Bolundsfjärden
5 Bedrock geology
5.2 Evaluation of primary data
5.2.5 Brittle deformation
Figure 5‑11. Location of outcrops where detailed fracture mapping has been carried out. Data that have been acquired according to the SKB method description are presented in /Hermanson et al. 2003a, 2003b, 2004, Leijon (ed) 2005, Cronquist et al. 2005, Forssberg et al. 2007, Petersson et al. 2007a/. Complemen-tary data from outcrops and across lineaments that do not strictly follow the SKB method description are presented and evaluated in /Stephens et al. 2003a, 2003b/ and /Petersson et al. 2007b/.
±
Local model area, stage 2.2 Candidate area
Outcrop where detailed fracture mapping has been carried out
Outcrop where mapping of fractures has been carried out in connection with the bedrock mapping programme
0 375 750 1,500metres
Outcrop where complementary lineament assessment has been carried out (two excavations)
© Lantmäteriverket Gävle 2007 Consent I 2007/1092 AFM100201
AFM001265 AFM001264
AFM001244 AFM001243 AFM001097
AFM000054 AFM000053
AFM001098
1630000
1630000
1632000
1632000
1634000
1634000 1634000
1636000
6696000 6696000
6698000 6698000
6700000 6700000
6702000 6702000
The detailed mapping across lineaments /Cronquist et al. 2005, Petersson et al. 2007ab/ confirmed that magnetic minima at four of the five excavations correspond to fracture zones (/SKB 2006a, Petersson et al. 2007b/ and see also section 5.2.7). The lineament defined by magnetic minima at the fifth excavation (AFM001244) corresponds to a swarm of Group D granite and pegmatite dykes with low magnetic susceptibility /SKB 2006a/. Excavation work also revealed a minor fracture zone that dips steeply to the NW at drill site 4 (AFM001097). The data from the other outcrops are judged to be relevant for the characterisation of fractures at the ground surface in the parts of rock domains unaffected by deformation zones or swarms of Group D intrusions, i.e. the background fracturing.
Fracture orientation from cored borehole data
The orientations of fractures outside possible deformation zones, which were identified in the single-hole interpretation of boresingle-holes during stage 2.2, have been compiled in /Stephens et al. 2007/. The modifications to the single-hole interpretations /Stephens et al. 2007, Table 3-2/ were taken into account in this work. Orientation data for sealed fractures, for open and partly open fractures, and for all fractures are shown for each borehole. No distinction was made in this analysis between fractures marked “visible in BIPS” and those marked “not visible in BIPS” (cf. geological DFN modelling work). Examples of orientation plots for four boreholes from the marginal and internal parts of the tectonic lens are shown in Figure 5-12. For purposes of simplicity in the text that follows, the strike of steeply dipping fracture sets is presented as a single direction (e.g. NE). The fractures in the different sets may dip in both directions (e.g. NW and SE) and can also be described as sub-vertical.
Thus, the procedure does not strictly follow the right-hand-rule method (see also section 2.4 in /Stephens et al. 2007/).
The data outside possible deformation zones show similar patterns in the different boreholes. Two steeply dipping fracture sets that strike approximately NE and NW are visible in nearly all the boreholes (Figure 5-12). However, especially the strike of the fractures in these sets varies, not least in the NE set (see, for example, KFM05A and KFM06A in Figure 5-12). In addition to the dominant NE and NW sets, other potential fracture sets with variable intensities in different boreholes are visible. For example, a steeply dipping set with NNW strike is present in boreholes KFM06A and KFM07A (Figure 5-12). Furthermore, all boreholes intersect fractures that are gently dipping to sub-horizontal (Figure 5-12). The concentration of open and partly open fractures is significantly
All (996 fractures) KFM01A
All (1,372 fractures) KFM07A
All (1,589 fractures) KFM05A
All (1,593 fractures)
KFM06A KFM06A
Open + Partly Open (360)
higher in this fracture set relative to that in the steeply dipping sets (see, for example, KFM06A in Figure 5-12). A compilation of the orientation of fractures inside different fracture domains as a prerequisite for the geological DFN modelling work is presented in section 5.6.1.
The orientations of fractures inside possible deformation zones have been presented in Appendix 9 in /Stephens et al. 2007/. Orientation data for sealed fractures, open fractures and partly open fractures are distinguished in each possible zone. All fractures that are not visible in BIPS were excluded from the analysis.
