Background

The occurrence of a dome of Proterozoic granites of the Trans-Scandinavian Igneous Belt (TIB) surrounded by a ring of Paleozoic sediments, as mapped by Stolpe (1872) and Hjelmquist (1966), posed an enigma never really discussed until the Siljan ring was suggested as an impact structure by Wickman et al. (1963) and Fredriksson and Wickman (1963). This suggestion was based on morphologic features, and it was ignored for many years until shock metamorphic evidence like planar deformation features (PDF) and shatter cones were found (Svensson 1973). Figure 1 shows the location of the Siljan structure together with other impact craters in Fennoscandia. Table 1 shows the stratigraphic column for the Siljan region and Fig. 2 gives an overview of the regional geology of the ring syncline and the central uplift. A view from the NW edge of the structure (Fig. 3) shows the still impressive crater in the landscape.

In a sector from west to north to northeast, the basement to the Paleozoic lithologies consists of mainly Proterozoic metavolcanic rocks intercalated with

Fig. 2. Geological overview. The pre-impact cover sequence is subdivided into late Proterozoic (>), Paleozoic limestone (brick pattern) and sandstone (coarse spaced dots). The crystalline core is marked with dense dots. Location of the drillings (black dots and numbers 1 - 7), MT measurement stations (marked +) and reflection seismic lines (gray lines) made for the Deep Gas Project. The deep wells Gravberg-1 and Stenberg-1 are located at (1) and (5), respectively.

Fig. 2. Geological overview. The pre-impact cover sequence is subdivided into late Proterozoic (>), Paleozoic limestone (brick pattern) and sandstone (coarse spaced dots). The crystalline core is marked with dense dots. Location of the drillings (black dots and numbers 1 - 7), MT measurement stations (marked +) and reflection seismic lines (gray lines) made for the Deep Gas Project. The deep wells Gravberg-1 and Stenberg-1 are located at (1) and (5), respectively.

Fig. 3. View of the Siljan structure from the NW slope towards the south. The ring depression with lakes is followed by the gently sloping central rise and the southern crater margin forms the horizon.

minor occurrences of sandstones. The central uplift contains different types of felsic intrusives within the Trans-Scandinavian Igneous Belt (TIB) and some minor mafic masses and dykes. The Jama and Siljan granite types are predominant and are part of a large batholith within the TIB extending for 300 km to the NW (Gaal and Gorbatschev 1987). In the crystalline core of the Siljan structure, all lithologies are overprinted by characteristic penetrative lineations. These lineations have been interpreted as shatter cone features in the geologic map of Kresten et al. (1991). Small-scale shatter cones occur in both felsic and mafic lithologies.

At three localities within the crystalline central uplift spatially limited xenoliths of Proterozoic sedimentary rocks were mapped. At one of these localities, close to Hattberg (close to locality 4 in Fig. 2) in the center of the uplift, a large clast of late Proterozoic sandstone is overprinted with complex striation patterns in several orientations.

Re-appraisal of the impact idea came with the launching of the Deep Gas Project by a consortium in 1982. A set of bedrock geological maps by Kresten et al. (1991) at the scale of 1:50 000 is based on detailed mapping of the sparse outcrops that occur within the impact structure, and water wells and local boulder finds. Also at that time detailed low altitude (terrain clearance 30 m) airborne geophysical surveys covered the Siljan region.

The Deep Gas Project aimed at finding potential abiogenic gas emanating from the mantle and accumulated in fractured crystalline rocks within an impact structure supposed to be sufficiently large to have penetrated the entire crust (Gold and Soter 1980).

The Deep Gas Project resulted in large amounts of data from 7 shallow (up to ca 800 m) and two deep drillings (Gravberg-1 with 6.8 km, and Stenberg-1 with 6.5 km vertical depth). The drill hole at Gravberg is located within the peak ring, whereas the drill hole at Stenberg is located at the center of the uplift. Much of these data are summarized in Juhlin (1991).

Table 1. Stratigraphy of the Siljan region. The Cambrian sequences are not represented in the area.

