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... A detailed historical overview of this research is given in a report of the 9th WOGOGOB meeting [1]. The results of geophysical studies, particularly a more detailed recent analysis [2][3][4][5][6], provide valuable information about the geological complexity of the area, which includes structural and stratigraphic disturbance. This complexity is a result of the high paleo-tectonic activity in the area and superimposed deformation due to Caledonian orogenic events. ...
... The central area comprises Proterozoic magmatic, metavolcanic, and metasedimentary rocks. The structure belongs to the Trans-Scandinavian Igneous Belt, and magmatic rocks are predominantly Dala granites (Jӓrna and Siljan types) with sporadically occurring mafic intrusions [2,8,13,[19][20][21]. ...
... The ring structure is identified as a ring graben [5,6] divided into mega-blocks by faults with significant horizontal displacements. On seismic reflection, it is clearly visible that basement blocks and sedimentary successions are often sharply inclined or overturned [2][3][4]. Such geological complications are interpreted to be the results of the Caledonian orogeny and the posterior Devonian impact event. ...
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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.
... Fredriksson & Wickman 1963;Svensson 1971;Wickman 1981;Reimold et al. 2005;Lindström et al. 2008). Reimold et al. (2015) dated the impact at 380 Ma (Frasnian, Devonian), but a number of slightly different ages can be found in the literature (Elming & Bylund 1991;Reimold et al. 2005;Jourdan & Reimold 2012;Muhamad et al. 2015). The impact led to considerable tectonic distortion of the Palaeozoic sedimentary cover of the Siljan region and difficulties to understand the sedimentary succession as exposures show only small parts and the connections of individual lithological units often are unknown. ...
... Rondot (1975) and Collini (1988) estimated a thickness of 350-500 m of Palaeozoic sediments through seismic data. Recent seismic investigations Muhamad et al. 2015Muhamad et al. , 2017 suggested the presence of a 400-m thick Ordovician limestone succession followed by ca. 200-m thick Silurian shales in the Mora region of the western part of the Siljan Ring, but equivalent thicknesses in outcrop do not exist and only a 21.57m thick succession of Ordovician limestones was found in the Mora 001 drill core (Lehnert et al. 2012, p. 88). ...
... 200-m thick Silurian shales in the Mora region of the western part of the Siljan Ring, but equivalent thicknesses in outcrop do not exist and only a 21.57m thick succession of Ordovician limestones was found in the Mora 001 drill core (Lehnert et al. 2012, p. 88). Muhamad et al. (2015) indicated that the thickness of the Silurian deposits varies considerably in the Mora area. It is estimated that a Palaeozoic overburden of up to 2500 m has been removed through erosion including also considerable thicknesses of post-impact strata (e.g. ...
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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 bears a graptolitic succession from the Monoclimacis crenulata Biozone to the Oktavites 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.
... Four of the Igrene AB wells in the Mora area were geophysically logged (Muhamad et al. 2015). Sonic velocity, temperature and electrical sondes were used for logging, and the data were circular-shaped depression formed by a Late Devonian meteorite impact (380.9 ± 4.6 Ma, Jourdan et al. 2012). ...
... This is exemplified by about 224 m of Silurian strata overlying a thin Ordovician unit in the Mora 001 core (see Figs. 1 and 2 for location). Analysis of geophysical well data from the Mora area (Muhamad et al. 2015) shows that there are rapidly changing lithologies in this structurally complex area and that the tectonically induced thickness of the Ordovician rocks in the compared to the lithology of the Mora 001 core. The logging data showed large differences in the log response between some of the wells, suggesting a complex structure in this area (Muhamad et al. 2015). ...
... Analysis of geophysical well data from the Mora area (Muhamad et al. 2015) shows that there are rapidly changing lithologies in this structurally complex area and that the tectonically induced thickness of the Ordovician rocks in the compared to the lithology of the Mora 001 core. The logging data showed large differences in the log response between some of the wells, suggesting a complex structure in this area (Muhamad et al. 2015). The Mora 001 sonic log was used to generate a synthetic seismogram which showed that, at least in this area, the boundary between Silurian clastic rocks and Ordovician limestone produces a strong seismic reflection (Muhamad et al. 2015). ...
Article
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.
