Ries–related seismite
We discovered sedimentary successions with distinct soft-sediment deformation structures in a temporary construction site near Ochsenhausen22,31, 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. 2-4), all with NW-SE-directed slump axes, convolute bedding, ball-and-pillow and flame structures, and clastic dikes. Such structures in continental deposits are typical of seismites caused by large earthquakes22,24,30. 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.
Distal Ries ejecta
The DREL10,11,22, 31, 34 was described from several outcrops in the Middle Miocene Upper Freshwater Molasse of the North Alpine Foreland Basin in Bavaria33,35,52 (SE Germany), Baden-Württemberg10,11,31 (SW Germany), and NE Switzerland34. During field 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 seismite-ejecta couplet. The ejecta horizon occurs either as a primary, in situ-preserved (Fig. 2), or secondary (fluvially reworked; Fig. 3) layer of ejecta. At the Tobel Oelhalde-Nord, Wannenwaldtobel (both Biberach; Figs. 2,3), and Kleintobel (Ravensburg; Fig. 4), angular clasts (Supplementary Fig. 3) of Upper Jurassic limestone often produced small dents (Fig. 4B) caused by the impact of Ries-ejected pebbles, cobbles, and boulders (Fig. 4C) into the soft sediment after ballistic air-travel over >100 km10,11,31,34. 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 first described at the flanks of the Tobel Oelhalde-Nord near Biberach22 and at the Kleintobel near Ravensburg (this study). Those clastic dikes are earthquake-produced structures22,24,32 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, Switzerland35. Those dikes also seem to postdate the Ries impact and, overall, the local facies and structural situation resemble those at Biberach and Ravensburg22. 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 impact22. Notably, this scenario – suggesting two spatially and temporally separate impacts – challenges the widely accepted binary asteroid hypothesis for the Ries-Steinheim event1,2,3,8,.
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 field 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 field from the SW1,8. While a precise and accurate 40Ar/39Ar age has been established for the Ries impact (14.808 ± 0.038 Ma12,13), 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 constraints5,8,38 (and references therein). From a biostratigraphic point of view, the Steinheim impact could postdate the Ries impact by as much as 1 Myr5,38. 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 738,40 (Serravallian stage of Miocene), thus representing a time gap of at least ~0.6 Myr5,38,40 (Fig. 6), in conflict with the double-impact scenario5,8,22. Moreover, a NW-SE-trending impact direction proposed for the Steinheim Basin8, as well as profoundly differing impactor traces at both impact sites (i.e., a fossil likely pallasite as the Steinheim meteorite6,8 vs. a missing or achondritic impactor signature for the Ries6,8) are at odds with the widely accepted double impact scenario.
Both the Nördlinger Ries and the slightly younger Steinheim impacts would have imparted significant 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 Biberach22, Ravensburg, and Bernhardzell34, 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 volcano22,65. Based on the age constraints for the dike-hosting sediments22,35,37, the dike is the product of a seismic event that occurred between ~14.81 Ma (Ries impact12,13) and approximately14.3 Ma (terminal sedimentation of the ‘Fluviatile Untere Serie unit35,37). In contrast to the precise age for the Ries12,13, the latter age is not very well constrained and may be associated with an error of a few kyr22. A seismo-tectonic (alpine tectonism) or volcano-seismic event (within the Paleogene to Quaternary European Volcanic Province) was recently discussed22 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 potential22, none of these earthquake centers can convincingly explain the formation of the post-Ries clastic dikes22.
The dimensions of the sandstone dikes significantly 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 energy22,24,32,36. Taking the comparable rock properties and the significantly 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 event22.
Supporting evidence for a major post-Ries seismic event comes from sediments of the Ries crater lake. A ~314 m-thick sequence of crater lake deposits was drilled in the scientific drilling project 1973. This sediment sequence, deposited in a lake that lasted for ~1 Myr43, contains olistoliths and sediments with intense slumping and convolute bedding42. 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.
