The Grand Canyon is one of the most magnificent – and enigmatic – erosional features on the Earth’s surface. At 277 miles (446 km) long, up to 18 miles (29 km) wide, and a mile (1.6 km) deep, the canyon has inspired awe in visitors for centuries. But beyond the aesthetic beauty and recreational opportunities that draw nearly 5 million people per year to its steep rim, the canyon contains immense geologic value and has been at the center of intense debate amongst geologists for over a century. Early geologists to visit the Grand Canyon, such as the intrepid John Wesley Powell, puzzled over its origin in the midst of being struck by its splendor (Powell, 1875, p. 394):
“The wonders of the Grand Canyon cannot be adequately represented in symbols of speech, no by speech itself. The resources of the graphic art are taxed beyond their powers in attempting to portray its features. Language and illustration combined must fail. The elements that untie to make the Grand Canyon the most sublime spectacle in nature are multifarious and exceedingly diverse.”
Early geologists like Powell and Clarence Dutton were particularly perplexed by the tendency of the canyon to cut through major (kilometer-scale) topographic obstacles, leading them to wonder about the timing and mechanisms of the canyon’s incision (e.g. Powell, 1875; Dutton, 1882). Decades of detailed geologic field observations eventually led to a consensus that the incision of the canyon did not occur until late Miocene time (~6 to 5 Ma) due to the integration of two previously separated drainage basins to form the modern Colorado River at that time (e.g. Longwell, 1946; McKee et al., 1967; Lucchitta, 1972; Pederson, 2008). In recent years, this consensus has been both supported (Karlstrom et al., 2008) and challenged (Wernicke, 2011; Flowers and Farley, 2012) by a wealth of new geologic data. The debate over the timing and mechanisms of incision in the Grand Canyon remains hotly contested, and has particular relevance to the Cenozoic tectonic history of the western United States.
The Grand Canyon preserves a geologic record extending back in time nearly the last two billion years – the age of the oldest rocks the bottom of the canyon (Fig. 1). These late Precambrian basements rocks (Vishnu Schist and Zoroaster Granite) are exposed in several portions of the canyon at river level. A thick (800-1500 m) sequence of Paleozoic sedimentary rocks – dominantly sandstones, shales, and limestones – unconformably overlies these basement rocks and forms the conspicuous layers that make up the canyon walls (NPS). Mesozoic strata found elsewhere in the Colorado Plateau region is rarely preserved in the Grand Canyon due to erosion. Overlying Tertiary volcanics and gravels, while sporadic, provide important age constraints on the age of the canyon (e.g. Young and Brennan, 1974; Elston and Young, 1991; Pederson et al., 2002). The canyon itself formed sometime in the Cenozoic (exactly when is debated and is the focus of this webpage) and cuts into four plateaus within the Colorado Plateau province of the southwestern U.S., a distinct region of high mean elevation (~2 km) but relatively minor internal deformation (Fig. 2). The timing of uplift of the Colorado Plateau remains debated, but has been proposed to occur in three discrete stages to varying degrees: Laramide (75-45 Ma), mid-Cenozoic (35-15 Ma), and Neogene (10 Ma-present) (e.g. Flowers et al., 2008; Cather et al., 2012).
The origin of the canyon itself has been inferred through a variety of geologic observations. Much of the debate over the canyon’s origin has historically centered on the “Muddy Creek problem” (see extended discussion in Wernicke, 2011). The name stems from the Muddy Creek Formation, a >500 m deposit of Miocene basin fill deposited between 13 and 6 Ma atop tilted fault blocks in Grand Wash Trough at the western exit of the canyon (Fig. 3; Longwell, 1936; Lucchitta, 1979). The absence of diagnostic detritus from the upper reaches of the Colorado River within this formation and the sudden appearance of such detritus between 5 and 6 Ma in both the Muddy Creek Formation (Lucchitta, 1972) and the newly opened Gulf of California (Dorsey et al., 2007; Kimbrough et al., 2007) is considered by many workers to indicate that the Grand Canyon had not been excavated prior to ~6 Ma (Karlstrom et al., 2008; Pederson, 2008). However, other workers maintain that this observation does not preclude canyon carving prior to 6 Ma, but rather only necessitates the integration of the Colorado River through its modern course at that time (Wernicke, 2011; Flowers and Farley, 2012; 2013). In addition, these workers have argued that a Miocene river running through a pre-6 Ma canyon would not have contributed substantial clastic sediment to Grand Wash Trough and the Muddy Creek Formation (Young, 2008; Hill and Ranney, 2008; Flowers and Farley, 2013).
