The information on this page is a summary description.
The full formal description is available here: Boncelles Formation
Abbreviation
Bn
Parent unit
Child units
Lithological description
The Boncelles Formation forms the fine-grained upper part of remnant Oligocene sand deposits on the eastern Hesbaye and Condroz plateaus, up to the Hautes Fagnes region (= Boncelles Formation sensu stricto, restricted to the ‘Chattian’). In the Boncelles type area (Figure 0-1) this lithostratigraphical unit consists of rather homogeneous well-sorted fine grained pale yellow sand, containing decalcified mollusc layers in some locations (= sables supérieurs on Figure 0-2). The top is weathered and rubified, a feature shared with the overlying Graviers liégeois (Liège Formation) (= limons et graviers on Figure 0-2). The base is formed by a thin double layer of red-stained glauconiferous coarse-grained sand with rounded quartz pebbles, overlying more heterogeneous coarser grained sand which is bleached white in colour (= sables moyens et inférieurs on Figure 0-2), resting on a residual flint deposit at the contact with the Palaeozoic bedrock (= silex on Figure 0-2). However, according to the latest revision of the Wallonian geological map, the Boncelles Formation in the Boncelles type area on the Condroz plateau south of Liège (Figure 0-1) contains the entire sand sequence, corresponding to the Om unit of the old geological map (Forir & Mourlon, 1897). This Boncelles Formation sensu lato thus includes both ‘Chattian’ and ‘Tongrian’ parts, cf.0 Age). The upper or ‘Chattian’ part of the Boncelles Formation sensu lato in the Condruzian type locality (= Boncelles Formation sensu stricto) is displaying a rhythmic succession in sets of 30 to 50 cm thickness, consisting of finely stratified sand with cross-bedding at their base and bioturbations at their top, exception made for the rubified top of the section. The sedimentary environment is open marine outside tidal influence, whereas the underlying ‘Tongrian’ sand has been deposited in tidal environments. The boundary between both units marks a hiatus with reorganisation of the sedimentary environment (Macar, 1934; Sierakowski, 1970). No sedimentary features were described from other occurrences. According to the proposal by Marion et al. (2018) the Boncelles Formation on the eastern Hesbaye plateau is restricted to the ‘Chattian’ part of the eponymous formation occurring on the Condroz plateau south of Liège, the ‘Tongrian’ part being assigned to the Sint-Huibrechts-Hern Formation. The Hesbayan Boncelles Formation has similar characteristics as the upper, Chattian part of the Condruzian Boncelles Formation, consisting of homogeneous yellow fine grained to silty sand, occasionally containing poorly preserved decalcified molluscs. The contact with the underlying Sint-Huibrechts-Hern Formation is sharp and marked by a horizon with quartz and flint pebbles. The Sint-Huibrechts-Hern Formation can be distinguished by its whitish-grey colour, coarser grain size and slight glauconite content. The Oligocene sand on the Hautes Fagnes plateau is well-sorted, fine to medium and typically shiny and polished as beach deposits; its colour varies from yellow to bleached white or rubified. It is equally divided in two parts, the lower unit is medium-grained and had its coastline along the northern rim of the Hautes Fagnes plateau, the upper unit is fine-grained and had its coastline further south. Compared to the Boncelles type locality both Hautes Fagnes sand units display more beach sand features, a difference explained by the more proximal coastal setting on the Hautes Fagnes plateau compared to the more distal open marine environment in the Boncelles type area (Demoulin, 1986, 1987, 1989). Based on this exclusively petrographical data, these units are proposed to correlate with the upper ‘Chattian’ and lower ‘Tongrian’ units at the Boncelles type locality and on the eastern Hesbaye plateau. Furthermore, all similar ‘Tertiary’ marine sand deposits preserved in N Eifel east of the Hautes Fagnes area are assigned to the Chattian ‘Kölner Schichten’ by German authors (Schäfer & Utescher, 2014).
Age
A mollusc fauna occurring in the Boncelles Formation of the type area, first described by Rutot (1907) is characterised by the presence of Meretrix (Callista) beyrichi, considered as a guide fossil for the Chattian. Although some doubts have been raised about this datation because of the poor quality of the fauna and its value for detailed correlations (discussion in Demoulin, 1989) similar faunas have been recorded from the Voort Member of the Veldhoven Formation in the Campine, and cannot be assigned to any other lithostratigraphical unit of different age. The stratigraphical assignment of the Boncelles Formation to the Oligocene is based on general palaeogeographical assumptions, supported by its heavy mineral association (Thibeau, 1960; Sierakowski, 1970; Demoulin, 1986, 1987, 1989; Juvigné et al., 2021a). In this way it is not dissociated from the underlying ‘Tongrian’ Oligocene sand, which is by all authors correlated to the Lower Oligocene Sint-Huibrechts-Hern Formation. Sierakowski (1970), thereby following Macar (1934) insisted on the twofold subdivision of the Oligocene sand in the Boncelles type area, whereas Demoulin (1986, 1987) also recognised two transgressive phases in the Oligocene sand on the Hautes Fagnes plateau and linked these to the subdivision already observed in the Boncelles type area. No firm Upper Oligocene, Chattian age can be established for the upper unit in the Oligocene sand of the Hautes Fagnes plateau but its link to the Boncelles type area and via this to the eastern Hesbaye plateau makes the attribution to the latest Oligocene transgressive phase and possible connection to the Chattian transgression in the Lower Rhine Graben more plausible, as already postulated by Hager (1980). Whatever more precise datation may reveal, it can be stated that the sediments attributed to the Boncelles Formation sensu stricto – as described in this LIS file – unequivocally represent the latest Oligocene transgression in eastern Belgium.
