Geophysical Investigations of the Siljan Impact Structure A Short Review

'Department of Land and Water Resources Engineering, Royal Institute of Technology, S-10044 Stockholm, Sweden (herbert kth.se) 2Geological Survey of Sweden, Division of Geophysics, Box 670, S-75128 Uppsala, Sweden (sven.aaro sgu.se) Abstract. 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 the...

Early Descriptions of the Main Crater Morphology

Parts Impact Crater

Johann Wilhelm Ludwig von Luce (1756-1842) is usually acknowledged as the discoverer of the Kaali craters. From the description of the main crater he compiled and published in 1827 (Luce 1827) appears that he had mapped the lake as early as 1780. However, it is obvious that islanders were aware of the existence of the exotic hollow much earlier. The morphology of the crater and uplifted dolomitic rocks gave rise to several legends and tales. It was believed that the lake had no bottom and its...

The Brim

The brim (Fig. 4) is identified as a body of more or less strongly fractured, crystalline rock that includes monomictic breccia (Tandsbyn Breccia). Where it is preserved from erosion, it extends continuously outwards from the rim of the inner crater. At its outer margin the brim rises slightly above the adjacent terrain which consists of either the well-preserved basement rocks of the sub-Cambrian peneplain, or Cambrian and Ordovician sedimentary rocks that rest on the peneplain. In the west...

Mjlnir Crater as a Result of Oblique Impact Asymmetry Evidence Constrains Impact Direction and Angle

Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway (filippos.tsikalas geo.uio.no) Abstract. The 40-km-diameter Mjolnir crater is proposed to have resulted from an oblique impact from the south southwest direction and at a -45 (possibly 30o-45o) angle from the horizontal. This is substantiated by several diagnostic structural and geophysical signatures related to obliquity and revealed through detailed re-assessment of Mjolnir's well-established structure,...

Purpose of This Study

The existence of polygonal craters, i.e., craters that are more or less angular instead of circular or ellipsoidal, has been known at least for about a century (Fielder 1961 Kopal 1966). Given their frequency on the surface of the Moon, they probably have been known as long as detailed observations of the lunar craters have been available. We have noted that polygonal craters are common, or at least present, not only on the Moon, but also on the Earth, Venus, Mercury, Mars and several asteroids...

Ries Crater Germany

So far, only a limited number of detailed studies of the products of impact-related hydrothermal alteration have been conducted. One of the first such investigations involved the mineralogical analysis of the complete impactite sequence of the interior of the Ries Crater, Germany, provided by the extensive analysis of the Nordlingen 1973 drill core (Stoffler et al. 1977). These authors concluded that it was possible that the crystallization of authigenic minerals observed in crater suevite...

Research History

Lockne was known since 1900 for its unique, coarse-clastic facies of Baltoscandian Ordovician rocks which were interpreted as the products of a high-energy coast (Wiman 1900, Thorslund 1940). According to Lindstr m (1971) there was hardly any place for a high-energy coast in the Baltoscandian Ordovician, and Lindstr m et al. (1983) regarded the Ordovician coarse clastics of Lockne as debris flows, to which the name Lockne Breccia was formally applied, and turbidites. A gravity-flow model was...

References

Bottomley D, Grieve RAF, York D, Masaitis V (1997) The age of the Popigai impact event and its relation to events at the Eocene-Oligocene boundary. Nature 335 365-368. Grieve RAF (1987) Terrestrial impact structures. Annual Reviews of Earth and Planetary Science 15 245-270 Kenkmann T (2002) Folding within seconds. Geology 30 231-234 Masaitis VL, Danilin AN, Mashchak MS, Raikhlin AI, Selivanovskaya TV, Shadenkov EM (1980) The geology of astroblemes (in Russian). Nedra Press, Leningrad, Russia,...

Impact Tectonics

The characteristic penetrative lineations observed in the crystalline rocks of the central uplift (Fig. 14) have been interpreted as shatter cone features thought to be formed during the passage of the shock and rarefraction waves. Alternative interpretations are the mass flow during the excavation stage or the collapse flow during the modification stage of the crater. The lineations have a distinct orientation (with the exception of the centrally located multi-striated Proterozoic sandstone...

Geophysical Data

The large geophysical database of the Siljan region contains a wealth of information about the impact structure and can be the basis for further studies of the effect of impact on the physical properties of the involved rocks. The documentation and reports of geoscientific data accumulated by the Deep Gas Project are available at Vattenfall (The Swedish State Power Board) Support AB in Stockholm. In the scientific summary report by Juhlin (1991), 17 sector reports and 41 publications concerning...

Further Magnetic Studies

From the aeromagnetic and rock magnetic data gathered so far, three interesting magnetic features are suggested to be studied further 1. The fragmented NNW-SSE trending dyke swarm within the central uplift dome is a very interesting set of marker structures for the analysis of the mass flow related to the formation of the central rise. With magnetic modeling, the orientation of these dyke fragments can be assessed. This requires, however, more data about their magnetic susceptibility and...

Fault Slip in a Deforming Rock Mass

A large, brittle, mass of rock under compression deforms internally along a large number of variously oriented, discrete slip zones that, together, contribute to the average strain of the mass. Each of these slip zones is a potential site for frictional melting. Evaluation of this potential requires information about the slip zones themselves and both the maximum displacement and velocity of slip across them. Tectonically generated pseudotachylites often form in well-defined orientations with...

Diamonds

Impact diamonds are the result of shock transformation of graphite or coal that have been shocked to shock pressures in excess of 30 GPa (Masaitis 1993 Koeberl et al. 1997b Gilmour 1998). Impact diamonds may appear as paramorphs of precursor crystals, often showing the crystallographic habits of these. Cubic and hexagonal impact diamond and lonsdaleite may occur as individual, nanometer to micrometer crystallites, but they can also occur in the form of large aggregates of up to 10 mm in size....

