Arizona’s Grand Canyon reveals an enormous sequence of rocks that represent more than a third of the 4.5-billion-year age of the Earth. The canyon itself, however, is quite young in comparison, with most or all canyon incision occurring over the past 5 million years according to most interpretations. Careful investigative work has refined this age determination, as reported in this brief article.
Basin and Range tectonic extension
Before arrival of the modern Colorado River, the Mojave and Sonoran Desert region of southwestern North America was subjected to severe tectonic extension. This produced the basin and range topography that we see today in southern and western Arizona, in southeastern California, and in Nevada and western Utah (the Basin and Range tectonic province). Tectonic extension and normal faulting are still occurring in much of Nevada, in the Death Valley and Owens Valley areas of California, and along the Wasatch Front in Utah. Playas and lakes are present in all of these areas because faulting lowers valley floors faster than they can be filled with sediment. When faulting and basin subsidence end, the basins will fill with sediment and drainages will spill over to eventually form integrated stream systems that end at the ocean.
The lower Pliocene Bouse Formation
When Colorado River water first entered the Basin and Range Province in what is now the Lake Mead area, it filled closed basins and the water spilled over the lowest point of each basin to enter adjacent basins, thereby producing a chain of lakes. Filling, spilling, and incision of outflow channels eventually led to the Gulf of California. Incision of outflow channels and sediment accumulation in basin keels produced the lower Colorado River valley. The extensive landscape flooding that occurred during this brief period left a record of lake sediments known as the Bouse Formation.
The lower Pliocene Bouse Formation consists of basal travertine, marl, and overlying siltstone deposited in the lower Colorado River valley of western Arizona, southeastern California, and southern Nevada. The interpretation that the Bouse Formation was deposited in a chain of lakes following first arrival of Colorado River water to closed basins inherited from earlier Basin and Range extension is supported by (1) Sr isotopic similarity of Bouse carbonates and shells to Colorado River water (Spencer and Patchett, 1997; Roskowski et al., 2010), (2) flood deposits, derived from northern sources, directly beneath Bouse strata near Laughlin, Nevada (House et al., 2008), and (3) maximum elevations of Bouse exposures that define individual lakes at maximum water level (Spencer et al., 2008a).
Maximum Bouse Formation elevations of ~330m in Blythe basin, which is the southernmost of the Bouse basins, indicate that Bouse lake water would have spilled westward into Bristol basin (Fig. 1). Bouse Formation exposures are extensive in the Colorado River valley where they have been exhumed by Colorado River incision. However, Bristol basin is closed, with gradual sediment accumulation rather than incision. As a result, there is only a single exposure of Bouse Formation where it appears to have been uplifted along a restraining bend in a minor, northwest-striking strike-slip fault (Reynolds et al., 2008).