New discovery of two seismite horizons challenges the Ries-Steinheim double-impact theory

This study addresses whether the Nördlinger Ries (~24 km diameter) and Steinheim Basin (~4 km diameter) impact structures in southern Germany formed simultaneously as a binary asteroid impact or represent temporally separate events. The context involves well-preserved impact features at both sites, including ejecta blankets and suevite at Ries and shatter cones and impact breccias at Steinheim. A distinctive distal Ries ejecta layer (DREL) occurs up to 180 km from Ries within North Alpine Foreland Basin sediments. Large impacts also generate significant earthquakes capable of producing seismites. Prior work identified clastic dikes cutting the DREL, hinting at post-Ries seismicity potentially linked to the Steinheim impact. The purpose of this study is to document two separate regional seismite horizons—one associated with the Ries impact and a younger one expressed by clastic dikes—and to evaluate their implications for the timing of the Ries and Steinheim events and the magnitude-distance relationships of impact-induced earthquakes.

• Distal impact ejecta layers are known from several events, notably around the Cretaceous-Paleogene (K-Pg) boundary in association with the Chicxulub impact, with documented occurrences thousands of kilometers from the source crater. Comparable distal deposits with shocked minerals and shatter-coned clasts are also reported from the Acraman impact in South Australia (>500 km from source).
• For the Ries impact, the coarse-grained DREL has been documented at numerous North Alpine Foreland Basin sites up to ~180 km from the crater and contains clasts (often Upper Jurassic limestone) including shatter cones, suggesting shock pressures of at least ~2 GPa.
• Impact-triggered earthquakes can produce soft-sediment deformation (slumps, convolute bedding, ball-and-pillow, flame structures) and clastic dikes, similar to tectonic seismites, with style governed by sediment properties and water saturation.
• The simultaneity of Ries and Steinheim has been questioned by biostratigraphic and structural evidence, and isotopic dating for Steinheim remains inconclusive, whereas a precise 40Ar/39Ar age exists for Ries (14.808 ± 0.038 Ma).

Field studies: Systematic field investigations over three decades focused on DREL localities in the North Alpine Foreland Basin (Germany and NE Switzerland). Intensive campaigns in 2019 in ravines near Biberach (Tobel Oelhalde-Nord and -Süd) and Ravensburg (Kleintobel) excavated up to ~15 m vertically and tens of meters laterally after heavy rainfall exposed structures above and below the ejecta horizon. Outcrops were logged for bedding, soft-sediment deformation, seismites, and cross-cutting clastic dikes.

Petrography: Clastic dike infill samples were stabilized with resin, prepared as polished thin sections, and examined by polarization microscopy. Unconsolidated infill was also studied under reflected light to assess fossil content.

Shock metamorphism: Mineral separates from DREL sands (Biberach, Ravensburg) were mounted in epoxy and prepared for universal-stage microscopy. Planar deformation features (PDFs) in quartz, including crystallographic orientations, were measured to assess shock levels. Due to rarity of highly shocked grains in distal ejecta, no comprehensive PDF statistics were compiled.

Earthquake magnitude estimation: Seismic magnitude estimates used a seismic efficiency factor of ~10^-4 to relate impact kinetic energy to earthquake magnitude (M ≈ 0.67 log10 E − 5.87). Moment magnitudes (Mw) were inferred from local magnitudes (ML) using empirical relations, acknowledging ML saturation at high magnitudes. Magnitude-distance relationships for liquefaction and clastic dike formation were compared with historical earthquake datasets to contextualize observed distal seismites and dikes.

