A review of the geomorphology of eastern Grand Canyon 2012 GSA SP 489.pdf · A review of the geomorphology of eastern Grand Canyon Joel L. Pederson Department of Geology, 4505 Old

119

The Geological Society of AmericaSpecial Paper 489

2012

A review of the geomorphology of eastern Grand Canyon

Joel L. PedersonDepartment of Geology, 4505 Old Main Hill, Utah State University, Logan, Utah 84322-4505, USA

Pederson, J.L., 2012, A review of the geomorphology of eastern Grand Canyon, in Timmons, J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth’s History: Geological Society of America Special Paper 489, p. 119–129, doi:10.1130/2012.2489(08). For permission to copy, contact [email protected]. © 2012 The Geological Society of America. All rights reserved.

INTRODUCTION

Grand Canyon is one of the great topographic features of our planet. Fittingly, this landscape has been the brewing pot of many fundamental ideas in geomorphology, and it remains a focal point of ongoing research. In this chapter the geomorphology of Grand Canyon is introduced and interpreted, as it is recorded in the Qua-ternary deposits of the eastern Grand Canyon map area, covered by the Geologic Map of Eastern Grand Canyon1. Besides explor-ing the stratigraphy and relations of the Quaternary map units, the history of canyon cutting and the effects of climate change on the landscape are highlighted.

OVERVIEW OF THE EASTERN GRAND CANYON LANDSCAPE

The Colorado River system has cut a series of canyons into a regional landscape of broad plateaus. Grand Canyon, anchor-ing the downstream end of this canyon sequence, creates an east-west transect through the southwestern Colorado Plateau, exposing Proterozoic through Permian bedrock, some of which is discussed elsewhere in this volume. The Colorado River is dis-tinctive in carrying water derived almost entirely from the distant Rocky Mountains, while transporting sediment produced from the erosion of the canyonlands of the Colorado Plateau.

Quaternary deposits in the map area are found along the Col-orado River corridor either as remnants fortuitously preserved along the lower Marble Canyon reach or as more completely pre-served deposits in the broader Furnace Flats reach downstream of the Butte fault. In addition, the major tributaries of the Chuar Val-ley region northwest of the river corridor (Nankoweap, Kwagunt, Carbon Canyon, Lava Chuar, Basalt, and Unkar) each contain a signifi cant stratigraphy of Quaternary deposits that can be cor-related between catchments.

Three types of surfi cial deposits—alluvial, colluvial, and eolian—are found in eastern Grand Canyon. Alluvium includes fi ne-grained Holocene fi ll terraces and fl oodplain deposits of the Colorado River, gravelly Pleistocene fi ll terraces of the Colorado River, and coarse fi ll terrace, debris fl ow, and fl uvial deposits of large tributaries. Colluvium includes landslides, talus, and debris fl ow deposits, which are commonly associated with remnant sed-iment mantles on slope-forming bedrock units or lie at the toe of canyon walls. Eolian deposits are relatively minor and consist of localized dune sand derived from Colorado River channel bars.

Hillslopes in Grand Canyon are dominated by the compound escarpments of alternating cliff and slope-forming bedrock that create the classic Grand Canyon vista (Fig. 1). Slope angle for a given geologic formation exposed in the canyon walls is likely related to a threshold set by rock strength, bedding thickness, and weathering processes. Under present climate conditions, hill-slopes are predominantly weathering limited; that is, most slopes have bedrock exposed, and their erosion is limited by the rate that rock is weathered into regolith. But there is abundant evidence in the map area that the gentler slopes were instead transport lim-ited and buried beneath mantles of regolith and talus during past climate episodes (Anders et al., 2005). In terms of modern pro-cesses, rock falls and rock avalanches are active on the cliffs of Grand Canyon, and overland fl ow is important for sediment trans-port on toe slopes and stream terraces. However, debris fl ows are probably the most important transport process delivering sedi-ment from hillslopes to drainages in Grand Canyon (Webb et al., 1989; Griffi ths et al., 1996; Melis, 1997). In this setting, debris fl ows are initiated when intense or prolonged rainfall combines with antecedent soil moisture, triggering failure and mass move-ment. Typical of Grand Canyon are cases where regolith stored in relict colluvial mantles is mobilized by a “fi rehose effect” of water pouring off cliffs and impacting slopes below (Griffi ths et al., 1996). It is noteworthy that most of the volume of Grand

1The map is available on inserts accompanying this volume and also as GSA Data Repository Item 2012287, online at www.geosociety.org/pubs/ft2012.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

on January 9, 2013specialpapers.gsapubs.orgDownloaded from

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120 J.L. Pederson

Canyon has been excavated by the hillslope processes involved in cliff retreat and canyon widening, not by river incision. Yet the potential for, and rates of, erosion are ultimately controlled by the incision of the Colorado River.