... The Stumsnäs 1 (Fig. 1) core section shows an intercalation between the Paleozoic rocks and the granitic basement towards the eastern part of the Siljan Ring, interpreted to represent large slabs of crystalline basement overlying the Ordovician succession in this area (Arslan et al., 2013). During the summer of 2013 four boreholes in the Mora area ( Fig. 1) were geophysically logged (Muhamad et al., 2015). Temperature, sonic and electrical logs were acquired down to as deep as 440 m. Results from these logs correlated well with core descriptions, in particular there was a good match between the gamma log and the Mora 001 core lithology (Muhamad et al., 2015). ...
... During the summer of 2013 four boreholes in the Mora area ( Fig. 1) were geophysically logged (Muhamad et al., 2015). Temperature, sonic and electrical logs were acquired down to as deep as 440 m. Results from these logs correlated well with core descriptions, in particular there was a good match between the gamma log and the Mora 001 core lithology (Muhamad et al., 2015). The logs show a large variation between the boreholes. ...
... Rapid changes in the Silurian succession suggest a complex sub-surface structure in the Mora area (Muhamad et al., 2015). Using the velocity log of the Mora 001 borehole a synthetic seismogram was generated which showed that the boundary between the Lower Silurian shale succession and Ordovician limestone succession should produce a strong reflection (Muhamad et al., 2015). ...
Article
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 20–30 km 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 500 m. 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.
... Näslund et al., 2005;Veikkolainen et al., 2017) confirmed that heat flow (and temperature gradients) may vary significantly throughout the crystalline rock of Sweden. Temperature gradients of 30 • C/km have been noted in central Sweden (Muhamad et al., 2015), although gradients between 15 and 20 • C/km are most common. A few exploratory wells for heat extraction, and in some cases circulation tests, have been made in Sweden. ...
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Enhanced geothermal systems (EGS) are a potential heat source in many parts of the world, even in locations where the temperature gradient is relatively low. We present here an integrated study of reflection seismic data, borehole logs and seismicity analysis performed in conjunction with a geothermal exploratory project operated by E.ON in Malmö, Sweden. In 2020, the pre-existing 2.1 km deep FFC-1 borehole through the sedimentary cover was deepened into the crystalline basement to about 3.1 km vertical depth. Combined interpretation of the reflection seismic data and geophysical wireline logs show that most of the reflectivity in the Precambrian basement is likely generated by lenses of mafic amphibolite embedded in a felsic gneissic matrix. The general structural bedding and foliation is gently dipping to sub-horizontal, similar to other locations in southwest Sweden. Fracture frequency is relatively high in the crystalline rock mass, with heavy fracturing in the uppermost part of the crystalline basement, obscuring a clear reflection from the top of the Precambrian. Highly fractured and hydraulically conductive intervals are also found between 2,562 and 2,695 m based on a temperature drop and the interpretation of the geophysical data. Open fractures, both natural and induced, have a clear N–S orientation, contrasting with the expected NW–SE direction based on the orientation of the Sorgenfrei-Tornquist Zone and earthquake fault plane solutions to the north. This difference may be partly explained by local variations in the stress field near the FFC-1 borehole and vairations in the stress field with depth. Despite this, the data from the FFC-1 well provide novel and unique information on the complex physical state of the crystalline basement on the margin of the Fennoscandian Shield, which further addresses the need for obtaining in-situ stress data to fully understand the local stress field prior to any stimulation. A temperature of 84°C measured at 3 km depth indicates that a desired EGS temperature of 120–140°C may be reached at 5–6 km depth, assuming a temperature gradient of about 20°C. If the relatively high fracture frequency and occurrence of fracture zones down to 3.1 km are also present at these target depths, then the FFC-1 location may be suitable for heat extraction if the rock mass is properly characterized before stimulation.
... The detailed historical overview of this research is given in a report of the 9th WOGOGOB meeting [7]. Results of geophysical studies, especially more detailed recent analysis [9,10,11,15,16], provide valuable information about the geological complexity of the area that includes structural and stratigraphy interrelations. That complexity results from the high paleo-tectonic activity of the area and superimposed deformation due to orogenic events. ...
