Siljan Ring Impact Structure Research Articles

Thermal structure of two deep boreholes in the Siljan Ring, Central Sweden, and palaeoclimatic implications

Heat-flow data from deep boreholes are of particular value as a number of factors, including climatic surface temperature variations, may disturb subsurface thermal conditions, especially at shallow depths. This study provides thermal results from two boreholes, Gravberg-1 and Stenberg-1, drilled to large depths in the Siljan Ring impact structure, Baltic Shield, Central Sweden. Taking several high-resolution temperature logs to a depth of more than 5000 m in Gravberg-1 and close to 2000 m in Stenberg-1, and up to a long time after drilling, ensures high-quality deep temperature data. A good-quality thermal conductivity profile was determined from mineralogical composition and radiogenic heat production from a spectral gamma-ray log. The observations show overall thermal conductive profiles with local non-conductive temperature variations. Observed present-day near-surface heat flow (depth range 200–500 m) in Gravberg-1 is 47 mW/m2 (std. dev. 1.5 mW/m2). From observed deep background heat flow, considering heat production, the equivalent steady-state, unperturbed heat flow is estimated at 66–68 mW/m2. Data from Stenberg-1 provide similar results. The observed reduction of heat flow at near-surface level by about 20 mW/m2 is interpreted as originating from long-term palaeoclimatic temperature variations. During the last glacial period, the area was, to varying degree, both ice-free and covered by the Fennoscandian ice sheet. Thermal modelling, with a ground-surface mean-temperature increase by 12±2 °C from glacial to post-glacial time, shows consistency between observed and modelled heat flow perturbations for the full depth range of observations. Heat flow perturbations may be significant to depths of about 2000 m. These results show that, uncorrected for palaeoclimate, depending on depths of measurements, observed continental heat flow values might be markedly underestimated. This applies to Northern Europe and, likely, to many other areas as well.

Structure and Composition of Basement and Sedimentary Cover in the Southwestern Part of the Siljan Ring, Central Sweden: New Data from the C-C-1 Drill Core

The results from the geological and geophysical investigations of the Siljan Ring impact structure (central Sweden) have shown that the Paleozoic sedimentary succession and the Precambrian basement were strongly affected by complex deformational processes. Studies of a new drill core from the C-C-1 well provide valuable additional information necessary for the reconstruction of the geological setting in the southwestern part of the Siljan Ring. It was found that the contact between the basement and the sedimentary cover is tectonic, not normal sedimentary, in origin. The basement interval comprises Precambrian metavolcanic and metasedimentary rocks with a single mafic intrusion (gabbro-dolerite) in the upper part. The rocks have only been partially metamorphosed. The intercalation of calcareous mudstones, skeletal wackstones, and black shales in the sedimentary cover interval is not consistent with the regional lithostratigraphy scheme. Thus, more likely that the sedimentary sequence is not complete as a result of tectonic displacements, and a significant part of the Lower and Middle Ordovician succession is missing. The Post-Proterozoic tectonic reactivation and impact event also caused the formation of four types of fracture. The third type of fracture is accompanied by cataclastic zones and probably have an impact-related nature. In the highly fractured basement rocks, a dissolution along the second type of fracture has resulted in the development of open vugs. Open vugs and microporosity in cataclastic zones have been considered to be an effective storage space for hydrocarbons.

Silurian stratigraphy and graptolite faunas of the Mora 001 and Solberga 1 drill cores, Siljan District, central Sweden

The Mora 001 and Solberga 1 drill cores provide the best available overview on the early Silurian (Llandovery, Rhuddanian to Telychian) graptolite succession available for the Siljan Ring impact structure of central Sweden. The Solberga 1 succession includes a nearly complete graptolite succession from the Pernerograptus revolutus Biozone (late Rhuddanian) to the Oktavites spiralis Biozone (late Telychian). Older graptolite faunas are unknown from the Siljan region. The Mora 001 drill core includes a graptolitic succession from the Monoclimacis crenulata Biozone to the O. spiralis Biozone, found in two lithostratigraphically separated lithological units here identified as the Kallholn Formation. A slice of the Orsa Sandstone Formation of possible later Silurian age is tectonically introduced into the Kallholn Formation in the Mora 001 drill core. The strong tectonic deformation of the Kallholn Formation in both drill cores can easily be understood through the Devonian impact history of the region.

Integrated interpretation of geophysical data of the Paleozoic structure in the northwestern part of the Siljan Ring impact crater, central Sweden

The Siljan Ring impact structure is the largest known impact structure in Europe and is Late Devonian in age. It contains a central uplift that is about 20–30km in diameter and is surrounded by a ring-shaped depression. The Siljan area is one of the few areas in Sweden where the Paleozoic sequence has not been completely eroded, making it an important location for investigation of the geological and tectonic history of Baltica during the Paleozoic. The Paleozoic strata in this area also provide insight into the complex deformation processes associated with the impact. In this study we focus on the northwestern part of the Siljan Ring, close to the town of Orsa, with the main objective of characterizing the subsurface Paleozoic succession and uppermost Precambrian crystalline rocks along a series of seismic reflection profiles, some of which have not previously been published. We combine these seismic data with gravity and magnetic data and seismic traveltime tomography results to produce an integrated interpretation of the subsurface in the area. Our interpretation shows that the Paleozoic sequence in this area is of a relatively constant thickness, with a total thickness typically between 300 and 500m. Faulting appears to be predominantly extensional, which we interpret to have occurred during the modification stage of the impact. Furthermore, based on the geophysical data in this area, we interpret that the impact related deformation to differ in magnitude and style from other parts of the Siljan Ring.

