The Cleanskin impact structure, Northern Territory and Queensland, Australia: A reconnaissance study

2021 ◽  
Author(s):  
T. Kenkmann ◽  
P.W. Haines ◽  
I.P. Sweet ◽  
K. Mitchell
Keyword(s):  
Remote Sensing ◽  
Northern Territory ◽  
Impact Structure ◽  
Impact Event ◽  
Deformation Features ◽  
Apparent Diameter ◽  
The Impact ◽  
Planar Deformation Features ◽  
Impact Origin ◽  
Quartz Grains

ABSTRACT We report on the Cleanskin structure (18°10′00″S, 137°56′30″E), situated at the border between the Northern Territory and Queensland, Australia, and present results of preliminary geological fieldwork, microscopic analyses, and remote sensing. The Cleanskin structure is an eroded complex impact structure of ~15 km apparent diameter with a polygonal outline caused by two preexisting regional fault sets. The structure has a central uplift of ~6 km diameter surrounded by a rather shallow ring syncline. Based on stratigraphy, the uplift in the center may not exceed ~1000 m. The documentation of planar deformation features (PDFs), planar fractures (PFs), and feather features (FFs) in quartz grains from sandstone members of the Mesoproterozoic Constance Sandstone confirms the impact origin of the Cleanskin structure, as proposed earlier. The crater was most likely eroded before the Cambrian and later became buried beneath Cretaceous strata. We infer a late Mesoproterozoic to Neoproterozoic age of the impact event. In this chapter, the Cleanskin structure is compared with other midsized crater structures on Earth. Those with sandstone-dominated targets show structural similarities to the Cleanskin structure.

scholarly journals Yallalie: a buried structure of possible impact origin in the Perth Basin, Western Australia

1999 ◽  
Vol 136 (6) ◽  
pp. 619-632 ◽  
Author(s):  
M. C. DENTITH ◽  
A. W. R. BEVAN ◽  
J. BACKHOUSE ◽  
W. E. FEATHERSTONE ◽  
C. KOEBERL
Keyword(s):  
Western Australia ◽  
Plan View ◽  
Digital Elevation ◽  
Elevation Data ◽  
Deformation Features ◽  
Digital Elevation Data ◽  
Seismic Reflection Profiles ◽  
Planar Deformation Features ◽  
Impact Origin ◽  
Quartz Grains

An enigmatic buried structure, located in Mesozoic sedimentary rocks in the Perth Basin, Western Australia, was discovered in 1990 by Ampol Exploration. The basin-like Yallalie structure (centred on 30° 26′ 40.3″ S, 115° 46′ 16.4″ E) is circular in plan view and about 12 km in diameter. High-resolution, seismic-reflection profiles across the structure show a basin-shaped area of chaotic reflections that extend to a depth of approximately 2 km below the surface. The structure has sharp boundaries with surrounding faulted, but otherwise relatively undisturbed, rocks. In the centre of the structure there is an uplifted area approximately 3–4 km across, similar to those described from complex meteorite impact structures. The seismically defined structure coincides with a circular topographic depression, and image processing of digital elevation data has allowed recognition of concentric and radial structures extending as far as 40 km from the centre of the depression. Gravity surveys show the structure to be associated with a positive gravity anomaly of about 30 gu. Aeromagnetic surveys have defined annular anomalies associated with the central uplifted section, and possibly margins, of the structure. A search for siderophile element enrichments (by neutron activation analysis) in the rocks of the structure, which would indicate the presence of a meteorite component, proved negative. Quartz grains in cores that penetrate the structure show the development of prismatic cleavage fractures and irregular, slightly curved planes formed by brittle fracture. An allochthonous breccia of Late Cretaceous rocks occurs a few kilometres west of the western margin of the structure. Quartz grains from a thin veneer of Tertiary sediments that drape the structure are essentially undeformed. However, multiple sets of closely spaced planar deformation features in quartz, characteristic of highly shocked rocks, have yet to be observed in the rocks of the Yallalie structure and the allochthonous breccia. The morphology of the Yallalie structure determined from topographic and geophysical data suggests strongly that it is of impact origin. Geological and geochemical evidence is equivocal, but is not inconsistent with this interpretation.

