The nature of groundmass of surficial suevite from the Ries impact structure, Germany, and constraints on its origin

Item Type Article; text

Authors Osinski, G. R.; Grieve, R. A. F.; Spray, J. G.

Citation Osinski, G. R., Grieve, R. A., & Spray, J. G. (2004). The nature of the groundmass of surficial suevite from the Ries impact structure, Germany, and constraints on its origin. Meteoritics & Planetary Science, 39(10), 1655-1683.

DOI 10.1111/j.1945-5100.2004.tb00065.x

Publisher The Meteoritical Society

Journal Meteoritics & Planetary Science

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Link to Item http://hdl.handle.net/10150/655911 Meteoritics & Planetary Science 39, Nr 10, 1655–1683 (2004) Abstract available online at http://meteoritics.org

The nature of the groundmass of surficial suevite from the Ries impact structure, Germany, and constraints on its origin

Gordon R. OSINSKI,1†* Richard A. F. GRIEVE,2 and John G. SPRAY1

1Planetary and Space Science Centre, Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick, E3B 5A3, Canada 2Earth Sciences Sector, Natural Resources Canada, Ottawa, Ontario, K1A 0Y7, Canada †Present address: Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Boulevard, Tucson, Arizona 85721–0092, USA *Corresponding author. E-mail: [email protected] (Received 11 June 2003; revision accepted 20 August 2004)

Abstract–Surficial suevites from the Ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy. The groundmass of these suevites comprises calcite, clay minerals, impact melt glass, crystallites (plagioclase, garnet, and pyroxene), francolite, and Ba-phillipsite. The latter zeolite is a secondary phase. Abundant textures have been observed: intricate flow textures between the various groundmass phases, globules of each phase in the other phases, spheroids of pyrrhotite in calcite, the “budding-off” of clay globules into silicate glass and/or calcite, euhedral overgrowths of francolite on apatite clasts, and quench-textured crystallites in the groundmass. Groundmass-forming calcite displays higher FeO, MnO, and SiO2 contents than limestone target material. The composition of suevite “clay minerals” is highly variable and not always consistent with montmorillonite. Three types of glasses are distinguished in the groundmass. Type 1 glasses are SiO2-rich and are clearly derived from sandstones in the sedimentary cover, while the protoliths of the other two glass types remains unclear. Analytical data and micro-textures indicate that the calcite, silicate glass, francolite, and clay minerals of the groundmass of the Ries suevites represent a series of impact-generated melts that were molten at the time of, and after, deposition. On cooling, plagioclase, pyroxene, and garnet crystallized from the groundmass. These results are at variance with the current, traditional descriptive definition of suevite. Given that Ries is the original type occurrence of “suevite,” some modification to the traditional definition may be in order. As the results of this study are most consistent with the groundmass of Ries surficial suevites representing a mix of several types of impact-generated melts, we suggest that a possible origin for these suevites is as some form of impact melt flow(s) that emanated from different regions of the evolving crater.

INTRODUCTION French 1998): 1) parautochthonous target rocks, dikes of allochthonous material, and pseudotachylyte in the crater Hypervelocity impact events generate pressures and floor/rim; 2) allochthonous crater-fill deposits within the temperatures that can vaporize, melt, shock metamorphose, crater (i.e., within the original transient cavity); 3) proximal and/or deform a substantial volume of the target. The ejecta deposits in the immediate vicinity of an impact crater transport and mixing of impact-metamorphosed rocks and (i.e., external to the original transient cavity and up to the minerals during the formation of impact craters produce a outer limit of the continuous ejecta blanket); and 4) distal wide variety of distinctive impactites (“rocks affected by ejecta deposits distant from the crater. impact metamorphism;” Stöffler and Grieve 1994). One of the least understood aspects of impactite Impactites from a single impact event have been classified formation on Earth is that of “ejecta.” This is due, in part, to into three major groups (Stöffler and Grieve 1994): 1) the lack of preservation of ejecta at almost all terrestrial shocked rocks; 2) impact melt rocks; and 3) impact breccias. impact structures, which is due to post-impact erosional These various types of impactites are found in different processes. The ~14.5 Ma-old Ries impact structure in locations within and around terrestrial impact structures (cf. Germany is one of the exceptions. Two main proximal ejecta

1655 © Meteoritical Society, 2004. Printed in USA. 1656 G. R. Osinski et al. deposits have been recognized at Ries (e.g., Engelhardt et al. while limestones dominate in the upper parts. The Hercynian 1995): 1) Bunte Breccia; and 2) “surficial” or “fallout” basement comprises a series of steeply dipping gneisses, suevite. It is generally accepted that the Bunte Breccia amphibolites, and ultrabasic rocks that are cut by a later series represents the remains of a continuous ejecta blanket of granitic intrusions (Graup 1978). emplaced along ballistic trajectories (e.g., Hörz et al. 1977, 1983). The emplacement mechanism(s) of the suevites, on PROXIMAL IMPACTITES OF THE the other hand, is less well-understood. Suevite was first RIES IMPACT STRUCTURE recognized at Ries and takes its name from the Roman name for the region, “Provincia Suevia” (Sauer 1920). Suevite has Four main types of proximal impactites have been been defined generally as a polymict impact breccia with a recognized at the Ries impact structure (Fig. 1): 1) Bunte clastic matrix/groundmass containing fragments of impact Breccia and megablocks; 2) polymict crystalline breccias; 3) glass and shocked mineral and lithic clasts (e.g., Stöffler et “surficial” or “fallout” suevites; and 4) coherent impact melt al. 1979; Stöffler and Grieve 1994). Although Ries is the rocks. The field relationships between these various type locality for suevite, this impact-generated lithology has lithologies were studied in detail as part of this study. A brief been cited as occurring at many other impact sites; although, overview of the important characteristics of these various typically not as an ejecta deposit. Therefore, it is important impactites is presented below. that the character, mode of formation, and emplacement mechanism of the classic Ries suevites are understood with Bunte Breccia and Megablocks as much clarity as possible. Here, we present the results of detailed field, optical, and The Bunte Breccia is a poorly sorted, glass-free polymict analytical scanning electron microscope (SEM) studies of Ries breccia, derived predominantly from the uppermost surficial suevites. Our aim was not to duplicate the many sedimentary target lithologies (Hörz 1982; Hörz et al. 1983). excellent and defining previous studies of impact glass clasts Bunte Breccia is volumetrically the most abundant type of (Hörz 1965; Engelhardt 1967, 1972; Stähle 1972; Pohl et al. proximal ejecta and has been interpreted as a continuous 1977; Engelhardt and Graup 1984; Engelhardt et al. 1995; See ejecta deposit (Oberbeck 1975; Morrison and Oberbeck 1978; et al. 1998; Vennemann et al. 2001; Osinski 2003) or crystalline Hörz et al. 1983). It consists of two main components: 1) rock clasts (e.g., Engelhardt and Graup 1984; Engelhardt 1997) primary ejecta excavated from the initial crater (~31 vol%; in the Ries suevite. Rather, this study focuses on the little- Hörz et al. 1983), which comprises predominantly studied and poorly understood groundmass of the suevites. To sedimentary rocks with minor admixtures of crystalline rocks, understand the origin of suevite, it is critical that the nature and of which granites are the most common (Hörz and Banholzer genesis of the groundmass be determined, as it forms ~50– 1980); and 2) local material or “secondary ejecta” (~69 vol%; 70 vol% of the total volume of this lithology. Hörz et al. 1983) derived from where the primary ejecta was initially deposited and then mobilized and incorporated by the GEOLOGICAL SETTING OF THE RIES IMPACT secondary cratering action of the primary ejecta (Hörz et al. STRUCTURE 1983). Megablocks have been defined as “displaced fragments of all stratigraphic units of the target rocks, which The ~24 km-diameter Ries impact structure in southern are larger than 25 m in size and can be mapped geologically” Germany possesses a sequence of impactites (Figs. 1 and 2), (Pohl et al. 1977, p. 354). including a thick series of crater-fill rocks (“crater suevite”), various types of proximal ejecta deposits (preserved up to a Polymict Crystalline Breccias radius of ~37 km from the crater center), and a tektite (“moldavite”) strewn field extending out to distances of 260– These impactites consist of a mixture of crystalline rock 400 km to the east and northeast of Ries (Hörz 1982). Recent fragments of different lithologies and shock levels (Pohl et al. 39Ar-40Ar isotopic age determinations on moldavites and 1977). Polymict crystalline breccias are rare and occur as impact glasses indicate an age between 14.3 and 14.5 Ma for irregular bodies a few 10s of meters in size, predominantly the Ries impact event (Schwarz and Lippolt 2002; Buchner et within the inner ring and the megablock zone of Ries al. 2003; Laurenzi et al. 2003). (Engelhardt 1990). These breccias are commonly observed to The target rocks at Ries consisted of a flat-lying overlie the Bunte Breccia, although Pohl et al. (1977) note sequence of predominantly Mesozoic sedimentary rocks that that the exact stratigraphic relationships are not always clear. unconformably overlay Hercynian crystalline basement. The sedimentary cover, at the time of impact, varied in thickness Suevite from ~470 m in the north to ~820 m in the south (Schmidt- Kaler 1978). The lowermost part of the sedimentary Isolated, surficial outcrops of suevite, from a few meters succession is characterized by sandstone, siltstone, and marl, to ~25–30 m thick, occur inside the morphological rim and up Groundmass of surficial suevite from Ries 1657

Fig. 1. Schematic cross section of the Ries impact structure indicating the nature and location of various impactites; modified from Hüttner and Schmidt-Kaler (1999). It should be noted that outcrops of impact melt rocks and polymict crystalline breccias are too small to be shown at the scale of the cross section. to radial distances of ~14 km beyond the rim to the south- The relationships between these units and the underlying southwest and east-northeast (Fig. 2; Engelhardt 1990). Bunte Breccia were studied in detail at several well-exposed However, ~80 m of suevite was penetrated in the sections in the Aumühle quarry (see Fig. 2 for the location). Wörnitzostheim drillhole (Fig. 2; Förstner 1967). Here, we Here, a transitional zone up to ~55 cm thick is locally use the term “surficial suevite,” following the terminology of developed between the basal suevite and the Bunte Breccia Engelhardt et al. (1995), to distinguish these deposits from (Fig. 3c) (cf. Chao et al. 1978). The thickest sequences of this other suevites at Ries (e.g., “crater suevite”) and from transitional layer are found in hollows in the underlying Bunte suevites at other terrestrial impact structures that occur in a Breccia. Importantly, this transitional layer contains clasts of variety of stratigraphic, structural, and lithological situations Bunte Breccia material (up to ~20 vol% of the clast (e.g., Masaitis 1999). Surficial suevites at Ries are also called population), with an increase in Bunte Breccia clasts “fallout suevites” in the literature (Engelhardt et al. 1995). downward (Fig. 3c). Similar observations were made by This terminology is not used here as it has clear, pre-judged Bringemeier (1994) at the Otting quarry (near the northeast genetic implications beyond simply describing a specific crater rim; Fig. 2) and by Chao et al. (1978) at outcrops, impact lithology. which are no longer exposed, in the Aumühle quarry. At all outcrops, suevites are seen to overlie Bunte Clasts of impact-generated glass are common in the main Breccia. The suevites were deposited on an uneven upper mass of the suevites. The glasses are typically vesiculated, Bunte Breccia surface that had several meters of relief schlieren-rich mixtures containing abundant mineral and (Fig. 3). No other impactites have been observed lithic fragments (Hörz 1965; Engelhardt 1967, 1972; Stähle stratigraphically above the suevites (Engelhardt et al. 1995). 1972; Pohl et al. 1977; Engelhardt and Graup 1984; Following the terminology of Bringemeier (1994), the Engelhardt et al. 1995; See et al. 1998; Vennemann et al. suevites in the crater rim area have been divided into two 2001). The results of a recent analytical SEM study reveal that units: “main suevite” and “basal suevite.” The main suevite four main glass types are present as clasts within the suevites constitutes the bulk of all suevite outcrops, is well- (Osinski 2003). In contrast to the underlying Bunte Breccia, consolidated, and contains abundant impact glass clasts (Figs. crystalline rocks generally dominate the lithic clast fraction of 3a, 3b, and 3d). No indications of sorting or layering have been the suevites (e.g., Pohl et al. 1977; Engelhardt and Graup observed in the main mass of the suevites (Engelhardt et al. 1984; Engelhardt et al. 1995). It is generally accepted that 1995). The only apparent textural regularity consists of a sedimentary rock clasts comprise <0.5 vol% of surficial preferred horizontal orientation of flat glass clasts (Hörz 1965; suevites (e.g., Pohl et al. 1977; Engelhardt and Graup 1984; Bringemeier 1994). Chilled margins, in which all glasses are Engelhardt et al. 1995), although Siebenschock et al. (1998) holohyaline, occur at the bottom and, where preserved, at the note that suevites from a new road cut in Hohenaltheim top of the main suevite mass (Engelhardt 1967). (~8 km south-southeast of Nördlingen) contain ~8 vol% In contrast, the basal suevite is fine-grained, typically limestone clasts. poorly consolidated, moderately to well-sorted, and deficient According to Engelhardt (1990), surficial suevites in clasts compared to the main mass (cf. Chao et al. 1978). comprise a clastic groundmass that forms ~80 vol% on 1658 G. R. Osinski et al.

