Groundbased Geophysics

Gravity measurements have been made in a coarse, irregular net, by the Geological Survey of Sweden, the Geodetic Survey, and in context with the Deep Gas Project by Uppsala University. All data have been collected in the national database and can be requested from the Geological Survey of Sweden. Figure 8 shows the compiled gravity with 1 mgal contours and gray tone shading in 5 mgal intervals. There is no clear gravity anomaly associated with the impact structure. Instead, local and regional differences in density of both near-surface and deep lithologies dominate the anomaly pattern. In Fig. 9, the regional field has been removed, which brings out the uppermost crustal features in the residual anomaly and suppresses the effects from deep and large structures.

The Deep Gas Project also provided 80 km of reflection seismic data traversing the central part of the structure and the northern and eastern parts of the sedimentary ring basin (Juhlin and Pedersen, 1987). Some of the seismic reflectors recorded within the central uplift were penetrated by the deep drilling and were found to be related to the occurrence of dolerite sills. The location of seismic lines is shown in Fig. 2. In Fig. 10, the seismic reflectors interpreted by Juhlin and Pedersen (1987 and 1993) are shown together with a topographic profile extending from west over the center to the north. The spatial characteristics of the reflective patterns have been summarized for this review.

Magnetotelluric (MT) measurements were made at 65 stations, mainly within the central uplift, traversing the ring and the nearest exterior areas to the NW and N. The magnetotelluric data were published in Zhang et al. (1989). The correlation with the impact structure is unclear, as the main low resistive feature that was found is off- center to the northwest with respect to the center of the impact structure. The location of MT stations is indicated in Fig. 2.

Fig. 11. Location of sampling sites for measurements of rock physical properties. The background map is based on digital elevation data with 50 m spacing from Lantmateriet (National Land Survey of Sweden).

Rock physical properties have been measured on samples from the centrally located 610 m deep drill hole (No 5 in Fig. 2) and from outcrops. The unpublished data are available on request from the national database at the Geological Survey of Sweden. The measurements comprise density, magnetic susceptibility and remanent magnetization. In addition, the Geological Survey of Sweden has a regional data set of such petrophysical measurements containing 2392 samples. For locations see Fig. 11. The data have been compiled in standard diagrams with some of the major lithologies related to the Siljan structure outlined (Fig. 12). Some results are summarized in Table 2.

Dsnilti (SI)

Fig. 12. Magnetic susceptibility - density plot of rock samples from the Siljan region. The variation field of some key lithologies is outlined with ellipses. The symbols refer to: Small squares - Paleozoic sandstone and limestone; Triangles: Siljan granite; Large squares: Jama granite. All other lithologies are marked with small + signs. Units are in SI (dimensionless and kgm-3, respectively).

Dsnilti (SI)

Fig. 12. Magnetic susceptibility - density plot of rock samples from the Siljan region. The variation field of some key lithologies is outlined with ellipses. The symbols refer to: Small squares - Paleozoic sandstone and limestone; Triangles: Siljan granite; Large squares: Jama granite. All other lithologies are marked with small + signs. Units are in SI (dimensionless and kgm-3, respectively).

A distinct density and magnetic contrast exists between different members of the TIB intrusives that occur within the central uplift of the Siljan structure (the Jarna and Siljan types, Hjelmquist 1966). There is a rather small density contrast between the Jarna granite and the Paleozoic limestones. Both features complicate the interpretation of the gravity measurements with regard to structural changes caused by the impact event.

In-situ electric resistivity measurements have been made in a coarse net mainly in the central part of the structure. These data were evaluated by Henkel (1992). A relation between impact-induced porosity (due to more intense fracturing) and the electric resistivity could be established and seems to be a characteristic feature of brecciated crystalline rocks, and the central uplift region of complex impact structures in particular.

Table 2. Compilation of petrophysical properties for rocks from the Siljan region. The total number of measured samples is 2392. The q-ratio is the ratio between remanent and induced magnetization.

Rock type

No. of samples

Density (kgm-3) mean st.dev.

Magnetic suscept. (10-5 SI)

q-ratio

All rocks (except dolerites and ores)

2212

2719

122

835

1.23

TIB granite (Jarna type)

65

2675

43

1188

0.31

TIB granite (Siljan type)

27

2618

24

361

0.35

Paleozoic limestone

20

2681

16

8

1.42

Table 3. Comparison of rock physical properties of Jama granite from the exterior and central uplift Samples from the outer part of the central uplift are from the drilling at locality 3 and samples from the central part of the central uplift are from drill hole 5 in Fig. 2, respectively. The q-ratio is the ratio between remanent and induced magnetization.

Table 3. Comparison of rock physical properties of Jama granite from the exterior and central uplift Samples from the outer part of the central uplift are from the drilling at locality 3 and samples from the central part of the central uplift are from drill hole 5 in Fig. 2, respectively. The q-ratio is the ratio between remanent and induced magnetization.

Source

Depth (m) below surface

Density (kgm-3)

Susceptibility (10-2 SI)

q-ratio

Exterior to impact structure

0

2712

1.4

0.21

0

2736

3.3

0.09

Outer part of central uplift

205

2695

0.15

0.37

225

2691

0.10

0.42

Central part of central uplift

110

2725

4.1

0.84

170

2715

5.0

0.65

290

2709

4.0

1.9

410

2662

3.2

1.3

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