When compared to many igneous and metamorphic rocks, sedimentary rocks often aren’t the most visually stunning. However, you’ll be hard pressed to find more vibrant and beautiful rocks than the sedimentary rocks exposed at Sedona, Arizona.
Rather than the standard hues of gray or beige, these sandstones, limestones and shales are breathtakingly vibrant shades of orange and red.
So, why then are these rocks so spectacularly red?
We can thank iron for that.
When the sediment that composes these rocks was deposited, iron-rich groundwater infiltrated the pore space between grains and left grains with a thin coating of iron oxide. This iron oxide that entered the groundwater system was derived from the chemical weathering of iron-bearing minerals (like hematite) in the rocks in the arid setting.
The oldest rocks at Sedona (the Supai Group) were first deposited 316 million years ago and are composed of alternating bands of sandstone, mudstone, conglomerate and limestone.The town itself is primarily built on the 285 million year old rocks of the Hermit Formation. The majority of the most well-known sites, like Bell Rock (upper right photo), are composed of the sandstone and siltstone of the Schnebly Hill Formation deposited 280 million years ago. The youngest Paleozoic rocks exposed at Sedona are the limestone, dolomite and chert of the Kaibab Formation deposited 270 million years ago.
There is a 225 million year unconformity between these units and the rocks that overlay them, meaning that the entire Mesozoic is not represented at Sedona.
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Emily Zawacki, Ph.D. 