The Ries-Steinheim crater pair and two major earthquakes – New discoveries challenging the double-impact theory


 The Nördlinger Ries and the Steinheim Basin are widely perceived as a Middle Miocene impact crater doublet. We discovered two independent earthquake-produced seismite horizons in North Alpine Foreland Basin deposits. The older seismite horizon,associated with the Ries impact is overlain by in situ-preserved distal impact ejecta, forming a unique continental seismite-ejecta couplet within a distance up to 180 km from the crater. The younger seismite unit, also triggered by a major palaeo-earthquake, comprises clastic dikes that cut through the Ries seismite-ejecta couplet. The clastic dikes were likely formed in response to the Steinheim impact, some kyr after the Ries impact, in line with paleontologic results. With the Ries and Steinheim impacts as two separate events, Southern Germany witnessed a double disaster in the Middle Miocene. The magnitude–distance relationship of seismite formation during large earthquakes suggests the seismic and destructive potential of impact-earthquakes may be significantly underestimated.


Ries-related seismite
We discovered sedimentary successions with distinct soft-sediment deformation structures in a temporary construction site near Ochsenhausen 22,31 , in three ravines at the 'Tobel Oelhalde-Nord' and 'Wannenwaldtobel' close to Biberach an der Riß (Fig. 2, Supplementary Fig. 1), and at the 'Kleintobel' near Ravensburg (Fig. 3, Supplementary Fig. 2). The soft-sediment deformation structures include meter-sized slumps (Figs. [2][3][4], all with NW-SE-directed slump axes, convolute bedding, ball-and-pillow and ame structures, and clastic dikes. Such structures in continental deposits are typical of seismites caused by large earthquakes 22,24,30 . The DREL that caps the seismite unit (Figs. 2,4,5) provides compelling evidence that the Ries impact was the source for this seismic event, causing soft-sediment deformation within a radial distance of ~100 to 180 km from the impact site. The potential reasons for the restricted occurrence of the seismite horizon within the study area (as opposed to an area-wide distribution in the surroundings of the Ries and Steinheim craters) are explained in detail in chapter 'Distribution of seismites' in the Supplementary Material.

was described from several outcrops in the Middle Miocene Upper Freshwater
Molasse of the North Alpine Foreland Basin in Bavaria 33,35,52 (SE Germany), Baden-Württemberg 10,11,31 (SW Germany), and NE Switzerland 34 . During eld work, we found additional outcrops of distal Ries ejecta in three ravines south of Biberach an der Riß and west of Ravensburg, recpectively. At all outcrop sites analyzed in this study, distal Ries ejecta overlie a seismite unit, thereby forming a distinct seismiteejecta couplet. The ejecta horizon occurs either as a primary, in situ-preserved (Fig. 2 4C) into the soft sediment after ballistic air-travel over >100 km 10,11,31,34 . Some of the clasts (mainly of Upper Jurassic limestones) contain shatter-cones (Fig. 4E). These observations suggest the seismite in the underlying pre-Ries deposits is genetically related to the Ries impact. The exposures of the seismite-ejecta couplet are situated within a distance of ~100 km (Ochsenhausen), ~110 km (Biberach), and ~140 km (Ravensburg) from the center of the Ries crater, respectively (Fig. 1). The most distant known occurrence of coarse-grained Ries ejecta occurs ~180 km SSW of the Ries crater, in an outcrop near Berhardzell in NE Switzerland (Fig. 1) from which shocked quartz grains were reported (pers.comm. Carl Alwmark). In this study, we present new evidence for shocked quartz grains with up to four sets of planar deformation features in loose sands constituting Ries ejecta exposed in the Tobel Oelhalde-Nord (Biberach; Fig. 4F, Supplementary Fig. 3), and with up to six sets of planar deformation features in Ries ejecta from the Kleintobel (Ravensburg, Supplementary Fig. 5). The Upper Freshwater Molasse deposits that overlie (i.e., postdate) the DREL are typically cross-bedded or horizontally layered and generally appear undisturbed and unaffected by dewatering processes.

