Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts

Abstract

Abstract— Clasts of deep‐seated crystalline basement rocks in suevites of the Ries crater, Germany, were catalogued lithologically and classified with regard to their degree of shock metamorphism. The sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior. These clasts can be grouped into seven types of metamorphic and nine types of igneous rocks. One hundred forty‐three clasts, representing these lithologies, were analyzed for major element bulk composition. The fallout suevite contains on average 4 vol% of crystalline basement clasts, 0.4 vol% of sedimentary rocks, 16 vol% of glass bodies (some of them aerodynamically shaped), and 79 vol% of groundmass. On average, 52% of all crystalline clasts are from metamorphic sources and 42% are of igneous origin. Using the shock classification of Stöffler (1974), 8% of all crystalline clasts appear unshocked (<10 Gpa), and 34, 30 and 27% of clasts are shocked to stages I (10–35 Gpa), II (35–45 GPa) and III (45–60 GPa), respectively. The bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations. This indicates that the glasses originate by shock‐fusion of a similarly composed basement. The crater suevite contains the same crystalline rock types that occur in the fallout suevites. The bore hole “Nördlingen 1973” yields an average of 62 vol% metamorphic and 38 vol% igneous rocks. The crater suevite differs from fallout suevites by a higher clast/glass ratio, by preponderance (65–95%) of clasts shocked to stage I only, and by the absence of aerodynamically shaped glass bodies. The source of crystalline clasts and melt particles of suevites is a volume of rocks, located deep in the crystalline basement, to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 GPa (deep‐burst impact model). Fallout suevites were ejected, propelled by an expanding plume of vaporized rock, and withdrew preferentially from this volume melt and highly shocked clasts, leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt.

Post‐impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?

“…Within the pipes, the fine-grained matrix of the suevite has been removed, leaving the filling of coarser fragments that are coated with a light brown layer of undetermined Fe-rich oxide material (Newsom et al 1986). Engelhardt (1997) concluded that there is no coherent impact melt sheet at the Ries crater. Moreover, Stöffler (1977) suggested if there exists such a sheet or lens, deep-seated in the crater centre, it would be of minor size.…”

“…The crater-fill suevites form a massive (up to 400 m thick) layer in the crater depression below the post-impact crater lake sediments (Engelhardt et al 1995). The crater suevite is a polymict breccia with lesser amount of glassy material as compared to surficial suevites (Engelhardt et al 1995;Engelhardt 1997).…”

“…Surfical suevites contain components from the crystalline basement at all stages of shock metamorphism and shockmelted glassy inclusions, specifically vitreous and devitrified impact glass (Engelhardt et al 1995;Engelhardt 1997). Surficial suevites are found in a large number of small isolated patches inside the morphological rim and up to radial distances of ∼14 km beyond the rim to the south-southwest and east-northeast (Fig.…”

“…1). The thickness of the layer is typically from few meters up to 25-30 m (Engelhardt 1990(Engelhardt , 1997; although substantial local variations are likely, given the presence of ∼80 m of surficial suevite in the Wörnitzostheim drillhole (Engelhardt 1990).…”

“…Newsom et al (1986) suggested that ∼10-15 vol% of the montmorillonite is of hydrothermal origin in the groundmass of surficial suevites. However, Engelhardt (1997) and Osinski et al (2004) reported that the suevite groundmass contains up to ∼50 vol% of "clayey" material that is evidently a hydrous silicate (montmorillonite, according to Engelhardt 1997); however, the exact nature and origin of this clayey groundmass remains poorly constrained. Similarly, Ames et al (2004) concluded from mineral assemblages of impactites in core Yaxcopoil-1, Chicxulub impact crater, that the alteration of impact glasses has resulted from low temperature seawater interaction rather than typical high temperature hydrothermal alteration.…”