| Abstract (ENG): |
Germany is a well-preserved complex impact crater with a prominent central uplift. The impact crater, known for its
shatter cones, is set in a sequence of Mesozoic sediments and lies some 40 km SW of the ~24 km-diameter Nördlinger
Ries [1]. While geochronologic studies of impact melt samples from the Ries crater (tektites) yielded a high-precision
Mid-Miocene (Langhian) Ar-Ar age of 14.81 Ma [2], several attempts to date the Steinheim impact using isotopic
techniques failed to produce a meaningful geologic age. This is partly due to the fact that, in contrast to the Ries,
impact melt lithologies at Steinheim are rather scarce and, if present, mainly of carbonatic composition [3] or intensely
altered to sheet silicates [4]. An Ar-Ar study of a pebble of fluidal, partially melted sandstone found at the Steinheim
central uplift [4] produced disturbed age spectra without any representative age signal. (U-Th/He) dating of zircon
crystals from the central uplift yielded partially reset (Moldanubian?) ages between ~282 and 247 Ma [5].
The double impact theory: The general notion that the Ries and Steinheim craters must have been produced
simultaneously by the impact of a binary asteroid [6,7] is, after all, mainly based on the assumption that two closely
spaced asteroid impacts around the same time are (higly) unlikely to occur at random [8]. However, as shown for other
suspected terrestrial impact crater pairs, such as the East and West Clearwater Lake impact structures in Canada [9]
and the Suvasvesi North and South impact structures in Finland [10], ‘false’ impact crater doublets do exist on Earth.
In such cases, misleading underlying theories can compromise an independent assessment of impact ages [11].
Biostratigraphic constraints: The most promising solution is offered by biostratigraphic constraints on the formation of the Ries and Steinheim impact structures and their respective crater lake deposits. One would assume both
crater depressions filled up with freshwater soon after their formation, as expected in a deeply water-saturated,
karstified paleolandscape with a high Mid-Miocene groundwater level [12]. Water saturation is, in fact, well demonstrated by aragonitic spring mound deposits (the ‘Wäldlesfels’) sitting on top of the Steinheim central uplift [1]. Likewise, the pervasive alteration of impact melt lithologies in the crater-filling impact breccia suggests the fresh Steinheim crater may have hosted a short-lived hydrothermal system [4]. For these reasons, we argue the basal crater lake
sediments at Steinheim more or less accurately reflect the timing of the impact. While the Mid-Miocene paleofauna
at the Ries crater lake has been assigned to the Mammal Neogene (MN) zone MN 6 (type locality: Sansan, France)
[13], with a coarse-grained Ries ejecta horizon in the North Alpine Foreland Basin sitting at a stratigraphic level close
to the MN 5/6 transition [14,15], Steinheim is known as a type locality for the somewhat younger MN 7 paleofauna
[1,13,16]. The basal lake sediments at Steinheim (the so-called kleini, steinheimensis, and sulcatus beds, named after
the gastropod Gyraulus) represent the upper MN 6 biozone (Langhian) and are overlain by younger lake sediments
(the trochiformis, oxystoma, revertens, and supremus beds) that belong to MN 7 (Langhian and Serravallian) [15,17].
In other words, the oldest, basal crater lake sediments at the Nördlinger Ries and Steinheim are separated by approximately one entire mammal biozone.
Discussion and conclusions: From a biostratigraphic point of view, the Steinheim impact appears to postdate the
Ries event by several kyr (and potentially ~1 Myr) [1,13]. While the Ries impact has a precise mid-Langhian Ar-Ar
age of ~14.81 Ma [2], the current best-estimate age for the Steinheim impact is derived entirely from biostratigraphic
constraints. Taking into account stratigraphic correlation and recent geochronologic results for the Langhian/Serravallian boundary (~13.8 Ma [18]), a late Langhian Steinheim impact scenario between ~14.3 and 14.0 Ma appears most
plausible. We, moreover, note the Ries and Steinheim impacts correlate well, biostratigraphically, with two
spectacular continental seismite horizons in Molasse deposits of the North Alpine Foreland Basin [19]. It remains
unclear at this point whether the two impacts are causally related to faunal turnover events in European land mammals
at the MN 5/6 (within uncertainty Ries-age) and MN 6/7 (approximately Steinheim-age) transitions [20,21]. |
| Citation: |
Schmieder, Martin and Buchner, Elmar and Sach, Volker J.
(2022)
How old is the steinheim impact crater (Germany)?
In: (Proceedings of the) 85th Annual Meeting of The Meteoritical Society 2022, August, 14-19, 2022, Glasgow, Scotland, Paper Abstract 6072.
(LPI Contribution; 2695).
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