By Wayne Ranney
The famous red rocks in and around Sedona are, for the most part, the same layers that make up the upper walls of the Grand Canyon. Even the formation names used between the two areas are similar and the reason for this is that it was Dr. Eddie McKee who first recognized and named the formations in Oak Creek Canyon. McKee had worked for many years at the Grand Canyon as a naturalist and geologist, and recognized the similarities and stratigraphic relationships between the two areas. He noted only that there seemed to be no Hermit Formation located in the Sedona area and he informally called the red rocks Supai “A”, Supai “B”, and Supai “C”. In 1979, Dr. Ron Blakey of Northern Arizona University revised McKee’s classification and noted that indeed the Hermit Formation was present in the Sedona/Oak Creek area and that McKee had failed to recognize the presence of an additional layer here not found at Grand Canyon. Blakey subsequently named this additional unit the Schnebly Hill Formation, which lies between the Hermit Formation below and the Coconino Sandstone above. See the accompanying diagram from the book “Sedona Through Time” to understand the relationship between McKee’s older classification and the one developed by Ron Blakey that we use today. Remember, the rock themselves have not changed only the way we describe them!
Why is the Schnebly Hill Formation present here in Sedona but not at Grand Canyon? The answer tells us a lot about how sedimentary rocks accumulate in the first place. Loose sediment is always being transported by water (or wind) to a depositional basin. An example of a modern basin is the Gulf of Mexico, where the Ohio and Missouri Rivers and delivering their load of sediment from the Appalachians and Rockies respectively to the gulf. The sediment can only come to rest and become buried where the crust of the earth is actively subsiding, creating space for it to be preserved. All along the course of the two rivers, there is no vertical space for the sediment to come to rest and so it is picked up and transported to the actively subsiding basin. In Schnebly Hill time (about 280 million years ago) the Grand Canyon area was not subsiding so the sediment was reworked and transported by wind to the Sedona area, which was actively subsiding. ere the sand grains could become buried as the crust was slowly lkowered through timeThat is how Sedona came to have a rock unit that the Grand Canyon does not.
But we get ahead of ourselves. The oldest red rock layer here in Sedona is the Supai Group (a Group is an assemblage of formations that are closely related to one another and the Supai Group contains four formations whose names are not important here). These lie on top of the Redwall Limestone and form a characteristic ledge and slope topography that is best observed beneath the span of Midgley Bridge on Highway 89A or along the trails in lower Carroll Canyon. The Supai Group records a gradual transition from the marine conditions that existed here for over 200 million years, to more continental environments. This transition is marked by the interplay between gray limestone and red mudstone in the lower part of the Supai Group. Continental deposition is more conducive to the creation and preservation of red rock strata since the iron component of the sediment is oxidized in earth’s atmosphere. The stage was set for more red rock.