Late Vendian and Cambrian

Sediments Cambrian

In the very beginning of the mature platform stage of development of the SCD, in Late Vendian and earliest Cambrian, the unconformity surface became tilted east, towards the Moscow sedimentary basin. East of the Baltic-Bothnian line, sand, silt, and clay sediments formed a huge uniform blanket that thickened eastwards (Puura et al. 1987, Fig. 3). At the second half of the Early Cambrian, the western part of the SCD was tilted south-and westwards (towards the Iapetus and Tornquist...

Ordovician and Silurian

During the Ordovician and Silurian Periods, a gently south-westerly and westerly tilted Baltic sedimentary basin developed within the SCD on top of the thin Cambrian platform. Early and Middle Ordovician sedimentation covered completely the Baltic area. Silurian sedimentation continued in the Baltic area, and in south and central Sweden. During the Early and Middle Ordovician, numerous impacts hit the target at sea. A disperse group of craters, located at and west of Baltic Sea, formed at the...

Clastbearing Dikes Formed by Infilling

Another group of dikes forms during the relatively long-term late modification stage of cratering. These dikes caused by doming of the brittle crater fill, its adjustment to slow upward movements and the development of the tension fractures in the crater floor. Open fissures and cracks in the apparent crater floor are later infilled with debris due to gravitation. This debris consists of poorly cemented clast-bearing sandy and clayey material forming the fillings of such openings, and was...

The Setting of Pseudotachylitic Breccia in Impact Craters

Veins of dark-matrix breccia resembling tectonic pseudotachylite have been described from many impact structures - generally from crater floor rocks, but occasionally from crater rim settings. Such reports are very rare for small, simple bowl-shape craters, and it must be questioned whether any bona fide pseudotachylite (friction melt) has ever been described from these. For example, the only breccia ever recovered from the crater rim of the Tswaing (Pretoria Saltpan) meteorite crater that...

Petrological and Geochemical Features

Petrological and geochemical features of the Siljan impact structure were investigated by Aldahan (1990, 1991) and Ramseyer et al. (1992). The igneous rocks used for these investigations include samples from outcrops, several shallow (down to 600 m depth) and the two deep bore holes Gravberg-1 and Fig. 13. Maps of observed alterations (left) of mineral cathodoluminescence. Drill hole locations are marked with diamonds. Stars denote outcrop samples. Profile across the Siljan central uplift dome...

Impact Direction and Angle

The Mjolnir crater lies on the Bjarmeland Platform in the central Barents Sea, which represents a stable sedimentary platform since Late Paleozoic times without evidence for considerable tectonism during its evolution (Faleide et al. 1993 Gudlaugsson et al. 1998). At the time of impact, 142 Ma (Jurassic-Cretaceous transition), the region contained upper Paleozoic strata, mainly carbonates and evaporites, overlain by 4-5 km thick Mesozoic siliciclastic marine sediments (Worsley et al. 1988...

Geology

The rocks of the vike Bay region have been mapped and described by Lundquist et al. (1990). Gneisses of meta-sedimentary rocks predominate, with ages older than 1.9 Ga. The gneiss is intruded by small local massifs of granite of 1.8 Ga age (Claesson 1987). Gneisses are predominant over the local granite bodies. These Svecofennian rocks are intruded by a large north-south striking and eastwards dipping Mesoproterozoic dolerite sill of Fig. 3. Interpretation of fracture patterns based on the...

Friction Melting in Shear Zones

The amount of friction melt around an impact crater can be computed, now that the displacement and slip velocity on internal shear surfaces are known as functions of the gross deformation of the rock mass. Instead of estimating the shear zone width from thermal diffusion, as in the physicists' model described above, I will take the width of the shear zone r to be some fraction of the grain size of the rock itself. I thus suppose that r is of the order of millimeters to a centimeter. This...

The Schematic Geology of the Greater Hellas Region

Our study area (latitude 2 N-66 S, longitude 8 W-208 W) covers a large portion of the southern hemisphere of Mars (see Fig. 1 and 2). The region is predominated by the Hellas impact basin some 2000 km wide and 9 km deep, possibly caused by an oblique impact with a trajectory from northwest (Leonard and Tanaka 1993, 2001 Tanaka and Leonard 1995). Another major impact basin, Isidis, is immediately to the north of our study area. The region around Hellas impact basin is a geologically diverse...

Methods

The two main methods are in situ electrical resistivity measurements with VLF-R technique along with fracture frequency measurements on outcrops. Electric Resistivity Measurements with the VLF-R Technique Electric resistivity is the inverse of conductivity, and the unit used is the ohmmeter (Qm). The aggregate rock resistivity is, in a first approximation, the weighted sum of the resistivities of the rock components. One of the components is water (the electrolyte) residing in the pores of...

Introduction

Shand (1916) first introduced the term pseudotachylite (variant spelling pseudotachylyte) to describe an enigmatic dark, flinty rock that apparently intruded granites near the Vaal River in what is now known as the Vredefort impact structure. Shand's early chemical analyses showed that the dark intrusive veins and pockets are compositionally similar to the enclosing granite, but he was unable to give a convincing explanation of their origin. After Shand's discovery, nearly a century of...

Frictional Heating of Rocks

The observation that friction can raise rocks to incandescence is an ancient one People have been starting fires with flint and steel for centuries. However, it took the science of the Industrial Age to properly relate mechanical work, heat and velocity. Amontons and Coulomb first formulated the law of solid friction (Palmer 1949 Scholz 1990). They discovered that the resistance to sliding of one block over another is proportional to the normal load and independent of the area of contact. In...