Direct geochronological constraints demanding post-6 Ma incision of the entire canyon do not currently exist; in particular, the upper ~70% of the western Grand Canyon is entirely unconstrained in this regard (Flowers and Farley, 2012). In the eastern Grand Canyon, Pliocene to Pleistocene U-Pb dates on cave speleothems imply that the bulk of incision in that portion occurred after 6 Ma (Polyak et al., 2008), although this data is contingent upon unproven paleo-hydraulic assumptions (Pederson et al., 2008) and remains highly contentious. Additional geologic constraints on canyon incision are indirect. Of particular interest is the existence of deeply incised early Tertiary paleocanyons on the plateau rim in the western Grand Canyon region that had >1200 m of relief prior to being infilled by early Eocene gravels with Late Cretaceous volcanic clasts (Fig. 4; Young, 1979; Elston and Young, 1991; Hill and Ranney, 2008; Flowers et al., 2008). These canyons provide relatively undisputed evidence that pre-6 Ma canyons did exist in the vicinity of the modern Grand Canyon. In the area of these paleochannels on the Hualapai Plateau, there are widespread fluvio-conglomerates directly overlain by 19 Ma volcanics (both units laterally continuous across modern tributary canyons in some cases, but are not found on the north rim of the Grand Canyon; Hill et al., 2008), implying that modern canyon incision on the plateau did not occur until after 19 Ma (Figs. 4 & 5; Lucchitta, 1972; Young and Brennan, 1974; Young, 2008). However, this does not necessarily preclude prior incision of those same canyons in the Early Tertiary. This history is complicated further by the possibility that the western and eastern sections of the canyon may have evolved independently of one another (Flowers and Farley, 2012).
In addition to providing an overview of previous work focused on the origin of the Grand Canyon, the above discussion emphasizes the complex and unresolved nature of this problem. Additional work is needed to fully decipher the origin of the canyon, and accordingly, the Cenozoic tectonic history of this region.
Geologists have being trying to understand the origin of this striking erosional feature for over a century, ever since it was first explored in detail by John Wesley Powell. The precise origin of the Grand Canyon has significant implications for the Cenozoic tectonic history of the western U.S. River incision is typically driven by a relative drop in base level, although climatic and other factors likely play a role as well (e.g. Pederson et al., 2002). A relative drop in base level can be accommodated by lowering the base level itself (e.g. Basin and Range faulting, rifting open the Gulf of California) and/or by raising the land surface somewhere along the river’s course (e.g. uplift of the Colorado Plateau). Thus, the timing and mechanisms of incision in the Grand Canyon should be genetically linked to regional tectonic processes to some degree. Therefore, this information could be used to assess the validity and relative importance of the various mechanisms and stages of uplift that have been independently proposed for the Colorado Plateau, which is a critical tectonic feature in this region.
Lastly, the formation of the Grand Canyon continues to pique the curiosity of geologists and non-geologists alike, as evidenced by the worldwide popularity of Grand Canyon National Park. Deciphering the origins of this spectacular natural feature therefore has relevance to anyone interested in how geologic processes shape our planet. Powell, a geologist, managed to broadly portray the uniqueness of the canyon in his expedition journal (1875, p. 397):
“The glories and the beauties of form, color, and sound unite in the Grand Canyon – forms unrivaled even by the mountains, colors that vie with sunsets, and sounds that span the diapason from tempest to tinkling raindrop, from cataract to bubbling fountain.”
To learn about the origin of the Grand Canyon in further detail, click here.
Joshua Johnson, 2013