A Volcanic Ash Layer in the Nördlinger Ries Impact Structure (Miocene, Germany): Indication of Crater Fill Geometry and Origins of Long‐Term Crater Floor Sagging

Abstract

The stratigraphy and sedimentary environments of impact crater interior sedimentary fill form valuable climate archives on Earth, in particular those in young impact structures such as Bosumtwi and El’gygytgyn. Only a few terrestrial impact structures with preserved sedimentary fill are known. These are primarily present-day crater lakes such as Pingualuit, Bosumtwi, El’gygytgyn, and Tswaing (Guyard et al., 2011; Melles et al., 2012; Partridge et al., 1993; Shanahan et al., 2009; Talbot & Johannessen, 1992). These crater fill deposits precisely record Quaternary climate changes, such as aridification or glacial cycles, as revealed by geochemistry, stable isotopes, and palynomorphs (Melles et al., 2012; Shanahan et al., 2009; Talbot & Johannessen, 1992). Fossil examples, however, are commonly buried (Boltysh: Gurov et al., 2006; Chesapeake Abstract Since its recognition as an impact structure 60 years ago, no volcanics were anticipated in the circular depression of the 14.8 Ma old Nördlinger Ries. Here, we describe for the first time a volcanic ash-derived clinoptilolite-heulandite-buddingtonite bed within the 330 m thick Miocene lacustrine crater fill. Zircon U-Pb ages of 14.20 ± 0.08 Ma point to the source of the volcanic ash in the Pannonian Basin, 760 km east of the Ries. The diagenetically derived zeolite-feldspar bed occurs in laminated claystones of the Ries soda-lake stage and represents the first unequivocal stratigraphic marker bed in this basin, traceable from marginal surface outcrops to 218 m below surface in the crater center. These relationships demonstrate a deeply bowl-shaped geometry of crater fill sediments, not explainable by sediment compaction and corresponding stratigraphic backstripping alone. Since most of the claystones formed at shallow water depths, the bowl-shaped geometry must reflect 134 +23/−49 m of sagging of the crater floor. We attribute the sagging to compaction and closure of the dilatant macro-porosity of the deeply fractured and brecciated crater floor during basin sedimentation and loading, a process that lasted for more than 0.6 Myr. As a result, the outcrop pattern of the lithostratigraphic crater-fill units in its present erosional plane forms a concentric pattern. Recognition of this volcanic ash stratigraphic marker in the Ries crater provides insights into the temporal and stratigraphic relationships of crater formation and subsidence that have implications for impact-hosted lakes on Earth and Mars.