Gravel-Capped Benches Above Northern Tributaries of the Escalante River, South-Central Utah

Home , Aquarius Plateau, Boulder, Escalante River, Gulch

CRevolution 2: Origin and Evolution of the Colorado River System II themed issue

Gravel-capped benches above northern tributaries of the Escalante River, south-central Utah

David W. Marchetti1,*, Scott A. Hynek2, and Thure E. Cerling2 1Geology Program, Western State College of Colorado, 600 N. Adams Street, Gunnison, Colorado 81230, USA 2Department of Geology and Geophysics, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA

ABSTRACT the Colorado Plateau to the Gulf of California age incision rates of these tributary drainages (Spencer et al., 2002; Dorsey et al., 2007; Peder- over the time since deposition and address the Andesitic boulder deposits mantle straths son, 2008; Dorsey, 2010; Lucchitta et al., 2011). signifi cance of our incision rates with regard to cut in sedimentary bedrock high above Thus, at ~5–6 Ma, a dramatic base-level fall was regional landscape evolution. the northern tributaries of the Escalante imposed upon the main-stem Colorado River River in south-central Utah. The andesitic system, likely causing a signifi cant increase in GEOLOGY AND GEOMORPHOLOGY gravel deposits are derived from the south- the bedrock incision rates of the Colorado River OF THE ESCALANTE ern escarpments of Boulder Mountain and and its tributaries. The mechanics and timing of RIVER DRAINAGE Aquarius Plateau. The sedimentology and how this incision was transferred upstream, and geomorphic expression of these deposits sug- its relationship to rock strength, major tectonic The Escalante River heads on the southwest- gest they are from slurry-fl ow mass move- structures, and mantle dynamics are areas of ernmost tip of Aquarius Plateau in south-central ments that have been reworked by fl uvial active research (Aslan et al., 2008; Cook et al., Utah (Fig. 1) and is a fi rst-order tributary of the processes. The andesitic boulders are sig- 2009; Crow et al., 2011; Karlstrom et al., 2011, Colorado River. The Escalante joins the Colo- nifi cantly tougher than the local sedimentary 2012). An essential component of this research rado 20 river miles upstream of the confl uence bedrock and cause boulder armoring and is determining the rate of vertical bedrock between the Colorado and San Juan Rivers . The topographic inversion. The andesitic boul- river incision at multiple spatial and temporal river presently sits in a deep canyon, which has ders are also effective tools for fl uvial incision scales in numerous Colorado Plateau drainages been cut into a broad valley between the Circle when transported across weaker bedrock. (e.g., Repka et al., 1997; Pederson et al., 2002; Cliffs uplift to the east and the Kai parowits Pla- Cosmogenic 3He exposure-age dating of some Wolkowinsky and Granger, 2004; Marchetti teau to the west (Fig. 1). The northern tributar- of the largest boulders exposed on the treads and Cerling, 2005; Darling et al., 2009; Hanks ies to the Escalante drain Aquarius Plateau and of four different deposits range from 303 ± 48 et al., 2011). With quantitative data constraining southern slopes of Boulder Mountain (Fig. 2). to 1395 ± 241 ka. The tallest boulders exposed the temporal interval and rates of river incision From west to east the largest of these tributaries on the deposit surfaces tend to yield the oldest for different segments of the Colorado River include: Pine Creek, Mamie Creek, Sand Creek, exposure ages, suggesting that boulder ero- system, we can test hypotheses about the nature Calf Creek, West and East Boulder Creeks sion and deposit erosion are controlling the and timing of incision and the processes that (which merge to form a singular Boulder Creek), exposure-age populations and indicating that affected incision. and Deer Creek. even the oldest exposure ages from a given In this paper, we present geomorphic, sedi- The bedrock geology of the Escalante River deposit are likely minimum age estimates. mentologic, and geochronologic data on four drainage basin includes more than 3 km of Using the oldest exposure age from each sur- different boulder-rich gravel deposits along trib- Permian to Tertiary sedimentary rocks com- face, we estimate maximum Escalante River utaries of the Escalante River. These deposits mon to the central Colorado Plateau (Doelling northern tributary incision rates of 151–323 represent the former active channels of streams et al., 2003). Along the northern tributaries of m Ma–1 for the period since 0.6–1.4 Ma. that were inundated with mass movements and the Escalante River, only strata from the Early presently sit high above the modern tributary Jurassic Kayenta Formation through Late Cre- INTRODUCTION drainages, demonstrating considerable bedrock taceous Tropic Shale are exposed (Weir and incision since deposition. The predominant Beard, 1990a, 1990b; Weir et al., 1990). Most There is recent controversy over the age, lithology of these deposits (trachyandesites) is of the geology around the northern tributaries and perhaps defi nition of, the Grand Canyon well characterized with respect to cosmogenic includes vast expanses of nearly bare Early (e.g., Flowers et al., 2008; Polyak et al., 2008; 3He exposure-age dating (Marchetti and Cerling, Jurassic Navajo Sandstone. The Navajo Sand- Wernicke, 2011); however, there is a strong 2005; Marchetti et al., 2005a, 2011). Following stone in these areas includes a wide variety consensus that prior to ~6 Ma there was no on that framework, we also present cosmogenic of topographic forms and highly varying iron clearly identifi able through-fl owing river sys- exposure-age estimates for the largest boulders mineralization (e.g., Beitler et al., 2003; Loope tem integrating and draining the majority of from each of the four surfaces we investigated. et al., 2011) that has caused observable differ- Finally, we use the age estimates and paleostrath ences in weathering. Additionally, the Navajo *Email: [email protected]. to modern-strath distances to estimate the aver- Sandstone acts as a shallow aquifer in the area,

Geosphere; August 2012; v. 8; no. 4; p. 835–853; doi:10.1130/GES00772.1; 12 ﬁ gures; 6 tables. Received 8 December 2011 ♦ Revision received 2 April 2012 ♦ Accepted 22 April 2012 ♦ Published online 26 June 2012

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Escalante River Plateau Kaiparowits Boulder Mountain km ′ le (Cook et al., 2009) and is discussed in the text. 111°45 UTAH Aquarius Plateau Figure 1. Satellite image of the Escalante River drainage basin. The pink square is the location of a convexity in Escalant The pink square drainage basin. 1. Satellite image of the Escalante River Figure tudinal proﬁ N ′ 01020 38°00

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k Deer Cree DCB DM

′ k A lf Cree lder Creek Ca Bou Boulder Town NHB B Escalante River eek Cr Sand Boulder Mountain

Mamie Creek refer to Figure 12 and are discussed in the text. 12 and are to Figure refer ′ and B to ′

Pine Creek Escalante limit glacial A km BMB Figure 2. Digital elevation model (DEM) of the upper Escalante River drainage basin showing the northern tributaries. Initials Escalante River 2. Digital elevation model (DEM) of the upper Figure Creek deposits that we investigated: BMB—Black Mesa Bench; NHB—New Home DM—Durffey Mesa; DCB—Deer boulder ferent A to A section lines Bench. Cross