Fig. 2. Simplified geological map of the Ries impact structure showing the sample locations of suevites (black) investigated in this study. “Ca” in circles represent localities where Graup (1999) documented evidence for the presence of carbonate melts within suevites. average. It apparently consists mainly of montmorillonite 1977). The microcrystalline groundmass comprises alkali (30–40 vol%) and glass (30–50 vol%), the remainder feldspar, plagioclase, quartz, and ilmenite (in decreasing comprising fine-grained lithic and mineral clasts. The glass order of abundance), with interstices filled by either a fresh or component of the groundmass is described as comprising devitrified glassy mesostasis (Osinski 2004). These angular or amoeboid particles (Engelhardt 1990). More impactites have been interpreted as coherent, and perhaps recently, however, it has been noted that the abundance of discrete, impact melt flows that emanated from the evolving calcite is highly variable and can amount to 40–50 vol% of crater during the modification stage of crater formation the suevite groundmass (Graup 1999). (Osinski 2004).

Impact Melt Rocks SAMPLES AND ANALYTICAL TECHNIQUES

Graup (1999) has documented seven outcrops of what is Optical microscopy was performed on 37 samples of considered to be coherent impact melt rock (or “red suevite”), suevites. This number includes two samples of an unusual sensu stricto, around the Ries structure, occurring as isolated type of black, vesicular suevite that was exposed in the spring bodies with lateral extents <10–50 m (Pohl et al. 1977). The of 2001, by new quarry operations, at the base of the north impact melt rocks overlie Bunte Breccia or megablocks and wall of the Aumühle quarry (samples 01-010a, b). Many of comprise a microscopic groundmass containing variably the suevite outcrops previously documented in the literature shocked lithic (predominantly granite) and mineral clasts are no longer accessible due to the closure and reclamation of (predominantly quartz) (Engelhardt et al. 1969; Pohl et al. quarries. This study also included samples from the impact Groundmass of surficial suevite from Ries 1659

Fig. 3. Series of field photographs of the east wall of the Aumühle quarry showing the relationship between suevite (light grey/green) and underlying Bunte Breccia (dark brown/red): a) view of the east wall near the entrance to the Aumühle quarry. Note the sharp contact between the suevite and Bunte Breccia; b) inset from (a). Note the (sub-) parallel alignment of elongate and disc-shaped glass clasts with the sub- vertical Bunte Breccia contact (arrows). A 40 cm-long rock hammer is shown for scale; c) a well-exposed sequence of transitional suevite that thickens from right to left. This unit forms a transitional zone to the main the suevite (fine-grained light grey/green lithology at top of image) and Bunte Breccia (dark brown/red lithology at the bottom right of the image). Note the clasts of Bunte Breccia material in the transitional suevite (arrows); d) view looking north along the east wall of the quarry showing a steep contact. The suevite has filled in a depression in the underlying Bunte Breccia. The height to the top of the outcrop is ~9.5 m. 1660 G. R. Osinski et al. collection of the Geological Survey of Canada. Polished thin of the different groundmass phases are presented below. sections were prepared from 30 samples and investigated Three important points are of note: 1) the main surficial using the JEOL 6400 digital scanning electron microscope suevites are typically groundmass-supported; 2) the (SEM) at the University of New Brunswick (see Fig. 2 for proportions of the various groundmass phases and clasts vary sample numbers and locations). The SEM was equipped with considerably, both between different localities and outcrops a Link Analytical eXL energy dispersive spectrometer (EDS) and over the scale of a thin section; and 3) vesicles, and Si(Li) LZ-4 Pentafet detector. Mobilization effects were commonly irregularly shaped, can comprise up to several reduced by using raster scan-modes and varying count times vol% of a particular sample (e.g., ~17 vol% in sample 00- from 60 to 100 sec. The beam operating conditions were 15 025b). kV and 2.5 nA at a working distance of 37 mm. The beam diameter was ~1 µm, with a beam penetration of ~2–4 µm, Calcite depending on the analyzed phase. Elements with an atomic number less than 9 (F) cannot be analyzed with this EDS As noted by Graup (1999), calcite is present in two system, so H2O and CO2-bearing phases will yield totals of distinct settings in Ries surficial suevites: 1) as a groundmass- less than 100 wt%. Each spot was analyzed for Si, Ti, Cr, Al, forming phase (Figs. 4 and 5); and 2) as globules within Mn, Fe, Mg, Ni, Ca, Na, K, P, S, and Cl. The SEM data were impact glass clasts. Graup (1999) presented detailed textural reduced using a ZAF procedure incorporated into the evidence and suggested that calcite has a primary origin by operating system. Analyses were calibrated using a multi- impact melting. This section focuses only on calcite that element standards block (type 202-52) produced by the C. M. occurs in the groundmass of the suevites. Taylor Corporation of Sunnyvale, California. SEM backscattered electron (BSE) imagery was used to investigate Distribution and Abundance the micro-textures of the impact glasses. The clast content and The distribution of calcite derived from this study agrees modal composition of the suevite groundmass were measured with results given by Graup (1999). Calcite is generally on representative digital BSE images using an image analysis abundant in suevites from the west to the southeast of the Ries program (Scion Image). structure (Figs. 2 and 4). Exceptions include the deposits at Amerdingen (sample # 00-056) and Bollstadt (sample # 00- PETROGRAPHY OF THE GROUNDMASS 060), south of the crater rim, where calcite is extremely rare (<1 vol%). It is notable that calcite globules are only present The groundmass of surficial suevites at Ries has in impact glass clasts in those suevites that also contain calcite previously been defined optically as all material with a grain in the groundmass. size of <1 mm (e.g., Stöffler et al. 1977; Engelhardt and Graup 1984). Optically unresolvable phases were termed Textures “matrix.” This study, using the analytical SEM, reveals that Based largely on BSE imagery, the following textures the grain size fraction of <1 mm comprises a number of have been observed: 1) irregular bodies of calcite, commonly discrete components (with their volumetric ranges in showing embayed outlines (Figs. 5a–5c and 5e); 2) individual parentheses) (Fig. 4): 1) silicate mineral and lithic fragments globules or groups of globules embedded in silicate glass and/ (8.9–50.1 vol%); 2) carbonate mineral and lithic fragments or clay minerals (Fig. 5d); 3) curved menisci with sharp (0–12.0 vol%); 3) angular impact glass clasts (0–18.3 vol%); boundaries between calcite and silicate phases (Figs. 5b–5e); 4) crystalline calcite (0–42.6 vol%); 5) fine-grained clay 4) coalesced, or partially coalesced, carbonate globules and minerals (1.6–70.6 vol%); 6) impact glass commingled with spheroids within silicate glass (Fig. 5d); and 5) isolated calcite and clay (0–16.6 vol%); 7) Fe-Mg-rich plagioclase (0– spheroids and cubic crystals of pyrrhotite (Fe1–xS) within 7.5 vol%) and rare garnet and pyroxene crystallites calcite (Fig. 5e). Calcite is usually not in direct contact with (<0.5 vol%); 8) francolite (carbonate-hydroxy-fluor-apatite) silicate glasses or silicate mineral clasts (cf. Osinski 2003). (0–5.3 vol%); and 9) Ba-rich phillipsite, a Ca-K-Ba zeolite There is typically a thin (<10–50 µm thick) layer of clay (0–34.2 vol%). minerals between these phases (Figs. 5b, 5c, and 5e). An Here, the “groundmass” of Ries surficial suevites is exception is sample 01-024, where clay minerals are rare in defined as the fine-grained material that encloses fragments the groundmass (<1.6 vol%; Fig. 4). of shocked/unshocked target material. This definition does not include any identifiable mineral and lithic clasts (>10– Composition 20 µm across), as was the case in previous studies. The Analyses of groundmass-forming calcite are presented in groundmass of the Ries suevites, as defined here, comprises Table 1. There can be considerable variation within individual calcite, clay minerals, impact melt glass, crystallites calcite globules and bodies (e.g., 0.8–2.4 wt% FeO, 0.2– (plagioclase, garnet, and pyroxene), francolite, and Ba-rich 0.8 wt% MnO, and 49.9–51.5 wt% CaO in series 3; Table 1) phillipsite. The distribution, chemistry, and textural attributes and between different regions of a single thin section Groundmass of surficial suevite from Ries 1661 ite contact. See Fig. 2 for the location of samples. The s (measured image areas range up to 5 × 7 mm in size). ined sample numbers represent those from the fine-grained ection at variable magnification distance of sample from Bunte Breccia-suev from the Ries impact structure. The underl >20 images per polished thin s tion <1 mm) of selected samples suevite underlying Bunte Breccia (BB). BB contact = basal suevite zone near the contact with Fig. 4. Modal composition (grain size frac modal compositions represent average values from measurements of 1662 G. R. Osinski et al.

Fig. 5. Plane-polarized light photomicrographs (a, d) and backscattered electron images (b, c, e, f) of calcite-rich regions of the suevite groundmass: a) image showing the groundmass-supported nature of suevite. Note the presence of a limestone clast with a well-preserved gastropod fossil; b) silicate glass and mineral fragments are invariably separated from the calcite groundmass by a thin layer of clay minerals. Also note that fragments of unshocked/shocked silicate target material are typically absent in calcite; c) globules and irregular patches of clay minerals within calcite and silicate glass; d) globules of calcite within a silicate glass-calcite groundmass; e) globules of pyrrhotite within calcite groundmass. Clasts are absent in calcite but are abundant in the small patches of clay minerals; f) suevite with a groundmass of calcite and francolite. Note the euhedral overgrowths of francolite on clasts of fluor-apatite (inset). Groundmass of surficial suevite from Ries 1663

Table 1. Average composition of groundmass-forming calcite from Ries surficial suevites.a Series # 1 2 3 4 5 6 7 Sample # 00-029 00-030 00-050 00-052a 00-052a 00-052a 00-052b # Analyses 7 9 4 7 3 4 10 wt%b s.d.c wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d.

