Workreport 2007-27



Geological Mapping of the Investigation Trench OL-TK11, the Storage Hall Area


Mattila, J., Aaltonen, I., Kemppainen, K. & Talikka, M.



Page count:



Geological mapping of investigation trench OL-TK11 was carried out in 2004 by geologists from Posiva Oy and the Geological Survey of Finland at the Olkiluoto study site, Eurajoki, Finland, as part of the Finnish deep repository project for the disposal of spent nuclear fuel. The investigation trench is located in the central part of the Olkiluoto study site, overlying the first part of the access tunnel to the ONKALO underground rock characterisation facility. The surface area of the this roughly equidimensional “trench” is approximately 1550 square metres and it has an average length of 35 metres in E-W-direction and 45 metres in N-S-direction.

The main rock type in the trench is diatexitic gneiss (60.6 %) while K-feldspar porphyry covers approximately 20.3 % of the total trench area. In addition, smaller occurrences of altered rock (11.5 %), pegmatitic granite (7.1 %) and mafic gneiss (0.5 %) were also observed. According to the observed crosscutting relations it is possible to infer that the K-feldspar porphyry is younger than the composite foliation in the surrounding gneisses. The altered rock is characterised by fracture-controlled sulphidisation and both pervasive and fracture-controlled silicification. The close association of the altered rocks with a brittle deformation zone indicates that either syn- or postectonic hydrothermal activity has occurred in relation to the faulting.

The foliation has quite a constant moderate dip towards SE in the trench and the type is mostly banded with moderate intensity. Small-scale chevron-type F3 folds were observed and their fold axis plunges moderately towards NE. In addition, very open, metre-scale D4 and D5 folds occur sporadically in the trench and their measured fold axes plunge mainly towards SE with a moderate dip. Two high-grade ductile shear zone intersections (HGI’s) were observed in the central part of the trench, characterised by elongated skarn inclusions and intensive foliation.

During the mapping, a total of 2174 fractures were investigated and based on the clustering of fracture orientations, three fracture orientation maxima could be distinguished: Maximum 1: vertical fractures with an average dip and dip direction 88º/192º; Maximum 2: vertical fractures with an average dip and dip direction 85º/092º; Maximum 3: low to moderately dipping fractures with an average dip and dip direction 37º/150º. P21 values show that pegmatitic granite and altered rock are the most fractured rock types (2.56 m/m2 and 2.28 m/m2,respectively) whereas diatexitic gneiss and mafic gneisses have the lowest fracture density (1.17 m/m2 and 1.38 m/m2 respectively). These values are most likely been affected by the observed fault, especially for the rock types located inside the fault zone. The average P21 value for the whole trench is 1.76 m/m2.

One brittle deformation zone (the “Storage Hall fault”) intersection occurs in the eastern part of the trench and has been identified as a sinistral strike-slip fault zone, formed during NNE-SSE-compression. The fault consists of a clearly definable core and with well-defined damage zones on each side, characterised by Riedel-fractures. The average dip and dip direction of the fault core is 65°/105°.

The sensitivity of using fracture frequency as a criterion in the definition of a fault zone was tested against the fracture data from the investigation trench. Based on the acquired information on fracture frequencies and types, it is possible to conclude that although fracture frequency may be high within the damage zones of some fault zones, this approach is not always sensitive enough to be used alone for fault zone identification. As a conclusion, the definition of a fault zone should be based on an overall analysis of geometrical and kinematic data, and particularly on an evaluation of fracture types in the damage zones, i.e. in the increase of fractures showing signs of movement (slickensides, slickenlines, fibrous growth of infilling minerals) and the geometrical arrangement of shear-related fracture populations.


investigation trench, lithology, brittle deformation, ductile deformation, fault zone, damage zone, disposal of spent nuclear fuel, Olkiluoto, Posiva Oy


Geological Mapping of the Investigation Trench OL-TK11, the Storage Hall Area (pdf) (3.2 MB)


Share article:
This website stores cookies on your computer. These cookies are used to improve our website and provide more personalised services to you.


To make this site work properly, we sometimes place small data files called cookies on your device. Most big websites do this too.

1. What are cookies?

A cookie is a small text file that a website saves on your computer or mobile device when you visit the site. It enables the website to remember your actions and preferences (such as login, language, font size and other display preferences) over a period of time, so you don’t have to keep re-entering them whenever you come back to the site or browse from one page to another.

2. How do we use cookies?

A number of our pages use cookies to remember your actions and preferences (such as login, language, font size and other display preferences.)

Also, some videos embedded in our pages use a cookie to anonymously gather statistics on how you got there and what videos you visited.

Enabling these cookies is not strictly necessary for the website to work but it will provide you with a better browsing experience. You can delete or block these cookies, but if you do that some features of this site may not work as intended.

The cookie-related information is not used to identify you personally and the pattern data is fully under our control. These cookies are not used for any purpose other than those described here.

3. How to control cookies

You can control and/or delete cookies as you wish – for details, see You can delete all cookies that are already on your computer and you can set most browsers to prevent them from being placed. If you do this, however, you may have to manually adjust some preferences every time you visit a site and some services and functionalities may not work.