Bedrock not only infl uences hillslopes but also exerts a strong control on the width of the canyon bottom along the river corridor, the width of the river channel itself, and ultimately on the preservation potenti al of the Quaternary deposits mapped (Howard and Dolan, 1981; Mackley, 2005). In the eastern Grand Canyon map area, there is a strong positive correlation between the measured strength of the bedrock in the lower Marble Can-yon, Furnace Flats, and the upper Granite Gorge reaches, and both the gradient of the river and the energy it expends. Both the gradient and stream power more than double downstream through the map area (Mackley, 2005). Hanks and Webb (2006) argue that the observed progression of a relatively fl at lower Mar-ble Canyon reach transitioning to the steep upper gorge reach defi nes a convexity in the river’s profi le that may be related to a disproportionate accumulation of geologically recent local debris brought to the river in Furnace Flats. In turn, debris fan loca-tion and frequency is itself established by regional joint and fault systems in the bedrock (Dolan et al., 1978), and the erodibility

of bedrock in the tributary catchments infl uences the volume of sediment transported to debris fans. Thus bedrock is both a direct and indirect control of the form of the river as well as the overall geomorphology of the eastern Grand Canyon landscape.

LANDSCAPE EVOLUTION AND INCISION OF THE GRAND CANYON REGION

The timing and causes of the uplift and the erosion of the Colorado Plateau are classic geologic problems that have been the subject of renewed scientifi c interest. We know that the region was at sea level in Late Cretaceous time (ca. 70 Ma) and that through much of middle Cenozoic time its landscape was relatively low in relief, but not necessarily in elevation. Now the plateau is a deeply incised landscape at an average eleva-tion of 1.9 km (Pederson et al., 2002b). The evolution of the landscape beyond these basic steps is much debated. In sum-mary, the uplift must be accounted for by some combination of (1) localized Laramide orogenic uplift in the early Cenozoic, especially in western Grand Canyon and the East Kaibab upwarp (e.g., Flowers et al., 2008), (2) middle–late Cenozoic epeirogenic uplift owing to modifi cations to the lithospheric mantle beneath

Figure 1. View looking downstream along Colorado River toward the mouth of Nankoweap Creek and the Nankoweap debris fan. Note classic escarpment form of canyon wall at left and the Pleistocene rock avalanche deposit along the debris fan (highlighted in sunlight in lower right).



A review of the geomorphology of eastern Grand Canyon 121

the plateau (e.g., Morgan and Swanberg, 1985; Humphreys and Dueker, 1994; Roy et al., 2009), (3) late Cenozoic regionalized support from upwelling asthenospheric mantle (Moucha et al., 2008; Karlstrom et al., 2008), and (4) broad isostatic rebound owing to late Cenozoic unloading by erosion and extensional faulting (Pederson et al., 2002b; Roy et al., 2009).

As the Colorado Plateau gained its high elevation through these mechanisms, drainages gained the potential to incise the landscape. But fi rst the trunk Colorado River had to become organized as it is today and fl ow off the high plateau into the low Basin and Range. This drainage integration itself is a mysteri-ous and long-debated issue. John Wesley Powell, the fi rst geolo-gist to complete a river expedition through Grand Canyon, in 1869, believed the Colorado River must be relatively old and that younger uplifts were raised across its path, including the Kaibab uplift in the map area, and he introduced the word “ante-cedent” to describe this (Powell, 1875). Clarence Dutton (1882) concurred and suggested that the East Kaibab monocline at the eastern edge of the Kaibab uplift was Pliocene in age, coincident with the uplift of the entire plateau and the incision of Grand Canyon. However, the much-revered G.K. Gilbert (1876) early on doubted the degree to which regional drainages are anteced-ent. It subsequently has been established that the Kaibab uplift and other monoclines of the Colorado Plateau are associated with the early Cenozoic Laramide orogeny, and that the younger rivers are not antecedent after all. The infl uential geomorpholo-gist William Morris Davis visited the region at the turn of the nineteenth century and asserted that the river was superimposed, not antecedent, across the uplifts of the region. He envisioned drainages at one time fl owing toward the northeast but then reversing direction and fl owing westward owing to block fault-ing (Davis, 1901). In Davis’s model, later uplift rejuvenated the stream and caused an episode of incision wherein the river became superimposed across older uplifts. In the broad sense, this model has been validated and supplemented by Chester Longwell (1946), Charles Hunt (1969), Edwin McKee (McKee and McKee, 1972), Ivo Lucchitta (1972), and others, with the general idea remaining that the Colorado River has integrated piecemeal over late Cenozoic time and is generally superim-posed onto the plateau landscape.

There have been recent and controversial hypotheses that westernmost Grand Canyon is an anomalously old erosional fea-ture (Polyak et al., 2008; Wernicke, 2011). Yet the major work-ing hypothesis for the past few decades has been that, although there were older paleo-canyons crossing the region, the Colo-rado River accomplished much of the incision of Grand Canyon in the past 6 m.y. (Lucchitta, 1972; McKee and McKee, 1972; Young and McKee, 1978; Karlstrom et al., 2008). Analysis of the sedimentary record of the Grand Wash trough, where the Colorado River exits the plateau, reveals a switch from internal-basin deposition to subsequent incision by a through-going river at 6 Ma (Longwell, 1946; Lucchitta, 1966). However, the story of how the river became integrated and reorganized to fl ow west, and what the drainage was like before that time, continues to be