SiO2 0.29 0.12 0.19 0.17 0.17 0.22 0.30 0.65 0.99 1.24 0.14 0.20 0.20 0.46 FeO 2.90 1.36 1.06 0.89 1.85 1.40 0.90 0.88 0.20 0.38 0.76 0.54 0.79 0.30 MnO 0.49 0.11 0.74 0.26 0.39 0.60 0.63 0.23 – n.d.d 0.66 0.27 0.66 0.25 MgO 0.82 0.34 0.72 0.44 0.81 0.34 1.40 0.89 0.41 0.07 1.19 0.39 1.27 0.42 CaO 49.29 2.93 52.09 3.53 50.68 1.49 49.63 3.16 51.23 1.28 50.92 1.53 51.90 1.82 K2O – n.d. 0.01 0.06 – n.d. – n.d. – n.d. – n.d. 0.56 0.06 P2O5 0.03 0.14 – n.d. – n.d. 0.10 0.25 – n.d. 0.06 0.25 0.04 0.16 SO3 0.12 0.22 – n.d. – n.d. – n.d. 0.06 0.21 – n.d. – n.d. Total 53.93 1.81 54.80 2.44 53.88 2.05 52.96 2.35 52.88 0.89 54.02 2.38 55.42 1.59

Series # 8 9 10 11 12 13 14 Sample # 00-052b 00-052b 00-056 00-061 00-061 00-061 01-028b # Analyses 8 3 4 3 7 6 10 wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d.

SiO2 0.33 0.86 0.17 0.31 0.20 0.10 0.27 0.04 0.29 0.09 0.27 0.14 0.10 0.20 FeO 1.10 0.31 0.86 0.39 1.83 1.38 0.37 0.12 – n.d. – 0.00 1.60 1.28 MnO 0.72 0.29 0.68 0.04 0.39 0.60 0.96 0.13 0.34 0.15 0.33 0.26 0.58 0.75 MgO 1.62 1.32 1.24 0.26 0.81 0.34 0.19 0.33 0.45 0.13 0.50 0.15 0.77 0.48 CaO 52.20 2.50 52.15 5.01 51.49 0.92 54.15 0.32 53.03 1.71 52.73 2.12 52.35 2.71 K2O 0.58 0.04 0.58 0.04 – n.d. 0.08 0.09 0.61 0.10 0.66 0.11 – n.d. P2O5 0.05 0.20 0.20 0.02 – n.d. – n.d. – n.d. – n.d. 0.03 0.15 SO3 0.08 0.28 – n.d. – n.d. – n.d. – n.d. – n.d. 0.04 0.19 Total 56.66 2.18 55.88 5.26 54.72 1.08 56.01 0.64 54.72 1.71 54.49 1.83 55.48 2.20 aAl, Ba, Cl, Na, Ti, and Sr were below detection for all analyses. bwt% = mean composition in weight percent. cs.d. = standard deviation (2σ). dn.d. = not determined.

(compare series 4, 5, and 6; Table 1). The high FeO (up to Textures ~4 wt%) and MnO (up to ~2 wt%) contents of groundmass- An unusual type of suevite from the Aumühle quarry forming calcite are in contrast to calcite in the limestone (samples 01-010a, b) comprises a black, fine-grained vesicular target lithologies (Table 2). The groundmass-forming calcite groundmass (Figs. 6a and 6b). The vesicular nature of this −3 can also contain up to 1.8 wt% SiO2 (71 out of 85 analyses in lithology results in a low density of <2.0 g/cm , compared with −3 Table 1). Trace amounts (up to ~1 wt%) of K2O, P2O5, and values of ~2.5–2.6 g/cm for other suevites (e.g., Siebenschock SO2 are also present in some samples (Table 1). The et al. 1998). Distinctive purple glass clasts are common ubiquitous presence of SiO2 in groundmass calcite is unusual (average 15.9 vol%; sample 01-010a). Co-genetic, unshocked and difficult to explain due to the charge imbalance it would plagioclase crystallites also occur in the clays of this lithology create in the calcite structure. (cf. basal surficial suevites at Aumühle, discussed below). A fine-grained basal layer of suevite (<40 cm thick) was Clay sampled at the Aumühle locality (samples 00-012a and 00- 025b; Fig. 2). This basal layer is very different from the main The presence of clay minerals in Ries suevites has been mass of the overlying suevites (cf. Figs. 6c–6f with Figs. 7 recognized for several decades. As with calcite, clays are and 8). The important properties of the basal layer are: 1) present in two distinct settings: 1) as groundmass-forming abundant vesicles (up to 25 vol%), typically irregularly phases (Figs. 6–8); and 2) as globules and schlieren within shaped (Figs. 4 and 6c); 2) high glass content of the impact glass clasts (see Osinski 2003). groundmass (average ~8–10 vol%; Figs. 4 and 6c–6f); 3) abundant plagioclase and rare garnet crystallites embedded in Distribution and Abundance clay minerals (Figs. 4 and 6c–6f); and 4) lack of calcite in the The groundmass of suevite is dominated by clay minerals groundmass (Figs. 6c–6f). It is notable that, under the SEM, in the majority of samples investigated, with modal these basal suevites resemble coherent impact melt rocks as abundances ranging from ~1 to 70 vol% (Fig. 4). described from Polsingen (Fig. 9; Osinski 2004). 1664 G. R. Osinski et al.

Table 2. Average composition of limestone target rocks.a Series #b 1 2 3 Sample # 00-015 00-050 01-028b # Analyses 10 3 2 wt%c s.d.d wt% s.d. wt% s.d.

e SiO2 – n.d. 0.01 0.01 0.01 0.03 TiO2 – n.d. – n.d. – n.d. Al2O3 – n.d. – n.d. – n.d. FeO – n.d. 0.06 0.16 0.21 0.61 MnO – n.d. – n.d. – n.d. MgO 0.87 0.40 0.54 0.28 0.71 1.14 CaO 52.96 1.03 54.13 0.41 53.08 2.16 K2O 0.01 n.d. – n.d. – n.d. P2O5 – n.d. – n.d. – n.d. SO3 0.03 0.16 0.30 0.15 0.25 0.05 Total 53.86 0.75 55.05 0.44 54.27 2.67 aBa, Cl, Na, and Sr were below detection for all analyses. bSeries 1 are analyses of monomict limestone (Bunte) breccia from Aumühle. Series 2 and 3 represent analyses of limestone clasts from suevites. cwt% = mean composition in weight percent. ds.d. = standard deviation (2σ). en.d. = not determined.

Figures 7 and 8 illustrate a representative selection of and 11) contents are not typical of montmorillonite clays textures from clay-rich areas of the suevite groundmass. There (e.g., Velde 1985). Notably, the platy clays always yield is a continuous transition from areas of “homogeneous” clay analyses conforming to montmorillonite. (Figs. 7a and 7b), through samples consisting of isolated globules (Figs. 7c–7g), to areas of densely packed globules, Silicate Glass commonly associated with impact-generated glass and/or calcite (Figs. 7h and 8). The following textures have been Distribution and Abundance observed: 1) curved menisci with sharp boundaries between Silicate impact glasses have been found in the silicate glass and clay minerals (Figs. 7c, 7f, and 8); 2) isolated groundmass of all but two of the 37 samples studied. globules of clay within silicate glass and/or calcite (Figs. 5c, Groundmass glasses may comprise up to ~20–30 vol% of the 5e, 7c, 7f, and 8); 3) the “budding-off” of clay globules into <1 mm-size fraction (Fig. 4). glass (Fig. 8) and/or calcite (Fig. 5c); 4) coalesced, or partially coalesced, clay globules within silicate glass, calcite, and/or Textures “host” clay (Figs. 5c, 7c–7e, 7g, 7h, and 8); and 5) highly BSE imagery reveals that glasses within the suevite deformed and streaked out clay “globules” (Fig. 8). In Fig. groundmass are always intimately associated with clay 10a, fine-grained groundmass-forming clays are cross-cut by minerals and calcite (Figs. 5c, 6c–6f, 7c, 7g, 7h, and 8). later, “platy” clay minerals. These platy clays typically Silicate glass is commonly found in the interstices between comprise <5–10% of the total clay fraction. globules of clay and calcite (e.g., Figs. 7c, 7h, and 8). These observations distinguish these glasses from the Ries impact Composition glass clasts studied previously by many workers. There is considerable variation in the composition of the clay minerals. Selected oxides are plotted in Fig. 11, and Composition individual analyses from two samples are presented in Tables Sixty-three analyses of groundmass glasses are presented 3a and 3b. X-ray diffraction analysis of studied suevites from in Fig. 12. The glasses can be divided into three main five localities suggest that montmorillonite is the only clay compositional groups (Fig. 12): 1) SiO2-rich (~80–100 wt%) mineral present (Engelhardt and Graup 1984; Newsom et al. glasses with low Al2O3 (<6 wt%), FeO (<3 wt%), MgO 1986). However, EDS analysis reveals that the composition (<3 wt%), CaO (<1 wt%), Na2O (<1 wt%), and K2O of the suevite “clays” can be highly variable and is not always (<3 wt%) contents (Fig. 11); 2) glasses with SiO2 contents of consistent with montmorillonite (Fig. 11). For example, high ~60–65 wt%, high Al2O3 contents (~17–23 wt%), low MgO Al2O3 (up to ~33 wt%; Table 3a, analyses 2, 4), TiO2 (up to (<0.8 wt%), FeO (<0.6 wt%) and CaO (<2.5 wt%) contents, ~2 wt%; Table 3a, analyses 1, 10, 12, and 14; Table 3b, and variable amounts of Na2O (1–8 wt%) and K2O (3– analyses 13 and 14), CaO (up to ~17 wt%; Table 3b, 11 wt%); 3) SiO2-poor (~50–62 wt%) glasses with high analysis 4), and P2O5 (up to 1.9 wt%; Table 3a, analyses 10 Al2O3 (~15–21 wt%) and FeO (~2–7 wt%) contents and Groundmass of surficial suevite from Ries 1665

Fig. 6. a and b) Backscattered electron photomicrographs of distinctive black, vesicular suevites from the Aumühle quarry (see text). Note the abundant irregularly shaped vesicles; c–f) suevites from the fine-grained basal layer at the Aumühle quarry (see text). In detail: c) the groundmass of the basal suevite is characterized by abundant vesicles, and the presence of plagioclase crystallites and glass intermingled with clay minerals; d) the glass-rich region of the groundmass. Plagioclase crystallites can be seen to have nucleated on a quartz clast in the upper center part of the image; e) image dominated by a clast of highly vesicular impact glass. Small (~2–10 µm-diameter) garnet crystals are present embedded in clay minerals; f) inset from (e) showing euhedral, zoned crystals of garnet. 1666 G. R. Osinski et al.

