Great Desert Tsunami

On the West Coast of Pangaea

This is a brief history of the geology of the Colorado Plateau, which introduces a spectacular theory to explain the sharp boundaries between the rock layers in this region. Think BIG!

The Pennsylvanian Period (320-286 MYA)

The origins of the Colorado Plateau began over 320 million years ago (MYA), when a piece of contintal crust called the Paradox Basin emerged from the Panthalassic Ocean north of the equator, and formed the west coast of the super-continent Pangaea. For tens of millions of years this distinct section of continent was a low-lying coastal plain.

Around 320 million years ago (MYA), a large mountain chain known as the Uncompaghre Uplift began rising east of the Paradox Basin as part of the extensive Ancestral Rockies. As the Uncompaghre rose, the Paradox Basin sank and formed a trough that was inundated by the sea.

The Paradox Basin and the Uncompaghre Uplift were formed as the result of a series of northwest-southeast trending faults. As these faults were by turns quiet and active over time, the shallow seas covering the Paradox Basin were occasionally cut off. As the water evaporated in the hot equatorial sun, the salts and other minerals were precipitated out and accumulated on the sea-floor. Geologists have counted approximately 29 cycles of inundation/evaporation.

Paradox Formation

These salt deposits are called the Paradox Formation, and they eventually accumulated to a depth of about 5,000 feet. As the sediments grew, their weight caused the Basin to sink further, and finally opened a long-lasting opening to the ocean.

Honaker Frail Formation

Another 1000 feet of fossil-filled limestones, marine sandstones, and siltstones were deposited as the sea levels continued to fluctuate. Highway 191 cuts through these grey Honaker Trail layers just in front of the Arches National Park visitor's center.

The Permian Period (286-245 MYA)

Cutler Group

As the sea advanced and retreated from the west, and the mountains of the Uncompaghre Uplift eroded, the white dune sands and red mountain sediments were deposited in alternating layers called the Cutler Group. These form the peppermint layers of the Needles in Canyonlands National Park.

Cedar Mesa Sandstone

A drop in sea level, possibly due to an ice age, allowed the wind to push sands into the Paradox Basin, which was now filled up.

Salt Under Pressure

As the weight of sediments accumulated, the underlying Paradox salts began to change and to move, like slow-motion toothpaste, under the pressure. The salt is less dense than rock, and began to accumulate in the weak zones of the faults; in some places the underlying Paradox salts are over 15,000 feet thick. Everything that happened afterwards was affected by this salt movement, as overlying rock layers were forced upward by the long ridges of salt.

Pangaea 255MYA
10MY before the Permian Extinction

An event or series of events that killed 95% of species on earth occurred about 245MYA. This was the Great Permian Extinction. Its cause is still unknown, but evidence points to an impact, or series of impacts, and their resulting climate changes. The Permian Extinction marks the boundary between the Permian and Triassic periods, which is visible in Canyon Country at the boundary between the White Rim Sandstone and the Moenkopi Formation.

Life gradually recovered from the Permian extinction. New species evolved, and the Canyon Country slowly became a land of lakes, streams, and abundant life. This period, laid down as the Moenkopi and Chinle Formations, lasted for nearly 50 million years.

Manicouagan Crater
Quebec, Canada

Approximately 208MYA, a large meteor impacted in what is now Manicouagan, Canada, causing a major extinction of approximately 60% of life on Earth. This impact occurred at the Chinle/Wingate sandstone transition. The following climate change turned the Colorado Plateau into a vast, arid, equatorial desert dominated by sand dunes, with virtually no land life. This desert lasted for millions of years, and laid down the Wingate Sandstone layer. This was the beginning of the Great Desert period of the Canyon Country.

There was some relief from the desert period when Pangaea broke up and the North American continent drifted northward. Between 196-191 million years ago, the Canyon Country became cooler and wetter. Life once again returned to the region, including the evolving dinosaurs and therapsids ("mammal-like reptiles") that left their footprints in stream bed and lake shore deposits of the Kayenta Formation. This relatively short period of time was the only break in the 40 million year history of the Canyon Country's Great Desert depositions.

