This account of a field trip taken to Death Valley for a sedimentology class I was taking dates back to Spring 2004, about halfway through my graduate career. I originally wrote this for Livejournal back when it was the big social network thing and hadn’t yet been eclipsed by Facebook.
This version has been modified slightly to be able to stand on its own as a combination travelogue and discussion of the geological past.
The Death Valley region is at once the hottest spot and lowest point in North America. The two are related; since the temperature of air is related to its height above sea level due to the change in pressure, air sunk below the level of the sea at the bottom of the valley compresses and warms (a process known as adiabatic heating). Death Valley is an ancient lakebed, where all that remains of Pleistocene Lake Manly today are the harsh mineral salts lining the bottom. The rocks that surround it are older still, the remains of ancient seas. We’re far enough inland to find very old strata, originally laid down on a world radically different than the one we know now. Atmospheres without oxygen, hot worlds without ice – or frozen worlds trapped beneath it, where North America bows under the weight of the Laurentide ice sheet, or the young Earth is buried under a kilometer of glacial ice. Hothouse and icehouse, cycling throughout the Phanerozoic through to the present day.
Whether our world will end in fire or ice, it was born in both.
Modern Day – Fire
The initial jumping-off point for this trip isn’t even in Death Valley proper, which we never actually spent time in, but the next valley over. We’re supposed to meet up in a little town called Shoshone. We are accompanied by an undergraduate student, Vicks, who isn’t in the class but is coming along as a combination of moral support and photographer. She is lucky enough to fly in with the ringmaster of this whole trip (one Martin Kennedy by name) but the rest of us are driving, and somewhere along the line half our little car caravan managed to inexplicably disappear. I spend as much time during this as possible asleep. What we’re driving through looks something like this, at least from the air:It’s counterintuitive, but desert landscapes such as this one are primarily shaped by water. Deserts don’t see much rain, so the land and the plants are not well adapted to large-scale rainfall. Rocks weather locally into clays that carpet the ground and make it hard for water to soak in. Plants are sparse, with deep instead of spreading roots, and they don’t do much to break up the soil. When it does rain in Death Valley, the rains become floods. Only heavy boulders and sticky clay, which require a huge amount of water energy to lift and transport, are left behind in the canyons between the mountains as a storm becomes a torrent down the slopes, spilling its sediment at the bottom as broad alluvial fans once it becomes unchannelled and loses steam.
This desert even has a river, the Amargosa. For reasons unknown it makes a sharp u-turn to the north just southeast of Death Valley, never reaching the sea, spilling out its last drops onto places like Badwater and Devil’s Golf Course. It’s too late in the year for there to be water in the Amargosa, so right now it’s dry.
14,000 years before present – Ice First stop is at a relatively young sedimentary environment, at least in terms of the rocks we’ll be looking at for most of this trip. We’re at the edge of a hot spring wetland – quite possibly the last thing I expected to be standing on the edge of, at least out here. Reeds and green algal mats grow in water heated from below by mantle warmth, and shade a shoreline crusted with mineral salts. Inexplicably, there are tiny fish living in this bubbling brew of microbial mats and geologic heat, probably desert pupfish. There’s also a horrific amount of flies, some of which are easily large enough to carry off the pupfish.
Death Valley is a pull-apart basin, a rip in the crust formed at the edge of two sliding crustal plates where they don’t quite shear smoothly against each other. Such tears reach very, very deep, more so than most geologic basins. There’s a vaguely inorganic smell in the air, the odor of mineral-rich water. It’s mostly dampened by the smell of rotting algae.
The hummocky rolling hills around the existing lake are the soft sediments that remain of a much larger body of water. Glacial lake Tecopa was born in the last glacial maximum, when the creeping ice sheets elsewhere in the country forced the jet stream south, covering the southwest with rain. The storms this region saw during that time were similar to those that fall over Northern California and Oregon in our modern interglacial climate. Death Valley was drowned by an Ice Age body of water known as Lake Manly, and the area was flush with greenery.
Onwards to more lake deposits near China Ranch. These are older, Miocene and Pliocene in age, somewhere between 24 and 2 million years old. We’re getting into some serious Wile. E. Coyote territory once we arrive:
We’re not looking at that right now, however. At the moment we’re poking at claystone lake sediments, chewing on the rocks and showing Vicks how to tell various clays apart. Smectites, when placed on the tongue, will stick; others do not; this leads a bit of a Dumb Geologist Trick whereby you stick bits of the outcrop to our tongue and then stick your tongue at someone else both to prove the type of clay it is primarily composed of and to gross them out slightly. The lake sediments are softer and more homogenous than the rocks making up the arches and the hoodoos we had seen before. These are soft claystones laid down in the bottom of an ancient lake, while the others are alluvial fan conglomerates (fanglomerates).
