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Abstract

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.

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... 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). ...
... Fig. 3). Lehnert et al. (2012) indicated the presence of megaslumps, debris flows and turbidites in their lower shale member and Muhamad et al. (2015Muhamad et al. ( , 2017 and Muhamad (2017, p. 16) referred the deformation in the Mora 001 and Solberga 1 drill cores to Caledonian collision tectonics and considered the successions to be 'largely unaffected by faulting and impact-related deformation'. Biostratigraphical information to date the lithostratigraphical column was not provided. ...
... Baarli et al. (2003) indicated a thickness of 50 m for the Kallholn Formation, overlain by the Styggforsen Limestone of uncertain thickness and a Nederberga Formation of more than 75 m (Fig. 2). Lehnert et al. (2012) and Muhamad et al. (2017) suggested over 200 m of Kallholn Formation in the Mora 001 drill core, but this investigation indicates a considerably lower thickness. ...
Article
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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.
... 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 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. ...
Article
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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.
... 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.
... Sweden: • Laisvall ( October 2013): Mineral exploration and geological mapping ( Malehmir et al., 2015b) • Stockholm ( November 2013): Förbifart Stockholm, site characterization and equipment quality control ( Brodic et al., 2015-Paper I) • Kristianstad ( April 2014): Contaminated site mapping ( Brodic et al., 2017b-Paper III) • Varberg ( May 2014): Planning of a double-track train tunnel ( Dehghannejad et al., 2017;Malehmir et al., 2015c) • Bollnäs (October 2014): Post-glacial fault imaging and characterization ( ) • Äspö ( April 2015): Tunnel-surface-tunnel and landstreamer seismics for fracture mapping ( Brodic et al., 2017a-Paper II) • Ludvika ( October 2015): Mineral exploration and geological site mapping ( Malehmir et al., 2017a) • Mora (October 2015): Sub-surface geological mapping ( Muhamad et al., 2017). • Malmberget ( Nov 2015): Mapping hazardous zones due to mining activities ( ) • Marsta ( January 2017): Bedrock and fracture zone imaging using different seismic sources-ongoing work • Varberg ( June 2017): Structural controls in mobilization of contaminants and remediation planning prior to start of the tunneling-ongoing work Norway: • Oslo ( June 2015): Planning of the E18-Oslo tunnel ( Bazin et al., 2016) Finland: • Turku (July 2014): Seismic imaging of esker architecture and water management ( Maries et al., 2017;and Paper IV) • Siilinjärvi ( July 2014): Mineral exploration/mine planning ( Malehmir et al., 2017b) Denmark: • Copenhagen ( May 2015): Chalk group mapping at Stevns peninsula and PhD training course ( Kammann et al., 2016) 1.2 MEMS-based landstreamer and outline of the thesis ...
Thesis
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To support urban infrastructure planning projects, along with various other near-surface applications, a multicomponent landstreamer was developed. The landstreamer was built with broadband (0-800 Hz), three-component (3C) micro-electro-mechanical system (MEMS) sensors. The digital nature of the MEMS sensors makes the developed landstreamer insensitive to electric/electromagnetic noise. The landstreamer’s design and its seismic imaging capabilities, along with the MEMS technical specifications, were evaluated in several studies. When comparing signals recorded with the streamer with planted MEMS sensors, no negative effects of the design were noted. Compared to different geophones tested, the streamer produced higher quality and broader signal bandwidth data. Additionally, a seismic study conducted in a tunnel demonstrated its electric/electromagnetic noise insensitivity. The streamer combined with wireless seismic recorders was used to survey logistically challenging areas for improved imaging and characterizations and avoid interference with traffic. For example, at the Stockholm Bypass site, the landstreamer recorded data were used for traveltime tomography with results showing a well delineated bedrock level and potential low-velocity zones matching with inferred poor-quality-class rocks. The seismic response of fractures and their extent between a tunnel and the surface was studied at the Äspö Hard Rock Laboratory site. The velocity model obtained using the traveltime tomography approach showed known well-characterized fracture systems and potential additional formerly unknown ones. Additionally, compressional- and shear-wave velocities, seismic quality factors, Vp/Vs and dynamic Poisson’s ratios of the known fracture zones were obtained. Fractures and/or weakness zones in the bedrock were imaged using refraction and reflection imaging methods at a site contaminated with a cancerogenic pollutant in southwest Sweden, illustrating the potential of the streamer for environmental-related applications. In southern Finland, the landstreamer was used for SH-wave reflection seismic imaging from a vertically oriented impact source with the results showing a well-delineated bedrock level and weak reflections correlating well with geology. At the same site, its potential for multichannel analysis of surface waves (MASW) was demonstrated. The surface-wave obtained shear-wave velocities match well with the borehole based stratigraphy of the site and are complementary to the SH-wave reflectivity and previous investigations at the site. Studies conducted in this thesis demonstrate the landstreamer’s potential for various near-surface applications and show the benefits and need for 3C seismic data recording.
Chapter
This volume represents the proceedings of the homonymous international conference on all aspects of impact cratering and planetary science, which was held in October 2019 in Brasília, Brazil. This volume contains a sizable suite of contributions dealing with regional impact records (Australia, Sweden), impact craters and impactites, early Archean impacts and geophysical characteristics of impact structures, shock metamorphic investigations, post-impact hydrothermalism, and structural geology and morphometry of impact structures—on Earth and Mars. These contributions are authored by many of the foremost impact cratering researchers. Many contributions report results from state-of-the-art investigations, for example, several that are based on electron backscatter diffraction studies, and deal with new potential chronometers and shock barometers (e.g., apatite). Established impact cratering workers and newcomers to this field will both appreciate this multifaceted, multidisciplinary collection of impact cratering studies.
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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
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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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A Bouguer anomaly map of Raguba oil field and the surrounding areas is presented and interpreted. The main features of the map are: 1) a belt of positive anomalies approaching Bouguer values of 3 mgal in the field area then increasing up 9 mgal toward the northwest. 2) Negative Bouguer values of -20 mgal on the east and west sides and a negative value of -13 mgal on the northeast side of the field. 3) Steep anomaly gradients trending north-south on both sides of the field. To the north, the trend takes a northwest direction. 4) All these anomalies are superimposed on a regional trend of 0.16 mgall/km negative toward the S. The positive belt is interpreted as a horst structure characterized by crystalline basement at shallow depths. The negative anomalies are due to the density contrast between the sediments and the basement. Structural sections along certain profiles are presented and used for constructing gravity models calculated by computer.-Authors.
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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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1468. Acta Universitatis Upsaliensis
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Muhamad, H., 2017: Geophysical Studies in the Western part of the Siljan Ring Impact Crater. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1468. Acta Universitatis Upsaliensis, Uppsala. 69 pp. ISBN 978-91-554-9794-1.