Fig. 7. Backscattered electron photomicrographs of clay-rich groundmass regions of Ries surficial suevites: a) sample in which the groundmass is comprised entirely of clay minerals; b) as shown here, vesicles are often common in the clayey groundmass of the suevites. Also note the presence of calcite in the groundmass and a glass spherule at the bottom of the image; c) globules of clay and interstitial glass showing evidence of deformation and flow. A large vesicle is present in the right of the image; d) the globule-rich region of the suevite groundmass. Note the presence of central vesicles in the larger globules. The very bright areas represent charging of the sample during SEM investigations; e) globules of clay in a groundmass of clay and plagioclase crystallites; f) pyroxene crystallites that have nucleated on a globule of clay; g) groundmass comprised entirely of clay minerals and impact glass. Several clay globules in the glass have been deformed and streaked out; h) flow textures developed between silicate glass and clay minerals. Note the zoned clay globules and the presence of calcite globules in the upper right area of the image. Groundmass of surficial suevite from Ries 1667

Fig. 8. Backscattered electron photomicrograph showing spectacular flow textures developed between silicate glasses and what are now clay minerals, indicating that both phases were in a liquid state at the same time. Two different glass compositions are also present and are intermingled with one another, as evidenced by their different grey scale in BSE mode. The “darker” glass is richer in SiO2 (~63.5 wt%) and K2O (~8 wt%) than the “brighter” glass (~54 wt% SiO2 and ~5 wt% K2O). The latter is enriched in TiO2 (~2 wt%), FeO (~6 wt%), and MgO (~2 wt%) compared to the former (<0.2 wt% TiO2, FeO, and MgO). The majority of large clay globules are concentrically zoned and possess a core that is darker in BSE images and which yields consistently lower totals, typically <70 wt% (Table 3b, analyses 3–5). The low totals could be due either to poor analyses or to a sub-microscopic porosity. However, if either were the case, the “darker” cores would be depleted in all elements. This is not the case, as the “darker” phase is commonly enriched in certain oxides (e.g., FeO, CaO, MgO) relative to the “lighter” outer rims (compare analyses 3–5 with analyses 6 and 7 in Table 3b). Thus, the lower totals of this “dark” clay may reflect the presence of higher amounts of volatiles in the darker phase.

variable amounts of MgO (up to 4 wt%), CaO (1–8 wt%), Na2O (2–5 wt%), and K2O (0–6 wt%). The oxide totals range from ~94 wt% to 100 wt%. The low totals for some glasses are likely due to volatiles (e.g., H2O) that cannot be analyzed with this SEM.

Plagioclase

A feature of the Ries surficial suevites is the presence of unshocked plagioclase crystallites in the groundmass clay minerals and impact glasses. The plagioclase laths are typically ~1–5 µm thick and up to ~40 µm long. BSE imagery reveals that plagioclase is invariably skeletal and typically displays hollow “swallow tail” terminations (Figs. 6d and 7e). Individual analyses of 14 “large” (~3–5 µm thick, >30 µm long) plagioclase crystallites are presented in Table 4. These Fig. 9. Backscattered electron photomicrograph of coherent impact plagioclases (An35−62Ab35−60Or2−6) are unusual, containing up melt rock from Polsingen (see Fig. 2 for the location). This lithology to 3 wt% FeO and MgO (Table 4). comprises a fine-grained vesicular groundmass comprising alkali feldspar, plagioclase, quartz, and ilmenite (in decreasing order of abundance), with interstices filled by either a fresh or devitrified Garnet glassy mesostasis (Osinski 2004). Note the similarity between the coherent impact melt rocks and the fine-grained basal suevites from Euhedral crystals of garnet ~2–10 µm in diameter were Aumühle (Figs. 6c and 6d). recognized in the clayey groundmass of two samples from 1668 G. R. Osinski et al.

Table 3a. Individual analyses of the clayey groundmass from Otting (wt%) (sample # 00-001).a Analysis # 1234567891011121314

SiO2 50.00 24.33 55.26 27.60 52.83 52.97 51.02 52.54 52.41 49.53 48.35 52.48 56.75 51.09 TiO2 0.96 0.30 0.43 0.24 0.34 0.31 0.42 0.50 0.93 1.66 0.18 1.19 0.27 1.43 Al2O3 16.83 33.54 18.34 28.12 16.92 12.50 18.38 18.49 17.51 17.77 16.80 17.80 8.87 17.88 FeO 5.36 11.14 6.20 13.13 6.41 8.01 5.22 5.11 6.48 6.60 6.81 6.79 3.71 7.10 MgO 1.83 6.17 2.22 5.68 2.69 6.13 2.21 2.58 2.20 2.39 2.06 2.49 5.66 2.47 CaO 1.29 0.44 1.39 1.00 1.59 0.60 1.28 0.64 1.04 3.36 3.53 1.25 0.91 2.39 K2O 0.10 – 0.14 – 0.17 0.38 0.17 – 0.14 0.19 0.10 0.18 0.28 0.41 P2O5 –––––––––1.851.49 – – 0.65 SO3 0.260.25––––0.22––0.290.22 – 0.25 0.20 Cl 0.28 0.19 0.15 0.13 0.14 0.14 – 0.08 – 0.19 0.21 0.17 0.23 0.20 Total 76.91 76.35 84.12 75.90 81.11 81.03 78.90 79.94 80.71 83.83 79.76 82.36 76.92 83.8 aBa, Na, and Sr were below detection for all analyses.

Table 3b. Individual analyses of the clayey groundmass from Seelbronn (wt%) (sample # 00-052a).a Analysis #b 12345678910111213c 14c

SiO2 51.35 57.68 35.57 32.40 34.06 50.44 51.62 54.36 56.65 55.54 56.73 57.79 56.21 56.18 TiO2 0.19–––0.15–0.19––0.15 0.23 0.30 1.01 1.18 Al2O3 17.95 18.33 7.61 7.12 6.98 15.90 16.44 17.36 18.96 19.12 18.57 19.31 17.98 17.69 FeO 2.96 2.92 3.76 4.22 5.60 3.68 3.39 3.63 2.59 3.00 3.37 3.24 5.19 5.29 MgO 3.91 4.04 6.38 6.61 6.67 4.73 4.39 4.17 4.41 4.41 4.46 4.52 4.03 4.41 CaO 2.19 2.35 7.48 12.65 6.93 3.81 2.28 2.41 2.39 2.39 2.44 2.37 3.12 3.27 K2O 0.38 0.36 0.25 0.25 0.25 0.41 0.47 0.37 0.28 0.42 0.35 0.35 0.28 0.32 P2O5 – – 0.68 – – – –––––––– SO3 – – 0.25 0.24 0.30 – ––––––0.20– Cl 0.08 0.37 0.20 0.14 0.14 0.19 0.08 0.23 0.10 – 0.07 0.13 0.19 0.18 Total 79.01 86.06 62.18 63.62 61.08 79.16 78.85 82.53 85.38 85.02 86.21 88.01 88.21 88.51 aBa, Na, and Sr were below detection for all analyses. bAnalyses 3–5 are from a phase that appears darker (in BSE images) than the surrounding groundmass clay (analyses 6 and 7). Analyses 8 and 9 are globules within groundmass (analyses 10 and 11). The remaining analyses are from random locations throughout the sample. cDuplicate analyses indicate that the data is reproducible.

Aumühle (00-012a, 00-025a; Figs. 6f and 6g). The larger (beam diameter ~1 µm). Nevertheless, EDS analysis reveals garnets display some compositional zoning, with a slight that they are similar in composition to aluminian subsilicic increase in almandine content and an associated decrease in pigeonites that are common in glass clasts (Osinski 2003). pyrope content toward the rims (Table 5, analyses 1–4). The garnet crystals are not fractured or shocked. Furthermore, the Francolite microscopic size of these garnets is very different from garnet in the metamorphic and intrusive igneous Ries target rocks Two suevite samples collected from Zipplingen are (typically 100s of µm to several mm in size). unusual in that impact glass is rare (<1–2 vol%) or absent within the groundmass, and the lithic clast population is Clinopyroxene dominated by granite. In addition, in sample 01-024, the groundmass is dominated by calcite, with minor (~4–8 vol%) Clinopyroxene crystallites are abundant in basement- francolite (Fig. 5f; Table 6). The francolite differs distinctly in derived impact glass clasts in the Ries surficial suevites (e.g., terms of textural setting and composition from the fluor- Stähle 1972; Engelhardt et al. 1995; Osinski 2003). BSE apatite that is present in the granitic clasts and as sporadic imagery reveals that clinopyroxene crystallites are also mineral clasts (Fig. 5f; Table 6). Clasts of fluor-apatite present in the groundmass clay minerals (samples 00-025b, (Table 6, analyses 2, 4, and 6) typically display euhedral to 00-050, 00-052b, and 00-053). Some appear to have subhedral overgrowths of francolite (Fig. 5f; Table 6, nucleated on clasts and globules within the groundmass (e.g., analyses 3, 5, and 7). Fig. 7f). These crystallites are typically <10 µm long and display irregular elongate, curved morphologies similar to Ba-Rich Phillipsite those seen in glass clasts (Osinski 2003). The small size (<0.5–1.5 µm in diameter) of the crystallites precluded an Zeolite minerals have been recognized in the groundmass accurate determination of their composition using the SEM of surficial suevites from four locations (Fig. 4). This phase Groundmass of surficial suevite from Ries 1669

Table 4. Individual analyses of plagioclase crystallites in the groundmass of surficial suevites (wt%).a Analysis # 1 2 3 4 5 7 8 9 10 11 12 13 14 Sample # 00-060 00-060 00-060 00-060 00-061 00-061 01-10a 01-28b 01-28b 01-28b 01-28b 01-28b 01-28b

SiO2 53.41 53.88 54.27 54.64 56.07 51.34 58.84 56.06 57.06 57.27 56.47 57.26 57.26 TiO2 0.63 0.48 0.20 0.46 0.22 0.44 – 0.51 0.48 0.56 0.49 0.25 0.42 Al2O3 27.04 24.68 26.32 22.75 25.41 26.06 24.97 23.63 24.32 21.90 22.56 24.97 25.53 FeO 2.45 2.86 1.30 2.39 2.21 1.91 0.32 2.38 2.47 3.13 2.55 1.43 1.57 MnO – – – 0.18 – – – – – – – – – MgO 1.90 1.96 0.79 2.66 1.29 1.36 0.40 1.70 1.86 2.25 1.68 0.82 0.82 CaO 11.36 11.06 10.99 10.74 6.82 11.57 6.88 8.88 9.21 8.11 9.31 8.61 9.31 Na2O 3.50 5.42 4.91 4.41 6.10 6.02 6.46 5.44 4.99 5.45 5.04 5.56 5.35 K2O 0.53 0.60 0.47 0.55 0.58 0.64 1.02 0.47 0.49 0.64 0.43 0.39 0.46 Total 100.82 100.93 99.25 98.76 98.69 99.33 98.88 99.06 100.88 99.33 98.53 99.27 100.71 Number of ions on the basis of 8 O Si 2.42 2.46 2.49 2.54 2.57 2.39 2.66 2.57 2.57 2.63 2.61 2.60 2.57 Ti 0.02 0.02 0.01 0.02 0.01 0.02 n.d. 0.02 0.02 0.02 0.02 0.01 0.01 Al 1.45 1.33 1.42 1.24 1.37 1.43 1.33 1.28 1.29 1.18 1.23 1.34 1.35 Fe 0.09 0.11 0.05 0.09 0.08 0.07 0.01 0.09 0.09 0.12 0.10 0.05 0.06 Mn n.d.b n.d. n.d. 0.01 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Mg 0.13 0.13 0.05 0.18 0.09 0.09 0.03 0.12 0.12 0.15 0.12 0.06 0.05 Ca 0.55 0.54 0.54 0.53 0.33 0.58 0.33 0.44 0.44 0.40 0.46 0.42 0.45 Na 0.31 0.48 0.44 0.40 0.54 0.54 0.57 0.48 0.44 0.48 0.45 0.49 0.47 K 0.03 0.03 0.03 0.03 0.03 0.04 0.06 0.03 0.03 0.04 0.03 0.02 0.03 Anorthite 62.00 51.25 53.81 55.41 36.76 49.80 34.79 46.04 48.94 43.28 49.15 45.04 47.66 Albite 34.54 45.46 43.45 41.21 59.51 46.91 59.06 51.05 47.99 52.64 48.14 52.57 49.56 Orthoclase 3.46 3.29 2.74 3.38 3.72 3.29 6.15 2.91 3.07 4.08 2.72 2.40 2.78 aBa, Cl, P, S, and Sr were below detection for all analyses. bn.d. = not determined.

