Allogenic Breccias Research Articles

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 Obolon impact structure, Ukraine, and its ejecta deposits

Abstract— The Obolon impact structure, 18 km in diameter, is situated at the northeastern slope of the Ukrainian Shield near its margin with the Dnieper‐Donets Depression. The crater was formed in crystalline rocks of the Precambrian basement that are overlain by marine Carboniferous and continental Lower Triassic deposits. The post‐impact sediments comprise marine Middle Jurassic (Bajocian and Bathonian) and younger Mesozoic and Cenozoic deposits. Today the impact structure is buried beneath an about 300‐meter‐thick sedimentary rock sequence.Most information on the Obolon structure is derived from two boreholes in the western part of the crater. The lowest part of the section in the deepest borehole is composed by allogenic breccia of crystalline basement rocks overlain by clast‐rich impact melt rocks and suevites. Abundant shock metamorphic effects are planar deformation features (PDFs) in quartz and feldspars, kink bands in biotite, etc. Coesite and impact diamonds were found in clast‐rich impact melt rocks. Crater‐fill deposits are a series of sandstones and breccias with blocks of sedimentary rocks that are covered by a layer of crystalline rock breccia.Crystalline rock breccias, conglomeratic breccias, and sandstones with crystalline rock debris have been found in some boreholes around the Obolon impact structure to a distance of about 50 km from its center. Those deposits are always underlain by Lower Triassic continental red clay and overlain by Middle Jurassic marine clay. The K‐Ar age of impact melt glasses is 169 Ma, which corresponds to the Middle Jurassic (Bajocian) age. The composition of crater‐fill rocks within the crater and sediments outside the Obolon structure testify to its formation under submarine conditions.

Terrestrial impact melt rocks and glasses

The effects of meteorite and comet impact on Earth are rock brecciation, the formation of shock metamorphic features, rock melting, and the formation of impact structures, i.e. simple craters, complex craters, and multi-ring basins. Large events, such as the 65-Ma Chicxulub impact, are believed to have had catastrophic environmental effects that profoundly influenced the development of life on Earth. In this review, an attempt is made to summarize some of the voluminous literature on impact melting, one important aspect of planetary impact, provide some comments on this process, and to make suggestions for future research. The products of impact melting are glasses, impact melt rocks, and pseudotachylites. Our treatise deals mainly with the geological setting, petrography, and major-element chemistry of melt rocks and glasses. Impact glasses, in several petrographic aspects, are similar to volcanic glasses, but they are associated with shock metamorphosed mineral and rock fragments and, in places, with siderophile element anomalies suggestive of meteoritic contamination. They are found in allogenic breccia deposits within (fall-back ‘suevite’) and outside (fall-out ‘suevite’) impact craters and, as spherules, in distal ejecta. Large events, such as the K/T boundary Chicxulub impact, are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris. Impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common. On a microscopic scale, however, impact glasses are commonly strikingly heterogeneous. Tektites are glasses ejected from craters over large distances. They are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal, sedimentary rocks. Four tektite strewn-fields are known, three of which can be tied to specific impact craters. Impact melt rocks form sheets, lenses, and dike-like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks. Dikes of impact melt rocks also intrude the rocks of the crater floor. They commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine- to coarse-grained. Chemically, they are strikingly homogeneous, but as with impact glasses, exemptions to this rule do exist. Large and thick melt bodies, such as the Sudbury Igneous Complex (SIC), are differentiated or may represent a combination of impact melt rocks sensu-strictu and impact-triggered, deep-crustal melts. A concerted, multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated. Impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes—especially of melting, excavation, and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater. Impact-triggered pressure-release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu-strictu. This controversial idea needs to be tested by a re-evaluation of existing data and models, be they based on field or laboratory research. Only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping. In this review, we summarize observations made on impact melt rocks and impact glasses in a number of North American (Brent, Haughton, Manicouagan, New Quebec, Sudbury, Wanapitei, all in Canada), Asian (Popigai, Russia; Zhamanshin, Kazakhstan), two South African structures (Morokweng and Vredefort), the Henbury crater field of Australia, and one European crater (Ries, Germany). Our tables listing major-element chemical compositions of impact glasses and melt rocks, however, include also data from structures not dealt with in further detail.

Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada

The Slate Islands impact structure is the eroded remnant of a ∼30–32 km-diameter complex impact structure located in northern Lake Superior, Ontario, Canada. Target rocks are Archean supracrustal and igneous rocks and Proterozoic metavolcanics, metasediments, and diabase. A wide variety of breccias occurs on the islands, many of which contain fragments exhibiting shock metamorphic features. Aphanitic, narrow and inclusion-poor pseudotachylite veins, commonly with more or less parallel boundaries and apophyses branching off them, represent the earlist breccias formed during the compression stage of the impact process. Coarse-grained, polymictic clastic matrix breccias form small to very large, inclusion-rich dikes and irregularly shaped bodies that may contain altered glass fragments. These breccias have sharp contacts with their host rocks and include a wide range of fragment types some of which were transported over minimum distances of ∼2 km away from the center of the structure. They cut across pseudotachylite veins and contain inclusions of them. Field and petrographic evidence indicate that these polymictic breccias formed predominantly during the excavation and central uplift stages of the impact process. Monomictic breccias, characterized by angular fragments and transitional contacts with their host rocks, occur in parautochthonous target rocks, mainly on the outlying islands of the Slate Islands archipelago. A few contain fragmented and disrupted, coarse-grained, polymictic clastic matrix breccia dikes. This is an indication that at least some of these monomictic breccias formed late in the impact process and that they are probably related to a late crater modification stage. A small number of relatively large occurrences of glass-poor, suevitic breccias occur at the flanks of the central uplift and along the inner flank of the outer ring of the Slate Islands complex crater. A coarse, glass-free, allogenic breccia, containing shatter-coned fragments derived from Proterozoic target rocks (upper target strata), observed at two locations may be analogous to the ‘Bunte Breccia’ of the Ries crater in Germany. At one of these locations, this breccia lies close to a crater suevite deposit. At the other, it overlies parautochthonous, monomictic breccia. The Slate Islands impact breccias are superbly exposed, much better than breccias in most other terrestrial impact structures. Observations, including those indicative of multiple and sequential processes, provide insight on how impact breccias form and how they relate to the various phases of the impact process. Eventually they will lead to an improved understanding of planetary impact processes.

Characteristics of large terrestrial impact structures as revealed by remote sensing studies

Remote sensing data sets of a spectrum of impact structures on Earth have been analyzed in an effort to determine if such structures have identifiable signatures which could be used as an aid in searching broad areas for the “missing craters” predicted by current estimates of the cratering flux. Meteor Crater, Arizona has been used as a “control” site, due to the extensive data available for this feature. Laboratory thermal infrared reflectance spectra of shocked sandstones indicate that shock processes alter the appearance of fundamental absorption features enough to permit such effects to be observed in airborne thermal infrared imagery collected with the NASA TIMS instrument. Analysis of this Tims imagery suggests that shocked and displaced target rocks can be readily identified. Aircraft multi-frequency imaging radar data collected with the NASA/JPL SAR polarimeter clearly reveals the textural pattern of the preserved components of the ejecta blanket. SPOT multi-spectral imagery of Bosumtwi, Ghana, illustrates the difficulties associated with remote sensing in highly vegetated terrains. Nevertheless, the imagery reveals the subtle expression of an outer tectonic ring at the crater, and may be sensitive to geobotanical variations associated with impact-related suevite deposits. In contrast, the Zhamanshin impact feature on the arid, sedimentary plains of Kazakhstan is ideal for multi-spectral remote sensing, although much of the expected morphologic expression of the complex crater is missing or subdued. Virtually all of the field-mapped impact-related deposits display a unique spectral pattern in decorrelated Landsat Thematic Mapper imagery. The extensive deposits of allogenic breccias, which cap the hills defining the inner ring of the crater, are very distinctive, as are certain of the impact glass deposits. A second, or outer, ring at 13–13.5 km is apparent in the decorrelated principal component TM imagery. The suggestion is that spectral disruption patterns for large, multi-ring impact features in arid settings should be identifiable from orbit. By examining a pristine 144 km diameter multi-ring impact feature on the Venus rolling plains, an impression of the original morphology of a continental impact crater of the magnitude of the Cretaceous-Tertiary (K/T) boundary impact structure can be deduced. The 35 Ma Popigai structure is the only known 100 km class impact crater on Earth approximately equal in age or younger than the K/T event, and its morphology is poorly expressed in single-band Landsat imagery. Thus, spectral anomalies associated with key impact deposits, such as allogenic breccias, suevite, melt, and glasses may have to form the basis for identifying candidate K/T impact sites in the absence of preserved morphologic elements of the crater. It is apparent that remote sensing is not a panacea for the discovery and validation of new impact craters, but it can contribute by serving as a pathfinder both for the identification of promising structures and the possible locations of key deposits for detailed fieldwork and sampling.

Impact craters and craterform structures in Fennoscandia

Sixty-two, nearly circular, topographic, morphological or geophysical structures in Fennoscandia and Estonia were studied to discover their origin. The structures were divided into five classes. Fifteen were caused by meteorite impacts (class A and a). Four of them (Lappajärvi, Jänisjärvi, Mien and Dellen) contain large volumes of impact melt. The age of the recognized impact craters varies from pre-historic (3500 B.C.) to 700 Ma and their diameters from 0.04 km to 55 km. The majority of the other structures are probable (class B) or possible (class C) craters for which there is not yet sufficient proof of impact origin. There is increasing evidence that some of the large arcuate morphological or geophysical structures (class E) represent the deeply eroded scars of very old craters but, owing to the lack of identified shock metamorphic features or impact-generated rocks (e.g., allogenic breccias), they cannot so far be classified as impact craters. So far, no craterform structure or ejecta layer of Archaean or Early Proterozoic age has been found in Fennoscandia, although, statistically, remnants of ancient cratering events should be found in the Fennoscandian shield. New ways of searching for these structures are proposed. The impact cratering rate in Fennoscandia is 2.4 × 10 −14 km −2 a −1 ( N = 12, the very small craters are omitted), which corresponds to about two events per 100 Ma for the last 700 Ma. This is a minimum estimate and is higher than the global cratering rate.