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Technischer Bericht NTB 86-01

Sondierbohrung Weiach Geologie

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The Weiach borehole was sited in the geological transition zone between the Tafeljura and the Molasse Basin. The location chosen was some 900 m to the S of the River Rhine in the Community of Weiach, Canton Zurich (coordinates: 676. 744/268.618, elevation 368.66 m above sea level). Drilling was carried out between January 10th and November 12th, 1983. Coring was continuous throughout the 2482 m deep borehole with the exception of four intervals in the crystalline basement (these together totalling 160 m). Cores were oriented mechanically using a "Multi shot" device. The borehole penetrated a 2020 m thick sedimentary succession; coring permitted the first recovery of productive Carboniferous strata recorded from the Swiss Alpine foreland to date. The core samples were studied in detail using a variety of scientific techniques, with the main aims of characterising lithologies and reconstructing the geological history.

The sedimentary succession is divided into two main packages:

  1. a 991.5 m thick sequence consisting of Quaternary (35.0 m), Lower Freshwater Molasse (140.27 m), Eocene (8.76 m) and Mesozoic (805.47 m); and
  2. a 1028.90 m thick Permo-Carboniferous succession.

The Mesozoic sequence mostly comprises muddy lithologies, with the exception of carbonate facies in the Upper Malm and Upper Muschelkalk. The Mesozoic is subdivided into 43 formations and members, some of which were biostratigraphically dated. The sequence could be correlated with the Jurassic of S Germany, in particular of the Wutach area. This underlines the lateral persistance of Mesozoic lithostratigraphic units in the region. The top of the mostly marly Malm (292.18 m) comprises the thick, karstified "Massenkalk" (90.27 m), made up of micrites and sucrosic carbonates (dedolomites). The Dogger (187.46m) is also developed in muddy ("Swabian") facies similar to those of S Germany. Ferruginous oolites provide marker horizons and thus permit stratigraphic subdivision. The Opalinus Clay (117.17 m) contains an average quartz content of 10-20 %, and can be subdivided into five subunits on the frequency of fine sand and silt horizons and the presence of sideritic concretions. The facies and thickness of the predominantly muddy Lias are similar to those of the Jurassic of eastern Aargau; however, the Middle Keuper again shows a similar facies to that displayed in Southern Germany. The mudstone-marl succession is interrupted by the Gansinger Dolomite (3.30 m) and the Schilfsandstein (11.93 m). In the underlying Gipskeuper (74.53 m), the anhydrite content decreases downwards, clearly in inverse relation to the corresponding increase in the mud component. The Gipskeuper is subdivided on the basis of sedimentary structures and textures into five subunits, each of which was also encountered in the Böttstein and Schafisheim boreholes. The dolomitic part of the Upper Muschelkalk (total 68.77 m) is relatively thick, and consists of the Trigonodu­-Dolomite (37.92 m) and the dolomitic portion of the Plattenkalk. In general, the Trigonodu-Dolomite shows lower porosity than at Böttstein on account of anhydrite pore-filling. The Middle Muschelkalk (56.66 m) is made up of the Dolomit der Anhydritgruppe (9.76 m) and the Sulfatschichten (46.90 m), halite horizons are absent. Although the Sulfatschichten could be subdivided into the same subunits in each of the six previously-drilled Nagra boreholes, there are nevertheless significant variations in sequence development and thickness between the individual boreholes. The Lower Muschelkalk (37.27 m) is made up of: the dark grey Orbicularis marls; the Wellenmergel; and the Wellendolomit. The Buntsandstein comprises a series of white, commonly poorly-cemented sandstones.