0510 Aquarius Plateau Aquarius N

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and interactions between the Navajo Sandstone (1997) and Marchetti et al. (2005a, 2007) found In the following sections, we describe the aquifer and the surface drainage system of the no evidence of Bull Lake–age glacial depos- location, sedimentology, and geomorphology Escalante River and its tributaries are potentially its and reinterpreted the Bull Lake–age glacial of the particular gravel deposits we studied. important for understanding long-term drainage deposits of Flint and Denny (1958) as mass For each deposit, we also report boulder-size development (e.g., Wilberg and Stolp, 2005). movement deposits. The lowest elevation of measurements and pebble lithology counts The northernmost reaches of the Escalante defi nitive glacial deposits in the upper reaches from various locations on each deposit’s tread. drainage basin include southern Aquarius of the Escalante drainage basin is between 2600 The largest boulders for each deposit were Plateau and Boulder Mountain. Both of these and 2700 m along both East and West Boulder determined by walking each deposit tread and high-elevation tablelands are capped by late Creeks (Fig. 2). On the Aquarius Plateau, there margins and measuring the dimensions of the Oligocene to Pliocene volcanic and volcani- is some evidence of periglacial activity in the largest clasts. We measured between 35 and 70 clastic rocks likely derived from the Marysvale form of nivation hollows, but there is no evi- boulders to determine the fi ve largest for each volcanic center located ~75 km to the northwest dence of glaciation. deposit. The dimensions of the fi ve largest boul- (Williams and Hackman, 1971; Billingsley ders from each surface are given in Table 1. et al., 1987; Mattox, 1991). The main volcanic GRAVEL DEPOSITS The pebble counts were done by identifying the unit that makes up the steep cliffs ringing south- lithology (andesite, chert, pedogenic carbonate, ern Boulder Mountain and Aquarius Plateau is The investigated gravel deposits sit high or sandstone) of all the pebble-sized clasts in a a porphyritic basaltic-andesite to trachy andesite above northern tributaries of the Escalante River 0.4 m2 circular area. The pebble-count data and (herein andesite). Recent work on this rock (Fig. 2). All of the deposits are dominated by composite lithologic percentages are given in unit on the nearby Fish Lake Plateau indicates boulder-sized clasts of andesite derived from Table 2. this widespread volcanic deposit is a densely the southern slopes of Boulder Mountain and welded ash-fl ow tuff emplaced ~26 Ma (Bailey Aquarius Plateau. The andesites range in color Black Mesa Bench et al., 2007; Ball et al., 2009). A mixture of col- from light to dark gray with occasional reddish- luvial, and in places glacial, surfi cial deposits brown clasts that have an oxidized groundmass. The Black Mesa Bench deposit is located composed primarily of andesitic boulders man- When exposed subaerially, the andesite clasts ~9 km NNW of Escalante, Utah (Fig. 2). The tles the southern slopes of Boulder Mountain typically acquire very dark desert varnish and deposit is 2 to possibly 8 m thick and is depos- and Aquarius Plateau. Although covered by appear black. Overall, boulder clasts in the ited on a strath of beveled Tropic Shale. Pine surfi cial deposits in the uppermost part of the gravel deposits range from rounded to angu- Creek has incised ~228 m below the level of the Escalante drainage basin, a weak unit of thinly lar, with many areas dominated by rounded to Black Mesa Bench strath (Fig. 3). The tread of bedded ashes, siltstones, chert pebble conglom- subrounded clasts. Where exposed, the gravel the deposit is covered by large areas of Pinyon- erates, and sandstones underlie the andesites. deposits can display cut-and-fi ll stratigraphy, Juniper woodland with two large sections that This unit has long been thought to be part of mild imbrication, and weak inverse grading. were recently drag-chained to remove trees and the Paleocene Flagstaff Formation (Smith et al., Although the deposits are dominated by andes- improve pasture (Fig. 4). Large expanses of the 1963; Williams and Hackman, 1971); however, itic clasts (~99%), they have occasional pebble- Black Mesa Bench tread have thin Av soil hori- recent work on the nearby Thousand Lakes to cobble-sized clasts of chert. The chert clasts zons composed of local eolian-derived silts and Mountain suggests that the unit is, in fact, much can be any one of a variety of colors, but yellow- sands. In many areas, especially where there younger and may be part of the Eocene Crazy brown and brick-red are the most common. The are overturned boulders, there are large pieces Hollow Formation (Deblieux et al., 2011). cherts are likely derived from the Tertiary unit of intact or broken pedogenic carbonate rinds Regardless of the age of this unit, its weak- that underlies the andesites. The matrix of the (alternatively crusts, coatings, or pendants) at ness and position immediately below the resis- gravel deposits is a light- to dark-gray mixture the surface or partly buried in the surface soil. tant andesites leads to a variety of large-scale of andesitic sand and silts. Maximum pedogenic carbonate rind thicknesses mass movements around Boulder Mountain and Aquarius Plateau (Fuller et al., 1981; Wil- liams, 1984; Marchetti et al., 2007). These TABLE 1. MEASURED BOULDER SIZES mass movements, and fl uvial reworking of a-axis b-axis c-axis Approximate Volume Surface (m) (m) (m) (m3) Buried? mass movement deposits, have transported and Black Mesa Bench 3.1 2.3 1.4 9.6 Y deposited extremely coarse volcanic (andesitic) 2.5 2.0 1.0 5.0 boulder gravels onto the upper drainage basin 2.0 1.6 1.0 3.3 Y 2.8 1.9 0.6 2.8 Y of the Escalante River. 2.0 1.4 0.8 2.4 Y During the Last Glacial Maximum (LGM; New Home Bench 2.1 1.7 1.7 6.1 ~21 ± 2 ka, ~MIS [marine isotope stage] 2), 2.4 2.0 1.1 5.5 Y Boulder Mountain had an ice cap with outlet 2.3 1.6 1.4 5.2 2.2 1.9 1.2 4.8 Y glaciers that fl owed radially from the center of 1.9 1.2 1.1 2.6 Y the ice mass (Flint and Denny, 1958; Marchetti Durffey Mesa 1.5 1.3 0.9 1.8 et al., 2005a, 2007). It is still unclear whether 1.2 1.0 0.8 1.0 Y 1.1 1.0 0.8 0.9 Y or not Boulder Mountain was glaciated during 1.0 0.8 0.7 0.6 the penultimate glaciation (~MIS 6). Flint and 1.1 0.8 0.6 0.5 Y Denny (1958) argue that there was a Bull Lake– Deer Creek Bench 2.2 2.0 0.6 2.6 Y 1.7 1.4 0.8 1.8 Y age (what they meant by “Bull Lake” age is also 1.4 1.2 0.8 1.3 unclear, likely either ~MIS 4 or ~MIS 6) glacia- 1.1 1.1 1.0 1.1 Y tion around Boulder Mountain. However, Waitt 1.8 1.1 0.5 0.9

