Crater-fill Deposits Research Articles

The Spectral Properties of Pitted Impact Deposits on Vesta as Seen by the Dawn VIR Instrument

Pitted impact deposits (PIDs) on Vesta have been shown to exhibit distinct spectral characteristics with respect to their surrounding host deposits and other typical Vestan areas regarding the first major pyroxene absorption near 0.9 μm. The PIDs, especially those in the ejecta blanket of the large crater Marcia, show higher reflectance and pyroxene band strength with respect to their impact deposit surroundings. This study complements the spectral characterization of the PIDs on Vesta with Visible and Infrared Spectrometer observations obtained by NASA’s Dawn mission. In particular, we focus on the second major pyroxene absorption near 1.9 μm. We analyze nine PIDs in the ejecta blanket of the crater Marcia, as well as PIDs within the crater-fill deposits of the craters Marcia, Cornelia, and Licinia. We find that the second pyroxene absorption behaves in similar ways as the first major pyroxene absorption. The PIDs in Marcia’s ejecta blanket show higher reflectance and pyroxene band strength with respect to their immediate impact deposit surroundings. The PIDs present in the crater-fill deposits of Marcia, Cornelia, and Licinia, however, do not show such spectral characteristics. This is also consistent with previous observations of the first pyroxene absorption band. Based on the experimental results of other studies, we speculate that the observed spectral distinctness arises from an oxidation process in the interior of impact deposits, where Fe cations migrate within their host pyroxene grains. Thus, the surfaces of (melt-bearing) impact deposits might be different from their interiors, apart from space-weathering effects.

THE NEPSKY-1 IMPACT CRATER AND ITS FILL DEPOSITS ON THE BASEMENT ROOF OF THE SIBERIAN PLATFORM

In the Earth studies, discoveries and investigations of impact craters buried beneath thick sediments are sporadic so far and may still remain inefficient, unless geophysical surveys of the areas of interest are initiated. Such studies can provide useful data to develop the knowledge about cosmic events of the past geological eras, as well as contribute to industrial development of the areas. It is known that an impact crater is filled with layered deposits that create specific sequences and are generally thicker that deposits outside the crater. We have investigated a Precambrian crater located in the southern part of the Nepa-Botuoba anteclise of the Siberian platform. This impact crater called Nepsky-1 was discovered by seismic surveys using the common depth point method (CDPM). In our study, we used the geological and geophysical deep-drilling data of three wells, including gamma-ray and neutron logs and lithological core descriptions. With reference to the concepts of regional geological conditions, we reconstructed the conditions of sedimentation in the study area based on a comprehensive analysis of the structural and textural features of the rocks, and logging and seismic survey data. By processing and interpretation of the CDPM 3D seismic survey data, we obtained the structural images and cross-sections of the impact crater and analysed the thickness of its fill deposits. The Nepsky-1 crater is a bowl-shaped structure with a rim composed of allogenic breccia. In the area around the crater, fault systems are detected. Based on the core sample analysis, we identified the lithological members of the crater and its rim and described them in detail. Active compensation with lacustrine-delta sediments took place in the Nepa period of the late Vendian. By the end of the Tira time, the crater was completely leveled up. In our study, we obtained the first data on the structure of the crater section of the Vendian deposits. Sedimentation in the study area was controlled by the sea level changes. The main terrigenous productive horizons are confined to sandstones that accumulated during the sea level low stand, and found at the bottoms of the lower and upper Nepa subsuites, as well as at the bottoms of the lower and upper Tira subsuites. It is established that the study area was tectonically active in the late Tira time. As a result, the sediments filling the crater were removed to subaerial conditions, and the edges of the allogenic breccia rim were partially destroyed and formed granite breccia outgrowths observed in the rim’s cross-section. We conclude that in the Nepa and Tira times, tectonic vertical movements initiated relatively fast weathering and transportation of the terrigenous material from the adjacent hills into the sedimentation basin. The Nepsky-1 crater gives evidence of meteorite bombing of the Siberian paleocontinent in Precambrian. Finding similar crater structures can be reasonably expected in the study region. Considering the increased thicknesses of crater fill deposits, buried craters are promising potential for discovering oil-source and overlying seal rocks, which is important for petroleum industry.

