Databank

Workreport 2007-27

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Name:

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

Writer:

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

Language:

English

Page count:

174

Summary:

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.

Keywords:

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

File(s):

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


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