The Slate Islands: a probable complex meteorite impact structure in Lake Superior

1976 ◽  
Vol 13 (9) ◽  
pp. 1301-1309 ◽  
Author(s):  
H. C. Halls ◽  
R. A. F. Grieve
Keyword(s):  
Country Rock ◽  
Lake Superior ◽  
Impact Structure ◽  
Early Paleozoic ◽  
Meteorite Impact ◽  
Erosion Level ◽  
Igneous Activity ◽  
Shatter Cones ◽  
Host Rocks ◽  
Sedimentary Unit

Shock metamorphic effects in samples from the Slate Islands, Lake Superior (48°40' N, 87°00' W) suggest that the islands are part of a meteorite impact structure. The islands form the central uplift of a complex crater and are ringed by a submerged trough and annular ridge with a diameter of 30 km. Precambrian bedrock units are locally brecciated and cut by allochthonous breccia dikes. These dikes contain clasts of identifiable country rock and also fragments of a sedimentary unit, possibly Upper Keweenawan in age, which is no longer present in outcrop. The orienta tions of shatter-cones present in the breccia host-rocks indicate the interior of the islands as the approximate shock centre. Microscopic planar features, equivalent to those described from other impact sites, occur in quartz and plagioclase and the level of shock deformation increases towards the interior of the islands. The shock event postdates Keweenawan igneous activity (about 1.1 b.y. old) and, on the basis of the erosion level, may be early Paleozoic in age.

Structural analysis of shatter cones from the Slate Islands, northern Lake Superior

1983 ◽  
Vol 20 (1) ◽  
pp. 1-18 ◽  
Author(s):  
R. M. Stesky ◽  
H. C. Halls
Keyword(s):  
Cross Section ◽  
Land Surface ◽  
Impact Crater ◽  
Elastic Anisotropy ◽  
Lake Superior ◽  
Meteorite Impact ◽  
Central Focus ◽  
Rock Types ◽  
Shatter Cones ◽  
Unusual Type

Shatter cones, an unusual type of fracturing produced by intense shock, are found widespread on the Slate Islands in northern Lake Superior. The islands have been interpreted as the central uplift of an eroded meteorite impact crater about 30 km in diameter. The cones are best developed in certain rock types: Keweenawan basalt flows and chilled margins of associated feeder dikes, and Archean diorite and foliated feldspar porphyry. At certain sites cones show an elongate cross section and anomalous orientation caused by foliation-induced elastic anisotropy in the host rock.In general the cones point upward and inward toward the centre of the Slate Islands group. After structural correction of some sites according to paleomagnetic data, there is increased convergence of cone axes to a central focus (the inferred impact point) that occurs at a height of about 1 km above the present land surface.

scholarly journals Lake Lappajärvi, Central Finland: a Possible Meteorite Impact Structure

Nature ◽  
1968 ◽  
Vol 217 (5127) ◽  
pp. 438-438 ◽  
Author(s):  
NILS-BERTIL SVENSSON
Keyword(s):  
Impact Structure ◽  
Meteorite Impact

scholarly journals Melting and cataclastic features in shatter cones in basalt from the Vista Alegre impact structure, Brazil

2015 ◽  
Vol 50 (7) ◽  
pp. 1228-1243 ◽  
Author(s):  
Lidia Pittarello ◽  
Fabrizio Nestola ◽  
Cecilia Viti ◽  
Alvaro Penteado Crósta ◽  
Christian Koeberl
Keyword(s):  
Impact Structure ◽  
Shatter Cones

Shatter cones at the Keurusselkä impact structure and their relation to local jointing

2016 ◽  
Vol 51 (8) ◽  
pp. 1534-1552 ◽  
Author(s):  
Maximilian Hasch ◽  
Wolf Uwe Reimold ◽  
Ulli Raschke ◽  
Patrice Tristan Zaag
Keyword(s):  
Impact Structure ◽  
Shatter Cones

The Lake Bosumtwi meteorite impact structure, Ghana-A magnetic image from a third observational level

2007 ◽  
Vol 42 (4-5) ◽  
pp. 793-800 ◽  
Author(s):  
Hernan UGALDE ◽  
William A. MORRIS ◽  
Christina CLARK ◽  
Brett MILES ◽  
Bernd MILKEREIT
Keyword(s):  
Impact Structure ◽  
Meteorite Impact ◽  
Magnetic Image

Genesis of the mafic granophyre of the Vredefort impact structure (South Africa): Implications of new geochemical and Se and Re-Os isotope data

2021 ◽  
Author(s):  
Wolf Uwe Reimold ◽  
Toni Schulz ◽  
Stephan König ◽  
Christian Koeberl ◽  
Natalia Hauser ◽  
...  
Keyword(s):  
South Africa ◽  
Country Rock ◽  
Order Estimate ◽  
Impact Structure ◽  
Impact Melt ◽  
Highly Siderophile Elements ◽  
Isotope Data ◽  
Rock Types ◽  
Siderophile Elements ◽  
Os Isotope

