Recently obtained seismic data and the results of stratigraphic drilling in southwestern Arizona indicate that several alluvium-covered valleys in this area are underlain by more than 3,000 m of Cenozoic deposits. These deposits, with the exception of the marine late Miocene clastic wedge of the Yuma basin and the Pliocene Bouse Formation of the lower Colorado River valley, are the result of continental sedimentation. For the most part, these continental rocks consist of locally derived clastic sediments and lesser amounts of interbedded volcanic rocks and, in some valleys, thick bodies of evaporites. On the basis of their position in the stratigraphic sequence in relation to regional or semiregional unconformities, the Cenozoic sequence of southwestern Arizona was subdivided into an older Unit I and a younger Unit II. The boundary between these two units is a widespread unconformity surface resulting from an important period of subsidence, block-faulting, and erosion that began in late Miocene time (13 to 12 m.y. ago). The two Cenozoic units have been dated and correlated on the basis of radiometric age determination of the inter-bedded extrusive volcanic rocks, on lithologic character, and with the help of seismic interpretation.

Sedimentation during early Cenozoic time (Unit I) took place in broad interior depressions under predominantly continental conditions. The late Miocene block-faulting episode changed the geography of southwestern Arizona and gave the area a typical basin-and-range structure of mountain-forming horsts separated by valleys underlain by grabens or half-grabens. The prevailing structural grain trends in a northwest direction. Unit II sediments were deposited in these troughs or grabens during late Cenozoic time. At least two of the five troughs located in the eastern part of the area studied contain thick sequences of evaporites that indicate interior drainage. These evaporites are assigned a late Miocene age on the basis of K-Ar ages for associated extrusive volcanic rocks and by their position in the stratigraphic sequence in relation to the late Miocene block-faulting episode. Exterior drainage systems were developed beginning sometime between 10.5 and 6 m.y. ago and have evolved progressively to give the area its present-day geomorphology.

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