Time Beyond Imagining - A Brief History of the Colorado Plateau: Old Salt

So, what do the Arches at Arches National Park, the Potash Mine just outside Canyonlands National Park, the tragic 2008 earthquake in the Sichuan Province of China, and the Southern Appalachians have in common?

Well, ok, not much, but bear with me. In the last post on the geology of the Colorado Plateau, we were exploring the formation of a rather extensive mountain range in southwest Colorado, where the Rocky Mountains are today (but these mountains, the Ancestral Rockies, were eroded away in the late Paleozoic era). We were left wondering why mountains could be rising in the interior of the continent, when most mountain ranges seem to develop along continental margins.

The breakup of the supercontinent Rodinia in late Proterozoic and Early Paleozoic time had a huge impact on the patterns of sedimentation in the Colorado Plateau, with a series of transgressions and regressions that gave rise to the formations of most of the recent posts. The continents remained as separate entities through much of Paleozoic time, but landmasses began to converge upon one another, and a series of collisions produced Himalayan-sized mountain ranges (which is coincidently the reason the Himalayas happened: Asia and India are colliding). A collision between a portion of Europe and the eastern part of North America raised the northern Appalachian Mountains. Asia and Europe connected to form the Ural Mountains, and in Pennsylvanian time, Africa crashed into the southeastern United States to raise the southern Appalachian Mountains. The supercontinent of Pangea was being constructed, bit by bit.

(As a side note, I should point out that when geologists speak of colliding and crashing continents, we are living in a totally different time reality. The colliding continents are moving no faster than 5 or 6 inches a year. Yes, India is crashing into Asia, but it has been happening now for 40 million years)

The collision of Africa and the SE United States explains the Appalachians, but what about the tragic earthquake in the Sichuan Province? The Ancestral Rockies formed at about the same time as the Appalachians, but the two ranges are separated by a thousand miles or so. In the same way, the mountains of central China are far from the Himalayas, but their origin is related to the buckling of the crust related to this continental collision. The process of buckling produces earthquakes, and in highly populated regions this produces tragedy. Peripheral buckling of the crust along ancient fault lines related to the collision with Africa is at least one explanation for the origin of the Ancestral Rockies.

The Potash Mine? Just south of the town of Moab, and on the edge of Canyonlands National Park, there is an exceedingly odd sight: the evaporation ponds of a potash mine. The turquoise blue, seen here from Dead Horse Point, clashes with the red beds of Permian formations. At the mine, holes are drilled into salt layers, and water is injected to dissolve the salts and bring them to the surface. The brine is evaporated, and the salts are harvested and transported away.

The salt is also related to the formation of the Ancestral Rockies. When the crust is buckled and mountains rise, in other places the crust is pushed downwards. A feature called the Paradox Basin developed southwest of the Ancestral Rockies. It flooded with seawater, but for some reason the connection to the ocean was cut off, and the water evaporated, leaving behind thick deposits of salt and other evaporite minerals. Water flooded in again, and the cycle was repeated, at least two dozen times. Eventually the salt layers were thousands of feet thick.

What is the connection to Arches National Park? Well, this post has become long, and I will discuss that issue next time!

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 7: Mountains Rise

Where did all the dirt come from??? During the previous post, we discovered that something had caused thousands of square miles of land across Arizona, Utah, and Colorado to be inundated with mud and sand more than 1,000 feet deep. These rocks formed above sea level, as deltas, floodplains, river channels, and sand dunes. In the Grand Canyon, these layers of red sediments are called the Supai Group and Hermit Shale. In southern Colorado and Utah, related rocks are referred to as the Hermosa Group, the Halgaito Formation, the Cedar Mesa Sandstone, and the Organ Rock Formation (the Organ Rock Formation makes up the base of the spires of Monument Valley).

On the other hand, today's photograph shows a canyon near the eastern edge of Colorado National Monument, one of those oft-overlooked gems that are scattered about the Colorado Plateau. The grayish scrub-covered rocks in the bottom of the canyon are composed of the same 1.7 billion year old metamorphic rocks that were seen in the Grand Canyon and Black Canyon of the Gunnison. Unlike the Grand Canyon, there is no Grand Canyon Supergroup, no Tonto Group, no Redwall Limestone, no Supai Group or Hermit Formation. None of the rocks of the Grand Canyon are found here at all. The rocks forming the canyon walls are Mesozoic in age (less than 245 million years old), with the lower slopes composed of Triassic Chinle Formation, and cliffs of Wingate Sandstone (more on these later). So we have a second mystery: why are the Paleozoic rocks missing in southwest Colorado when they are thousands of feet thick in adjacent regions?

Pioneering geologists in the region soon recognized that the source of the sediments found in Arizona were mountains that had risen in southwest Colorado during Pennsylvanian and Permian time. The mountains were pushed so high that all the overlying Paleozoic sediments and a considerable amount of the Proterozoic metamorphic rocks were washed onto the coastal plains, leaving a denuded surface of metamorphic rocks that were not covered by sediments until Mesozoic time. These mountains are referred to today as the Ancestral Rocky Mountains or the Uncompahgre Uplift. As we look at the unconformity between the Proterozoic and Mesozoic rocks, we are looking at the eroded roots of what must have been an extraordinary mountain range rising from the interior of the North American continent.

Which leads to the next vexing question: what were mountains doing within the continental interior? Mountains most often form along a continental margin (think of the Andes, Cascades, or Coast Ranges). For a long time, no particularly good explanation could be offered to explain the development of these mountains.

More in the next post! And a salty surprise!