Workreport 2016-63



Lithology and Structure of the Investigation Trench OL-TK10


Kärki, A.



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Geological mapping of the investigation trench OL-TK10 was carried out in 2005 by the employees of Posiva Oy and subsequently augmented by the methods of digital photograph interpretation. The results of the studies are presented in this report. The area of the trench is approximately 900 square metres and it has the length of ca. 35 metres in the N-S direction and 30 metres in the E-W direction.

The bedrock in the area of the trench consists mainly of supracrustal high-grade metamorphic meta-pelites, meta-arenites and intermediate, pyroclastic metavolcanic rocks which were metamorphosed under upper amphibolite facies conditions to various metatexites and diatexites. Layer-structured metatexites cover the most part of the trench whereas dike-structured metatexites and schlieren-structured and nebulitic diatexites exist in lesser amounts. In addition to those, wide leucosome bodies or granitic pegmatoids occur at places.

Ductile deformation structures of four major stages are described and a complicated structural sequence is identified from the trench. Metamorphic banding, S1 was isoclinally folded by the stage D2 simultaneously with strong migmatization. These structures were overprinted by the F3 folding leading to the generation of the close and open fold elements. By the stage D4 the bedrock was once more deformed and often close fold elements were developed as the latest components of the final interference structures.

Zones of extremely strong fracturing are missing in the area of the trench but some regional variation in the fracture density and in their directions is detectable. Two sets of steeply dipping fractures can be distinguished from the whole fracture population. The first set is approximately in the ENE – WSW direction and the second set in the NW – SE direction. As a whole, the fractures detected in the area of the trench are rather short, often less than 2 m in length, the measured dips of those are mostly steep and their aperture is not often detectable from the surface section.


Investigation trench, lithology, ductile deformation, fracturing, nuclear waste disposal.


Lithology and Structure of the Investigation Trench OL-TK10 (pdf) (16.1 MB)


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