Vein fillings and past fluid flows

The summary of the petrographic description of the vein material sampled from cores of the Schlattingen borehole from the Effingen Member down to the Posidonienschiefer is given in Tab. 4-38. The detailed description can be found in Appendix E. Tectonic veins are abundant in the upper part of the profile (Effingen Member – 'Brauner Dogger'), whereas Opalinus Clay is devoid of such veins. Veins are found infrequently in the underlying Lias. Dip-slip normal movement was identified along many of the veins (Albert et al. 2012).

Sampled veins are all filled with calcite. In addition to this ubiquitous mineral, chlorite is found in veins hosted in the Effingen and Birmenstorf Members, accompanied by pyrite and dolo-mite(?) in the Effingen Member. Celestite is found in minor amounts in veins hosted in the Parkinsoni-Oolith, as small tabular crystals showing rolling extinction and dendritic growth.

This habitus suggests fast precipitation in supersaturated conditions. Celestite is in part replaced by a second generation of calcite in one of the samples. Pyrite is rarely found. Ferruginous mud apparently fills the drusy void left after calcite precipitation in a vein hosted at the boundary between the Anceps-Athleta-Oolith- and Macrocephalus Beds. This might correspond to fault gouge produced from the iron-rich oolithes.

Aside from the veins related to tectonic activity (faulting), two samples correspond to contrac-tion cracks in septaria concrecontrac-tions and are filled with ankerite (?), calcite, and minor pyrite.

These structures are related to diagenetic processes (probably bacterially driven) during the induration of the rock. The sample located in the Blagdeni Beds (Humphriesioolith Formation) bears large flat carbonate structures that are not veins but big mollusk shell fragments.

Tab. 4-38: Summary of the vein samples petrography.

Depth [m] Formation Vein fillings Tectonic regime

Post-mineralisation tectonic stress?

750.00 Effingen Member - dolomite (?) - chlorite 757.24 Birmenstorf Mb. - calcite Normal or reverse

fault?

Moderate 759.19 Wutach Fm. - calcite Normal fault (?) Moderate 760.65 Wutach Fm. - calcite

- ferruginous mud Unclear No

763.07 Variansmergel Fm. - calcite

- fault gouge Normal fault (?) Yes

765.88 Variansmergel Fm. - calcite Extension No

769.11

Parkinsoni-Württemb. Beds - calcite

- celestite Inverse fault (?) Yes 838.57 Opalinuston Septaria infills

(carbonates) No fault No 838.85 Opalinuston Septaria infills

(carbonates) No fault No 952.33 Jurensis Beds - calcite Extension No 957.20 Posidonienschiefer - calcite

- trace pyrite(?) Normal fault No 957.70 Posidonienschiefer - calcite Normal fault (?) No

959.12 Posidonienschiefer - calcite Tectono hydraulic? No

Some samples show zebra textures or fibrous calcite that indicate syn-tectonic mineral precipita-tion. When clear, the tectonic regime of the vein structures is always normal and/or extensional, the only exception being one sample hosted in the Parkinsoni Oolith, where inverse movement is probable. Many vein samples show post-mineralisation tectonic reactivation, particularly in the upper part of the profile (above 790 meter drill length). This is reflected by the presence of fault planes crosscutting vein minerals, fault gouge, and polysynthetic twinning of vein calcite.

4.10.2 Isotope composition of rocks and vein minerals

The isotopic compositions of C, O, and Sr of whole rock carbonate and vein minerals are given in Tab. 4-39.

Oxygen data for whole rock carbonate plot close to the expected range for equilibrium with seawater at tropical climate (20 – 30 °C), in the 24 to 29 per mil range (Fig. 4-81). Lower values are recorded in the USM, which is consistent with a continental detrital sedimentation (a large part of the carbonate is inherited from older Mesozoic marine sediments) under freshwater conditions. One sample in the Opalinus Clay shows very low δ¹⁸O value, at around 21 per mil VSMOW.

The δ¹⁸O values of vein calcite are systematically lower than the host rock carbonate, and range from about 16 to 21 per mil VSMOW.

Carbon isotope composition of the whole rock carbonate shows a relatively complex depth curve ranging from about -6 to 3 per mil VPDB, with the highest values in the Malm and the lowest in the Keuper and USM (Fig. 4-81).

The δ¹³C values of the vein calcite are apparently similar to their host rock.

Strontium isotope composition of the whole rock carbonate shows a relatively well defined profile (Fig. 4-82a), which mostly correlates with the sheet silicate content (data from samples in Malm to Lias), as shown in Fig. 4-82b. The Sr content is given in Tab. 4-39 with respect to whole rock and to calcite content. The Sr leached by acetic acid is not the total Sr of the rock, as silicates (including clay minerals) are normally not affected (N. Clauer, personal communica-tion 2011). Acetic acid leach is the routine method for the extraccommunica-tion of Sr from carbonate, and this is why the Sr contents are normalized to calcite. In the section 'Brauner Dogger' – Lias, the Sr content normalized to calcite content follows the same depth trend as the ⁸⁷Sr/⁸⁶Sr ratio (Fig. 4-82a and 4-83a) and also correlates with the clay-mineral content Fig. 4-83b). This is not compatible with Sr being uniquely sourced from the carbonate. At this stage, it is difficult to interpret the Sr concentrations, and sequential leaching experiments may provide more clarity.

The ⁸⁷Sr/⁸⁶Sr ratio of calcite and calcite-celestite veins follows a different profile than the whole rock carbonate (Fig. 4-82a) with a significant scatter at the boundary between the Malm (value close to the whole rock carbonate) and the 'Brauner Dogger', reaching a maximum at the top of the Lias. The sample distribution is unfortunately not regular and reflects the abundance of vein material in the drillcore. The Sr content of the vein calcite apparently decreases with depth (Fig. 4-83a). Fig. 4-83b shows that the ⁸⁷Sr/⁸⁶Sr ratio correlates inversely with the Sr content in vein calcite, but positively for whole-rock carbonate.

(a) (b)

Fig. 4-81: δ¹⁸O (a) and δ¹³C (b) depth profile along borehole for whole rock carbonate and vein calcite.

(a) (b)

Fig. 4-82: (a) ⁸⁷Sr/⁸⁶Sr ratio depth profile for whole rock carbonate and vein calcite (+celestite); (b) ⁸⁷Sr/⁸⁶Sr ratio of whole rock carbonate versus sheet silicate content of the rock.

Green bars indicate the ⁸⁷Sr/⁸⁶Sr ratio of contemporary seawater.

(a) (b)

Fig. 4-83: (a) Sr content depth profile for whole rock carbonate and vein calcite (celestite-bearing vein samples are not considered); (b) ⁸⁷Sr/⁸⁶Sr ratio of whole rock carbonate and vein calcite (upper) and sheet silicate content of the rock (lower) versus Sr content (normalized to calcite content for whole rock samples).