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TABLE 2. PEBBLE-COUNT DATA either side of the deposit (deposit strath to top Surface and location of alluvium). The tread of the Durffey Mesa (Latitude and longitude Pedogenic Volcanic North American datum 1983) Chert carbonate clasts Sandstone Total deposit is covered with areas of Pinyon-Juni- Black Mesa Bench per woodland and has areas of locally derived 37.8524 °N 111.6425 °W 54 32 56 0 142 eolian sand and silt deposition and thin, silty 37.8526 °N 111.6479 °W 38 70 12 1 121 Av horizons. The surface of the deposit also has 37.8446 °N 111.6399 °W 115 131 9 0 255 large areas of thick pieces of intact or broken Surface totals 207 233 77 1 518 and overturned pedogenic carbonate rinds and (%) 40 45 14.8 0.2 laminar plates (Fig. 8). Maximum pedogenic New Home Bench 37.8930 °N 111.4689 °W 22 282 12 0 316 carbonate-rind thicknesses are 8–9 cm. Clasts 37.8869 °N 111.4568 °W 52 3 15 0 70 in the deposit range from rounded to angular 37.8631 °N 111.4409 °W 43 0 104 0 147 but are dominated by rounded to subrounded 37.8469 °N 111.4320 °W 165 0 55 0 220 37.8656 °N 111.4374 °W 75 0 39 1 115 clasts, and surface clasts range from slightly to extremely weathered. The composite pebble- Surface totals 357 285 225 1 868 (%) 41.1 32.8 25.9 0.1 sized fraction of the Durffey Mesa surface is Durffey Mesa completely dominated by pedogenic carbonate 37.8681 °N 111.3928 °W 23 272 93 0 388 clasts. No sandstone was found in the total of 37.8717 °N 111.3948 °W 1 924 1 0 926 1543 pebble-sized clasts identifi ed. 37.8647 °N 111.3939 °W 8 206 15 0 229 Surface totals 32 1402 109 0 1543 Deer Creek Bench (%) 2.1 90.8 7.1 0 Deer Creek Bench The Deer Creek Bench is located ~6 km SE 37.8664 °N 111.3698 °W 29 0 41 0 70 37.8675 °N 111.3710 °W 4 66 20 0 90 of Boulder, Utah, and is ~2 km ESE of Durffey 37.8628 °N 111.3708 °W 4 162 33 0 199 Mesa. The Deer Creek Bench deposit is cut into 37.8643 °N 111.3688 °W 54 0 79 0 133 Navajo Sandstone and ranges from 2 to pos- Surface totals 91 228 173 0 492 sibly 8 m thick (Figs. 9 and 10). Deer Creek (%) 18.5 46.3 35.2 0 has incised ~210 m below the level of the Deer Creek Bench strath on the E side of the deposit, while on the W side of the deposit, the drainage are 5–6 cm. The Black Mesa Bench surface is covered with Pinyon-Juniper woodland inter- is full of andesite colluvium and sandy alluvium, located just west of the W-dipping Escalante spersed with large areas of locally derived and the elevation of the modern strath is unclear. monocline, which is the steep western limb eolian silts and sands. The New Home Bench The tread of the Deer Creek Bench deposit is of the Escalante anticline (Lidke and Sargent, tread also has large areas of thin Av soil hori- covered with areas of Pinyon-Juniper woodland 1983). The location of the deposit just down- zons and sizeable pedogenic carbonate rinds and has large areas and thick accumulations of stream of where Pine Creek emerges from a nar- littering the surface. Maximum pedogenic car- locally derived eolian sands and silts, including row canyon cut into the westward-dipping limb bonate rind thicknesses are 7–8 cm. Exposures small dune fi elds (Fig. 10). Smaller areas of the of the Escalante monocline suggests the deposit into the New Home Bench deposit show stage tread have thin, silty Av soil horizons and areas may be in part structurally controlled. Clasts in II–III soil carbonate development (determined of intact or broken pedogenic carbonate rinds. the deposit range from rounded to angular, and using criterion of Machette, 1985). Clasts in Maximum pedogenic carbonate rind thick- surface clasts range from slightly to heavily the deposit range from rounded to angular but nesses are 6–7 cm. Clasts in the deposit range weathered. The composite pebble-sized fraction are dominated by subangular to subrounded, from rounded to angular, and surface clasts of the Black Mesa Bench surface is dominated and surface clasts range from slightly to heavily range from slightly to heavily weathered. The by chert and pedogenic carbonate clasts. Only a weathered. The composite pebble-sized fraction composite pebble-sized fraction of the Deer single clast of sandstone was found in the total of the New Home Bench surface is dominated Creek Bench is mostly pedogenic carbonate of 518 clasts identifi ed. by chert and pedogenic carbonate clasts. Only a clasts with slightly fewer andesitic pebbles. single clast of sandstone was found in the total No sandstone clasts were found in the total of New Home Bench of 868 pebble-sized clasts identifi ed. 492 pebble-sized clasts identifi ed. Around the margins of the Deer Creek Bench deposit are The New Home Bench deposit is located Durffey Mesa numerous “pede stal boulders,” where andesite ~5 km SW of Boulder, Utah, and State Highway clasts sit atop small pedestals of Navajo Sand- 12 is atop part of the ~7-km-long deposit. The Durffey Mesa is located ~4 km SSE of Boul- stone preserved underneath the andesite clasts New Home Bench deposit is cut into Navajo der, Utah, and is just to the W of the Burr Trail. because they were protected from weather- Sandstone in the lower reaches and Carmel For- The Durffey Mesa deposit is cut into Page ing (Fig. 10C). These pedestal boulders dem- mation in the upper reaches (Figs. 5 and 6) and Sandstone and ranges from 1 to possibly 4 m onstrate the very different weathering rates ranges in thickness from ~2 m to possibly greater in thickness (Figs. 7 and 8). The drainages on between the Navajo Sandstone and the Oligo- than 9 m. In the lower reach of the deposit, Calf either side of the Durffey Mesa deposit are cene-age andesites, and are good examples of Creek has incised ~206 m below the W side of choked with a mixture of andesitic colluvium meter-scale boulder armoring. These precari- the New Home Bench strath, and Dry Gulch has and sandy alluvium, and so it is diffi cult to ously balanced rocks also provide some con- incised ~142 m on the E side (Figs. 5 and 6). determine where the modern bedrock straths straints on the seismic history of the region (cf. The tread of the New Home Bench deposit is are. However, there is ~140–200 m of relief on Balco et al., 2011).