• Discovery of two distinct seismite horizons in the North Alpine Foreland Basin:
  • An older seismite horizon with soft-sediment deformation (slumps, convolute bedding, ball-and-pillow, flame structures) capped by in situ distal Ries ejecta (DREL), forming a primary continental seismite–ejecta couplet. Occurs 100–180 km from the Ries crater (e.g., Ochsenhausen ~100 km; Biberach ~110 km; Ravensburg ~140 km; Bernhardzell ~180 km).
  • A younger horizon expressed by clastic dikes that cross-cut the Ries-related seismite and overlying DREL, and extend into post-Ries deposits, indicating a later major earthquake.
• The DREL includes coarse clasts (Upper Jurassic limestone, some with shatter cones) and finer quartz- and feldspar-rich sands showing fining-upward trends; quartz grains often angular with weak-to-moderate shock overprint, and rare grains with up to 4–6 sets of PDFs, indicating derivation from deeper crystalline basement.
• Seismite geometry and slump-axis orientations (NW–SE) point to a seismic source in the Ries–Steinheim region, not the Alps or local volcanic fields.
• Timing constraints:
  • Ries impact precisely dated to 14.808 ± 0.038 Ma; the overlying DREL is primary at several sites.
  • Clastic dikes formed after the Ries event and before terminal sedimentation of the Fluviatile Untere Serie (~14.3 Ma), implying a ~0.5–0.6 Myr gap consistent with biostratigraphic differences between Ries (MN5–MN6) and Steinheim (MN6–MN7) crater lake faunas.
  • Suggested best-fit age for Steinheim ~14.3 Ma (within ~14.8–14.1 Ma from climate and stratigraphic constraints).
• Magnitude–distance implications:
  • Ries earthquake estimated at Mw ~8.5 (possibly higher), consistent with observed liquefaction and seismites out to ~180 km.
  • Formation of clastic dikes at radial distances up to ~150 km implies a triggering earthquake of at least Mw ~8.5, higher than prior estimate for Steinheim (Mw ~6.6), suggesting impact-induced seismicity may be underestimated.
• Environmental sequence at 100–180 km from Ries (minutes after impact): initial seismic shaking (15–60 s), arrival of fireball/air blast (2–4 min) causing truncation and charred wood, ballistic ejecta deposition (3–5 min) of pebbles to boulders producing small impact pits, plume fallout sands for minutes to hours, followed by heavy rainfall/flash floods reworking ejecta into channels.
• Evidence within Ries crater lake cores shows mid-sequence slumps and convolute bedding potentially recording a later strong earthquake, consistent with a post-Ries event (likely Steinheim).

The documented seismite–ejecta couplet unequivocally ties the older soft-sediment deformation horizon to the Ries impact, establishing that a large impact generated seismites at least 100–180 km from the crater. Cross-cutting clastic dikes demonstrate a second, younger high-magnitude earthquake that postdates Ries. Biostratigraphic differences between crater lake deposits and regional stratigraphy, together with climate-driven groundwater changes, support temporal separation of the impacts by ~0.5–0.6 Myr (potentially up to ~1 Myr). This challenges the widely held binary asteroid double-impact model for Ries–Steinheim and favors two independent impacts.

Magnitude–distance comparisons with historical tectonic earthquakes suggest that both the extent of Ries-related liquefaction and the distal occurrence of clastic dikes require very large magnitudes (≥Mw ~8.5), implying that the seismic efficiency and destructive potential of impact-induced earthquakes may be higher than previously assumed. Spatial trends in dike dimensions (largest near Biberach, diminishing southwards) argue for a seismic source north of Biberach, consistent with Steinheim’s location, and make alpine tectonic or phreatomagmatic sources unlikely given their positions and typical seismic efficiencies.

Climatic conditions likely modulated the style of deformation: Ries occurred during warm, humid conditions with high groundwater, promoting widespread soft-sediment deformation; the later event occurred under drier conditions with deeper groundwater, favoring fracture propagation and clastic diking over pervasive liquefaction. Together, the observations integrate stratigraphic, sedimentologic, structural, paleontologic, and shock metamorphic evidence to support two separate mid-Miocene impacts in southern Germany.

This work identifies and documents two distinct paleoseismic horizons in the North Alpine Foreland Basin: (1) a Ries impact-related seismite capped by in situ distal ejecta forming a unique continental seismite–ejecta couplet up to ~180 km from the crater, and (2) younger, cross-cutting clastic dikes attributed to a subsequent high-magnitude earthquake, most plausibly the Steinheim impact. The results imply that Ries and Steinheim were temporally separate events, approximately 0.5–0.6 Myr apart, thereby overturning the commonly assumed binary-asteroid double-impact model. Magnitude–distance relationships indicate impact-induced seismicity can be extremely energetic and more far-reaching than many endogenic analogs.

Future research should: obtain precise and accurate geochronology for Steinheim; expand regional mapping of the seismite–ejecta couplet and clastic dikes; refine impact seismic efficiency and wave propagation models; develop quantitative magnitude–distance calibrations for impact-induced liquefaction and diking; and further investigate crater lake records for evidence of post-impact seismic events.

• The seismic efficiency factor and near-surface seismic wave propagation for impacts are poorly constrained (uncertainty over several orders of magnitude), affecting magnitude estimates.
• Isotopic dating for the Steinheim impact is lacking; its age is inferred indirectly from biostratigraphy and regional stratigraphy.
• Shocked quartz grains are scarce in distal ejecta; no comprehensive PDF statistical dataset was produced, limiting quantitative shock pressure reconstructions.
• Alternative seismic sources (alpine tectonism, regional volcanism) cannot be absolutely excluded, though considered unlikely based on distance and expected seismic efficiencies.
• Seismite distribution likely depends on local sediment properties and groundwater conditions, limiting spatial generalization of observations.