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Dossier II: Sedimentological and tectonic conditions
Dossier II discusses the sedimentological and tectonic conditions in the geological siting regions and their surroundings. It describes the stratigraphic and sedimentological characteristics of the host rocks and their confining units as well as the structural geology and the tectonic situation in the siting regions. Maps of the depth and thickness of the host rocks in the siting regions are also presented.
The information on the sedimentological and tectonic conditions in the geological siting regions provides input for the safety-based comparison in several respects. The stratigraphic and sedimentological features of the host rocks and confining units are important for evaluating their thickness, barrier properties, ease of characterisation and partly also explorability. The latter aspects are discussed in Dossiers VI on the barrier properties of the host rocks and confining units and VIII on ease of characterisation and explorability, based on the background information provided here. Information on the structural geology and tectonic regime in the area of the siting regions is used to define regional tectonic elements that are important for delimiting the geological siting regions and the disposal perimeters located within these regions; they also influence the underground space available for the repository. Structural and tectonic characteristics, particularly the tectonic regime in the siting regions, are also considered in the evaluation of various aspects of long-term geological evolution and geomechanical properties. These aspects are discussed in Dossiers III on long-term evolution and IV on geomechanical information. The depth of the host rocks in the siting regions is important for defining the disposal perimeters that will be evaluated as part of the safety-based comparison. Aspects of long-term evolution and geomechanics as well as construction technology are taken into consideration when considering optimum depth; these are discussed in more detail in the Dossiers III and IV mentioned above (and the reports cited therein).
The database for characterisation of the sedimentological and tectonic conditions in the siting regions was improved significantly during Stage 2 of the Sectorial Plan process. The borehole data were expanded, particularly with information from a large number of new boreholes of third parties; these were mostly shallow, but geophysical logging campaigns were able to provide key data for improving the stratigraphic and sedimentological characterisation of the host rocks and the confining units. The regional 2D seismic measurements for Northern Switzerland also represent an important element of the information base. The seismic dataset already available from Stage 1 was completely reprocessed in Stage 2 and its interpretability improved. With the 2D seismic campaign 2011/12 the exsisting data were furthermore supplemented with 20 new seismic profiles with an improved imaging quality. The thereby increased density of the data, which is comparable for all the siting regions in Northern Switzerland, made it possible to clarify regional tectonic elements and to lay the groundwork for local revision of the geological models for the siting regions.
The host rocks in Northern Switzerland are the Opalinus Clay, the 'Brauner Dogger' claystone sequence and the Effingen Member. Together with their under- and overlying confining units, they form the so-called potential effective containment zone; this is characterised briefly in the following from stratigraphically underlying to overlying formations.
The lower confining units of the Opalinus Clay host rock consist mainly of clayey-marly and sulphate-rich sediments of the Keuper and Lias (Staffelegg Formation). Calcareous, dolomitic and sandy units occur within these sediments (Arietenkalk, Gansingen Dolomite, Stubensandstein and Schilfsandstein Formations). They are up to several metres thick and form weathering-resistant 'hard beds' in outcrops. In the Jura-Südfuss region there is a sequence of sandy limestones and calcareous sandstones up to more than 10 m thick directly beneath the Opalinus Clay.
The Opalinus Clay consists mainly of partly strongly sandy claystones and, compared to other Mesozoic formations of Northern Switzerland, it forms a homogeneous thick layer package. There are distinct indications in the Jura-Südfuss siting region that the Opalinus Clay is less thick than in the other regions.
In the siting regions Südranden, Zürich Nordost and Nördlich Lägern, the Opalinus Clay is overlain by the 'Brauner Dogger' claystone sequence. This formation is considered as a potential host rock in the Zürich Nordost and Nördlich Lägern regions. The sequence is mainly argillaceous in composition, but contains micritic, sandy or biodetrital limestones and iron oolites (so called 'hard beds') that occur isolated or in sequences and are summarised together here under the heading sandy limestone sequences. Some of these less clay-rich sequences can laterally not be correlated unambiguously. The most clay-rich homogeneous package within the 'Brauner Dogger' is located in the upper part of the unit and consists of the Parkinsoni-Württembergica strata and the Variansmergel Formation. In the Nördlich Lägern siting region, there are indications of a thicker, less clay-rich rock body in the western and eastern parts, besides the claystone sequence encountered in the Weiach borehole. In the Jura Ost and Jura-Südfuss siting regions, the upper confining units of the Opalinus Clay consist of the Passwang Formation and the marly Lower Acuminata strata. The Passwang Formation consists of several sequences of claystones, sandy and/or bioclastic marls, limestones and iron oolites. It shows clear lateral and vertical facies variations and is generally less clayey compared to the 'Brauner Dogger'.