Clastic dikes
In addition to the seismite capped by distal Ries ejecta, we discovered large-scale clastic dikes rst described at the anks of the Tobel Oelhalde-Nord near Biberach 22 and at the Kleintobel near Ravensburg (this study). Those clastic dikes are earthquake-produced structures 22,24,32 that crosscut the Ries-related seismite, ejecta (the DREL), and undisturbed post-Ries deposits and, hence, clearly postdate the Ries impact event and earthquake. A horizon of distal Ries ejecta associated with smaller clastic dikes is also known from Bernhardzell, Switzerland 35 . Those dikes also seem to postdate the Ries impact and, overall, the local facies and structural situation resemble those at Biberach and Ravensburg 22 . The genetic relationship between the seismite-hosting deposits and the Ries impact is evidenced by the primary, in situ-preserved ( Supplementary Fig. 4) DREL sitting right on top of these deposits. The large clastic dike that cross-cuts both the Ries seismite and ejecta near Biberach was recently tentatively linked to the somewhat younger Steinheim impact 22 . Notably, this scenario -suggesting two spatially and temporally separate impacts -challenges the widely accepted binary asteroid hypothesis for the Ries-Steinheim event 1,2,3,8, .

Another popular 'double impact' fails inspection
The distinct SW-NE alignment of the Steinheim Basin, the Nördlinger Ries impact structures, and the Central European tektite strewn eld supports the general notion that both impact structures represent an impact crater doublet formed by an incoming pair of asteroids entering the Earth's gravitational eld from the SW 1,8 . While a precise and accurate 40 Ar/ 39 Ar age has been established for the Ries impact (14.808 ± 0.038 Ma 12,13 ), isotopic dating has, thus far, failed to yield a geologically meaningful age for the Steinheim impact. Several studies pointed out that the simultaneous formation theory for the two impact structures is, in fact, not supported by palaeontologic and structural geologic constraints 5,8,38 (and references therein). From a biostratigraphic point of view, the Steinheim impact could postdate the Ries impact by as much as 1 Myr 5,38 . The oldest lake deposits inside the Ries crater contain a fossil fauna that belongs to the mammal zone MN 6 (Langhian stage of Miocene), whereas fossils in the basal lake deposits of the Steinheim Basin correspond to the transition of mammal zones MN 6 to MN 7 38,40 (Serravallian stage of Miocene), thus representing a time gap of at least ~0.6 Myr 5,38,40 (Fig. 6), in con ict with the double-impact scenario 5,8,22 . Moreover, a NW-SE-trending impact direction proposed for the Steinheim Basin 8 , as well as profoundly differing impactor traces at both impact sites (i.e., a fossil likely pallasite as the Steinheim meteorite 6,8 vs. a missing or achondritic impactor signature for the Ries 6,8 ) are at odds with the widely accepted double impact scenario.
Both the Nördlinger Ries and the slightly younger Steinheim impacts would have imparted signi cant energy into the sedimentary target, causing at least regional-scale disturbances. The occurrence of a laterally extensive seismite in sandy deposits of the Upper Freshwater Molasse of pre-Ries age, exposed near Biberach, Ochsenhausen, and Ravensburg and capped by a primary horizon of in situ-preserved distal Ries ejecta and undisturbed younger deposits, proves that the seismite is the product of a Ries impact-induced giant earthquake. At Biberach 22 , Ravensburg, and Bernhardzell 34 , clastic dikes cut through the Ries-related seismite-ejecta couplet and portions of the overlying Upper