New Evidence Surrounding Twin Craters Rocked by Mid-Miocene Impact Earthquakes
December 28, 2020
For decades, the well-preserved 15-mile-wide Ries crater in Germany, hosting the picturesque medieval town of Nördlingen, and its smaller companion crater twenty-five miles to the southwest, the 2.5-mile-wide Steinheim Basin, have been regarded as a textbook example of a terrestrial impact crater doublet. According to this hypothesis, an asteroid pair slammed into the Earth about 14.8 million years ago in mid-Miocene epoch, thereby creating the two craters simultaneously within fractions of a second. It was deemed unlikely that two impact craters could potentially form that close to one another, around the same time, purely at random.
A new study by geologists Elmar Buchner (Neu-Ulm University of Applied Sciences, Germany), Volker Sach (Sigmaringen, Germany), and former LPI Postdoctoral Fellow Martin Schmieder, published recently in Scientific Reports, offers evidence for an alternative hypothesis of the mid-Miocene double disaster in central Europe. Investigation of two separate layers of seismite — rock chaotically deformed by a seismic wave — found within a distance up to 120 miles south of the impact craters suggest there were two major earthquakes that once rocked the region. While the lower seismite unit is capped by ejected pebbles and cobbles of mostly Upper Jurassic limestone launched from the distant Ries impact site (some of them even contain ammonites, brachiopods, and shatter cones as a hallmark of asteroid impact), the upper seismite unit occurs in the form of dikes that cut through the Ries seismite and overlying strata. “The Ries impact-quake probably had a magnitude around 8.5 and was similar in its strength to the most devastating earthquakes mankind has witnessed,” says lead author Buchner. “But that’s only the first half of the story. Within the first two minutes after the seismic shaking, a massive air-blast and fireball leveled the entire region, forests were blown off and ignited, and for several more minutes, baseball- and football-size rocks came falling from the sky. This impact havoc was then followed by the prolonged fallout of airborne sand and dust, blanketing the landscape. Days and weeks of heavy rain and flash floods ensued.”
“It was a veritable Miocene disaster, and in addition to the chaotic traces of the Ries earthquake we can still see charcoaled trees, displaced animal bones, and evidence of catastrophic flooding in the outcrops,” adds Sach, who had discovered, carefully cleared, and meticulously documented the unique exposures and their delicate sediment features. “This is very exciting because these layers are the first geologic evidence for an impact-triggered earthquake — that is, seismite capped by impact ejecta — that we know of in a continental setting, and they allow us to reconstruct the exact sequence of events on the day the Ries asteroid hit Europe,” Buchner explains.
But there wasn’t only the Ries earthquake. The eye-catching dikes of liquefied rock also found in the geologic sections, squeezed upward from a water-rich bottom layer into temporary open fractures as the Earth’s crust was still shaking, help reconstruct an extended history of two disaster events. “These dikes clearly cross-cut the Ries-seismite unit, its ejecta cap, and a few meters of the overlying strata. They are, therefore, definitely younger than the Ries impact and probably related to the Steinheim impact, which seems to have occurred a few hundred thousand years after the Ries event,” says Buchner, contradicting the widely popular double-impact theory. In fact, independent paleontologic evidence from the two impact craters themselves supports the new hypothesis of a later Steinheim impact in a region that had just been hit by the Ries asteroid. Both crater depressions filled up with water shortly after impact, forming two annular crater lakes perhaps somewhat similar in appearance to Crater Lake in Oregon. However, the deepest and oldest fossiliferous crater lake deposits at the Ries crater seem to be several hundred thousand years older than those inside the Steinheim Basin, as recent studies suggest.
“It adds up pretty nicely with the new discovery of the Ries-produced seismites and the cross-cutting seismic dikes that are, on a geologic timescale, just slightly younger,” summarizes Martin Schmieder. “And we are not even really surprised. Our crater-age studies at other impact structures worldwide have, in several instances, shown that most impact craters that were once considered crater doublets or even ‘chains’ of multiple impact craters turned out to have greatly different geologic ages, making them unrelated to one another. Two good examples are the two Clearwater Lake impact structures in Canada that almost overlap each other or the closely spaced Suvasvesi craters in Finland.”
The Ries and Steinheim craters may have, finally, been divorced from one another. But there may still be a true crater pair up in central Sweden, one that Buchner and his colleagues are hoping to study in more detail one day, too.
For more information, visit https://www.nature.com/articles/s41598-020-79032-4.