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 Rights Copyright © The Meteoritical Society Download date 26/09/2021 08:44:05 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final published version 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