Fracturing

The intensity of fracturing has been addressed in the interpretation of MT, airborne VLF measurements, and drill hole loggings and direct observations on outcrops. In all these data more intense fracturing was found within the crystalline central uplift. In the case of MT data, a decreased electric resistivity was interpreted as increased fracturing. In the case of airborne VLF data, the pattern of steep conductive zones (fractures) is distinctly different in the crystalline core compared to...

Discussion and Conclusions

In many of the rose diagrams the dominance of hexagonal craters is manifested by peak intervals of about 60 (see, e.g., Fig. 8b and 9b). Most polygonal craters in greater Hellas region have a tendency towards hexagonal outline, whereas clearly square-shaped craters are more or less a rarity, even among small polygonal craters. Large square-shaped craters are absent. Perfect pentagons are no anomaly, although they are not nearly as common sight as perfect hexagons. Typical are also craters where...

Brittle Deformation of a Large Rock Mass

Both numerical models and many geologists' depictions of the movement of the rocks beneath a large impact crater often idealize the motion as if it were the flow of a homogeneous fluid. Nothing could be further from the truth, as both modelers and geologists are usually well aware. Cold, rapidly deformed rock masses respond to applied forces in a brittle manner. That is, deformation is not spread uniformly throughout the rock mass but is, instead, concentrated in narrow zones that separate...

Impacts into the Mesoproterozoic and Neoproterozoic Local Sedimentary Areas

Meso- to Neoproterozoic sedimentary basins have survived in downlifted positions within grabens (Satakunta, Gavle, Muhos, Aland Sea), halfgrabens (Bothnian Bay, Lake Ladoga district, Landsort Trench district) or gentle syncline-type depressions (central and northern part of the Gulf of Bothnia) (Amantov et al. 1996, Puura et al. 1996, Puura and Floden 1997). None of craters is located within the survived Meso- to Neoproterozoic sedimentary basins. However, several craters are located near to...

BP Structure

Upon approach in the field, the structure shows - in comparison to the mesa and butte terrane of the region - limited, but clearly discernible, topography (Fig. 8). The inner ring is a central block of ca. 500 m width, with steeply upturned rock units that are intensely folded, faulted, and - in places - brecciated (Fig. 9). A variety of microdeformation has been noted in quartz from these breccias and from other sandstone samples, including a few bona fide planar deformation features (work in...

BP Impact Structure

The BP structure is located in the Gebel Dalma area of Southern Cyrenaica, 165 km northeast of Kufra Oasis, and is centered at 25 19' N and 24 20' E. It occurs in a broad northeast-trending syncline of Nubian Sandstone. Flanking the syncline are hills composed of Upper Paleozoic shelf clastics, in which neither volcanics nor evaporites have been observed. French et al. (1974) and Underwood and Fisk (1980) described BP as consisting of two discontinuous rings that surrounded a central bloc the...

Resurge Gullies

The brim is crosscut by radial channels (Fig. 2 and 4), at the bottom of which there are Locknee Breccia and loftarstone, deposited by the resurge from the collapsing water crater. The Nyckelberg gully to the north is neither deep nor particularly extensive. The Tandsbyn gully to the west is the best preserved and exposed. The Bergb le-Loke gully to the south is large, but its western margin is distorted tectonically. Its eastern margin is missing owing to erosion in late geologic time and,...

Svecofennian Crustal Domain as a Target Crystalline Basement and Sedimentary Cover

Formation Svecofennian

The Svecofennian Crustal Domain is divided into 2 sub-regions (see Fig. 1) (a) SCD within the Fennoscandian Shield and (b) SCD within the Russian Platform in the Baltic States and adjacent areas of Belarus and north-western Russia. The Baltic Sea together with the Gulf of Finland determines the NE-SW-trending boundary between the two sub-regions. Restricted areas with remnants of Meso- to Neoproterozoic (Gulf of Bothnia seabottom with grabens extending into the mainland, Lake Ladoga district)...

Morphology and Crater Diameter

The Siljan structure, centered at 14.9 E and 61.1 N (Fig. 1), is one of the larger impact structures on earth. It is clearly visible in the morphology, as demonstrated by digital elevation data (Fig. 4). An interpretation of these data with respect to tangentially oriented steps in the terrain is given in Fig. 5. The structure has been subjected to post-impact differential erosion due to tectonic activity of regional crustal blocks and tilting of the entire region. Especially the block to the...

Formation of the Crater

Isohypse Crater

According to Reinwald (1933), the meteorite had pierced the dolomite massif by 2.5 m. The mighty impact was accompanied by heat which turned the water in the rocks instantaneously into steam and heated the gases, which had formed, so far that they caused burning of the rock not only at the level of explosion, but also at the depths they reached through a dense net of impact-generated cracks. At the same time, these huge quantities of instantaneously formed gases caused an acute explosion which...

Large Masses of Crystalline Ejecta

Some of the most spectacular outcrops of Tandsbyn Breccia occur outside the brim at distances of 6-9 km from the centre of the crater. The Tandsbyn Breccia has resisted erosion better than the surrounding lithologies and commonly forms hillocks that are several meters high. Thorslund (1940) thought that these local rises of monomictic crystalline breccia are rooted in the basement vertically beneath them. He developed a widely accepted model of a local granitic archipelago in the Ordovician sea...

Shallow and Deep Drillings

The Deep Gas Project started off with a series of seven shallow core drillings to characterize the uppermost part of the impact structure (down to ca. 800 m). Their locations are indicated in Fig. 2. Five drill holes were located on the central rise and two in the eastern sedimentary ring basin. From these investigations information was obtained of the porosity of the involved rocks and their physical properties (density, magnetic susceptibility and remanent magnetization). From rock magnetic...