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to the deposits. These include: the predominant rounding of clasts in some of the deposits or Escalante monocline areas of deposits, imbrication in some exposures, and occasional cut-and-fi ll stratigraphy in some exposures. Taken together, these observations suggest that most of the deposits are likely fl uvially reworked debris-fl ow or hyperconcen- trated fl ow run-out deposits. The deposits are A′ clearly composite and likely the result of multiple episodes of mass movement deposition and BMB fl uvial reworking. The treads of all four of the deposits are cov- Pine Creek A ered with locally sourced eolian silts and sands and large areas of intact, broken, and sometimes overturned pedogenic carbonate rinds and parts of carbonate laminar horizons. These two observations suggest that the absolute elevation of the deposit treads have been both eroding and aggrading through time. The pedogenic carbonate littering the surfaces suggests a previously thicker mantle of deposit in which the carbonate precipitated that has since been eroded away. The sometimes thick accumulations of eolian- derived silts and sands suggest that at least 2400 A A′ recently, the deposits may have been increasing in thickness in some areas. Exposures in the New Home Bench deposits showed stage II–III pedogenic carbonate morphology; how- 2300 ever, pedogenic carbonate rind thicknesses sug- cline gest that perhaps a higher stage was achieved in most of the deposits. The Durffey Mesa deposits in particular have extremely thick pedogenic 2200 carbonate rinds (commonly 8–9 cm) and large, thick (sometimes 11–12 cm) plates of blocky- Escalante Mono BMB laminar pedogenic carbonate. These platy-laminar carbonate clasts may have been the upper Elevation (m) 2100 laminar part of a former K horizon, suggesting ~228 m possible stage IV carbonate development some- time in the past.

2000 Boulder Armoring

The geomorphic process of topographic V.E. = 6× Pine Creek inversion, or inversion of relief, has been rec- 1900 ognized for some time, and can be important 0 1 2 3 4 in understanding the long-term evolution of Distance (km) many landscapes (Brunsden, 1993; Pain and Figure 3. Digital elevation model close-up and topographic profi le across the Black Mesa Ollier, 1995). Typically, the most common Bench (BMB). V.E.—vertical exaggeration. processes that cause the armoring essential for topographic inversion are lava fl ow deposition (e.g., Wahrhaftig, 1965; Ollier, 1988; Aslan Synthesis deposits; the nearly monolithologic nature of the et al., 2008) or in situ duricrust (extreme silica, deposits in outcrop, surface exposure, and peb- iron, aluminum, or carbonate accumulations) Our observations of the four gravel deposits ble counts (Table 2); occasional inverse grading formation in soil profi les (e.g., Twidale, 1984; described above allow us to make some conclu- in some exposures; and fi nally, the sharp con- Ollier, 1991; Eppes et al., 2002). Only a few sions about the mechanisms and paleoenviron- tact between the underlying bedrock and the studies have looked at boulder armoring as a ments of deposition. Several observations argue deposits (e.g., Fig. 6C) shows no signifi cant possible mechanism for topographic inversion for the deposits being from mass movements. weathering or alteration (soil formation) and (Bryan, 1940; Mills, 1981, 1990). Granger et al. These include: the extremely large boulder so suggests rapid deposition. However, several (2001) demonstrated that both boulder armor- sizes (Table 1); the thickness of the gravel-rich observations argue for some fl uvial component ing and soil cover are important components of

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B C

Sample BMB-03

Figure 4. Photographs of the Black Mesa Bench (BMB) deposit. (A) Digitally stitched photograph of the W side of the BMB deposit (deposit caps the hill) and Pine Creek; photograph looking SW. (B) Photograph of the BMB tread. (C) Photograph of the BMB tread showing cosmogenic sample BMB-03.

the long-term landscape evolution of granitic ing can be seen at many different spatial scales so it is better for attempting to date very long terrains. around the upper Escalante River basin—rang- exposure durations. The cosmogenic production Mass movement deposits composed of coarse ing from the benches we studied (Figs. 1–10) rate of 3He is the highest of all the routinely used volcanic boulders are common throughout the to the volcanic pedestal boulders that commonly nuclides, and He isotopes can be measured rela- upper Escalante River drainage basin. These form around the margins of gravel benches cut tively easily using conventional noble gas mass deposits cover the paleovalley fl oors (straths) into Navajo Sandstone (Fig. 10C). spectrometry and only require pure mineral sep- where they were deposited. Due to the large size arates. Since He has a low atomic mass, it can of the clasts in these deposits, the local main- Cosmogenic 3He Exposure-Age Dating leak out of many mineral phases at Earth surface stem drainage and tributaries cannot easily of Boulder Clasts temperatures; however, it is fully retained in remove these deposits, and so they can persist dense mineral phases such as olivine, pyroxene, in the landscape as remnants of the former posi- Cosmogenic exposure-age dating is a rela- garnet, Fe-Ti oxides, zircon, and apatite (e.g., tions of valley fl oors and active river channels tively new age dating technique that uses the Farley et al., 2006; Dunai, 2010). The andes- (Williams, 1984; Williams et al., 1990). Addi- buildup of cosmic ray–derived isotopes, mea- ites in the gravel deposits we investigated have tionally, over long periods of time, the volcanic sured in certain mineral phases or pieces of abundant phenocrysts of pyroxene (~10%–30% rocks are relatively more resistant to physi- whole rock, to determine the exposure dura- pyroxene phenocrysts in most samples) and cal and chemical weathering causing them to tion of rock surfaces (Gosse and Phillips, 2001; have had very long exposure durations; so 3He erode more slowly than the local sedimentary Dunai, 2010). The “method” is actually a series is a favorable cosmogenic method. bedrock (Selby, 1993; McLelland et al., 2008). of methods based on the isotope being measured There can be several sources of noncosmo- Vol canic gravel deposits are resistant to weath- (commonly 3He, 10Be, 21Ne, 26Al, and 36Cl) and genic He in volcanic minerals that need to be ering and occupy transport-limited geomorphic the mineralogy of the rocks being investigated. assessed in order to determine an accurate 3He settings, and once the deposits are isolated The 3He method is similar to other cosmogenic exposure age. The fi rst of these is He from the from the active drainage, these effects magnify. methods in basic principle but has several differ- magma (magmatic He) that becomes trapped Ultimately, both of these factors can lead to ences that make it the most suitable choice for in fl uid inclusions during crystallization (Kurz, topographic inversion, where the former valley the andesite-rich gravel deposits in the northern 1986; Cerling and Craig, 1994). This compo- fl oors become preserved higher-elevation land- Escalante drainage basin. 3He is a stable nuclide, nent of He is most commonly corrected for by scape elements because they were armored with which means its cosmogenic buildup in a min- crushing the minerals in vacuo, measuring the coarse volcanic boulder deposits. This armor- eral is not affected by radioactive decay, and 3He/4He ratio of the released gasses and using

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duced directly from the decay of U and Th as alpha particles (α) incorporate electrons; this 4 4 He is known as radiogenic helium ( Herad). Non cosmo genic 3He can be produced through a mechanism where alpha particles from radiogenic decay interact with common light elements (Na, Mg, K, and Ca) and produce slowed or NHB “thermalized” (thermal) neutrons via (α,n)