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Results of geological and geophysical investigations of the Siljan Ring impact structure (central Sweden) revealed complicated relationships between Paleozoic sedimentary succession and the Precambrian basement. Tectonic and depositional evolution caused complex geology. Studies of a new drill core from the C-C-1 well provide information necessary for the reconstruction of the geological setting in the southwestern part of Siljan Ring. The whole interval of the core section is from 32.60 to 634.90 m with almost no breaks. The sedimentary cover is 373.55 m thick in total. The sedimentary sequences are predominantly composed of wackestones, mudstones, and shales. In the lower part of the sedimentary section, limestone layers intercalate with black shales. In a result of the investigations, it has been suggested that sedimentary layers represent Late Ordovician and Silurian deposits and have disturbed stratigraphic relations. The basement section is composed of Precambrian meta-volcanic and meta-sedimentary rocks. The contact between the basement and the sedimentary cover is tectonic, not normal sedimentary, in origin. Tectonic processes caused intensive rock fracturing. Four generations of fractures were identified with analysis of fracture relations and mineralization sequence. Only two of them occur in sedimentary rocks that probably belong to the latest stages of tectonic activity. Highly fractured basement rocks in some cases contain open vugs developed along the fractures. Rock matrix is tight either in sedimentary and basement rocks and only micro-porosity space is recognized in cataclastic zones. Single evidence of bituminous filling of micro-porosity zone and partly cemented vug is established in limestone from the lower part of the sedimentary section. These findings are particularly valuable for stratigraphy refinement and tectonic setting reconstructions as well as oil and gas reservoir forecasts.
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Recent discoveries of extant and fossilized communities indicate that eukaryotes, including fungi, inhabit energy-poor and anoxic environments deep within the fractured igneous crust. This subterranean biosphere may constitute the largest fungal habitat on our planet, but knowledge of abyssal fungi and their syntrophic interactions with prokaryotes and their concomitant metabolisms is scarce. Here we report findings of fossilized, chitin-bearing fungal hyphae at ~540 m depth in fractured bedrock of the Siljan impact structure, the largest crater in Europe. Strong ¹³C-enrichment of calcite precipitated with and on the fungi suggests formation following methanogenesis, and that the anaerobic fungi decomposed dispersed organic matter producing for example H2 that may have fueled autotrophic methanogens. An Eocene age determined for the calcite infers the first timing constraint of fossilized fungi in the continental igneous crust. Fungi may be widespread decomposers of organic matter and overlooked providers of H2 to autotrophs in the vast rock-hosted deep biosphere.
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Three-dimensional finite element numerical model calculations have been carried out to investigate the quantitative effect of the eccentric position of a normal resistivity borehole probe used in practice. Detailed calculations were done between the point-wise analytical solution and numerical solution to verify the results obtained from the finite element method for a normal probe with finite-length cylindrical electrodes. In the borehole the pattern of the current flowing out from current electrode A is efficiently influenced by the eccentricity. For high-resistivity rock the current density is decreased, while for low-resistivity rock it is increased toward the wall side. On the other hand, the eccentricity does not affect considerably the apparent resistivity calculated from electrode potentials. In most geological situations the deviation is less than 2%. However, in infrequent cases when the true resistivity of the rock is extremely low and/or the distance between the potential and current electrodes is very small the effect of the eccentricity can exceed even 10%.
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New drill cores from the largest known impact structure in Europe, the relict of the Siljan meteorite crater, provide new possibilities to reconstruct Early Palaeozoic marine environments and ecosystems, and to document changes in sedimentary facies, sea level and palaeoclimate in Baltoscandia. The impact crater is an important target of the project “Concentric Impact Structures in the Palaeozoic” within the framework of the “Swedish Deep Drilling Program”. Two core sections, Mora 001 and Solberga 1, have been analysed. The sedimentary successions of these core sections include strata of late Tremadocian through late Wenlock ages. Our preliminary studies show not only that several of the classical Palaeozoic units of Sweden are represented in the area, but also that other significantly different facies are preserved in the Siljan district. An erosional unconformity representing a substantial hiatus occurs between Middle Ordovician limestone and a Llandovery-Wenlock (Silurian) shale succession in the western part of the Siljan structure and suggests an extended period of uplift and erosion. This may be related to forebulge migration due to flexural loading by the Caledonian thrust sheet to the west. Thus, this part of Sweden, previously regarded as a stable cratonic area, presumably was affected by the Caledonian collision between Baltica and Laurentia.