The two major paleoseismic events recorded at various sites across the North Alpine Foreland Basin seem to have occurred close in time in the Miocene, yet during markedly different climatic and paleoenvironmental conditions. Soft-sediment deformation caused by the Ries earthquake at ~14.81 Ma occurred when the climate was warm and humid38-41,44-46 (during or slightly after the Miocene Climate Optimum at 14.9 Ma44-46) and the palaeo-groundwater level reached the former land surface. While the Ries-triggered earthquake caused extensive stirring of water-saturated sediments, the earthquake presumably induced by the Steinheim impact seemingly did not cause any widespread soft-sediment deformation, but generated clastic dikes. This suggests a rather dry state of the sedimentary bedrock, with a deeper palaeo-groundwater level locally above water–logging clay horizons. A significant episode of climate change during the Middle Miocene in Central Europe was recently dated at ~14.48 to 14.13 Ma46 through the analysis of palaeosoils in the North Alpine Foreland Basin. That change in climate led to a stronger seasonality and less humid conditions in Central Europe44-46. Assuming the Steinheim impact and the Biberach clastic dike are genetically linked, the age for the Steinheim impact would most likely fall between ~14.8 and ~14.1 Ma. Taking the biostratigraphic, sedimentologic, and climatologic findings into account, the suggested best-fit impact age for Steinheim is approximately 14.3 Ma. This age sits well with the time frame of the terminal sedimentation of Fluviatile Untere Serie at 14.3 Ma35 and the initial phase of Mid-Miocene cooling at 14.43 Ma46. The time gap of approximately 0.5 Myr also fits the purported age difference between the crater lake deposits at both impact structures, as well as the post-Ries timing of active slumping within the Ries crater lake deposits. All these arguments, combined with the lack of an effective seismic source for a high-magnitude earthquake postdating the Ries event, lead us to conclude that the Ries and Steinheim impact structures are likely the result of two spatially and temporally separate impact events in southern Germany, occurring ~40 km and ~0.5 to 1 Myr apart.
In the past decade, many of the seemingly well-established terrestrial impact crater doublets and chains were discredited. 40Ar/39Ar dating results for several impact structures14,23,47-49 contradict the hypothesis that planet Earth experienced the formation of a giant ‘impact crater chain’ during a major Late Triassic multiple impact event47. Recent work, moreover, revealed that apparent crater pairs, for instance the partly overlapping East and West Clearwater Lake impact structures (Québéc, Canada)48 or the two Suvasvesi impact structures (Finland),49 are not the crater doublets they seem. To date, the only terrestrial crater pair that survived closer inspection is the Lockne–Målingen pair in Middle Sweden50, which was produced during an active period of Mid- to Late Ordovician asteroid bombardment of the Earth14,23,50.
In the light of two spatially and temporally separate impact events, the occurrence of the distinct and well-preserved Ries-related seismite topped by primary distal ejecta near Biberach, Ravensburg, and Bernhardzell is explained as follows: 1. Thick, fine-grained, and homogenous sandy deposits intercalated with clays22 promoted water-saturation within the Upper Freshwater Molasse in the study area, facilitating dewatering processes and soft-sediment deformation22,24 triggered by the Ries impact. 2. 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 event22. The occurrences of the seismite near Biberach, Ochsenhausen, Ravensburg, and Bernhardzell are the first 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 first known occurrence of a primary continental seismite-ejecta couplet preserved in situ.
Magnitudes of impact-earthquakes
The magnitude of earthquakes induced by meteorite impacts is still somewhat uncertain, and the seismic efficiency (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 impacts19,20,25,29, calculated magnitudes may, in cases, be too conservative22. The magnitude of the ‘Chicxulub earthquake’ was probably approximately ML 9.918,19. Endogenic (tectonic) earthquakes may not reach such an extraordinary magnitude, and the strongest earthquakes ever recorded correspond to a Richter magnitude ML 8.6 to 8.7 (e.g., the great Valdivia, Chile, or Alaska earthquakes62,64).
An earthquake of at least ML 5 to 6 and a moment magnitude of MW 6.5 is required for the formation of seismites22,24. The systematic relation between specific 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 Earth57,62-64. 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 database16,21,22,24-303436. However, the earthquake magnitude–distance relationship for liquefication effects is currently still underexplored and needs to be evaluated from the perspective of geologically younger major earthquakes.