Freshwater Molasse. This provides tangible evidence for a second, high-magnitude earthquake in the region that had previously been affected by the 'Ries earthquake'. The Biberach clastic dike exposed at the Tobel Oelhalde-Nord reached the former land surface forming an extrusive fossil sand volcano 22,65 . Based on the age constraints for the dike-hosting sediments 22,35,37 , the dike is the product of a seismic event that occurred between ~14.81 Ma (Ries impact 12,13 ) and approximately14.3 Ma (terminal sedimentation of the 'Fluviatile Untere Serie unit 35,37 ). In contrast to the precise age for the Ries 12,13 , the latter age is not very well constrained and may be associated with an error of a few kyr 22 . A seismo-tectonic (alpine tectonism) or volcano-seismic event (within the Paleogene to Quaternary European Volcanic Province) was recently discussed 22 as a potential source for the younger earthquake some ~0.5 Myr after the Ries impact. However, considering their geographical position and rather low seismic potential 22 , none of these earthquake centers can convincingly explain the formation of the post-Ries clastic dikes 22 .
The dimensions of the sandstone dikes signi cantly decrease towards the South, from the giant Biberach clastic dike in the North and the dikes near Ravensburg to the dm-long clastic dikes of Bernhardzell in Switzerland. These localities are situated at 80 km, 110 km, and 150 km south of the Steinheim crater, respectively. Dike dimensions are a function of host rock properties and seismic energy 22,24,32,36 . Taking the comparable rock properties and the signi cantly different dimensions of the clastic dikes at the three localities into account, the seismo-tectonic epicenter was located closer to, and likely north of, the Biberach area. This suggests the Steinheim impact may have been the trigger mechanism of the post-Ries seismic event 22 .
Supporting evidence for a major post-Ries seismic event comes from sediments of the Ries crater lake. Ã 314 m-thick sequence of crater lake deposits was drilled in the scienti c drilling project 1973. This sediment sequence, deposited in a lake that lasted for ~1 Myr 43 , contains olistoliths and sediments with intense slumping and convolute bedding 42 . Somewhat surprisingly, the slumped deposits do not occur at the basis of the lake deposits, which would have been favored by the steep relief of the newly formed, precipitous impact crater; but soft-sediment deformation appears to be dominant in the middle of the sedimentary succession. The slumps and convolute bedding within the crater lake could well represent a long-distance effect of a strong earthquake some hundred kyr after the Ries impact, potentially triggered by the Steinheim impact ~40 km SW of the Ries crater. In the past decade, many of the seemingly well-established terrestrial impact crater doublets and chains were discredited. 40  Distal Ries ejecta blanketed the Ries seismite, was locally preserved in situ, and presently crops out in ravines and a river bank. 3. As an additional feature, clastic dikes cutting through the Ries-related seismite-ejecta unit appears to have been caused by a second high-magnitude earthquake presumably linked to the Steinheim impact some kyr after the Ries impact event 22 . The occurrences of the seismite near Biberach, Ochsenhausen, Ravensburg, and Bernhardzell are the rst deposits in which evidence for earthquake-induced soft-sediment deformation structures caused by the Ries impact has been documented. To our knowledge, this is also the rst known occurrence of a primary continental seismiteejecta couplet preserved in situ.

