Sunday, March 16, 2014

Out of the Valley of Death: When a Broken-up World Turned to Ice

Our grand exploration of the Valley of Death took us briefly out of the valley itself where we could legally collect some Paleozoic fossils, and to do some training on outcrop interpretation (but no pictures of the Charlie Brown outcrop this year; see this for a previous year's version). We crossed Salsberry Pass at 3,315 feet, drove down a long drainage towards the south end of Death Valley, then turned north and crossed Jubilee Pass at 1,290 feet and drew close to Ashford Mill at sea level.
We were looking for evidence of the cold times in Death Valley. Not the cool times of the last ice age when a freshwater lake filled the valley floor. That was a blink of the eye in geologic time. We were looking at events nearly a billion years ago, when the continents of the world were breaking up, and the surface of the planet nearly froze over. Maybe completely frozen over.
Rodinia was a supercontinent that predated the better known Pangea. The break-up of Rodinia just prior to the beginning of the Paleozoic era heralded the appearance of complex multicelled animals on Earth, and the ragged torn edges of the continent here and there contain evidence of huge changes in world climate. The canyon below Jubilee Pass preserves the remnants a fault-controlled valley that developed along the margin of the newly formed Pacific Ocean and slowly filled with thousands of feet of sediment.

The rocks that filled the rift are collectively called the Pahrump Group, and are divided into three formations, the Crystal Springs Formation, the Beck Springs Dolomite, and the Kingston Peak Formation (oldest to youngest). The older two are interesting because they contain the oldest fossils known from California: structures in the mud that record the daily collection of mud and clay on algal slime that covered pebbles and boulders in a shallow sea that filled the fault trough. The concentrically layered structures are called stromatolites. But the Kingston Peak Formation is strange.

As can be seen in the picture above, the Kingston Peak is composed in large part of conglomerate, a rock with large numbers of boulders, cobbles and pebbles in a matrix of sand and mud. Most conglomerates form in alluvial fans or river channels, and much of the Kingston Peak seems to have formed that way. But some of the boulders show the scratches and grooves that indicate that they were dragged across a hard surface, and some look as if they were dropped into the mud of ocean floor. One of the few situations where this sort of thing happens is when icebergs melt and drop their load of boulders onto the sea floor. Such rocks of possible glacial origin are called diamictites

This would be pretty strange because the presence of carbonate rocks in the adjacent layers and reconstructions of continental distribution at the time suggests the Kingston was deposited in tropical or equatorial regions. No glacial episode in the last 500 million years covered that much of the planet. Not even close. It could be that these rocks at Death Valley formed in some other way, such as extensive mudflows or some other explanation. It's just that such diamictites have now been found at a few dozen places around the world, and that can't be ignored as a local circumstance. It looks like the Earth froze over around 700-800 million years ago. All the way from the poles to near the equator.
How could life on Earth survive such a calamity? Environmental refuges probably existed in the shallow oceans beneath the ice - cold, but not solid ice. Warm chemical-rich water could have been found in the hot springs around the mid-ocean ridges. There may have been freshwater lakes under the glacial ice. In any case, life had to adapt or die, and one of the strategies that evolved may have been multicellular organization and formation of tissues and organs. All complex life on the planet today may owe its existence to the "Snowball Earth".

The concept of a "snowball Earth" must still be considered an hypothesis. Although the glacial episode most certainly took place, many researchers feel that a strip of open ocean still persisted near the equator. Those details remain to be worked out. But on this day, we had a chance to walk across rocks that formed in a time when the Earth turned cold. Very, very cold. And we were observing those rocks from the hottest and nearly the driest place on Earth today.

We walked up a valley eroded along the boundary between the Beck Springs and the Kingston Peak Formations, and walked onto a ridge with a stunning view of the south end of the Death Valley graben. The high ridge of the Panamint Mountains rose in the distance. In the foreground was a black hill called Shoreline Butte, which would be our next stop. It was an incredible view, and it was an incredible story that was told by the rocks beneath our feet. We hiked back to the vehicles and headed up the road to Badwater.

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