Colorado Plateau uplift and erosion evaluated using GIS fact, less uplift on the plateau than pro- Cenozoic epeirogeny (e.g., Hunt, 1956; Joel L. Pederson, Rob D. Mackley, posed sources can supply. This suggests Morgan and Swanberg, 1985; and James L. Eddleman, Department Laramide uplift of the plateau was signifi- Humphreys, 1995; Spencer, 1996; of Geology, Utah State University, cantly less than that of the Rocky McQuarrie and Chase, 2000). The earliest Logan, Utah 84322-4505, USA Mountains, consistent with its prevalent researchers visiting the Colorado Plateau sedimentary basins, and/or that there has saw the deep incision of its spectacular been little or no post-Laramide uplift be- canyons and concluded that erosion has ABSTRACT yond erosional isostasy. been driven by recent—and ongoing— Study of the interaction between uplift uplift (e.g., Powell, 1875; Dutton, 1882; and erosion is a major theme of our sci- INTRODUCTION Davis, 1901; Hunt, 1956). This conclusion ence, but our understanding of their in- Pioneering geologists such as John was in keeping with W.M. Davis’ influen- terplay is often limited by a lack of Wesley Powell, Clarence Dutton, G.K. tial model that large-scale cycles of uplift quantitative data. A classic example is Gilbert, and William Morris Davis pon- and erosion end with landscapes de- the Colorado Plateau, for which the dered the Colorado Plateau landscape nuded to a peneplain near sea level, and starting and ending points are well evolution, and questions still puzzle re- with the related assumption that incision known: The region was at sea level in searchers today. In particular, what is the must be driven by subsequent uplift the Late Cretaceous, and now, the explanation for the plateau’s mild struc- rather than other means of lowering base deeply eroded land surface is at ~2 km. tural deformation compared to surround- level for streams. The concept of a The path of the landscape between ing areas, its high average elevation, and plateau denuded to near sea level after these endpoints is less clear, and there its dramatically incised landscape (Fig. 1)? Laramide time, and then epeirogenically has been longstanding debate on the Hypotheses for uplift of the Colorado uplifted later in the Cenozoic to account mechanisms, amounts, and timing of Plateau include mechanisms such as flat- for canyon incision, has persisted, but uplift and erosion. We use a geographic slab subduction, crustal thickening, and most workers recognize that the eleva- information system to map, interpolate, anomalous mantle properties during two tional history and the timing of erosion of and calculate the Cenozoic rock uplift stages of activity: (1) early Cenozoic the Colorado Plateau are still unknown. and erosional exhumation of the (Laramide) uplift; and (2) middle-late Colorado Plateau and gain insight into its landscape development through time. Initial results indicate uplift and erosion are highly spatially variable with mean values of 2117 m for rock uplift and 406 m for net erosional exhumation since Late Cretaceous coastal sandstones were deposited. We estimate 843 m of erosion since ca. 30 Ma (a larger value because of net deposition on the plateau over the early Cenozoic), which can account for 639 m of post-Laramide rock uplift by isostatic processes. Aside from this iso- static source of rock uplift, paleobotanical and fission-track data from the larger re- gion suggest the early Cenozoic Laramide orogeny alone should have caused more than the remaining rock uplift, and geo- physical studies suggest mantle sources for additional Cenozoic uplift. There is, in Figure 1. Physiographic extent of the Colorado Plateau according to Hunt (1956), which is used here for all discussion and analyses. Mean elevation is taken from merged 90 m digital elevation models (NAD83). 4 AUGUST 2002, GSA TODAY Isostatic response to the erosional ex- humation of the plateau over the middle- late Cenozoic should itself result in sig- nificant rock uplift, but this has not been adequately considered in evaluating the above ideas. Several well-known studies have investigated this effect in other areas using approaches different from those used here (e.g., Molnar and England, 1990; Montgomery, 1994; Tucker and Slingerland, 1994; Small and Anderson, 1995, 1998; Whipple et al., 1999). Here Figure 2. Illustrations of terms as described in text: U = rock uplift, U = we use a straightforward approach of di- R S ε rectly measuring rock uplift and exhuma- surface uplift (surface lowering when negative), = exhumation. A: The case of “active” tectonic rock uplift. B: “Passive” rock uplift due to isostatic tion using information preserved in the response to exhumation. landscape and the stratigraphic record. We first quantify mean Cenozoic rock uplift for the plateau. Then, by estimating erosional exhumation, we evaluate how the plateau’s upper crust in the Laramide patterns on the central and northern much of this uplift can be accounted for orogeny (Spencer, 1996), but McQuarrie plateau are thought to have