The Steinheim Basin impact crater (SW-Germany) – Where are the ejecta?

The ∼24km Nördlinger Ries and the ∼3.8km Steinheim Basin in southern Germany are thought to represent a ∼14.8Ma old impact crater doublet. The complex craters of the Steinheim Basin with its crater fill deposits and the Nördlinger Ries and its voluminous impact ejecta blanket are still widely preserved. Although located in an environmental setting that presumably underwent the same erosional history as the Ries crater, field geologic studies suggest that no proximal or distal ejecta of the Steinheim impact event are presently preserved. Generally, the lack of the ejecta blanket around the crater could be explained either by intense erosion, the scarcity of outcrops, or it never formed. In contrast to the lack of ejecta, fluvial and lacustrine Middle Miocene sediments deposited prior to, synchronous with, and shortly after the impact are preserved in many places in the surroundings of to the Steinheim Basin.On low-density asteroids or planets with highly porous target rocks (⩾30–40% effective porosity), impact structures can form without significant ejecta outside the craters due to the compaction of porosity and a concordant drastic reduction of the ejecta velocity. In the Steinheim area, the target rocks comprised loose, porous Miocene sands, Upper Jurassic limestones and Middle Jurassic porous sand- and claystones. The average porosity of the entire sedimentary target suite may have reached 20–30% or even higher values assuming the existence of open karst cavities in the Upper Jurassic carbonates. Compaction of the porous target rocks, resulting in the reduction of ejected material, in combination with erosion could explain the apparent lack of impact ejecta in the wider periphery of the Steinheim impact structure. The Steinheim Basin represents the first proposed terrestrial example of an impact crater characterized by porosity-related ejecta suppression, and it is suggested that other sediment-hosted impact structures on Earth might exhibit analogous excavation-process characteristics.

The transition of spatter to lava-like body in lava fountain deposits: features and examples from the Cabezo Segura volcano (Calatrava, Spain)

The Cabezo Segura II volcanic cone (Calatrava volcanic province, Iberian microplate) comprises proximal wall deposits with a well defined crater wall unconformity and crater-fill deposits. The complex volcanic succession, that shows evidence of several eruptive episodes, was built by magmatic and hydrovolcanic explosions of different styles (Strombolian, Hawaiian, sub-Plinian and phreato-Strombolian) generated from a multiple feeder ultrabasic dyke. Intra-crater rock units at the volcano summit include spatter deposits together with up to 10m thick and more than 200m long lava-like bodies. Geological logs for the main lava-like bodies define a characteristic facies model that involves a central lava-like mass which grades vertically into a transition zone of apparently coherent spatter, then dense spatter and, finally, into vuggy spatter deposits. These units are inferred to have formed during pulsating lava fountain-type explosive eruptions; the depicted facies distribution being the result of progressive increase in welding grade and densification of the spatter in response to variations in the accumulation rate. Their field features may be used as a guide for the precise identification of vent sites in deposits of Hawaiian eruptions. Also, structures like those here recognised, that might have survived in lava-like flows, could be of help to identify when lava-producing eruptions represented an explosive Hawaiian event (lava fountains) and not a purely effusive event.

Introduction to Special Issue of Bulletin of Volcanology on “Advances in Kimberlite Geology and Volcanology”

