WYOMING (SHEEP MOUNTAIN) “Multiple Craters”
by: Charles O’Dale
- Type: Multiple Simple
- Centered on: N 42° 40’ 38″ W 105° 28’ 00″a
- Max Crater Diameter: ~80 m
- Age Ma: ~280b – PALEOZOIC
aThe craters are on Wyoming private lands.
b Geologic dating: The impact age is inferred to be immediately after Casper Formation deposition and before the deposition of the Goose Egg Formation Opeche Member. This sedimentological boundary indicates that the impact event occurred in the Lower Permian in the Leonardian North American Stage at +/−280 Myr that correlates with the Kungurian stage of the International chronostratigraphic chart (Kenkmann, T. et al 2018)
A large number of small impact structures have been discovered in Wyoming, USA, and we raise the question of how this accumulation occurred. We document 31 crater structures of 10−70 m diameter with corresponding shock features but missing meteorite relics. All craters occur along the outcrops of the uppermost Permo-Pennsylvanian Casper Sandstone Formation and are ∼280 m.y. old. Their spatial arrangement shows clusters and ray-like alignments. Several craters have elliptical crater morphologies that allow the reconstruction of impact trajectories. The radial arrangement of the trajectories indicates that the craters are secondary craters formed by ejecta from a primary crater whose likely position and size are reconstructed. Modeling ballistic trajectories and secondary crater formation indicates that impacts occurred at around 700−1000 m/s and caused small shock volumes with respect to crater volumes. This is the first field of secondary craters found on Earth, and we disentangle its formation conditions. (Kenkmann, T. et al 2022)
Boulder, Colo., USA: Several dozen small impact craters, 10–70-m in size, have been discovered in southeastern Wyoming. A team of U.S. and German geoscientists found these ancient craters in exposed sedimentary layers from the Permian period (280 million years ago). After discovering the first craters, the team initially suspected that they are a crater-strewn field, formed by the breakup of an asteroid that entered the atmosphere. However, with the discovery of more and more craters over a wide area, this interpretation was ruled out.
Many of the craters are clustered in groups and are aligned along rays. Furthermore, several craters are elliptical, allowing the reconstruction of the incoming paths of the impactors. The reconstructed trajectories have a radial pattern.
“The trajectories indicate a single source and show that the craters were formed by ejected blocks from a large primary crater,” said project leader Thomas Kenkmann, professor of geology at the University of Freiburg, Germany. “Secondary craters around larger craters are well known from other planets and moons but have never been found on Earth,”
The team calculated the ballistic trajectories and used mathematical simulations to model the formation of the craters. All of the craters found so far are located 150–200 km from the presumed primary crater and were formed by blocks that were 4–8-m in size that struck the Earth at speeds of 700–1000 m/s. The team estimate that the source crater is about 50–65 km in diameter and should be deeply buried under younger sediments in the northern Denver basin near the Wyoming-Nebraska border.
2017 GEOLOGICAL EXPLORATION by Jack Schmitt et al
Evidence for a large Paleozoic Impact Crater Strewn Field in the Rocky Mountains
Thomas Kenkmann, Kent A. Sundell, Douglas Cook
The Earth is constantly bombarded by meteoroids of various sizes. During hypervelocity collisions a large amount of energy is coupled to the Earth’s atmosphere leading to disruption of decimeter to hundred meter-sized meteoroids. Smaller meteoroids may form meteorite strewn fields while larger initial bodies and high-strength iron meteoroids may form impact crater strewn fields. Impact crater strewn fields are ephemeral and none documented to date are older than about 63,500 years. Here we report on a newly discovered impact crater strewn field, about 280 Myr old, in tilted strata of the Rocky Mountains near Douglas, Wyoming. It is the oldest and among the largest of impact crater strewn fields discovered to date, extending for a minimum of 7.5 km along a SE-NW trajectory. The apparent width of the strewn field is 1.5 km, but the full extent of the crater strewn field is not yet constrained owing to restricted exposure. We probably see only a small section of the entire crater strewn field. The cascade of impacts occurred in an environment that preserved the craters from destruction. Shock lithification aided this process (2018).
[Images with permission from Doug Cook – paper author]
This research started as a part of the American Association of Petroleum Geologists (AAPG) Eclipse Seminar organized by Doug Cook. Kent Sundell of Casper College had a line on the possible impact craters so we included some field work there before the 2017 solar eclipse. Our headliners were astronaut-geologists Jack Schmitt and Jim Reilly (now Director United States Geological Survey – USGS).
[see – METEORITE]
Thomas Kenkmann, Louis Müller, Allan Fraser, Doug Cook, Kent Sundell, Auriol S.P. Rae; Geological Society of America Secondary cratering on Earth: The Wyoming impact crater field
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Kenkmann, T. et al. Experimental impact cratering: A summary of the major results of the MEMIN research unit Met. Planet. Sci. 53