Called “The Great Unconformity”, the Grand Canyon has a 1.2 billion year gap in the rock record. Above the gap is 500 million-year-old sandstone (Cambrian age), a time of billions of shelled marine creatures. Below the gap is metamorphic rock of the Slatherian period aged 1.75 billion years where no fossils exist at all. But in between, more than a billion years of history have left no account on this spot on Earth – or in other areas all over the planet.
Unconformities in general tend to reflect long-term patterns of sediment flow, collecting in low areas, then being uplifted and eroded, then subsiding to be covered by younger sediments. They demonstrate that the layers between sedimentary formations could be of significantly different ages. This strange pattern exists world-wide.
Scientists are not sure why the break in the fossil record exists or what occurred across the planet to cause the gap. Possibly this area was under water and then thrust upward by some seismic force. Possibly this area was at a very high altitude and hence, no deposition would occur. Maybe rock did indeed form but then for some reason, eroded away. Or maybe a global catastrophe (e.g. global flood, world-wide comet impact, glacier) sheered the surface of the Earth destroying the world as it was.
Most interestingly, the unconformity separates rocks with familiar fossils, such as shells, from rocks with no fossils at all (or only fossil bacteria). The gap is interesting because during that era, a blink in geological time, a burst of evolution lead to a flurry of organisms and the first appearance of shells and skeletons. Whatever caused the gap may have provided the impetus for the burst of biodiversity that blossomed during the early Cambrian age.
Images courtesy Lunar and Planetary Institute