The claystones are packed chock full of gypsum, indicating a lake subject to periodic drying, further reinforced by the fact that elsewhere, the fibrous gypsum can be seen filling in v-shaped mudcracks. We’re on the edge of what may have been a desert playa lake. Perhaps the ice sheets were retreating, and the region was drying out.
This place may have seen more than one lake, perhaps related to another pulse of glacial advance. Dry out a lake, uplift its sediments and warp them from tectonic stress so they no longer lie horizontal, erode them down and put more sediments on top, and you have yourself an angular unconformity such as this one, made up of lake sediments both above and below.
To lithify, uplift, tilt and erode strata takes enormous amounts of time. The deposition of soft claystones, which settle as millimeter-thick layers in still, quiet water, takes more time still. James Hutton, the father of modern geology, once said he could see the dizzying depths of geologic time in an angular unconformity such as this one – “No vestige of a beginning, no prospect of an end.” This one’s not quite as dizzying, but that’s because the west coast of North America is geologically very active compared to the beaches of Scotland.
We break for one of China Ranch’s famous date shakes before leaving behind sediments left by a modern-looking Earth. Enough of these young Cenozoic deposits; back we go in time, skipping most of the Phanerozoic in the process.
545 million years before present – Fire
Most of the west coast of North America disappears, as it has not accreted yet. The young North American craton stops much farther east than it does today (roughly around here) and most of it is flooded by warm, shallow oceans.The entire continent rests near the equator. We stop on the road at the boundary between Phanerozoic and Proterozoic, early in the Cambrian, at the bottom of a very long slope leading up to this:
The steel-gray cap atop the hillside is the early Cambrian Bonanza King Formation. It represents the remains of an ancient tropical carbonate platform – think Bahama Banks, I suppose, with clear blue water and white carbonate sands. Where scleractinian corals build tall, complex reefs today, ancient calcified sponges called archaeocyathids built their slender branching frameworks, held together by calcified microbes. They’re too small to be called reefs, so they’re typically known simply as “bioherms.”
But that formation and its shallow seas lie far upslope. Right now we’re down in the Stirling Quartzite, the boundary between the Cambrian and the Precambrian, cursing and arguing and trying to measure the first 300-odd meters that represents the initial shoaling cycle, where deepwater sediments give way to progressively shallower, lower-energy ones as the sedimentary basin becomes filled over time. Vicks wandered off and found evidence that someone else had been out here before.
Sun’s going down. We take a break back at Shoshone for dinner and beer at the Crow Bar, since it’s too dark to get much done in the way of work and we’re all tired anyway. While the other students (and the teacher) play pool, Vicks and I crouch out back to alternately photograph and annoy a stray Western toad, Bufo borealis.
Time to go make camp.
Ugh. It’s too damned early to be awake, but we’re up anyway. Now that there’s actual light to see it becomes obvious that we camped among cliffs of soft ash-fall tuff, probably generated by Pleistocene volcanism. I’m not sure we’re actually supposed to be camping here, but as they say, it’s easier to beg forgiveness than permission. This seems to have been the approach everyone else took, as people have been busily carving caves out of the cliffs here for some time now.
A a bit of coffee and breakfast later, and we’re off.
I see too many damned sunrises in this line of work. Don’t know why people tend to make a big deal out of them, but I guess that’s because other people aren’t expected to be crawling on rocks just after dawn. Then again, it may also have something to do with the fact that I take to mornings like a lead duck to water.
1 billion years before present – Fire
Once again we stand on the remains of an ancient carbonate shelf, but this one is much older than yesterday’s Bonanza King. Tiny photosynthetic microbes built reef-like bioherms on what was then a tropical passive margin – think modern-day Florida and you probably have something about right, at least in terms of tectonics and water temperature. The air that far back would have been even more oxygen-poor than the Cambrian; in the Paleozoic, after the rise of molecular oxygen, atmospheric oxygen levels were thought to be half what they are today. These reefs are much older. The land was still lifeless, the atmosphere starved of oxygen, but the oceans held the first stirrings of life. Then came the rifting, pulling this passive margin apart, sinking it deep as the basin expanded and tore. Dark, iron-rich lava called diabase came up and pooled in the carbonate country rock, providing the only numerical age for the entire succession: 1.09 Ga. This is the Crystal Spring Formation.
Multiple shoaling cycles are recorded above it as the basin subsides, begins filling to the surface, and subsides again. The resistant beds that stand out are shallow-water carbonates, while the softer stone in between is deeper-water mud. Notice how the cycles are getting shorter upsection. We past a karst surface where the carbonate has been aerially exposed and silica has replaced the limestone; apparently, this is where the silicified remains of the first microbial life on land was found. The eroded karst surface marks the end of the Beck Springs Formation, and now we’re in no-man’s land.
Sometime after the rocks were exposed, the sea returns but the warm-water carbonates do not. Nobody’s entirely sure why, but we end up climbing on fissile, sharp-edged sandstones for much of the rest of the succession. Carbonates weather to razor edges (a process called karstification) so they’re terrible to climb around on, but I’m not sure these are better:
I spent much of my time at this succession chasing undergrad students with band-aids.
