Alain Herbosch

A new geological map of the Brabant Massif, Belgium

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.

The Cambrian to mid Devonian basin development and deformation history of Eastern Avalonia, east of the Midlands Microcraton: new data and a review

Abstract 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.

Microfacies, sequence stratigraphy and clay mineralogy of a condensed deep-water section around the Frasnian/Famennian boundary (Steinbruch Schmidt, Germany)

Abstract A multidisciplinary analysis (microfacies, sequential stratigraphy and clay mineralogy) was made on Frasnian/Famennian (F/F) boundary strata of the Steinbruch Schmidt section in Western Germany. Three major microfacies are recognised. Their succession records a shallowing-upward evolution from deep, quiet and poorly oxygenated environments, below the storm wave base, to environments influenced by current activities close to the storm wave base. The Kellwasser Horizons correspond to the deepest microfacies. The shallowest microfacies correspond to fine-grained calcareous tempestites or turbidites coming from a distant shelf of northwest Germany. The sequential pattern through the F/F boundary shows the succession of seven systems tracts. Two sequence boundaries are located just above the Lower Kellwasser Horizon and at the F/F boundary itself. These are underlined by hardgrounds suggesting time gaps. The Kellwasser Horizons correspond to sea-level highstands and the overlying beds record a transition from lowstand to transgressive systems tracts. Illite and kaolinite are the dominant clay minerals associated with mixed layers and traces of chlorite. Illite abundance is maximal during Kellwasser Horizons. Illite and kaolinite were probably inherited from a highly weathered source area although part of the illite is diagenetic. Kaolinite is the second most abundant clay mineral and is particularly well represented (up to 50%) between the Kellwasser Horizons. An unusual clay assemblage of illite and mixed layers is associated with a bentonite layer. Kaolinite increases during times when thin tempestites or turbiditic microbioclastic layers come from a distant shelf during sea-level falls. The kaolinite percentage reaches its maximum at the top of the lowstand systems tract. The high percentage of kaolinite suggests a hot–wet climate and could be related to global warming.

Faults in the Asquempont area, southern Brabant Massif, Belgium

The literature suggests that the Asquempont fault, a supposedly important reverse fault forming the limit between the Lower to lower Middle Cambrian and the Ordovician in the Sennette valley, is poorly understood. Nevertheless, this fault is commonly equated with a pronounced NW-SE-trending aeromagnetic lineament, the Asquempont lineament, and both the geometry of the Asquempont lineament and the supposed reverse movement of the Asquempont fault are used to develop large-scale tectonic models of the Brabant Massif. New outcrop observations in the Asquempont area, the “type locality” of the Asquempont fault, in combination with outcrop and borehole data from surrounding areas, show that the Asquempont fault is not an important reverse fault, but instead represents a pre-cleavage, low-angle extensional detachment. This detachment formed between the Caradoc and the timing of folding and cleavage development and is not related to the aeromagnetic Asquempont lineament. The Asquempont area also contains several relatively important, steep, post-cleavage normal faults. Apparently, these occur in a WNW-ESE-trending zone between Asquempont and Fauquez, extending westward over Quenast towards Bierghes. This zone coincides with the eastern part of the WNW-ESE-trending Nieuwpoort-Asquempont fault zone, for which, on the basis of indirect observations, previously a strike-slip movement has been proposed. Our outcrop observations question this presumed strike-slip movement. The Asquempont fault may be related to the progressive unroofing of the core of the Brabant Massif from the Silurian onwards. Possibly, other low-angle extensional detachments similar to the Asquempont fault occur in other parts of the massif. Possible candidates are the paraconformity-like contacts depicted on the most recent geological map of the Brabant Massif.

Review of the stratigraphy of the Ordovician in the Brabant Massif, Belgium

A review of isolated data from the literature and our own observations make it possible to establish a model of the stratigraphical history of the Ordovician on the southern margin of the Brabant Massif. The succession is composed of seven informal formations, totalling a minimum of 2500 m of sediments, mainly comprising mudstones, siltstones and sandstones. This succession is compared and partly correlated with the sections observed in the adjoining areas, in the Bande de Sambre-et-Meuse, the Stavelot and Rocroi Massifs and the Remscheid–Altena and Ebbe anticlines (western Germany).

