BUNTE BRECCIA REVISITED: THE DISTRIBUTION AND SOURCE

46th Lunar and Planetary Science Conference (2015)
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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 [1].
A recent 3D compilation of the morphology and thickness distribution of the continuous Ries ejecta deposits
(Bunte Breccia, BB) [2] 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
[3]. 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 [4])
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, [4]). 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 [4].
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 [5] and
are a common texture of Keuper sands in the BB. Mechanical abrasion, the mechanism proposed for the
formation of mortar texture [7] is not sufficient alone,
since no clasts are observed. We propose decompres-
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sion vaporization of porewater as additional mechanism [9] 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:. [1] Stöffler D. et al. (2012) MAPS.,48,
590-627. [2] Sturm S. et al. (2013) Geol.,41, 531-534.
[3] Kenkmann T. and Wittmann A. (2010) LPI Contribution No. 1559, p.16. [4] Chao E.C.T. et al. (1977)
LPSC.VIII, 163-165. [5] Hörz. et al. (1983) Rev.
Geophys. Space Phys. 21, 1667-1725. [7] Hüttner R.
and Schmidt-Kaler H. (2005) Geol. Bav., 104, 7-76.
[8] 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.