debated (e.g., Pederson, 2008; Douglass et al., 2009). Recent results from thermochronology confi rm the older hypothesis that the Kaibab uplift was bisected by signifi cant paleo-canyon drainages in middle Cenozoic time (Lee, 2007; Flowers et al., 2008), making that particular barrier easy for the eventual Colo-rado River to become integrated across (Pederson, 2008). Anal-ysis of the greater Muddy Creek Formation in the Basin and Range indicates that the Miocene paleo–Colorado River did not fl ow on some different path into that region at the western mar-gin of the Colorado Plateau before 6 Ma, as had been previously suggested (Pederson, 2008). Instead, it has been hypothesized that some signifi cant drainage in the western Grand Canyon area captured the upper Colorado River near the west end of the Kai-bab uplift (Lucchitta, 1990; Polyak et al., 2008; Young, 2008), whereas others suggest that a relatively meager paleo–upper Colorado River terminated and infi ltrated in the western Grand Canyon area, feeding a Miocene karst-groundwater connection into the Grand Wash trough prior to the surface-drainage inte-gration (Hunt, 1956; Pederson, 2008).

The standard story, until recently, was that the latest Ceno-zoic pulse of Grand Canyon incision occurred between 6 and 1.2 Ma, having mostly concluded by that latter time, when lava famously fl owed into and nearly to the base of western Grand Canyon (Hamblin, 1994). However, we now know that the river has continued to incise signifi cantly in the past million years, and that the western-canyon basalt fl ows are substantially younger than previously thought (Dalrymple and Hamblin, 1998; Lucchi-tta et al., 2000; Fenton et al., 2002; Pederson et al., 2002a; Crow et al., 2008). The emerging new picture suggests that river-damming fl ows clustered into episodes of volcanic activity at ca. 500 ka and ca. 300 ka along the Toroweap and Hurricane faults, respectively (Crow et al., 2008). Also, these “dams” were short-lived, hetero-geneous, and leaky constructs that occasionally fed great outburst fl oods (Fenton et al., 2004; Crow et al., 2008). In addition, we now have a better understanding of down-to-the-west slip rates along the active Hurricane-Toroweap fault zone in this same area, and slip rates are approximately equal to the difference between Quaternary incision rates upstream and directly downstream of the faults (Fenton et al., 2001; Pederson et al., 2002a; Karlstrom et al., 2007). Geometry dictates that this pattern is due to subsidence along these normal faults, offsetting incision along their hanging walls (Pederson et al., 2002a; Pelletier, 2010).

In summary, the overall incision of Grand Canyon has been a response to a combination of Cenozoic uplift with multiple sources and varied timing, as well as the elevation difference between the edge of the Colorado Plateau and the Basin and Range, produced mostly in Miocene time by extensional fault-ing. It was necessary for the Colorado River to take advantage of precursor canyons and to get organized and integrated off the Grand Wash escarpment at 6 Ma before most incision of Grand Canyon could begin, but how this happened is still debated. Sig-nifi cant excavation of Grand Canyon has been accomplished in the Quaternary and continues, with incision rates varying along the length of Grand Canyon as a result of neotectonism.



122 J.L. Pederson

QUATERNARY DEPOSITS OF THE EASTERN GRAND CANYON MAP AREA

Background

The longer term geomorphic history of Grand Canyon is fas-cinating. But, in fact, the bulk of Quaternary deposits preserved and mapped in eastern Grand Canyon are not the result of tecto-nism or volcanism but rather of erosion and deposition cycles con-trolled by glacial-interglacial-scale climate changes. Considering this, it is important to review previous studies in the region relat-ing to both the Quaternary stratigraphy and to paleoclimate. The presentation, further below, of the character and stratigraphic rela-tions of the mapped deposits will be relatively brief, with the sup-porting geochronologic methods and results deferred to existing publications (Pederson et al., 2002a, 2006; Anders et al., 2005).

Previous Quaternary Stratigraphic Work

Previous work in the map area includes studies by Machette and Rosholt (1991), who identifi ed seven gravelly Colorado River terraces in eastern Grand Canyon that formed episodically in the last 700 ka, based on uranium-trend dating. In the same area, Lucchitta et al. (1995) also identifi ed seven main-stem ter-races and employed preliminary cosmogenic surface-exposure dating to obtain ages. The pertinence of the chronologic data from these studies is unclear because the uranium-trend dating method is now abandoned, and the Lucchitta et al. (1995) study was early in the use of cosmogenic dating and did not utilize amalgamated sampling or inheritance corrections. Lucchitta et al. (1995) concluded that the Pleistocene aggradation episodes in eastern Grand Canyon occurred as a result of sedimentologic and hydrologic changes in the headwaters of the Colorado River system during major glacial epochs.