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The Stumsnäs 1 core, drilled in 2011 in the southern part of the Siljan Ring, provides new insights into the stratigraphy of the Ordovician succession in central Sweden. The core section shows evidence of the structural complications caused by the late Devonian meteorite impact in the area. In the core, about 90 m of Tremadocian to Darriwilian strata are sandwiched between Proterozoic igneous basement rocks. At the lower contact, the sedimentary succession starts with glauconitic, fine-grained siliciclastics and carbonates directly resting on the weathered basement. The basal siliciclastic unit is overlain by a ca. 19-m-thick limestone succession, well-known from this part of the Baltoscandian Basin, includes strata corresponding to the Latorp Limestone through the upper Holen Formation. This succession is covered by a limestone–marl alternating sequence, representing some yet unknown deeper water facies, presumably coeval to the topmost Holen Formation through upper Dalby Limestone. The upper third of the sedimentary succession in the core includes numerous slices of partly overturned strata, including relatively thick successions of the Slandrom Formation, Fjäcka Shale and Jonstorp Formation, as well as carbonate and siliciclastic units of yet unknown stratigraphic origin.
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The Ordovician age Lockne and Devonian Siljan craters are two of the largest impact structures in northern Europe. Both formed in targets with a thick, low-strength upper layer. This target configuration is known to generate concentric crater structures with an outer, shallow crater in the low-strength layer, surrounding a central, deeper crater in the more resistant substrate. The concentric craters of Lockne and Siljan are excellent models for studies of similar concentric craters on Earth and elsewhere in the Solar system. Several structural issues remain, and drilling through the craters within the Swedish Deep Drilling Program intends to address the following: the extent of the craters with respect to the time of impact; the effects of cratering on the basement; and the role of basement structure for the crater formation. A problem for the Lockne crater is the relation to the Caledonian orogeny and the lateral extension of the ejecta blanket - the rim is interrupted by a radial depression that has been interpreted both as primary and secondary, tectonically induced. A second feature to study is the deeper and older (1.82-1.80 Ga) NNW-SSE shear zones that cut the basement. In the Siljan area the development of mega block associations comprising the infilling of the graben is disputed. The blocks may either be formed by sagging of peripheral parts of the fault blocks or alternatively by major radial movement involving kilometre long transport.
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The Stumsnäs 1 core, drilled in 2011 in the southern part of the Siljan Ring, represents a structurally complex section and provides new insights into the impact-induced local structure in central Sweden. The Siljan Ring was formed around the central uplift of a Late Devonian meteorite crater, the largest known impact structure in Europe. The Stumsnäs 1 core section reveals that about 90 m of Palaeozoic (Lower to Upper Ordovician) sedimentary rocks are sandwiched between Proterozoic igneous basement rocks. The sedimentary contact to the underlying 260 m of Proterozoic basement is a prominent unconformity. The contact to the overlying 190-m-thick slab of Proterozoic basement is a few metres wide complex fault zone, comprising alternating thin slices of sedimentary and granitic rocks together with fault breccia and gouge. The tectonic emplacement of basement rocks over the Palaeozoic sedimentary succession is apparently impact-related and caused folding and faulting of the underlying sediments, some of which were overturned and cut out. Minor fault zones occur throughout the Stumsnäs 1 core section and have large damage zones with intense fracture networks along which alteration and mineralisation took place, likely also impact-related. Small-scale faults and fractures are common and are critical for fluid migration and hence for ongoing exploration for natural gas and geothermal energy reservoirs in the Siljan impact structure.
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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 Sollerön 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 average rock lead. Relatively heavy sulfur isotope values in the galena may point to a sulfate source in the Paleozoic sediments for the sulfur. The schalenblende texture of the sphalerite ore suggests repeated pulses of rapid precipitation from supersaturated ore solutions.
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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.
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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.
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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. 10 km 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 200 m thick in the central part of the profile. Weak reflections from about 600 m depth suggest a 400 m 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. 12 km 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 200–300 m along most of the profile. A deeper reflection from about 2 km 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.