The Effingen Member follows above the 'Brauner Dogger' in the Südranden, Zürich Nordost and Nördlich Lägern siting regions and belongs here to the confining units. It is considered as a potential host rock in the Jura-Südfuss siting region. The Effingen Member can be divided into calcareous marl sequences and limestone sequences up to more than 10 m thick. The correlation of these sequences between boreholes is partly possible.
The host rock in the Wellenberg siting region (Palfris Formation, Vitznau Marls and Tertiary marl formations) consists mainly of marls. Due to a special tectonic constellation, these have been subject to imbrication and folding to form an unusually large accumulation. Intercalations of limestone sequences are difficult to correlate and limestone wedges of considerable size could be present as as exotic blocks.
The discussion of the structural geological and tectonic conditions in the geological siting regions in the present Dossier is focused on aspects that are significant for the safety-based comparison of the siting regions in Stage 2 of the Sectorial Plan process. This involves characterising the tectonic regime in the siting regions, the identification of regional tectonic elements and the local structural and tectonic conditions in the siting regions.
The term tectonic regime as used in Stage 1 of the Sectoral Plan process relates to a zone with a characteristic tectonic location and formation history. Areas within a tectonic regime can be expected to have similar structural geological and tectonic properties and overprinting patterns. The differentiation of the tectonic regime for the five siting regions in Northern Switzerland carried out in Stage 1 of the Sectorial Plan process was confirmed by the intensive investigations in Stage 2. The Südranden siting region and the central and northern sections of the Zürich Nordost region are located in the eastern Tabular Jura and the north-eastern Swiss Molasse Basin respectively and hence to the north of the seismically mappable zone of influence of the compression tectonics associated with the Alpine "distant push" (Fernschub). This region was subject to comparatively low tectonic strain. The southern part of the Zürich Nordost siting region and the Nördlich Lägern and Jura Ost regions lie in the area of influence of the Precursory Folding Zone, while the Jura-Südfuss siting region lies in the Subjurassic Zone. While the precursory folding zone shows only slight overprinting due to the tectonics associated with the Alpine "distant push", the Subjurassic Zone is more strongly deformed. In contrast to the other geological siting regions, the Wellenberg siting region lies within the central Alps between the Drusberg and the Axen nappe and was subject to much stronger tectonic deformation than the siting regions in Northern Switzerland.
Regional tectonic elements can only be used for defining the disposal perimeters in the siting regions in Northern Switzerland. The trend of the regional fault zones already identified in Stage 1 was verified by the denser 2D seismic database, which has also been improved in terms of its interpretability. In addition, tectonic zones to be avoided were newly defined in Stage 2; these follow the trend of basement faults that were reactivated in post-Palaeozoic times, and anticlines within the Mesozoic sedimentary succession.
The local structural and tectonic conditions in the siting regions differ considerably in detail. In Südranden, the investigations in Stage 2 confirmed that the overburden has undergone relatively little tectonic disturbance; the same applies for the Zürich Nordost region. The southern part of this siting region is located above the northern boundary zone of the Permo-Carboniferous Trough of Northern Switzerland which was reactivated in post-Palaeozoic times and, according to the 3D seismic analysis, has less favourable structural geological properties. When optimising the disposal perimeter for the safety-based comparison this part of the siting region is identified as a tectonic zone to be avoided. For the Nördlich Lägern siting region, the explorations in Stage 2 showed a stronger influence of regional fault zones, particularly in the western part of the region. The northern part of the siting region is located above the northern boundary zone of the Permo-Carboniferous Trough of Northern Switzerland which was reactivated in post-Palaeozoic times. Similarly to the situation for Zürich Nordost, it is designated as a tectonic zone to be avoided. The Jura Ost siting region is affected by regional tectonic elements only in its outermost marginal areas and the current interpretation of the seismic profiles shows only slight disturbance. In contrast, the influence of regional fault zones is more clearly recognisable in the Jura-Südfuss siting region than previously assumed. In addition, there is also a comparatively more pronounced small-scale tectonic overprinting in this region. In Wellenberg, cored boreholes show a higher density of small-scale fault zones compared with the siting regions in Northern Switzerland. These cannot be recorded systematically because of the difficulties with exploring this region using reflection seismics.
To characterise the geological conditions in the siting regions, the geological models for the regions in Northern Switzerland were updated compared to Stage 1, taking into account all available data. An updated 3D geological model was also prepared for Wellenberg. For the siting regions in Northern Switzerland, there is a significant change in the depth of the relevant geological horizons compared to Stage 1 in some cases, even taking into account methodological uncertainties. The base of the Opalinus Clay which can serve as regional reference horizon is significantly shallower in Südranden and, in the Nördlich Lägern region, mainly in the western part, significantly deeper than previously assumed. Depth maps were also used to prepare thickness maps for the host rocks. These are based mainly on seismic data and contain greater uncertainties compared to information from boreholes. Nevertheless, they confirm the indications from boreholes concerning the regional bandwidth of the stratigraphic host rock thicknesses.