The discrepancy pops up in the Grand Canyon, where ancient rock abuts, well, less ancient rock. TENJI / CC BY-SA 3.0
Atlas Obscura has a story about the The Great Unconformity. In geology, an unconformity is a break in the sequence of time in a continuous rock record. There is usually a large gap of missing rock layers at the contact point between a much older layer and a younger one. It's caused by a period of erosion or a pause in sediment accumulation. The Great Unconformity is a sequence of many missing rock layers in the early Cambrian Period about 540 million years ago. It's cause has stumped geologists all over the world as the gaps is pretty much worldwide. A new paper in the journal Proceedings of the National Academy of Sciences proposes that this erosion didn't happen all at once. There may have been many events... a whole bunch of small unconformities.
GEOLOGY FORMS THE CONTEXT FOR many other fields, including archaeology, paleontology, climatology, and more, but that doesn’t stop this bedrock science from throwing a curveball here and there. In some outcrops around the world there’s a gap, a hint of time gone missing, a huge swath of geological data that should be there but isn’t.
It’s called the “Great Unconformity,” and it has long vexed geologists from Nevada to Scotland. Geology is often the study of layers, set one on top of each other for billions of years and compressed into sequences that provide geologists insight into how the Earth has evolved through the eons. Under the best circumstances, that sequence is more or less uninterrupted, but there can be gaps—sometimes big ones, like the Great Unconformity, which can be seen all over, from the Rockies to southern Africa to northern China. This gap spans one of the murkier periods in Earth’s history, before the Cambrian explosion, around 540 million years ago, when the diversity of life on Earth went wild.
Erosion is one natural process that wears layers away from the stack of geological deposits, but how so much was wiped out across such a wide range of places in one go has remained unknown. Even the unconformity isn’t uniform, ranging in scale from over a billion years of missing time to a mere couple hundred million. In the Grand Canyon, the timeframe of the unconformity jumps multiple times along its length.
