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

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Abstract

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 stratigraphy. 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 description 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 Megasequence 2 begins in a shelf environment with the Abbaye de Villers and Tribotte formations. The Rigenée Formation marks a rapid deepening that leads to the slope and/or deeper water deposits of the Rigenée, Ittre, Bornival, Cimetière de Grand Manil, Huet and Fauquez formations. Thereafter, an abrupt change of bathymetry marks the top of Megasequence 2 and leads to the shallow shelf deposits of the Madot and Brutia formations. The igneous activity, represented by interbedded volcanic to volcano-sedimentary rocks and magmatic intrusions, reached a peak in the Madot Formation, which forms the base of Megasequence 3. This group shows a moderately thick, between 1500 to 2000 metres, mostly siliciclastic succession. Some shelly facies in the Upper Ordovician show the rapid drift of Avalonia to low latitude and the warm Boda Event that precedes the Hirnantian glaciation. A chronostratigraphic comparison with the Central Condroz Inlier shows that the succession there is also almost complete from the Middle Ordovician (Huy Formation) to the lowermost Silurian (Bonne-Espérance Formation) and that a short stratigraphic hiatus marks the top of Megasequence 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.

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Over : Alain HERBOSCH

Over : Jacques VERNIERS