Magnitudes of impact-earthquakes
The magnitude of earthquakes induced by meteorite impacts is still somewhat uncertain, and the seismic e ciency (i.e., the portion of the impactor's kinetic energy transformed into seismic energy) is only constrained within two orders of magnitude (for the theoretical background and calculations see Methods section) 18,22,57 . Accordingly, taking into account global-scale seismic effects (tentatively) linked with terrestrial impacts 19,20,25,29 , calculated magnitudes may, in cases, be too conservative 22 . The magnitude of the 'Chicxulub earthquake' was probably approximately M L 9.9 18,19 . Endogenic (tectonic) earthquakes may not reach such an extraordinary magnitude, and the strongest earthquakes ever recorded correspond to a Richter magnitude M L 8.6 to 8.7 (e.g., the great Valdivia, Chile, or Alaska earthquakes 62,64 ).
An earthquake of at least M L 5 to 6 and a moment magnitude of M W 6.5 is required for the formation of seismites 22,24 . The systematic relation between speci c styles of crustal deformation (e.g., clastic dikes and soft-sediment deformation) and radial distance from the seismic source depending on the earthquake magnitude was studied for many regions on Earth 57,62-64 . Liquefaction and the concomitant formation of seismites caused by impact-induced earthquakes is preserved in the sedimentary record at a number of localities worldwide and summarized in a comprehensive database 16,21,22, . However, the earthquake magnitude-distance relationship for lique cation effects is currently still underexplored and needs to be evaluated from the perspective of geologically younger major earthquakes. minutes after the impact event, a hot air blast 56 reached the study area blowing off woods, soil, and the upper portions of the slumps and deformed soft-sediments. A typical feature of the DREL is that it commonly lies on deformed Upper Freshwater Molasse sediments that are truncated at the top and exhibit an almost perfectly at paleosurface (Fig. 2). This 'disaster topography' does not correspond with the original, unaffected palaeolandscape that was dominated by rivers, lakes, and damp forests 5,53 .
Charred wood, reported for instance from the Unterneul sandpit 53 , suggests high temperatures of the airblast. Within three to ve minutes, an episode of bombardment by pebbles, cobbles, and boulders mainly of Upper Jurassic limestones, many of them shatter-coned (Fig. 2), ensued 2,3,10,11,22,31,34 . The ballistically transported components stem from the uppermost tens of meters of the Ries target rocks 2,10 . They directly overlie the seismite in Upper Freshwater Molasse deposits and sometimes penetrate these sediments by a few cm or dm, thereby forming small funnel-like depressions (Fig. 2). Accordingly, these features can be described as small-scale secondary impact pits (i.e., formed by ejecta projectiles), an impact-related feature rarely seen on Earth 11,22,31 .
In addition to the larger cobbles and bolders at the base of the cm-to dm-thick primary ejecta horizon, the ejecta layer also consists of sand and small pebbles mainly made up of grains of limestone, quartz, and feldspar 22 . These ner-grained deposits locally show a distinct ning-upward trend. Quartz grains in the ejecta horizon are often very angular and show a weak to moderate shock overprint (e.g., indistinct planar deformation features in one or two directions) in agreement with pressures at the lower end of the shock metamorphic regime (mostly <5 GPa). Only a small proportion of quartz grains in the distal Ries ejecta horizon of the study area show higher degree of shock-metamorphic overprint in the form of planar deformation features in up to six optically visible directions (Fig. 2F, Supplemetary Fig. 5). These highly shocked quartz grains were probably derived from the crystalline basement and, hence, from deeper parts of the Ries target (at least ~600 m below the former land surface).
The ejected material temporarily reached a height of ~50 to 100 km above the land surface 2 . In contrast to the coarser ejecta fragments, the highly shocked quartz grains were not ballistically transported, but are more likely part of the fallout from the Ries impact plume that began to collapse roughly two minutes after the impact 2,3 . Fallout from the impact plume may have rained down for minutes to hours 2,3 . Similar to crustal materials dispersed during volcanic eruptions 51,60,69 , small ejecta particles and ash from the impact plume probably reached the higher troposphere and stratosphere and caused havy rainfall for days (and possibly for weeks or months due to the atmospheric disturbance) after the impact event.
Finally, the Ries impact event was followed by havy rainfall and ash oods, as known from volcanic eruptions 59,60 . Fluvial channels were incised into the seismite-bearing Upper Freshwater Molasse in the study area (Fig. 3) and now contain a mix of reworked DREL and locally-derived rock material that can be correlated across several exposures within the North Alpine Foreland Basin. The reworked layers sometimes lack obvious sorting or grading and clasts are matrix-supported. These debritic layers show similarities to lahars to a certain degree. Most of the reworked layers, however, show indistinct sorting, and rounding and imbrication of clasts indicate transport and deposition in fast-owing, high-energy ood streams (Fig. 3). Logs and pieces of wood up to 2.6 m in length 61 , relics of the impact-blasted wet forest 61 , are abundant in the reworked uvial deposits. Moreover, well-preserved skeletal remains of the Miocene rhinoceros Brachypotherium brachypus was reported in ash ood deposits near Ravensburg 61 . It can be speculated wheater this very big animal was killed by the hot airblast, struck to death by incoming Ries ejecta boulders, or whether it drowned in the 'tsunami-like' continental ash ood following the impact event. In the Biberach and the Ravensburg area, the primary DREL resembles a bone bed owing to the high concentration of fossil wood, remnants of amphibians, reptiles (e.g., turtles, small alligators), and mammals amongst other bones and teeth of rhinoceroses, peccaries, deers (Fig. 2D), aquatic musketeers, and other hoof animals 61 . The intact nature of bones and teeth document that these fossils were not signi cantly reworked and that the nding situation is more or less in situ. Some 500 kyr later, the same region was affected by a second giant earthquake, presumably induced by the Steinheim impact event, that produced large dikes cutting through the Ries seismite-ejecta couplet and the overlying layers of Upper Freshwater Molasse. With the Ries and Steinheim impacts as an extraterrestrial one-two punch, Southern Germany seems to have witnessed a veritable double disaster in the Middle Miocene.

