Geology is all around us, scarcely thought of as we go about our lives. Yet, it affects everything we do as a civilization, as a society and as individuals. While barely appearing to change from day to day, it works to alter
Geology is all around us, scarcely thought of as we go about our lives. Yet, it affects everything we do as a civilization, as a society and as individuals. While barely appearing to change from day to day, it works to alter the course of evolution. Preserving a record of creatures and landscapes both ancient and forgotten, the story of our past is written in stone and waiting to be read. I offer a view of how I see our world and its inhabitants, both past and present, as seen through my lens.
In my previous post on the Great Unconformity of the Grand Canyon, I discussed its contiguous stratigraphy and the regional events involved in their formation. In this follow-up post, in the light of current thought, I wanted to offer a more global interpretation of the events that occurred during its interval.
In the following photo we are peering into the Inner Gorge of the Grand Canyon, as seen from the South Rim. Can you specify the location of the Great Unconformity?
THE LATE PROTEROZOIC-CAMBRIAN INTERVAL Our Earth system experienced enormous change during the Late Proterozoic through the Cambrian. This is the time span that encompasses the events that lead up to and include the Great Unconformity. Recorded in the Earth's lithosphere is a history of the unconformity's missing time, of fragmented and lost ancient continents, of a planet frozen solid within a ball of ice, of a global flood of biblical proportions, and an epic tale about the creation, diversification and radiation of radically different new forms of life that appeared suddenly in a biological "Big Bang." It sounds religious, but it's not (although many believe that it is). Its geological, tectonic, climatological and biological.
Where shall we begin? Let's redefine the Great Unconformity of the Grand Canyon, briefly reprising my previous post.
THE GREAT UNCONFORMITY OF THE GRAND CANYON The photo below is taken of the Great Unconformity at arm’s length in a side canyon called Blacktail within the Grand Canyon. All unconformities are gaps-in-time, geological horizons where time and rock layers are forever lost (having been consumed by the forces of erosion) or simply never even formed. The only difference is that this one is massive. It's the mother of all unconformities. It's so important that it's written with capital letters, and that's not all. It's found not only in the Grand Canyon, but all across the globe. But not just anywhere. You have to know where to look. You have to know the geology, and plate tectonics helps. If you know that, you can truly go back in time.
The Great Unconformity resides within the contact between the horizontally-bedded, Early Cambrian Tapeats Sandstone and the vertically-folded, crystalline basement of the Early Proterozoic Vishnu Schist. Notice a conglomeritic lens of Tapeats "trapped" above the contact. Here and there, large chunks of schist and granite have been caught up in the flood that created the shoreline of the Tapeats.
You can almost hear the waves of the gradually rising Panthalassic Ocean (proto-Pacific Ocean) lapping on the shores of a new supercontinent (Laurentia), a remnant of the larger one (Rodinia) that has broken into pieces and whose pieces will one day reunite (Pangaea). One billion two hundred million years, give or take. That's the size of the time gap. That's the amount of time that's unaccounted for within that paper-thin space! A quarter of the Earth's history. Its magnitude is incomprehensible and it's missing!
MORE QUESTIONS THAN ANSWERS
What events happened during the Great Unconformity? How do we even begin to find out, if the strata holding the clues doesn't exist? Or does it? Why is the end of the Precambrian in the Grand Canyon marked by an unconformity? Why was the interval so long? Is the unconformity found globally? Are any aspects of a Snowball Earth glaciation recorded in the Chuar Group? What is Snowball Earth? What lifeforms, if any, existed during the prolonged interval of the unconformity? How did they survive such a crisis? Did the geological events of the period favor or disfavor their evolution and diversification? What caused the global high seas following the rifting of Rodinia that flooded the Grand Canyon region and 