Catastrophic origins: geologists have been puzzling over the cause of the cone sands for almost 200 years – striking tadpole-shaped structures in the sediments of southern Germany and California. Now researchers have revealed the secret of these structures. Accordingly, they only arise where strong earthquakes or other seismic tremors have compressed the sandy subsoil. The cone sands could therefore serve as an indicator of seismic risks.
At first glance, they are quite unspectacular: The cone sands that occur in some sedimentary formations are a few dozen centimeters to a meter long and mostly consist of compressed sandstone. They typically have a thickened head, followed by a tapering tail. “This striking shape seems to be unique for the cone sands, it is not known from other concretions,” explain Elmar Buchner from the Neu-Ulm University of Applied Sciences and his colleagues.
Stalagmites? Fossil Mushrooms? Flash tubes?
The first representatives of these strange structures were found south of the Swabian Alb around 200 years ago – around Ravensburg, Biberach, Ulm and Günzburg. Geologists discovered very similar cone sands on Mount Signal in California in the 1930s. “Thousands of such cone sands were found there, but almost all of them were collected in the 1950s or destroyed by construction work,” report Buchner and his team.
But what created these strange sandstone formations? “Earlier theories interpreted the cone sands as buried stalagmites, as petrified seaweed, beets or mushrooms, as fulgurites formed by lightning strikes or as sand-filled crab ducts,” the researchers explain. The elongated formations were also seen as concretions from the tidal zone of the former seashore. Unlike many other compressed sand formations, however, the peg sands do not have an internal structure of concentric rings or otherwise recognizable layers.
Strikingly uniform alignment
Buchner and his team, on the other hand, have uncovered evidence of a far more dramatic origin of the cone sands. The basis for this was formed by more than 311 cone sands and their find layers from the Molasse basin south of the Swabian Alb, which the researchers thoroughly geologically examined and mapped. In addition, they subjected individual cones sands to a detailed petrographic analysis.
The investigations revealed: The cone sands in southern Germany are not oriented arbitrarily. Instead, their tails seem to be pointing in the same direction almost everywhere – roughly south, the researchers found. The cones near Augsburg tend to face southeast, while those near Ochsenhausen and Biberach are slightly southeast. If you lengthen these lines, they come together roughly at the border between the Swabian and Franconian Jura.
Seismic tremors in the game
Also striking: The German cone sands mostly occur in layers from the time around 15 million years ago – the time when a large asteroid struck the Nördlinger Ries and hurled debris, molten rock and seismically induced shock crystals into the Alps. The cone sands in California, on the other hand, are all around 40 kilometers west of the Imperial Fault, part of the San Andreas Fault – and they too all point away from this center of strong earthquakes.
According to Buchner and his team, this suggests that the formation of the cone sands must be closely linked to strong seismic tremors. You see in them a special type of sand-based seismite – deposits formed by shock deformation. The shock waves of an earthquake or other strong vibration ensure that soil granules are pressed together and solidified.
Strong earthquake indicator
However, the cone sands only appear to form under very special conditions, as the researchers explain. On the one hand, this requires a loose layer of sand on partially moist sediment. On the other hand, however, the triggering event must have a certain minimum strength. “Cone sands only seem to form in very high-energy and destructive earthquakes of at least magnitude 7,” writes the team. Otherwise, these structures would also have to have emerged more frequently along other faults.
But this means: The cone sands are suitable as potential indicators for areas in which there have been particularly strong earthquakes or other seismic events. This could help narrow down the earthquake risk in these regions. So far, only earthquakes up to magnitude seven have been known at the Imperial Fault in California. The cone sands now suggest, however, that there has been a significantly stronger quake there in the recent geological past.
The new findings not only solve the riddle about the formation of the cone sands, they could also prove useful. With their help, the hazard potential of certain regions could be assessed more precisely than before. (Nature Communications, 2021; doi: 10.1038/s41467-021-27061-6)
Source: Neu-Ulm University of Applied Sciences