Abstract— The circular Cloud Creek structure in central Wyoming, USA is buried beneath ˜1200 m of Mesozoic sedimentary rocks and has a current diameter of ˜7 km. The morphology/morphometry of the structure, as defined by borehole, seismic, and gravity data, is similar to that of other buried terrestrial complex impact structures in sedimentary target rocks, e.g., Red Wing Creek in North Dakota, USA. The structure has a fault‐bordered central peak with minimum diameter of ˜1.4 km, composed predominantly of Paleozoic carbonates thickened by thrust faulting and brecciation, and is elevated some 520 m above equivalent strata beyond the outer rim of the structure. There is a ˜1.6 km wide annular trough sloping away from the central peak (maximum structural relief, 300 m) and terminated by a detached, fault‐bounded, rim anticline. The youngest rocks within the structure are Late Triassic (Norian?) clastics and these are overlain unconformably by post‐impact Middle Jurassic (Bathonian?) sandstones and shales. Thus, the formation of the Cloud Creek structure is dated chronostratigraphicly as ˜190 ± 20 Ma. Reported here for the first time are measurements of planar deformation features (PDFs) in shocked quartz grains in thin sections made from drill cuttings recovered in a borehole drilled at the southern perimeter of the central peak. Other, less definitive microstructures consistent with impact occur in samples collected from boreholes drilled into the central peak and rim anticline. The shock‐metamorphic evidence confirms an impact origin for the Cloud Creek structure.
Abstract— Historically, there have been a range of diameter estimates for the large, deeply eroded Vredefort impact structure within the Witwatersrand Basin, South Africa. Here, we estimate the diameter of the transient cavity at the present level of erosion as ∼124–140 km, based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted Transvaal outliers. Taking erosion into account (<6 km) and scaling to original final rim diameter, an estimate of close to 300 km for the rim diameter is obtained. Independent estimates of the final rim diameter, based on an empirical relation of central uplift diameter to rim diameter, spatial distribution of pseudotachylites, and concentric large scale structural patterns, give a similar estimate of close to 300 km for the original final rim diameter. An impact structure of this size is expected to have had an original multi‐ring form. At this size, the Vredefort impact structure encompasses the bulk of the Witwatersrand Basin, which appears to owe its preservation to the Vredefort impact. In addition, the Vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area, which, in recent interpretations, is believed to be a component in the creation of the world‐class gold deposits of the Witwatersrand Basin.
The Vredefort impact structure contains a suite of granophyric dykes, referred to as the Vredefort Granophyre, occurring within and at the edge of the Archaean basement core. New whole-rock chemical analyses, together with previous data, represent a complete suite of the Granophyre occurrences. These data show that the Vredefort Granophyre has a remarkable chemical homogeneity, within and between dykes, on a regional scale, and a unique composition ( approximately 67 wt.% SiO 2 , approximately 1 wt.% TiO 2 , approximately 13 wt.% Al 2 O 3 , approximately 7 wt.% Fe 2 O 3 , approximately 3 wt.% MgO, approximately 4 wt.% CaO, approximately 3 wt.% Na 2 O. approximately 2 wt.% K 2 O). Five volumetrically abundant regional lithologies are: Transvaal carbonate, Ventersdorp lava. Witwatersrand quartzite, Witwatersrand shale, and Outer Granite Gneiss. These lithologies are used as components in both harmonic and least-squares mixing calculations to reproduce the Vredefort Granophyre composition. The best-fit mixture is made up of the five target rocks used and corresponds to: approximately 40% lava, approximately 30% quartzite, approximately 25% gneiss, approximately 3% shale, and approximately 2% carbonate. These results are geologically reasonable, given our knowledge of the pre-impact regional stratigraphic succession at Vredefort, but they do not conform with the clast population in the dykes. This is easily reconcilable. however, since previous studies demonstrated that the clast population in impact melt rocks is not necessarily representative of the components, or their proportions, melted to form the melt. Characteristics of the Vredefort Granophyre are similar to those of terrestrial impact melt rocks. Our previous interpretation that the Vredefort Granophyre dykes are injections, and the only remaining evidence of the existence, of the impact melt rocks produced during the formation of the Vredefort impact structure about 2 Ga ago is confirmed by this geochemical study.
Abstract— Hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near‐surface volume of the target body. Local geology of the target area tends to be of secondary importance, and the net result is that impacts of similar size on a given planetary body produce similar results. This is the essence of the utility of observations at impact craters, particularly terrestrial craters, in constraining impact processes. Unfortunately, there are few well‐documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time. Nevertheless, observations of the terrestrial impact record have contributed substantially to fundamental properties of impact. There is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling, in particular from recent hydrocode models. The terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that Z‐models are, at best, first order approximations. Observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures. There are, however, major observational gaps and cases, e.g., Vredefort, where current observations and hydrocode models are apparently inconsistent. It is, perhaps, time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s, which were fueled by observations at terrestrial impact structures. Filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