Age

Event

Lithology

Quarternary

Glaciations

Till

Paleozoic

Devonian

Siljan impact 377 Ma

Impact melt and breccias

Silurian

sandstone

Limestone and siltstone

Ordovician

Limestone and shale

Limestone

Discontinuity

(Vendian / Riphean /Jotnian)

Mafic sills and dykes

Sandstone

Discontinuity

Middle

Felsic -intermediate volcanics

Felsic intrusives

Svecofennian orogeny

Meta supracrustal gneisses

The previous age determinations by 40Ar-39Ar dating were based on a few samples of suggested impact melt and ranged from 342 to 368 Ma (Bottomley et al. 1978). A collection of recently found impact melt breccias from 5 localities within the central uplift have been dated with laser-argon and argon step heating methods. A new, older age for the Siljan impact event was established at 377±1.7 Ma (Reimold et al. 2004). The chances to find more impact melt is very restricted, as the area within the central portion of the structure is basically devoid of outcrops.

The complex mass flow in connection with the transient crater collapse is discussed in Henkel and Reimold (1997 and 1998) for the Vredefort structure. In the numerical models of impact rater collapse in Melosh and Ivanov (1999), the complexity of this mass redistribution is envisaged as thinning of marker layers in the ring depression and thickening towards the central rise. The collapse flow results in radial inward mass concentration of the exterior, near-surface lithologies, and a radial outward dispersion of the central uplift lithologies. The collision of these opposed radial mass flows causes the up-and overturning of lithologies as observed in the Vredefort and Siljan structures. The collision of radial mass flows also results in the imbrication and repetition of lithologies that were located at the excavated crater edge and the central uplift, respectively. In the Siljan structure, repetitions of marker horizons of Vendian sandstone have been reported from along the central crystalline core in the northeastern part of the central uplift (Thorslund and Collini (1980). It can also be seen on the bedrock map by Kresten et al. (1991) by the distribution of Paleozoic mega-blocks in the southeastern and northwestern part of the central uplift. In the outer part of the crystalline central uplift, seven Paleozoic mega-blocks have been mapped, ranging in size from hundreds of meters to several km. In many of these occurrences limestone has been quarried for cement production. These quarries are excellent study sites for the tectonic overprint caused by the collapse flow. The lithologies of the sedimentary ring are locally strongly tectonised and broken into clasts with sizes up to several tens of meters, and are occasionally surrounded by micro-brecciated rock material.

In four places along the ring syncline, the Proterozoic basement rocks are in a high structural position and the Peleozoic lithologies appear to be missing. Kenkmann and von Dalwigk (2000) have suggested, as a working hypothesis, that these sectors represent radial transpression ridges that have formed by mass concentration by the radial inflow from the collapsing crater wall. Within individual clasts, the stratigraphic succession is often remarkably well preserved and several occurrences of Paleozoic pre-impact cover rocks have been reported as stratigraphic type localities. The occurrence large blocks of cover rocks immersed in brecciated crystalline basement have, for example, also been observed in one of the deep drill holes 3 km outside the peak ring of the Popigai structure, some 400 m below the allogenic breccia level (Masaitis et al. 1998)

The collapse of large craters has been modeled numerically for a series of structures with increasing diameters (Melosh and Ivanov 1999). In these models, transient craters with diameters >30 km collapse into peak ring structures. The near-surface structures from beyond the edge of the excavated crater are thickened in the ring basin and up- and overturned by the collapsed peak ring.

Despite huge efforts that have been put into investigations of the Siljan structure, several key issues are still unresolved. The original size of the structure, the vertical and lateral extent of different types of impact overprint (shock, tectonic, thermal, magnetic), the structure of the ring basin (depth, tectonic, stratigraphic), and the occurrence and nature of impact melt rocks, all remain to be further investigated.

In this short review emphasis is on the morphology as seen in digital elevation data and on the geophysical data from airborne, surface and drillhole measurements. The most characteristic result is that the very large and morphologically well-expressed Siljan crater largely lacks dramatic geophysical anomalies (as opposed to many smaller structures were both distinct gravity and magnetic anomalies occur).

This review concentrates on the morphological, geophysical and petrophysical aspects of the Siljan impact structure.

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