Steeply dipping fractures that strike WNW to NW, ENE to NNE and NNW as well as gently dipping to sub-horizontal fractures are conspicuous. Along some zones, there is also an indication that the steeply dipping fractures with ENE to NNE strike can be divided into two sub-sets /Stephens et al. 2007/. These features are strongly reminiscent of the orientation of fractures outside possible deformation zones (see above). A second characteristic feature is that more than one set of fractures occurs along many of the possible deformation zones. It is inferred that the different sets of fractures, in particular the steeply and gently dipping fractures, are genetically related and formed close in time during the geological history. Compilations of the orientations of fractures along the different sets of modelled deformation zones are presented in section 5.5.4.
Fracture frequency from cored borehole data
The fracture frequency statistics for each of the cored boreholes used in model stage 2.2, both for the whole length of a borehole and for the part that lies outside possible deformation zones, has been presented in /Stephens et al. 2007, Tables 3-11 and 3-12, respectively/. Apart from borehole KFM01C, the frequency of fractures per metre in the bedrock outside possible deformation zones ranges between 1.24 and 5.55. These values decrease somewhat, if borehole intervals that are situ-ated between or very close to possible deformation zones and that are judged to have been affected by the brittle deformation along the zones /Olofsson et al. 2007/ are excluded. Borehole KFM01C, designed to investigate lineaments, is a notable anomaly and the entire length of this borehole has possibly been affected by the brittle deformation along the zones in this borehole. For this reason, the data from KFM01C were omitted from the geological DFN intensity modelling work /Fox et al.
2007/.
Moving average and cumulative frequency plots for each of the cored boreholes used in model stage 2.2, which show the mode of variation for different types of fractures with depth in each borehole, were presented in Appendix 10 in /Stephens et al. 2007/. This type of analysis is one of the tools that has been used in the single-hole interpretation work, as an aid in the identification of possible deformation zones (see section 5.2.2). Furthermore, it has had major implications for the identification of fracture domains at Forsmark (/Olofsson et al. 2007/ and section 5.6.1).
On the basis of analyses initially in /SKB 2005a/ and with the help of more borehole data in /Olofsson et al. 2007/, it has been inferred that the north-western and south-eastern parts of the can-didate volume differ from each other, where it concerns the variation in the frequency of particularly open and partly open fractures down to approximately 1,000 m depth (Figure 5-13). Furthermore, inside the targeted, north-western part of the candidate volume, the boundary between bedrock with an anomalously high frequency of open and partly open fractures (upper part), and bedrock with a strong dominance of sealed fractures (lower part), does not occur at the same elevation in the boreholes. This boundary varies between c. 40 m (e.g. drill site 8) and c. 200 m (e.g. drill site 1) below sea level. Boreholes KFM01A and KFM03A provide type logs that distinguish the contrasting variations in fracture frequency with depth inside the candidate volume (Figure 5-13).
Fracture mineralogy from cored borehole data
Various aspects of the minerals that coat and fill fractures at Forsmark have been addressed in seven primary data reports and in two peer-reviewed publications /Sandström et al. 2006, Sandström and Tullborg 2007/. An overview report that integrates the results of this work has been presented in /Sandström et al. 2008/. Work that documents more quantitatively the amount of different minerals along fractures is in progress and will be presented in a separate, complementary document. The necessity of these data for the purposes of bedrock transport properties was recognised at a late stage during the site investigation programme.
Figure 5‑13. Fracture frequency plots for the cored boreholes KFM01A and KFM03A. The upper diagram
Fracture frequencyFracture frequency
Fracture frequency
0 200 400 1,000
0 1
0 600 800
0 200 400 1,000
0 10 20 30 40
50 0
10 20 30 40 50 60 70
Fracture frequency
Elevation (m.b.s.l.)600 800
200 400
Elevation (m.b.s.l.)
1,000
800
Cumulative density functionCumulative density function
Elevation (m.b.s.l.)
Elevation (m.b.s.l.) Elevation (m.b.s.l.) Elevation (m.b.s.l.) 0.8
0.6 0.4 0.2
600
KFM01A
0 200 400 600 800 1,000
0 0.2 0.4 0.6 0.8 1
0 200 400 600 800 1,000
0 200 400 600 800 1,000
0 10 20 30 40
50 0
10 20 30 40 50 60 70
1
KFM03A
Open_total fractures Sealed fractures Sealed fracture network All fractures Deformation zone, SHI Deformation zone, modelstage 2.2
Open_total fractures Sealed fractures Sealed fracture network All fractures Deformation zone, SHI Deformation zone, modelstage 2.2
Relative and absolute ages of fracture minerals
Different generations of fracture minerals have been recognised at Forsmark (Figure 5-14). The relative time relationships between these different generations as well as the data bearing on absolute age determinations have been evaluated and used in the establishment of the geological evolution of the site (/Söderbäck (ed) 2008/ and section 3.1).