For the impact that formed the 24 km-diameter Ries crater in southern Germany (impact energy ~5 × 1020J; equivalent to ~120,000 megatons of TNT), an earthquake of Richter magnitude ML 8.0 was calculated18,22. The most distal exposures of a seismite in the form of soft-sediment deformation structures and clastic dikes caused by the Ries impact-induced earthquake occur within a distance of at least 180 km from the center of the crater (Bernhardzell, Switzerland). According to the mapping of distal ground failure effects caused by large earthquakes up to ML 7.5, clastic dikes and soft-sediment deformation structures may occur at a distance of ~70 to 130 km from the epicenter of major earthquakes62-64. Even the giant 1964 Alaska earthquake that had a magnitude of ML 8.6 caused significant ground failure only within a radius of 130 km62,64. All earthquakes that cause liquefication of sediments within a radial distance of more than 150 km had magnitudes of ML 8 or higher62-64. A magnitude of ML 8 for the eroded, ~8 km-diameter Upheaval Dome impact structure20,21 in Utah, USA was proposed on the basis of the earthquake magnitude-distance relationship for synsedimentary deformation in Jurassic rocks in the wider surroundings of the impact site21. Taking these arguments into account, a magnitude in the range of ML ~8 to 9 for the ‘Ries earthquake’, producing seismites within a 180 km radius, appears geologically plausible. Based on the comparison with distal ground effects of modern earthquakes62-64, a local magnitude of ~8.6 might be the best fit for the Ries earthquake and its distant effects.
The nearby Steinheim impact event (impact energy ~2.3 × 1018J; equivalent to ~550 megatons of TNT) formed a much smaller, complex impact crater about 4 km in diameter. The magnitude of the Steinheim earthquake was estimated at about ML 6.418. The most distal seismites in the form of soft-sediment deformation and clastic dikes presumably linked with the Steinheim impact earthquake occur within a radial distance of at least 150 km from the source crater. While ground failure due to earthquakes of ML ≥7.5 may occur within a radial distance of 100 km or more, the outer limit for the occurrence of seismogenic clastic dikes dramatically decreases for earthquakes of ML <7.562-64. The most distal ground effects of an earthquake with ML 6.8, for instance, reach radial distances of only ~23 km from the epicenter57,62-64. The formation of clastic dikes at a radial distance of 150 km, therefore, requires a palaeo-earthquake of the magnitude ML 8 or higher. From this point of view, we speculate whether the magnitude of the postulated ‘Steinheim earthquake’ may have been closer to ML 8. This estimate clearly exceeds previous estimates for impact-earthquake magnitudes calculated for smaller-size impact events.
Environmental effects of the Ries and Steinheim events
The Ries impact caused a series of events that affected the wider surroundings of the crater within a minimum radial distance of 180 km2-4,9-11,22,31,33,34,41,42,52,53,61. Some of the effects overlap and initiated the near-complete destruction of the surface-near environment within this radial distance. The impact-induced earthquake immediately followed the impact event when P-waves reached radial distances of 110 km from the crater center ~15 seconds after the impact. The earthquake would have lasted for ~45 seconds until P-waves and S-waves passed this damage zone54,55. The seismic energy would have caused intense slumping, soft-sediment deformation, and locally clastic dikes in the upper meters of the water-saturated Upper Freshwater Molasse (Figs. 2 to 5, Supplementary Figs. 1 and 2). Approximately 2 minutes after the impact event, a hot air blast56 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 flat paleosurface (Fig. 2). This ‘disaster topography’ does not correspond with the original, unaffected palaeolandscape that was dominated by rivers, lakes, and damp forests5,53. Charred wood, reported for instance from the Unterneul sandpit 53, suggests high temperatures of the airblast. Within three to five minutes, an episode of bombardment by pebbles, cobbles, and boulders mainly of Upper Jurassic limestones, many of them shatter-coned (Fig. 2), ensued2,3,10,11,22, 31, 34. The ballistically transported components stem from the uppermost tens of meters of the Ries target rocks2,10. 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 Earth11,22,31.
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 feldspar22. These finer-grained deposits locally show a distinct fining-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 surface2. 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 impact2,3. Fallout from the impact plume may have rained down for minutes to hours2,3. Similar to crustal materials dispersed during volcanic eruptions51,60,69, 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 flashfloods, as known from volcanic eruptions59,60. 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-flowing, high-energy flood streams (Fig. 3). Logs and pieces of wood up to 2.6 m in length61, relics of the impact-blasted wet forest61, are abundant in the reworked fluvial deposits. Moreover, well-preserved skeletal remains of the Miocene rhinoceros Brachypotherium brachypus was reported in flash flood deposits near Ravensburg61. 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 flashflood 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 animals61. The intact nature of bones and teeth document that these fossils were not significantly reworked and that the finding 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.