The volcanology of kimberlite pipes and volcanoes has been a neglected area of mainstream volcanology research, until the last several years. Previously, published kimberlite research focused largely on petrological and geochemical aspects, with few studies on the physical eruption processes, the volcanic facies characteristics and their volcanological significance from the perspective of modern volcanology. To some extent this situation arose because of the constraints of confidentiality associated with the research, which was commonly controlled by major diamond mining and exploration companies in a very competitive industrial environment, as well as a lack of awareness of the opportunites for research amongst the volcanological community. The Canadian kimberlite fields, newly-discovered in the early 1990’s, featured diamondiferous pipes with a greater range of geometries and deposit types, including crater fill deposits, than occurred in the traditional southern African pipes. These deposits were diverse in componentry, texture, and origins and, more often than not, diamond grades varied greatly across the different lithofacies. The variations in diamond distributions within these deposits created an immediate need to develop a better understanding of the factors that control diamond grade distribution and variations in economic kimberlite pipe systems and provinces. Thus, several years ago, the diamond industry began to invite interest from parts of the academic volcanological research community to undertake research on the volcanological aspects of kimberlite volcanoes and pipes. Kimberlite pipes or bodies occur in geographic and age clusters, much like small intraplate basaltic monogenetic volcanic provinces and fields. From what we can tell many kimberlite bodies appear to have been monogenetic volcanoes, although there are also pipe complexes, which consist of coalesced, overlapping pipes, resulting from a succession of eruption events, although the temporal spacing of these is generally unclear. In general, however, each eruption phase appears to have been short-lived, much like many monogenetic volcanoes. Most kimberlite pipes that reached the Earth’s surface were largely explosive in their behaviour, although the Igwisi Hills complex in Tanzania is an exception because lavas have been found in parts of a preserved edifice (R.J. Brown, M.Field and R. S.J. Sparks, pers. comm.). There are few kimberlite bodies or pipes where surface deposits and the supracrustal volcanic edifice are preserved in situ and intact, for documentation and study as is commonly the case with Cenozoic volcanoes of other, more common magma suites. The youngest kimberlite has an age of 32.3 Ma. Thus, for most kimberlites, their volcanic edifices have been eroded. Exceptions include the Igwisi Hills complex in Tanzania, and the Fort a la Corne kimberlite field in Saskatchewan, Canada. For most kimberlite bodies, understanding of the eruption behaviour This paper constitutes part of a special issue:

Structure and evolution of the Rockeskyllerkopf Volcanic Complex, West Eifel Volcanic Field, Germany

The Rockeskyllerkopf Volcanic Complex (RVC) comprises three overlapping monogenetic volcanic centers: Southeast Lammersdorf (SEL), Mauseberg (M) and Rockeskyllerkopf (RKK). Each volcanic center comprises proximal wall deposits with a well defined crater wall unconformity and crater fill deposits that partially to completely cover the outer crater wall. The SEL Center is a phreatomagmatic tuff ring composed of lithic rich tephra deposited by pyroclastic falls and surges. The second center, Mauseberg, with its crater to the northwest of the SEL Center is predominantly magmatic. Topographic and outcrop patterns suggest that this center may have formed a series of overlapping scoria cones along a N–S trending fissure. The youngest center, RKK, which lies on a poorly developed palaeosol within the earlier Mauseberg deposits, comprises a well developed proximal crater wall sequence. This sequence of magmatic, likely Strombolian, fall and grain avalanche deposits passes upward into a crater fill sequence that comprises variably welded bombs. The final eruptions in the center were massive lava flows that were ponded within the RKK crater. Ar–Ar age dating of reequilibrated fragments of phlogopite megacrysts in the SEL lavas indicates volcanic activity began at 474 ± 39 ka. Literature K–Ar dates for the youngest lava flows in the RKK Center give ages of 360 ± 60 to 470 ka. Our interpretation of the age data and the presence of the poorly developed palaeosol between the Mauseberg and RKK centers indicates that volcanism in the RVC began around 470 ka with the eruption of the SEL and Mauseberg centers followed a few thousand years later by the eruption of the RKK Center.