Analysis of borehole geophysical data from the Mora area of the Siljan Ring impact structure, central Sweden

The Siljan impact structure is the largest known impact structure in Europe, the result of a Late Devonian meteorite impact (380.9±4.6Ma). It is outlined mainly by a ring of lakes and Palaeozoic sedimentary rocks of Ordovician to Devonian age. The Palaeozoic successions are generally poorly exposed, but often well preserved with clear stratigraphy. At some locations they are strongly tectonised with sharply inclined or nearly overturned packages of crystalline basement and/or sediments. Down-hole logging data were acquired in the western part of the Siljan impact structure to determine some of the physical properties of the Palaeozoic sedimentary rocks in the area. Boreholes Mora 001 (356m logged depth), Vattumyra Production (420m logged depth), Mora VM 2 (94m logged depth) and Mobillyft (437m logged depth) were logged for temperature, sonic velocity and electrical resistivity. Logging data were compared to the lithology in the Mora 001 core, which had been mapped in detail, and information from other cores in the area. Good agreement between the natural gamma log and the core lithology was found. The sonic log shows a marked difference in velocity for the more clastic Silurian succession compared to the Ordovician succession and the Precambrian basement. A synthetic seismogram shows that a high amplitude reflection is expected at the Silurian–Ordovician boundary, raising some questions concerning interpretation of a seismic profile located about 6–7km north of the study area. Correlation of the borehole logs shows that the thickness of the Silurian succession varies rapidly in the area and that its composition differs over distances of less than 1km. These rapid variations suggest that the study area may be located in a megablock zone that was highly influenced by the impact. Caledonian tectonics and changing depositional environments may also play a role in explaining the present-day borehole lithologies. Even though the boreholes are relatively far from the seismic profile and the geology is complex, the new data confirm that the Silurian has significant thickness along parts of the seismic profile. Potentially, the Silurian can be up to 450m thick on parts of the profile. Further geophysical investigations in the area, including seismic surveying and gravity measurements, may help in mapping the complex structures away from the boreholes and discriminating between possible geological models.

A new interpretation of the sedimentary cover in the western Siljan Ring area, central Sweden, based on seismic data

Two new reflection seismic profiles over the Paleozoic successions of the western part of the Siljan Ring impact structure show a contrasting seismic signature. The more southerly c. 10km long Mora profile reveals a highly disturbed structure, with only a few kilometers of relatively horizontally layered structures observed. However, interpretations of refracted arrivals in the data, that can be correlated to reflections, indicate the Silurian clastic rocks to be about 200m thick in the central part of the profile. Weak reflections from about 600m depth suggest a 400m thick Ordovician limestone sequence to be present. Cores from the area show a mainly shale lithology for the Silurian and only a thin sequence of Ordovician strata, suggesting a rapid thickening of the Ordovician towards the north. On the more northern c. 12km Orsa profile clear reflections from the Paleozoic successions are seen along the entire profile, except on the southernmost few kilometers. Based on interpretations of refracted arrivals, the Silurian succession appears to be considerably thinner here, and possibly absent at some locations. The Ordovician is also interpreted to be thinner in this area, with a maximum thickness of about 200–300m along most of the profile. A deeper reflection from about 2km within the crystalline basement may represent a dolerite sill. The lack of clear basement reflections on the Mora profile can be attributed to near-surface conditions and the acquisition geometry. The seismic data and recent coring in the area suggest the presence of a deeper paleo-basin towards the southwest with significantly more shales being deposited and the Paleozoic successions being severely disturbed. The shallow coring and seismic data will help form the basis for locating future boreholes for deeper drilling to study impact processes and the Paleozoic evolution of central Sweden.

Interpretation of reflections from the central part of the Siljan Ring impact structure based on results from the Stenberg-1 borehole

In 1984, 1985 and 1990, several multichannel seismic reflection profiles were shot over the Siljan Ring, a meteorite impact structure believed to have been formed approximately 360 Ma ago. The bedrock of the area consists mainly of gneisses and granites and the ring itself of Palaeozoic sedimentary rocks lying on top of granites. Dolerite dikes of different age and orientation have been mapped in the area. The existence of dolerite intrusions at depth has been verified through the drilling of two deep boreholes, Gravberg-1 and Stenberg-1. Interpretation of seismic data and borehole data from the Gravberg-1 borehole showed a strong correlation between high-amplitude subhorizontal reflections and dolerite sills. Geophysical and geological well-logging in the Stenberg-1 borehole showed the occurrence of dolerites in the borehole. Profile 4, running E-W across the borehole, has been reprocessed in order to improve the seismic image. Several thick dolerites below 5.7 km in the borehole correlate with high-amplitude reflectors on the seismic section. Both the logging data and the seismic interpretation suggest that these intrusions are subhorizontal and laterally continuous. Above 5.7 km in the borehole, the dolerites are thinner and are in some cases associated with fracture zones. On the seismic section at these depths there is a complex system of weaker dipping reflectors. Some of these dipping reflectors correlate with either fracture zones, thin dolerites or a combination of both.