Navajo Sandstone

Navajo Sandstone deposit
The Four-Corners region approximately 191MYA

By around 191MYA, the Great Desert had returned. This was another low coastal desert, next to the tongue of the sea that came down into present-day Utah from the north. The environment was dominated by rolling sand dunes that were continuously shifted by the strong winds that brought massive amounts of beach sands into the Colorado Plateau. This material was deposited as the Navajo Sandstone, shown in yellow in the map above.

Unlike the desert of the Wingate, however, the Navajo was home to many plant and animal species. These were supported by the numerous scattered oasis, springs, and playas. The latter were broad, shallow, highly-mineralized drylakes, which were occasionally inundated by rain from fierce tropical storms. When the plentiful playas were wetted by storm runoff, they supported a food chain that began with algae and cryptobiotic species that provided food for crustaceans, which in turn fed the small therapsids and the predatory dinosaurs that visited the oases and playas for food and water. The desert also supported primitive trees, horsetails, and other vegetation around the water centers.

Although very few fossils have been found in the Navajo, we know life was there because it left its footprints, fossil burrow castings, and other evidence of its existence in the playa muds and oases shorelines. Unfortunately, the lack of fossils in the Navajo makes it difficult to date the deposition timeframe.

The Navajo desert lasted for millions of years, and eventually no more new material was brought in. Its huge dunes flowed across the lithified subsurface and were gradually worked into a flattened plain by the constant winds, not losing much to erosion, slowly lithifying into sandstone.

The Great Desert Tsunami

Think Big!

Then one day, approximately 172 million years ago, a large comet or meteor impacted somewhere in the Panthalassic ocean to the southwest of Pangaea. It came straight in, traveling at many hundreds of miles per second. The column of water directly beneath the impact acted like a giant hydraulic piston, and transferred its energy directly into the planet's lithosphere and deeper. It rang the earth like a stupendous bell, and set off global tectonic havoc, triggering massive, planet-wide earthquakes, rifting and faulting, accelerating activity along all the continental plate boundaries, activating volcanos and creating new ones.

At the impact site, the energy not passed on into the planet was converted into other forms, per the laws governing the conservation of energy. Most of the energy released by the resulting explosion, as the object bore into the ocean and possibly the sea bed itself, was converted into heat, which, in turn, was absorbed by the surrounding oceanic water and atmosphere, ejecting megatons of water and marine minerals - soluble salts - into the earth's atmosphere. The heat absorbed by the ocean water resulted in an epic El Nino of great scale and magnitude. Thus the stage was set for long-term effects on the earth's global climate, weather, and ecosystems that lasted for decades, possibly centuries. All the water placed into the atmosphere as super-heated steam upon impact came back down to earth, much of it as catastrophic rains.

A more immediate side-effect of the impact, however, was the stupendous tsunami waves radiating in all directions from the impact site, one series after another. The first set was caused by the direct force of the impact and water displacement outwards, the second and subsequent sets were caused by the immediate closing of the momentary hole in the ocean that resulted from the impact's explosion.

The rings of tsunamis traveled around the globe, and the entire shoreline of Pangaea was soon ravaged by these super-violent waves, including its west coast and hundreds of miles of ancient dune-sand desert.

As the first wave hit the shore, it raced inland, sweeping away anything movable in its path. In the Canyon Country this included masses of wave-abraded, sun-dried driftwood picked up along the shore by the tsunami. Any inland dune sand not tightly bound to the upper lithified surface was also picked up. As the wave reached the soft, unlithified playa deposits they were distorted and most of the animal tracks, so abundant in lower-layer playas, were erased. The loose dune sand mixed with driftwood filled the low-lying areas. The flat desert was planed even flatter, scrubbed clear of any signs of dunes or life.