Thickness
The thickness of the Boncelles Formation is 7 -11 m in Sart-Haguet sand pit type locality in Boncelles, for the Boncelles Formation sensu stricto, corresponding to the upper or ‘Chattian’ part of the formation (cf. Figure 0-2, Figure 0-3). The Boncelles Formation sensu lato (including the underlying sand correlated to the Sint-Huibrechts-Hern Formation) reaches about 20 m in thickness, similar to the maximal thickness reached on the eastern Hesbaye plateau by the Boncelles and Sint-Huibrechts-Hern formations combined (ca 25 m). On the Hautes Fagnes plateau thickness of both the upper and lower units of the Oligocene sand is limited to 2 to max 5 m. Due to post-Oligocene erosion the remaining thickness may be more reduced, resulting in limited conservation of the Boncelles Formation sensu stricto.
Area of occurrence
Geological mapping did not differentiate between the Oligocene sand. Hence, the Boncelles Formation is included either in the Sint-Huibrechts-Hern Formation (e.g. at Maurissen sand pit in Elst) on the Flemish geological maps (Claes et al., 2001) or in ‘undifferentiated Oligocene sand’ of the geological maps of Wallonia, except for Marion et al., 2018 and Delcambre, 2018 (unpublished), who include the Boncelles Formation in the proposed Rocourt Group on the Hesbaye plateau. Although the occurrence of the Boncelles Formation sensu stricto (meaning the deposits correlated with the Chattian) is poorly constrained and its known occurrences are limited to within the preservation area of the underlying sand correlated to the Sint-Huibrechts-Hern Formation, its conservation and the Hesbaye plateau is probably linked to dissolution of the underlying Cretaceous. Moreover, its depositional area will not far exceed the western boundary of the Roer Valley Graben Shoulder (Rauw, Beringen and Mal faults, cf. Dusar & Vandenberghe, 2020), along a limit extending further south towards the Liège region. Identified occurrences of the Chattian Boncelles Formation thus seem to be in geographical continuation with the Voort Member of the Veldhoven Formation (see LIS Veldhoven Formation) further north in the Campine. This means that assignment to the Chattian of the Rond Péry mollusc fauna on the adjoining mapsheet Jehay-Bodegnée – Saint-Georges indeed could be questioned (Delcambre, 2018, unpublished). Linking the Boncelles Formation sensu stricto to the Voort Member of the Veldhoven Formation means also linking to the Köln Formation of the Lower Rhine Graben (Hager, 1980), despite the assumed absence of equivalent Chattian deposits in the Netherlands South Limburg and on the Herve plateau (Demoulin, 1989). Yet, the Oligocene sand deposits on the Hautes Fagnes plateau can equally be subdivided in two units. Although both units represent coastal barrier sand the upper one is consistently finer grained and more deprived of heavy minerals (Demoulin, 1986, 1987). Moreover, its occurrences extend across the Hautes Fagnes culmination up to the Weisser Stein, whereas the lower unit had its coastline along the Spa – Solwaster ridge along the northern flank of the Hautes Fagnes plateau (Figure 0-4). This means that the upper unit represents the latest Oligocene transgression, separate from the previous one which is associated with the Lower Oligocene ‘Tongrian’ transgression, and that this upper unit may be linked to the upper ‘Chattian’ part in the Boncelles type area (Demoulin, 1989). It also would mean that Miocene erosion has removed most of the easily erodible Chattian Boncelles Formation.
Type locality
The former Sart-Haguet sand pit in Boncelles forms the type section (134W0011); the former Gonhis (or Les Gonhirs in older documents) sand pit where Rutot (1907) first recorded a Chattian fauna (named Aquitanian by Rutot) can be considered as an auxiliary stratotype (134W0010). The entire ‘lambeau de sables oligocènes’ at Boncelles forms the type locality (Rutot, 1908; Fraipont, 1908; Destinez, 1909; Fourmarier, 1931, 1934; Ancion & Van Leckwijck, 1947; Calembert, 1954; Sierakowski, 1970; Juvigné et al., 2021a, b) – see Figure 0-1, Figure 0-2 from Sierakowski (1970) and the sand pit profiles by Rutot (1907) on Figure 0-3.
Alternative names
Abbreviation ‘BCL’ on the geological maps of the Walloon region. Formerly part of ‘sables oligocènes indifférenciés’ or ‘Om (dépôts inférieurs marins (Tongrien?) du système oligocène)’ on the old 1/40.000 geological maps (e.g. Forir & Mourlon, 1897).
Authors
Dusar, M., Vandenberghe, N. & Demoulin, A.
Date
01/09/2023
Cite as
Dusar, M., Vandenberghe, N. & Demoulin, A., 2023. The Boncelles Formation, 01/09/2023. National Commission for Stratigraphy Belgium. http://ncs.naturalsciences.be/lithostratigraphy/Boncelles-Formation