ABSTRACT This contribution is concerned with the debated origin of the impact melt rock in the central uplift of the world’s largest confirmed impact structure—Vredefort (South Africa). New major- and trace-element abundances, including those of selected highly siderophile elements (HSEs), Re-Os isotope data, as well as the first Se isotope and Se-Te elemental systematics are presented for the felsic and mafic varieties of Vredefort impact melt rock known as “Vredefort Granophyre.” In addition to the long-recognized “normal” (i.e., felsic, >66 wt% SiO2) granophyre variety, a more mafic (<66 wt% SiO2) impact melt variety from Vredefort has been discussed for several years. The hypothesis that the mafic granophyre was formed from felsic granophyre through admixture (assimilation) of a mafic country rock component that then was melted and assimilated into the superheated impact melt has been pursued here by analysis of the two granophyre varieties, of the Dominion Group lava (actually metalava), and of epidiorite mafic country rock types. Chemical compositions, including high-precision isotope dilution–derived concentrations of selected highly siderophile elements (Re, Os, Ir, Pt, Se, Te), and Re-Os and Se isotope data support this hypothesis. A first-order estimate, based on these data, suggests that some mafic granophyre may have resulted from a significant admixture (assimilation) of epidiorite to felsic granophyre. This is in accordance with the findings of an earlier investigation using conventional isotope (Sr-Nd-Pb) data. Moreover, these outcomes are in contrast to a two-stage emplacement model for Vredefort Granophyre, whereby a mafic phase of impact melt, derived by differentiation of a crater-filling impact melt sheet, would have been emplaced into earlier-deposited felsic granophyre. Instead, all chemical and isotopic evidence so far favors formation of mafic granophyre by local assimilation of mafic country rock—most likely epidiorite—by a single intrusive impact melt phase, which is represented by the regionally homogeneous felsic granophyre.

scholarly journals Precise radiometric age establishes Yarrabubba, Western Australia, as Earth’s oldest recognised meteorite impact structure

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Timmons M. Erickson ◽  
Christopher L. Kirkland ◽  
Nicholas E. Timms ◽  
Aaron J. Cavosie ◽  
Thomas M. Davison
Keyword(s):  
Western Australia ◽  
Impact Structure ◽  
Meteorite Impact

scholarly journals Shatter cones: (Mis)understood?

2016 ◽  
Vol 2 (8) ◽  
pp. e1600616 ◽  
Author(s):  
Gordon R. Osinski ◽  
Ludovic Ferrière
Keyword(s):  
Accurate Method ◽  
Small Region ◽  
Impact Craters ◽  
Meteorite Impact ◽  
Geological Features ◽  
Cone Formation ◽  
Apparent Diameter ◽  
Shatter Cones ◽  
Evidence Of Impact

Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and “double” cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationshipDsc= 0.4Da, whereDscis the maximum spatial extent of in situ shatter cones, andDais the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact.

Crustal structure northwest of the Sudbury meteorite-impact structure (LITHOPROBE reflection-seismic AGT Line 42)

1997 ◽  
Vol 1 (4) ◽  
pp. 189-201 ◽  
Author(s):  
W. M. Moon ◽  
X. G. Miao
Keyword(s):  
Crustal Structure ◽  
Impact Structure ◽  
Meteorite Impact ◽  
Reflection Seismic

Geochronology in the Lake Superior region

1968 ◽  
Vol 5 (3) ◽  
pp. 715-724 ◽  
Author(s):  
S. S. Goldich
Keyword(s):  
World Wide ◽  
Lake Superior ◽  
Decay Constants ◽  
Minnesota River ◽  
The Past ◽  
Daughter Nuclide ◽  
Igneous Activity ◽  
Radiometric Ages ◽  
Minnesota River Valley

During the past 10 years many radiometric ages have been determined on minerals and rocks in the Lake Superior region. The oldest known rocks are the granitic gneisses in the Minnesota River Valley, which have been dated at 3300 to 3550 m.y. ago. Both K–Ar and Rb–Sr methods have been applied to samples from a number of the metasedimentary formations in the region. The ages, however, appear to be the time of folding or of metamorphism rather than of deposition for which only limits or ranges can be given from the ages for associated igneous and metamorphic rocks.Although considerable progress has been made, significant uncertainties remain in the decay constants and in the analytical measurements. More serious problems, however, are geologic ones, such as the effects of metamorphism and of weathering on the parent–daughter nuclide ratios. Both analytical and geological considerations must enter into any proposal for a time classification of the Precambrian.A three-fold division of the Precambrian with time boundaries at 2600 and 1800 m.y. serves well for the Lake Superior region. In addition, the Keweenawan igneous activity is well dated at approximately 1100 m.y. ago. Terms such as Keweenawan, Huronian, and others are best used locally, and time units of a Precambrian classification that might have world-wide utility should not be tied closely to geographic localities. A single radiometric method, as for example K–Ar largely on micas, is not a satisfactory basis for a classification.

Sign in / Sign up

Export Citation Format

Share Document