The distinct, sandy Holocene alluvial stratigraphy of the main-stem corridor has been relatively well studied because of its relations to archaeology, and it provides an important record of more recent fl uvial history. Hereford et al. (1996) mapped and described the Holocene stratigraphy as a suite of inset sandy deposits, or fi ll terraces, including prominent late Holocene ter-races they called the “striped alluvium” and the “alluvium of Pueblo II age,” which are associated with archaeological sites. Hereford (2002) focused upon correlations of these deposits with those found in tributaries upstream in the Colorado Plateau and to past climate changes, for example, correlating the latest Holocene “upper mesquite terrace” with the Naha Formation of Hack (1942), which was deposited in many of the valleys of the southwestern United States during the Little Ice Age (ca. A.D. 1400–1880). This model views the Holocene stratigraphy in the context of inset cycles of aggradation and incision. But in a nota-bly different approach, O’Connor and others (1994) interpreted some of these same deposits in the Lees Ferry area in the context of paleofl ood hydrology. These workers identifi ed fi fteen fl ood-event beds spanning the past 4500 years, and they estimated dis-

charges for those paleofl oods, assuming no aggradation or inci-sion over that time (O’Connor et al., 1994). Finally, historic and modern deposits are inset into these Holocene terrace deposits and have been heavily studied in relation to the effects of Glen Canyon Dam (e.g., Howard and Dolan, 1981; Schmidt, 1990; Schmidt and Rubin, 1995; Topping et al., 2000).

The new surfi cial mapping of eastern Grand Canyon builds upon this previous work along the main-stem river. More impor-tantly, though, it includes the fi rst detailed mapping of tributary deposits. Also, in contrast to most previous work, the focus of the new mapping is on the Pleistocene record rather than the Holo-cene and historic deposits. This is partly prescribed by the scale of the map, with Pleistocene deposits covering a much larger map area than Holocene deposits.

Paleoclimate of Eastern Grand Canyon

The newly mapped deposits range in age back to ca. 500 ka, but paleoclimate studies in the immediate region resolve just the changes over the past ca. 50 k.y. at best. Though not in the immediate region, the important Devils Hole paleoclimate record is an exception, also extending back to ca. 500 ka. Correlations between this record and the mapped stratigraphy are presented in the discussion below.

Regional paleoclimate records from pollen and plant mac-rofossils indicate that from 50 to 25 ka, prior to the last glacial maximum, mean annual temperature was 2.9–4.3 °C cooler than the present, precipitation was greater than present, a winter pre-cipitation regime dominated, and the altitudinal zones of plant communities were depressed 460–900 m relative to modern conditions (Anderson et al., 2000; Coats, 1997). During full gla-cial conditions, 25–15 ka, pollen, macrofossils, and the isotopic composition of herbivore teeth indicate that annual precipitation was ~13% more, mean temperature was 6.7 °C lower, and some plant species were depressed ~1000 m (e.g., Cole, 1990; Mead and Phillips, 1981; Connin et al., 1998). The glacial- interglacial transition began by ca. 15 ka and was characterized by a major vegetation disturbance as plant communities migrated upslope (Cole, 1990; Weng and Jackson, 1999). There is a signifi cant signal in the Grand Canyon region for the Younger Dryas cold climate oscillation from ca. 13–11.5 ka (Cole, 1990; Wurster et al., 2008), and to a lesser degree for the short global cooling event at ca. 8.2 ka (Wurster et al., 2008). Early Holocene precipita-tion was likely somewhat greater than present owing partly to a stronger summer monsoon. Local records suggest that the middle Holocene (8–4 ka) had an annual temperature ~1 °C warmer than today, potentially still with a more active summer monsoon (Cole, 1990; Weng and Jackson, 1999; O’Brien et al., 2006).

Pleistocene Main-Stem Stratigraphy

Recent mapping along the main-stem Colorado River has focused on Pleistocene deposits, where geomorphic and strati-graphic relations between stream terraces and side canyon and




hillslope landscape components have been investigated. At least seven Pleistocene and one compound Holocene fi ll deposit of the Colorado River have been identifi ed along the main-stem corridor, and each can have multiple terrace levels developed on it (Fig. 2). These are thick Pleistocene fi ll terraces (up to 64 m of gravel) with irregular basal contacts representing buried valley topography.

The sandy middle–late Holocene deposits have been mapped and studied in much greater detail by others, as described above. These appear on the map accompanying this volume as Qg1, but herein this main-stem deposit is referred to as M1 to keep it dis-tinct from tributary deposits, and likewise, all main-stem deposits are referred to with M. Distinct from the Holocene deposits are the Pleistocene clast-supported, pebble-cobble gravel to gravelly sand deposits of M7–M3. Based on samples from M3, Pleistocene clast compositions are ~85% limestone, sandstone, dolomite, and chert derived from the nearby Paleozoic bedrock in Grand Canyon. Only ~15% is quartzite, volcanic porphyry, and granite clasts from farther upstream in the Colorado River system. The modern gravel bars in Furnace Flats are slightly more dominated by local rock types (Anders, 2003). Two facies in these deposits are (1) clast-

supported, imbricated, cross-bedded, pebble-cobble gravel; and (2) cross-bedded silty sand. Bedding is generally medium scale and has tabular to lenticular geometry (Figs. 3 and 4). Gravel-bar bed forms and the dominance of bed load in preserved sediment suggest that the Pleistocene river at times may have been more braided in form than the present-day Colorado River.

M3 is the best preserved main-stem terrace in eastern Grand Canyon and was deposited during an episode that began prior to ca. 71 ka and proceeded until after 64 ka on the basis of optically stimulated luminescence (OSL) dating (Pederson et al., 2006). It is especially prevalent in the broader reaches of the Colorado River between the Palisades and Unkar tributary mouths, form-ing the main gravels seen adjacent to the modern river. The base of the M3 gravel is below present-day river level, and terrace treads are up to 38 m high.

