| Among the ~180 impact structures currently known on Earth, the ~24 km Nördlinger Ries
crater in southern Germany is unique in terms of the state of preservation of the crater shape
and its impact ejecta blanket and offers a considerably wide age data set achieved by various
types of material for isotopic dating and analytical methods [1]. The ~3.8 km Steinheim
Basin is a well-preserved, complex impact structure hosted by a sequence of Triassic
to Upper Jurassic sedimentary rocks of the eastern Swabian Alb, SW Germany
[2]. The specific alignment of the Steinheim Basin, the Nördlinger Ries crater,
and the Central European tektite strewn field had led to the assumption that both
impact structures formed simultaneously [3] during the ‘Ries-Steinheim event’
~14.59 Ma ago [1]. In contrast to the voluminous isotopic dataset for the Ries,
however, no isotopic dating of rock or mineral samples from the Steinheim Basin has
been carried out successfully and all efforts made did not yield reliable ages so
far.
In addition to the extensive isotopic dataset, we recently performed the first 40Ar/39Ar dating
of the Nördlinger Ries impact structure using monomineralic (recrystallized K-feldspar) melt
separated from a shocked crystalline target rock clast (granite) in impact melt rocks. This
40Ar/39Ar step-heating analysis yielded a 14.37 ± 0.30 (0.32) Ma (2Ïă) age for the Nördlinger
Ries supported by the age plateau characteristics, as well as the robust data set
statistics and a concordant total fusion age. Therewith, we added a new age to the
extensive isotopic age data set for this impact structure [1]. As a concept study on the
well-studied Ries impact structure, we (U-Th)/He dated zircons from Aumühle quarry
suevite samples and from Polsingen quarry impact melt rock yielding an age of
14.26 ± 0.31 Ma (2Ïă, n = 10) [4]. Among the recently discovered impact melt
lithologies at Steinheim is a pebble of partially molten, fluidally-textured Middle Jurassic
sandstone recovered from the central uplift (‘Steinhirt’) [2]. 40Ar/39Ar dating of
white to reddish feldspathic cryptocrystalline to glassy domains of the partially
molten sandstone failed to yield any statistically representative age but resulted in
strongly scattered age data with individual steps ranging from ~0 Ma to ~600 Ma,
inconsistent with the local Miocene crater lake biostratigraphy and earlier stratigraphic
age estimates [5]. (U-Th)/He dated zircons extracted from Jurassic and Triassic
sandstones that were drilled in the central uplift of the Steinheim Basin yielded ages that
range from 226.14 ± 8.16 to 281.97 ± 9.36 Ma (2Ïă) [5], thus indicating that the He
systematics in the clear zircon grains were not reset by the Steinheim Basin impact
event.
The isotopic ages for the Nördlinger Ries impact of 14.37 ± 0.30 (0.32) Ma (40Ar/39Ar
step-heating) and of 14.26 ± 0.31 Ma ascertained by (U-Th)/He dated zircons are
indistinguishable within error from the majority of the Nördlinger Ries ages obtained from
suevite glasses and Ries tektites in recent years. However, we suggest a slightly older age of
14.59 ± 0.20 Ma (2Ïă) as best value for the Ries impact event, based on current decay
constant values for 40K/40Ar and 40Ar/39Ar dating and 30 individual Ries ages obtained so
far [1]. Thus, the ~14.59 Ma impact age of the Nördlinger Ries crater can be regarded as
well-established. In the case of the Steinheim Basin, (U-Th)/He dating failed to resolve a
reasonable age for this impact event. On that account and also from an argon isotopic point
of view, the Steinheim impact age still remains elusive. From a biostratigraphic
point of view [6], it is likely that both craters (and their crater lake infill) formed
simultaneously but it cannot be ruled out that the two impact structures are the products
of two independent impact events in the Miocene. Therefore, the concurrency of
the Ries and Steinheim impacts still remains to be confirmed by isotopic dating
methods.
References: [1] Buchner et al. (2010) MAPS 45, 662-674. [2] Buchner and Schmieder
(2010) MAPS 45, 1093-1107. [3] Stöffler et al. (2002) MAPS 37, 1893-1907. [4] van
Soest et al. (2010) AGU abstract ID# P44B-08. [5] Schmieder et al. (2010) Paneth
Kolloquium abstract no. 45. [5] Buchner et al. (2010) 41 LPSC abstract no. 2151.
[6] Heizmann and Reiff (2002) Der Steinheimer Meteorkrater, Pfeil, Munich, 160
pp.
*Martin Schmieder, Francis J. Cooper, Kip V. Hodges, Fred Jourdan, Christian Köberl,
Gisela Pösges,
Uwe Reimold, Winfried H. Schwarz, Mario Trieloff, Matthijs C. van Soest and Jo-Anne
Wartho |