On elevated fluoride and boron concentrations in groundwaters associated with the Lake Saint-Martin impact structure, Manitoba

Abstract Hydrogeological investigations conducted by the Geological Survey of Canada in the Lake Saint-Martin region of Manitoba have confirmed earlier reports of naturally elevated F − and B concentrations in local groundwaters. Fluoride and B concentrations are highly correlated ( r 2 = 0.905) and reach 15.1 mg/L and 8.5 mg/L, respectively. Virtually all groundwaters with F − concentrations greater than the drinking water limit of 1.5 mg/L are from wells within the Lake Saint-Martin impact structure, a 208 Ma complex crater 23 km in diameter underlying a large part of the study area. The high-F − groundwaters can be classified into two groups according to their anionic and isotopic compositions. Group I samples consist of Na-mixed anion groundwaters, with Cl greater than 100 mg/L and highly depleted 18 O compositions indicative of recharge under much cooler climatic conditions than at present. Samples belonging to this group exhibit a striking relationship to crater morphology, and are found in an arcuate belt within the southern rim of the impact structure. Group II high-F − samples consist of Na–HCO 3 –SO 4 groundwaters, with little Cl, and less depleted 18 O compositions. Samples belonging to this group are associated with groundwaters recharged locally, on a low ridge within the impact structure. This paper traces the probable source of high-F − groundwaters to phosphatic pellets in shales of the Winnipeg Formation, a regional basal clastic unit which sub-crops at shallow depth beneath the crater rim as a result of more than 200 m of structural uplift associated with the impact event. This extensive aquifer is known elsewhere in southern Manitoba for its naturally-softened groundwaters and locally elevated F − concentrations. Group I groundwaters are interpreted as discharge from the Winnipeg Formation where it abuts against crater-fill deposits. Group II high-F − groundwaters are interpreted as modern recharge from within the impact structure, displacing Group I groundwaters. Thus, elevated F − and B concentrations observed in groundwaters of the Lake Saint-Martin area represent the geochemical signature of upwelling from a deep regional aquifer. The previously unsuspected discharge zone occurs within an isolated sub-crop of the aquifer formed as a result of structural uplift caused by the impact event.

The geology of Darwin Crater, western Tasmania, Australia

Darwin glass is a siliceous impact glass found in a 400 km 2 strewn field near Mt Darwin, western Tasmania, Australia. It has been dated by Ar–Ar methods at 816 ± 7 ka. A 1.2 km diameter circular depression, named Darwin Crater (42°18.39′S, 145°39.41′E), is the assumed source crater for the glass. Darwin Crater is situated in a remote rain forested valley developed within Siluro–Devonian quartzite and slate (Eldon Group). Earlier geophysical investigations demonstrated that the structure is an almost circular bowl-shaped sediment-filled basin. This paper provides the first detailed description of the geology of Darwin Crater. The centre of the crater has been penetrated by two drill cores, the deeper to a maximum depth of ∼ 230 m. The drill cores intersected fine-grained lacustrine sediments (∼ 60 m thick) overlying poorly sorted coarser crater-fill deposits. The pre-lacustrine crater-fill stratigraphy comprises an uppermost polymict breccia (∼ 40 m thick) of angular quartz and country rock, which contains very rare (≪ 1%) fresh glass fragments (Crater-fill Facies A). Beneath the polymict breccia facies, the drill core intersected monomict sandy breccias of angular quartz (Crater-fill Facies B), and a complicated package of deformed slate clasts (Crater-fill Facies C). Quartz grains in the crater-fill samples contain abundant irregular fractures. In some of the most deformed quartz grains, sub-planar fractures define zones of alternating extinction that superficially resemble twinning. Kinked micas are also present. While the deformation observed in clasts of the crater-fill facies is far greater than in rocks cropping out around the crater, no diagnostic shock indicators, such as planar deformation features (PDF's) in quartz, were observed. If the crater is of impact origin, as seems likely due to the close association with Darwin glass, this is another example of a simple crater where diagnostic shock indicators appear to be absent, preventing confirmation of an impact origin by petrographic analysis of crater-fill samples alone.

