By Wayne Ranney
Oak Creek Canyon, although not as deeply cut as Grand Canyon, is still a very pleasant place to hike with many trails leaving the canyon floor and accessing the rim on either side of it. One will note that Oak Creek Canyon is asymmetrical in nature with its eastern side about 800 feet lower in elevation than the western side, and is composed almost entirely of basalt lava rock that rests on the Schnebly Hill Formation near the canyon’s mouth. The west side stands higher and has a thin cap of lava rock that rests on the Kaibab Limestone. An understanding of this odd arrangement reveals the geologic history of the canyon.
First, a large embayment was carved back into the edge of the Mogollon Rim shortly after it formed 30 million years ago. It was during the formation of this embayment that the upper formations in what would become the eastern walls of Oak Creek Canyon were removed. The embayment eventually became inundated with basalt lava originating in vents on top of the Rim and flowing downhill into the embayment. These flows terminated on the floor of the Verde Lake basin. Today, Interstate 17 drops off of the Mogollon Rim on the upper surface of these flows, which are dated between 6 and 8 million years old.
Sometime after 6 million years, the Oak Creek Fault became active and dropped the east side down about 1,000 feet relative to the west side. The fault served to direct groundwater along its trend to the south, and springs likely issued from the south facing wall of the Mogollon Rim near Uptown Sedona. Spring flow initiated a process known as sapping, whereby the spring water weakened and compromised the integrity of the rocks adjacent to the springs. With spring water concentrated both along the fault and the contact of the Schnebly Hill and Hermit Formations, the weakened rock caused the springs to headward erode back into the edge of the Rim. Through time, the process acted like a zipper being undone in the upstream direction, as this spring originating gash migrated northward along the fault line, positioning Oak Creek in the process. The sapping interpretation for the creation of Oak Creek Canyon is hypothetical at this time but can explain why the lower end of Oak Creek Canyon (Sedona to Grasshopper Point) is not positioned directly along the fault, but is exactly parallel to it upstream from Grasshopper Point. Only time will tell if the hypothesis will survive closer scrutiny.
After Oak Creek became confined within the fault zone, the deepening of the canyon ensued within the last 4 million years. This was accomplished by the relentless rolling of durable basalt boulders on the bedrock in flash floods and mega-floods associated with increased precipitation during the ice age. These boulders acted like a large rock tumbler to carve the canyon. Eventually, Oak Creek will incise down through the Redwall Limestone, the Martin Formation, the Tapeats Sandstone, and ultimately the meta-volcanic rocks that presently are only exposed near Jerome. This will certainly change the profile and look of Oak Creek Canyon and the Sedona area but outdoor enthusiasts need not fear such changes – the rate of canyon cutting, although episodic, is only about one foot every 1,600 years.