Rock Talk By Larry Larason

The Pennsylvanian Effect

The first settlers in western Colorado were used to seeing valleys carved by rivers flowing through them. When they encountered a valley in what is now Montrose County where the Dolores River cut across, rather than through it, they were puzzled and named it Paradox Valley. How this valley, and others like it, formed is an interesting story that begins in the Pennsylvanian Period, a time when giant insects like dragonflies with three-foot wingspans, flitted through swampy forests of tree ferns.

During the Pennsylvanian Period North America was caught in a tectonic vise as all Earth's land masses congregated into the super-continent of Pangaea. Africa and South America were pressing into the southeastern portions of the North American continent, raising the Appalachian and Ouachita Mountains. Subduction was occurring along the West Coast, and island arcs were being added to the continental margin. Another effect of these compressional forces was the creation of the Uncompagre Mountains, which rose in an arc from east central Utah through southwestern Colorado into north central New Mexico. In addition, other "Ancestral Rockies" rose along the present location of the Front Range in Colorado.

The Uncompagres were raised to 12-15,000 feet in only a few million years. The onset of erosion was immediate, of course, but they were not erased until the Jurassic Period. The present day Uncompagre Plateau was pushed up again at a later time. As the mountains rose, a basin, just to the west, sank to depths equivalent to the height of the mountains. It was named the Paradox Basin because it became the source of the paradoxical valleys in the region as it filled with salt. Where did the salt come from?

In the Pennsylvanian Period sea levels all over the world rose and fell. It was a glacial period with polar ice caps and continental glaciers forming and melting in cycles. North America was near the equator with tropical temperatures, so no glaciation occurred here, but the sea rose and fell across its western region. Salt water inundated the Paradox Basin. Later, when the sea fell, water stayed behind, in part because the Monument Upwarp and some other areas stood high as a result of the same forces that raised the mountains. The uplift during the Pennsylvanian was probably the second time that the Monument Upwarp had appeared. The present version is its third incarnation. The Monument Upwarp extends from Kayenta, AZ to as far north as Canyonlands National Park west of Moab, UT. The upwarp may seem to end at the Abajo Mountains, but they were superimposed on the landscape, long after the most recent uplift, which took place during the Laramide Orogeny. Water trapped in the Paradox Basin evaporated slowly to become supersaturated with salts. First the carbonates precipitated and fell to the bottom to become limestone covering the sandstones and shale, formed of outwash from the mountains. Then the sulfates fell out to become gypsum or anhydrite. As the briny water continued to evaporate, halite [salt] and, finally, some potassium and magnesium chlorides precipitated. All these types of rock are called "evaporites" because of their origin. At some point in the cycle the sea returned to refresh the water, and the sequence began again. There were nearly 30 of these cycles. The result was a basin about 190 miles long and 95 miles wide filled up to 15,000 feet deep with a mélange of evaporites, limestone, shale and sandstone. More coal was created all over the world during the Pennsylvanian than at any other period in Earth's history. Not in the Four Corners, however. Our coal comes from the Cretaceous. But we did get petroleum from Pennsylvanian events. Although the Monument Upwarp was high, at times when the sea level was highest it was submerged. In the shallow waters algae grew into huge mats, which were fossilized into bioherms. These structures have been compared to piles of cornflakes, that is, they are very porous. Remains of the sea life that came when the water level was high fell into the shale at the bottom of the sea as the creatures died. To become petroleum, such organic remains must be "cooked" at no more than 150 degrees for several million years. If temperatures are higher, the result is natural gas. After the oil formed, it migrated into the porous bioherms. Oil is now extracted from such structures in several oil fields in Southeastern Utah.

Now, let's get back to those paradoxical valleys. Under pressure, salt flows, rather like glacier ice. As the sediment continued streaming off the Uncompagre Mountains, the pressure grew. Pushing upward the salt mounded the overlying rock into salt anticlines. The arched-up strata were fractured as they deformed, allowing water to seep below the surface and dissolve the upper layers of salt. At some point the rocks in the anticline were no longer supported and collapsed into valleys. Since these valleys were formed by subsurface factors, they have nothing to do with surface drainage patterns. Thus rivers may cross them at nearly right angles. Examples are Spanish Valley, where Moab is located, Lisbon Valley, Paradox Valley, and half a dozen others in east-central Utah and western Colorado. Weathering along the faults and joints in the fractured rock above the salt produces fins. Arches National Park is a good place to see the fins created by bulging and cracking of brittle rock caused by salt pushing up an anticline.

Salt, easily dissolved, appears nowhere on the surface in the region, but in some places gypsum that was pushed up with the salt appears in jumbled piles. Potash [potassium chloride] is mined near Moab.

Although the Pennsylvanian ended 286 million years ago, it had a profound effect on much of the modern-day Colorado Plateau.