Lithological and structural bedrock model of the Haestholmen study site, Loviisa, SE Finland

The Haestholmen study site is located within the anorogenic Wiborg rapakivi granite batholith, 1640 1630 Ma in age. The bedrock consists of various rapakivi granites, which can be divided into three groups or lithological units: (1) wiborgite and pyterlite, (2) porphyritic rapakivi granite, and (3) even-grained or weakly porphyritic rapakivi granite, pyterlite being the dominant rock type. The evengrained and weakly porphyritic rapakivi granite has been interpreted to form a younger intrusive unit with a thickness of ca. 500 m, dipping approx. 20 deg to the NNW-NNE. Surface fractures form a distinct orthogonal system, with three perpendicular fracture directions: fractures dipping steeply (dip >75 deg) to the NE-SW and NW-SE plus subhorizontal (dip <30 deg) fractures. The fracturing in the outcrops is sparse, the average fracture frequency being 0.6 fractures/m. The majority of the fractures in the drill cores are horizontal or very gently dipping and there is no difference in fracture orientations in regard to rock type or depth. Core samples are usually slightly fractured (1 - 3 fractures/m), even-grained rapakivi granites being in places abundantly fractured (3 10 fractures/m. The broken sections in Haestholmen core samples represent about 4.6 % of the total length of the samples. Calcite, dolomite, Fe- hydroxides and clay minerals (illite, montmorillonite and kaolinite) form the most typical fracture mineral phases throughout the drill cores. Core discing is locally seen as repeated fracture-like subparallel cracks in core with spacing of about some millimetres to tens of millimetres. The structural model contains 27 structures (denoted by the term R+number), more than half of which have been verified by direct observations from boreholes or from the VLJ repository. The remaining structures are mainly based on the geophysical interpretation, and have been classified as probable or possible fracture zones. In addition, local structures with uncertain orientation and continuity occur in the rock mass. They are not classified as R-structures but may still have hydraulic significance. The most significant features of the bedrock are the subhorizontal structures R1, R3, R18 and R19 located in boreholes over the depth ranges of 50 - 150 m, 150 - 350 m, 300 - 500 m and 700 - 950 m, respectively. Transmissivity values for the R-structures measured by the double packer system and the flowmeter lie in the range 1*10-3 m2/s to 1*10-7 m2/s, the average being 1-10-5 m2/s. (orig.)