The formation, cooling, and alteration of the impact ejecta deposits at the Ries crater are an analog for impact processes and the formation of regoliths and soils on Mars. We have studied the suevite deposit, an impact‐melt‐bearing breccia found outside of the crater rim. This suevite contains large numbers of chimney‐like degassing pipes, apparently formed by high gas pressures in the hot (>500°C) fluidized suevite immediately after deposition of the suevite. The likely sources of the gas are volatiles from the melted and shocked basement inclusions. The suevite outside of the crater rim contains about 15 wt % clay, indicating a large amount of alteration of the suevite. The clay is an Fe‐rich montmorillonite with less than 5% interstratified illite, indicating that the temperature at which most of the alteration occurred was less than 100°–130°C. The duration of this low‐temperature period may have been extended by additional heating from the exothermal alteration reactions. The surfaces of the degassing pipes are not altered to clays, however, but have a thin Fe‐rich oxide or hydroxide coating. The absence of alteration in the pipes suggests that the main alteration event outside the crater occurred under unsaturated conditions, such that the walls of the pipes were dry. Suevite deposits are likely to exist on Mars, because of the presence of volatile‐rich target rock. Martian suevites may have also been fluidized and hydrothermally altered, contributing significantly to the formation of the clay‐like phases inferred to exist in the Martian soil.

ZusammenfassungSuevite sind Impaktbreccien, die in einer montmorillonitischen Matrix geschockte und ungeschockte Mineral-und Gesteinsfragmente aus dem kristallinen Untergrund, Glas-Einschlfisse und eine geringe Menge von sediment~iren Fragmenten enthalten. Es werden Daten fiber die modale Zusammensetzung der Fall-out-Suevite (abgelagert an isolierten Punkten rings um den Krater) und des Krater-Suevits (der eine Schicht unter den nach-Riesischen Seesedimenten im Krater bildet) mitgeteilt. Fall-out-Suevite enthalten aerodynamisch geformte Bomben, die im Krater-Suevit fehlen. Unter Berficksichtigung der feineren Glaskomponenten -nicht nur der groBen Bomben und Fragmente -erh~ilt man ffir den Glasgehalt der Fall-out-Suevite 47 vol%, fiir den des Krater-Suevits 29 vol%. Die kristatlinen Einschtfisse in den Sueviten geh6ren zu den magmatischen und metamorphen Gesteinstypen, die den Untergrund des Riesgebietes aufbauen. Dieser besteht aus einer h6he-ren Lage magmatischer Gesteine (haupts/ichtich Granite) und einer tieferen Serie von Gneisen und Amphiboliten. Auf Grund einer Aufsammlung von 1 200 Kristallineinschliissen aus 13 Suevit-Vorkommen wurde die H~iufigkeit der einzelnen Gesteinstypen in den Sueviteu bestimmt. Die Suevite enthalten im Mittel 46 % magmatische und 54 % metamorphe Gesteinseinschlfisse. Im Gegensatz dazu bestehen die nicht oder nur wenig geschockten kristallinen Ejekta um den Ries-Krater aus 82 % magmatischen und 18 V0 metamorphen Gesteinen.Die mittleren chemischen Zusammensetzungen der kristallinen Gesteinstypen wurden auf Grund yon 138 chemischen Analysen bestimmt. Ein Vergleich mit der Zusammensetzung der Suevit-Glaser ergibt, dab diese durch die Aufschmelzung der Gneise gebildet wurden, welche in einem tieferen Niveau des kristallinen Untergrundes lagen.Die AbstractSuevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement, glass inclusions and a small amount of sedimentary clasts. Data are given of the modal composition of fall-out suevites (deposited at isolated points around the crater) and crater suevite (forming a layer below post-impact lake sediments in the crater cavity). Fall-out suevites contain aerodynamically shaped bombs which are absent in crater suevite. Taking into account not only large glass fragments and bombs, but also the finer fractions, the glass content of fall-out and crater suevites amounts to 47 and 29 vol%, respectively. Crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks (mainly granites) and a lower series of gneisses and amphibolite. Based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined. Suevites contain on the average 46 % igneous and 54 % metamorphic clasts. In constrast, weakly shocked and unshocked crystalline ejecta of the Ries structure consist of 82 % igneous and 18 % metamor...

Early events during the formation of the Ries crater include ejection of moldavite tektites and deposition of the Bunte breccia outside of the uplifted inner crystalline ring of the crater. The restricted location of the moldavites may suggest an oblique impact at the Ries. The Bunte breccia ejecta was emplaced relatively cold and was probably a continuous ejecta blanket. Impact-melt-bearing breccias (suevite) were deposited inside the crater (crater suevite) and deposited on top of the Bunte breccia outside the crater (fallout suevite). The presence of aerodynamically shaped glass bombs in the fallout suevite indicates solidification during high-speed travel through the atmosphere, while the existence of fluid-drop chondrules and glass spherules in the fallout suevite and the upper sorted suevite within the crater indicates solidification of some of the melt in the absence of atmosphere or at low relative velocity to the atmosphere. Accretionary lapilli are also found in the upper sorted layer of the crater suevite, suggesting condensation of water vapor in a cloud over the center of the crater, similar to those produced by the rise of a nuclear fireball, or a Plinian eruption column. A thin, fine-grained layer at the base of the fallout suevite could be a lateral extension of the sorted crater suevite, or a depositional feature, similar to those seen in ignimbrite deposits. Following deposition of the suevite, formation of low-temperature (<100°C) clays by hydrothermal alteration associated with cooling of the melt-bearing deposits is supported by recently obtained paleomagnetic data. Several of the features inferred for the Ries crater, such as an oblique impact, the cloud over the crater, and the ignimbrite-like depositional features may also have been present during larger events such as the Cretaceous/Tertiary boundary event.

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