Considerations and Evidence Regarding the Size of the Vredefort Structure and its Spatial Relation to the Witwatersrand

Transvaal Basin

Originally, Vredefort researchers focused purely on the Vredefort Dome. Most early size estimates for the Vredefort Structure ranged from 70 (only the inner parts of the Vredefort Dome) to 140 (Dome plus surrounding Potchefstroom Synclinorium) kilometers (e.g., Bishopp 1941 Dietz 1961 Simpson 1978 Grieve and Pesonen 1992). Grieve and Masaitis (1994), however, estimated - on the basis of a map with concentric structural features published by McCarthy et al. (1990) - that the original diameter of...

Avike Bay a km Diameter Possible Impact Structure at the Bothnian Sea Coast of Central Sweden

Herbert Henkel1, Vaino Puura2, Tom Floden3, Juho Kirs2, Mare Konsa4, Ulla Preeden2, Robert Lilljequist5 and Joanne Fernlund1 1Royal Institute of Technology, Department of Land and Water Resources Engineering, S-10044 Stockholm, Sweden (herbert kth.se) 2Institute of Geology, University of Tartu, Vanemuise 46, EE-51014 Tartu, Estonia (puura ut.ee) Department of Geology and Geochemistry, Stockholm University, S-10691 Stockholm, Sweden (tom geo.su.se) 4Institute of Geology, Tallinn Technical...

Devonian Permian

Out of four Late Phanerozoic structures, only the Devonian to Early Carboniferous Siljan structure survived in the shield area. It is the largest in the SCD and most deeply eroded (PL 7) suggesting removal of thick sedimentary cover. Strong regional erosion in late Carboniferous-Permian destroyed its superstructure and infrastructure except deep roots. Therefore, Cambrian and Ordovician deposits are survived only in the down-faulted outer moat of the Siljan impact ring structure, presently...

Post Impact Tectonic Structure of the Popigai Crater

During its 35 Myr-long history, the Popigai crater has been slightly modified by denudation and deposition. The maximum depth of erosion is estimated to vary from about 100 m in the inner depression to 250-500 m at the southwestern rim of the crater. According to Plotnikova (1990), four periods of the late modification can be distinguished (1) Miocene - Early Pliocene denuda- Fig. 7. Aerial photograph of a suevite field, showing well-exposed linear drainage pattern caused by faulting, in the...

Stresses and Strains in an Impact Event

The details of how stresses and strains vary as an impact crater opens and then collapses are highly complex functions of many variables, including details of the flow field, time after impact, rock structure and properties (Collins et al. 2004). Nevertheless, it is possible to give a general overview of the general magnitudes of stress and strain through this process. Impact crater formation is broadly divided into three major stages Contact and compression, excavation, and collapse (Melosh...

Conclusion

Long-distance downward and upward displacements of rock fragments induced by impact cratering may be distinguished using their specific lithology compared with source layers of target rocks in their original position. These fragments are used as markers of transportation of fragmented material, and are called wandering clasts. 1. Wandering clasts are found in dikes of monomict and polymict lithic breccias and impact melt rocks, that penetrate crater floors, crater fills and in the surroundings...

Age of the Avike Bay Structure

Based on the appearance of the magnetic anomaly associated with one of the sills of Asby-type dolerite (occurring at the shore to the south and to the north of the bay and continuing displaced into the bay) the Avike Bay structure seems to be younger than this dolerite, i.e., < 1.2 Ga. The occurrence of brecciated dykes of Alno type (Lundquist et al. 1990) may place the age younger than the 600 to 210 Ma interval of ages for the Alno magmatism. Dykes of Alno type contain both angular and...

References cited

Aaloe A (1958) Kaalij rve meteoriidikraatri nr. 5 uurimised 1955. aastal Studies of Kaali meteorite crater no.5 in 1955 . ENSV TA Geoloogia Instituudi Uurimused 2 105-117 (in Estonian, with English and Russian summaries) Aaloe A (1981) Erinevused Kaali kraatrite vanuse m rangutes Discrepancies in dating the Kaali meteorite craters . Eesti Loodus 4 236-237 (in Estonian, with English and Russian summaries) Aaloe A, Eelsalu H, Liiva A, Lougas V (1975) Voimalusi Kaali kraatrite vanuse t...

List of contributors

Geological Survey of Sweden Division of Geophysics Box 670 S-751 28 Uppsala, Sweden (sven.aaro sgu.se) Department of Land and Water Resources Engineering Royal Institute of Technology S-100 44 Stockholm, Sweden (annb kth.se) Department of Geology and Geochemistry Stockholm University S-10691 Stockholm, Sweden (ilka geo.su.se) Raymond A. Donelick Apatite to Zircon Inc. 1075 Matson Road Viola, ID 83872-9709, USA Laboratory of Palaeobotany and Palynology Utrecht University Budapestlaan 4...

Petrography

Under the optical microscope cones can be seen to originate from layers of dark ferruginous material including unidentified shell fragments and thin layers of detrital quartz (Figs. 10 and 11). In most studied fragments, cones show a uniform growth direction, but in a few layers several millimeter-sized cones have grown in opposite directions from two different substrate laminae (Fig. 11). The conical layering is marked by argillaceous and carbonaceous material including minute, disseminated...

Clastbearing Dikes Formed by Injection

Lithic Hole Driller

The most widespread group of injected dikes forms at the excavation stage during the transient cavity growth and radial tension of its floor. In the true crater floor two varieties of such dikes may be distinguished a DI -impactite dikes impact melt matrix , and, 2 DM - mylolisthenite from Greek myle - mill, and olistainein - to slide, Rondot 1994 and polymict lithic breccia dikes matrix composed of clastic material, which is fluidal in the case of mylolisthenite , but some transitional types...

Problematics

On the basis of what we now know about pseudotachylitic breccias in impact structures, in comparison to tectonic pseudotachylite, a number of important unresolved problematics, A - H, can be listed A. The need to distinguish between bona fide pseudotachylite friction melt and other breccia types in general, and in impact structures in particular B. How to distinguish in impact structures between bona fide pseudotachylite friction melt and injections of impact melt breccia into the crater floor...