scholarly journals Shocked quartz in distal ejecta from the Ries impact event (Germany) found at ~ 180 km distance, near Bernhardzell, eastern Switzerland

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Sanna Holm-Alwmark ◽  
Carl Alwmark ◽  
Ludovic Ferrière ◽  
Matthias M. M. Meier ◽  
Sofie Lindström ◽  
...  
Keyword(s):  
Impact Structure ◽  
Impact Cratering ◽  
Impact Ejecta ◽  
Deformation Features ◽  
Different Depths ◽  
Shocked Quartz ◽  
Depositional Age ◽  
Impact Events ◽  
Planar Deformation Features ◽  
Quartz Grains

AbstractImpact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size.

Further evidence for an impact origin of the Tsenkher structure in the Gobi-Altai, Mongolia: geology of a 3.7 km crater with a well-preserved ejecta blanket

2017 ◽  
Vol 156 (1) ◽  
pp. 1-24
Author(s):  
GORO KOMATSU ◽  
JENS ORMÖ ◽  
TOGOOKHUU BAYARAA ◽  
TOMOKO ARAI ◽  
KEISUKE NAGAO ◽  
...  
Keyword(s):  
Noble Gas ◽  
Gas Analysis ◽  
Volcanic Origin ◽  
Large Size ◽  
Deformation Features ◽  
Order Of Magnitude ◽  
Ejecta Blanket ◽  
Planar Deformation Features ◽  
Impact Origin ◽  
Related Sample

AbstractThe Tsenkher structure in the Gobi-Altai, Mongolia is a c. 3.7 km diameter crater with a well-preserved ejecta blanket. It has been hypothesized to be either of impact or volcanic origin in our previous work. Observations during our 2007 expedition and related sample analyses give further support for an impact origin. The evidence includes the presence of a structurally uplifted near-circular rim surrounded by an ejecta blanket, and abundant breccias, some of which are melt- and millimetre-scale spherule-bearing. Planar deformation features (PDFs) were found in one quartz grain in a breccia sample. Fe-rich grains are found in a vesicular melt sample that is also characterized by elevated platinum group element (PGE) abundances with respect to the sedimentary bedrock of the area (approximately an order of magnitude). Noble gas analysis of one breccia sample yielded an elevated 3He/4He value of (5.0±0.2) × 10−6. Although not conclusive alone, these geochemical results are consistent with a contribution of meteoritic components. A volcanic origin, in particular a maar formation, would require explanations for the unusual conditions associated with Tsenkher, including its large size occurring in isolation, the structurally uplifted rim and the lack of a bedded base surge deposit. A pronounced rampart structure observed at the eastern ejecta is also unusual for any volcanic origin. 40Ar–39Ar dating of a vesicular melt sample gives an age of the Tsenkher structure of 4.9±0.9 Ma. The rampart structure could provide insights into the formation of similar ejecta morphologies associated with numerous impact craters on Mars.

Shatter cones and planar deformation features confirm Santa Marta in Piauí State, Brazil, as an impact structure

2014 ◽  
Vol 49 (10) ◽  
pp. 1915-1928 ◽  
Author(s):  
Grace Juliana Gonçalves de Oliveira ◽  
Marcos Alberto Rodrigues Vasconcelos ◽  
Alvaro Penteado Crósta ◽  
Wolf Uwe Reimold ◽  
Ana Maria Góes ◽  
...  
Keyword(s):  
Impact Structure ◽  
Planar Deformation ◽  
Deformation Features ◽  
Shatter Cones ◽  
Planar Deformation Features