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The gravity-driven collapse of complex impact craters induces the mass transfer of large rock volumes. In distal parts of a crater inward movements dominate, whereas in the centre of an impact structure outward movements occur if the central uplift collapses. The particle trajectory field is centro-symmetric, which signifies that the conditions for plane-strain deformation are not fullfilled. Converging particle trajectories can be compensated either by a bulk thickening of inward sliding masses (folding, repetition of rock units along thrust faults, plastic flow) or by the formation of localised radial transpression ridges (RTR) at the edges of individual landslides. In these transpression ridges material is uplifted to accommodate the converging mass flow. Different modes of uplift are possible including radial folding, lateral overthrusting and the formation of positive flower structures. A simple geometric model is used to estimate the amount of transpression thickening and bulk thickening of inward sliding masses on the base of volumetric considerations. The existence of RTRs is confirmed by structural investigations at the Siljan impact structure, Sweden, as well as at other complex impact craters on Earth.
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Isotopic data on the Proterozoic Svecofennian bedrock of south-central Sweden are reviewed. Interpretation of the results by various radiometric methods establishes the ages of the syn-orogenic and late-orogenic igneous events to be 1.89-1.85 Ga and 1.80-1.78 Ga, respectively. The origin of the Svecofennian rocks is discussed on the basis of Sm-Nd data. Genetic models imply that the late-orogenic igneous rocks crystallized from mobilized granitic material formed by partial melting of Svecofennian metasedimentary rocks. These models are not supported by the εNd(T) values. At 1.65 Ga, the present bedrock surface is estimated to have been at a depth of 8 km with a temperature below 280°C. With continued uplift the deformation character of the bedrock changed from ductile to brittle. -Author
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Shatter cones have been found at five localities on the dome of the Siljan structure, northwest of Stockholm. This provides further support for the interpretation that this structure was formed by a meteorite impact.
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The Bathurst Mining Camp, northern New Brunswick, Canada is a major base-metal producing region that includes the Brunswick No. 12 and smaller past-producing Brunswick No. 6 deposit. Sustaining mining activity in the camp requires exploration of orebodies near and adjacent to existing mines. In this paper, we recovered, re-processed, modeled and interpreted a 2D high-resolution reflection seismic profile in the vicinity of the Brunswick No. 6 open pit with the aim of providing key information on the geological structures associated with mineral deposits at depth. The seismic data quality is good to excellent with numerous strong reflections in raw shot gathers, resulting from a careful survey design that included test shots and a priori considerations for the geological environment. Physical properties of lithological units and mineralized zones including gamma–gamma, conductivity, magnetic susceptibility, density and sonic data were studied from borehole geophysical measurements conducted near the mine. This helped to correlate seismic data and shows that the Brunswick Horizon, a key ore-prospecting horizon, and associated lithological contacts are strongly reflective. The results demonstrate that reflection seismic imaging has been particularly effective for imaging steeply dipping structures of the Brunswick No. 6 deposit many of which intersect the surface and thus allow for correlation with surface geology. Massive sulfides and iron formation of the Brunswick Horizon are identifiable locally within a distinctive reflective package that can be used as a broad guide in the region for the exploration of deep base-metal deposits. In several locations, a change from highly reflective to semi-transparent seismic character is explained by the presence of faults that juxtapose highly reflective lithological units with seismically transparent lithologies.
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The Siljan impact structure in Sweden is the largest confirmed impact structure in Western Europe. Despite this, the structure has been poorly studied in the past, and detailed studies of shock metamorphic features in the target lithologies are missing. Here, we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at Siljan. At 21 localities from an area approximately 20 km in diameter located centrally in the structure, the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured. Observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter. The most strongly shocked samples, recording pressures of up to 20 GPa, occur at the very central part of the structure, and locally in these samples, higher pressures causing melting conditions in the affected rocks were reached. Pressures recorded in the studied samples decrease outwards from the center of the structure, forming roughly circular envelopes around the proposed shock center. Based on the distribution pattern of shocked quartz at Siljan, the original transient cavity can be estimated at approximately 32-38 km in diameter. After correcting for erosion, we conclude that the original rim to rim diameter of the Siljan crater was somewhere in the size range 50-90 km.
