Siljan Ring Structure Research Articles

Lithological Investigation of The Drill Core from a Sedimentary Cover in the Area of the Siljan Ring, Central Sweden

The ring structure of Siljan, located in the central part of Sweden, is considered by many researchers to be a meteorite (impact) crater. Impact craters are among the most complex geological objects on the Earth. The origin and formation of these structures still raises many questions. To find answers to these questions we need reliable geological information about the structure of the crater and the composition of the rocks. Information about the thickness and geological structure of the Siljan Ring area sedimentary cover will help to understand the process of the Siljan Ring structure’s formation as well as other similar geological formations on the Earth. Here, we present the results of laboratory studies of sedimentary rock samples taken from four exploration wells drilled in the vicinity of the Siljan Ring crater, which made it possible to compile their detailed lithological description. The laboratory studies included a structural analysis of the samples, and a texture and mineralogical analysis in thin sections. A structure analysis was carried out visually, while structural and mineralogical analyses were carried out on thin sections using a polarizing microscope and a scanning electron microscope. The main components of the rocks (minerals and fragments), along with their ratio and secondary transformations, were determined. The results of the structural analysis of the samples, and the textural and mineralogical analysis of the rocks in thin sections, showed that the sediments’ composition in the sedimentary cover near the Siljan Ring structure changes in different areas in accordance with their facies and stratigraphic characteristics. Furthermore, a change in the thickness of the sections and the succession sequences of rock units was established. A change of this nature is presumably caused by tectonic disturbances of an endogenous or impact source.

High-resolution seismic imaging of Paleozoic rocks in the Mora area, Siljan Ring structure, central Sweden

The Late Devonian Siljan Ring structure in Sweden is the largest known impact structure in Europe. The present-day structure comprises a central dome that is about 20–30 km in diameter, which is surrounded by a ring-shaped depression. In this study, we focus on the southwestern part of the Siljan Ring with the aim to map the structure of the Paleozoic sedimentary rocks. Four 2D high-resolution seismic lines with a total length of about 3 km were acquired in the Mora area. A three component eighty-unit land streamer, combined with wireless recorders, was used for data acquisition along with a weight drop source. Processing of the data shows that clear reflections are present, but results are less distinct where external noise was present during acquisition or the maximum source-receiver offset was too short. Petrophysical measurements on core samples, core log data and a density model along one line were used to guide the interpretation of the seismic sections. These data demonstrate that fault blocks are present in the study area and that the individual blocks have been affected differently by impact-related tectonics.

A hydrothermal system associated with the Siljan impact structure, Sweden--implications for the search for fossil life on Mars.

The Siljan ring structure (368 +/- 1.1 Ma) is the largest known impact structure in Europe. It isa 65-km-wide, eroded, complex impact structure, displaying several structural units, including a central uplifted region surrounded by a ring-shaped depression. Associated with the impact crater are traces of a post-impact hydrothermal system indicated by precipitated and altered hydrothermal mineral assemblages. Precipitated hydrothermal minerals include quartz veins and breccia fillings associated with granitic rocks at the outer margin of the central uplift, and calcite, fluorite, galena, and sphalerite veins associated with Paleozoic carbonate rocks located outside the central uplift. Two-phase water/gas and oil/gas inclusions in calcite and fluorite display homogenization temperatures between 75 degrees C and 137 degrees C. With an estimated erosional unloading of approximately 1 km, the formation temperatures were probably not more than 10-15 degrees C higher. Fluid inclusion ice-melting temperatures indicate a very low salt content, reducing the probability that the mineralization was precipitated during the Caledonian Orogeny. Our findings suggest that large impacts induce low-temperature hydrothermal systems that may be habitats for thermophilic organisms. Large impact structures on Mars may therefore be suitable targets in the search for fossil thermophilic organisms.

The Siljan ring impact structure: Possible connections with minor ores in its neighborhood

Abstract Martanberg and Slattberg are two old, small mines situated outside, but near the Siljan Ring structure and possibly both are influenced by the impact. The bedrock formed in the main c. 1700 Ma ago. Martanberg is known as a small abandoned copper mine with chalcopyrite and pyrite. Its ore seems to have been formed during two widely separated periods. Slattberg is an old nickel mine which has been worked intermittently, most recently during the two world wars. The host rock is an amphibolite dike which possibly has changed its mineralogical composition during a later event. Both these ores may be explained as a result of the Siljan impact about 360 Ma ago. Wickman, F.E., 1994: The Siljan Ring impact structure: possible connections with minor ores in its neighborhood. GFF, Vol. 116 (Pt. 3, September), pp. 145–146. Stockholm. ISSN 1103–5897.

