Influence of inherited structural domains and their particular strain distributions on the Roer Valley graben evolution from inversion to extension

The influence of strain distribution inheritance within fault systems on repeated fault reactivation is far less understood than the process of repeated fault reactivation itself. By evaluating cross sections through a new 3D geological model, we demonstrate contrasts in strain distribution between different fault segments of the same fault system during its reverse reactivation and subsequent normal reactivation. The study object is the Roer Valley graben (RVG), a middle Mesozoic rift basin in western Europe that is bounded by large border fault systems. These border fault systems were reversely reactivated under Late Cretaceous compression (inversion) and reactivated as normal faults under Cenozoic extension. A careful evaluation of the new geological model of the western RVG border fault system – the Feldbiss fault system (FFS) – reveals the presence of two structural domains in the FFS with distinctly different strain distributions during both Late Cretaceous compression and Cenozoic extension. A southern domain is characterized by narrow (<3 km) localized faulting, while the northern is characterized by wide (>10 km) distributed faulting. The total normal and reverse throws in the two domains of the FFS were estimated to be similar during both tectonic phases. This shows that each domain accommodated a similar amount of compressional and extensional deformation but persistently distributed it differently. The faults in both structural domains of the FFS strike NW–SE, but the change in geometry between them takes place across the oblique WNW–ESE striking Grote Brogel fault. Also in other parts of the Roer Valley graben, WNW–ESE-striking faults are associated with major geometrical changes (left-stepping patterns) in its border fault system. At the contact between both structural domains, a major NNE–SSW-striking latest Carboniferous strike-slip fault is present, referred to as the Gruitrode Lineament. Across another latest Carboniferous strike-slip fault zone (Donderslag Lineament) nearby, changes in the geometry of Mesozoic fault populations were also noted. These observations demonstrate that Late Cretaceous and Cenozoic inherited changes in fault geometries as well as strain distributions were likely caused by the presence of pre-existing lineaments in the basement.

Upper Oligocene lithostratigraphic units and the transition to the Miocene in North Belgium

The presence of Chattian deposits in Belgium was confirmed in the early 20th century by correlation of their mollusc faunas with the type Chattian in Germany. Consequently, the Voort Formation in the Campine Basin and the Boncelles Sand on the northeastern Ardennes were established and assigned a Chattian age. Contacts with underlying Rupelian and overlying Burdigalian formations are marked by hiatuses, linked mainly to end-Oligocene Savian tectonics and reactivation of the Roer Valley Graben (RVG). On the Campine Block, only the lower part of the Chattian, the Voort Sand is deposited, increasing in thickness in the direction of the RVG and including a geophysically traceable clayey marker horizon allowing the mapping of this unit in the Campine Basin, into the Netherlands and even possibly link it to the hydrostratigraphic subdivision of the Chattian in the Lower Rhine Graben. Lithologically, these uppermost Paleogene Chattian deposits form the base of the Neogene sequence along the Southern Bight of the North Sea, characterised by predominantly glauconite-bearing sand. The Chattian sediments rapidly become thicker in the strongly subsiding RVG, resulting in a more continuous sedimentation with the development above the Voort Sand of a clay unit and another sand unit, forming together the Veldhoven Formation. In Belgium such sequence is only found in the RVG without biostratigraphic data. However, it can be demonstrated that lithostratigraphically this sequence is comparable to the better-studied Veldhoven Formation in the Netherlands where biostratigraphy revealed that the Veldhoven Formation grades into the Aquitanian to Burdigalian, crossing the Paleogene–Neogene boundary and separated from middle Miocene deposits by the Early Miocene Unconformity (EMU). It is proposed to harmonise Belgian and Dutch stratigraphic nomenclatures, making the more complete Veldhoven Formation applicable both in the Campine Basin and the Roer Valley Graben, and further north in the Netherlands. Within this scheme, the Belgian Voort Formation becomes the Voort Member as the lower part of the Veldhoven Formation, of which the middle Wintelre clayey and upper Someren sandy members are only recognised in the graben.