The Lockne Crater Revision and Reassessment of Structure and Impact Stratigraphy

Maurits Lindstrom1, Jens Ormo2, Erik Sturkell3, and Ilka von Dalwigk4 department of Geology and Geochemistry, Stockholm University, S-10691 Stockholm, Sweden Maurits.Lindstrom geo.su.se 2Centro de Astrobiolog a CSIC INTA , Ctra de Torrej n y Ajalvir, km 4, E-28850 Torrej n de Ardoz, Madrid, Spain ormo inta.es 3The Icelandic Meteorological Office, Bustadarvegur 9, IS-150 Reykjavik, Iceland erik vedur.is Department of Geology and Geochemistry, Stockholm University, S-10691 Stockholm, Sweden ilka...

Iron Ore at Ternovka Ukraine

The Krivoi Rog region of Ukraine is world-famous for its gigantic iron ore deposits. A 10-11 km diameter or gt 8 km, according to Krochuk and Sharpton 2002 , perhaps originally 15-18 km large, now deeply eroded impact structure, known as Ternovka or Terny Nikolskiy 1991 Nikolskiy et al. 1981, 1982 Val'ter 1988 , is located in the Proterozoic fold belt of the Krivoi Rog basin, at 49o01'N 33o05'E. Ternovka was formed about 375 25 Ma ago. Both iron and uranium ores have been mined here from...

Background and General Characteristics

Multiply Striated Joint Sets Msjs

The conical fracturing phenomenon known as shatter cones Figs 16 and 17, 20 and 21 is widely considered shock impact diagnostic. Since the early pioneering work by Dietz 1947, 1959, 1961, 1963, 1968 , Hargraves 1961 , Manton 1962, 1965 , Milton 1977 , Milton et al. 1972, also 1996 , and Roddy and Davis 1977 , shatter cones have been described from many impact structures and have been, in many cases, the first indication for the presence of an impact structure. French 1998 defined shatter cones...

Microscopic Features Related to Shatter Cones

A comprehensive review of petrographic observations on shatter cones was presented by Nicolaysen and Reimold 1999 . Lilly 1981 concluded that shock effects due to shatter coning are only present to a depth of one or two grain diameters away from the shatter cone surface. Carter 1965 investigated planar microdeformation features in shatter cone specimens, work that resulted in one of the first detailed studies of PDFs. A related recent study is that by Hargraves and White 1996 on shatter cones...

Fracture Deformations of Impact Rock Bodies

Apart from the block faulting of the Popigai crater, numerous smaller faults and fractures, which cut separate bodies of impact rocks, were discovered during large-scale mapping of selected areas within the crater these fractures are well-exposed on aerial photographs of areas composed of different impact lithologies, for example, allogenic megabreccia or suevites Fig. 5, 6 . The photogeologic guides of fractures and faults are lineaments of drainage pattern Fig. 7 zones of abundant vegetation...

General Geology of BP and Oasis Impact Structures

Both features are located in the Al Kufrah basin of southeastern Libya. At the present erosion level, the rocks that crop out at both structures are sandstones more or less ferruginous with minor conglomerates and siltstones. These rocks are strata of probably Early Cretaceous age Goudarzi 1970 Tawadros 2001 Hallett 2002 , which is generally characterized as a ferruginous, fine- to medium-grained sandstone containing abundant cross-beds, mud cracks, ripple marks and silicified wood. There has...

Paleoenvironmental Significance of Unstructured TypeIII Kerogens

The lack of major chemical differences between the S0 size fractions may be partly a result of the depositional history of the organic matter. Usually, kerogens, which appear unstructured under the petrographic microscope, are assigned as type-II kerogens and are attributed to an algal or marine origin. Type III kerogens commonly exhibit a high level of structural integrity. However, the S0 kerogen appears to have originated from a terrestrial accumulation of decomposing land plant debris,...

Hydrothermal Processes in Impact Structures

Impacts on land will involve volatiles and water of the lithosphere, and those in oceanic environments will directly affect water bodies as well as rock-bound volatiles. Even impact into arid environments will potentially result in release of volatiles through shock dissociation of hydrous minerals of the target. Interaction of aqueous solutions and other volatile components with hot, shocked rock volumes will result in hydrothermal activity, leading to effective alteration processes and...

Hydrocarbons

A 5985 m deep borehole, Gravberg 1, was sunk in 1986-1987 into the 65 to 75 km diameter Siljan impact structure in Sweden. The project endeavoured to investigate the possibility that commercial volumes of mantle-derived abiogenic methane gas could be trapped in the fractured and brecciated lower parts of the impact structure and could provide a sustainable source of energy for Sweden Casta o 1993 . This country derives much of its energy supply from hydrotechnological facilities. It was hoped...

Background

The Lockne Area, Jamtland, Sweden, provides unique opportunities to study the interior structure of a relatively large impact crater. Relatively recent erosion has exposed many outcrops that can be studied geologically and geophysically. The purpose of this investigation is to investigate the correlation between electric resistivity and fracture frequency of crystalline rocks using the electro-magnetic, Very Low Frequency-Resistivity VLF- R , method. I also present the result of a modified...

Ejecta Distribution

Geological field data show that, apart from the overturned flap of crystalline ejecta, which occurs within roughly 2.5 km from the boundary of the inner crater, fragments of granite ejecta with sizes from 0.1 to 10 m lie at distances r of up to 10 to 15 km from the crater center. This seems to be the norm for land impacts, where ejecta are continuously distributed over a large area around the crater with a density roughly decreasing with distance as r McGetchin et al. 1973 . However, this is...

