DOUGLAS WYOMING (SHEEP MOUNTAIN) “Multiple Craters”

Type: Multiple Simple
Centered on: N 42° 40’ 38″ W 105° 28’ 00″^a
Max Crater Diameter: ~80 m
Age Ma: ~280^b – 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)

Secondary cratering on Earth: The Wyoming impact crater field

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.

GROUND EXPLORATION

Coincidentally, while 2017 the exploration of the Sheep Mountain Craters was proceeding, my lady and I witnessed the solar eclipse just north of the crater field.

280 MILLION YEARS AGO A HUGE ASTEROID SLAMMED INTO THE WYOMING/NEBRASKA BORDER

By Phil Plait

The directions of elongated craters and crater chains at the different secondary cratering sites can be extended to triangulate on the probable location of the primary impact, on the Wyoming/Nebraska border. Photo: Kenkmann et al. (2022)

Drone shot of several secondary craters confirmed near Sheep Mountain in Wyoming. Note the elongated shape and how the craters align along it. The crater at the left is about 25 meters wide. Photo: Kent Sundell, Casper College

Drone shot of a 20-meter-wide secondary crater at the Sheep Mountain site. The crater is surrounded by a halo of whitish quartzite, what was originally quartz sandstone before heating and compression from the impact. Photo: Kent Sundell, Casper College

2017 GEOLOGICAL EXPLORATION by Jack Schmitt et al

More than 40 circular to ellipsoidal possible impact structures (centered at the red circle) have been identified on the northeast flank of the Sheep Mountain anticline near Douglas, Wyoming, USA. The position indicators are from my SPOT Personal Tracker.

Evidence for a large Paleozoic Impact Crater Strewn Field in the Rocky Mountains
Thomas Kenkmann, Kent A. Sundell, Douglas Cook

ABSTRACT
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]

Simplified geological sketch and location of confirmed and possible impact craters at the NE slope of Sheep Mountain anticline, WY, USA. All craters occur in the uppermost Casper Fm. (+/−280 Myr) at the immediate contact to the Goose Egg Formation, Opeche Shale Member. The change in depositional environment may have allowed the shock lithified craters to be submerged and preserved by muds in a quiescent paralic lagoon transgression. The soft mudstone cap was easily eroded recently to expose the shock hardened craters. Note that the entire crater strewn field on the anticline is tilted by about 15°NE (vertical exaggeration of 1.5).

(a) Eastern slope of Sheep Mountain anticline. Selective erosion exposed the contact between Casper Sandstone and the Opeche Member of the Goose Egg Formation, where the craters were discovered. (b) Panorama view of crater 1, observed from the SSW crater rim. (c) Downrange front of the meter thick, coherent ejecta flap of crater 4. (d) Partly eroded crater 15 displays uplifted and folded Casper sandstone along the crater rim. (e) Injected dike with sub-rounded pebbles and vesicles, (crater 78). (f) Pocket with sub-rounded cohesive sand fragments in a fine sand matrix. This type of monomict soft-sediment brecciation occurs frequently along the raised rims of craters 34 and 36.

Drone images of craters 1 to 5 (a) Crater 1 is 60 m in diameter along the NW-SE trajectory. Its NE flank is eroded. Crater 1 samples have shocked quartz grains. (b) Crater 2 has a 31 m long axis and ovoid shape. The apparent overturned flap is well-preserved downrange. (c) Craters 3, 4 and 5 interfere with each other and together form a highly elliptical cavity in NW-SE direction. Crater 5 possibly formed by ricochet of the projectile. Dotted red and blue lines outline the crater cavities and the preserved ejecta blankets, respectively.

Deformation microstructures in quartz grains of Crater 1. Except for (e) all photomicrographs were taken under crossed polarizers, path difference is added in f, g, and h. (a) Cross-cutting and fluid-decorated planar deformation features (PDFs) in quartz grain. Spacing is 3.5 µm on average. (b) Crystallographic orientation of lamellae of (a). (c) Quartz grains with fluid-decorated basal PDF lamellae along (0001), spacing is 5–6 µm. (d) Narrow-spaced PDF lamellae parallel to the c-axis. The spacing is 2.5 µm. (e) Planar fracture along (0001). (f) Concussion fracture (right) is massively decorated with fluid inclusion. Boehm lamellae on the left. (g) Grain with high density of fluid-decorated fractures. Note that the fractures end at the round shaped original grain surface and do not extend into the syntactic overgrowth seams suggesting that the impact occurred prior to diagenesis. (h) Indentation and interlocking of quartz grains led to shock lithification. Hertzian-type concussion fractures follow point-to-point contacts and stress chains through the grains. These fractures are tensile fractures and are filled with fluid inclusions.

SIDE NOTE

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).

Jack Schmitt at Douglas crater SM-1. Photo credit Doug Cook (current Chair of the AAPG Astrogeology Committee).

References

[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