One of the many stops on my final undergraduate geology field trip this March was at Mosaic Canyon in Death Valley National Park, California. This canyon is fascinating because it illuminates the history of this location over the last 700 million years.
The canyon was formed as a result of erosion in the local basin and range topography. Here the crust is slowly being pulled apart by extensional forces. The crust cracks and forms faults, and some blocks drop along these faults relative to the surrounding blocks. The dropped blocks form valleys (basin) and the higher blocks form mountains (range). Mosaic Canyon is located on the transition from a dropped block (Death Valley) and an elevated block (the Panamint Range). The elevation difference is key to promoting erosion.
The mouth of the canyon is covered in thick beds of gravel and breccia (the appearance of the breccia is what gives it the name Mosaic Canyon). These deposits are the result of erosion from the action of rainwater flowing down the canyon. Rainwater slowly erodes the walls of the canyon while carrying gravel, mud, and sand down to the mouth. Here the sediments reach wider, flatter ground and spread out into an alluvial fan. During the Quaternary Period (2.6 million years ago to present) this has been a major geologic activity in this area (along with the ongoing crustal extension of the basin and range province).
Where this gets really interesting, however, involves the source of this erosional detritus. As you enter the mouth of the canyon, the scenery changes considerably. The loose gravel and breccia present at the mouth give way to smoothly polished and intricately carved canyon walls. This is part of the Noonday Formation, formed during the Cryogenian Period around 700 million years ago.
These sediments were originally deposited as limestone. Calcium carbonate settled out of shallow ocean waters to form this rock. Over time, the calcium was partially replaced by magnesium, forming dolomite. Subsequent burial by additional sediments lead to the compaction and metamorphism of this dolomite into marble. The erosion that has occurred since then has stripped away the overlying beds, revealing these ancient rocks.
What’s fascinating about this Noonday dolomite is that it’s a cap carbonate, formed at the end of a glacial cycle. Deglaciation often ends in the accumulation of calcium carbonate, “capping” the glacial event. Carbon dioxide built up in the atmosphere from volcanic outgassing, and then ended up in the ocean. Here it accumulated and converted to calcium carbonate, where it precipitated and collected on the ocean floor.
The Cryogenian Period contained perhaps the most severe ice ages known on earth. Repeated intense glaciation may have even resulted in the entire earth becoming completely or almost completely covered in ice, an episode suggested by the “snowball earth” hypothesis.
What’s really cool is that following this intense, long glacial period life on earth finally began a significant increase in complexity. For around three billion years leading up to this point, life was generally unicellular. The Ediacarian Period (630-542 million years ago) saw the appearance of bizarre large multicellular organisms, famously preserved in the Burgess Shale in western Canada. The Cambrian Period (542-488 million years ago) saw an even greater increase in biodiversity, which has continued to fluorish to this day (with a few pesky mass extinctions along the way).
The success of life on earth hinged largely on the growing abundance of atmospheric oxygen which made large, complex, oxygen-breathing organisms possible. This increase in oxygen was courtesy of the growing numbers of photosynthetic unicellular marine organisms (like green algae), and later, land plants. At the same time, the end of severe ice ages like those seen in the Cryogenian were also a requisite for the success of life. Although glacial periods have come and gone many times since, they have never been so severe as to eliminate the warm equitorial climates that are most favorable to life. If the snowball earth hypothesis is correct, this is exactly what happened around 700 million years ago. In retrospect, it was really exciting to walk among rocks from this period in earth history.