The mystery of the pre–Grand Canyon Colorado River— Results from the Muddy Creek Formation

The Colorado River’s integration off the Colorado Plateau remains a classic mystery in geology, despite its pivotal role in the cutting of Grand Canyon and the region’s landscape evolution. The upper paleodrainage apparently reached the southern plateau in the Miocene, and recent work supports the longstanding idea that the river was superimposed over the Kaibab uplift by this time. Once off the plateau, the lower river integrated to the Gulf of California by downstream basin spillover from ca. 6–5 Ma. An unknown link remains: the history of the river in the western Grand Canyon region in Miocene time. One of the viable hypotheses put forward by previous workers—that the late Miocene Muddy Creek Formation represents the terminal deposits of the paleo–Colorado River in the Basin and Range northwest of Grand Canyon—is tested in this paper. Results indicate instead that local drainages along with the paleo–Virgin River are the likely sources of this sediment. The remaining hypothesis—that the paleo-upper Colorado River dissipated and infiltrated in the central-western Grand Canyon area—has modern analogs, provides a potential source for extensive Miocene spring and evaporite deposits adjacent to the southwestern plateau, and implies a groundwater-driven mechanism for capture of the upper drainage. INTRODUCTION—THE GRAND DEBATE The path of the pre–Grand Canyon Colorado River and how it came to its present course off the Colorado Plateau and into the lowlands of the Basin and Range have been debated for decades. Solving the problem of how the river came to flow off the plateau is key to understanding the formation of Earth’s most famous erosional landscape—the Grand Canyon region. The Colorado River did not tap into its potential energy for erosion until it was integrated over the Grand Wash Cliffs and off the Colorado Plateau, dropping its base level ~1500 m and driving much of the subsequent canyon incision upstream (Pederson et al., 2002a). Understanding how this happened has been hampered by a lack of data, mostly the result, ironically, of erosion having removed much of the geologic record of the river’s history. Based on his early river expeditions, John Wesley Powell believed the Colorado River to be antecedent—relatively old, with younger uplifts raised across its path (Powell, 1875). However, it was soon thereafter established that the monoclinal uplifts of the Colorado Plateau formed in the early Cenozoic Laramide orogeny and that the Colorado River and its major tributaries are younger. Walcott (1890) first suggested that the river was superimposed across the southern flank of the Kaibab uplift while still flowing in overlying, less resistant Mesozoic strata (Fig. 1). William Morris Davis likewise recognized that the Colorado River is superimposed across the exhumed older uplifts of the plateau (Davis, 1901). He envisioned ancestral drainages that flowed northeast and then reversed direction due to down-faulting to the west, with recent uplift of the plateau rejuvenating the Colorado River and forming Grand Canyon. Broadly speaking, Davis’ model was validated and supplemented by the work of subsequent workers. It was recognized early on that the upper basin fill in the Lake Mead region, southwest of the plateau margin, is relatively young and that it precludes the existence of the present Colorado River in that area (Blackwelder, 1934; Longwell, 1946). Closer analysis of the sedimentary record of the Grand Wash Trough, where the Colorado River enters the Basin and Range, revealed a switch from internal basin deposition to subsequent dissection upon the arrival of the exotic Colorado River at 6 Ma (Lucchitta, 1966). Thus, the Colorado River was integrated ca. 6 Ma and carried out most of the incision of Grand Canyon since then (Lucchitta, 1972; McKee and McKee, 1972). How the river reorganized to flow west and what the drainage was like before that time are elusive problems. After the Laramide orogeny, drainages in the Grand Canyon region were directed northeast off the ancestral Mogollon highland (Young and McKee, 1978). In Oligocene time, these consequent drainages were disrupted by magmatism in the central plateau, by the onset of arid climate, and potentially by epeirogenic uplift (e.g., Cather et al., 2008). In Charlie Hunt’s extensive work (1956, 1969), he concluded that much of the upper Colorado River drainage had established itself on the northern and central plateau by Miocene time. So where did the drainage go when it reached the southern plateau? Research along the Mogollon Rim indicates that there was no Miocene exit route for the river to the southwest (Young and Brennan, 1974). Therefore, the main working hypotheses have been that the ancestral upper Colorado River: (a) issued southeast along the Little Colorado River trough either off the plateau or into the Bidahochi Formation of the south-central plateau (McKee et al., 1967); (b) crossed the Kaibab uplift and terminated in the southwestern Colorado Plateau (Hunt, 1956); or (c) crossed the Kaibab uplift and continued northwest across the low plateaus and into the Basin and Range (Fig. 1; Lucchitta, 1990). Regarding hypothesis (a), researchers proposed that the Miocene river exited the plateau to the southeast to account for earlier Miocene erosion in the area (McKee et al., 1967). But there is no evidence for such a river, and the presence of the Bidahochi Formation along this path is a problem (e.g., Hunt, 1969). The Bidahochi dates to the time in question, but it