The evolution of the Upper Ordovician to Silurian basin in the Condroz Inlier and the Brabant Massif from a litho- and biostratigraphical point of view

Twelve years ago Verniers et al. (2002a) summarized what was known at the time about the Cambrian to Middle Devonian basin evolution and deformation history of Belgium, eastern England and surrounding areas east of the Midlands Microcraton. The combination of the data concerning the basin development, stratigraphy, sedimentology, subsidence history, magmatism, tectonic deformation and metamorphism led the authors to define three megasequences, each megasequence being characterized by a different geodynamic setting. In the present study only the third megasequence, starting in the middle Katian, is considered. This megasequence is heralded by the first signs of the collision, or better docking, of Avalonia and the continent Baltica. During the middle Silurian, these two continents collide with Laurentia. A counterclockwise rotation of the Midlands Microcraton with respect to the Lake District from the Caradoc (Sandbian-middle Katian) to the Emsian led to the convergence of the Midlands Microcraton and the Luneberg-North Sea Microcraton, resulting in the Brabantian orogeny and the Anglo-Brabant Deformation Belt. Stratigraphical data of the Condroz Inlier during the third megasequence are restricted, especially those for the Silurian part, this in contrast with our knowledge of the Brabant Massif. The data concerning the Condroz Inlier are mainly based on studies dating from the first half of the previous century. Some years ago, Vanmeirhaeghe (2006b), in his PhD study, revised the lithostratigraphy of the Ordovician and parts of the lower Silurian of the Condroz Inlier in combination with a biostratigraphy using chitinozoans. Detailed field observations of the lithofacies in the Condroz Inlier led to the reconsideration of the lithostratigraphical units present in the inlier. Biostratigraphical analysis mainly with chitinozoans, but also with graptolites and brachiopods, allowed to place the lithostratigraphical units in the chronostratigraphy. Once the stratigraphy of the Condroz Inlier was established, correlation with the Brabant Massif became possible. During megasequence 3 the sediments of the Condroz Inlier were deposited on the shelf, but the location of the sedimentation on the shelf changed through time. During the same megasequence sedimentation of the Brabant Massif starts with shelf deposition, but slope conditions prevail during the Silurian with deposition of turbidites. The thickness of the lithostratigraphical units generally increases through the Llandovery with thick deposits starting from the upper Telychian. The basin of the Brabant Massif begins to deepen already in the upper Katian and we prefer to place the start of the development of the foreland basin at the onset of the deepening in the upper Katian. From the lower Telychian on oxic-anoxic changes in the stratigraphical column of the Condroz Inlier are noted. From the upper Telychian on to the end of the Wenlock anoxic sedimentation takes place almost continuously, with deposition of dark grey, finely laminated mudstone; oxic intervals are limited. Such mudstone, known as laminated hemipelagites, have already been recorded from the Brabant Massif and are here described for the first time from the Condroz Inlier.