A Beginner's Guide to the Geology of Saguaro-Juniper area
For an overall perspective, bear in mind that the whole of Arizona south of its Colorado Plateau and Central Highlands is part of the Basin and Range Province of western North America (map below adapted from Nations & Stump 1996 -- see Credits & Bibliography):
The typical mountains and valleys which we see today in this Province have been most recently produced by extensional (horizontal, roughly East-West pulling-apart) faulting of long blocks of continental crust. The map of southern Arizona shown below (adapted from Nations & Stump1996, p. 176, see Credits & Bibliography) illustrates the generally NW-SE direction of the alternating mountain blocks and valleys of our region, and reflects the roughly east-west expansion of the landscape underlying the fault-block process:
When you drive along the San Pedro River Valley (see this link for an introductory tour), you ride atop thousands of feet of basin sedimentary deposits which overlie this down-faulted block: coarse gravel and sand formed as alluvial fans, fine mud and ash where lakes once stood, all of these depositions flowing down from the nearby mountains even as the mountain-blocks arose.
[In the following discussion, you may want to refer to an authoritative geological time scale; for one adapted from the University of California Berkeley Museum of Paleontology (cited in Credits & Bibliography, click here.]
In our area, the mountains, terraces, and basins we see resulted from two distinct mountain-building eras. The earlier began late Cretaceous in age (beginning some 75 million years ago), and is known as the Laramide Orogeny. Later, in the Cenozoic (Tertiary) Era, more recent mountain building in the Galiuro region began in the late Oligocene and early Miocene Epochs as the Mid-Tertiary Orogeny (named by our own late Saguaro-Juniper member Paul Damon), a regionally-widespread period of volcanism and extension that started some 28 million years ago. The sediments flanking these mountains were deposited as debris even as the volcanic mountains were rising. Then, at the end of this second major period of volcanism (represented by the Galiuro Volcanics in our area -- see below), the flanking sediments were faulted and tilted, the greatest period of such faulting and tilting ending about 20 million years ago. This was followed by a transition to Basin and Range faulting (Orogeny), with near-vertical block faulting beginning 15 million years ago and producing the pattern one sees today: long narrow, tilted fault block ranges characterized by sharp escarpments, and separated by deep basins -- the sinking of basins being as pronounced as the raising of the ranges. These basins continued to subside from 15 million years ago until about 8-6 million years ago (and in far Southeastern Arizona and adjacent Sonora this subsiding apparently continues to the present day). Our own area, however, is now apparently structurally quiescent
Today, only the tops of the mountain blocks can be seen: the Galiuro Mountains escarpment which forms a spectacular backdrop of our lands is merely the highest tip of one of these up-tilted fault blocks, mostly buried by the sediments which were carved out of it as it arose. (For a comparable example, the sediments in the Tucson basin are up to 12,000 feet deep, and the Catalina mountains we see from Tucson today are only the visible tip of a much larger structure.)
This fault-block lifting-and-dropping entirely changed regional drainage patterns. At first, the basins were often occupied by lakes.Gradually, between 10 and 6 million years ago, a series of separate basins in our region filled with sedimentary debris, prominently the Quiburis Formations, until streams began to flow from basin to basin, eventually integrating the basins into a more regional drainage network. This process has apparently continued since the end of the Miocene Epoch (about 5 million years ago), though the Willcox and Lordsburg playas well to our east in the Sulphur Springs Valley still remain closed basins. The San Pedro basin, including a great lake where Benson now stands (the pink sedimentary beds visible in the badlands surrounding Benson are remains of its depositions), was gradually drained between 3.5 and 1.6 million years ago to become a captured tributary of the Gila River to the north, as the river patterns we now observe for southern Arizona became established.
This increased downcutting of basins, and -- during the last 2 million years -- alternating rainy and dry cycles, have produced a complicated series of terraces which can be seen as one drives along the San Pedro valley. Soza Mesa is one such terrace of Quiburis Formation, its flanks exposing older sedimentary deposits below more recent ones as the river has cut down into the basin sediments. (Again, for initial illustration of these features, see the San Pedro River Tour link above, and also the Landscapes pages, which includes images of our main mountain ranges, etc.)
For a summary placing all of these processes in their wider geohistorical context, see