Many Solar System bodies, including our Moon, are covered by enormous numbers of impact craters. On Earth, in contrast, the total number of such craters discovered stands at 208. This is largely due to the Earth's active surface, with the continents and continental shelves being subjected to constant erosion and deposition of sediments, and the ocean floors being constantly recycled through subduction and seafloor spreading. However, the situation is more complicated than it seems at first sight; many of the craters seen on the Moon and other Solar System bodies are in fact secondary craters, formed by debris ejected from larger impact events. Once understood, such craters have been relatively easy to identify, frequently being elliptical rather than circular in shape, shallower than primary craters, and arranged radially around the initial impact crater. The implications of this are debatable, with some planetary scientists arguing that as many as 95% of small craters on some bodies may be secondary in origin, whereas others see them as a insignificant proportion of the total number.
In a paper published in the journal GSA Bulletin on 11 February 2022, Thomas Kenkmann and Louis Müller of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, Independent Consultants Allan Fraser and Doug Cook, Kent Sundell of the School of Science at Casper College, and Auriol Rae, also of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, describe the discovery of a secondary impact field comprising at least 31 craters, and possibly as many as 60 more, in southeastern Wyoming, USA.
The presence of a field of impact craters in the Rocky Mountains in Wyoming was first reported in 2018 by Thomas Kenkman, Kent Sundall, and Douglas Cook. At the time they reported about 40 circular-to-elliptical structures on a tilted Permian exposure on the northeast flank of the Sheep Mountain, eight of which showed sufficient grain deformation to be confirmed as impact structures. These were initially interpreted as the result of a single large object which broke up as it entered the atmosphere, resulting in a group of closely clustered craters.
The new paper by Kenkman et al. describes the presence of several other craters on Sheep Mountain Ridge, and other exposures of the same age at Wagonhound Ridge, Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road, as well as possible craters at several other locations. These range from 10 to 80 m in diameter, and while many are circular, some are as much as 1.7 times as long as they are wide. All of the structures which are firmly established as being craters lie at the top of the Casper Formation, which is immediately overlain by the Opeche Shale member of the Permian Goose Egg Formation, dating the impacts to about 280 million years ago, making them late Early Permian in origin.
The Casper Formation is made up primarily of aeolian sandstones (i.e. sands laid down in a terrestrial desert or dune environment), although the uppermost portion of the section, where the craters are preserved, represents a marine transgression into this environment, forming a lagoon or sabkha environment (a sabkha being a coastal saltpan regularly refilled by tidal waters and emptied by evaporation). The craters sometimes form pedestals standing above the eroded surface of this formation, resulting from lithification processes of associated with the impacts, such as shock fusion (welding together of particles by a sudden impact), and cementing by a glassy melt formed within the crater.
The Casper Formation, on the upper surface of which the craters are preserved, has very little surface exposure, 1% of in Converse County and only 4% in Albany County, suggesting that many more craters may be preserved buried beneath the centre. This area lies within the Laramide Mountains, an area within the wider Rocky Mountains where Late Cretaceous to early Eocene (~75-50 million years ago) reverse faulting and folding uplifted an area of Archean basement rocks and the material overlying it. Most of the known craters lie upon the Sheep Mountain flank of the Sheep Mountain anticline, an extended fold-ridge mountain running from northwest-to-southeast, where the Opeche Shale has eroded back revealing the upper surface of the Casper Formation; it has been calculated that about 2 km of overlying rocks have been eroded away here since the onset of the orogeny; strata that are still likely to be in place in other areas, covering up other craters associated with this field.
Wagonhound Ridge is a similar, and associated, structure to the south of Sheep Mountain, showing slightly less uplift. The Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road exposures are found on the southwestern slope of the basement uplift of the Laramie Mountains in transition to the Shirley Basin, where the Casper Formation has largely been eroded away, but is exposed on several remaining buttes.
