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

This work is dedicated to the memory of Prof. Dr. Wolf von Engelhardt.Abstract-The 3.8 km Steinheim Basin in SW Germany is a complex impact crater with central uplift hosted by a sequence of Triassic to Jurassic sedimentary rocks. It exhibits a well-preserved crater morphology, intensely brecciated limestone blocks that form the crater rim, as well as distinct shatter cones in limestones. In addition, an impact breccia mainly composed of Middle to Upper Jurassic limestones, marls, mudstones, and sandstones is known from drilling into the impact crater. No impact melt lithologies, however, have so far been reported from the Steinheim Basin. In samples of the breccia that were taken from the B-26 drill core, we discovered small particles (up to millimeters in size) that are rich in SiO 2 (50 wt%) and Al 2 O 3 (28 wt%), and contain particles of Fe-Ni-Co sulfides, as well as target rock clasts (shocked and unshocked quartz, feldspar, limestone) and dropletshaped particles of calcite. The particles exhibit distinct flow structures and relicts of schlieren and vesicles. From the geochemical composition and the textural properties, we interpret these particles as mixed silicate melt fragments widely recrystallized, altered, and ⁄ or transformed into hydrous phyllosilicates. Furthermore, we detected schlieren of lechatelierite and recrystallized carbonate melt. On the basis of impactite nomenclature, the melt-bearing impact breccia in the Steinheim Basin can be denominated as Steinheim suevite. The geochemical character of the mixed melt particles points to Middle Jurassic sandstones (''Eisensandstein'' Formation) that crop out at the center of the central uplift as the source for the melt fragments.

Section: Discussionmentioning

confidence: 99%

Steinheim suevite—A first report of melt‐bearing impactites from the Steinheim Basin (SW Germany)

Buchner

Schmieder

2010

“…, 2008; Kring ; Kenkmann and Schönian ; Wittmann and Kenkmann ; Muttik et al. , ; Pohl et al. ; Jung et al.…”

Section: Short History Of Suevite Researchunclassified

“…; Osinski ; Muttik et al. , ). In the crater suevite, various zeolites (erionite, clinoptilolite, wellsite [Na‐harmotome], chabasite, stilbite, and philippsite) and analcite are common in addition to montmorillonite, some chlorite, calcite, and pyrite (Förstner ; Stähle and Ottemann ; Stöffler et al.…”

Section: Petrography Of Suevitementioning

confidence: 99%

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Ries crater and suevite revisited—Observations and modeling Part I: Observations

Stöffler

Artemieva

Wünnemann

et al. 2013

154

246

We report results of an interdisciplinary project devoted to the 26 km‐diameter Ries crater and to the genesis of suevite. Recent laboratory analyses of “crater suevite” occurring within the central crater basin and of “outer suevite” on top of the continuous ejecta blanket, as well as data accumulated during the past 50 years, are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations (Artemieva et al. 2013). The properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting. In a new approach, we reconstruct the minimum/maximum volumes of all allochthonous impact formations (108/116 km3), of suevite (14/22 km3), and the total volume of impact melt (4.9/8.0 km3) produced by the Ries impact event prior to erosion. These volumes are reasonably compatible with corresponding values obtained by numerical modeling. Taking all data on modal composition, texture, chemistry, and shock metamorphism of suevite, and the results of modeling into account, we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement. Numerical modeling indicates that only a very small fraction of suevite can be derived from the “primary ejecta plume,” which is possibly represented by the fine‐grained basal layer of outer suevite. The main mass of suevite was deposited from a “secondary plume” induced by an explosive reaction (“fuel‐coolant interaction”) of impact melt with water and volatile‐rich sedimentary rocks within a clast‐laden temporary melt pool. Both melt pool and plume appear to be heterogeneous in space and time. Outer suevite appears to be derived from an early formed, melt‐rich and clast‐poor plume region rich in strongly shocked components (melt ≫ clasts) and originating from an upper, more marginal zone of the melt pool. Crater suevite is obviously deposited from later formed, clast‐rich and melt‐poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper, central zone of the melt pool. Genetically, we distinguish between “primary suevite” which includes dike suevite, the lower sublayer of crater suevite, and possibly a basal layer of outer suevite, and “secondary suevite” represented by the massive upper sublayer of crater suevite and the main mass of outer suevite.

“…K‐metasomatism has been reported as a process related to impact‐induced hydrothermal activity in the crater suevites of the Ries impact structure, Germany (Osinski ; Muttik et al. ). During K‐metasomatic alteration, K‐rich fluid interacts with the host rock and may replace Ca and Na in feldspars.…”

Section: Discussionmentioning

confidence: 99%

The melt‐bearing impactites of the Ritland structure, Norway–Implications for melt formation in small impact craters

Kalleson

Dypvik

Riis³

et al. 2013

Abstract-A melt-bearing impactite unit is preserved in the 2.7 km diameter shallow marine Ritland impact structure. The main exposure of the melt-bearing unit is in an approximately 100 m long cliff about 700 m southwest of the center of the structure. The melt and clast content vary through this maximum 2 m thick unit, so that lithology ranges from impact melt rock to suevite. Stratigraphic variations with respect to the melt content, texture, mineralogy, and geochemistry have been studied in the field, and by laboratory analysis, including thin section microscopy. The base of the melt-bearing unit marks the transition from the underlying lithic basement breccia, and the unit may have been emplaced by an outward flow during the excavation stage. There is an upward development from a melt matrix-dominated lower part, that commonly shows flow structures, to an upper part characterized by more particulate matrix with patchy melt matrix domains, commonly as deformed melt slivers intermingled with small lithic clasts. Melt and lithic fragments in the upper part display a variety of shapes and compositions, some of which possibly represent fallback material from the ejecta cloud. The upper boundary of the melt-bearing impactite unit has been placed where the deposits are mainly clastic, probably representing slump and avalanche deposits from the modification stage. These deposits are therefore considered sedimentary and not impactites, despite the component of small melt fragments and shocked minerals within the lowermost part, which was probably incorporated as the debris moved down the steep crater walls.