Contr. Tert. Quatern. Geol. 34(3-4) 53-67 10 figs., 1 tab. Leiden, October 1997 Review of the lithostratigraphy of the Middle Eocene in northern Belgium Bart Fobe GEOLOGISCH INSTITUUT UNIVERSITEIT GENT GENT, BELGIUM Fobe, Bart. Review of the lithostratigraphy of the Middle Eocene in northern Belgium. — Contr. Tert. Quatern. Geol., 34(3-4): 53-67, 10 figs, 1 tab. Leiden, October 1997. The lithology of Middle Eocene deposits, and that of stone layers in particular, was studied in subcrops in northwestern Belgium, allowing the conclusions their be drawn. On of its brown the of the Pittem following regarding lithostratigraphy to account colour (lignite), upper part Member (Gent Formation) has proved to often be a conspicuous key horizon. On the basis of a study of the geometry of the deposits in various boreholes and of the sandstone facies it can now be confirmed that the Beemem Member (previously referred to the Aalter or Knesselare formations) in fact belongs to the Vlierzele Formation. The Oedelem Formation(new name) consists of fossil-rich sand (Aalter Member or Aalter sands on of which the which sand. sensu stricto) top rests Evergem Member (new name), comprises glauconitic In borehole interpretations, this unit has been confused with the Vlierzele sand or with the Wemmel sand. Previous records of Lede Formation strata in the area between Gent and the North Sea coast are here confirmed. Key words — Eocene, Belgium, lithostratigraphy. Fobe, Instituut, Universiteit B. dr. sc., Geologisch Gent, Krijgslaan 281 S8, B-9000 Gent, Belgium; and, Herentalsesteenweg 66, B-2220 Heist-op-den-Berg, Belgium. Contents aspects of these. Stone layers in the Belgian Eocene often show typical petrographic features, which makes them useful in and correlations. Introduction p. 53 lithostratigraphic interpretations studies have been made of the Lithostratigraphic framework p. 54 Comparable recently borehole lithostratigraphy of the Kallo 1988), Ursel- Study of sections and stone layers p. 55 (Fobe, of stone 59 Maldegem (De Breuck et al., 1989), Mol, Oostham and Stratigraphy layers p. Kwaadmechelen Beerzel, Booischot and Discussion p. 62 (Fobe, 1989), Rillaar boreholes. The stone facies recorded Conclusions p. 65 (Fobe, 1993a) from boreholes with those in the Acknowledgements p. 66 were compared occurring in References 66 outcrop area northern Belgium. p. Both calcite- and silica- (opal or chalcedony-) cemented stone layers occur in the deposits studied. Their Introduction origin will not be discussed in the present paper; the reader is referred to Fobe (1990) for a discussion of silica- This paper presents a new interpretation of the cemented sandstones and to Fobe (1986) for that of sandy lithostratigraphy of Middle Eocene strata in the area limestones. between Antwerp and the North Sea coast (NW Belgium), Thin sections were stained following the method where these deposits are almost exclusively known from described by Evamy (1963) in order to distinguish ferroan boreholes. from non-ferroan calcite. Whenever necessary, X-ray In results here based part, presented are on a diffraction analysis (XRD) was used to check the of cemented which petrographic investigation layers occur mineralogy. in these with reference the lateral shows the location of the and deposits, particular to Figure 1 outcrops and vertical distribution of stone facies and their boreholes studied, while Fig. 2 gives the key to the lithostratigraphic significance rather than to diagenetic lithology of all other figures, except Fig. 6. 54 (1988) to comprise the Merelbeke, Pittem and Vlierzele the Gent Formation members. In the present study, use of in which it follows Fobe's (1996) new definition, comprises only the Merelbekeand Pittem members. 3 - Vlierzele Formation Formation defined Steurbaut & Nolf The Vlierzele was by (1986), who showed that in its type section, the Ursel- Maldegem borehole, this unit comprised the Pittem sandy clay and the Vlierzele sands (sensu stricto) of previous authors. Fobe (1996) presented a new definition and 1. of northwestern Belgium and contiguous French Fig. Map excluded the Pittem sandy clay. In the area between Gent territory, showing the location of outcrops and boreholes and the North Sea coast, the Vlierzele Formation was studied for the Abbreviations are as present paper. established in in described from and auxiliary stratotypes follows: AA - Aalter, BA - Balegem, BE - Beerlegem, the Hijfte and Ursel-Maldegem boreholes. According to EG - Egem, EV - Evergem, HY - Hijfte, KA - Kallo, KL formation has lower and Fobe (1996), the a sharp upper - Kluizen, KN - Knokke, KR - Kruibeke, MO - Mol, NK boundary and can be subdivided into five members, from - - Nieuwkerke-Waas, OE - Oedelem, RI - Rillaar, UR bottom to top: Hijfte (gravel and sand), Lochristi (clayey Ursel, WA - Waarschoot, WI - Wippelgem, ZE - Oosterzele Beernem and Zeebrugge. sand), (sand), (clayey sand) Drongengoed (sand). When compared with the legend to the the new geological map (Jacobs et al., 1993b), Vlierzele Formation corresponds to the (undifferentiated) Vlierzele Member. Fobe (1996) argued that the concept of undifferentiated Vlierzele sand unit which an passes downwards gradually into the Pittem Member (sandy clay) used in the the lead as legend to geological map, may to correlation errors. The validity of the Vlierzele Formation as proposed by Fobe (1996) will be further discussed in the present paper. 