The largest cluster of impact-related structures is found on the northeastern flank of the Sheep Mountain, where a series of circular, irregular-shaped, and ellipsoidal have been confirmed as impact craters. These vary in their preservation quality from pristine to heavily eroded, with erosion apparently linked to the recent exposure of structures which were rapidly buried after their formation; i.e. the most eroded structures are located higher on the flanks of the mountain. The most pristine structures show steep crater walls, raised rims with overturned ejecta flaps, and remains of proximal ejecta blankets. These craters have floors are covered by soil and filled with muds derived from the overlying Opeche shales, making it hard to establish their depth-to-diameter ratio. Many of these craters are elliptical-to-ovoid in shape, with their long axes having fairly consistent orientations of 315–328°. The distribution of ejecta around the craters is uneven, with well-developed overturned flaps on their northwestern sides, suggesting they were caused by debris thrown from a primary impact to the southeast. Four of the craters form a chain, with a similar orientation. The more eroded structures further upslope tend also to be more rounded, with an internal ring structures. These often stand proud of the eroded surface, being more resilient to erosion due to the shock-fusion of the sandstone.
Ten possible craters have been found on the exposed surface of the Casper Formation at Wagonhound Ridge. Two of these have been confirmed as definite craters due to elevated rims. These are again filled with soil, and slightly elliptical.
At Mule Creek a large elliptical crater measuring 56 m by 44 m has an orientation of 284±5°. A second structure, measuring 30 by 27 m is adjacent to this. Neither of these are elevated above the surrounding rock surface, and neither preserves any rim structure or surrounding breccia. It is thought that these represent the lower portions of larger craters that have been mostly eroded away; the larger of them appears to be surrounded by a larger ring at a distance of about 100 m, possibly representing underlying rocks that were consolidated by the impact. This area is cut through by a north-south and northwest-southeast–trending tectonic joint system visible in remote sensing images, which would have served to hasten erosion in this area. This joint system extends about 300 m to the northwest of the main crater, and contains at least nine irregular, soil-filled depressions which might represent further impacts. Another cluster of possible craters, one of which has been confirmed as an impact structure, is found about 2.5 km to the southeast of the main crater at Mule Creek. These structures have crater rims composed of sandstone breccia sealed with chert matrix.
At Fetterman Ridge a series of erosional buttes have exposures of the upper surface of the Casper Formation which have been eroded away from much of the surrounding landscape. One of these, a hill measuring roughly 200 m by 100 m, hosts three impact craters, measuring 30 m by 22 m, 28 m by 17 m, and 10 m by 10 m. The long axes of the two elliptical craters trend west-north-west to east-south-east, although their southern rims are more eroded. The 28 m by 17 m is distinctly elevated on its northwestern rim, with a visible ejecta flap on its western side. Again, the brecciated and fragmented rocks are sealed by microcrystalline silica, making them resistant to erosion. Other buttes to the northwest and southeast show possible additional impact craters, although these are more heavily eroded.
The Fetterman Road cluster comprises six possible craters 7–30 m in diameter, some of which are distinctly eliptical. The most distinct of these is 25 m by 15 m and has an orientation of 296°. The rim of this crater is elevated about a metre above its interior.
Eight possible craters are present at Palmer Canyon Road, about 10 km to the southeast of the Fetterman Road cluster. The two most conspicuous of these measure 42 m by 40 m, and 28 m by 19 m. Again, elevated rims are composed of quartzitic breccia with microcrystalline chert fill.
All of the discovered craters are on the upper surface of the Casper Formation, and all are in sandstones, but the nature of these sandstones varies slightly from site-to-site, reflecting an environment which was fully sub-aerial in the southwest, passing through a braided-river system into a shallow marine environment with some carbonate deposition in the northeast. The presence of water in the sands in some environments does not appear to have led to degradation of the crater rims. In all cases the craters were buried beneath Opeche Shale Member red beds of the Goose Egg Formation rapidly after their formation.
Brittle deformation, indicative of sudden physical shocks, can be seen on both large and small scales. The crater rims and ejecta all show brecciation (breaking into angular fragments) and brittle deformation, while individual grains are often intensely fractured.
