The Wyoming Crater Field: more secondaries, a continuous ejecta blanket, and a possible source crater.

Introduction: Since the initial finding of a large number of secondary craters in southeast Wyoming [1, 2], the search for a source of the secondaries has been a major goal. Two potential locations of the primary crater, in the Denver basin and near the Hartville uplift near the town of Guernsey were proposed [3].

Recent field work was carried out in April 2026 in SE Wyoming, USA to investigate these potential sites of the primary and to prove and sample more secondary crater sites. Generally, accessibility to outcrops is challenging due to the prevalence of privately owned land; permission from landowners is mandatory for access to sites. Here, we report on our findings near the Glendo Reservoir, where Pennsylvanian to Permian lithologies were mapped as the Hartville Formation. We targeted the upper portions of this formation (Hartville divisions 1-3) as they are correlated to the upper Casper Formation, which contains secondaries further to the North and West.

Results: 15 new potential secondary craters were found north of the Glendo Reservoir and are currently being evaluated for shock effects. These secondaries occur as morphologically raised features in the field. Rings of partly brecciated quartzitic sandstones with diameters up 100 m were found. The rings show a stronger quartzitic overprint than surrounding sandstone layers. These quartzitic sandstones commonly form meter-high walls that delineate the rims of the circular to elliptical craters. A tightly spaced cluster of four secondaries with diameters of ~50 m each was found, where crater rims partially overlap. Within the craters the sandstones show strong brecciation and occasional folding, and thus resemble the secondary fields found, e.g., near Douglas [1].

To the south of the secondary craters, zones of intense deformation are found in sedimentary layers of the Hartville Formation. Subvertical fault zones consist of mostly monomict (and rare polymict) breccias that can reach over 10 meters in width. Surprisingly, only minor displacement of the local stratigraphy is visible and is confined to at most a few meters of throw. The intensity and width of these breccia zones do not match typical tectonic fault zones with minor displacement but could be explained through repeated, oscillating movement, e.g., seismic activation during the cratering process. These breccia zones have spacings of several 100 m.

Alongside the subvertical breccia zones, a horizontal bed of ~10 m thick polymict breccias could be traced over a lateral extent of roughly 1 km. The breccias are subdivided into a lower red clay- and silt-dominated zone and an upper grey carbonate-dominated zone. The breccias are located roughly 50 m below the stratigraphic horizon containing the secondaries. The unbrecciated sediment layers immediately above the breccias show a shallow thrust ramp with 2-3 m throw, indicating that the breccia layer may have served as a detachment zone for lateral displacement where softer, unconsolidated clays and silts were more easily deformed.

Finally, polymict breccias are found at the top of the Hartville Formation at a stratigraphic level corresponding to that of the secondary craters. We interpret these breccias as a potential continuous ejecta blanket. At the reservoir, the breccias consist mainly of carbonate clasts in a red silty matrix. Clast sizes here are usually in the cm to dm range and show intense deformation, with breccia thicknesses reaching a few meters. These breccias could be laterally traced over 1 km. Further to the north, decameter thick exposures of polymict breccias are locally found, with one noteworthy outcrop containing block sizes of over 10 m. In all cases, polymict breccias contain only sedimentary clasts, while metamorphic and igneous rocks of the Precambrian basement were not found.

Discussion: The widespread occurrence of monomict and polymict breccias along with some strata displacements and local strata tilting suggests that a section of a larger impact structure and a continuous ejecta blanket is exposed at the Glendo Reservoir. We assume that the largest portion of the crater is buried beneath younger strata. As only sedimentary rocks and no crystalline basement clasts were found within the polymict breccias, this gives a constraint on the maximum possible crater size. Profiles in geological maps from the area show that the thickness of sedimentary layers from the top of Precambrian igneous and metamorphic lithologies to the Permian is between 300-450 m (1000-1500 feet). Using a maximum excavation depth of 1/10 of the transient crater diameter [4], the potential crater should have a transient crater diameter of 4.5 km or less. Applying a maximum complex to transient crater diameter ratio of 2 [5] indicates a primary crater of less than 9 km diameter.

In summary, field results suggest a potential primary crater in close proximity to the Glendo reservoir, with numerous monomict and polymict breccias that are difficult to explain through endogenic or tectonic processes. The primary crater is presumably partially or fully buried in this region. Shock metamorphism is required for the recognition of impact craters [6]. Therefore, thin sections of sampled rocks are currently being scanned for shock features.

 

Acknowledgments: We acknowledge funding by the German Research Foundation, grants KE 732/30-1 and 732/30-2. We express our gratitude to the Reese family and Glendo State Park for granting us access for field work.

 

References: [1] Kenkmann, T. et al. (2018) Scientific Reports 8:13246; [2] Kenkmann, T. et al. (2022) GSA Bulletin, 134(9-10):2469-2484. [3] Sturm, S. et al. (2026) Met. Planet. Sci, submitted. [4] Melosh, H. J. (1989) Impact cratering. A geologic process. [5] Collins G. S. et al. (2020) LPSC #2028. [6] Cavosie, A. J. et al. (2026) Met. Planet.Sci., accepted.