Trace elements and their relation to the mineral phases in the lateritic bauxites from southeast Guinea Bissau

Abstract Fourteen trace elements (Sr, Ba, La, Ca, Mg, Cu, Zn, V, Ni, Co, Mn, Ga, Cr, Zr) were determined in 120 auger- and core-drill samples from one bauxite plateau situated in the SE part of Guinea Bissau. Following the quantitative determination of minerals identified in the samples, all geochemical and mineralogical data were statistically treated. The use of several statistical tools enabled us to identify two distinct populations of samples, which were found to be related to the shale and dolerite parent rocks, respectively. The dispersion patterns of the elements: Mn, Zr, La, Zn, Cu, Cr and Ni yield strong contrasts between these two populations. Bauxites developed on Devonian shales were found to contain more Cr and Ni than bauxites derived from Cretaceous dolerites which were richer in Cr and Ni. It is postulated that the difference in duration of the bauxitization processes which affected the two parent bedrocks and/or the different resistance to weathering of the primary mineral host phases in the two rock types are responsible for this inversion. R -mode factor analysis revealed the conspicuous affinity of the Mn-Zn-Ni-Cu-Co-Mg group and V to goethite. Gibbsite, boehmite and anatase were found to be associated with “resistate” trace elements like Zr and to a lesser extent, Cr. Gibbsite also reveals a certain affinity to more mobile, strongly depleted elements like Sr and Ba. The study proved the existence of numerous statistically significant correlations between the trace elements like the groups Zr-Ti, Zn-Cu-Ni-Co-Mn and Sr-Ba-La.

L'écaille des Gaux (Ardennes beiges): un exemple d'évolution tectono-sédimentaire complexe lors du développement d'un pli de propagation

The kinematics of internal structuration in the ≪Gaux thrust sheet≫ (Charleroi Coal basin; Belgium) is explained with the help of a geometric model. This model is controlled by the distribution of ≪ Landelies red breccia ≫ outcrops that was syntectonically deposited, probably during the Namurian. We suggest that the tectono-sedimentary evolution was controlled by (1) the growth of a Fault-propagation fold, (2) the formation of collapse structures and (3) localized interbed extension.

Search for Magnetite in Lunar Rocks and Fines

Magnetite crystals larger than 2 micrometers are absent from rocks and fines. Smaller opaque spheres in the fines can tentatively be identified as magnetite. Their concentration is not higher than 1 x 10-6 particle per particle smaller than 1 millimeter. In the fines from the sampling site, the contribution of material similar to type 1 carbonaceous meteorites is insignificant, either because it never existed, or because it was evaporated or comminuted by impact or was diluted by indigenous material. Other magnetite habits typical of carbonaceous meteorites or possibly of cosmic dust or comets were also sought without success-such as rods, platelets, framboids, spherulites, and idiomorphic crystals.

Sedimentology and palaeo-environment of the Upper Visean anhydrite of the Franco—Belgian Carboniferous basin (Saint-Ghislain borehole, southern Belgium)

Abstract The Upper Visean (Carboniferous) evaporitic basin of the Franco—Belgian Variscan domain is represented by the thick anhydrite series cored at Saint-Ghislain (Hainaut, southern Belgium) and Epinoy 1 (northern France). Although the sedimentary record has been modified by post-depositional events (early and late diagenesis, tectonics), a study based on sedimentology, stable isotope data (sulphates and carbonates) and micropalaeontology of the Upper Visean (V3) stratigraphic interval from Saint-Ghislain, allows reconstruction of the evaporitic sedimentation processes and the establishment of a palaeogeographic framework. Most of these Ca-sulphate deposits were subtidal. Deposition occurred in a shallow-water, marine-fed lagoon or “salina” filled by a cyclic shoaling-upward sequence mainly composed of Ca-sulphates associated with carbonates.