Microstructures related to impact. (A) Crosscutting {1013} and (0001) PDF lamellae in sample from crater SM-19 are decorated by fluid inclusions. (B) {1013} PDF lamellae in sample from crater MC-1. A + B show that shock effects are restricted to the detrital grains while the overgrowth is undeformed. (C) Relatively wide-spaced planar fracture lamellae in sample from crater WR-4. (D) Concussion fractures in adjacent quartz grains emanate from initial grain contacts. (E) Crater SM-28 contains abundant chert layers and chert lumps that are embedded in the sandstone. Some of the chert lumps contain spherical lapilli. For interpretation, see text. (F) Close-up of a spherical lapillus that is interpreted as an accretionary lapillus. The concentric rings are composed of microcrystalline quartz around a dark-colored center. All photomicrographs were taken with crossed polarizers. Kenkman et al. (2022).However, shocked grains are somewhat rare in the crater sediments, with slides made up from samples taken in the field typically showing only two-or-three shocked grains, and these usually being surrounded by unshocked grains rather than clustered together. Shocked grains were found in all parts of the crater structures, and at a much lower fequency level, outside the craters in undeformed sands, probably indicating their having been blown from craters by winds shortly after their formation. The degree of fracturing implies that these grains were subjected to pressures in excess of 10 gigapascals.
Cherty (amorphous) silica is present at all sites, often forming the matrix which binds the sand grains together. Investigation of one of the craters at Sheep Mountain found a variety of structures within this chert, including elongated shapes and wavy layers. Within the chert were spherical structures resembling accretionary lapilli; glassy grains which are typically associated with violent volcanic eruptions, formed by the accretion of glassy siliica layers onto grains suspended in hot, turbulent air.
The Wyoming Crater Field shows a number of features that help the reconstruction of the direction being travelled by impactors. The most obvious of these are oval or elliptical shaped craters, and craters arranged into chains, which gives the orientation of the direction of travel, but not the actual direction. However, direction of travel can be determined by using the following lines of evidence: (1) a steeper crater wall uprange; (2) a preserved overturned ejecta flap downrange, with a preferred deposition of ejecta downrange; (3) an ovoid crater shape with the strongest curvature downrange, and (4) V-shaped herringbone patterns of ejecta pointing up-range.
The area has been subjected to some deformation since the impact craters formed, but none appears to have had its orientation changed by more than about 5%, enabling the use of craters from different sites to attempt to relocate the site of the original impact. Based upon this, Kenkman et al. suggest that the original impact happened at a site with map co-ordinates close to 41°28′N, 103°59′W; all of the craters lie between 150 km and 200 km from this site.
Modelling of possible trajectories of objects thrown from a primary impact crater suggest that boulders with a diameter of 2 m would need an initial velocity of 3-4 km per second to reach 150-200 km from the initial impact, while 4 m objects could reach this distance with an initial speed of 2 km per second would also fall within this zone. Such objects would deliver energy in the range of 12 to 400 gigajoules when they impacted, arriving at angles of between 45° and 60°.
Kenkman et al. calculate that an object with a density of 2500 kg per cubic meter (typical for many rock types), with a 4 m radius impacting at 1 km per second would create an impact crater about 45 m in diameter. A similar object 2 m in radius would create a crater 25-30 m in diameter, depending on its angle of approach. A range of objects between 1 and 4 m in diameter, travelling at between 500 and 1500 m per second, would generate craters between 8 and 55 m in diameter, with larger impacts releasing more energy and creating more shocked material as a proportion of the impactors mass.
Kenkman et al.'s findings reveal a series of clusters of craters across a wide area of southeast Wyoming, all of which appear to have formed simultaneously about 280 million years ago. These are best explained as secondary craters caused by material thrown from a large primary impact crater. No obviously foreign material was found in any of the craters, making it likely that the impacting material was similar in composition to the rocks of the areas impacted, and reconstructions of the direction of travel suggest the primary impact was between 150 and 200 km from the discovered craters.
Based upon the reconstructed size and impact speeds of the ejecta material, Kenkman et al.
predict that the original impactor would have been 2.0-2.7 km in
diameter, and to have hit the ground at about 20 km per second, creating
a crater 50-65 km in diameter.
This would place the location of the original impact in Goshen or Laramie counties in Wyoming, or Kimball, Banner, or Cheyenne counties in Nebraska. These areas fall within the Denver Basin, and Permian strata are deeply buried beneath subsequent Mesozoic and Cainozoic deposits. Despite this apparent lack of accessibility, these deposits have been heavily boreholed by hydrocarbons exploration companies, which may enable reconstruction of the original impact site. To date, Kenkman et al. have not found evidence of distorted or missing sedimentary strata which might be associated with such an impact in any well log data examined, but one drill core, 1–35 Hawk Fee, does show a breccia layer at 3023–3066 m below the surface, and Kenkman et al. are hoping to visually inspect this core in the near future.
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