Impactites and ejecta 10

Impactites and ejecta deposits are exceptionally well preserved at the Ries crater. The sequence of impactites preserved at the Ries crater include: a) a thick series of crater-fill rocks (‘crater suevite’); b) various proximal ejecta deposits preserved up to a radius of ~37 km from the crater centre (Fig. 2.1); and c) a tektite strewn field extending out to distances of 260 – 400 km east and northeast of Ries (Hörz 1982). The allochthonous crater-fill units occur within the inner basin and consist of ‘crater suevite’ overlain by ~400 m of post-impact lacustrine sedimentary rocks (Pohl et al., 1977) reflecting the existence of a post-impact crater lake. There are four main types of proximal ejecta identified at the Ries impact crater which overlie the outer zone of the structure: 1) Bunte Breccia and megablocks; 2) polymict crystalline breccias; 3) ‘surficial’ suevites; and 4) coherent impact melt rocks (Engelhardt 1990; Osinski 2004; Fig. 2.1).

The Bunte Breccia is the most abundant proximal ejecta unit by volume. Outcrops of this poorly sorted, glass free, polymict breccia have been interpreted as remnants of a continuous ejecta blanket which was emplaced along ballistic trajectories (Oberbeck 1975; Morrison & Oberbeck 1978; Hörz 1982; Hörz et al. 1983), The Bunte Breccia is derived predominantly from the uppermost sedimentary target sequences (Hörz 1982; Hörz et al. 1983). The Bunte Breccia is comprised of two main components: a) primary ejecta excavated from the initial crater that is dominantly sedimentary rock with subordinate admixtures of crystalline material (predominantly granites); and b) local material or secondary ejecta. The secondary ejecta zone includes deposits of primary ejecta that have been re-mobilized and incorporated by the secondary cratering action of the primary ejecta (Hörz et al. 1983). Megablocks are defined as “displaced fragments of all stratigraphic units of the target rocks, which are larger than 25 m in size and can be mapped geologically” (Pohl et al. 1977, p. 354).

The Polymict crystalline breccias are mixtures of crystalline rock fragments of different lithologies and shock levels (Pohl et al. 1977). Rare irregular outcrops (a few tens of meters in size) of the polymict crystalline breccias occur overlying the Bunte Breccia in the inner ring and megablock zone (Engelhardt 1990). Stratigraphic relationships between

the polymict crystalline breccias and the Bunte Breccia are not always clear (Pohl et al. 1977).

The surficial suevite (after Engelhardt et al. 1995) is distinct from the crater-fill suevite. Isolated outcrops of surficial suevite overlie the Bunte Breccia inside the morphological rim of the Ries Crater and up to radial distances of ~14 km beyond the rim to the south- southwest and east-northeast (Engelhardt 1990; Fig. 2.1). The surficial suevite was deposited on the uneven surface of the upper Bunte Breccia. Deposits of the surficial suevite range in thickness from a few meters to ~25 – 30 m (Engelhardt et al. 1990). The Wörnitzostheim drill hole within the megablock zone penetrated ~80 m of suevite. The surficial suevite contains lithic, mineral and glass clasts hosted within a dominantly montmorillonite (30 – 40 vol%) and glass (30 – 50 vol%) groundmass which constitutes ~ 80 vol% of the suevite units (Engelhardt 1990). The remainder of the groundmass is composed of fine-grained lithic and mineral clasts. The abundance of calcite within the groundmass is variable accounting for up to 40 – 50 vol% (Graup 1999), In contrast to the Bunte Breccia, crystalline material dominates the lithic clasts hosted within the suevite (e.g., Pohl et al. 1977; von Engelhardt & Graup 1984; Engelhardt et al. 1995). However, a new road cut exposes suevites that conation ~8 vol% limestone clasts (Srebenschock et al. 1998). Glasses within the surficial suevite occur as either angular or amoeboid particles (Engelhardt 1990). Bringemeier (1994), divided the surficial suevite into two distinct lithologic units: 1) dominant main suevite that represents a clast-rich impact melt rock emplaced via impact melt flows (Osinski et al. 2004); and 2) subordinate basal suevite, a fall-out suevite, sensu stricto.

The well-consolidated main suevite forms the bulk of all surficial suevite outcrops. The main suevite contains abundant glass, mineral and lithic clasts. There are no indications of sorting or layering (Engelhardt et al. 1995) and the preferred horizontal orientation of flat glass clasts constitutes the only observed textural regularity (Engelhardt & Hörz 1965; Bringemeier 1994). Glasses within the main suevite occur both as groundmass phases and as discrete glass clasts (Osinski 2004). Glass clasts are typically vesiculated, schlieren- rich mixtures containing abundant mineral and lithic fragments (Engelhardt & Hörz 1965; Engelhardt 1972; Stähle 1972; Pohl et al. 1977; von Engelhardt & Graup 1984;

Engelhardt et al. 1995; Vennemann et al. 2001; Osinski 2003, 2004), identified four main glass types present within the main suevites.

The groundmass of the main suevites is defined after Osinski (2004) as the fine-grained material that encloses fragments of shocked/unshocked target material exclusive of any identifiable mineral and lithic clasts (>10 – 20 µm across). In a recent study Osinski et al. (2004) characterized the groundmass, sensu stricto, of the main suevites and has interpreted the groundmass phases as a series of impact melts on the basis of observable textures in SEM BSE. The discrete groundmass components include: silicate mineral and lithic fragments (8.9 – 50.1 vol%), carbonate mineral and lithic fragments (0 – 12.0 vol%), angular impact glass clasts (0 – 18.3 vol%), crystalline calcite (0 – 42.6 vol%), fine-grained clay minerals (1.6 – 70.6 vol%), impact glass comingled with calcite and clay (0 – 16.6 vol%), Fe-Mg-rich plagioclase (0 – 7.5 vol%), rare garnet and pyroxene crystallites (<0.5 vol%), francolite (carbonate-hydroxy-fluor-apatite; 0 – 5.3 vol%), Ba- rich phillipsite (Ca-K-Ba zeolite; 0 – 34.2 vol%). Vesicles can comprise up to several vol% of a sample. The main surficial suevites are typically groundmass supported, however the proportions of the various groundmass phases and clasts vary from thin- section scale to outcrop scale (Osinski et al. 2004).

Osinski et al. (2004) presents textural evidence that the groundmass phases of the main suevite were in a liquid state at the time of deposition. Furthermore, the observation that the clays are the host phases for the vesicles suggests the generation of volatile-rich melt with vesicles forming following deposition (Osinski et al. 2004). The main mass of surficial suevite was emplaced as a high temperature (580o_{C – >900}o_{C; Engelhardt et al.}

1995; Harker & Tuttle 1955) melt-rich flow containing entrained glass and lithic clasts that emanated from different regions of the evolving crater during the formation of the central uplift during the modification stage of crater formation (Osinski et al. 2004).