An early period of precipitation of a high-temperature mineral assemblage, which includes epidote (Figure 5-14a), chlorite and quartz was followed by a period (or periods) of hydrothermal precipita-tion of different, lower temperature minerals, including adularia (older generaprecipita-tion), hematite, prehnite, laumontite and calcite (Figure 5-14b, c). The fractures that bear epidote formed prior to the Sveconorwegian tectonothermal event, i.e. they formed prior to 1.1 Ga. The fractures that contain the older generation of adularia formed either during or prior to this event. A combination of these two possibilities is also viable.
Figure 5‑14. Drill core photographs showing fracture minerals in the four different generations recognised at Forsmark. a) Generation 1. Epidote-bearing cataclasite (KFM06A, 268.77–268.82 m, /Sandström and Tullborg 2005/). b) Generation 2. Fracture filled by brick-red, hematite-stained adularia cut by a fracture filled with prehnite (KFM05A, 689.33–689.61 m, /Sandström and Tullborg 2005/). c) Generation 2.
Laumontite-sealed breccia (KFM04A, 244.46–244.58 m, /Sandström and Tullborg 2005/. d) Generation 3.
Calcite and pyrite crystals on top of a fracture surface coated with quartz (KFM01A, 267.0 m, /Sandström et al. 2004/). e) Generation 3. Asphaltite in voids in older, partly dissolved calcite along a steeply dipping fracture (KFM06A, 106.94–107.14 m, /Sandström and Tullborg 2005/). f) Generation 4. Open fracture with a thin coating of calcite. This generation of calcite often occurs together with clay minerals (KFM08B, 97.37–97.43 m, /Sandström and Tullborg 2006/).
f d c
b a
e
asphaltite epidote
Precipitation of younger low-temperature minerals along fractures – including sulphides, oily asphaltite, clay minerals and calcite (Figure 5-14d, e, f) – occurred during Phanerozoic time. The asphaltite was derived from Cambrian to Lower Ordovician oil shale. Together with limestone and other clastic sedimentary rocks, this oil shale covered the crystalline bedrock at Forsmark during much of the Phanerozoic (/Söderbäck (ed) 2008/ and section 3.1). Furthermore, growth of adularia (younger generation) occurred during the Permian. The youngest generation of calcite occurs in hydraulically conductive fractures and zones. This mineral may have precipitated over a long period extending to the present. Fluids, which transported glacial sediment, also migrated downwards and filled new or reactivated fractures at Forsmark during the later part of the Quaternary /Carlsson 1979, Leijon (ed) 2005/.
Distribution of fracture minerals
The identification and significance of different fracture minerals along each possible zone in the single-hole interpretations have been addressed in /Stephens et al. 2007, section 3.6.5/. The number of occurrences of each mineral in each zone are shown in Appendix 11 in /Stephens et al. 2007/ and the orientation of fractures divided on the basis of fracture mineralogy are presented in Appendix 12 in /Stephens et al. 2007/. Identical analyses have also been completed for fracture minerals in frac-ture domains /Sandström et al. 2008/. The occurrences of fracture minerals along different sets of deformation zones have been addressed in /Stephens et al. 2007, sections 5.3.2 and 5.5/ and a sum-mary is provided here in section 5.5.4. For comparison purposes, a sumsum-mary of the occurrence and orientation of fracture minerals in fracture domains, based on /Sandström et al. 2008, section 6.1/, is presented in section 5.6.1.
The occurrence of similar minerals and wall-rock alteration along different fracture orientation sets within a single deformation zone provides support to the inference drawn earlier that the different sets are genetically related. Although, for example, laumontite is conspicuous along steeply dipping fractures that strike approximately NE (see also /SKB 2005a/), the older mineral epidote and the younger minerals pyrite and clay minerals are also present along fractures with this orientation.