A 1028.90 m thickness of Permo-Carboniferous continental clastic sediments were penetrated beneath the Buntsandstein succession (between 991.50 and 2020.40 m depth). These clastics chiefly consist of an alter­nation of muds, siltstones, sandstones and minor conglomerates. Above 1188 m depth the sediments encountered were typical red-bed facies deposited under semi-arid, oxidising conditions. Light grey sandstones and grey to black muds occur below this depth, as weIl as a thick sedimentary sequence containing coal horizons, which indicate deposition in a warm, humid climate. Four palynological biozones could be distinguished running successively from the upper Stephanian A to the Autunian. This dating is confirmed by numerous plant occurrences. The Permian-Carboniferous boundary occurs between 1443.0 and 1451.5 m depth. This boundary does not correspond with the observed colour change from red (above) to grey (below) which is thought to mark the transition from Upper to Lower Rotliegendes. The lithologically variable Permo­-Carboniferous sequence is divided on the basis of sedimentary features into 11 lithofacies units. The uppermost unit („Fine-grained red-bed playa deposits") occurs between 991.50 and 1058.03 m, and consists of a monotonous succession of red-brown, silty muds with green reduction spots, rare anhydrite and caliche nodules as weIl as abundant desiccation cracks and adhesion ripples. In each case, the reduction spots display enrichment of metallic mineral complexes (chiefly nickel and cobalt arsenides). The chemical mobility of elements such as U, V, Co, and Ni indicate that the enrichment occurred during diagenesis. The next unit ("Polymict proximal alluvial fan breccias") occurs between 1058.03 and 1086.48 m and in turn overlies the "Cyclic red-brown alluvial fan Series" of dm- to m-thick coarse sandstone to mudstone cycles. Following beneath this is the lithologically similar "Cyclic grey-brown to grey-black alluvial fan Series (1169.62 – 1251.07 m) . The "Lacustrine Series" (1252.07 m  – 1387.95 m) displays a variable, sedimen­tologically complex assemblage of conglomerates, fining-upwards sequences, greenish turbiditic siltstone alternations and bituminous black mudstones or calcareous marls with stromatolites, fish and ostracods. The "Coarse sandstone-mudstone megacycle Series" (1387.95 – 1474.80 m) is made up of sandstone units up to 8 m thick, which together with their overlying siltstones make up cycles 15 m in thickness. The "Upper cyclic sandstone-mud Series" (1474.80 – 1551.39 m) contains rare yellow-brown tuff horizons up to 18 cm in thickness. These tuffs indicate volcanic activity during latest Stephanian times. The "Coal Series" (1551.39 – 1751.60 m) is punctuated by six groups of coal seams several metres thick and large-scale sand­stone-mudstone cycles up to 38.62 m in thickness. The coal rank in the Permo-Carboniferous as a whole ranges from high volatile bituminous coals („Gasflammkohlen") to low volatile bituminous coals ("Fettkohlen"). With the exception of two seam groups the coals are non­commercial carbonaceous shales, on account of the high ash contents. Most of the coals represent forest peat which was re-deposited together with siliciclastic detritus and living algae in bog lakes. Beneath the "Coal Series" are three cyclic series: the Middle ­(1751.60 – 1840.91 m) and the Lower- (1950.98 – 1010.40 m) cyclic sandstone-mudstone Series, separated by the "Finely conglomeratic coarse sandstone Series". This latter unit consists of an irregular succession of coarse-grained to finely conglomeratic sandstones several metres thick with rare mudstone to fine sandstone horizons. The Permo-Carboniferous sediments were deposited in a late Variscan intramontane basin in a variety of sedimentary environments ranging through the sequence from anastomosing river systems with swamps in the lower part of the succession, through lake complexes to alluvial fans and playas.

High total and effective porosities were encountered in four formations from the Tertiary to the base of the Mesozoic sequence. These formations (Lower Fresh­water Molasse, Massenkalk, Trigonodus-Dolomite, Bunt­sandstein) differ in this respect from the otherwise generally muddy facies, although the Opalinus Clay also possesses an average porosity of 10 %. The Lower Freshwater Molasse contains very poorly-cemented siltstones and sandstones with porosities up to 30 %; likewise the Buntsandstein strata display high secon­dary porosity. Of the two carbonate formations, the Massenkalk is noteworthy for its strong karst dissolu­tion and the Trigonodus-Dolomite for its dissolution pores. The total porosity decreases through the Permo­Carboniferous sequence from approximately 10 % at the top to about 2 % at the base. Despite this trend, porosity maxima of 20 % were measured around 1100 m depth. Permeability values are highly variable, but clearly decrease until 1400 m depth, and remain almost throughout the Permo-Carboniferous sequence below the detection limit of 0.5 mD.