Boulder Creek A′ reactions. The thermal neutrons can then react 6 3 NHB Dry with Li to produce tritium ( H), which quickly 3 Gulch decays (T1/2 = 12.3 a) to He via the following reaction: 6Li (n,α) 3H→3He (Andrews and Kay, 3 3 A 1982). This He is known as nucleogenic ( Henuc)

C and is a problem in rocks with high U and Th alf C concentrations, high Li concentrations in the reek target mineral phase, older crystallization ages, and short exposure durations (Dunai, 2010). In two previous studies, we successfully used two different suites of completely shielded andesite samples, where cosmogenic 3He production is assumed to be nil, to correct for both radiogenic 2000 4 3 A A′ He and nucleogenic He in cosmogenic exposure-age samples (Marchetti and Cerling, 2005; Marchetti et al., 2005a). The shielded sample pyroxenes from those studies yielded 3He con- 1950 NHB centrations ranging from 3.7 × 106 to 10.9 × 106 atoms g–1, 4He concentrations ranging from 18.25 × 1012 to 46.54 × 1012 atoms g–1, and a very consistent 3He/4He ratio of 2.08 ×10–7. In this 1900 study, concentrations of cosmogenic 3He (3He ) ~142 m c were determined from the total measured 3He and 4He concentrations (3He and 4He ) using ~206 m tot tot

Elevation (m) the following relationship: 1850

3 3 4 3 4 Hec = Hetot – [ Hetot × ( He/ He)s], (1)

where the shielded (3He/4He) ratio is 2.08 × 1800 s Dry Gulch 10–7 (Marchetti et al., 2005a).The 1σ uncertainty associated with the shielded correction is Calf Creek V.E. = 7 × ~14% and is propagated through the determina- 3 1750 tions of the Hec concentrations. This shielded 0 0.5 1.0 1.5 2.0 2.5 4 Distance (km) correction fully accounts for radiogenic He, nucleogenic 3He, and any possible magmatic 3He Figure 5. Digital elevation model close-up and topographic proﬁ le across the New Home or 4He missed during crushes or split measure- Bench (NHB). V.E.—vertical exaggeration. ments of crushed powers and whole crystals.

Sampling

that ratio to correct the total He released from fully crushed powders yielded no statistical dif- In sampling the boulders for surface-expo- furnace heating (e.g., Scarsi, 2000; Blard and ference in the resulting He isotopic concentra- sure dating, we chose boulders that had the best Farley, 2008). We crushed numerous splits of tions—again indicating little to no magmatic He combination of height above the deposit sur- pyroxenes from Boulder Mountain andesites in the pyroxenes. We suspect that the magmatic face, relative lack of weathering of the boulder for this and previous studies (Marchetti and He in pyroxene fl uid inclusions was degassed crown, and position within the surface. Higher Cerling, 2005; Marchetti et al., 2005a, 2005b, after eruption and during the high temperatures and larger boulders are less likely to have been 2007, 2011) and found that Boulder Mountain associated with the welding of the ash-fl ow tuffs buried and then exposed, and to have been sig- and Aquarius Plateau pyroxenes had little to no (Ball et al., 2009). nifi cantly eroded. Clasts that sit higher above He released from crushing and therefore ana- In volcanic rocks with older crystallization the surface are also less likely to have their tops lytically unresolvable 3He/4He ratios. Splits of ages (>1 Ma), another source of noncosmogenic affected by fi re-induced spalling or diffusional several cosmogenic samples analyzed both as He is radioactive decay of U and Th in the target losses of He from wildfi re heating (e.g., Bier- whole uncrushed crystals of pyroxene and as mineral phase and the whole rock. 4He is pro- man and Gillespie, 1991). Many of the deposit

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A B

C D

Figure 6. Photographs of the New Home Bench (NHB) deposit. (A) Photograph of cosmogenic sample NHB-02. (B) Photograph of the W edge of the NHB deposit; photograph looking N–NW. Calf Creek in valley to W, Boulder Mountain–Aquarius Plateau in background. (C) Photograph of the contact between the NHB deposit and underlying Carmel Formation; photograph taken in the NW part of the NHB deposit. Hammer for scale. (D) Photograph of the NHB tread.

treads we sampled had patches of eolian sand gible. Sample locations and elevations were Pyroxene separates were completely degassed in and silt deposits. Sampling the highest boul- determined using hand-held global positioning a modifi ed Turner furnace at ~1400 °C. Helium ders would also insure against the possibility system (GPS) units with the North American isotope concentrations and isotopic ratios were of transient boulder burial due to surface infl a- Datum of 1983 (NAD 83 datum) (Table 3). determined using a MAP-251 mass spectrom- tion. Boulders that had heavily spalled, fl aking, eter with electron multiplier (3He) and Faraday or chipped crowns were avoided. Good samples Analytical cup detectors (4He) at the Noble Gas Laboratory were slightly polished, likely by wind, and were of the University of Utah. Reactive gasses were often coated with rock varnish. Where possible, Andesite samples were crushed to a uni- removed using SAES getters, while Ar and Ne we tried to sample clasts away from the edges form grain size, and 1–2 mm pyroxenes were were trapped and removed cryogenically using of gravel deposits. Boulders near the edges separated using standard magnetic and heavy a cold head held at 10K. Measured counts of 3He could be affected by creep and are more likely liquid techniques. The pyroxene separates were and 4He were furnace blank corrected and then

to have been buried and subsequently exposed etched in dilute (~5%) HNO3/HF multiple times standardized against multiple analyses of puri- by erosion that is concentrated near the edges to remove the outer few tens of microns of the ﬁ ed Yellowstone Park gas (MM; 3He/4He ratio 3 4 of the deposits. Samples were removed using grains. This etching should effectively remove of 16.5 RA, where RA is the He/ He ratio in air a hammer and chisel or airless jackhammer. any areas of implanted or recoil-loss He on the ~1.384 × 10–6) or puriﬁ ed local Wasatch Moun- Topographic shielding at all sites was negli- surface of the grains (Blard and Farley, 2008). tain atmosphere (Little Mountain air). Proce-

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dural furnace and crusher blanks in this system 5 3 range from 0 to 2 × 10 atoms for He and 1–5 × Boulder Town 109 atoms for 4He. The helium isotopic data and 3 Hec concentrations determined using Equation 1 above are given in Table 4.

