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Van Tuyl Lecture: Starting with a blast from the Devonian

Very few events in the history of the world mirror the shear destruction brought by a meteor impact. While there have likely been countless collisions in the history of the planet, it was the proposed Alamo event that was discussed this past week at the Geology and Geological Engineering’s Van Tuyl lecture. Dr. John Warme, an Emeritus Professor from Colorado School of Mines and a team of other geo-scientists including the late Dr. Jared R. Morrow, to whom the lecture was dedicated, have spent many years interpreting the observed structures of the Alamo Breccia in Nevada and have come to the conclusion that there was likely a very large impact in the area in the Devonian Period of Earth’s history.

At the time of the Devonian, the area of the impact was not the mountainous terrain of Southern Nevada as it is today, but was instead a shallow carbonate platform much like what exists off the coast of Florida today. “There’s a lot of rock in Nevada [currently],” said Warme, “its a great place to do geology.” It is this large amount of history preserved in the rock that has allowed Warme and others to investigate this event as much as they have.

To imagine the Alamo impact, one must first understand the nature of meteor collisions. Impacts have a much broader impact than just the asteroid hitting the ground. No matter where the asteroid strikes, there are sure to be breccias, a geological term for broken-up rocks, in addition to the seismic and other major environmental effects; when an event like this happens over a body of water it becomes necessary to count in the effects of tsunamis and structural collapse on the seafloor. “When a big event like this happens, there are a lot of adjustments that happen over a long period more than the immediate catastrophes,” stated Warme. In the case of the Alamo event, Warme and his team have determined six different regions that have been affected differently by the impact, where the effects can range from basic tsunamis to terrific rains of molten high-velocity limestone lapilli.

Though it is difficult to sum up the exact order of events in all their intricate relationships, a brief timeline is as follows. As the bolide hits the ground it burrows in forming a transient crater, abnormally intense seismic waves, and material is ejected at speeds above the speed of sound. This cloud of material plummets to the ground followed by the formation of concentric ring structures. All this time the heat of the impact vaporizes some of the water in the crater, pushes the rest away as a tsunami, then the crater is filled by water back-flow. This re-surge of water will form a central spout that then collapses to form a second tsunami and the process repeats several times while damping down. These processes cause huge volumes of material to be disrupted and displaced including boulders that are larger than some houses. The stresses exerted by the impact can warp the some of the preexisting sedimentary and limestone beds causing some of them to fold over each other. All this time more tsunamis occur as the sea level fluctuates and more of the airborne material crashes back to the ground. On a longer range timescale of a few days debris flows begin to pour down the side of the continental margin that deposit shallow water features such as sponges in deep water settings.

When asked why it was that Warme originally believed there to be an impact at the location he responded, “Well the first time I saw it, I knew it had to be an impact, the main thing was that there was a great big breccia sitting on top of a long flat platform.” In geology, for the most part, bedding is deposited horizontally and logically. If there is a large oceanic platform full of carbonates, the moment something like a breccia is found, with no local breccia source, it is clear indication that some sort of event happens that was out of the normal. After Warme and his team began looking closer at the details, more features like large folded beds and tsunamiites, a form of rock deposited by tsunamis, were found.

To add to the reality of the find, it became clear through an analysis of the surrounding geology that there were signs of multiple catastrophic processes happening at once. For the most part, the observed features matched the models that had been proposed for a large impact. Of course there are still hidden features that may further bolster the claim that the Alamo Breccia is, in fact, an asteroid impact. While a crater center has not been found yet, Warme expressed no concern about its lack of presence. “[The impact] happened along the margin of the paleo-Pacific Ocean, the crater could have self-destructed.” To back up his thoughts on the idea of self-destruction, Warme displayed an example of the rim of a smaller crater on a planetary body which had slid down the rim of a much larger preexisting crater, suggesting that the Alamo impact occurred on the edge of the Devonian North American continent, then the crater slid off into deep water.

If a meteor really did hit off the coast of the Devonian North America, it could have some interesting ramifications for the current understanding of the relationships between impacts and extinctions. It is known that not all large impacts result in massive extinctions like that present at the boundary between the Cretaceous and Tertiary periods and given the timing of the Alamo event it is not likely that it resulted in an extinction, still more knowledge can help understand these catastrophes. In a follow up to the lecture, the idea that the Alamo event may have brought about ore mineralization was entertained. While most of the features have been worn away or have yet to be found, at other impact sites such as Sudbury in Canada, there is associated mineralization via the cracks that would occur from the stress of the impact.



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