900 million years before present – Ice
Atop the Beck Springs Dolomite and the unnamed siliciclastic succession lie rocks generated in cold, glacial waters. Back at this morning’s stop we saw them exposed as a glacial diamictite, a jumbled mess of mud and cobbles and pebbles that showed the striations of grinding by ice. Here the same formation shows its glacial colors in the form of dropstones – boulders and cobbles dropped by glaciers into deep-water sediments as they calve off and melt. But this is no ordinary diamictite. It’s red.
What you are looking at now is one of the last of Earth’s banded iron formations – enormous deposits of oxidized iron laid down as ocean beds. In the modern-day ocean iron is scarce (it’s actually a limiting resource for phytoplankton), and the scarcity is related to the oxygenated water and atmosphere. In the presence of molecular oxygen, iron oxidizes and becomes solid, precipitating out of the water. Reducing conditions are necessary to keep iron in solution in seawater, but how do you keep this much iron out of reach of the atmosphere?
Cap it with ice, or so the theory goes. The glacial sediments we saw elsewhere as a diamictite record one of the last of the great Neoproterozoic glacial episodes, which some scientists (such as Paul Hoffmann) think represent a glacial advance that covered the entire planet, all the way up to the equator. Paleomagnetic data does indicate that the continent was in the lower latitudes when it was freezing over, but that doesn’t necessarily mean the entire planet was frozen. After all, once the entire Earth is frozen over there is no way to break the positive feedback loop of increasing albedo and falling temperature and force back the ice – yet somehow it must have retreated, or else this planet would to this day resemble Europa, the frozen ocean moon of Jupiter. At any rate, the banded iron formations represent the glacial retreat and exposure of the waters to the atmosphere; elsewhere, similar retreats are marked by thick dolomite horizons, known as cap carbonates. This one has one too, called the Noonday Dolomite.
800 million years before present – Fire
Even if you haven’t frozen the world solid, how do you beat back glaciers that have covered most of the rest of the planet in ice? Carbon dioxide would still be outgassing from the planet, but it’s actually a fairly weak greenhouse gas compared to others present in Earth’s atmosphere. Water vapor is stronger and represents a significant amount of heat capture in the modern atmosphere, but freezing temperatures plunge the carrying capacity of air down too low to hold a significant amount of this natural greenhouse gas. But we have another, much more powerful source of greenhouse emissions locked up in the depths of Earth’s oceans. Methane clathrates are molecules of methane caged in special crystals of ice. They lie buried in Earth’s ocean sediments, a pool of natural carbon larger than the entire biomass and all the fossil fuel emissions on Earth combined – set the entire living world on fire and you still won’t release enough carbon to match the methane hydrate pool. Methane is also 27 times more effective an atmospheric greenhouse gas than is carbon dioxide. The clathrates are metastable at modern-day deep ocean conditions, balanced on a razor’s edge of cold temperatures and high pressure, and destabilizing any of this pool is liable to bring about a positive feedback that cannot be stopped. More methane release warms the planet more, which makes remaining hydrates unstable. A world that was once flash-frozen becomes a furnace, and the ice retreats. That’s what Martin Kennedy works on, and the methane in the oceans is probably what this was growing on:
That entire little peak at the center of the image is a single stromatolite. Or rather, it’s one of them; it’s one of the odd bioherms from the Noonday Dolomite, a cap carbonate for the Kingston Peak succession that shows evidence of methane release. There are multiple bioherms growing up on the Noonday platform, but these are growing down in a deep-water succession of black, organic-rich clays, well below the reach of sunlight. These probably weren’t built by cyanobacteria (as they are thought to have grown in water to deep for sunlight to penetrate) but may instead represent methanotrophic organisms, the inhabitants of ancient cold seeps. The organic nature of the rock becomes clearer on closer examination, despite the unusual weathering that makes the surface of the rock appear to run like wax.
Modern Day – Ice
We think of our modern climate as warm and equable, but we live in the midst of an ice age. The surface of our planet is shaped in a large part by the large-scale continental ice sheets that dominate our poles. Ice creates a strong thermal gradient, equator-to-pole, that gives the wind its strength and creates the jet stream that brings storms to the temperate regions. The formation of sea ice generates the subfreezing water that stirs and ventilates the deep seas. Our human society has built itself up based on the climate of the interglacial Holocene, a world balanced on the edge of glacial ice and greenhouse warmth; but if these ancient sediments are indicators of anything, it is that such climates are themselves intrinsically unstable. The center cannot hold, and the warmth – or cold, in our case – can not and does not last. Hothouse and icehouse, glacial advance and methane release, cycling over and over through Phanerozoic time.
Our world will end in fire someday, billions of years from now when the sun becomes unstable. In between it will shift forever between the extremes, fire and ice, greenhouse and icehouse – no vestige of a beginning, no prospect of an end.