Table 5. Individual analyses of garnet crystallites in the groundmass of surficial suevites (wt%).a Analysis # 1 2 3 4 5 6 Core Rim Core Rim Core Core

SiO2 38.31 36.42 38.15 36.58 38.16 38.09 Al2O3 21.24 20.58 21.10 20.75 21.69 21.35 FeO 23.25 26.73 23.07 25.93 23.59 23.59 MnO 0.81 0.77 0.79 0.84 0.69 0.82 MgO 14.44 13.98 14.64 13.50 14.82 14.58 CaO 0.85 0.58 0.57 0.83 0.61 0.63 K2O 0.43 0.46 0.41 0.57 0.29 0.39 Total 99.32 99.52 98.73 99.00 99.84 99.44 Number of ions on the basis of 24 O Si 5.86 5.68 5.86 5.71 5.80 5.82 Al 3.83 3.78 3.82 3.82 3.88 3.85 Fe 2.97 3.48 2.96 3.39 3.00 3.02 Mn 0.10 0.10 0.10 0.11 0.09 0.11 Mg 3.29 3.25 3.35 3.14 3.36 3.32 Ca 0.14 0.10 0.09 0.14 0.10 0.10 K 0.08 0.09 0.08 0.11 0.06 0.08 Almandine 45.69 50.27 45.52 49.95 45.83 46.06 Spessartine 1.61 1.47 1.58 1.64 1.36 1.62 Pyrope 50.57 46.87 51.47 46.35 51.30 50.75 Grossular 2.13 1.40 1.43 2.06 1.51 1.57 aBa, Cl, Na, P, S, Sr, and Ti were below detection for all analyses. 1670 G. R. Osinski et al.

documented in drill cores from Ries suevites (Wörnitzostheim drill hole [Förstner 1967]; Deiningen and Nördlingen 1973 drill holes [Förstner 1967; Jankowski 1977; Salger 1977; Stähle and Ottemann 1977; Stöffler et al. 1977; Pfannschmidt 1985]). Several authors have reported the presence of vesicles lined by zeolites in impact glass clasts from suevite outcrops; however, the only previous report of zeolites in the groundmass appears to be a reference to “tiny prismatic zeolite crystals (probably erionite)” by Engelhardt and Graup (1984, p. 455).

IMPACT GLASS CLASTS

A distinctive component of the Ries surficial suevites are “aerodynamically shaped” glass bombs or “fladen” (Hörz 1965). In a detailed study of glass particles >3 cm in size, Hörz (1965) recognized eight different forms ranging from “bowl” and “dish” shapes to irregular lumps and elongated forms. Based on their aerodynamic form, the interpretation has been that the suevite glass bombs were transported through the air and deposited at temperatures of <700 °C (i.e., below the glass transition temperature; Hörz 1965; Engelhardt 1967, 1972; Stähle 1972; Pohl et al. 1977; Engelhardt et al. 1995). Our SEM studies of the morphology of glass clasts over a wide range of sizes (<50 µm, >10 cm) reveals that, in the smaller particle size range, there is a marked disparity between the shape of holohyaline and hypocrystalline varieties. The former always occur as angular shards with outlines of broken vesicles readily discernable (Fig. 13a) (cf. Osinski 2003). In contrast, hypocrystalline glasses are more Fig. 10. a) Backscattered electron (BSE) photomicrograph showing variable in their morphology and commonly lack vesicles fine-grained, groundmass-forming clays cross-cut by later “platy” (Figs. 13b–13e). Clear, holohyaline glass rims are present on montmorillonite; b) suevite sample in which the groundmass is predominantly Ba-rich phillipsite. Note the pronounced alteration of a number of particles (e.g., Figs. 13b and 13c). These glass a glass clast in the left half of the image. Alteration typically begins rims can be seen to cross-cut earlier flow textures and differ along perlitic fractures. in composition compared to the host glass (Figs. 13b and 13c). Where glass rims are absent, the contact between typically accounts for <1–2 vol% of a particular sample, with hypocrystalline glass particles and the groundmass can be the exception of samples 00-050 and 00-056, which have difficult to discern, and the glass appears to “intermingle” 9.5 vol% and 34.2 vol% zeolites, respectively (Fig. 4). In with the suevite groundmass (e.g., Figs. 13d and 13e). regions where zeolites are abundant in the groundmass, Fragment-rich rims are also present around some glass clasts impact-generated glass clasts typically show pronounced (Fig. 13f). Clast-rich globules of calcite (and clay) are alteration along perlitic fractures and on clast margins common in clasts of basement-derived silicate glass (e.g., (Fig. 10b). Zeolites also form amygdales within vesicular Figs. 13g and 13h). impact glasses. Analyses indicate that the zeolite mineral present in the DISCUSSIONS AND CONCLUSIONS samples is Ba-rich phillipsite (Table 7). However, Deer et al. (1963) note that phillipsite is often associated with Origin of Groundmass Phases harmotome (Ba-zeolite), with the two minerals occurring as interpenetrating complex twins. Thus, the high Ba content The groundmass of the Ries suevites, as defined here, may be due to a component of harmotome in the phillipsite comprises calcite, impact melt glass, francolite, clay (Table 7). minerals, crystallites (plagioclase, garnet, and pyroxene), and Phillipsite and harmotome, along with analcime, Ba-rich phillipsite. The latter is a replacement mineral and is clinoptilolite, erionite, wellsite, and chabasite, have been not considered further. For comparison, Engelhardt (1997) Groundmass of surficial suevite from Ries 1671

Table 6. Average composition of fluor-apatite and francolite (carbonate-hydroxy-fluor-apatite) phases in the groundmass of surficial suevites from Zipplingen (sample #01-024).a Series # 1 2 3 4 5 6b 7b Phase Francolite Fluor-apatite Francolite Fluor-apatite Francolite Fluor-apatite Francolite Description Groundmass Clast Rim Clast Rim Clast Rim # Analyses 6 3 3 3 3 3 3 wt%c s.d.d wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d.

e SiO2 2.09 1.69 0.33 0.11 2.80 1.25 0.24 0.04 0.22 0.01 0.22 0.11 – n.d. f TiO2 0.54 0.71 – n.d. – n.d. – n.d. – n.d. n.a. n.d. n.a. n.d. Al2O3 0.79 0.49 – n.d. – n.d. – n.d. – n.d. n.a. n.d. n.a. n.d. FeO 0.70 0.78 0.22 n.d. 0.27 0.10 0.25 0.06 0.29 0.08 n.a. n.d. n.a. n.d. MnO 0.23 n.d. – n.d. – n.d. – n.d. – n.d. n.a. n.d. n.a. n.d. MgO 0.53 0.46 – n.d. – n.d. – n.d. – n.d. n.a. n.d. n.a. n.d. CaO 51.16 2.42 53.93 0.26 52.25 1.98 53.81 0.19 51.98 0.15 53.93 0.64 53.00 0.54 Na2O 0.76 n.d. – n.d. – n.d. – n.d. 0.74 n.d. n.a. n.d. n.a. n.d. K2O 0.86 0.31 0.17 0.09 0.12 0.02 – n.d. 0.10 n.d. n.a. n.d. n.a. n.d. P2O5 34.14 1.14 42.80 0.47 33.74 2.44 42.38 0.45 33.81 1.46 41.30 0.28 33.74 0.44 SO3 0.30 n.d. – n.d. 0.49 0.43 – n.d. 0.64 0.06 n.a. n.d. n.a. n.d. Cl – n.d. 0.10 0.02 – n.d. 0.10 0.01 – n.d. n.a. n.d. n.a. n.d. F n.a. n.d. n.a. n.d. n.a. n.d. n.a. n.d. n.a. n.d. 2.82 1.25 3.26 0.60 Total 89.79 2.17 97.40 0.61 88.60 1.59 96.62 0.28 87.14 1.26 98.27 0.48 90.00 0.57 aBa and Sr were below detection for all analyses. bAnalyses carried out with the beryllium window removed to allow for the detection of fluorine. cwt% = mean composition in weight percent. ds.d. = standard deviation (2σ). en.d. = not determined. fn.a. = element not analyzed.

Table 7. Average composition of Ba-phillipsite in the groundmass of surficial suevites.a Series # 1 2 3 4 5 6 Sample # 00-056 00-056 00-056 00-50 01-025a 01-028b # Analyses 9 5 5 4 3 3 wt%b s.d.c wt% s.d. wt% s.d. wt% s.d. wt% s.d. wt% s.d.

SiO2 57.58 4.01 58.03 1.47 59.99 1.62 55.62 1.71 54.48 1.28 52.77 2.61 Al2O3 18.80 1.47 19.60 0.45 18.30 0.52 15.97 0.33 17.22 0.64 16.35 1.28 FeO 0.06 0.24 – n.d.d – n.d. – n.d. 0.13 0.22 0.19 0.65 MgO 0.04 0.22 – n.d. – n.d. – n.d. 0.43 0.09 – n.d. CaO 3.91 1.74 3.83 1.35 3.36 0.72 4.43 0.67 6.85 0.12 5.14 0.62 Na2O 0.60 0.94 0.77 0.94 0.83 0.94 0.16 0.65 – n.d. – n.d. K2O 6.57 1.77 6.69 1.23 6.71 0.59 4.08 1.94 2.92 0.28 2.71 0.69 BaO 1.22 0.36 2.70 0.45 2.59 0.64 2.90 0.17 0.05 0.18 3.31 0.38 Total 88.79 3.54 91.62 1.60 91.79 2.59 83.16 1.51 82.08 1.36 80.46 0.65 Numbers of ions on the basis of 32 O Si 11.70 0.41 11.62 0.21 11.92 0.10 12.11 0.16 11.67 0.14 11.91 0.45 Al 4.51 0.48 4.63 0.12 4.29 0.13 4.10 0.14 4.35 0.16 4.35 0.39 Fe 0.01 0.04 – n.d. – n.d. – n.d. 0.02 0.04 0.04 0.12 Mg 0.01 0.07 – n.d. – n.d. – n.d. 0.14 0.03 – n.d. Ca 0.85 0.41 0.82 0.30 0.72 0.17 1.03 0.15 1.57 0.05 1.24 0.17 Na 0.23 0.36 0.30 0.37 0.32 0.36 0.07 0.28 0.00 n.d. – n.d. K 1.70 0.41 1.71 0.30 1.70 0.13 1.13 0.54 0.80 0.08 0.78 0.20 Ba 0.10 0.03 0.21 0.03 0.20 0.05 0.25 0.02 0.00 0.02 0.29 0.03 aCl, Mn, S, and Sr were below detection for all analyses. bwt% = mean composition in weight percent. cs.d. = standard deviation (2σ). dn.d. = not determined. 1672 G. R. Osinski et al.

