TY - BOOK
T1 - Lithological and structural bedrock model of the Hästholmen study site, Loviisa, SE Finland
AU - Front, Kai
AU - Paulamäki, Seppo
AU - Ahokas, Henry
AU - Anttila, Pekka
PY - 1999
Y1 - 1999
N2 - 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.
AB - 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.
KW - precambrian
KW - rapakivi granite
KW - fracturing
KW - hydraulic conductivity
KW - bedrock model
KW - structural model
KW - final disposal
KW - nuclear waste
UR - https://www.posiva.fi/en/index/media/reports.html
M3 - Report
SN - 951-652-086-3
T3 - Posiva Report
BT - Lithological and structural bedrock model of the Hästholmen study site, Loviisa, SE Finland
PB - Posiva
CY - Helsinki
ER -