Geological Setting

The 80-90-km-wide Vredefort Dome Gibson and Reimold 2001a,b is located in Archean and Paleoproterozoic rocks of the Kaapvaal craton of southern Africa Fig. 1 . The Dome comprises a central core of about 4550 km diameter that is composed of poly-deformed, pre-3.1 Ga, Archean granitoid gneisses and granites, with fragments of upper amphibolite to granulite facies, mafic and felsic ortho- and paragneisses. The core is surrounded by a circa 20-km-wide collar of greenschist to amphibolite facies,...

Illinois

Map showing fission-track sample locations, location of the Newton County stone quarry in Kentland, Indiana, and sampled counties in Illinois LS-La Salle and Indiana C-Clark, Cl-Clay, F-Franklin, J-Jackson, L-Lake, N-Newton, and W-White . All samples collected are shown only those listed in Tables 1-5 yielded enough apatite grains gt 5 to be analyzed and used in this study. Fig. 1. Map showing fission-track sample locations, location of the Newton County stone quarry in Kentland,...

Economic Mineral Deposits in Impact Structures A Review

Wolf Uwe Reimold1, Christian Koeberl2, Roger L. Gibson1, and Burkhard O. Dressler1,3 1Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa reimoldw geosciences.wits.ac.za gibsonr geosciences.wits.ac.za Department of Geological Sciences, University of Vienna, Althanstrasse. 14, A-1090 Vienna, Austria christian.koeberl univie.ac.at 3185 Romfield Circuit, Thornhill, Ontario, Canada, L3T 3H7 burkhard...

Oblique Impact

To investigate the influence of the impact angle we have performed 3D numerical simulations of an oblique 45o impact of a 300-m-radius granite projectile into a target consisting of crystalline basement overlain by a water layer that is 800 m deep. We used the same numerical model based on a 3D version of the SOVA hydrocode Shuvalov 1999 . The spatial resolution was lower than in the 2D simulations. The initial cell size in the central high-resolution region 100x50x80 cells was 0.025 km in both...

Epigenetic Mineralization Related to the Sudbury Structure

Ames and Gibson 1995 and Ames et al. 1997, 1998 described extensive, regional hydrothermal alteration that pervasively affected the breccia accumulations in the Sudbury basin of the Sudbury impact structure, in particular the Onaping and lower Onwatin formations. They reported evidence for a regional sub-seafloor hydrothermal system that included vertically stacked, basin-wide, semi-conformable alteration zones, in which the rocks had undergone from base to top, silicification, albitization,...

Other Deposits in and Benefits from Impact Structures

Dellen Impact Structure

Besides metallic and mineral deposits, as discussed in detail in the above, impact structures may have much potential for other commercial or economic interests. Building materials have - for centuries - been obtained from impact structures, as, for example, witnessed by the century-old churches in Nordlingen in the Ries Crater of southern Germany and in Rochechouart in the meteorite crater of same name in the Haut-Limousin province of southwestern France. Like the medieval castle at...

Rock Physical Properties

Physical Properties Gniss

In-situ measurements of magnetic susceptibility and electric resistivity Fig. 7, for location see Fig. 2 were made on outcrops of gneiss and dolerite. The gneisses have low susceptibility of ca 1.8-104 SI and the dolerite has gt 100 times higher susceptibility, 3-102 SI Fig. 7a . Rock samples were measured with respect to density, magnetic susceptibility, and remanent magnetization. The gneisses and granites are generally low-magnetic, in contrast to the high-magnetic dolerite, whose magnetic...

Marquez Dome Crater

Currently some 170 impact structures are known on Earth - presumably representing a mere fraction of the entire terrestrial cratering record for a meteorite impact structure list e.g. Impact database Other solid bodies of the Solar System display surfaces that have been thoroughly cratered, but have barely been accessible for detailed impact geological study. Only the Moon and Mars have been - and will in future be - targets of direct geological study, besides probing of large, impact-cratered...

General Geology of the Popigai Crater

The 100-km-diameter Popigai impact structure Masaitis et al. 1980, 1998, 1999, and references therein formed 35.7 Ma ago Bottomley et al. 1997 at the northeast edge of the Anabar Shield, Northern Siberia. The crater was excavated within Archean crystalline rocks with overlying Proterozoic to Permian sedimentary cover with a thickness of up to 1.5 km. The crater is filled by various kinds of lithic breccias and impact melt rocks - both fragmental suevites and massive tagamites Fig. 1 . The total...

Post Impact Ore Mineralization in the Vredefort Witwatersrand Structure

At Vredefort, a curiously complicated ore geological situation prevails. As explained earlier, the Archean Witwatersrand Supergroup strata of exceptional gold and uranium concentration were impacted by the Vredefort bolide some 700-900 Ma after their deposition. The current erosion level has been estimated for geological and metamorphic reasons McCarthy et al. 1986 Gibson et al. 1998 Gibson and Reimold 2000 at ca. 7-10 km. Consequently, only a deep cross-section through the central uplift is...

Conein Cone Petrography

Most cone-in-cone structures are composed of fibrous calcite and films of argillaceous material. The cone structure consists of nested and interfering cone-shaped, plumose aggregates of calcite fibers. The individual fibers are generally less than 6 m wide but may be up to 3 cm long plumose aggregates of fibers may attain 7 cm in length. The long axes of the plumose aggregates are oriented parallel to the long axes of the cones. Individual cones and cone segments are separated by argillaceous...

Newton County Stone Quarry Kentland Geology

Gutschick's 1961, 1972, 1983, 1987 detailed, high-quality maps and published descriptions show that bedding and the major faults in the Kentland quarry generally dip steeply to the north and west of the apex of the Kentland dome Fig. 3 . The major fault in the quarry, the Kentland Quarry Fault, is a folded, bedding-parallel, reverse fault between the St. Peter Sandstone and overlying Platteville Group Fig. 3 and below . The Kentland Quarry Fault and surrounding strata are folded into a steeply...

