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

Kai Front, Seppo Paulamäki, Henry Ahokas, Pekka Anttila

Research output: Book/ReportReport

Abstract

The Hastholmen 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) evengrained 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 ea. 500 m, dipping approx. 20° to the NNW-NNE. Surface fractures form a distinct orthogonal system, with three perpendicular fracture directions: fractures dipping steeply (dip >75°) to the NE-SW and NW-SE plus subhorizontal (dip <30°) 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 Hastholmen 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 bore holes 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.
Original languageEnglish
Place of PublicationHelsinki
PublisherPosiva
Number of pages100
ISBN (Print)951-652-086-3
Publication statusPublished - 1999
MoE publication typeD4 Published development or research report or study

Publication series

SeriesPosiva Report
Volume99-31
ISSN1239-3096

Keywords

  • precambrian
  • rapakivi granite
  • fracturing
  • hydraulic conductivity
  • bedrock model
  • structural model
  • final disposal
  • nuclear waste

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    Front, K., Paulamäki, S., Ahokas, H., & Anttila, P. (1999). Lithological and structural bedrock model of the Hästholmen study site, Loviisa, SE Finland. Posiva. Posiva Report, Vol.. 99-31 http://www.posiva.fi/files/2713/POSIVA-99-31_web.pdf