The Siljan Ring impact structure: possible connections with minor ores in its neighborhood: F. E. Wickman, GFF, 116(3), 1994, pp 145–146

The Siljan Ring impact structure: possible connections with minor ores in its neighborhood: F. E. Wickman, GFF, 116(3), 1994, pp 145–146

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.

Scientific Summary Report of the Deep Gas Drilling Project in the Siljan Ring Impact Structure

Scientific Summary Report of the Deep Gas Drilling Project in the Siljan Ring Impact Structure

The Siljan Ring Impact Structure as a Target for Hydrocarbon Exploration: Implications from the Gravberg 1 Well, Sweden

The Siljan Ring Impact Structure as a Target for Hydrocarbon Exploration: Implications from the Gravberg 1 Well, Sweden

Deep drilling at the Siljan Ring impact structure: oxygen-isotope geochemistry of granite

The Siljan Ring is a 362-Ma-old impact structure formed in 1700-Ma-old I-type granites. A 6.8-km-deep borehole provides a vertical profile through granites and isolated horizontal diabase sills. Fluid-inclusion thermometry, and oxygen-isotope compositions of vein quartz, granite, diabase, impact melt, and pseudotachylite, reveal a complex history of fluid activity in the Siljan Ring, much of which can be related to the meteorite impact. In granites from the deep borehole, δ18O values of matrix quartz increase with depth from near 8.0 at the surface to 9.5‰ at 5760 m depth. In contrast, feldspar δ18O values decrease with depth from near 10 at the surface to 7.1‰ at 5760 m, forming a pattern opposite to the one defined by quartz isotopic compositions. Values of δ18O for surface granites outside the impact structure are distinct from those in near-surface samples from the deep borehole. In the deep borehole, feldspar coloration varies from brick-red at the surface to white at 5760 m, and the abundances of crack-healing calcite and other secondary minerals decrease over the same interval. Superimposed on the overall decrease in alteration intensity with depth are localized fracture zones at 4662, 5415, and 6044 m depth that contain altered granites, and which provided pathways for deep penetration of surface water. The antithetic variation of quartz and feldspar δ18O values, which can be correlated with mineralogical evidence of alteration, provides evidence for interaction between rocks and impact-heated fluids (100–300° C) in the upper 2 km of the pluton. Penetration of water to depths below 2 km was restricted by a general decrease in impact-fracturing with depth, and by a 60-m-thick diabase sill at 1500 m depth that may have been an aquitard. At depths below 4 km in the pluton, where water/rock ratios were low, oxygen isotopic compositions preserve evidence for limited high-temperature (>500° C) exchange between alkali feldspar and fluids. The high-temperature exchange may have been a post-impact event involving impact-heated fluids, or a post-magmatic event.

The Siljan Deep Well: Helium isotope results

The Siljan Ring impact structure in Sweden is the scene of current deep-drilling exploration for methane trapped in a crustal reservoir. The initial hole, drilled to a depth of 6100 m through Precambrian granite, has been augmented by three branching “side-track” holes beginning at −4700 m that have extended the drilled depth to 6957 m. Forty-four gas samples extracted from the circulating drilling fluid have been analyzed for He, Ne, and 3He 4He ratios. Granite chips recovered from the well and eight natural gas seeps from the Siljan Ring and other localities in Sweden have also been analyzed to provide ambient helium comparisons. The 3He 4He ratios in the well gases are mixtures of three components: (1) atmospheric helium dissolved in recirculated drilling fluid, (2) “carrier” He used for well-site gas chromatography that contaminated some of the samples taken for He isotope analysis, and (3) a “crustal” radiogenic component from the granite with a 3He 4He ratio of 0.015 times the atmospheric ratio (1.40 × 10 −6). Although helium concentrations are generally low throughout the hole (up to 4800 ppm), hydrofracturing at the termination of drilling in Side-track Hole 3 yielded gas with 23,100 ppm of helium and the “crustal” isotope ratio for Siljan granite fluids. Isotope ratios in the gas seeps are in agreement with the deep “crustal” value, but the granite itself has lower 3He 4He ratios due to preferential extraction of 3He from Li-bearing micas. No evidence of a mantle helium component is observed in the isotopic data: an upper-limit calculation based on the difference between He isotope ratios in the granite and in the formation waters gives a maximum of 0.15% of MORE helium in the well gas helium.

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.