In contrast, the older M4 and M5 deposits are remnants preserved in both the wide Furnace Flats reach as well as the narrower lower Marble Canyon reach above the Little Colorado confl uence, where they are locally armored by resistant travertine carbonate deposited by springs. M4 was deposited starting just

M1

M6 M5385-320 ka

M7180

150

120

90

60

30

0

Hillslope sediment

M375-60 ka

Met

ers

abov

e re

fere

nce

stag

e

M220 ka

M4130-90 ka

Figure 2. Composite main-stem Colo-rado River stratigraphy in eastern Grand Canyon with numerical age ranges from data in Pederson et al. (2006).

M3

55 ± 1269 ± 9

15 m

TCNOSL

71 ± 11OSL Colorado River

Figure 3. Main-stem Colorado River fi ll terrace M3, with absolute dates ob-tained from this locality (Pederson et al., 2006). Base of the deposit lies be-low the present river surface; note the gravel-bar foresets preserved below the M3 label. OSL—optically stimulated luminescence; TCN—terrestrial cos-mogenic nuclides.



124 J.L. Pederson

before 124 ka, and aggradation proceeded until well after 118 ka, based on U-series dating of this same interbedded travertine and deposits at tributary mouths in the map area (Anders et al., 2005; Pederson et al., 2006). Its base is at river level, and although the true top of this deposit is commonly obscured by colluvium or is poorly preserved, it is estimated to be 60 m high (Fig. 4). Older episodes of main-stem deposition are poorly preserved in the map area, and there is evidence (at Unkar delta for example) for deposits dating between the times of the M4 and M5 map units. Yet a more clearly defi ned M5 was deposited beginning prior to 343 ka and proceeding until sometime after 322 ka, also based on dating of travertine. The base is 28 m or higher above river level, and the poorly preserved tread is estimated to be 94 m above present river stage. M6 and M7 are groups of deposit remnants mapped in the Furnace Flats reach and are the highest and oldest preserved, but they have not been dated. M6 is ~60 m thick, and its strath is 117 m above river level. M7 is ~50 m thick, with a strath 133 m above the river. The true bases of these older fi ll ter-races may have been lower, and the fi lls were likely thicker, than what we can observe in the present-day remnants.

Notably, no exposed main-stem deposits have been identifi ed that date to the last glacial maximum (MIS2, 25–15 ka). However, terrace deposits of this age in the Colorado River headwaters sug-gest that a similar deposit probably should exist in eastern Grand Canyon (e.g., Phillips, et al., 1997; Reheis et al., 1991). The loca-tion for such a deposit in the study area is under the present-day channel. Data for the composition of the Colorado River’s channel bottom and dam-site exploration drilling records indicate signifi -cant thicknesses of sub-channel sediment, at least in some reaches

of the river through Grand Canyon (Hanks and Webb, 2006). This possible deposit is designated M2, though nowhere in the study area is it unambiguously exposed or mapped.

Tributary Stratigraphy

The larger tributary catchments of eastern Grand Canyon, like Nankoweap, Lava Chuar, Comanche, and Unkar Canyons, have been a focus of this mapping as well as sedimentological and geochronological study. These catchments hold a spectacular and well-exposed Quaternary stratigraphy that records processes responding to climatic changes. The Chuar Valley and Furnace Flats tributaries are all coarse-bedload–dominated streams that have relatively low base discharges or are ephemeral. Five or more distinct alluvial-fi ll deposits have been recognized in these tributary canyons, and they are associated with at least 11 dis-tinct terrace surfaces (Fig. 5). These can be correlated confi dently between the different eastern Grand Canyon tributaries by land-scape position and surface and soil-profi le characteristics, and this is confi rmed by geochronology. These older Pleistocene terraces are vegetated by desert scrub, grass, and cacti, and have desert pavement in places and are associated with greater soil-profi le development with increasing terrace height (Anders, 2003). Hill-slope colluvial remnants of the last major sedimentary hillslope and valley-fi ll episode can be traced directly to related tributary fi ll terraces (Fig. 6), and this alluvium can be traced downstream to the Colorado River corridor.

Tributary terrace deposits range in thickness and height both along the length of a single drainage and between different

Figure 4. View looking across Colorado River from Kwagunt camp. U-series dating of travertine provides data on the erosion preceding M4 deposition (153 ka) and the timing of the deposition of the M4 unit.




drainages. They consist of thick lenticular beds of immature pebble-to-boulder gravel with a sandy matrix. Two distinct, inter-bedded facies are present in tributary stream deposits in varying proportions: (1) a predominance of clast-supported gravel with local clast imbrication, and (2) generally subordinate, matrix-supported gravel with randomly oriented clasts in a sandy matrix (DeJong, 2007). The matrix-supported facies is interpreted to be

debris fl ow deposits, whereas the clast-supported facies is inter-preted to be alluvium derived in large part from the reworking of debris fl ow deposits. In general, the lowest suite of tributary deposits and terraces (incorporated into Qal) are heavily veg-etated, have clear bar-and-swale topography, and have weakly developed desert soils. These represent the dynamic, event- dominated recent histories of these channels studied by others

Qal

S5

S220-7 ka

S15 ka

200

170

80

60

40

20

0S3y

50-30 ka

S4110-90 ka

S3o65-55 ka

Met

ers

abov

e th

alw

eg

Figure 5. Generalized tributary drainage stratigraphy, with age ranges from Anders et al. (2005) and DeJong (2007). A stratigraphic connec-tion exists between hillslope deposits and S3y and S4, but the S2 and Holocene S1 are instead derived largely from the reworking of older deposits. Qal—Quaternary alluvium.