Evidence for the shock melting of sulfates from the Haughton impact structure, Arctic Canada

Sulfate minerals can form an important component of the matrix to carbonate-rich impact melt breccias at the 24 km diameter, 23 Ma Haughton impact structure, Arctic Canada. The textural and chemical features of the matrix-forming sulfates indicate that these phases, in addition to co-existing carbonates and silicates, crystallized directly from an impact-generated melt. Evidence for this includes (1) the matrix-supported nature of the crater-fill lithologies, (2) sulfate–carbonate–silicate liquid immiscible textures, (3) possible quench textures in anhydrite, and (4) flow textures developed between anhydrite and silicate-rich glasses. Further supporting evidence includes the presence of Si, Al and Mg in the anhydrite structure, which were probably ‘trapped’ by quenching from a melt. Irregular blebs and globules of shock-melted carbonates within anhydrite also suggest a common origin for the two phases. Field studies reveal that clasts of anhydrite-bearing target material are also present in the crater-fill deposits. Several clasts of anhydrite–quartz lithologies exhibit evidence for incipient shock melting in both phases. Previous assumptions about the response of sulfates and carbonates to hypervelocity impact (i.e., lack of melting) may, therefore, be incorrect.

The clay mineralogy of sediments related to the marine Mjølnir impact crater

Abstract— The 40 km diameter Mjølnir Crater is located on the central Barents Sea shelf, north of Norway. It was formed about 142 ± 2.6 Myr ago by the impact of a 1–2 km asteroid into the shallow shelf clays of the Hekkingen Formation and the underlying Triassic to Jurassic sedimentary strata.A core recovered from the central high within the crater contains slump and avalanche deposits from the collapse of the transient crater and central high. These beds are overlain by gravity flow conglomerates, with laminated shales and marls on top. Here, impact and post‐impact deposits in this core are studied with focus on clay mineralogy obtained from XRD decomposition and simulation analysis methods. The clay‐sized fractions are dominated by kaolinite, illite, mixed‐layered clay minerals and quartz. Detailed analyses showed rather similar composition throughout the core, but some noticeable differences were detected, including varying crystal size of kaolinite and different types of illites and illite/smectite. These minerals may have been formed by diagenetic changes in the more porous/fractured beds in the crater compared to time‐equivalent beds outside the crater rim.Long‐term post‐impact changes in clay mineralogy are assumed to have been minor, due to the shallow burial depth and minor thermal influence from impact‐heated target rocks. Instead, the clay mineral assemblages, especially the abundance of chlorite, reflect the impact and post‐impact reworking of older material. Previously, an ejecta layer (the Sindre Bed) was recognized in a nearby well outside the crater, represented by an increase in smectite‐rich clay minerals, genetically equivalent to the smectite occurring in proximal ejecta deposits of the Chicxulub crater. Such alteration products from impact glasses were not detected in this study, indicating that little, if any, impact glass was deposited within the upper part of the crater fill. Crater‐fill deposits inherited their mineral composition from Triassic and Jurassic sediments underlying the impact site.

40Ar/39Ar age of the Manson impact structure, Iowa, and correlative impact ejecta in the Crow Creek Member of the Pierre Shale (Upper Cretaceous), South Dakota and Nebraska

