The Miocene-Pliocene Verde Formation (~ 5 million years old) is a complex formation consisting of several different facies (distinct units within the formation that represent different depositional character from other units). These facies vary from limestone to mudstone to sandstone. They are interpreted as lacustrine (deposited in a lake) in origin, and the presence of evaporites in the mudstone layers indicates that the lake was periodically dry. The age of the Verde Formation has been constrained by fossil evidence as well as K-Ar (Potassium-Argon) dating techniques.

 

References:

Owen-Joyce, S.J. and Bell, C.K., 1983 Appraisal of Water Resources in the Upper Verde River Area, Yavapai County, Arizona, Arizona Department of Water Resources Bulletin 2 (Prepared by the U.S. Geological Survey), available online at: http://www.verde.org/usgs/usgsstud.html.

Nations, J.D., 1981

McKee and Anderson, 1971