[en] Geological mapping of trench OL-TK9 was carried out the by the Geological Survey of Finland at the Olkiluoto study site, Eurajoki, southwestern Finland, as a part of Posiva Oy's site investigation programme for the development of an underground repository for nuclear waste. The NW-SE striking trench, which is about 371 metres in length and 0.4-2.5 metres in width, is situated to the west from the OL-TK3 in the northwestern part of the study site. The rock types were determined macroscopically and the main rock types were further investigated microscopically. On the basis of the migmatite structure, the migmatitic gneisses at Olkiluoto can be divided into three groups: veined gneiss, stromatic gneiss and diatexitic gneiss. Most of the migmatitic gneisses in the trench are veined gneisses and only a few short sections of the diatexitic gneiss or K-feldspar porphyry exist. Stromatic gneisses were not encountered. The leucosome of the veined gneiss show vein like, more or less linear traces with some features similar to large-scale augen structures, whereas the migmatitic structure of the diatexitic gneiss is more asymmetric and irregular. The southernmost part of the trench is dominated by mica gneiss and pegmatitic granite. The mica gneiss is homogenous mica-rich rock that contains less than 10 % granite leucosome material. The pegmatitic granite shows variations in grain size, colour, and mode of occurrence. The veined gneiss dominates the central part of the trench. The northernmost part of the trench is mainly comprised of tonalitic-granodioritic-granitic gneiss, which has an igneous character. Most rocks in the trench are altered, the intensity of the alteration varies from weak to strong, and the most common alteration type is chloritisation. The intense alteration has resulted in pervasive softening and weathering of the bedrock in places and up to four meters thick layers of strongly weathered rocks were observed on the edges of the trench. The foliation S2B and associated granitic leucosome veining usually strike NE-SW and dip gently to the southeast (the mean dip/dip direction is 38/158 deg). The foliation gradually turns from NE-SW striking in the southern part of the trench to E-W striking in the northern part. No pre-D_2B structures were identified in the OL-TK9. The leucosome veins show isoclinal D₂ folds in places. During the third deformation phase (D₃), the isoclinal F2 folds of the mica gneiss and the granite leucosome veins were refolded by F₃ folding mainly producing small-scale, tight folds with fold axis moderately plunging towards east and rarely to the west. During the fracture mapping, all fractures intersecting the central thread were measured. Dip direction and dip, rock type, length, form, undulation, character, aperture, and infilling if present were recorded for each fracture. A total of 1197 fractures were measured including several fracture clusters (cluster = many identical fractures in a small area). The mean fracture density of the trench is 3.23 fractures/m. From the fracture orientation data, three different fracture sets were interpreted: Set (I) fractures dip gently to the south-southeast, set (II) fractures dip steeply to the east or west, and set (III) fractures dip moderately towards the north. Partly due to the width of the trench most fractures have trace length less than two metres. Most fractures (51,4 %) are open or partially open and 32.4 % of the fractures have infillings. Fracture infillings include chlorite, calcite, kaolinite, epidote, pyrite, illite, mica minerals, quartz, Fe-oxide (hematite), and soil. Several deformation zone intersections were observed in the investigation trench. Four Brittle Joint cluster Intersections (parallel to the foliation), one N-S striking Brittle Fault zone Intersection, one Semi-brittle Fault zone Intersection, and one N-S striking Ductile Shear zone Intersection intersect the trench. (orig.)