Geologic Mysteries

By Mimi Diaz, Phoenix Branch Chief, Arizona Geological Survey

Picture it: Payson, 1.73 billion years B.C., when there was truly ocean front property in Arizona. The adjoining seas were spawning first life of amoebas and algae, and explosive volcanoes dominated the coast, dispensing thick, viscous flows of red rhyolite that billowed great clouds of steam as the lava encountered water. Stream systems wore down the mountain ranges, dropping off rounded remnants of the formerly towering rocks along the way, intermixed with coarse grains of sand, only to be baked as another flow of lava coated the sediments. Earthquakes would periodically rock the region as the adjacent oceanic plate was shoved underneath the continent, feeding the volcanic frenzy. This went on for roughly 80 million years, until the rock record pulls us up short at what geologists call The Great Unconformity, 1.2 billion years of missing time, almost a quarter of Earth’s amazing history, gone, wiped from the region forever.

Visitors can hike down to an incredible view of Tonto Natural Bridge. Visitors may also see the bridge from a number of parking lot viewpoints.

Fast forward to roughly 545 million years ago, when the rock record picks back up, telling us that the restless seas spent the next 20 million years encroaching on, and covering, the former beachfront property. The seas filled in the lowlands and eroded away the cliffs, leaving naught but sand, cobbles, the occasional ripple mark, and another unconformity (spanning about 100 million years).

Little is known about this timeframe; however, the overlying rock units (and those in other parts of the Southwest indicate that this was probably a time of great inactivity—very little shaking and baking going on. Eventually, though, the ocean reclaimed its dominance, returning with a vengeance again and again, depositing layers of limestone and shale, and preserving examples of sea creatures such as coral and scallops. Fish were present by now, quite different than today’s fish, which, had humans been present, would have given the unwary snorkeler quite a surprise with their massive, monstrous bony heads.

About 330 million years ago, the seas settled down to some steady work, allowing crystals of lime to steadily accumulate and sea life to flourish. Sea snails and clams arrived on the scene, trawling the shallow marine floor for sustenance, while corals built up a reef offshore. Just to the north of this area, the winds were busy carving out a landscape of sand dunes that were dissected by the occasional river, supplying the ocean with fresh water and sediment.

Evidence suggests the travertine bridge used to be longer, perhaps even 1,000 feet in length.

But then, something happened rather abruptly here. By about 290 million years ago, all record of geologic activity vanished yet again. What happened in Payson? Why are rock units to the north and west present to attest to what happened for the next 225 million years, but not here? Were they eroded away? Or were they even deposited in the first place? A giant supercontinent, known as Pangaea, formed and broke up, and dinosaurs roamed the planet until extinction, but no record of any of those monumental events was left behind here for us to interpret.

We can infer from other parts of the western US that volcanic activity increased, as did the number of earthquakes, when the great collision known as the Laramide Orogeny occurred 50- 70 million years ago. This series of events kickstarted the formation of the Rocky Mountains, a time of repeated grinding, crunching, and warping as two tectonic plates collided, causing rocks to break and slide over and under each other.

The next exciting phase for this area didn’t start to occur until probably about 35 million years ago when another mountain building event started up—explosive volcanoes revisited the state, and great blobs of molten rock worked their way from deep within the bowels of earth ever closer to the surface, forming giant granite plutons. This was a different type of mountain building than that which created the Rockies, however. This time, the continent was trying to pull itself apart, extending its edges further and further outward in an attempt to annex as much of the planet as possible.

As the crust stretched, great faults tore through the earth to accommodate the massive stress; mountain ranges rose, and valleys (or basins) dropped. These basins trapped materials falling inexorably off mountain slopes and filled up with runoff from rainwater that no longer had an outlet, causing the landscape to be dotted with ephemeral lakes that left gypsum crystals behind every time they evaporated.

Mysteriously, the northwest side of the canyon that Tonto Natural Bridge crosses was shoved and twisted around so that the previously horizontal rocks were dipping steeply to the northwest, leaving giant corners of limestone jutting out like the back end of a sinking ship. The rocks on the southeast side are nearly horizontal still, and Pine Creek flows right smack down the dividing line between the two orientations. Could this have been caused by a special type of fault that occurred when the Mazatzal Mountains were building?

During the last two million years, Arizona experienced a time that could aptly be described as fire and ice; the gradual cycling through several ice ages along with eruptions of a different breed of volcanoes that spewed out copious amounts of thin, runny lavas. The dark basaltic lavas flowed repeatedly on top of each other, capping the tops of what would become mesas as the rivers cut down into canyons. In fact, the caprock of Buckhead Mesa above the park covers fully nine to ten such lava flows.

Meltwater from the mountain glaciers and from the cooler, wetter climate alternately gushed and trickled into the valleys and determinedly seeped downward through tiny pore spaces in the rock units. In places, the water seeped out when it encountered a rock unit that was too difficult to flow through, creating springs. As the water oozed through several hundred feet of limestone, it carried dissolved particles of lime (calcium carbonate) with it. When the spring water emerged into air, some of the water evaporated, forcing the lime to precipitate (settle out like happens if you put too much sugar in tea), coating the surface with a fine, nearly imperceptible veneer. Eventually, though, the veneers of lime coalesced into a visible unit referred to as travertine. Oftentimes, travertine has a translucent and crystalline appearance; other times, as in the case at Tonto Natural Bridge, organic material clouds the crystals, creating a white gummy substance that is still called travertine (had it formed underground, though, we would have cheerfully called it caliche!).

As it turns out, this organic content is a critical component of the bridge system: algae and vegetation roots provide structural support for the travertine to be deposited aerially! The water cascaded (back then; today it drips) down the roots and algal mats, growing in length and width (similar to how icicles grow). The steady action of Pine Creek, a perennial stream, constantly ate away at the underside of the growing deposit; however, the travertine continued undeterred across the canyon, until it glued itself to the opposite wall and began dripping downward, covering the ancient rhyolite below it. If you walk under the bridge, you can put your hand on the contact between the rhyolite and the travertine and get a chill in your spine: your hand is spanning 1.6 billion years of history!

During its heyday, the bridge was probably close to 1000 feet long, as evidenced by the remains of travertine lining the edges of the canyon. Today, though, it is still impressive at 400 feet long, 150 feet wide, and 180 feet tall, giving it the well-deserved ranking of the largest travertine bridge in the US! Mystery still surrounds the bridge, though. Exactly how long ago did the bridge start forming, and what were the growth patterns? How long did it take for it to grow to its full size? There are other possible hypotheses to explain how the bridge formed; is the one presented here the correct one? Standing at viewpoint 4, gazing raptly at the rainbow-ensconced waterfall rushing over the arch into the gaping maw of the canyon below is a great place to contemplate the mysteries of how Tonto Natural Bridge came to be.

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The views, conclusions, findings and opinions expressed in this Feature Story are the personal views of the author and do not necessarily reflect the views or policies of Arizona State Parks, the Arizona State Parks Board, or the government of Arizona.

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Geologic Mysteries

By Mimi Diaz, Phoenix Branch Chief, Arizona Geological Survey