Article
Reflection seismic measurements were carried out during 1985 in the area of the Siljan impact structure of Sweden. A total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the process that formed them than could be obtained from surface observations or theoretical modeling studies. Standard stacked sections have been interpreted in terms of current impact models. The transient crater diameter is found to be approximately 22 km, considerably smaller than current estimates based on interpretation of surface geology. The structural uplift is estimated to be 3 km at the 5- to 8-km level, and the material flow necessary to support the uplift is mainly horizontal
Article
During 1985 about 80 km of surface multichannel Seismic reflection data were collected across the meteorite impact in the Siljan Ring area in central Sweden. The area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of Palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 Ma, thereby forming a circular ring about 40 km in diameter. Dolerite intrusions ranging in age from 850 to 1700 million years are also present. The Seismic data revealed several high-amplitude, laterally continuous, sub-horizontal reflections in the northern part of the structure. The high-amplitude reflections and a possible intermediate low-velocity zone were contributing factors in choosing the site for the Gravberg-1 Deep Earth Gas test well. Drilling and vertical Seismic profiling (VSP) found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre-impact area extent of at least 800 km2. Studies of amplitude and frequency versus offset (AFVO) show the observations are compatible with a model of simple granite/dolerite/granite layering.
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STRONG evidence for the occurrence of explosion craters caused by the impact of large meteorites on the surface of the Earth through geologic time has been accumulating from several distinct lines of search within the past ten years1. The Dominion Observatory of Canada has now examined sixteen structures of possible impact origin by combinations of aerial photography, geophysical methods, field mapping, drilling and laboratory investigations2. Dence grouped the impact structures into simple craters, characterized by diameters of less than 10 km and by the absence of a central uplift, and complex craters having a diameter of more than 10 km and a central uplift surrounded by an annular depression3.
Article
The velocity-stress formulation for propagation of elastic seismic waves through 2D heterogeneous transversely isotropic media of arbitrary orientation is presented. The equations are recast into a finite-difference scheme and solved numerically using fourth-order spatial operators and a second-order temporal operator on a staggered grid. Absorbing, free-surface and symmetry boundary conditions have been implemented. Test cases compare well with other published solutions. Synthetic seismograms are calculated over two idealized models: (i) vertical fractures in granite with a dolerite sill reflector and (ii) a dipping anisotropic shale. Comparisons with the isotropic counterparts show significant differences which may have to be accounted for in seismic processing in the future.
Chapter
Siljan in southwest Sweden is the largest impact structure in western Europe, with a present topographic diameter of ca. 75 Siljan in southwest Sweden is the largest impact structure in western Europe, with a present topographic diameter of ca. 75 km. Recent age determinations indicate an age of 377 Ma. The bedrock geology of the region has recently been re-mapped by km. Recent age determinations indicate an age of 377 Ma. The bedrock geology of the region has recently been re-mapped by the Geological Survey of Sweden in the scale of 1:50 000. There is now complete coverage with airborne geophysics. New maps the Geological Survey of Sweden in the scale of 1:50 000. There is now complete coverage with airborne geophysics. New maps of the geophysical data have been prepared for this review. In connection with the Deep Gas Project, further geophysical studies of the geophysical data have been prepared for this review. In connection with the Deep Gas Project, further geophysical studies were made and two drill holes were sunk to over 6 km depth in the central uplift of the structure. The Deep Gas Project produced were made and two drill holes were sunk to over 6 km depth in the central uplift of the structure. The Deep Gas Project produced a large number of reports and publications, which are listed in the summary report of Juhlin (1991). Some of the results are a large number of reports and publications, which are listed in the summary report of Juhlin (1991). Some of the results are compiled and shortly summarized here. Digital elevation data are available with 50 m spatial resolution, and a gray tone mappatial resolution, and a gray tone map has been prepared with the regional trend removed. A profile of these data shows that the peak ring of the structure is still has been prepared with the regional trend removed. A profile of these data shows that the peak ring of the structure is still visible in the morphology. visible in the morphology.