This observation suggests that fractures with this orientation have both an older history prior to the growth of laumontite and a younger history following the growth of this mineral. Thus, the analytical work provides strong evidence for the presence of different generations of minerals, which formed under different metamorphic conditions, along a single zone, i.e. zone reactivation occurred. It needs to be kept in mind that the temperature along the zones, with their hot hydrothermal fluids, can be higher than that in the surrounding bedrock.
The variation in the occurrence of different fracture minerals with depth was addressed in /Stephens et al. 2007, section 3.6.5/, while a more focused analysis of the variation of minerals along open fractures, in the upper 100 m of the bedrock, was presented in /Sandström et al. 2008, section 6.2/.
In order to assess the variation of fracture minerals with depth, the frequency of occurrence of a particular mineral in a particular type of fracture, as well as the overall frequency of occurrence of this fracture type, has been documented for each 10 m vertical depth interval along a borehole /Stephens et al. 2007/. Nearly 900 mineral distribution plots have been generated in this analysis.
A few examples are presented in Figure 5-15.
As for the wall-rock alteration referred to as oxidation (see section 5.2.3), the minerals in the older parageneses (epidote and adularia-prehnite-laumontite) show no relationship between frequency of occurrence and depth (Figure 5-15a). By contrast, apart from borehole KFM12A, which intersects the regionally important Forsmark deformation zone, the younger mineral asphaltite is restricted entirely to the uppermost part of the bedrock (Figure 5-15b). In some boreholes, other younger minerals, for example pyrite and clay minerals (Figure 5-15c), show a similar concentration in the upper part of the bedrock. It needs to be kept in mind that the current ground surface is more or less the same surface that existed prior to c. 540 million years ago (the sub-Cambrian unconformity or sub-Cambrian peneplain) and, for this reason, the correlation with depth may have been established
Figure 5‑15. Variation with depth of a) adularia (older generation) in KFM01D, b) asphaltite in KFM01B, c) clay minerals in KFM01B and d) fractures with no minerals in KFM02A. For each mineral, the analysis has addressed the occurrence in all fractures. The total number of fractures/10 m borehole interval is also shown.
Clay Minerals in all fractures in KFM01B
0 50 100 150 200 250 300 350
10 30 50 70 90 110 130 150 170 190 210 230 250 270 290 310 330 350 370 390 410 430 450 470
Vertical depth (m.b.s.l)
Number of fractures / 10m vertical depth Fractures with Clay Minerals Total number of fractures
Adularia in all fractures in KFM01D
0 20 40 60 80 100 120 140
70 100 130 160 190 220 250 280 310 340 370 400 430 460 490 520 550 580 610
Vertical depth (m.b.s.l)
Number of fractures / 10m vertical depth Fractures with Adularia Total number of fractures
Asphaltite in all fractures in KFM01B
0 50 100 150 200 250 300 350
10 30 50 70 90 110 130 150 170 190 210 230 250 270 290 310 330 350 370 390 410 430 450 470
Vertical depth (m.b.s.l)
Number of fractures / 10m vertical depth Fractures with Asphaltite Total number of fractures
No_mineral in all fractures in KFM02A
0 50 100 150 200
90 130 170 210 250 290 330 370 410 450 490 530 570 610 650 690 730 770 810 850 890 930 970
Vertical depth (m.b.s.l)
Number of fractures / 10m vertical depth Fractures without Mineral Total number of fractures
d c
b a
concentrated in the upper part of the borehole, at shallow depths, but they are also locally present at a depth of 400 to 500 m. This observation indicates some relationship to the establishment of the current ground surface, at least for these fractures. By contrast in boreholes KFM02A (Figure 5-15d) and KFM03A, there is no simple relationship with depth in the borehole, but there is a clear
concentration along sections where possible deformation zones with gently dipping fractures have been inferred in the single-hole interpretations.
Several hypotheses were discussed in /Stephens et al. 2007, section 3.6.5/ that may account for the occurrence of these structures, and further work on the occurrence and origin of particularly open fractures, which neither contain a mineral coating nor any wall-rock alteration, was recommended /Stephens et al. 2007, p. 91/. This work has recently been initiated and the results of this study will be reported separately in a complementary document. At this stage, it is premature to make any firm conclusions concerning their significance or origin. However, errors in the mapping procedure, the occurence of drilling-induced fractures, particulary where problems with drilling were encountered, or the possibility that at least some of the fractures without mineral coating represent a relatively young geological feature, i.e. they formed after the establishment of the sub-Cambrian unconformity, merit particular attention.
5.2.6 Character and kinematics of deformation zones