Fig. 11. Harker variation diagrams illustrating the composition of selected oxides of clay minerals within the suevite groundmass. Oxide totals can range from ~70% to ~85%, and interpretation of trends is not warranted. The white ellipses represent the range of composition of montmorillonite (data from Olphen and Fripiat [1979]). suggested that the groundmass comprises ~35 vol% clay individual globules, or groups of globules, within silicate minerals, ~40 vol% fragments and amoebic bodies of glass, glass. and 22 vol% lithic and mineral fragments. There is, however, Further evidence for an impact melt origin for a distinction here between glass clasts and groundmass- groundmass-forming calcite comes from the high FeO, MnO, forming glass. The latter may encompass the “amoebic” and SiO2 contents, which contrast with calcite from the glasses of Engelhardt (1997). The origin of these Malmian limestone target lithologies at Ries (cf. Tables 1 groundmass-forming phases will be discussed below. and 2). It was not possible, using the SEM, to detect any inclusions within calcite, which suggests that the high FeO, Calcite MnO, and SiO2 are not due to contamination from It has been shown previously that shock-melted calcite is surrounding phases during EDS analysis. This compositional present as globules within silicate glass clasts (Graup 1999; difference cannot be explained by recrystallization of Osinski 2003). The results of this study are consistent with the limestone fragments or hydrothermal alteration but is typical hypothesis of Graup (1999) that calcite within the suevite for carbonate melts (e.g., Woolley and Kemp 1989). Indeed, groundmass is also an impact-generated melt phase. Evidence high SiO2 contents are consistent with studies of carbonatites, for this includes: 1) the groundmass-supported nature of which show that the solubility of SiO2 in carbonate melts calcite-rich samples; 2) sharp and curved menisci between increases with increasing temperature (e.g., Brooker et al. calcite, silicate glass, and clay minerals; 3) flow textures 2001). Rapid crystallization (quenching) of a high developed between calcite and various phases; and 4) temperature SiO2-rich carbonate melt can, therefore, produce Groundmass of surficial suevite from Ries 1673

Fig. 12. Box plots showing the range of major oxide contents for glasses in the suevite groundmass recognized in this study. The composition of the four types of impact glass clast present in suevites are plotted for comparison (data from Osinski 2003). The boxes define the interquartile range, with the median value shown as a horizontal line. Whiskers extend from the boxes to the highest value in the data set or to a distance of 1.5× the interquartile range, whichever is less. The outliers are plotted as solid diamonds. 1674 G. R. Osinski et al.

Fig. 13. a) Holohyaline glass particle displaying an angular, bubble-wall outline (arrows); b) irregularly shaped hypocrystalline glass clast that displays a holohyaline glass rim of different composition; c) holohyaline glass rim that cross-cuts flow textures (i.e., schlieren) in a hypocrystalline glass clast; d) irregularly shaped glass clasts. Note the lack of angular outlines; e) globules of clay in a glass clast appear to be intermingling with the clayey groundmass; f) clast-rich rim on a hypocrystalline glass clast. Note the well-developed perlitic fractures in the glass; g) impact-generated glass clast containing a clast-rich globule of calcite. The clasts are predominantly quartz, alkali feldspar, and angular silicate glass shards; h) clay globules with suspended mineral fragments in an impact glass clast. Groundmass of surficial suevite from Ries 1675

relatively SiO2-rich carbonates (e.g., carbonates with ~3– Osinski 2003). These glasses could be derived from the 10 wt% SiO2 in the experiments of Brooker et al. [2001]). chemically heterogeneous “mixed” gneisses from the However, it is not clear if the Si and Al is incorporated into the crystalline basement at Ries (~52–64 wt% SiO2, 14–19 wt% calcite structure or if Si and/or Al is present in nm-sized Al2O3, 3–6 wt% FeO, 0–3 wt% MgO, 2–5 wt% CaO, 3–4 silicates that crystallized from the CaCO3-rich melt during wt% Na2O, and 1–5 wt% K2O; Vennemann et al. 2001). The quenching. high Al2O3 and CaO contents are also consistent with an Additional evidence for the impact melt origin of the origin from the impact melting of clay-rich sedimentary rocks calcite comes from the presence of isolated spheroids and such as shales, claystones, and/or marls in the sedimentary crystals of pyrrhotite within calcite (Fig. 5e). The spheroids cover at the time of the Ries impact. are not vesicle fillings, as the calcite does not contain any vesicles. Identical pyrrhotite spheroids have been observed in Francolite impact melt rocks from the Popigai impact structure, Russia, Phosphates are common accessory minerals in a wide where they have been interpreted as immiscible globules variety of rock types. Francolite, however, is only present in (e.g., Whitehead et al. 2002). Immiscibility requires that both those suevites that also contain granitic fluor-apatite-bearing the spheroids and the surrounding calcite were liquid at the clasts. Importantly, clasts of fluor-apatite typically display same time, which is consistent with the shock melting of euhedral to subhedral overgrowths of francolite (Fig. 5f). pyrite-bearing limestones present in the Ries target sequence. These textures are not consistent with hydrothermal alteration These findings agree with the phase relations of CaCO3, but are indicative of the phenocryst-groundmass relationships which suggest that carbonates shocked to temperatures common in endogenous igneous rocks, thus indicating that >1500–2000 K should undergo melting (Ivanov and Deutsch francolite crystallized from a melt. 2002). Indeed, by considering the phase relations with respect to the Ries modeling results of Stöffler et al. (2002), ~0.4 km3 Clay of carbonate melt is predicted (cf. volume of Ries suevites of Fine-grained “clay minerals” are the dominant ~0.084 km3; Engelhardt and Graup 1984). Thus, the presence constituents of the groundmass in the majority of samples of carbonate impact melts at Ries should not be unexpected investigated, ranging up to ~70 vol% (Fig. 4). Previous (cf. Osinski 2003). workers have suggested that the clays formed by post-impact hydrothermal alteration of fine-grained glass particles Glass (Engelhardt 1972; Stähle 1972; Newsom et al. 1986, 1990) Silicate glasses in the groundmass undoubtedly represent and/or crystalline rock flour (Engelhardt and Graup 1984). quenched impact melts. With respect to the three main Textural evidence documented in this study confirms the view compositional groups of groundmass glasses (Fig. 12), the that the fine-grained clayey groundmass was originally most likely protolith for the SiO2-rich (~80–100 wt%) glasses impact melt glass (cf. Engelhardt 1972). Evidence for this are the Jurassic and Triassic sandstones present in the includes: 1) intricate flow textures between the various lowermost parts of the sedimentary succession (>350 to groundmass phases (Figs. 7c, 7g, 7h, and 8); 2) the “budding- <750 m in depth; Schmidt-Kaler 1978). This is the same off” of clay globules into glass and/or calcite (Figs. 5c and 8); precursor as the SiO2-rich glass clasts in the suevites (Fig. 12; 3) the presence of plagioclase, garnet, and pyroxene Osinski 2003). This view is supported by results of numerical crystallites in the clays (e.g., Figs. 6c–6g, 7e, and 7f); 4) the modeling studies, which predict that ~4 km3 of melt and vapor presence of vesicles in the groundmass clay (Figs. 6a–6c and would have been produced from sandstones in the Ries 7b–7d); 5) coalesced, or partially coalesced, clay globules sedimentary cover (Stöffler et al. 2002). within silicate glass and/or calcite (Figs. 7c–7e, 7g, 7h, and Type 2 groundmass glasses are feldspar-like in 8); 6) sharp and curved menisci between clay minerals, composition (Fig. 12), which could suggest an origin from the calcite, and silicate glasses of different composition (Figs. 7c, shock melting of a feldspar-rich lithology in the crystalline 7g, 7h, and 8); and 7) abundant globules of clay in the basement (e.g., granite and/or orthogneiss). However, the groundmass. Na2O and K2O contents of these glasses are highly variable, It has been noted that the excellent preservation state of which is also consistent with the leaching of K+ and Na+ due glasses and delicate textural relationships in the Ries to the passage of meteoric water (e.g., Garrels et al. 1962; suevites “simply rules out large-scale [hydrothermal] Grieve et al. 1987). Thus, the analyzed Na2O and K2O replacement processes” (Graup 1999, p. 433). This agrees contents of type 2 glasses may not reflect their original with our work and that of Newsom et al. (1986), which composition. shows that montmorillonite is the only identifiable clay The third group of glasses exhibits considerable mineral present and that it comprises <10–15 vol% of the compositional variation (Fig. 12). The SiO2-poor (~50– groundmass. Thus, a substantial portion of the “clayey” 62 wt%) nature of these glasses distinguishes them from groundmass (up to ~50 vol%) is not montmorillonite, which basement-derived impact glass clasts in the suevites (Fig. 12; is consistent with the considerable variation in the 1676 G. R. Osinski et al. composition of this material (Fig. 11). Chemical contact with the host silicate glass. Similarly, in this study, the heterogeneity is not typical of hydrothermal montmorillonite fragments of silicate glass and minerals are rarely seen to be clays (Olphen and Fripiat 1979; Velde 1985) but is consistent in direct contact with calcite. There is typically a thin (<10– with an impact melt origin for this phase (cf. Dence et al. 40 µm thick) layer of clay minerals between silicate glass/ 1974). The complete absence of Na is also atypical of mineral/lithic fragments and calcite (e.g., Figs. 5b and 5e). montmorillonite (Olphen and Fripiat 1979). Notably, “platy” Fragments of silicate target material are typically absent in clay minerals with a composition close to montmorillonite calcite but are concentrated in the clay (e.g., Figs. 5b and 5e). are present in amounts up to ~10 vol% (Fig. 10a). This By analogy with experiments on carbonate and silicate melts, montmorillonite displays open-space filling textures and these phenomena can be explained by the lower melt-solid cross-cuts the more homogenous groundmass “clays,” interfacial energy of silicate melts relative to coexisting suggesting that it formed from the circulation of carbonate melts (Minarik 1998). This results in the silicate hydrothermal fluids along fractures in the already lithified melt “selectively wetting the grain-edge channels between suevite (cf. Newsom et al. 1986). solid phases; thereby excluding the carbonate melt to the If hydrothermal montmorillonite accounts for <10– center of melt pockets, away from grain edges” (Minarik 15 vol% of the Ries suevites, then what is the nature and 1998, p. 1965). This would explain the pockets of clast-free origin of the remainder of the clayey groundmass? It is carbonate in the groundmass. Carbonate melts are also highly suggested that this material is either X-ray amorphous clay reactive (e.g., Treiman 1989). Thus, it is also possible that, in and/or an as yet unidentified hydrous phase(s) and/or that it some cases, the thin layer of clay represents a glassy (and remains (at least in part) in its original glassy state. While the subsequently devitrified) reaction zone between silicate and exact nature of this material remains unclear, it is apparent carbonate melt. that it is hydrous and that this hydration cannot be solely due to alteration by post-impact hydrothermal fluids, for the Summary reasons outlined above. Thus, it is evident that hydrous The proposal of Graup (1999), that calcite within the impact melts were generated during the Ries impact event, groundmass of Ries surficial suevites crystallized from an either by direct melting of H2O-rich target rocks and/or impact-generated carbonate melt, is confirmed in this study. through concentration of H2O in certain areas in the residual Silicate glasses in the groundmass must also represent impact melt during cooling (cf. Osinski 2003). Potential sources of melt, and the quenching to glass occurred after deposition. H2O in the Ries target sequence were sedimentary rocks (>5– Crystallites, vesicles, and impact-generated glass and/or 20 wt% H2O) and/or crystalline basement rocks (up to calcite intermingled with clay minerals indicate that the ~10 wt% H2O; Engelhardt et al. 1969). Upon quenching of progenitor to the clays was also originally an impact melt these hydrous melts, any glasses would have been highly phase. Thus, the calcite, francolite, silicate glass, and clay unstable due to the high H2O contents and would, therefore, groundmass, which form ~50–70 vol% of the total volume of have quickly devitrified to form clays and other hydrous the Ries suevites, originated as a variety of impact-generated phases. It is well-known that devitrification accompanies melts (cf. calcite and clay globules within the basement- cooling of hot glass and involves the nucleation and growth of derived glass clasts in the suevites; Osinski 2003). Similar crystals at subsolidus temperatures (e.g., McPhie et al. 1993). globules of montmorillonite in impact glasses from the West Clearwater Lake impact structure, Canada, have likewise Crystallites been interpreted as impact melt phases (Dence et al. 1974). A previously unrecognized feature of Ries suevites is the presence of plagioclase, garnet, and pyroxene crystallites in Physical State of the Groundmass Phases at the Time of the groundmass-forming clays and glasses (e.g., Figs. 6c–6f, Emplacement 7e, and 7f). Plagioclase is invariably skeletal and typically displays hollow “swallow tail” terminations, indicating rapid In the previous section, we have established that the crystallization from a melt in response to high degrees of groundmass of Ries surficial suevites contains a series of undercooling and supersaturation, and low nucleation impact-generated melt phases. Determining the physical state densities (e.g., Drever et al. 1972; Lofgren 1974; Donaldson of the various phases at the time of emplacement is critical for 1976). Such quench crystal morphologies are common in understanding the origin of the surficial suevites. The two impact melt rocks (e.g., Carstens 1975; Grieve et al. 1987). possible end member scenarios are that the groundmass Thus, the presence of crystallites within the clay minerals phases were either solid (i.e., clasts and shards) or liquid (i.e., indicates that this part of the groundmass was originally a melt. still molten) upon deposition. It is generally believed that impact-generated silicate Relationship Between Calcite and Clay glasses in the suevites were deposited as solid, fragmented In a study of the impact glass clasts in surficial suevites, particles and are, therefore, clasts (e.g., Hörz 1965; Osinski (2003) noted that calcite globules are never in direct Engelhardt 1967, 1972; Stähle 1972; Engelhardt et al. 1995). Groundmass of surficial suevite from Ries 1677