Distinguishing shock-related microstructures in gneisses from the Vredefort impact structure, South Africa

2021 ◽  
Author(s):  
Fabian Dellefant ◽  
Claudia Trepmann ◽  
Stuart Gilder ◽  
Iuliia Sleptsova ◽  
Melanie Kaliwoda ◽  
...  
Keyword(s):  
South Africa ◽  
Magnetic Properties ◽  
Frictional Heating ◽  
Rapid Quenching ◽  
Impact Structure ◽  
Impact Event ◽  
Lightning Strikes ◽  
Fine Grained ◽  
The Impact ◽  
Drill Cores

<p>Shocked gneiss (~8 GPa) from the Vredefort impact structure (South Africa) contain planar fractures in quartz decorated by magnetite and ilmenite, which are commonly attributed to the impact event. However, the surface at Vredefort is riddled by lightning strikes, which also produce rapid pressure-temperature pulses that can modify the microstructure and the magnetic properties of the rocks. To understand the differences between lightning and impact-related shock effects, we investigated samples from two, 10 m-deep drill cores by Raman spectroscopy, polarized light microscopy/U-stage and electron microscopy/electron backscatter diffraction techniques. Magnetite and ilmenite within planar fractures in quartz occur at all depths, and are therefore intrinsic to the impact event, independent of lightning. Primary iron-bearing minerals were locally heated by the generation of shear fractures in neighboring quartz, leading to small volumes (micrometer scales) of melt intruding into nearby fractures. Frictional heating and rapid quenching of feldspar and quartz is indicated by localized, fine-grained aggregates along intragranular planar fractures as well as transgranular pseudotachylytic veins. On the other hand, altered ilmenite grains with exsolved magnetite occur only in gneisses from the uppermost 80 cm of both drill cores. When in contact with biotite, the ilmenite-magnetite boundaries are altered to chlorite, and the ilmenite is partly transformed to anatase. These alteration products contain fine-grained magnetite. It appears that lightning strikes altered the existing ilmenite-magnetite in the Vredefort samples to produce smaller, more single-domain like magnetite grains, consistent with the observed magnetic properties of the samples</p>

Impact-induced hydrothermal dissolution in pyroxene: Petrographic and geochemical characterization of basalt-dominated polymict impact breccias from the Vargeão Dome, Brazil

2021 ◽  
Author(s):  
Jennifer Epstein ◽  
Lidia Pittarello ◽  
Álvaro P. Crósta ◽  
Christian Koeberl
Keyword(s):  
Hydrothermal Alteration ◽  
Hydrothermal Fluids ◽  
Terrestrial Planets ◽  
Impact Structure ◽  
Impact Event ◽  
Southern Brazil ◽  
Geochemical Characterization ◽  
Heterogeneous Nature ◽  
The Impact

ABSTRACT Constraints on impact-related hydrothermal alteration are important to enable the reconstruction of the possible processes affecting the surface of other terrestrial planets, such as Mars. Terrestrial impact structures excavated in basaltic targets provide the opportunity for analog studies. In Brazil, seven impact structures have been confirmed so far. Three of them, Vargeão Dome, Vista Alegre, and Cerro do Jarau, were formed in the same basaltic province belonging to the Paraná Basin, and they have several common characteristics. Oxidized basaltic breccias locally containing sandstone clasts occur in all these structures. In this work, selected samples of such breccias from the Vargeão Dome impact structure in southern Brazil were petrographically and geochemically investigated to further constrain the effects of the postimpact hydrothermal alteration. The breccia matrix shows typical oxidation effects induced by postimpact hydrothermal fluids, which highlight its heterogeneous nature, related to the impact event, and mixing components from different pre-impact stratigraphic formations. The detection of partially dissolved exsolution lamellae in pyroxene and of related alteration products constrains the effects of hydrothermal alteration in the basalts of the Vargeão Dome, which could serve as a terrestrial analog for planetary studies.

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