Kenkmann, and two co-authors, reported in 2018 that the Earth is constantly bombarded by meteoroids of various sizes, as late as the 1960s professional were debating whether surface circular structures were either igneous/volcanic intrusions or meteor/comet impact sites. The arguments were generally settled when impact criteria (shocked quartz, cone-in-cone structures, microtektites, and other microscopy features (see: Wise and Campbell (2018), pp. 128-131, and other discussions on the local impact craters in Wyoming in the cited report) became well known and accepted by the geoscientific profession. There are a few remote sites that are still under debate, e.g., the eye of Africa, etc.
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. The authors report herein 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.
The authors concluded that the buried 7 km diameter Cloud Creek impact crater lies exactly downrange on the inferred impact trajectory some 120 km NW of the Douglas crater strewn field. However, the age of this buried crater, constrained by well logging, paleontology, and seismic surveying, is 190±20 Myr. So a relationship to the Douglas crater strewn field is ruled out if the age of Cloud Creek is correct. The current state of evidence indicates a large 7.5×1.5 km impact crater strewn field formed by a violent disruption of a single body of yet unknown size and composition during passage through the atmosphere. In case the strewn field is wider than a few kilometers alternative scenarios should probably be taken into account. Among them are:
i) The craters represent a section of a secondary crater field around a large, yet unknown crater, that should exist several hundreds of kilometers SE of the current exposure. Secondary craters are intensely studied on the Moon and on Mars. Their origin is obvious if the fields of small craters surround large primary craters and exhibit distinctive morphologies such as shallow, irregular shapes and occurrence in chains and clusters, sometimes with distinctive herringbone patterns. Secondary impact velocities can generate stress pulses exceeding elastic limits of rocks and may cause shock. However, secondary crater fields on Earth are as yet unknown and the geophysical evidence of a large buried crater is so far lacking in the suspected region.
ii) Multiple air bursts of a single meteoroid at a very high altitude may form a very wide, unprecedented impact crater strewn field. The fragmentation process would be affected by the atmospheric density at the 280 Myr impact event. It is known that the Earth experienced a major O2 pulse during the Late Paleozoic with an O2 concentration of 35%.
iii) A paired meteoroid or an asteroid break-up prior to atmospheric entry or an asteroid shower may have been responsible for a wider extent of the crater field. There are recent examples of asteroids with companions. The Ries-Steinheim impact is an example of a terrestrial-paired impact. As long as the extent of the impact-crater field is not fully constrained, modeling of the event is not meaningful. Additional craters on Sheep Mountain and outlier areas must demonstrate shocked rocks. This ongoing study will also apply geophysical and geochemical means to address these intriguing questions.
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