Isotopic and paleontologic dates published for 41 Tertiary sedimentary units in Arizona provide a basis for statewide correlation and interpretation for the Tertiary tectonic history of the state. Locations of the units and depositional basins are mapped, and their absolute and relative ages are plotted in a correlation chart, based primarily on isotopic dates of intercalated volcanic units.

The general stratigraphic and structural relationships of the sedimentary units provide evidence of the Tertiary evolution of structure and drainage systems in Arizona. Early Tertiary sedimentary rocks of southern Arizona are predominantly well-rounded, coarse-grained stream gravels that were derived from nearby uplands formed during the Laramide and mid-Tertiary orogenies. In northern Arizona, comparable sediments were deposited in drainage systems flowing northward from central Arizona, across the present Colorado Plateau. Local ponding occurred in both areas, wherein fine clastics and carbonates were deposited.

Extensional tectonics of the Basin and Range disturbance resulted in volcanic activity and normal faulting, creating deeply subsiding basins in which thick sequences of fluvial, lacustrine, and evaporite sediments were deposited. Sedimentation continued until Pliocene time when tectonism and volcanism abated, allowing the establishment of through-flowing drainage and downcutting of the more elevated basins. The fossil record in these basins contributes valuable information relative to their depositional history and is used for preliminary interpretations of Tertiary paleoclimates, land elevations, and paleodrainages.

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