developed by “passive” isostatic response to erosion. and Chase (2000) have suggested that by the Oligocene (Hunt, 1969), though The remaining amount of uplift is what weak midcrustal material flowed east- development of present-day drainage off must be accounted for by Laramide ward from the thick Sevier orogen pro- the southwest margin of the plateau, and events and other mechanisms for post- viding Laramide crustal thickening and probably most erosion, postdates Laramide epeirogeny. uplift. Changes in lithospheric buoyancy Miocene structural differentiation be- Our focus here is specifically on rock have been attributed to low-angle sub- tween it and the Basin and Range. uplift and erosional exhumation, and we duction of a relatively buoyant slab and Studies of Cenozoic deposits on the use definitions of these terms after its aftereffects (Humphreys, 1995). This southern and southwestern margins of England and Molnar (1990), as illustrated includes mechanical thinning of the man- the Colorado Plateau document that in Figure 2. Rock uplift (UR) is the vertical tle lithosphere and its subsequent modifi- drainages flowed northeast away from displacement of rock relative to a datum cation by upwelling asthenosphere (Bird, Laramide highlands onto the plateau dur- (e.g., the geoid), exhumation (ε) is the 1984; Humphreys, 1995; Spencer, 1996). ing the early Cenozoic. These drainages thickness of rock removed through tec- Bird (1984) suggested complete removal were disrupted by normal faulting along tonism and/or erosion, and the resultant of mantle lithosphere during low-angle the Basin-and-Range transition zone, and change in ground-surface elevation con- subduction, but isotopic and geophysical then the Colorado River was integrated − ε stitutes surface uplift (US = UR ) or studies suggest preservation of some off this escarpment, reversing surface thickness of mantle lithosphere (Livaccari drainage to the southwest after 6 Ma lowering (when US is negative). In an erosional setting, rock uplift may drive and Perry, 1993; Spencer, 1996; Lastowka (e.g., Lucchitta, 1972; Young and significant exhumation and thus results in et al., 2001). Paleobotanical studies from Brennan, 1974; Young and McKee, 1978; less surface uplift (Fig. 2A). Likewise, after the region, including those from the Pierce et al., 1979; Cather and Johnson, “active” uplift, exhumation is typically northern plateau, suggest that in the mid- 1984; Potochnik and Faulds, 1998). This several times the resultant surface lowering dle-late Eocene after the Laramide uplift, drainage change resulted in a significant because of isostatic response (Fig. 2B). regional surface elevations were as high effective base-level drop for streams, or higher than now (Gregory and Chase, driving late Cenozoic incision that proba- Early Cenozoic Rock Uplift 1992; Wolfe et al., 1998). Middle Eocene bly accounts for most erosion on the The Paleozoic and Mesozoic sedimen- flora from the Green River Formation are plateau. A key debate has been whether tary sections of the Colorado Plateau con- interpreted to indicate surface elevations neighboring Basin-and-Range basins tribute ~3 km to its 40–45-km-thick crust of 1.5–3 km (Wolfe, 1994; Wolfe et al., have undergone faulting and subsidence (Keller et al., 1998; Lastowka et al., 2001). 1998). This provides a minimum estimate relative to an already high plateau, This sedimentary package was generally of early Cenozoic rock uplift, since sur- whether the plateau has been uplifted formed near sea level but now outcrops face elevation at the close of the relative to the Basin and Range, or far above sea level, and thus there has Laramide orogeny would be less than to- whether both have been uplifted (cf. been uplift (quantified below) since the tal rock uplift because of concurrent ex- Lucchitta, 1979; Hamblin, 1984; Pederson Mesozoic. Proposed mechanisms for up- humation (Fig. 2A). et al., 2002). Discussion hinges on geo- lift of the region in the early to middle physical evidence (e.g., Morgan and Cenozoic include both crustal and mantle Middle-late Cenozoic Epeirogenic Uplift Swanberg, 1985; Parsons and McCarthy, modifications to provide crustal thicken- Post-Laramide events have continued 1995), the marine versus continental ori- ing or changed lithospheric buoyancy. to alter the landscape of the western gin of Neogene deposits along the lower There was no appreciable thickening of United States. The broad-scale drainage Colorado River corridor (cf. Lucchitta, GSA TODAY, AUGUST 2002 5 externally drained at about this time, but it arguably occurred in Miocene time in the southwestern plateau. For our pur- poses, the paleosurface we reconstruct, generalized as ca. 30 Ma, everywhere predates the major incision that has sub- sequently defined the landscape. In up- lands of the neighboring Rocky Mountains, the late Eocene–early Oligocene is represented by the “late Eocene erosion surface” that formed as Laramide highlands were eroded to rela- tively low relief and basin sedimentation slowed (e.g., Epis and Chapin, 1975).