4 - Oedelem Formation In the area between Gent and the North Sea coast, resting Fig. 2 Key to the lithological logs in Figs 3-5 and 7-10. sands on top of the Vlierzele are poorly exposed sands, Abbreviations used in these figures are as follows: Fm. - which were referred to as Paniselian' on the old Formation, Mbr. - Member, We - Wemmel Member, 'Upper - 1:40 The name 'Aalter sands' B.N. - Bande Noire, EV - Evergem Member, AA Aalter geological map (scale 000). - - introduced Leriche Member, DR Drongengoed Member, BE Beernem was by (1941). Member, OO - Oosterzele Member, LO - Lochristi Nolf (1972) established the Den Hoorn Formation to Member, HY - Hijfte Member, PI - Pittem Member, ME comprise the Oedelem and Aalter members, with the latter - MerelbekeMember and EG - Member. Egem occurring in gullies incised into the former. Jacobs & the Geets (1977) added the Beernem Member as the Knesselare lowermost unit in what they defined as LITHOSTRATIGRAPHIC FRAMEWORK Formation, concluding that only the Oedelem Member extended throughout nearly the entire area between Gent 1 - Ieper Formation and the North Sea coast. Jacobs & Geets (1977) described the Aalter Member (corresponding to the Aalter sands s. the studied in the of local at Aalter and Underlying stratigraphic sequence str.) as an infilling (i.e. Gent) gullies present paper is the Egem Member (sand) of the leper in the Oedelem Member. The areal extension of the Formation of Early Eocene age. Beernem Member was limited to the region south/southeast of Brugge, which means that the Knesselare Formation mainly comprised only the Oedelem 2 - Gent Formation Member. For a number of years now, the name Aalter Formation The Gent Formation introduced Marechal & it is also used in the was by Laga has appeared in the literature; legend 55 Wemmelsand in the to the geological map (Jacobs et al., 1993a, b). The reason same area. is that the The of the Lede Formation has been why the Knesselare Formation was abandoned presence recently recorded Aalter & Nolf, and in the fossiliferous sands exposed in the Aalter type section and at (Steurbaut 1989) Breuck the Oedelem Member of boreholes north of Aalter were Ursel-Maldegem area (De et al., 1989). assumed to be equivalent deposits (Jacobs et al., 1993a; Maréchal, 1993). The Aalter Formation of the new geological map (scale 1:50 000; Jacobs et al., 1993a, b) 7 - Maldegem Formation comprises the Beemem and Oedelem members. However, This unit referred Formation the Aalter Formation has never been defined formally (formerly to as Meetjesland consists of alternation of according to the guidelines for lithostratigraphic or Kallo Formation) an sandy & At its nomenclature (Hedberg, 1976). Occasionally, reference clay and clay deposits (Jacobs Sevens, 1988). the Asse Member clay), has been made to Steurbaut & Nolf's (1989) base, below (glauconitic sandy comprehensive bio- and chronostratigraphic study of the occurs either a glauconitic sand, with upward-increasing Aalter sands at the type locality. However, Steurbaut & clay content (Wemmel Member) or a glauconite-rich, fossil-rich sand few Nolf (1989) did not provide a definitionnor a discussion of occasionally gravelly layer (a of these did decimetres known the 'Bande Noire'. the lithostratigraphic position sands, nor they thick) as Brussels discuss their relationship with previously defined The Wemmel Member is known from the and from the between Gent and the coast; the units, the Oedelem Member. area area lithostratigraphic e.g. latter is of interest in the On the Somewhat confusing also is the use of the facies name area present study. the Wemmel 'Aalter equivalent' as a part of the Aalter Formation, geological map (Jacobs et al., 1993b), and, as is covering the Oedelem Member in boreholes in the North Member rests upon the Aalter Formation usual, overlain the Member. in the Sea (Jacobs & Sevens, 1993). in turn by Asse However, the Fobe (1996) proposed to reassign the Beemem Ursel-Maldegem boreholes, the lower part of sands the Wemmel Member and probably also the basal part of the Oedelem glauconitic traditionally assigned to Formation Member (Drongengoed Member) to the Vlierzele Member have been correlated with the Lede on and evidence Breuck Formation, in view of the fact that the Beemem Member palaeontological petrographic (De et this is discussed further. has also been encountered (as a sandy clay or clayey sand al., 1989). Below, problem intercalation) outside the area designated by Jacobs et al. confines of (1993b) on the geological map, but within the LAYERS the Vlierzele Formation, and within the Vlierzele Member STUDY OF BOREHOLE SECTIONS AND STONE considered to be of the geological map, there undifferentiated.