3He Exposure Ages

3He exposure ages were determined using the 3 Hec concentrations from Table 4 and the sample DM and site data from Table 3 with the CRONUS A′ Deer Creek online 3He exposure-age calculator (Goehring A et al., 2010; http://www.cronuscalculators.nmt Boulder Creek .edu/he/he_age.xhtml). This online exposure-age calculator determines 3He exposure ages and

age uncertainties for ﬁ ve different scaling rou- Calf Creek tines and different boulder surface erosion rates (Table 5). The sea-level, high-latitude (SLHL) production rates used in the CRONUS online calculator depend upon the scaling routine and range from 119 to 136 atoms g–1 yr–1 (Table 4 in Goehring et al., 2010). The resulting exposure ages vary based on which scaling routine is applied and whether or not boulder surface 2100 erosion is taken into account. Since the boulders A A′ almost certainly experienced some erosion, we prefer the exposure ages determined with the DM 0.00001 cm yr–1 erosion rate. We use the expo- 2050 sure ages determined with the Lal (1991) and Stone (2000) time varying production rate for discussion and arguments in the rest of the paper 2000 because the ages ± uncertainty of that scaling routine capture the full range of the rest of the exposure ages determined with other scaling 1950 routines (Table 5).

Several geomorphic factors can affect boulder Elevation (m) exposure ages from allochthonous deposits. The fi rst of these is pre-exposure or “inheritance,” 1900 where the sampled rock surfaces were exposed alluvium ? ? to cosmic rays prior to being incorporated in the deposit that was sampled. In this scenario 1850 alluvium the exposure-age estimate of deposition would ? ? be “too old” because some of the cosmogenic V.E. = 10× 3He was acquired before deposition. Several fac- 1800 tors argue against this possibility for the gravel 01234 deposits we investigated. First, the large boul- Distance (km) ders in the deposits we sampled are likely from mass movements, which suggest geologically Figure 7. Digital elevation model close-up and topographic profi le across Durffey Mesa (DM). rapid excavation, transport, and deposition. Sec- V.E.—vertical exaggeration. ondly, the gravel deposits are extremely close to their source. All of the deposits are less than ~20 km from the modern andesitic outcrops siderably less of an effect than deposit erosion pre-exposed had only ~8 ka of exposure age at along the southern cliffs of Aquarius Plateau for surfaces older than a few 100 ka (Anderson the site of deposition. Since the boulders were and Boulder Mountain. Given a modest rate of et al., 1996; Repka et al., 1997; Hancock et al., likely pre-exposed up drainage, at a higher cliff retreat, the deposits were likely even closer 1999; Hidy et al., 2010; Heyman et al., 2011; elevation with a higher 3He production rate, the to the volcanic escarpment during deposition Schmidt et al., 2011). Previous work by us in actual pre-exposure duration of those clasts was ~0.6–1.4 Ma (Table 5). Finally, several studies the nearby Fremont River drainage (Marchetti less than ~8 ka. Ultimately, we argue that pre- have directly measured the amount of cosmo- and Cerling, 2005) showed that most shielded exposure is unlikely for the deposits we investi- genic inheritance in fl uvial systems and found boulders in a debris-fl ow deposit had no pre- gated given that they are from mass movements that it is typically quite low (~5–60 ka) and con- exposure, and two boulders that were possibly and are close to source. Most importantly, if

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Sample DM–03

C D

Figure 8. Photographs of the Durffey Mesa (DM) deposit. (A) Photograph of cosmogenic sample DM-03. (B) Photograph of the DM deposit tread; the white material is either pedogenic carbonate rind or plate. (C) Photograph of the DM deposit (treed ridge on skyline) taken from the SE of the DM deposit and looking NW. (D) Photograph of a thick plate of pedogenic carbonate from the DM deposit.

there is some inheritance, it should be negligible over glacial moraines in that they do not start deposits have moderate to strong positive cor- when compared to the exposure durations and with sharp relief and the resulting concen- relations (r = 0.73–0.98) between exposure age uncertainties that we determined (Table 5). trated erosion that a steep-sided glacial moraine and boulder height, and for each surface the old- Allochthonous geomorphic deposits that are would. However, deposit and boulder erosion est exposure ages are associated with the highest quite old (~100 ka and older) are diffi cult to date are by far the most signifi cant limiting factors in boulders. The Black Mesa Bench surface shows with cosmogenic techniques because of boulder establishing an accurate cosmogenic age of our no correlation (r = –0.07) between exposure age and deposit erosion (Gosse and Phillips, 2001). deposits. Here, we assume a slow rate of boulder and boulder height likely because the range of This is especially true for boulders in glacial erosion (0.000001 cm yr–1) in the CRONUS heights for sampled boulders from that deposit moraines older than LGM in age because of cosmogenic age calculations to correct for slow is small (0.5 to 0.7 m). Taken together, these moraine crest erosion exposing previously boulder surface erosion (Table 5). To estimate data suggest that in most cases the higher boul- buried clasts (Putkonen and Swanson, 2003; the effects of deposit erosion, we collected ders above a deposit surface will yield the old- Heyman et al., 2011). Easterbrook et al. (2003; multiple boulder samples from each surface and est exposure ages, and therefore, either extreme p. 49–56) demonstrated that the highest boul- recorded the height of each sample relative to boulder erosion (of select boulders) or deposit ders above a Sacagawea Ridge–age (pre–Bull the modern deposit surface (tread) (Table 3). By erosion and boulder exhumation are the most Lake–age; ~350–610 ka) moraine yielded the plotting the exposure age of each clast versus its strongly controlling factors of the exposure-age oldest cosmogenic exposure ages and the clos- height above the deposit tread, we can qualita- populations. We, therefore, interpret the oldest est age estimate to the time of moraine emplace- tively assess the effect of deposit erosion (Fig. exposure ages from each deposit as the best ment. The fl uvially reworked mass movement 11). Figure 11 demonstrates that the New Home estimate of the age of deposition. Furthermore, deposits we sampled have a slight advantage Bench, Durffey Mesa, and Deer Creek Bench since the boulders and boulder deposits are

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clearly eroded, the oldest exposure ages provide Boulder Town a minimum age estimate for deposition and surface abandonment.

INCISION RATE ESTIMATES

We use the depth of incision around each deposit and the exposure age of the oldest boulder from each deposit to estimate the long- A′ term incision rate of the drainages surrounding Deer Cre

each deposit (Table 6). We subtract an estimate DCB of each deposit’s thickness and make depth of A ek incision measurements in areas where we have observed the modern ﬂ oodplain sediment to be thin or negligible (~2 m or less). This allows us to determine a paleostrath to modern-strath estimate of bedrock incision over the time duration since deposition. The depth of incision around the deposits is quite large, and so small errors (few m) in estimating deposit thicknesses or in measuring incision depths would only slightly affect the resulting incision rates. We did not determine incision rates around the Durffey 2000 Mesa surface because both drainages around the A A′ deposit are ﬁ lled with an unknown amount of sediment, and we could not identify the modern straths. Since the oldest exposure ages are DCB likely minimum age estimates of the deposi- 1950 tional age of each deposit, the incision rates we determine are maximum rates. The incision rates we estimate for northern tributary drainages of the Escalante River range from 151 to 1900 323 m Ma–1 and are determined over a time interval of 0.6–1.4 Ma (Table 6). Comparing the

rates of geologic process, such as river incision, Elevation (m) ~210 m may not be appropriate when the time interval 1850 over which the rates are determined varies sig- niﬁ cantly (Gardner et al., 1987; Mills, 2000). With that caveat in mind, we compare our rate data with previously published incision rate 1800 data using ages in the ~0.5–1.5 Ma timeframe to avoid some of the complications of making V.E. = 9× Deer such comparisons. Creek 1750 DISCUSSION 0 0.5 1.0 1.5 2.0 2.5 3.0 Distance (km)

