46th Lunar and Planetary Science Conference (2015) 1534.pdf BUNTE BRECCIA REVISITED: THE DISTRIBUTION AND SOURCE OF WATER IN THE EJECTA OF RIES CRATER; GERMANY. A. Pietrek1, T.Kenkmann1 and D.Jung2 ,1Institute of Earth and Environmental Sciences- Geology, Albert-Ludwigs-University, Germany( [email protected]), 2Bavarian Agency of Geology, Hof, Germany Introduction: The Ries crater is a 26 km Miocene complex impact structure in Southern Germany. It formed in a target of a horizontally layered ~ 750 m thick sedimentary succession of limestones, sand and siltstones, claystones and marls overlying a Variscian basement (mainly gneiss, granite and amphibolite . A recent 3D compilation of the morphology and thickness distribution of the continuous Ries ejecta deposits (Bunte Breccia, BB)  revealed a concentric ejecta depression and a massive accumulation of ejecta similar to the morphology of Martian double layer (DLE) craters. The Ries crater formed both in the presence of an atmosphere and target volatiles. Vertical vents observed directly at the Suevite-BB contact document that substantial amounts of water were present in the BB and were vaporized after coverage with hot suevite . This study aimed to (1) detect where and to what amount water was present during the excavation and emplacement of BB, (2) to link that water to its preimpact source reservoir, and (3) to characterize the influence of water on the emplacement process. Two fully cored drillcores (Itzing, 7 km ESE of crater rim, 1.56 Rc, Otting, 3,5 km E of crater rim, 1.28 Rc) from 1976 were reevaluated (previously described by ) described in full length and studied in detail based on 36 handsamples and 31 polished thin sections (Fig.1). Observations: Drillcore description: Itzing drill core has a total length of 64 m with >52 m of BB. The BB is a succession of interlayered monomict or diamict crater derived breccias deposited over a polymict basal layer enriched in local material in contact with the authochthonous Malmian limestone. Unit I is a 16,55 m thick crystalline Breccia mixed with red and white Keuper sandstone and claystones. Unit two (16,5522m) is a Liassic Black shale Breccia with inclusions of red Keuper clay fragments, interlayered with a monomict marl breccia with mortar texture. The Liassic black shale has retained its original sedimentary stratification and shows mainly brittle in situ fracturing with subordinate local weak plastic deformation. Unit III (22-37,8m) is a several decameter thick swirly mixture of different white and red Keuper sands and clays, interrupted by solid blocks of a pale green-red mottled Keuper clay. Unit IV (37.8-52,1m) is a succession of monomict or diamict limestone breccias and a polymict polymict breccia phase with a clayey to silty, brownish matrix and between estimated 20 and 40% clasts from the whole range of the sedimentary range of the target. Over 80% of clast are limestone or Tertiary clays, with an increase of Tertiary clays with depth. Clasts are angular except for finely laminated Tertiary clays, which are intensely plastically deformed. The ratio of Tertiary increases with depth, both in matrix and clast content of the polymict breccia.The boundary to the autochthonous Malm is not distinctive and was estimated to lie at around 52,1 m based on the degree of fragmentation and displacement of the limestone. Otting drill core has a total length of 69, 5 m with ~47 m of BB . It is topped by 9 m Suevite (Unit I, ). Unit II (0,9-11,3m) is a succession of a polymict breccia phase, a Keuper sandstone Breccia and silty to fine sandy marls deformed by ductile flow. The shallower polymict phase 1 (Unit III, 11,3-26,0m) has a brownish-gray clayey matrix and contains angular clasts from the whole sedimentary range of the target. The clast content is estimated 20-25 %. Unit IV (26-28,2m) is an interlayering of the polymict breccia 1 and 2 with sharp contacts. The polymict phase 2 (Unit V, 28,2-38,7m) has a light whitish to reddish silty to sandy matrix with a similar clast content than phase 1 with the additional rare occurrence of crystalline clasts. Unit VI (38,758,4m) is a polymict breccia phase comparable to the basal polymict breccia phase of Itzing drillcore. The contact to the authochthonous limestone forms a distinct surface with striations . Fig. 1: A: Ductile flow of white and red Keuper sands and clays. B: Angular limestone clasts, mantled by plastically deformed Tertiary clay. Note how the clay is welded into the crevacces of the clast. C: Banded marl deformed by ductile flow. The marl injects into a polymict breccia phase. D: Intensely deformed Tertiary clay. Note how clasts of other lithologies are subangular to angular. Indicators for water: Tertiary clays generally are intensely plastically deformed (Fig 1D). They are often found wrapped or welded around competent clasts of 46th Lunar and Planetary Science Conference (2015) different lithologies (Fig.1B) or act as cohesive binding agent to form aggregates between different breccia phases or clasts. Tertiary clays are enriched in both basal polymict breccia layers and are rarely observed in the upper breccia phases. Keuper sandstones are rarely observed as competent clasts, but form several meter thick deposits of swirly and turbulent mixing patterns of different texture in Itzing drillcore (Fig.1,2A). A thin marly phase in Otting drillcore shows the same plastic deformation behaviour and furthermore has formed several injections into a polymict breccia body (Fig. 2C). Discussion: The loamy properties of Tertiary clays are indicative of water saturation during emplacement. The clays are enriched in the basal polymict breccia layer derived mainly from the local substrate and top of the target. The clays possibly acted as lubricant for the ground hugging flow of crater derived breccias.The ductile flow of several meter of Keuper sand requires the complete destruction of the grain matrix and virtual pulverization. Similar textures are described by  and are a common texture of Keuper sands in the BB. Mechanical abrasion, the mechanism proposed for the formation of mortar texture  is not sufficient alone, since no clasts are observed. We propose decompres- 1534.pdf sion vaporization of porewater as additional mechanism  enhancing fragmentation and formation of the flowpatters by granular flow. The injection features found near the Suevite-BB contact in Otting drillcore are similar to injection features of water saturated unconsolidated sediments which were sealed and buried. This implies that the marly phase was fluidized and overpressured, probably due to the heat of the hot Suevite emplaced few cm above. From our observations, surface water and pore water bound to Tertiary clays were present in substantial quantities in the basal polymict layer reworking surface material. Water was introduced into the crater derived upper part of the BB through pore water of deaper seated lithologies, at least from Keuper sandstone. References:.  Stöffler D. et al. (2012) MAPS.,48, 590-627.  Sturm S. et al. (2013) Geol.,41, 531-534.  Kenkmann T. and Wittmann A. (2010) LPI Contribution No. 1559, p.16.  Chao E.C.T. et al. (1977) LPSC.VIII, 163-165.  Hörz. et al. (1983) Rev. Geophys. Space Phys. 21, 1667-1725.  Hüttner R. and Schmidt-Kaler H. (2005) Geol. Bav., 104, 7-76.  Rager A.H. et al. (2014) EPSL, 385,68-78. Fig. 2: Schematic profiles of Itzing (left) and Otting drillcores (right). Both drillcores are topped by impactites derived mainly from the crystalline basement. The BB can be divided into an upper deposit derived from primary crater material resting upon a basal polymict layer with an increasing content of local material. The occurrence of fluid/plastic flow textures and plastic deformation features in samples is marked to the left of the columns.
© Copyright 2016 ExploreDoc