Further constraints on the formation of the Siljan impact crater from seismic reflection studies

Abstract Recently acquired seismic reflection data over the Siljan Ring structure support previous estimates of the dimensions of the excavated crater made from earlier seismic data. The data are consistent with a present-day excavated crater diameter of 22 km and a minimum depth of 3 km. Allowing for erosion, these depths correspond to a maximum excavated crater diameter of 26 km and a minimum depth of about 4.5 km upon impact. The excavated crater diameter is at the lower limit or slightly less than what would be expected based on empirical relations from crater mechanics using a final rim diameter of 52 km. The structural uplift is less well defined on the new data, with the profile running closer to the center of the crater, but the data indicate an uplift of about 2 km at 9 km depth. The current data set together with previous reflection data strongly support an impact origin for the Siljan Ring structure and indicate that the crust below 15 km was not involved in post-impact vertical movements.

Microtektites and Mass Extinctions: Evidence for a Late Devonian Asteroid Impact

Glass spherules, similar to microtektites, have been found near the Frasnian-Famennian boundary (F/F) (Upper Devonian) at Senzeilles, Belgium, contemporaneous with one of the largest marine mass extinctions of the Phanerozoic. These spherules exhibit a wide range of compositions and display teardrop, dumbbell, and compound morphologies analogous to microtektites. In addition, they lack crystallites, have few or no vesicles, and have a low content of volatile material. These characteristics are supportive of an impact origin. The Siljan Ring (Sweden) and Charlevoix structure (Quebec,Canada) are candidate craters of this age. The presence of microtektites near the F/F boundary supports the hypothesis that an impact caused the Upper Devonian worldwide benthic mass extinctions.

Electric resistivity in the Gravberg‐1 Deep Well, Sweden

The Siljan impact structure located in central Sweden has been the object of intense geophysical and geological studies for several decades. When The Swedish State Power Board decided to drill a deep borehole in the area, investigations increased dramatically around and in the hole. In this paper we present the results of electrical resistivity measurements on the surface, in the hole, and on core samples from shallow coreholes distributed over the whole impact structure. Magnetotelluric data constrain the central pan of the structure to be essentially onedimensional with an upper crustal unit of about 10000 Ωm followed by a less resistive impact related unit of 1000 Ωm from a depth of about 6 km to a depth of 20 km. The lower crust is a homogeneous unit of about 300 Ωm extending down to about 40 km, where the upper mantle is marked by an increase in resistivity of about 1000 Ωm. The borehole resistivity log extends down to about 6100 m, i.e., the expected depth where the resistivity drops by an order of magnitude. Major fracture zones, some of which coincide with the horizontal dolente sills, known to extend over tens of kilometers, are probably the main carriers of current. Fracture zones are typically 10m thick surrounded by more intact rock masses, which show a general tendency of increasing resistivity with depth. In fracture zones the electrical resistivity lies between 1000 and 5000 Ωm, with the interesting trend that below 4000 m depth the resistivity is smaller than in the fracture zone in the depth interval 2000–4000 m. Saline fluids recovered during testing below 5.4‐km depth and the magnetotelluric results lead us to conclude that pore fluids are highly saline below this depth even on a regional scale, and the zone of decreased resistivity found in the upper crust within the Siljan ring structure is interpreted as a zone of increased porosity relative to outside the ring.

Geological results of the drilling in the Siljan Ring Structure

Geological results of the drilling in the Siljan Ring Structure

Drillhole results to be discussed

Vattenfall, the Swedish State Power Board, is searching for a predicted reservoir of abiogenic methane beneath the floor of a meteorite crater in central Sweden. Some of the early scientific results from the drilling project at the Siljan Ring impact structure will be presented on Thursday, May 21, at the 1987 AGU Spring Meeting in Baltimore, Md.Thomas Gold of Cornell University (Ithaca, N.Y.) has predicted that large amounts of methane from deep within the earth may move closer to the surface beneath sites where large meteorites have hit the earth, such as the Siljan Ring structure (Eos, July 9, 1985, p. 537). The site is known for its gas seeps, according to Paul Westcott of the Gas Research Institute (GRI, in Chicago, Ill.). The institute is putting up 15% of the costs of the drillhole in return for access to samples and data. Seismic surveys at the site revealed horizontal structures in the granite, which may suggest the presence of gas‐liquid interfaces, Westcott said.