Reinterpretation of the Neogene sediments of the Bree Uplift, NE Belgium

The present geological map of the Flemish Region shows a small lens-shaped isolated outcrop of the Miocene Bolderberg, Diest and Kasterlee Formations, surrounded by younger formations, in an area that coincides with the tectonic Bree Uplift segment, on the southwestern border of the Roer Valley Graben in NE Limburg. The fault, bordering the segment at its SW side, had been interpreted to be tectonically active throughout the Neogene. Now, it is argued that an erroneous lithostratigraphic interpretation of the outcropping strata supported that view. Field observations of some of the outcrops and sampled drill holes show that the sediments do not belong to an Opitter member of the Bolderberg Formation, a Gruitrode Mill member of the Diest Formation and a Dorperberg member of the Kasterlee Formation, but most probably to the lower, latest Miocene or early Pliocene part of the Mol Formation and an unknown Pliocene marginal marine deposit not unlike and at about the stratigraphic position of the Poederlee Formation. That glauconiferous sand deposit, which has always been interpreted as consisting of two successive sedimentary cycles, is now accommodated in a single cycle, using the sedimentary model of deposition in a confined, backbarrier tidal basin subject to marine sand input and local stages of flow constriction and intraformational incision. Like already proposed by Rossa (1986) and Demyttenaere (1989), reprocessed seismic sections show only minor movements along the southwestern fault of the Bree Uplift since the Paleocene, and no inverse tectonic movements at all since the Middle Miocene.

A review of the lower and middle Miocene of northern Belgium

The stratigraphy, sedimentology and paleogeography of the lower and middle Miocene Berchem and Bolderberg Formations from northern Belgium have been extensively studied during the last decades, a.o. in the framework of doctoral research, as parts of subsurface mapping and interregional geological correlation initiatives by governmental organizations. The last formal stratigraphical revision on formation level, however, almost dates from two decades ago, notwithstanding the fact that a wealth of new data has become available. A compilation and assessment of the stratigraphical data of the lower and middle Miocene has been carried out and a refined stratigraphical framework—based on dinoflagellate cyst stratigraphy—is presented. Recommendations for the National Commission for Stratigraphy of Belgium are proposed. A new member, the Molenbeersel member, is proposed for the glauconite-bearing silts and fine sands in the upper part of the Bolderberg Formation in the Roer Valley Graben.

A reappraisal of the stratigraphy of the upper Miocene unit X in the Maaseik core, eastern Campine area (northern Belgium)

The stratigraphy of the Tortonian-Messinian sequence from the Maaseik core, located on the shoulder of the Roer Valley Graben (RVG) in the eastern Campine area in northern Belgium, was improved. The analysis of the marine palynomorphs (dinoflagellate cysts and acritarchs) from the uppermost part of the Breda Formation, the unnamed unit X and the basal part of the Lower Waubach Member led to the recognition of the mid to upper Tortonian Hystrichosphaeropsis obscura biozone. Therefore deposition of this entire analyzed sequence took place sometime between 8.8 to 7.6 Ma. Paleoenvironmental interpretation of the palynomorphs points to shallow marine conditions and most probably a stressed environment during the deposition of unit X. A comparison with the time equivalent stratigraphy in the nearby Belgian Campine, the Dutch RVG and the German Lower Rhine Basin allowed the identification of the Inden Formation and required a shift in the base of the Kieseloolite Formation compared to the earlier lithostratigraphic interpretation of the Maaseik core. The regional stratigraphic scheme shows the progressive northwestward extension of the river facies from the Lower Rhine during the late Tortonian.