Figure 6. Overlooking part of the Car-bon Canyon tributary drainage. A mas-sive remnant hillslope deposit that once covered the entire toe slope of the es-carpment grades laterally to become the Qg3 (S3) valley fi ll.



126 J.L. Pederson

(e.g., Webb et al., 1989; Griffi ths et al., 1996). Additionally, some S2 terraces are physically correlative with older Holocene depos-its mapped along the Colorado River corridor (Lucchitta et al., 1995; Hereford et al., 1996).

Relatively abundant new age control, especially for younger fi ll terraces, is provided mostly by OSL dating (Anders et al., 2005; DeJong, 2007). The most recent major fi ll episode affect-ing the hillslopes and tributary streams of Grand Canyon dates from 50 to 30 ka (Fig. 6). A subordinate, yet signifi cant, inset alluvial deposit in tributaries dates to 20–7 ka, and this S2 fi ll ter-race is important in that fi eld relations indicate that it is derived from incision and reworking of sediment stored in the thick S3 hillslope remnants (Fig. 7). These two sets of deposits are partic-ularly well exposed and preserved, and are key to interpreting the mechanisms driving the erosion and deposition cycles recorded in the stratigraphy, as discussed below.

In terms of older tributary deposits, the S3 of the map actu-ally combines a more recently recognized older deposit and a younger deposit that are found in close association (DeJong, 2007). The older S3o is most distinct in the Lava Chuar drain-age and is ca. 65–55 ka. The S4 is well preserved, specifi cally up the Nankoweap and Lava Chuar drainages, and has been dated in Lava Chuar to between 110 and 90 ka (Anders et al., 2005). Finally, a few remnants of very high gravels in the Nankoweap and Kwagunt drainages that do not necessarily correlate are grouped into the S5 unit.

Relations between Tributary and Main-Stem Stratigraphy

An interesting sedimentological disconnect exists between the Pleistocene main-stem system and local catchments in east-

ern Grand Canyon, as more fully presented in Anders et al. (2005). For example, tributaries contain a greater number of dis-tinct fi ll-terrace gravels over the same length of Pleistocene time, and generally preserve only younger deposits relative to the main stem. Another example is that, at the confl uences of side sys-tems and the main-stem river, the very base of tributary deposits of the younger S3y and the S4 interfi nger with Colorado River deposits, but most of their thickness progrades out over, and is younger than, related Colorado River terraces. In fact, existing age control and stratigraphy indicate that M3 deposition signifi -cantly preceded the younger S3y tributary aggradation, and the main stem actually began incising during the later stages of tribu-tary aggradation (Anders et al., 2005). A fi nal example is that the Colorado River does not appear to have a deposit correlating with the transitional late Pleistocene–early Holocene S2 deposits of local catchments at all. S2 deposits of tributaries are observed diving below the tributary channel grade near their mouths at the heads of tributary debris fans, where the S2 becomes overlain by younger, sandy Holocene deposits.

DISCUSSION

Overall Incision of Grand Canyon

The incision and aggradation of the Colorado River in east-ern Grand Canyon through middle-to-late Quaternary time can be traced in detail using the mapped main-stem fi ll-terrace stra-tigraphy and the series of absolute dates from them. The overall trend of lowering, integrated through this cyclic fl uvial history, provides the best bedrock incision rate for this important land-scape (Fig. 8). In eastern Grand Canyon, the complications of

Figure 7. Example of the stratigraphic relation of S2 valley fi ll issuing from the erosion and reworking of the thick S3y (younger) valley fi ll in the upper Comanche tributary drainage.




this cyclic history of alluviation and incision has led to various incision rates reported by different workers. Our bedrock incision rate for eastern Grand Canyon of 142 m/Ma is greater than rates from the subsiding hanging wall of the Hurricane and Toroweap faults in western Grand Canyon (Lucchitta et al., 2000; Pederson et al., 2002a), and are average rates for the western United States.

At longer time scales, incision along the Colorado River since the most recent pulse of incision started at 6 Ma, using our rate of 142 m/Ma, could carve less than half the ~1800 m gorge of eastern Grand Canyon. This is consistent with the idea that incision rates have slowed through time, starting with a sudden pulse of incision after river integration that has been tapering off with time (Pederson et al., 2002a). It is also consistent with evi-dence that signifi cant canyon cutting happened via paleodrain-ages prior to the Colorado River taking its full present path at 6 Ma and creating the Grand Canyon we see today (Young and McKee, 1978; Flowers et al., 2008; Pederson, 2008).

