Late Cenozoic basins of Arizona
— Robert B. Scarborough and H. Wesley Peirce

After about one hundred years of geological research, it has become apparent that the physiography of the Basin and Range province is strongly related to a late Cenozoic relief- producing tectonic event. This event established a complex array of disconnected mountain ranges and rather broad, intervening valleys. However, the detailed relationships between modern physiography and Cenozoic structural events are just beginning to be understood. The purpose of this paper is to explore some of these relationships.

 

A few terms need to be defined. In this paper a temporal distinction will be made between "basin" and "valley." A basin is a feature that acted in the past as a center of deposition or a trapping mechanism for sediments, whereas a valley (bolson) is an extant, topographically low feature which is contained between adjacent mountains. Most of the modern valleys in southern Arizona are spatially related to basins produced during the late Cenozoic (fig. 1). The two features are thus related, as most valleys are topographic forms which owe their distribution to the position of earlier-formed late Cenozoic basins. Valleys are, however, always more extensive than basins because of pedimentation (erosion) of adjacent mountain blocks.

 

For the purpose of categorizing discrete Cenozoic geologic episodes, a name should be applied to the important tectonic event that was initiated during the late Miocene (14-6 m.y. ago). We shall call this event the "Basin and Range disturbance." "Basin fill" is the sedimentary group that was deposited in basins created by the Basin and Range disturbance. However, we exclude all deposits formed by relatively modern, integrated stream systems; these deposits are generally coarser- grained. It is likely that these latter materials are no older than Pleistocene.