Field studies
In the last three decades, the DREL 10,11,22,31,34 was systematically investigated in the North Alpine Foreland Basin. We paid particular attention to ravines in the areas of Biberach and Ravensburg in SW Germany and Bernhardzell (St. Gallen, Switzerland). After heavy rainfall in the Biberach and Ravensburg area in spring 2019, deposits with soft-sediment deformation structures and clastic dikes were partially exposed below and above the distal ejecta horizon along the valley sides of the ravines. The structures were excavated during eight eld campaigns from spring to winter 2019. We excavated the sandy foreland basin deposits over a vertical extension of 15 m along the ank of the 'Tobel Oelhalde-Nord' (Biberach) and over tens of meters laterally along the anks of the ravines 'Tobel Oelhalde-Nord and -Süd' (Biberach) and Kleintobel (Ravensburg).

Petrography
Samples of the dike's in lls were taken, stabilized by synthetic resin, and processed to polished thin sections. Thin sections of the dike's in ll were analyzed for their petrographic properties using a polarization microscope. Additional unconsolidated samples of the in ll were investigated by re ectedlight microscopy to assess their fossil content (e.g., Miocene mammal bones, invertebrates, and plant remnants).

Shock metamorphism
Mineral grains separated from the distal Ries ejecta horizon from the Tobel Oelhalde-Nord south of Biberach and Kleintobel near Ravensburg were mounted in epoxy blocks from which thin sections were prepared, then studied using a four-axis universal stage mounted on an optical microscope. Planar deformation feature (PDF) planes in quartz grains and their crystallographic orientation were determined using the universal stage microscope 66,67 . The resulting PDF orientations are indicative of shock pressures that affected the impacted bedrock 66,67 . However, this method requires the detailed analysis of a large number of shocked quartz grains. Due to their rare nature in the distal Ries ejecta horizon, this study does not provide systematic PDF statistics.

Estimated magnitude of impact earthquakes
Seismic e ciency (i.e., the fraction of the impactor's kinetic energy that is transformed into seismic wave energy) is thought to range between 10 −5 and 10 −3 . Using a mean value of 10 −4 for that e ciency 18,57 (and references therein), an equation that correlates the impact energy with the resultant seismic magnitude was derived: Declarations Tables   Table 1 List of the environmental effects of the Ries event from seconds to days after impact affecting the wider surroundings of the impact structure as to be observed in the study area 100 to 180 km from the center of the crater.