Morphology and Crater Diameter

The Siljan structure, centered at 14.9° E and 61.1° N (Fig. 1), is one of the larger impact structures on earth. It is clearly visible in the morphology, as demonstrated by digital elevation data (Fig. 4). An interpretation of these data with respect to tangentially oriented steps in the terrain is given in Fig. 5. The structure has been subjected to post-impact differential erosion due to tectonic activity of regional crustal blocks and tilting of the entire region. Especially the block to the NW of the Siljan structure is more elevated, although it still contains late Proterozoic (Riphean/Jotnian/Vendian) cover rocks (sandstone). In Fig. 4 this regional tilt has been removed by subtraction of a planar surface from the elevation data. The residual relative elevation shows the crater structure more clearly. The terrain profile based on elevation data with 50 m spacing (see section on ground-based geophysics, Fig. 10) shows that the Siljan structure has a peak ring of ca. 20 km diameter.

From the present morphology, the erosional crater diameter can be estimated to ca. 75 km based on the prominent escarpments marked in Fig. 5 and the highest topography shown in the profile in Fig. 10. This erosional

Fig. 4. The Siljan impact crater seen in digital elevation data with 50 m spacing, from Lantmateriet (National Land Survey of Sweden). A linear regional terrain gradient has been removed.

Fig. 4. The Siljan impact crater seen in digital elevation data with 50 m spacing, from Lantmateriet (National Land Survey of Sweden). A linear regional terrain gradient has been removed.

edge, as demonstrated in Fig. 5, is highly irregular as opposed to the more circular outline of the central uplift of granitic rocks and pre-impact cover rocks. This uplift of crystalline rocks has a diameter of ca. 30 km at the present erosion level, and it has still, a partial peak ring shape, where elevations are up to 110 m higher than in the interior (Fig. 5 and 10). The height of the ring uplift is up to 265 m above the sedimentary ring basin, which, in turn, lies up to 415m below the exterior terrain. This ring basin is dominated by a series of lakes, which are wide in the south (Siljan), narrow in the north (Skattungen), and almost absent in the east (a few very small lakes). The elevation of the large lake is 162 m above the present sea level.

Fig. 5. Morphological interpretation. The elevation of the lake system is 162 m above sea level. Numbers indicate point elevations in m. The thick lines mark prominent topographic escarpments. Shading outlines the ring of pre-impact cover rocks and the lakes (thin lines).

Previous estimates of the crater diameter range from 52 km (Grieve 1988), i.e., just including the sedimentary ring basin, to 65 km (Kenkmann and von Dalwigk 2000) based on the occurrence of pseudotachylitic dykes assumed to be related to the impact event.

The Paleozoic sedimentary rocks are completely eroded in the surroundings of the Siljan structure. In the northwestern part of the structure, late Proterozoic rocks were also part of the pre-impact cover sequences. To the west, outside the Paleozoic ring, Proterozoic sedimentary and volcanic rocks are preserved over large areas. The minimum of erosion was, therefore, estimated to be the assumed thickness of the Cambro-Silurian sediments, i.e. 400 to 500 m (Rondot 1975). In addition, an unknown amount of younger Paleozoic cover rocks could also have been removed. Another estimate of

Fig. 6. Aeromagnetic total intensity anomaly map of the Siljan region, compiled by the Geological Survey of Sweden.

1 to 2 km is given in Collini (1988) based on the complete erosion of an assumed impact melt thickness of ca. 200 m in addition to the removal of the exterior Paleozoic cover rocks.

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