CLOUD CREEK IMPACT STRUCTURE

Type: Complex
Age Ma: ~190 +/- 30 ^a – JURASSIC
Diameter: 7 km
Location: N 43° 7’ W 106° 45’
Shock Metamorphism: Planar deformation features (PDFs) in quartz

^a The Cloud Creek structure is dated chronostratigraphicly, formed sometime between the Late Triassic and Middle Jurassic (Tr-J). The structure lies under the Tr-J unconformity.

The approximate location of the Cloud Creek structure in central Wyoming, USA

The circular Cloud Creek structure in central Wyoming, USA is buried beneath ~1200 m of Mesozoic sedimentary rocks. The red circle indicates the dimension of the ~7 Km crater. (Google Earth)

The wing of my bird GOZooM is pointing directly the center of the 7 km wide Cloud Creek structure. For scale, the distance l-r in the image is 4 Km.

Abstract–

Located near the center of the Casper Arch in central Wyoming and buried below the Late Triassic-Middle Jurassic (Tr-J) unconformity at a depth of ~1130 m, the 7-km-diameter Cloud Creek structure displays the morphological characteristics of a complex terrestrial impact crater. Geologic interpretation of the Cloud Creek structure is based on reflection seismic data, supplemented by gravity and magnetics mapping, and by data from ten exploratory wells that penetrated some part of the circular Cloud Creek structure. There is a 1.4-km-diameter central peak enclosed by a ring fault zone, and a 1.6 km-wide encircling skirt sloping into an annular trough that is overlain by a fault-bounded, partial rim anticline. Within the central peak, strongly deformed Paleozoic strata are centripitally uplifted ~520 m (Precambrian uplifted ~200 m) above extracrater levels and truncated by members of the Middle Jurassic Sundance Formation. Crater morphology has been modified by Early Jurassic erosion and an important Laramide overprint. The Cloud Creek impact event occurred near the lower end of the 40-m.y. time gap spanned by the Tr-J unconformity, i.e., near the close of Triassic (~190 +/- 30 Ma). (Stone)

Location of the Cloud Creek impact structure (CL) on a regional tectonic map of central Wyoming, USA. Contours (in ft) are on the Cretaceous Dakota Formation. the stippled areas are Precambrian crystalline rocks. the inset shows the continental US with Cloud Creek in relation to other impact structures as black dots. (Modified from Stone 1999.)

Structural contour map on the pennsylvanian Tensleep Formation (modified from Stone 1999). Contour interval is 200 ft (60 m). Base level datum is mean sea level. The trace of the outer rim fault zone just below the Tr-J unconformity is marked by the largest dashed circle. The smallest dashed circle near the center of the map is the annular fault zone that encircles the central peak. Within this annular peak-ring fault zone (the area marked by a gray stippled pattern), the coutoured tensleep horizon is missing (by ejection and/or erosion) and Amsden Formation rocks are uplifted and truncated at the Tr-J unconformity.

A number of structural elements are visible to identify this complex crater, such as the central raised zone, the faulted rim with anticlinal horizon, and the ring-like trough [Stone and Therriault, 2003]. However, the relative coherent seismic events in the central peak of the Cloud Creek structure reveal a uniqueness. This uniqueness might be suggestive of the existence of the significant erosion, and that what we observe now are the remaining roots of the impact structure. In addition, the TR-J unconformity displayed on the borehole well logs and the severely fractured zone below the unconformity provide the evidence of erosion. Furthermore, based on the scaling relationship pointed out by Melosh [Melosh, 1989], for a crater which is 7 km wide, the central peak is supposed to be 700 m instead of the currently observed 520 m. This evident removal of estimated 200 m strata might be another hint of active erosion events during the deposit environments.(Wei Xie, 2014)

Planar deformation features (PDFs) in quartz have been identified and measured in samples collected from reported intervals 30 m and 330 m below the TR-J unconformity.

Comparison of the “standard” stratigraphic column on the Casper Arch and the section found in the central peak of the Cloud creek impact structure. Stratigraphic uplift (SU) of the Madison Formation in the central peak area above its normal elevation outside the crater is ~520 m. At the Tr-J unconformity, there is progressive onlap of Sundance members over and around the central peak (Stone et al 2003).

Structural cross-section B-B’ restored by flattening on the Sundance datum to remove the effects of Laramide deformation. Formation symbols are identified in the figure to the left. (Stone et al 2003).

References

[see – METEORITE]

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

Melosh, H. J. (1989), Impact cratering: A geologic process, Research supported by NASA. New York, Oxford University Press (Oxford Monographs on Geology and Geophysics, No. 11), 1989, 253 p., 1.

Stone, D. S., Therriault, A. M., The Cloud Creek structure, central Wyoming, U.S.A. – Impact origin confirmed, Meteoritics & Planetary Science, v.38, p.445-455. 2003.

Stone, D. S., Cloud Creek: A possible Impact Structure on the Casper Arch, Wyoming, The Mountain Geologist, vol. 36, No. 4, Rocky Mountain Association of Geologists. 2000.

Xie W. Seismic Characterization of A Possible Buried Impact Structure near Bow City in Southern Alberta Department of Physics University of Alberta

University of New Brunswick