Article
The continental crust of the Baltic Shield was formed between 3.5 and 1.5 Ga ago during four periods of orogenic activity. In the Archaean, an extensive area of tonalitic-trondhjemitic crust was generated. Very little is known about the nature of the oldest recognizable, Saamian orogeny ⩾ 3.1-2.9 Ga ago. The Lopian (Rebolian) orogeny 2.9-2.6 Ga ago, however, generated two different types of terrain compatible with plate tectonic concepts. One is a belt of high-grade gneisses formed in a regime of strong mobility, while the other is a region of granitoid intrusions and greenstone belts surrounded by the remnants of a Saamian substratum.In the early Proterozoic, a period of intracratonic sedimentation and volcanism followed. Eventually, a passive continental margin was formed by rifting along the present boudary between the Archaean and Proterozoic Domains. Activation of this margin occurred at the beginning of the Svecofennian orogeny 2.0-1.75 Ga ago. Large-scale formation of new continental crust took place during both the Svecofennian and Gothian (1.75-1.5 Ga ago) orogenies which are characterized by the early-orogenic formation of tonalitic-granodioritic plutonic rocks and calc-alkaline volcanites.The Sveconorwegian-Grenvillian orogeny 1.25-0.9 Ga ago was essentially a period of reworking of previously formed crust.
Article
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.
Article
In earlier studies, the 65-75 km diameter Si1jan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by K-Ar and Ar-Ar chronology at 342-368 Ma, with the commonly quoted age being 362.7 +/- 2.2 Ma (2 sigma, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Si1jan age. Here we report new Ar-Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Si1jan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 +/- 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 +/- 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% Ar-39 release of 377.5 +/- 2.4 Ma (2 sigma). Our conservative estimate for the age of Siljan is 377 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 +/- 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Si1jan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine-grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earth's biostratigraphic record.
Deep Drilling in Crystalline Rock
  • A Boden
  • K G Eriksson
Boden, A., Eriksson, K.G., 1988. Deep Drilling in Crystalline Rock: Vol. 1: The Deep Gas Drilling in the Siljan Impact Structure, Sweden and Astroblemes. Springer Verlag, Berlin, Germany (364 pp.).
Deep Gas, Swedish Premises: The Siljan Ring Project: Independent Expert Evaluation
  • R R Donofrio
  • K H Olsen
  • F W Vlierboom
  • F Witschard
  • G Petersson
Donofrio, R.R., Olsen, K.H., Vlierboom, F.W., Witschard, F., Petersson, G., 1984. Deep Gas, Swedish Premises: The Siljan Ring Project: Independent Expert Evaluation. Vattenfall, Stockholm (63 pp.).
Scientific summary report of the deep gas drilling project in the Siljan ring impact structure
  • C Juhlin
Juhlin, C., 1991. Scientific summary report of the deep gas drilling project in the Siljan ring impact structure. Vattenfall FUD Report no. UG 1991(14).
Ordovician stratig-raphy and sedimentary facies of the Stumsnäs 1 core from the southern Siljan Ring, central Sweden
  • O Lehnert
  • G Meinhold
  • A Arslan
  • J O R Ebbestad
  • M Calner
Lehnert, O., Meinhold, G., Arslan, A., Ebbestad, J.O.R., Calner, M., 2013. Ordovician stratig-raphy and sedimentary facies of the Stumsnäs 1 core from the southern Siljan Ring, central Sweden. Geol. Fören. Stockh. Förh. 135, 204–212.
Introduction to borehole geophysics
  • J H Williams
Williams, J.H., 1998. Introduction to borehole geophysics. United States Geological Survey (http://ny.usgs.gov/projects/bgag/intro.text.htms, 4 pp.).
Dating terrestrial impact structures
  • F Jourdan
  • W U Reimold
  • A Deutsch
Jourdan, F., Reimold, W.U., Deutsch, A., 2012. Dating terrestrial impact structures. Elements 8, 49-53.
Applications of shallow geophysics in a regional geological and hydrogeological investigation
  • S E Pullan
  • A Pugin
  • J A Hunter
  • S D Robinson
  • M A Annecchione
  • G E Leblac
Pullan, S.E., Pugin, A., Hunter, J.A., Robinson, S.D., Annecchione, M.A., Leblac, G.E., 2001. Applications of shallow geophysics in a regional geological and hydrogeological investigation, Oak Ridges Moraine, southern Ontario. Proceedings of a Symposium on the Application of Geophysics to Engineering and Environmental Problems, Annual meeting of the Environmental and Engineering Geophysical Society, Denver, 4-7 March 2001, CO, p. 17 (published on CD-ROM and http://sta.gsrc.nrcan.gc.ca/page1/ envir/orm/orm.htm).