Mjolnir Impact Obliquity Constrains Models for Near Field Perturbations

Numerical simulations and experimental analogues have shown that obliquity is accompanied by less energy transfer from the projectile to the target e.g., Gault and Wedekind 1978 Hayhurst et al. 1995 Schultz 1996 Burchell and Mackay 1998 Ivanov and Artemieva 2002 . The Mjolnir energy release estimates of Tsikalas et al. 1998a were made considering an elevation impact angle of 45 based, at that time, on well-known probability arguments Shoemaker 1962 Shoemaker et al. 1990 . The energy release was...

Experimental Shock Compression of Rocks

We start with a brief summary of the laboratory studies, which motivated the present numerical work. In these experiments Kenkmann et al. 2000 sized-sized two-component cylindrical rock samples were enclosed in an iron container and impacted by an iron flyer plate at 2540 m s. The experimental setup is shown schematically in Fig. 1. The individual samples consisted of two lithologies, namely quartzite and dunite, which have been chosen because they show a strong contrast in their behavior under...

The Sense of Pseudotachylite

Modern textbook definitions of pseudotachylite are still basically descriptive but do sometimes recognize that such breccia appears to occur in two different geological settings - tectonic fault and shear zones on the one hand, and impact structures on the other. A typical example is found in Bates and Jackson 1987 , who define pseudotachylite as a dense rock produced in the compression and shear associated with intense fault movements, involving extreme mylonitization and or partial melting....

Offsets in Brecciation and Structural Uplift

Impact craters on sedimentary targets have the advantage, in comparison with similar structures on crystalline targets, that the regular, pre-impact stratification of these targets provides reference horizons against which the impact-induced structures can be identified and mapped by seismic reflection studies. Such studies have provided an effective means of mapping the large-scale geometrical structure at depth with a high-degree of horizontal and vertical resolution e.g., Morgan and Warner...

Relation to Archean Witwatersrand Gold and Uranium Ore Deposits

Witwatersrand Golden Arc

The Vredefort impact structure encompasses the bulk of the Witwatersrand Basin Fig. 3 . The synclinal structure of the supracrustal sequence from the Witwatersrand to the Transvaal Supergroup in the environs of the Vredefort Dome represents effective downwarping of the stratigraphic units including those of the Witwatersrand Supergroup McCarthy et al. 1990 . This entire succession containing the gold- and uranium-rich conglomerate horizons known locally as reefs of the Witwatersrand Supergroup...

Origin of Conein Cone Structures

The origin of cone-in-cone structures has been strongly debated, but many aspects of their formation have remained unclear. A common definition Bates and Jackson 1987 states The structure appears to be due to pressure aided by crystallization and weathering solution along intersecting conical shear zones. The important hypotheses for the formation of calcareous cone-in-cone structures can be divided into two groups those favoring early displacive formation of concretions in soft sediment and...

BP and Oasis Impact Structures Libya Remote Sensing and Field Studies

Christian Koeberl1, Wolf Uwe Reimold2, and Jeff Plescia3 'Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria christian.koeberl univie.ac.at 2Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Johannesburg 2050, South Africa 3MP3-E163, Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA jeffrey .plescia jhuapl.edu Abstract. Remote sensing images and recent...

Shallow Excavation

Shallow Excavation

To study the effect of shallow excavation we performed numerical simulations of an impact against a three-layered target. We used the same numerical model as in Ormo et al. 2002 , but increased the spatial resolution of the numerical grid 80 cells across water column and explicitly included a sedimentary layer 10 cells thick . Based on the results of previous simulations we considered the impact of a 300-m-radius stony asteroid into 800-m-deep water at a velocity of 20 km s. The crystalline...

Ynntjrnen Breccia

Since the publication of the paper of Lindstr m et al. 1983 those Middle Ordovician lower Caradoc rocks occurring in the Lockne area that contain isolated bodies of reorientated or physically disintegrated limestone, have collectively been called Lockne Breccia. The original assumption was that all of these rocks have a shared origin as debris flows. The idea of a common origin remained even after it was realized that debris flows, properly defined, had not been active. The discussion of von...

The Lockne Breccia and Loftarstone

As remarked above, the definition of the Lockne Breccia should be restricted to resurge deposits that contain clasts of crystalline ejecta. The content of limestone clasts is mostly of diverse provenance. Transitions to the overlying loftarstone may be either sharp Fig. 12 and erosional, or gradual Fig. 13 . The principal difference from the loftarstone is the smaller grain size in the latter. The sharpening of recognition criteria has led to a considerable reduction of the distribution on the...

The Mids Problematics A B S E types Tagamite and Shock Veins

Right through to the beginning of the previous decade, many workers -especially in South Africa - had remained reluctant to accept the long proposed e.g., Dietz 1960 impact origin of the Vredefort Dome. Within this context, the origin of pseudotachylite in Shand's type locality was not considered significant. It took the confirmation of bona fide shock deformation in quartz Leroux et al. 1994 and zircon Kamo et al. 1996 , and the identification of a meteoritic component in the Vredefort...

The Duobblon Structure A Small Segment of a Large Precambrian Impact Structure

Robert Lilljequist1 and Ulla Preeden2 'Ecominas, Calle Horno 9, Estepuma, Spain robertlilljequist yahoo.se 2Institute of Geology, University of Tartu, Vanemuise 46, EE-51014 Tartu, Estonia ullap ut.ee Abstract. The Duobblon Structure is located in northern Sweden, near the Scandinavian mountain range in the county of Vasterbotten. The structure constitutes a 30 km long formation, slightly concave to the NW. The Revsund Granite 1.87 Ga makes up most of the basement, and grades into a 100 m thick...

