Hydrogeology

How did Pipe Spring National Monument become a welcome oasis in the Arizona Strip?

Geology

In the Colorado Plateau region, Pipe Spring National Monument (PISP) is located north of the Grand Canyon. You can find it sitting at the base of Winsor Point of the Vermillion Cliffs (Sharrow, 2009). Here at PISP, the vibrant rock layers of the Navajo sandstone and Kayenta Formation are exposed and can be seen (Image 1). The Navajo sandstone is a porous rock, which allows water to move and gather underground. The rock layer underneath the Navajo sandstone is of the Kayenta Formation. The topmost part of this layer is made up of sandstone and siltstone. As you continue down, the layer becomes more clay-like. This rock type, called shale, does not let water pass through it as easily as sandstone. This prevents water from moving downward. Instead, the water finds a weakness in the rock and gets pushed out onto the land surface, forming the springs we see today!

The Sevier Fault is another important geologic structure that passes through PISP. The Sevier Fault originally formed as a reverse fault (Graham 2010). It was the result of compressive forces from the Sevier Orogeny, which happened about 140 million years ago (Ma) to 55 Ma. Then from about 65 Ma to 35 Ma, the Laramide Orogeny occurred. Deep faulting on the west block of the Sevier Fault caused the rock layers to fold. This uplift formed the Moccasin Monocline. The Miocene extension that occurred 15 Ma, was another major geologic event. During this event, the Earth's crust started pulling apart. This reactivated the Sevier Fault into the normal fault that we see today. The resulting structural geology of these major events caused a syncline, a U-shaped trough (Image 2), and many cracks in the bedrock to form. Both play an important part in how water accumulates and moves within our aquifer.