However, impact-generated silicate glasses also form a that the calcite, silicate glass, and clay minerals in the locally important component (up to ~20–30 vol%; Fig. 4) of groundmass of Ries surficial suevites represent a series of the groundmass in the Ries suevites (i.e., silicate impact melt impact-generated melts and that the bulk of these phases were was a component of the groundmass in the suevites, which molten on deposition. Upon cooling, plagioclase, garnet, and only quenched to a glass after deposition). Previous workers pyroxene crystallized from the groundmass. Given that the have noted that some glass clasts seem to merge into the Ries structure is the original type occurrence of “suevite,” matrix such that a rigorous distinction between clasts and some revision of the terminology surrounding suevite may be groundmass is not always possible (Engelhardt and Graup in order, particularly as breccias conforming to the traditional 1984). Here, it is suggested that these “clasts” may not, in definition of suevite occur as dikes and as units roofing and fact, be clasts but are exclusively areas of groundmass glass flooring coherent impact melt sheets at many impact sites that were quenched after deposition. (e.g., Dence 1971; Grieve 1975). Moreover, there is evidence that what is now silicate glass, clay minerals, and calcite, were all in the liquid state at the same Origin and Emplacement of Surficial Suevites time. The evidence includes: 1) intricate flow textures between the various phases (Figs. 7c, 7g, 7h, and 8); 2) globules of each It is generally accepted that the Bunte Breccia represents phase in the other two phases (Figs. 5c, 5d, 7d, 7e, 7g, 7h, and the continuous ballistic ejecta blanket at Ries (e.g., Morrison 8); 3) the “budding-off” of clay globules into glass and/or and Oberbeck 1978; Hörz 1982; Hörz et al. 1983). The ballistic calcite (Figs. 5c and 8); 4) coalesced, or partially coalesced, emplacement of primary crater-derived ejecta resulted in clay globules within silicate glass and/or calcite (Figs. 7c–7e, secondary cratering, the incorporation of local material 7g, 7h, and 8); 5) sharp and curved menisci between clays, (“secondary ejecta”), and considerable modification of the calcite, and silicate glasses of different composition (Figs. 7c, local substrate (Hörz et al. 1983). Conversely, several models 7g, 7h, and 8); and 6) the presence of plagioclase, garnet, and have been proposed for the origin of the Ries surficial suevites pyroxene crystallites in the groundmass-forming clays and (e.g., Chao 1977; Stöffler 1977; Engelhardt and Graup 1984; glasses (e.g., Figs. 6c–6f, 7e, and 7f). Engelhardt 1990; Newsom et al. 1990; Engelhardt et al. 1995). Many vesicles (up to 16.8 vol%) in the groundmass- These models fall into two main groups, based on the mode of forming clays retain a (sub-) rounded shape, while others have transport and subsequent deposition: been deformed (Figs. 6a–6c and 7b–7d). The spherical shape 1. Various authors have suggested that suevite material was indicates that they are not pore spaces between clasts. A ejected ballistically (Chao 1977; Stöffler 1977; fundamental interpretation of the origin of vesicles is that the Engelhardt and Graup 1984; Engelhardt 1990, 1997; host phase (i.e., what is now clay) must initially have been a Engelhardt et al. 1995). Stöffler (1977, p. 448) proposed volatile-bearing melt. Furthermore, the (sub-) rounded vesicles that, during ejection, melt and rock fragments formed a must have been formed after deposition; otherwise, they would dense cloud that “is expected to leave the crater in a have been destroyed and/or deformed during transport. sheet-like conical plume.” However, Hörz (1982, p. 46) It is notable that many regions of the groundmass are rich noted that “the secondary cratering mechanism in clay globules (e.g., Figs. 7c–7h). In many cases, the postulated for the Bunte Breccia does not apply to globules are completely enclosed by glass or other clay suevite because (a) no substrate incorporation is minerals with no apparent channels for the introduction of observed and (b) most glass bombs occur in a well- later hydrothermal fluids. Numerous globules retain a (sub-) preserved, unfractured state.” Thus, the mode of suevite spherical shape (Figs. 7c–7h). A fundamental interpretation deposition must have involved fundamentally different of this texture is that the globules must have been solid or processes than that of the Bunte Breccia (Hörz 1982; highly viscous at the time of incorporation, which can only be Engelhardt 1997). To account for the differences explained by an impact melt origin for the globules. Similar between the models and the observed field relations, features at the West Clearwater Lake impact structure, Engelhardt (1997, p. 552) modified his original Canada, were used as evidence for a primary impact melt hypothesis and suggested that ballistic ejection of origin for the clay globules (Dence et al. 1974). “suevite materials was superimposed by the propelling action of an expanding plume of vaporized rocks from Redefinition and Classification of Ries Surficial Suevites? which they settled as a turbulent suspension of melt lumps and solid particles in a medium of hot gases, According to the traditional nomenclature of Stöffler and without disturbing the ground.” Grieve (1994), suevite is described as “polymict impact 2. Newsom et al. (1986, 1990), while not discounting the breccia with a clastic matrix containing lithic and mineral models above, suggested another possibility, namely, that clasts in various stages of shock metamorphism, including after ballistic ejection, the impact plume collapsed with cogenetic impact melt clasts, which are in a glassy or subsequent deposition of surficial suevite from a ground crystallized state.” However, the results of this study reveal surge or “suevitic flow” similar to a pyroclastic flow. 1678 G. R. Osinski et al.

The results of our field and analytical studies generally uniform thickness over restricted areas and drape the support the proposals of Newsom et al. (1986, 1990) and underlying topography (Fisher and Schmincke 1984). Bringemeier (1994) that suevites were emplaced in the form a flow. Ballistic ejection and subsequent “fallout” from an Different “Phases” of Suevite ejecta plume is not compatible with the observations as Hüttner (1969) described the presence of different outlined below. “phases” or “facies” of surficial suevite that he interpreted as individual “flows.” Unfortunately, the outcrops documented Lack of Sorting by Hüttner (1969) are no longer exposed. However, several It has long been recognized that the main mass of Ries different suevite phases that could be interpreted as individual surficial suevites are unsorted to poorly sorted. This is not flows have been investigated in the Aumühle quarry as part of predicted by subaerial deposition (i.e., fallout from an ejecta this study (e.g., black, vesicular suevite and “normal” suevite). plume), as these deposits are typically well-sorted and display normal grading, as is the case in pyroclastic fall deposits Synthesis (Fisher and Schmincke 1984). Indeed, true “fall” deposits do Thus, it appears that main mass of surficial suevites were occur in the center of the Ries structure in the form of a emplaced as some form of flow, as first suggested by Newsom ~17 m-thick unit that is superbly graded (Stöffler 1977). et al. (1986, 1990) and Bringemeier (1994). However, we Moreover, we suggest that the thin, fine-grained, sorted basal suggest a different model to these workers as to the type of layers of suevite present at some localities may represent true flow involved. These previous studies were based largely on “fallout” suevites, but these occurrences are minor (<1 m the premise that the groundmass to these surficial suevites is thick) and are overlain by the main mass of surficial suevites. clastic, resulting in the comparisons that were drawn with In this respect, this sorted basal layer may represent lateral pyroclastic flows. Based on the SEM observations that the extensions of the sorted fallback layer from the crater interior groundmass of Ries suevites contains a series of impact- (cf. Newsom et al. 1990). generated melts, it is suggested that another mode of origin of suevites is possible. Namely, they represent melt-rich flows Orientation of Elongate Glass Clasts with entrained clasts that emanated from different regions of It is well-documented that there is a preferred horizontal the evolving crater. Exterior impact melt flows have been orientation of flat glass clasts in surficial suevites (Hörz 1965; recognized around lunar and venusian impact structures (e.g., Bringemeier 1994). This requires a predominantly horizontal Howard and Wilshire 1975; Hawke 1976; Hawke and Head mode of transport (i.e., a flow; Bringemeier 1994; F. Hörz, 1977; Grieve and Cintala 1995; Cintala and Grieve 1998). It personal communication). would be surprising if such impact melt flows are not produced on Earth, given that venusian and terrestrial impacts Chilled Margins produce approximately equivalent amounts of melt relative to The presence of chilled margins at the bottom and, where the diameter of the transient crater (Grieve and Cintala 1995). preserved, at the top of some outcrops (Engelhardt 1967) In addition, terrestrial impacts generate approximately six suggests that suevites were deposited as a single unit (cf. F. times more melt for equivalent-sized transient craters than do Hörz, personal communication). This is compatible with lunar impacts (Cintala and Grieve 1998). This begs the emplacement as a flow, but not fallout from an ejecta plume, question: if exterior impact melt flows occur on the Moon, which would entail deposition over a significant length of time. then why not on Earth?

Bunte Breccia-Suevite Relationship Characteristics of Suevites that Constrain Any Proposed The sharp contact between suevites and underlying Flow Model Bunte Breccia indicates that there was a substantial temporal hiatus and a change in the emplacement mechanism between Impact-Generated Melt Phases in the Groundmass the deposition of the two lithologies. Hörz (1982) calculated The calcite, silicate glass, francolite, and clay minerals of that the emplacement time for the Bunte Breccia was ~5 min. the groundmass of Ries surficial suevites represent a series of For a complex crater the size of Ries, this necessitates that the impact-generated melts that were molten at the time of, and deposition of suevite did not occur until after the excavation after, deposition. In previous models, which all involve some stage had ceased and the modification stage was well component of ballistic ejection and transport through the underway (e.g., Melosh 1989). atmosphere, silicate melt would be quenched to a glass before A further argument against a fallout origin for the deposition, and carbonate melt would have crystallized (e.g., surficial suevites is the observation that they infill depressions the feathery-textured carbonates in ejecta at the Chicxulub (up to several meters deep) in the underlying Bunte Breccia impact structure, Mexico; Jones et al. 2000). These surface. This is a characteristic of pyroclastic and lava flows observations, therefore, suggest that suevites originated as but not of fall deposits, which typically produce layers of a melt-rich flows. Groundmass of surficial suevite from Ries 1679