Accuracy of Our Dating Attempts Figure 9. Digital elevation model close-up and topographic proﬁ le across Deer Creek Bench (DCB). V.E.—vertical exaggeration. The largest uncertainty associated with our exposure ages is deposit and boulder surface erosion—which makes our exposure ages minimum age estimates. How much of a minimum et al., 2011) could provide further temporal con- new burial ages give us pause, and reinforce the estimate they may be is unknown but could be straints on the age of the deposits. A recently argument that we demonstrate above, namely constrained. Employing additional independent published isochron-based cosmogenic burial age that exposure ages of surface boulders in older dating techniques such as U-series disequilib- of a gravel deposit on top of a strath near Bull- deposits (likely >300 ka from our experience rium or U-Pb on pedogenic carbonate rinds frog Marina (1.5 ± 0.1 Ma; Darling et al., 2011) elsewhere; Marchetti et al., 2005b) are minimum (e.g., Sharp et al., 2003; Woodhead et al., 2006; is signiﬁ cantly older than previously published age estimates of deposition. These two data sets Rasbury and Cole, 2009; Cliff et al., 2010) or cosmogenic 10Be exposure ages of surface clasts suggest that for the Bullfrog deposit, the differ- cosmogenic burial techniques (Wolkowinsky exposed on the deposit tread (0.48 ± 0.01 Ma; ence between surface boulder exposure ages and and Granger, 2004; Granger, 2006; Darling Davis et al., 2001). If robust and repeatable, these burial ages is a factor of 3. However, given the

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A B

C D

Sample DCB–04

Figure 10. Photographs of the Deer Creek Bench (DCB) deposit. (A) Photograph of the DCB deposit (tree-covered surface in the middle ground) taken from W of the DCB deposit looking E. (B) Photograph of the DCB deposit tread showing an area of signiﬁ cant eolian sand accumulation. (C) Photograph of a pedestal boulder just to the S of the DCB deposit. The boulder eroded out of the deposit and is now armoring a small area of Navajo Sandstone. (D) Photograph of the DCB tread and cosmogenic sample DCB-04.

extremely thick nature of the Bullfrog Marina that the deposit and its underlying bedrock strath Geomorphic Evolution of the Northern deposits, we are uncertain that the surface boul- were active parts of the fl uvial system and since Tributaries of the Escalante River— der exposure ages are necessarily as severe of incised below. In that sense, cosmogenic burial Ideas and Speculations underestimates of the terrace depositional age as ages might be best thought of as maximum age Darling et al. (2011) suggest. Hanks and Finkel estimates of the abandonment of fl uvial depos- In this section, we discuss some ideas regard- (2005) make a similar argument regarding the its and straths due to fl uvial incision. Similarly, ing the long-term landscape evolution of our cosmogenic burial results of Wolkowinsky and when cosmogenic burial ages are used to deter- fi eld area with regard to the exposure ages and Granger (2004) at a site along the San Juan mine incision rates, those rates could be con- incision rates that we determined. It is impor- River. In aggrading deposits, the burial age of sidered minimum rates because the burial ages tant to remember that even our oldest exposure sediments deeper in a fi ll terrace should be older represent the longest period of time over which ages are likely minimum age estimates, and so than those near the surface. Whether the differ- the rates could be determined. This is especially our incision rates are maximum rate estimates. ence is a short or long period of time depends the case when the burial ages are determined on Therefore, some of the following discussion is upon whether the deposits are from single or sediments deep in the deposit near the deposit speculative in nature because we make argu- multiple events, and the length of time of each and/or strath contact. Ideally, the best age dating ments that assume our incision rates are indeed depositional event. Regardless of the length of attempts would obtain cosmogenic burial ages 151–323 m Ma–1 and not slower. Our goal is time of aggradation, the surface, and not the base deep in a deposit and multiple, independent sur- to explore ideas about possible controls on the of the deposit, is the best record of the last time face or tread age estimates. deep incision seen in our fi eld area and by exten-

