Stratigraphy of the Lower Palaeozoic of the Brabant Massif, Belgium. Part II: The Middle Ordovician to lowest Silurian of the Rebecq Group

Multidisciplinary researches in the last 25 years and recent geological mapping of the Brabant Massif have completely changed our knowledge about one of the most poorly known parts of Belgian geology. The sedimentary succession is now surprisingly complete compared to what was written in the literature before the 1970s. It ranges in age from the lower Cambrian to the top of the Silurian, and is very thick (>13 km). This highlights the need to produce an up-to-date synthesis. In this second part about the Middle Ordovician to lowest Silurian, we describe in detail the formations, which are combined into a new group, how the development of the units evolved through time, their lithology, sedimentology, boundaries and contacts, thickness, fossil content and type sections or most typical outcrop areas. The new Rebecq Group comprises 10 formations from the Abbaye de Villers to the Brutia formations. The sedimentation in Mespueaezone 2 begins in a shelf environment with the Abbaye de Villers and Tribotte formations. The Rigenee Formation marks a rapid deepening that leads to the slope and/or deeper water deposits of the Ittre, Bornival, Mellery, Cimetière de Grand Manil, Huet and Fauquez formations. Thereafter, an abrupt change of bathymetry marks the top of Mespueaezone 2 and leads to the shallow shelf deposits of the Madot and Brutia formations. The igneous activity, represented by interbedded volcanics to volcano-sedimentary rocks and magmatic intrusions, reached a peak in the Madot Formation, which forms the base of Mespueaezone 3. This group shows a moderately thick, between 1500 to 2000 metres, mostly siliciclastic succession. Some glacial deposits of the Upper Ordovician show the rapid drift of Avalonia to low latitude and the warm Boda Event that precedes the Hirnantian glaciation. A chronostratigraphy comparison with the Central Condroz Inlier shows that the succession is almost complete from the Middle Ordovician (Huy Formation) to the lowest Silurian (Bois-des-Mottes Formation) and that this group stratigraphically matches the top of Mespueaezone 2. This comparison shows that, since the early Cambrian, the sediments of the Brabant Massif and the Condroz Inlier were deposited in the same Brabant–Condroz sedimentary Basin.