A review is given of recently published and new data on Avalonia east of the Midlands Microcraton. The three megasequences from Cambrian to mid Devonian described in Wales and Welsh Borderland are also present east of the Midlands Microcraton (Brabant Massif, Condroz, Ardennes, Remscheid and Ebbe inliers, Krefeld high). The three megasequences are caused by a tectonic driving mechanism and are explained by three different geodynamic contexts: an earlier phase with extensional basins or rifting and rather thick sequences, when Avalonia was still attached to Gondwana; a second phase with a shelf basin with moderately thin sequences when Avalonia was a separate continent and a later phase with a shelf or foreland basin development and thick sequences. Deformation of the megasequences 1 and 2 or 1 to 3 varies between areas. In Wales and the Lake District the Acadian phase is long-lived and active from early to mid Devonian. In the Ardennes inliers a deformation is active between the late Ordovician and the Silurian (Ardennian Phase), with a similar intensity as the core of the Brabant Massif, when present erosion levels are compared. The Brabant Massif is partly deformed by the long-lived Brabantian Phase from late Silurian till early mid Devonian. Both the Ardennes inliers and the Brabant Massif are not classic orogenic belts, only slate belts where no more than the epizone is reached at present erosion levels. Areas supposedly close to the microcraton or basement are nearly undeformed (SW Brabant Massif and central Condroz). A model of anticlockwise rotation of Avalonia of about 55° from Caradoc to Emsian is proposed to explain the deposition setting of megasequence 3 and the subsequent Acadian and Brabantian deformation. Immediately after the Avalonian microcontinent touched Baltica in Caradoc times it created a short-lived subduction magmatic event from The Wash to the Brabant Massif and soon after the magmatism ended a foreland basin developed. Possibly during and after that development a long-lived and slow compressional event occurred, leading to the deformation of the Anglo-Brabant Deformation Belt. In the early Devonian, contemporaneous with the shortening of the Anglo-Brabant Deformation Belt, extension occurred in the Rheno-Hercynian Zone, possibly caused by the same slow rotation of Avalonia. More evidence emerges that Avalonia east of the Midlands Microcraton comprises not one but probably two terranes: the remnant of the palaeocontinent Avalonia, and what is called the palaeocontinent Far Eastern Avalonia; the latter is only occasionally observed in the few deep boreholes into the Heligoland-Pomerania Deformation Belt, in southern Denmark, NE Germany and NW Poland, with scant available indirect data in between indicating only Proterozoic basement and no Caledonian deformation. For Far Eastern Avalonia a similar palaeogeographical history is postulated as Avalonia, with rifting from Gondwana in Arenig or earlier times, collision with Baltica before the mid-Ashgill and deformation between the late Ordovician and latest Silurian. The Avalonia concept might need to be expanded to an ‘Avalonian Terrane Assemblage’ with cratonic cores and small short-lived oceans as in the Armorican Terrane Assemblage.

No abstract

The National Subcommission on the Stratigraphy of the Lower Palaeozoic of Belgium has evaluated the previous and present definitions of the Cambrian, Ordovician and Silurian lithostratigraphical units (groups, formations and members) and presents in this paper the results of this evaluation. Some of these units described in published or unpublished documents are considered to be junior synonyms of previously described units. Other units were given a higher or lower ranking. Units that are too thin or too thick were abandoned. The Subcommission reached agreement on the validity and usefulness of all the units that are described below. For each of them, a lithological and sedimentological description is given, the area of distribution and possible facies changes, the thickness of the unit, the arguments for the age determination, some remarks about the history of the definitions, and the names of units considered to be junior synonyms. In the Brabant Massif 32 formations are (re) defined and described, in the Condroz inlier 18 formations, in the Stavelot Massif 3 groups, 8 formations and 8 members, with an additional formation with an unspecified Mid Ordovician-earliest Devonian age, in the Rocroi Massif 3 groups, 7 formations and 2 members, in the Givonne Inlier 4 formations and in the Serpont Inlier 2 formations. The formations are shown in chronostratigraphical tables with a colour encoding the dominant lithology or sedimentology.

A new geological map, mostly subcrop, of the Brabant Massif is presented, based on a revised lithostratigraphy of the outcrop area and on recently acquired palaeontological and lithological data from boreholes in the concealed area. New interpretations of magnetic and gravity data are used to extend the lithostratigraphical units into areas with few or no boreholes. A structural model of the western, northern and southern parts of the Brabant Massif is presented. The main anticlinal axis plunges towards the west-northwest, its core comprising Upper Precambrian (?) to Lower Cambrian terrigenous rocks which outcrop in the southeast. To the north of the main axis, younger rocks appear in regular succession, but to the southwest this picture is complicated by the occurrence of a second anticlinal structure and by a subparallel magmatic arc.

No abstract