Coal rank, as measured by vitrinite reflectance determinations on the organic component, increases downwards from 0.46 %Rmax at 365 m depth to 1.85 %Rmax at 2015.75 m. Nevertheless, the rank profile shows a kink between 1000 and 1400 m depth. The Palaeozoic part of the profile shows a distinctly higher downward rate of rank increase than the Mesozoic sequence. Calculations of organic maturity led to the conclusion that abnormally high geothermal gradients of around 104°C/km must be inferred for the pre-Lower Permian sequence. Thus the observed thermal maturity of the organic matter was already attained during the Permo-Carboni­ferous. Furthermore, this thermal discordance above 1400 m depth implies the stripping at that horizon of a now-removed cover sequence of about 1000 m thickness. Structural analyses permitted location of this supposed discordance at about 1309 m; below this depth the fracture intensity was five times greater than above. Fracturing above this depth is generally weak throughout the sedimentary succesion up to the Tertiary, although fracture intensity is variable. An examp­le of a more strongly fractured formation is the "Massenkalk". Fracturing is generally weak in comparison with the Böttstein borehole -this is a product of the muddier succession and the relatively stable tec­tonic situation at Weiach. 81 % of fractures encountered in Tertiary and Mesozoic strata are filled; more than half of these are lined with clay minerals and a quarter with calcite. In the Permo-Carboniferous 90 % of the fractures possess fillings, of which 83 % consist of clay minerals.

The succession contains four separate aquifers, namely the Quaternary gravels, the "Massenkalk", the Upper Muschelkalk and the Buntsandstein. The Massenkalk is strongly karstified to a greater or a lesser extent, with many clay-bearing karstic fissures. Average permeabilities, as determined by double packer tests, lie between 10-7 and 10-6 msec-1. The Muschelkalk aquifer is specifically located in the upper Trigono­dus-Dolomite and the Dolomit der Anhydritgruppe. Fluid migration paths are provided by those horizons with greater secondary porosity and more intense cleavage. The Buntsandstein includes some poorly ­cemented beds containing artesian groundwater.

There seems to exist a sharp boundary between the Carboniferous sandstones and the crystalline basement (70 cm core loss). No basal conglomerate was encountered although the top five metres of the crystalline basement show slight surface weathering. The basement consists of a monotonous, banded series of high graded, partly hornblende-bearing biotite-plagioclase gneisses, and acid dykes. These dykes make up 15 % of the drilled crystalline sequence. The parent rocks of the gneisses were argillaceous, Al-poor dolomitic greywackes probably of Precambrian age. The gneisses do not obviously correspond to one of the lithologic units of the southern or central Black Forest, but show a distinct similarity with the gneisses from borehole Urach 3.

The whole rock sequence suffered an intense tectonic-­hydrothermal overprint during the Upper Carboniferous (ca. 300 – 320 M.a.) regional tectonic event, which was accompanied by important granite plutonism. A combination of tectonic movements and convective circulation of hot H2O-NaCI fluids, probably under local overpressures, produced a strong cataclasis and hydrothermal alterations of primary metamorphic minerals. The whole rock chemistry remained mostly unaltered, except for an increase in H2O and in some cases CO2 concentration, and therefore ore enrichment forming processes were unimportant.

The tectonic-hydrothermal overprint at Weiach occurred under retrograde temperatures below 400°C and pressures below 1 kb, i.e. at conditions corresponding to a strongly elevated geothermal gradient, probably in association with the plutonism. Most of the intense fracturing (mean value approx. 11 fractures/meter) also dates from this event.

The intense Permian brittle deformation and argillaceous alteration observed in the crystalline basement of other Nagra drillholes is lacking at Weiach. The only traces of the Permian event are probably represented by calcite filled fractures, CaCl2-rich fluid Ca2+-rich inclusions and exchangeable cation associations in the interlayers of sheet silicates.

More than 90 % of the fractures are filled and mostly contain chlorite and calcite deposits. Open fractures are extremely rare and only occur in some aplites. No clearly identifiable fractures systems could be detected. Consistent with this, only very little formation water was observed and the mean value for the hydraulic permeability amounted to less than 10-11 ms-1 (provisional value, cf. NTB 85-11).

Most of the calcites examined from fracture fillings do not exhibit isotopic equilibrium with the recent formation waters. Uranium and Thorium decay series suggest geochemical stability for at least 1.5 – 2 million years.

The following mean values were obtained for selected petrophysical parameters:

parameterunitfresh
gneisses
altered
gneisses («Typ 2»)
acid
dyke rocks

Density g/cm3 2.74 2.68 2.61

Absoluteporosity Vol% ca. 1.5 1.0 3 – 4

Open porosity Vol% 2.3 1.7 1 – 1.5

Head conductivity W/moK 2.7 2.5 3.1

Specific suface m2/g n.b. 46 n.b.

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