Landscape Response to Climate Change in Eastern Grand Canyon

Tectonism and drainage integration are the long-term mech-anisms controlling incision of Grand Canyon, as discussed in the fi rst sections of this chapter. But climate variability drives the short-term aggradation and incision episodes recorded in the thick Quaternary deposits of eastern Grand Canyon. Among other things, this is evident from the facts that the cyclic sedimentation varies at glacial-interglacial time scales, that fi ll terraces are cor-relative and contemporaneous in several local drainages, and that an upslope change in weathering, hydrology, or sediment trans-

port is needed to account for the thick hillslope deposits grading into valley-fi ll alluvium (specifi cally the younger tributary S3y and the S4).

Research greatly empowered by new geochronologic tools is revealing increasing complexity in how the Grand Canyon landscape as a whole is responding to changes in climate over the Quaternary (Anders et al., 2005; DeJong, 2007). Our knowl-edge of the Grand Canyon landscape’s responses to climate change are based especially on the most recent, best preserved, and best dated set of Pleistocene to early Holocene deposits (S3 and S2). In summary, main-stem Colorado River aggradation appears to be linked time-wise to glacial advances in its distant headwaters, which could have modulated the hydrology of the river greatly, and potentially increased sediment yield, at least in the headwaters, by an order of magnitude (Hallet et al., 1996). According to the Devil’s Hole paleoclimate record (Winograd et al., 1992), M3 aggradation in eastern Grand Canyon began roughly at the peak of a glacial advance and continued over the transition to warmer climate conditions (Fig. 9). The same tim-ing relative to glacial-interglacial climate changes seems to hold true for both the M4 and M5 fi ll terraces, suggesting a consistent response of the main stem to full-glacial and deglacial changes. Yet a consistent and simple relation to global climate changes may be spurious and not borne out elsewhere along the length of the Colorado River.

In contrast, tributary sedimentologic responses to glacial-interglacial climate changes appear to be more numerous and out of phase in comparison to the main stem (Fig. 9). The younger S3y tributary deposit, for example, occurs in the variable but gen-erally cold period of MIS3, when the main stem was generally

142 m/my

-20

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0 100 150 200 250 300 350 400

Hei

ght a

bove

ref

eren

ce s

tage

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)

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bedrock baseof fill terrace

Time (ka)

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Figure 8. Curve representing the height of the Colorado River’s bed through time, tracing cycles of deposition and erosion superimposed upon the overall trend of incision. Stippled pattern represents fi ll deposits, and gray represents bedrock cut during the troughs of cycles. Diamonds and brackets mark chronological sample heights, dates, and ana-lytical errors from Pederson et al. (2006). Squares represent fi eld measurements not associated with a numerical date. The slope of the black line is the background incision rate.



128 J.L. Pederson

incising. Stratigraphic relations in the upper catchments of local tributaries clearly indicate that tributary stream deposition at this time was linked to increased sediment production and storage on local hillslopes (Anders et al., 2005). In contrast, deposition of the tributary S2 deposit during the most recent glacial-inter-glacial transition matches the record elsewhere in the Mojave and Sonoran Deserts, and is consistent with a common model of deposition in desert streams during vegetation disturbance and intense summer monsoon precipitation at this time (Bull, 1991). Again, evidence thus far indicates that this response to climate change during the last glacial-interglacial transition is not evident in the record of the main-stem Colorado River. Thus, it may be a

case that the specifi c climate changes and the specifi c responses undergone in the desert catchments of Grand Canyon result in a tributary record that is quite independent of the main-stem Colo-rado River, which is under the infl uence of a range of integrated climate changes over a huge and varied upstream catchment.

Lack of Evidence for Lava Dams

All evidence argues against the idea proposed by Hamb-lin (1994) that lava dams in western Grand Canyon caused the aggradation of the Colorado River and its tributaries in eastern Grand Canyon. The distinctive internal stratigraphy of topsets,

Figure 9. Tributary and main-stem Colorado River sedimentary activity through time in eastern Grand Canyon com-pared with the Devil’s Hole regional climate record (modifi ed from Winograd et al., 1992) and the global SPECMAP paleoclimate curve tracking continental ice volume from isotopes in marine sediment cores. Marine isotope stages are highlighted, with gray bars marking even-numbered glacial epochs.

S3yS2 S4

Devil’s Hole

SPECMAP

Marine Isotope Stage1 2 3 4 5 6

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redlocrehgi h

ecie

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Colorado River in eastern Grand Canyon reach

