Tripping Over Catastrophes: Outcrop Serendipity

Dr. John Warme, a professor of geology here at Mines from 1979-2002 and a professor emeritus since then, came to give a special talk about some of his more interesting research projects from the past. The unifying concept of the three different projects that he presented on was the idea that anomalies in the geologic record often record great catastrophes, and following up on these anomalies can lead to striking discoveries. A geologic catastrophe signifies something that happened rapidly on a grand scale, rather than something destructive (though the two definitions often go hand in hand).

The first catastrophe Warme described is a unique landslide in the Grand Canyon that Warme nicknamed “Pancho’s Radical Runup,” because of its proximity to a rafting campsite named Pancho’s Kitchen. Along the Grand Canyon’s three hundred kilometers, there are many documented landslides, especially where the Bright Angel Shale is exposed. The Bright Angel is a weak unit which sits between two strong, cliff-forming rock units; because of this, many water seeps form at this point in the stratigraphy, which in turn instigate the landslides. These landslides have significantly affected the geography of the canyon walls; for instance, the only waterfall in the canyon at river level, Deer Creek Falls, is the result of a young landslide which blocked the creek’s original route to the Colorado River. At a spot called “The Narrows”, where the Grand Canyon is at its narrowest, Dr. Warme (who used to lead rafting field trips down the canyon) noticed something odd about the way the bedding on the high banks was weathering. The location was fairly inaccessible, located atop a two hundred foot cliff, but Warme was determined. After making it up to the strange bedding to look at it up close and taking photos from an airplane with a good camera, he found something surprising: the same rock units were overlying more of themselves, at an angle. Furthermore, the overlying, tilted section of the units was “shattered” but not dispersed, as if it had been pulverised in situ, and contained “relaxation faults”, as if it had been in motion and settled back down. Conversely, normal talus – chunks of rock that broke off of the cliffs and rolled down the slope – remained in chunks called “boxcars”, which maintained internal cohesion, not showing any of the breakage structures of the anomalous beds.
After examining all of these clues, Warme realized that this was a landslide deposit… from the opposite side of the canyon. Such a phenomenon, called a runup, occurs when a particularly high-powered landslide forms an air cushion under itself when it hits the bottom of a slope and continues forward, up the opposite side of the valley, much as water poured down one side of a bowl will splash up the opposite side. This particular runup consists of five separate “slabs” or tongues, covers a few square kilometers of area, and went about seven hundred feet up the canyon wall: the highest “cross-canyon” runup in the lower 48 states. At 750 thousand to 1.25 million years old, this is also one of the oldest landslides in the canyon. The frictional heat of the slide was so great that it naturally converted the limestone into quicklime, so that when it came to a halt and the pore water in the rocks bubbled out, the shattered slide acted like man-made cement, freezing itself in place for Dr Warme to find it and unlock its secrets.

Next, Warme spoke of a landslide on the opposite side of the world, in the Atlas Mountains of Morocco. During the Jurassic period, this was a rift zone (a place where a continent is breaking in two). Rifts often exude basalt lavas and form depressed areas called wrench zones which tend to catch water. The rhomboid wrench zones in this area were first “dry rifts”, filled with basalts, evaporite minerals and “redbeds”, but once subsidence was deep enough, the ocean was able to flow into the zone and a carbonate shoal developed. Later, when the Alps formed, the compression forced the rift faults to reverse direction, creating mountains out of a sea. In the midst of these mountains are three “jebels”, or ridges, separated by faults, which comprise pieces of a single large carbonate grainstone block thirty kilometers in length. The block represents a former reef, with many fossils in it. Reefs require sunlight, so the jebel bedrock must have formed in shallow water… but there are deepwater landslide deposits, or turbidites, over the top of the reef. The turbidites are themselves full of coral fragments, as well as chunks of dark deepwater sediments and debris flows. In the past, researchers were baffled by what appeared to be a reef that had somehow sunk to the bottom of the ancient sea. After Dr Warme examined the rocks, however, he determined the true story behind the jebels. This is not a reef but a piece of a reef shed from the shelf edge in one coherent block, called an olistolith, which fell down the continental slope into deep water. As it fell, the block destabilized the sediments it ran over, which then came tumbling down after it, filled in the space behind it, and topped over it, spilling across and over. The coral fragments in the turbidites are olistostromes – small broken pieces of the reef’s edge knocked off during the collapse – and the dark bits were ripped up from the seafloor as the landslide passed. This simple explanation failed to be recognized based merely on the sheer size of the olistolith.

Finally, Warme discussed the largest of his catastrophes: the breccia (or broken rock) left behind by a Devonian meteorite impact in present-day Nevada. At the time of the impact, Nevada was the Pacific coast, a tropical carbonate ramp. Subsequent compression caused a thrust belt to form along the former seacoast, and the area is now made up of hundreds of subparallel mountain ranges. The Alamo impact breccia is found in at least twenty-five of these ranges, and had long stumped researchers before Warme began his investigation. As he explained, “the broken rocks were variously interpreted as coral-reef talus, karst cave-fillings, and fault or solution-collapse breccias.” The thickness of the breccia varies, but gets up to three hundred meters in places.

The breccia contains many unique characteristics which are signatures of a bolide (or meteor) impact. Shocked quartz, for instance, which records the pressure wave from an impact, is the classic sign of a meteor strike. There are also carbonate impact lapilli – “rock hailstones” – which form the same way as volcanic lapilli, aggregating about some nucleus in a superheated debris cloud and falling back to earth. The lapilli have fossil fragments as their nuclei; the process by which they were cemented is the same quicklime formation phenomenon seen in Pancho’s Radical Runup. The unit contains intraformational folding and a smashed “fallback” layer between unbroken, undeformed rocks, an unusual observation that records the debris from the impact falling back to earth. All of these clues pointed Warme and his students towards an impact, so they began searching for a crater. Sure enough, they found one. Though the crater has since been disrupted by tectonics (the whole area is now heavily faulted), it can be seen in the rock record. The bolide struck about 150 km north of present-day Las Vegas. Because the impact was underwater, it caused a tsunami, which also left a record in the geology. Successive graded beds, each thinner than the last, signify successive tsunami waves as water sloshed from the crater to the seacoast and back. Desiccation cracks in the tsunami deposits show that the wave travelled inland far enough to get above sea level. Graded “resurge” beds record the same sloshing phenomenon below sea level. Slumped normal sedimentation records the collapse of the oversteepened crater walls back to a stable slope after the turmoil had ended. Finally, a reef which grew atop the breccia suggests that the sea in this area was deepened enough by the impact to make room, or accommodation space, for corals to grow.

Warme concluded his talk by pointing out that he was only able to recognize these strange anomalies in the rock record because he had seen so many rocks and had so much geological experience that he knew what was normal and what was not. In other words, he said, “One must understand the expectable in order to recognize the anomalous.”