INTRODUCTION TO SEDONA-AREA GEOLOGY    Sedona Rocks

• Sedona’s rocks are red because they contain iron oxide (hematite). Most of the red rock layers are sandstones whose red color comes from the iron-rich cement that binds the sand grains together. Less than one percent of the cement is iron oxide, but this is enough to color the rocks red. The origin of the iron is often from iron-bearing minerals that were present in the sediment.
• Faults are important in Sedona geology. A fault is a break in rocks where one side moves relative to the other—up, down, or sideways. Faults cause earthquakes and weaken rocks, promoting erosion.
• Sedona’s main rock layers formed before dinosaurs, in the Paleozoic Era.
• Sedona is built on the soft shale of the Hermit Formation.

 

Stage 1: 1,800-1,700, then 515-251, then 251-65 Million Years Ago

 

Southwest                                                                  Northeast

Three Eras, Three Sets of Layers       The Yavapai Supergroup (bottom layers) formed by plate tectonic processes as volcanic mountains collided with western North America. After a billion years of erosion, the mountains were reduced to a flat layer of schist (gray), granite (pink), and volcanic rocks containing rich copper deposits (black). Above these mountain roots Paleozoic sedimentary layers (blue, red, tan) and Mesozoic layers (green) formed. Most of these layers accumulated on top of a slowly subsiding western margin of North America.

   

Stage 2: 80-40 Million Years Ago

 

Uplift       During this time, as the Rocky Mountains rose to their heights, central Arizona also rose south of Sedona. This formed a highland known as the Central Arizona Highlands. The uplift resulted in total erosion of overlying Mesozoic rocks (green layers in the diagram above—no longer present in this diagram). The Paleozoic layers, including Sedona’s red rocks, bent and tilted toward the northeast as the highlands rose. Eventually the deep-seated schist and granite, the Yavapai Supergroup, reached the surface. Pebbles eroded from the schist and granite, forming a gravel layer above the Paleozoic strata.

   

Stage 3: 28-15 Million Years Ago

 

Erosion       Near the Central Arizona Highlands, erosion removed rock strata, stripping everything away down to the Hermit Shale. Inch by inch, removal of the easily-eroded, soft shale undermined the harder cliffs above. This caused the overhanging strata to cave in. As this process progressed, a cliff formed, the ancestral Mogollon Rim, and the cliff slowly retreated toward the north.

   

Stage 4: 15 Million Years Ago to Present

 

Lava Flows and Faulting        This most recent stage started with volcanoes and lava flows. These are still visible today at the tops of cliffs near Sedona (red-orange layer). A little later the Verde Fault, along the west side of the Verde Valley, caused a large lake to form there. This fault also exposed the copper deposits of Jerome. A smaller fault occurred along the future path of Oak Creek Canyon, promoting erosion and development of the canyon.

     Entire page copyright Dave Thayer 2007, all rights reserved.