Not a Good Unconformity; a Great Unconformity


It’s been a while since WC wallowed in some geeky, geological goodness, so let’s talk about unconformities, specifically the Great Unconformity, because there’s some news on it.

An unconformity is a place where there is a gap in the geological layer cake, where old rocks and newer rocks are layered against each other, with the intervening layers missing. The most famous unconformity in geology is Hutton’s Unconformity at Siccar Point, Scotland. That unconformity spans 80 million years, an unimaginably long time.

The Great Unconformity is 15 times that old, 1.2 billion years. More than a quarter of the life of the planet.

The Great Unconformity is exposed in two places that WC knows of. At Frenchman Mountain, in southern Nevada, and at the bottom of the Grand Canyon, in Arizona. In both cases, the basement rock is 1.7 billion years old, the Vishnu Formation, Precambrian granite and schist that formed deep in the Earth’s crust. The Vishnu Formation was exposed by erosion, a lot of erosion, with everything above it eroded away. Seriously, geology doesn’t have a clue what the intervening rocks are; they are completely gone.

Sea level rose and the Vishnu Formation sank a bit, and the exposed Vishnu Formation surface became sea bottom. The sea bottom in turn was covered with sediments – that’s what happens on ocean bottoms –  creating sandstone and limestone about 500 million years ago.

Much more recently, at Frenchman Mountain, the whole assemblage was lifted and tilted to an angle of about 50°. That tilted block of rock is Frenchman Mountain today.

Anatomy of the Frenchman Mountain Unconformity

Anatomy of the Frenchman Mountain Unconformity

The Great Unconformity is exposed on the lower shoulder of Frenchman Mountain. There’s even an interpretive sign.

In the Grand Canyon, it as more a matter of the Colorado River cutting down through those marine rocks and sediments.

The Great Unconformity near the bottom of the Grand Canyon

The Great Unconformity near the bottom of the Grand Canyon

But the result is the same, in less than the width of your little finger, the geological record jumps 1.2 billion years.

Which leads to the question, how in the world did that happen? Especially, how did it happen over a wide area? Because the Great Unconformity can be found in various places all around the world. Where is all the missing rock, and how was it removed?

Some very clever work measuring the Hafnium-Luteium ratios in zircon crystals may have given us the answer. Okay, that takes some explanation.

Both Hafnium and Luteium are radioactive elements, but decay very, very slowly. 176Lu decays to 176Hf with a half-life of approximately 37 billion years. Readers who have made it this far will agree that’s pretty damned slow. But it makes the ratio of the two isotopes ideal for measuring very long geological intervals.

Zircon crystals are a kind of geologic time capsule, nearly indestructible and sealed tightly. Once Luteium gets sealed into a zircon crystal lattice, it stays there, very slowly decaying into Hafnium. By measuring the Hf/Lu ratios in thousands of zircon crystals, scientists found an unusually large number of zircons with their clock set to 550-600 million years ago. That implies a lot of erosion in a relatively short time; math models suggest about two miles of erosion, on average, across all the continents and continental shelves.

The timing for all that erosion lines up nicely with the Snowball Earth theories. Those were periods when, some geologists claim, most of the earth was covered in ice. The three periods of epic cold snaps in the 180 million years leading up to the start of the Cambrian are sometimes referred to as the “Snowball Earth” periods, and they appear to be the key. The first two of these episodes, in particular, are thought to have seen huge ice sheets draped over every continent for millions of years.

Glaciers are pretty incredible erosive forces. On top of that, the growth of ice sheets on land comes with a lowering of global sea level, exposing vast areas of former seafloor to erosion. Think Beringia. That also lowers the base level that glaciers and streams flow down to, giving them even more downhill energy, making them still more erosive.

So the implication is that the Great Unconformity was created by the Snowball Earth intervals, with massive continental glaciers carving off those two miles of rock, and that the massive amounts of eroded rock wound up in the ocean. All those sediments would have put a lot of nutrients in the ocean. Which may have been a factor in the Cambrian Explosion, the geologically sudden evolutionary explosion of complex life, which is just as remarkable as the sudden change in the rock in the Great Unconformity.

Serious props to University of California, Berkeley’s C. Brenhin Keller. His zircon work may have tied together the Great Unconformity, the Cambrian Explosion and Snowball Earth.

For a geology fan, it doesn’t get much geekier than that.

 

 

 

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