Retention of U and Pb in zircons from shocked granite in the Siljan impact structure, Sweden

The Siljan Ring structure, Central Sweden was formed by a Devonian impact. The U−Pb zircon isotopic system of the shocked Proterozoic granite basement appears to be undisturbed, although the zircons were mechanically disrupted by the impact.

Fluid inclusions in quartz from a quartz breccia in the Siljan Ring structure, central Sweden

Abstract The Siljan structure represents a deep erosion level of an astrobleme. Post-impact formation of minerals as various types of vein fillings is common. Non-shocked milky quartz from a large vein revealed fluid inclusions of three types:(1) one-phase, regularly shaped inclusions, < 1 to 10 μm in size, (2) two-phase, regularly to irregularly shaped inclusions, 2 to 7 μm, (3) larger irregular two-phase inclusions, 8–20 μm. The one-phase inclusions dominate; density of inclusion occurrence is estimated at 46 × 104 inclusions per cm2. Melting temperatures indicate a total salinity of 1.0 eq. wt. % NaCl. Homogenization temperatures range between 165 and 309°C. The fluid inclusion data imply a complex origin of the quartz. It consists in part of breccia fragments formed after the impact. The quartz vein was cemented by deep circulating meteoric waters at successively lowered temperatures initially over 300°C.

The Proterozoic rock pebbles of the basal Ordovician conglomerate in the Siljan Ring structure and their significance

Abstract It has been postulated that during the formation of an impact crater, fragments from the bedrock mainly move radially with respect to the impact center. The result of rock counts in the Ordovician basal conglomerate from five localities in the Siljan Ring structure is consistent with this hypothesis. The distribution of rock types occurring as pebbles in the allochthonous conglomerate varies from locality to locality, reflecting the Proterozoic bedrock of each radial sector from the impact center.

Geochemical studies on the Boda Pb-Zn deposit in the Siljan astrobleme, central Sweden

Abstract The Boda Pb-Zn deposit is located in the eastern part of the Siljan impact structure in central Sweden. It consists of calcite, baritc, sphalerite, galena, pyrite and marcasite, mainly as fracture and breccia fillings along a thrust zone in Ordovician limestones. Boda, and the smaller but similar Solleron deposit, probably formed in close connection with the tectonic disturbance following the Devonian meteorite impact considered to have created the Siljan Ring structure. A geochemical profile through the ore-hosting limestone at Boda shows that wall rock alteration is not very marked. Lead isotope data point to the Proterozoic basement as the direct or indirect source of the ore lead, and to a large contribution of redeposited Proterozoic lead in the Ordovician limestone, presumably contained in the clastic silicate fraction. The extreme radiogenicity of the Boda ore lead may be explained by selective leaching of the more radiogenic lead fraction from the source rocks, rather than leaching of ave...

Rb-Sr dating of a postorogenic Svecokarelian composite dike within the Siljan ring structure, central Sweden

Abstract Rb-Sr dating of a postorogenic Svecokarelian composite dike from a huge clast within the Siljan ring structure, central Sweden, gives a two point whole rock reference line of 1646 Ma and a whole-rock-mineral isochron age of 1515 ± 21 Ma. Thus, the meteorite impact during Devonian time does not seem to have greatly affected the Rb-Sr whole rock and mineral systems but only indirectly through secondary effects.

Displacements and the Siljan Ring Structure

The formation process of an impact structure is usually divided into two stages. During the first one a transient crater is formed, this stage is characterized by centrifugal displacements. The second stage, the modification stage, is characterized by centripetal displacements. Accordingly, in large eroded impact structures, where in general only displaced blocks and megablocks can be observed, geological studies should be made on separate sectors, not on the whole structure as a unit (the sector principle). Geological observations in the Siljan ring structure are discussed from this point of view.

Shock effects in pegmatitic quartz from the Siljan ring structure, central Sweden

Abstract Shocked milky quartz from the core of a granite pegmatite occurs at Brunnvasselbodarna, about 10 km from the impact center of the Siljan ring structure. The quartz displays well-developed cleavage fractures dominantly parallel with {1011}, some of which are filled with secondary laumontite. Planar features are common, the most frequently observed are oriented parallel to {1013}, {1011}, {0001}, but {1012}, {1122}, {2131}, {2241} and {5161} are also observed. An estimate of the peak pressure of the shock wave in Brunnvasselbodarna gives ca. 16 GPa. This value is probably too high because the grain size of the pegmatite quartz is large compared to the quartz grains of the rocks for which the method used has been devised.

An Eocambrian sequence in the Siljan ring structure

"An Eocambrian sequence in the Siljan ring structure." Geologiska Föreningen i Stockholm Förhandlingar, 102(2), p. 188