The Diest Formation: a review of insights from the last decades

Research conducted since the 1960s on the upper Miocene Diest Formation in NE Belgium is reviewed and integrated. Their lithology unites the deposits of the glauconiferous Diest Sand in one formation, though biozones and internal sedimentary structures strongly suggest the formation may agglomerate the deposits of two separate, successive sedimentary cycles. The lowermost cycle is thought to have deposited the "Hageland Diest sand" during the early or middle Tortonian. It contains the Diest Sand in the main outcrop area in Hageland, Zuiderkempen and central Limburg, and probably also the Deurne Member near the city of Antwerpen. It furthermore includes the lower part of the Dessel Member in the central Kempen and in the Belgian part of the Roer Valley Graben (RVG). The Hageland Diest cycle represents the infill of a large tidal inlet tributary to the southern North Sea bight, then situated over the southern Netherlands and the Lower Rhine embayment. The Hageland Diest sand has the composition of a marine deposit, yet the confined area of occurrence and the presence of tens of metres deep incisions at the base, set it apart. The confinement of the embayment, strong tides and a steady supply of coastal‐marine sand are invoked as the main driving forces that resulted in the distinctive geometry and internal architecture of the unit. The upper cycle is associated with the "Kempen Diest sand", which is found in the subsurface of the RVG and the Noorderkempen. It has a late Tortonian to earliest Messinian age with progressively younger ages occurring to the NW. It encompasses the upper part of the Dessel Member and the overlying, coarser Diest Sand, and correlates to most or all of the thickly developed Diessen Formation in The Netherlands. It is the deposit of a prograding marine delta, containing both marine components and continental components fed by the palaeo‐Meuse/Rhine river mouths. Accommodation space kept increasing during deposition, due to subsidence of the deposition area, especially inside the RVG but also in the Noorderkempen. Although there is a fair consensus on the above, many concrete points about the geometry and depositional history of the Diest Formation and even a definitive decision on its single or dual character remain to be sorted out. In addition, this review excludes the Flemish Hills sand and the Gruitrode Member from the Diest Formation.

Structural framework: a new way to organise and communicate geological information

&lt;p&gt;A structural framework is a well-defined concept, being used primarily to add structural understanding to a geological model. Within GeoConnect&amp;#179;d, a new approach is used, i.e. the structural framework concept is modified to become the leading model, in which geological maps and models can be inserted and related to. This structural framework is being developed and implemented for two areas of interest - Roer-to-Rhine in northwest Europe and Pannonian Basin in eastern Europe - and will soon be implemented in two pilot areas, Ireland and Bavaria. The organisation of information is strongly linked to different scales of visualisation, starting from the pan-European view (1:15,000,000) with the possibility to zoom in to the scale of local geological models and maps in these four areas.&lt;/p&gt;&lt;p&gt;The GeoConnect&amp;#179;d structural framework reorganises geological information in terms of geological limits and geological units. Limits are defined as broadly planar structures that separate a given geological unit from its neighbouring units, e.g. faults (limits) that define a graben (unit), or an unconformity (limit) that defines a basin (unit). Therefore, the key relationship between these two structural framework elements is that units are defined by limits i.e. all units must be bounded by limits. It is important to note that this relationship is not necessarily mutual: not all limits have to be unit-defining.&lt;/p&gt;&lt;p&gt;A first test of the structural framework methodology was carried out in the Netherlands and Belgium for the Roer Valley graben, as the faults in this area were already modelled in a cross-boundary project (H3O-Roer Valley Graben). Displaying different elements according to scale of visualisation coupled with vocabulary information (definition, grouping and semantic relations between elements, etc.) following the SKOS-system proved a powerful tool to display geological information in an understandable way and improve insights in large-scale geological structures crossing national borders. Additionally, links with other GeoERA projects such as HIKE and its fault database are being successfully established. We consider the outcomes of this test promising to fulfil one of the main goals of GeoConnect&amp;#179;d, i.e. preparing and disclosing geological information in an understandable way for stakeholders. We also consider this as the way forward towards pan-European integration and harmonisation of geological information, where the ultimate challenge is to correlate or otherwise link information from different geological domains and of different scales.&lt;/p&gt;&lt;p&gt;This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 731166.&lt;/p&gt;

Influence of inherited structural domains and their particular strain distributions on the Roer Valley Graben evolution from inversion to extension

