Impact ejecta page
Terms written in italics are in general explained in the Impact Structure and Meteorite Crater Glossary
Impact ejecta is a special group of sediments comprising material that is thrown out from an impact crater in the excavation stage and deposited around it. Impact ejecta are in general extremely heterogeneous due to the target conditions (e.g., continental, marine), target lithostratigraphy (crystalline, sedimentary, mixed target), the complex excavation processes (including, e.g., the hypothesized superficial spall plate ejection, the hypothesized jet to form tektites), and the complex ejecta landing and emplacement processes. The latter include secondary cratering and mixing with local material, and computer modeling suggests that in very large craters excavated ejecta may even mix with collapse material in the modification stage of impact cratering.
Globally, impact ejecta are subdivided into proximal and distal ejecta, and it is easily understandable that the size of the ejected clasts on the whole decreases with increasing distance of deposition. Whereas the size of proximal ejecta clasts may reach to the order of kilometer, microscopic distal ejecta may in very large impacts extend worldwide (e.g., in the K/T boundary Chicxulub impact).
Although the thickness of proximal impact ejecta (by definition, exposed < 5 crater radii from the point of impact) may be considerable (several hundred meters, e.g., at the Mid-Tertiary Azuara impact structure, Spain - the Pelarda Formation), impact ejecta in contrast to the Moon and other planets are rare on Earth primarily as the result of rapid erosion.
In many cases, proximal impact ejecta can sedimentologically well be described as diamictites and breccias including megabreccias (e.g. the Alamo and the Azuara megabreccias) and multiple breccia generations (breccias-within-breccias). The diamictic character of many impact ejecta has established the hypothesis (e.g., Rampino M.R.: Tillites, diamictites, and ballistic ejecta of large impacts. - J. Geology, 120, 439–456, 1994) that glacigene tillite deposits may in fact have originated from large impacts (the ice age paradox). This hypothesis is thus far not supported by compelling evidence.
Impact ejecta may clearly be identified by impact melt rocks, glass, suevite breccias, characteristic high-pressure/short-term deformations and the occurrence of shock metamorphism and macroscopic shock effects like shatter cones. Distal ejecta may show spherulitic texture. In old ejecta deposits, the impact signature may largely be overprinted by weathering and various metamorphic processes.
Figs.1 - 4. Various aspects of the Ries (Nördlinger Ries) impact ejecta (Bunte breccia ejecta) resting on autochthonous Jurassic limestones. In the Ries crater Bunte Breccia ejecta, components that were stratigraphically separated by hundreds of meters in the target can now be observed in close contact. This is due to the ejection trajectories in the complex excavation flow field.
Fig. 1. Gundelsheim quarry (about 20 km away from the crater center).
Fig.2. Gundelsheim quarry (about 20 km away from the crater center).
Fig. 3. Ronheim quarry (about 14 km away from the crater center).
Fig. 4. Ronheim quarry (about 14 km away from the crater center).
Fig. 5, 6. Ries crater ejecta in the Aumühle quarry (roughly 12 km from the crater center) comprising suevite (greyish) and multicolored Triassic and Jurassic sediments. The strongly shocked and glass-rich suevite was deposited after the emplacement of the Mesozoic ejecta.
Fig. 6. Ries crater ejecta in the Aumühle quarry.
Fig. 7. Ries impact crater: dislocated Malmian megablock as part of the ejecta blanket. Iggenhausen quarry, about 12 km from the crater rim. Although the limestone is fractured and grit-brecciated through and through, the megablock sized of the order of a few hundred meters has remained coherent despite a transport range of at least 12 km. For early geologists, at the time familiar with the volcanic origin of the Ries crater, these enormous megablock displacements were a worry.
Fig. 8. Rubielos de la Cérida (Spain) impact ejecta tunneling well-bedded autochthonous Cretaceous sediments near Escucha. Impact ejecta emplacement can be strongly errosive as shown in the image.
Figs. 9 - 12. Azuara and Rubielos de la Cérida (Spain) impact ejecta exhibiting typically diamictic facies.
Fig. 9. Puerto Mínguez ejecta, Rubielos de la Cérida
Fig. 10. Puerto Mínguez ejecta, Rubielos de la Cérida.
Fig. 11. Puerto Mínguez ejecta, Rubielos de la Cérida.
Fig. 12. Pelarda Formation ejecta, Azuara.
The Spanish impact ejecta shown here in typical exposures demonstrate that many geologists are simply helpless when confronted with these sediments. In the official geological maps the Puerto Mínguez ejecta are generally described as conglomerates. Geologists from the Zaragoza university now as before consider them to be fluvial deposits, and even experienced sedimentologists have classed the Spanish impact ejecta to be conglomerates (J. Smit, 2000, written communication).
Impact structures with well preserved proximal ejecta are listed below together with a few links:
Ries http://www.impact-structures.com/germany/ries.htmChicxulub http://adsabs.harvard.edu/abs/2003EAEJA.....6925O
Boltysh http://www.lpi.usra.edu/meetings/impact2000/pdf/3042.pdf
Azuara http://www.impact-structures.com/spain/controversy/pelarda.html
Rubielos de la Cérida http://www.impact-structures.com/spain/rubie/ejecta.html
Haughton http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1579.pdf
Tüttensee http://www.chiemgau-impact.com/artikel2.pdf
The Impact ejecta page is so far provisional and will be considerably enlarged.