A set of 34 laser total-fusion 40 Ar/ 39 Ar analyses of sanidine from a melt layer in crater-fill deposits of the Manson impact structure in Iowa has a weighted-mean age of 74.1 ± 0.1 Ma. This age is about 9.0 m.y. older than 40 Ar/ 39 Ar ages of shocked microcline from the Manson impact structure reported previously by others. The 74.1 Ma age of the sanidine, which is a melt product of Precambrian microcline clasts, indicates that the Manson impact structure played no part in the Cretaceous-Tertiary (K-T) mass extinction at 64.5 Ma. Moreover, incremental-heating 40 Ar/ 39 Ar ages of the sanidine show that it is essentially free of excess 40 Ar and has not been influenced by postcrystallization heating or alteration. An age spectrum of the matrix of the melt layer shows effects of 39 Ar recoil, including older ages in the low-temperature increments and younger ages in the high-temperature increments. At 17 places in eastern South Dakota and Nebraska, shocked quartz and feldspar grains are concentrated in the lower part of the Crow Creek Member of the Pierre Shale (Upper Cretaceous). The grains are largest (3.2 mm) in southeastern South Dakota and decrease in size (0.45 mm) to the northwest, consistent with the idea that the Manson impact structure was their source. The ubiquitous presence of shocked grains concentrated in a thin calcarenite at the base of the Crow Creek Member suggests it is an event bed recording an instant of geologic time. Ammonites below and above the Crow Creek Member limit its age to the zone of Didymoceras nebrascense of earliest late Campanian age. Plagioclase from a bentonite bed in this zone in Colorado has a 40 Ar/ 39 Ar age of 74.1 ± 0.1 Ma commensurate with our sanidine age of 74.1 Ma for the Manson impact structure. 40 Ar/ 39 Ar ages of bentonite beds below and above the Crow Creek are consistent with our 74.1 ± 0.1 Ma age for the Manson impact structure and limit its age to the interval ±74.5 0.1 to 73.8 ± 0.1 Ma. Recently, two origins for the Crow Creek have been proposed—eastward transgression of the Late Cretaceous sea and a Manson impact-triggered tsunami. We conclude that most data are in accord with an impact origin for the Crow Creek Member and are at odds with the marine transgression hypothesis.

The Gardnos impact structure, Norway: Petrology and geochemistry of target rocks and impactites

The Gardnos structure, Norway is an approximately circular area of anomalously fractured and brecciated rock, about 4.5 km in diameter, emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite. The original recognition of Gardnos as a deeply eroded impact structure between 900 and 400 Ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks (impactites). Deep erosion of the structure has erased the original rim, removed much of the crater-fill deposits, and exposed large areas at or near the original crater floor. However, a wide variety of distinctive impactites —fractured and blackened quartzites in the sub-crater basement rocks, lithic breccias, and melt-bearing breccias—are still preserved. These impactites show petrographic and chemical characteristics that confirm an impact origin: distinctive Planar Deformation Features (PDFs) in quartz and feldspar, incipient melting of feldspar clasts in the melt-bearing breccias, close matches between the chemical composition of the breccias and mixtures of the target lithologies, and the detection of an extraterrestrial component. A minor extraterrestrial component (≤0.15%) was detected in the melt-bearing breccias, based on significantly elevated Ir and Os contents and lower 187Os/ 188Os ratios compared to those in the target rocks. The Gardnos impactites are significantly enriched in C (5–10X) over the exposed target rocks. This may reflect the presence of a C-rich shale overlying the metamorphic basement at the time of impact; this idea is supported by δ 13C values of −28.1 to −31.5%. measured in the impactites. Mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60–90 wt% granite gneiss, 0–30 wt% amphibolite, 0–12 wt% quartzite, and 3–19 wt% of a C-rich shale component. The deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult. One possible scenario involves the impact of a 300 m diameter stony meteorite that released 10 19 J of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m. The original crater was filled with at least 0.3 km 3 of allochthonous melt-matrix breccias containing about 0.06 km 3 of impact melt. The structure underwent low-grade (greenschist?) metamorphism in Caledonian time (about 400 Ma ago) and was subsequently eroded to its present appearance. The value of the Gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone, in the preservation of a possibly complete sequence of crater-fill breccias beneath a cap of elastic sediments, and in the unusual carbon enrichment of its impactites.

New constraints on the Slate Islands impact structure, Ontario, Canada

The Slate Islands in northern Lake Superior represent the eroded remains of a complex impact crater, originally approximately 32 km in diameter. New field studies there reveal allogenic crater fill deposits along the eastern and northern portions of the islands indicating that this 500-800 Ma impact structure is not as heavily eroded as previously thought. Near the crater center, on the western side or Patterson Island, massive blocks of target rocks, enclosed within a matrix of fine-grained polymict breccia, record the extensive deformation associated with the central uplift. Shatter cones are a common structural feature on the islands and range from less than 3 cm to over 10 m in length. Although shatter cones are powerful tools for recognizing and analyzing eroded impact craters, their origin remains poorly constrained.