see Table 1 and Pl. 6, Figs 7-9) showing Dinantian conodont zonation suggested the evolutionary relation between Gnathoclus for application in Europe. semiglaber and Gnathodus texanus. The former species gave rise to G. texanus by the re­ The conodont zonal scheme for duction of the inner parapet and platform the Lower Carboniferous currently used ornamentation. The reduction trend of in Belgium was established by GROESSENS morphological changes in the conodont (1975). Recently, it was in part modif­ platform, as late Tournaisian gnathodon­ ied to produce some new detailed sub­ tan Pa elements display (BELKA 1984 : fig. divisions (PAPROTH et al., 1983 : Tab. 10; 1985b) is also exemplified by the 2). Although the usefulness of this origin of Gnathodus pseudosemigZaber from G. scheme for the Belgian stratigraphy cuneiforms. This is indicated by transi- "'remains unquestionable, i ts global appli­ tional forms (Pl. 5, Figs. 4-9) as well as cation proves to be limited. This is by ontogenetic development of small spe­ because some species used for definition cimens of G. pseudosemiglaber which recapi­ of subzones (e. g. Dollymae hassi, Dollymae tulate a character of G. cuneiformis. To bouckaerti, Eotaphrus bultyncki) are rather show phyletic sequence of above mentioned rare out of Belgium while other index species of Gnathodus, the transitional forms as Polygnathus communis carinus and forms are separately presented in Table 1, Eotaphrus burlingtonensis appear in Belgium but they are not considered herein to be significantly later than their lower the separate species. ranges recognized in many Lower Carboni­ ferous sections (see LANE, SANDBERG & The other anomaly which is ZIEGLER, 1980). Undoubtedly, this is worthy to note, is the record of Pseudo­ why the framework of the world-wide pre­ polygnathus pinnatus. This species (Pl. 2, limina ry standard conodont zonation Fig. 9) yields earlier appearance relative (SANDBERG et al., 1978; LANE, SANDBERG to Pseudopolygnathus oxypageus, Dollymae & ZIEGLER, 1980) differs considerably bouckaerti, and Eotaphrus bultyncki. It is from that recognized in Belgium by much earlier than was recognized by LANE, GROESSENS (1975). SANDBERG & ZIEGLER (1980 : Tab. 2) but identical range of Ps. pinnatus reflecting, The investigated section yields most probably, an actual stratigraphie all diagnostic forms which allowed to range of this species is recorded from identify all the conodont zones of Upper Britain and Ireland (VARKER & SEVASTOPULO Tournaisian and Lower Viséan strata sensu 1985: Tab. 6). GROESSENS (1975), i. e. the carinus Zone, the anchoralis Zone, and the beckmanni The genus Scaliognathus is repre­ Zone (Text-fig. 3), and also to correlate sented by Sc. praeanchoralis (Pl. 1, Fig. this section with other successions of 7), Sc. anchoralis fairchildi (Pl. 1, Figs, Belgium (GROESSENS, 1975; GROESSENS, 8-11) and Sc. anchoralis europensis (Pl. 2, CONIL & LEES, 1976). Fig. 2). LANE & ZIEGLER (1983 : 205-206) suggested that Sc. a. europensis probably Of special interest in this developed from Sc. a. fairchildi although classical section, however, was also to the appearance of Sc. a. fairchildi before test up the preliminary standard cono­ Sc. a. europensis was not documented at that dont zonation. The results we received time. Now, such an evidence is recorded. show the scheme is to be corrected. In the "Route de Salet" section Sc. a, fairchildi cames 1. 5 m be fore Sc.