The presence of globules of calcite and clay is further An impact melt flow model also solves a longstanding evidence for an impact melt flow origin. Droplets and globules enigma, as detailed by Hörz (1982, pp. 46–47), namely, “the have been observed in some igneous rocks derived from aerodynamically sculptured bombs attest to some eruptions of very low viscosity magmas. Examples include interaction with an atmosphere while extremely poor sorting “spatter deposits” produced by some Hawaiian lava fountains and significant abundance of fine-grained matrix material (e.g., Heiken 1972). BSE images indicate that many of the . . . are incompatible with severe aerodynamic interactions; larger Ries globules of clay have been deformed after there is no winnowing of fine-grained components.” We deposition (e.g., Figs. 7c–7e, 7f, 7g, and 8). This deformation suggest that no “aerodynamic sculpting” is required, as the may take the form of simple compaction or may result in equivalent shapes could be produced by transport in an spectacular folding, stretching, and even rupturing of globules impact melt flow. due to shear movement (e.g., Fig. 8). Several smaller globules have retained their original, presumably (sub-) spherical, shape Temperature of Deposition (e.g., Figs. 7h and 7g). This suggests that some globules were A universal temperature of deposition of >580 °C (Curie solid or highly viscous at the time of incorporation into the point of magnetite) is required by the fact that all suevites groundmass. Similar observations were made for clay globules show a reversed remanent magnetization (Pohl and in glass clasts from Ries suevites by Osinski (2003). This is Angenheister 1969). Similar temperatures are indicated by compatible with the bulk of the material in the interior of the the complete loss of fission tracks in titanite from suevites flow remaining liquid, while droplets of calcite and “clay” melt from Otting (Miller and Wagner 1979). An upper in the upper reaches were dispersed and cooled somewhat temperature limit of ~750 °C has been suggested by before being reincorporated into the flow. Indeed, this is to be Engelhardt et al. (1995) based on crystallization experiments expected due to the volatile-rich nature of the melt flow (i.e., on impact glasses. This represents the glass transition vesiculation and volatile loss during transport would lead to temperature for the large glass clasts. The occurrence of quenching of droplets in the upper reaches of the flow). fragmented glasses has, therefore, been interpreted as indicating that they were deposited at temperatures close to Morphology and Orientation of Glass Clasts their transition temperature (Engelhardt et al. 1995). A characteristic of Ries suevites is the occurrence of However, “most glass bombs occur in a well-preserved, apparently “aerodynamically” shaped glass bombs or “fladen” unfractured state” (Hörz 1982, p. 46). Furthermore, (e.g., Hörz 1965; Engelhardt 1967, 1972; Stähle 1972; Pohl et groundmass glasses commonly show evidence for flow after al. 1977; Engelhardt and Graup 1984; Engelhardt et al. 1995). deposition. A consistent interpretation for these observations Based on their aerodynamic form, the interpretation has been is that some basement-derived melt bodies were quenched in that “viscous melt lumps were hurled at high velocities the earlier stages of the cratering process and subsequently through a resistant medium” (Engelhardt et al. 1995, p. 282). fragmented during transport, while other basement-derived This “medium” was considered to be the atmosphere. melts were transported and deposited in a (partially) molten However, this is unlikely, as the atmosphere would have been state at temperatures >750 °C. This allowed time for the post- blown away in an impact event the size of Ries. This is deposition crystallization of pyroxene and plagioclase consistent with the presence of fluid-drop chondrules and crystallites and the intermingling of these basement-derived glass spherules in the Ries suevites that solidified in the melts with the groundmass. absence of an atmosphere (Graup 1981; Newsom et al. 1990). Evidence for even higher temperatures for suevites Impact melt itself is also a “resistant medium.” Thus, the comes from the presence of decarbonated rims on some same “aerodynamic” or streamlined shapes could also form if limestone clasts (Baranyi 1980). Decomposition of calcite viscous coherent bodies of basement-derived melt were requires temperatures >900 °C (Harker and Tuttle 1955). incorporated and transported in impact melt flows. Indeed, Importantly, the decarbonated rims are neither cut by or impact melt glass clasts from breccias at the Mistastin Lake diffuse into the suevite groundmass, indicating that impact structure, Canada, have streamlined shapes, occurring decomposition occurred after deposition. Such high as “streaks, blobs, or contorted lenses” (Currie 1971, p. 28). temperatures are consistent with an impact melt flow origin These breccias conform to the definition of suevite but form a for the surficial suevites. discontinuous layer beneath the coherent impact melt sheet and/or as dikes in the crater floor and were, therefore, never Groundmass “Inclusions” airborne (Currie 1971; Grieve 1975). Suevite dikes at Globules of fragment-rich calcite and clay are present in Mistastin often “show well developed flow structure, many basement-derived impact glass clasts (Figs. 13g and commonly with complex eddies and swirls” (Currie 1971, 13h). That is, the glass clasts contain inclusions of the p. 27). That is, streamlined shapes may equate with transport surrounding groundmass. For such textures to be preserved, in a less dense medium, but the medium may not necessarily both the inclusions and the host must have been in a molten be the atmosphere. state at the time of incorporation. Furthermore, calcite 1680 G. R. Osinski et al. globules are only present in impact glass clasts at locations Model where calcite also occurs in abundance in the groundmass. The evidence provided by investigations at the SEM This would imply physical contact and intermingling of the scale points toward “suevites” at Ries as being more similar to various molten phases during transport. In an ejection origin, clast-rich impact melt rocks, or impact melt breccias, than the atmosphere is not a particularly confining medium, and it previously considered. We suggest that true “fallout” suevites is, therefore, difficult to envisage such intimate mixing of occur only as minor, discontinuous deposits (<1 m thick) melt phases, except on landing. But, there is no evidence of underlying the main mass of the surficial suevites. mixing of suevite and Bunte Breccia to support such a high Furthermore, in agreement with the work of Newsom et al. energy depositional environment. An impact melt flow would (1986, 1990) and Bringemeier (1994), it appears that suevites provide a confining medium. were emplaced in the form of a flow. While we cannot completely discount a pyroclastic flow, we suggest that the Degassing Pipes melt-rich nature of the suevites and other evidence detailed Vertical degassing pipes have been recognized in some above are more consistent with an impact melt flow origin. suevite deposits (e.g., Aumühle and Otting). These have been This should not be surprising given that exterior impact melt studied by Newsom et al. (1986), who noted that they flows occur on the Moon and Venus. The stratigraphic resemble features observed in pyroclastic flow deposits and relationship between the suevites and the continuous ejecta suggested that a significant source of volatiles could have (Bunte Breccia) blanket at Ries is also consistent with an come from melted and shocked crystalline basement impact melt flow origin for suevites. Many workers have inclusions. These authors did not discount other sources of documented that exterior impact melt deposits typically volatiles. The majority of large glass clasts in the suevite have overlie the continuous ejecta deposits of lunar craters (e.g., smooth surfaces (Hörz 1965), with vesicles typically being Howard and Wilshire 1975; Hawke and Head 1977). As absent from the surface of the glasses. Therefore, it is likely Hawke and Head (1977) note, these features suggest that most that substantial vesiculation and release of volatiles occurred of the exterior melt flows were emplaced during the before the deposition of what are now the glass clasts. modification stage of complex crater formation. This explains However, the consistently low totals of clay analyses in the the temporal hiatus between the deposition of Bunte Breccia groundmass of the surficial suevites (~75–88 vol%) indicate and suevite. substantial (~12–25 vol%) amounts of volatiles. The presence During the modification stage, the floor of the transient of abundant (up to ~16 vol%) vesicles in the clay groundmass crater at Ries was uplifted, resulting in the so called “inner of the surficial suevites suggests that exsolution of volatiles ring” of uplifted crystalline basement. Movements associated from this phase continued after emplacement. with this uplift could have imparted an outward-directed vector, resulting in the transportation of some of the melt Comparison with Coherent Impact Melt Rocks at Ries phases still within the trace of the original transient cavity Discrete outcrops of reddish, vesicular impact melt rocks outward as flows and toward and beyond the final crater rim. are present in the vicinity of the northeast and southwest In recent numerical modeling of large impacts, impact-melted crater rims (Engelhardt et al. 1969; Pohl et al. 1977; material drapes over the overheightened central uplift (Ivanov Engelhardt and Graup 1984; Graup 1999). These lithologies and Melosh 2003). Impact melt lithologies would, therefore, have been interpreted recently as impact melt flows (Osinski be expected to flow off the overheightened central uplift 2004). Importantly, both suevites and the coherent melt rocks during both the initial upward movement and the subsequent occur in the same stratigraphic position (i.e., overlying Bunte collapse of the central uplift. Breccia), suggesting that both lithologies were emplaced at similar times during the impact cratering process. The basal CONCLUDING REMARKS suevites from Aumühle also closely resemble the coherent impact melt rocks from Polsingen (cf. Figs. 6c and 6d, with The original classification of Ries suevites as breccias Fig. 9). The simplest explanation for these observations is that with a clastic matrix/groundmass was mainly based on hand both the surficial suevites and the reddish impact melt rocks specimen and optical microscope studies. The evidence represent impact melt flows. The difference between the two provided by investigations at the more detailed SEM scale lithologies can be explained as being due to differences in the points toward “surficial suevites” at Ries as being more akin protolith. The impact melt rocks represent an end member to mixed impact melt breccias. Given that Ries is the original derived entirely from the crystalline basement (Engelhardt et type occurrence of “suevite,” some redefinition of the al. 1969; Engelhardt and Graup 1984; Osinski 2004). The terminology, as traditionally used, may be required, at least surficial suevites, however, represent more volatile-rich melt for the spatial and genetic equivalents of surficial suevite. For flows due to incorporation of varying amounts of shock- these lithologies, the term “impact melt breccia” is more melted sedimentary rocks and/or more volatile-rich appropriate in place of suevite. This is not to say that there are crystalline rocks. not lithologies at terrestrial impact craters that correspond to Groundmass of surficial suevite from Ries 1681 the traditional definition of suevite (i.e., that have a clastic Chao E. C. T., Hüttner R., and Schmidt-Kaler H. 1978. Principal matrix and contain lithic, mineral, and impact melt glasses as exposures of the Ries meteorite crater in southern Germany. clasts, with a range of recorded histories). Such lithologies Munich: Bayerisches Geologisches Landesamt. 84 p. Cintala M. J. and Grieve R. A. F. 1998. Scaling impact melting and occur as units flooring and roofing coherent impact melt rock crater dimensions: Implications for the lunar cratering record. sheets at complex craters, as dikes in crater floors, and within Meteoritics & Planetary Science 33:889–912. the interior breccia lenses at simple craters. Currie K. L. 1971. Geology of the resurgent cryptoexplosion crater This study has shown the value of the SEM in at Mistastin Lake, Labrador. Geological Survey of Canada determining the nature of the fine-grained groundmass of Bulletin 207. Ottawa: Geological Survey of Canada. 62 p. Deer W. A., Howie R. A., and Zussman J. 1963. Rock-forming surficial suevites at Ries. Care should be exercised when minerals, vol. 4: Framework silicates. London: Longmans, interpreting seemingly “clastic” textures based on optical Green and Co. Ltd. 435 p. studies alone. We suggest that use of the SEM for microscopic Dence M. R. 1971. Impact melts. Journal of Geophysical Research imaging and analysis of impactites may prove similar to the 76:5552–5565. breakthrough in our understanding of planar deformation Dence M. R., Engelhardt W. v., Plant A. G., and Walter L. S. 1974. Indications of fluid immiscibility in glass from West Clearwater features (PDFs) that was achieved using the transmission Lake impact crater, Quebec, Canada. Contributions to electron microscope (TEM) (e.g., Langenhorst 1994; Leroux Mineralogy and Petrology 46:81–97. et al. 1994). This work also highlights the value of re- Donaldson C. H. 1976. An experimental investigation of olivine examining previous concepts in light of new imaging and morphology. Contributions to Mineralogy and Petrology 57: analytical techniques. 187–213. Drever H. I., Johnston R., Butler P., Jr., and Gibb F. G. F. 1972. Some textures in Apollo 12 lunar igneous rocks and in terrestrial Acknowledgments–This work is part of G. R. Osinski’s Ph.D. analogs. Proceedings, 3rd Lunar and Planetary Science thesis and was funded by the Natural Sciences and Conference. pp. 171–184. Engineering Research Council of Canada (NSERC) through Engelhardt W. v. 1967. 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