M4M3M2

Eastern Grand Canyon tributaries

S3o




foresets, and bottomsets seen in coarse-grained deltas are clearly absent, and deposits are signifi cantly younger than the potentially dam-forming lava fl ows of western Grand Canyon (Crow et al., 2008). Though it is a famous geologic story, the existence of several signifi cant Quaternary lava-dam lakes in Grand Canyon must be questioned on the basis of recent studies of Hamblin’s (1994) proposed lake deposits upstream (Kaufman et al., 2002; Anders, 2003). Recent research efforts of the basalt fl ows them-selves seem to concur. As an example, the Prospect Canyon fl ow sequence was hypothesized by Hamblin (1994) to be the oldest (1.8 Ma) and highest of the lava dams, extending across the gorge and fi lling the Prospect and Toroweap side canyons. Recent research indicates that this instead represents part of a composite of eruptions that lasted from 725 to 475 ka on both sides and within the canyon instead of one of several large dam edifi ces (Crow et al., 2008). Other recent research supports a revised story of lava dams based on study of basalt-dominated, bouldery gravel in the reach below the proposed lava dams, which are interpreted as outburst-fl ood deposits from failed lava dams (Fenton et al., 2002). Using correlations based on cosmogenic dating and basalt chemistry, Fenton et al. (2004) identify fi ve distinct fl ood depos-its from 525 to 100 ka, which thin and fi ne strongly downstream. In summary, a model of more dynamic basalt-river interactions, with the river eroding and circumventing lava fl ows and fewer, smaller, and leakier dams, is more valid (Fenton et al., 2002; Crow et al., 2008).

Nankoweap Debris Fan and Stanton’s Cave

Hereford (1984) fi rst recognized the anomalous nature of an unusually coarse and immature deposit that underlies much of the prominent high ridge-terrace along the axis of the debris fan at the mouth of Nankoweap Creek (Fig. 1). The high ridge on the debris fan south of Nankoweap Creek has a terrace that parallels and is 20–30 m above the slope of Nankoweap Creek as it crosses the debris fan from west to east. Correlative, but less extensive, deposits are exposed to the north by the cutbank of Nankoweap Creek near the head of the debris fan. These remnants actually contain two distinct deposits: the immature, angular pebble- boulder diamicton, and an underlying, fi ner gravel.

The underlying gravel has clasts that are moderately to well rounded and composed of Proterozoic and Paleozoic (espe-cially Redwall Limestone) sedimentary rocks, indicating that the source of this typical tributary gravel is mostly the Nankoweap side drainage. Considering this unit’s landscape position, it is tentatively correlated with tributary deposit S4 or S3, and could therefore be 100–40 ka, based on OSL ages in tributary canyons (Fig. 6). The coarser inset deposit appears to infi ll erosional topography developed in the underlying gravel, and it accounts for the entire thickness of the exposed ridge at its distal end. It is massive, poorly sorted, subangular to angular pebble to boul-der gravel, with a notable abundance of large angular boulders of chert-bearing carbonates of the Kaibab Formation. This deposit is best interpreted as the result of a rock avalanche. Hereford

(1984) suggested that the mass movement originated on the upper part of the escarpment on the left bank of the Colorado River across from the present-day Nankoweap fan, and that it would have covered the entire canyon fl oor at this locality. The presence of very large, angular Kaibab Formation blocks in the deposit is key, with the nearest outcrops of the Kaibab Formation being 1 km above the river on the east wall of Marble Canyon. In contrast, the nearest possible source up the Nankoweap and Little Nankoweap drainages is several kilometers away.

Hereford et al. (1998) also note that more-rounded clasts of Nankoweap Creek provenance, scattered main-stem Colo-rado River clasts, and other evidence indicate that the surface of the ridge has been reworked by the Colorado River. Similarly, Machette and Rosholt (1991) interpreted the stratigraphy of the main ridge as having inset strath terraces at its eastern end. They used U-trend dating to obtain an age of 40 ± 24 ka for one of these terraces and 210 ± 25 ka for the rock avalanche deposit itself at the upper end of the ridge, but the validity of this dat-ing method is questioned. Considering that the rock avalanche is stratigraphically younger than the S4 deposit, and the presence of Holocene deposits inset into it on the Nankoweap debris fan, it must be somewhere between 70 and 10 ka.

Intriguingly, Hereford also proposed that the resultant dam-ming of the river was responsible for the deposition of driftwood in Stanton’s Cave, 32 km upstream. The mouth of Stanton’s Cave lies ~45 m higher than the Colorado River at river mile 31.7R. Archaeological excavation and other studies of the fl oor of the cave were undertaken occasionally from 1969 to 1982 (Euler, 1984). The deeper stratigraphy in the cave contained logs from cottonwood and Douglas fi r trees interpreted as driftwood, mostly from a single depositional event (Euler, 1984). Radiocarbon dat-ing of this wood in the mid-1980s resulted in an age determina-tion of 43,700 ± 1800 14C yr B.P., although this is a minimum age estimate, considering the limits of the method at the time (Machette and Rosholt, 1991).

Hereford’s (1984) suggestion that the wood was deposited when the Colorado River was dammed by the Nankoweap rock avalanche is problematic. The original top of the rock avalanche is not preserved, and the reworked top (at 900 m elevation) lies ~30 m below the elevation of the wood in Stanton’s Cave. Importantly, the new mapping and geochronology presented above demonstrate that the Colorado River clearly has fl owed at a higher grade episodically in the late Pleistocene, as noted by Machette and Rosholt (1991) in this context. It was depositing channel gravel and sand ~40 m higher than present river stages at ca. 60–40 ka (Fig. 3), potentially just before or during the rock avalanche. At this elevated late Pleistocene grade, the Colorado River could have easily fl ooded Stanton’s Cave. Thus, changes in the form and grade of the Colorado River during the climate changes of the late Quaternary provide a stronger hypothesis to account for the driftwood in Stanton’s Cave.

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