The first wave continued inland until it reached the higher land to the north and east, it rushed up the steep uplift of the Uncompaghre Plateau and salt anticline slopes, until its energy was dissipated, and the great mass of water retreated back to the sea. As it flowed back, it carried immense amounts of soils – now muds – from the highland slopes, and deposited them over the planed surface of the Navajo sandstone. This initial layer protected the Navajo sandstone underneath from buffeting by the subsequent waves that washed up and over the continent's shoreline.

Each succeeding tsunami wave carried down still more iron-rich sediments from the highlands, but no more bits of life. Driftwood was deposited far inland in the first layer, but there are no signs of life, with the exception of a few mollusk shells in the west, closer to the sea.

Aftermath

The clean interface between the Dewey Bridge muds
and the lighter Navajo sandstone. Above the Dewey Bridge
is the Slickrock member of the Entrada formation.

The red, iron-rich highland muds deposited over the Navajo form the Dewey Bridge Member of the Entrada Sandstone group. They are approximately 100 feet thick, and bear no signs of life, which means they were laid down before life could take hold, i.e., very quickly, probably over mere decades, a century or two at the most.

The juncture between the Navajo and the Dewey Bridge is absolutely clean; there is no mingling between the two, and the upper few feet of the Navajo has obviously been water-worked.

The water-saturated Dewey Bridge deposition was laid down in two distinct phases. The first, the "Tsunami Unit", is the direct result of wave-deposited muds. The second, the "Deluge Unit", is the result of additional muds washed down from the highlands by the torrential rains that were among the long-term effects of the impact and subsequent explosion.

The immediate impact aftermath period does not end abruptly. It tapers off over decades as the original climate conditions that had existed in the region for millions of years gradually returned to normal. Slowly, inevitably, the desert returned. The top Dewey Bridge sediments were reworked by hot equatorial winds, and new sands were brought into the region.

A New Desert

The new desert had a different geographic configuration from the great Navajo desert that lay dead and buried beneath a hundred feet or more of lifeless mud. The impact tsunamis and tectonic effects such as accelerated sea-floor rifting raised sea levels. The sands of the new desert were oxidized-red from high iron content, and bear very little signs of life. This desert is the Slickrock member of the Entrada sandstone.

During the millions of years of its existence, this desert was occasionally flattened. This is indicated by horizontal bands of angled dune-sands separated at intervals by thin parallel water-layers of the same material. The impact resulted in periodic tectonic activity, much of which caused subsequent smaller tsunamis and huge storms. Each horizontal water layer in the Slickrock member may represent a regional earthquake great enough to trigger a tsunami sufficiently powerful to sweep hundreds of miles inland across the Slickrock sand dunes. There is evidence of additional impacts towards the end of this deposition, as well.

The desert eventually lay between the open ocean to the west, and an inland sea, the Sundance Sea, to the east. For awhile, the Summerville deposits intruded into the region from the west; there is a Summerville layer between the Slickrock and the Moab Tongue west of Highway 191, before being pushed back and covered briefly by the white dune and shoreline sands of the Moab Tongue Member.

And Still More Impacts

There is ample evidence that the end of the Moab Tongue deposition was also marked by periodic multiple impacts, which preceeded the "Big One" that finally ended the age of the dinosaurs at the end of the Jurassic, 65 million years ago. For example, there are abundant footprints of therapsid dinosaur tracks on the upper surface of the Moab Tongue. These dinosaurs did not generally travel in groups, and in order to be preserved, their tracks had to have been covered almost immediately by invading Summerville muds or they would have been washed away by the next tide. Where did all those creatures come from, and where did the go in such great numbers in such a short timeframe? How did the Summerville cover the Moab Tongue so quickly and so permanently?

Canyon Country enigmas abound...

Time Passes

The Canyon Country was gradually buried under thousands of feet of sediments and the planet continued its story of impacts, tectonic activity, sea-level changes, and resulting climate change.

Today, 144 million years after the end of the Jurassic, the surfaces of the ancient deserts are exposed once again to the light of the sun. The bipedal descendants of great apes that evolved in the arid grasslands west of the Great Rift in Africa approximately 3.5 million years ago walk across the ancient sands and marvel at the stories they tell.