After their first development in the middle Mesozoic, the overall NW-SE striking border fault systems of the Roer Valley Graben were reactivated as reverse faults under Late Cretaceous compression (inversion) and reactivated again as normal faults under Cenozoic extension. In Flanders (northern Belgium), a new geological model was created for the western border fault system of the Roer Valley Graben. After carefully evaluating the new geological model, this study shows the presence of two structural domains in this fault system with distinctly different strain distributions during both Late Cretaceous compression and Cenozoic extension. A southern domain is characterized by narrow ( 10 km) distributed faulting. The total normal and reverse throw in the two domains was estimated to be similar during both tectonic phases. The repeated similarities in strain distribution during both compression and extension stresses the importance of inherited structural domains on the inversion/rifting kinematics besides more obvious factors such as stress directions. The faults in both domains strike NW-SE, but the change in geometry between them takes place across the oblique WNW-ESE striking Grote Brogel fault. Also in other parts of the Roer Valley Graben, WNW-ESE striking faults are associated with major geometrical changes (left-stepping patterns) in its border fault system. This study thereby demonstrates the presence of different long-lived structural domains in the Roer Valley Graben, each having their particular strain distributions that are related to the presence of non-colinear faults.

An updated and revised stratigraphic framework for the Miocene and earliest Pliocene strata of the Roer Valley Graben and adjacent blocks

In the Netherlands, the bulk of the Miocene to lowest Pliocene sedimentary succession is currently assigned to a single lithostratigraphical unit, the Breda Formation. Although the formation was introduced over 40 years ago, the definition of its lower and upper boundaries is still problematic. Well-log correlations show that the improved lecto-stratotype for the Breda Formation in well Groote Heide partly overlaps with the additional reference section of the older Veldhoven Formation in the nearby well Broekhuizenvorst. The distinction between the Breda and the overlying Oosterhout Formation, which was mainly based on quantitative differences in glauconite and molluscs, gives rise to ongoing discussion, in particular due to the varying concentrations of glauconitic content that occur within both formations. In addition, the Breda Formation lacks a regional-scale stratigraphic framework which relates its various regionally to locally defined shallow marine to continental members. In order to resolve these issues, we performed renewed analyses of material from several archived cores. The results of archived and new dinocyst analyses were combined with lithological descriptions and wire-line log correlations of multiple wells, including the wells Groote Heide and Broekhuizenvorst. In this process, the updated dinocyst zonation of Munsterman & Brinkhuis (2004), recalibrated to the Geological Time Scale of Ogg et al. (2016), was used. To establish regionally consistent lithostratigraphic boundaries, additional data was used along a transect across the Roer Valley Graben running from its central part (well St-Michielsgestel-1) towards the southern rift shoulders (well Goirle-1). Along this transect, chronostratigraphic and lithostratigraphic analyses were integrated with well-log correlation and the analyses of seismic reflection data to constrain geometrical/structural relationships as well. The results led to the differentiation of two distinct seismic sequences distinguished by three recognisable unconformities: the Early Miocene Unconformity (EMU), the Mid-Miocene Unconformity (MMU) and the Late Miocene Unconformity (LMU). The major regional hiatus, referred to as the Mid-Miocene Unconformity, occurs intercalated within the present Breda Formation and compels subdivision of this unit into two formations, viz. the here newly established Groote Heide and the younger Diessen formations. Pending further studies, the former Breda Formation will be temporally raised in rank to the newly established Hilvarenbeek subgroup, which comprises both the Groote Heide and Diessen formations. Whereas these two sequences were already locally defined, a third sequence overlying the LMU represents two newly defined lithostratigraphical units, named the Goirle and the Tilburg members, positioned in this study at the base of the Oosterhout Formation. Besides their unique lithological characteristics, in seismic reflection profiles the Goirle and the Tilburg members stand out because of their distinct seismic facies. Use of an integrated, multidisciplinary and regional approach, an improved southern North Sea framework and more comprehensive lithostratigraphic subdivision of Neogene successions is proposed for the Netherlands, to make (cross-border) correlations more straightforward in the future.