The Sub Cambrian Peneplain and the Crater

When marine sedimentation began in large parts of Sweden in the Cambrian, it covered a surface that had been land for long enough time to cut extensive peneplain facets into the deeper, crystalline parts of Proterozoic orogens. A sub-Cambrian peneplain can thus be identified in most parts of the country where Paleozoic sedimentary rocks are preserved. In Jamtland this peneplain has got a north-westward slope of 1.5 through mainly Caledonian deformation of the lithosphere Karis and Stromberg...

Sudbury Ore Mineralization

The Cu-Ni and platinum group element ore bodies of the Sudbury Structure occur in the Sublayer, the Footwall Breccias, footwall rocks beneath the SIC, and the Offset Dikes. They are of extraordinary value. The total ore reserve is estimated at 1.65 x 109 metric tons at 1.2 Ni and 1.05 Cu. The cumulative value of ore extracted from the Sudbury Structure in more than 100 years of production has been estimated at over US 100 billion. The current production is valued at between US 1.5 and 2 billion...

Silicified Conein Cone Structures from Erfoud Morocco A Comparison with Impact Generated Shatter Cones

Reimold 2 and Christian Koeberl 3 'Dipartimento di Scienze della Terra, Universita degli Studi di Modena e Reggio Emilia, Largo S. Eufemia 19, 41100 Modena, Italy lugli.stefano unimore.it 2Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa reimoldw geosciences.wits.ac.za 3Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria...

Uranium at Carswell Canada

The outer limit of the Carswell impact structure in Sasketchewan Canada 58o27'N 109o30'W is only vaguely defined, but the diameter of the exposed structure is generally estimated at 39 km, although Grieve and Masaitis 1994 suggested that the original diameter of the Carswell Structure could have been as large as 50-55 km. The structure comprises an annular, about 5 km wide trough around a 20-km-wide core terrane composed of metamorphic basement rocks Fig. 6 . Despite its relatively young age of...

The Preliminary Analysis of Polygonal Impact Craters within Greater Hellas Region Mars

Teemu hman1, Marko Aittola2, Veli-Petri Kostama2 and Jouko Raitala2 institute of Geosciences, Department of Geology, P.O. Box 3000, FIN-90014 University of Oulu, Finland. teemu.ohman oulu.fi 2Planetology Group, Division of Astronomy, Department of Physical Sciences, P.O. Box 3000, FIN-90014 University of Oulu, Finland Abstract. The polygonal planimetric shape of impact craters has been known for a long time, but has not been discussed much in the past. Polygonal craters exist on all kinds of...

The Sudbury Igneous Complex SIC

The various units of the SIC, from bottom to top, comprise the Sublayer, Norite, Quartz-Gabbro and Granophyre Fig. 9 . Apparently, there are several phases of Sublayer which, in general, consist of a noritic-quartz dioritic rock containing mafic and ultramafic inclusions as well as inclusions derived from the target rocks Naldrett et al. 1982, 1984 . Sulfide mineralization is associated mainly with the mafic and ultramafic inclusion rich Sublayer. The Sublayer is the main ore-bearing unit of...

Shatter Cone Formation

Different mechanisms have been proposed for the formation of shatter cones. This includes the idea by Johnson and Talbot 1969 that the elastic precursor of a shock front is scattered by a heterogeneity in a rock see also Sharpton et al. 1996 . Gash 1971 proposed that shatter cones were produced from the interaction of an incident shock wave with a tensile wave reflected from a reflective source such as a local heterogeneity in a rock for example a relatively large grain, pore space, or fracture...

The Combined Petrographic and Chemical Analysis of endPermian Kerogens

Visscher2, H. Brinkhuis2 and J.W. de Leeuw1,3 'Department of Geochemistry, Institute of Earth Sciences, Utrecht University, PO Box 80.021, NL-3508 TA, Utrecht, The Netherlands 2Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, NL-3584 CD, Utrecht, The Netherlands 3Department of Marine Biogeochemistry and Toxicology, Royal Netherlands Institute for Sea Research NIOZ , PO Box 59, NL-1790 AB, Den Burg, Texel, The Netherlands Current...

The Carbonado Conundrum

Carbonados are polycrystalline diamond aggregates of generally irregular shapes that have been observed in placer deposits and low-grade metamorphic rocks of mainly Brazil, Russia, South Africa, Ukraine, Venezuela, and the Central African Republic. Some major occurrences have been exploited commercially for the production of industrial diamond e.g., Trueb and de Wys 1971 Kaminskiy et al. 1978 . Carbonados occur in crustal lithologies and have crustal signatures in terms of stable isotope...

Siljan Impact Structure Sweden

A prime example of an impact-related hydrothermal deposit with inherent economic importance are the large Pb-Zn deposits located in the 377 Ma old Reimold et al. 2004 , ca. 65-km-diameter Kenkmann and von Dalwigk 2000 others favour ca. 75 km, Henkel and Aaro this volume . Siljan impact structure in Central Sweden. A major deposit, estimated at 0.3 million metric tons of ore at grades of 3 Pb, 1.5 Zn, and 70 ppm Ag, is located in Ordovician and Silurian rocks of the rim syncline around the...

Pseudotachylite or Pseudotachylitic Breccia Comparison of Vredefort Witwatersrand Breccias and Tectonic Friction Melt

In this section, a number of aspects of breccias occurring in the Vredefort Dome-Witwatersrand Basin VWB and in tectonic fault shear zone TFS settings are compared. Pseudotachylitic breccias in the Vredefort Dome -Witwatersrand Basin can be divided into two groups 1. the breccias in the goldfields gt 80 km from the center of the structure and well outside the zone of shock metamorphism 2. the breccias in the Vredefort dome, where shock-induced features like shatter cones, PDFs and coesite and...