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Workreport 2007-25

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Name:

Geological Mapping of Investigation Trench OL-TK13 at the Olkiluoto Study Site, Eurajoki, SW Finland

Writer:

Talikka, M.

Language:

English

Page count:

91

Summary:

Geological mapping of investigation trench OL-TK13 was carried out by the Geological Survey of Finland at the Olkiluoto study site, Eurajoki, as a part Posiva Oy's site investigation programme for the development of an underground repository for nuclear waste. The east-west striking, ca. 250 m long trench is located in the vicinity of boreholes OL-KR23 and OL-KR27, ca. 250 m east of the ONKALO research facility. The mapping was performed from washed bedrock surface and rock types were determined macroscopically.

The main rock types in OL-TK13 are diatexitic gneiss, veined gneiss, pegmatitic granite and K-feldspar porphyry. Mica gneiss and granite/granitized mica gneiss exist to a lesser extent. The diatexitic gneiss is the dominant rock type in the western part and the veined gneiss in the eastern part of the trench.

The veined gneiss consists of pelitic mica gneiss paleosome and pegmatitic granite leucosome veins that are parallel to the foliation. In the diatexitic gneiss, the proportion of the leucosome veins and patches is over 50 % and the rock has an ambiguous texture. The pegmatitic granite also occurs as wider sections in the western part of the trench. The K-feldspar porphyry is characterized by potassium feldspar phenocrysts (Ø<5 cm) in matrix comprising mica gneiss and leucosome material. In the granite/granitized mica gneiss, medium-grained granite veins cut fine-grained granitized mica gneiss resulting in a breccia type appearance. The mica gneiss is mainly present as inclusions. Elongated skarn inclusions are present throughout the trench. Softening of the bedrock due to weathering and alteration (chloritisation, kaolinitisation, hematisation) is a common feature in OL-TK13. Up to 1.5 m of strongly weathered bedrock was removed during the excavation in places.

All rock types, except late pegmatitic granite veins, have been subjected to a polyphase deformation. The dominant tectonic feature is foliation S2B and associated leucosome veining. Mean orientation of foliation is 116/52º and the dip direction is in the western part to the E or SE and in the eastern part to the SE. This transition is due to compressional event during the D4 deformation phase. The migmatitic gneisses were folded during the D3 deformation phase resulting in small scale, tight and asymmetrical F3 folds plunging moderately to the NE.

During the fracture mapping, all fractures longer than one metre and all fractures intersecting the central thread were investigated. Measurements including orientation, length, fillings, Jr-value, Ja-value and undulation were recorded for a total of 860 fractures. The mean fracture density is 3.5 fracture/m. From the orientation data, three fracture sets were identified: (1) fractures parallel to the foliation, (2) subvertical N-S trending factures and (3) fractures dipping steeply to the N. ~45 % of all fractures are 0.5-1.5 m in length and ~44 % are longer than 1.5 m. Either or both of the fracture ends are under Quaternary deposits in more than half of the fractures, one third of fractures combine to another fracture and both ends are visible in a quarter of all fractures. Three quarters of all fractures contain fillings, the most common being chlorite, hematite, kaolinite and illite. Three water-conducting fractures or fracture zones were detected.

A total of fifteen deformation zone intersections have been determined OL-TK13. Seven zones were classified as Brittle joint intersections (BJI), seven as Brittle fault intersections (BFI) and one as a Semi-brittle deformation zone (SFI). Most deformation zones are parallel to the foliation.

Keywords:

Investigation trench, rock types, ductile deformation, fracturing, nuclear waste disposal, Olkiluoto

File(s):

Geological Mapping of Investigation Trench OL-TK13 at the Olkiluoto Study Site, Eurajoki, SW Finland (pdf) (1.5 MB)


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