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TABLE 3. EXPOSURE-AGE SAMPLING DATA cutting. At some point, presumably after sig- Boulder Topographic Sample nifi cant exposure of the Navajo Sandstone, the Location and Latitude Longitude Elevation height/longest shielding thickness sample number (°N) (°W) (m) axis (m) factor (cm) landscape morphology of the northern tributar- Black Mesa Bench ies area changed and deep incision started and BMB-01 37.8504 111.6419 2180 0.6/1.7 1.00 4.0 is currently still going on. BMB-02 37.8502 111.6435 2180 0.7/1.0 1.00 3.0 Our maximum incision rates of 151–323 m BMB-03 37.8538 111.6477 2194 0.5/1.3 1.00 4.0 –1 BMB-04 37.8485 111.6434 2177 0.5/1.2 1.00 4.0 Ma demonstrate that this incision may have New Home Bench been proceeding at a faster rate than other long- NHB-02 37.8856 111.4661 2096 0.7/1.3 1.00 3.0 term (since 0.6–1.4 Ma) rates from around the NHB-03 37.8844 111.4629 2074 0.5/1.4 1.00 3.0 Colorado Plateau (~40–150 m Ma–1: Wolkow- NHB-04 37.8846 111.4629 2075 0.6/1.1 1.00 3.0 NHB-05 37.8828 111.4539 2061 0.4/1.1 1.00 2.0 insky and Granger, 2004; Darling et al., 2009, NHB-06 37.8710 111.4408 2037 0.5/1.3 1.00 6.0 2011) and the regional background incision rate NHB-07 37.9009 111.4693 2116 0.8/1.5 1.00 7.0 of 100 m Ma–1 assumed by Cook et al. (2009). Durffey Mesa Bench In a broad sense, both our incision rate estimates DM-01 37.8683 111.3929 2074 0.5/1.0 1.00 3.0 (from surface-exposure ages, likely maximum DM-02 37.8762 111.3968 2091 0.4/0.9 1.00 3.0 DM-03 37.8693 111.3930 2071 0.6/1.0 1.00 4.0 rates) and the other Colorado Plateau rates since DM-04 37.8758 111.3967 2084 0.7/1.1 1.00 6.0 the time period of 0.6–1.4 Ma (Wolkowinsky Deer Creek Bench and Granger, 2004; Darling et al., 2009, 2011) DCB-01 37.8627 111.3678 1992 0.2/0.9 1.00 2.0 (from cosmogenic burial ages, perhaps mini- DCB-02 37.8650 111.3662 1989 0.2/0.8 1.00 2.0 DCB-03 37.8659 111.3654 1993 0.7/1.2 1.00 3.0 mum rates) may be approaching the same value, DCB-04 37.8638 111.3714 2002 0.5/1.1 1.00 3.0 especially given the large uncertainties associated with both dating techniques. In a more nar- row sense, our rates do exceed the other regional sion, much of the Colorado Plateau, during the high-level gravel deposit along the northern estimates by 2–8 times and warrant some dis- Quaternary. tributaries of the Escalante River in the past cussion. We suggest several possible explana- An important observation regarding the boul- ~0.6–1.4 Ma. We hypothesize that prior to deep tions for the potentially faster rates of incision der deposits we studied is their relationship to canyon incision, the overall relief of the distal and deep overall landscape dissection seen in the Navajo Sandstone. The Black Mesa Bench Aquarius Plateau–Boulder Mountain piedmont our fi eld area since deposition of the high-level deposit is just 1.5 km to the west of a steeply was more subtle and the geomorphology was andesitic gravels at 0.6–1.4 Ma: westward-dipping outcrop of Navajo Sandstone dominated by scarp retreat of the Aquarius (1) Retreat of the Aquarius Plateau–Boulder in the western limb of the Escalante anticline Plateau–Boulder Mountain margin, colluvia- Mountain escarpment effectively shut off col- (Fig. 4A). It is also located at the downstream end tion and fl uvial reworking, and effective lateral luviation to the distal piedmont allowing more of a deeply incised reach of Pine Creek, known planation. Since abandonment of the high-level stream power to go into incision rather than sedi- as The Box, which is cut into Navajo Sandstone gravel deposits, the geomorphology has been ment transport, deposition, and reworking. Effec- (Figs. 1 and 2). The New Home Bench, Durffey dominated by fl uvial incision and deep canyon tively cutting off signifi cant deposition of coarse Mesa, and Deer Creek Bench deposit straths are all cut close to the upper stratigraphic boundary of the Navajo Sandstone. The very northernmost TABLE 4. HELIUM ISOTOPE DATA FOR EXPOSURE-AGE SAMPLES 4 3 3 4 3 § 3 part of the New Home Bench deposit is actually Location and Hetot Hetot He/ He R/Rair* Hec Hec 12 –1 6 –1 6 –1 cut into Carmel Formation (Fig. 6C), while most sample number (10 atoms g ) (10 atoms g ) fusion fusion (10 atoms g ) (%) Black Mesa Bench of the deposit is cut on the uppermost Navajo BMB-01 30.35 ± 0.18 392.0 ± 7.3 1.29 × 10–5 9.3 385.7 ± 7.5 98.4 Sandstone. The Durffey Mesa deposit is cut into BMB-02 30.82 ± 0.14 285.5 ± 3.7 9.27 × 10–6 6.7 279.1 ± 3.8 97.8 Carmel Formation (which interfi ngers with Page BMB-03 29.49 ± 0.09 337.0 ± 5.3 1.14 × 10–5 8.3 330.8 ± 5.4 98.2 BMB-04 30.26 ± 0.05 283.4 ± 3.3 9.37 × 10–6 6.8 277.1 ± 3.3 97.8 Sandstone, which is sedimentologically similar to Navajo Sandstone) just above the top of the New Home Bench NHB-02 27.36 ± 0.11 356.2 ± 15.4 1.30 × 10–5 9.4 350.0 ± 15.4 98.4 Navajo Sandstone (Fig. 12). The Deer Creek NHB-03 14.34 ± 0.06 236.2 ± 10.1 1.65 × 10–5 11.9 233.2 ± 10.1 98.7 Bench deposit is clearly cut onto Navajo Sand- NHB-04 26.01 ± 0.11 258.2 ± 7.9 9.93 × 10–6 7.2 252.8 ± 8.0 97.9 –6 stone in the upper part of the unit. Other fi eld- NHB-05 30.38 ± 0.28 206.3 ± 14.2 6.79 × 10 4.9 200.0 ± 14.2 96.9 NHB-06 29.65 ± 0.12 245.5 ± 5.6 8.28 × 10–6 6.0 239.3 ± 5.9 97.5 and image-based observations made by us indi- NHB-07 21.01 ± 0.21 273.0 ± 5.5 1.30 × 10–5 9.4 268.6 ± 6.3 98.4 cate that many of the other high-level, vol canic Durffey Mesa boulder-rich deposits in the area are cut near the DM-01 16.48 ± 0.18 410.7 ± 9.1 2.49 × 10–5 18.0 407.3 ± 10.1 99.2 stratigraphic top of the Navajo Sandstone. DM-02 15.57 ± 0.18 190.6 ± 8.4 1.22 × 10–5 8.9 187.3 ± 8.6 98.3 DM-03 31.98 ± 0.02 493.2 ± 9.0 1.54 × 10–5 11.1 486.6 ± 9.0 98.7 Although the andesitic boulder deposits sit DM-04 19.69 ± 0.03 399.8 ± 11.1 2.03 × 10–5 14.7 395.7 ± 11.1 99.0 at a variety of elevations, the relief between Deer Creek Bench different boulder deposit treads (and straths) is DCB-01 32.25 ± 0.13 193.6 ± 12.2 6.0 × 10–6 4.3 186.9 ± 12.0 96.5 relatively small when compared to the variable DCB-02 38.01 ± 0.15 163.2 ± 10.3 4.3 × 10–6 3.1 155.3 ± 10.1 95.2 –5 but extreme incision of many of the northern DCB-03 35.20 ± 0.14 645.8 ± 40.8 1.8 × 10 13.3 638.4 ± 40.5 98.9 DCB-04 35.09 ± 0.13 386.0 ± 8.4 1.1 × 10–5 8.0 378.7 ± 8.4 98.1 tributaries (Fig. 12). This suggests a possible Note: Measurement uncertainties are ±2σ. 3 4 3 4 –6 change in overall landscape morphology since *R/Rair is the He/ He ratio of the sample divided by the He/ He ratio of air: 1.384 × 10 . deposition and abandonment of most of the §Determined using Equation 1.

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TABLE 5. EXPOSURE AGES FOR DIFFERENT SCALING ROUTINES WITH 1 mm BOULDER SURFACE EROSION –1 3 PER 10 ka (0.00001 cm yr ) AND Hecosmo FROM TABLE 4 (AGES IN ka) Location and Constant production Time-varying production sample number Lal (1991); Stone (2000) Desilets et al. (2006) Dunai (2001) Lifton et al. (2005) Lal (1991); Stone (2000) Black Mesa Bench BMB-01 781 ± 114 679 ± 101 667 ± 99 673 ± 100 706 ± 105 BMB-02 548 ± 78 480 ± 70 471 ± 68 476 ± 69 499 ± 73 BMB-03 656 ± 95 568 ± 83 558 ± 82 562 ± 82 591 ± 87 BMB-04 550 ± 78 482 ± 70 473 ± 69 478 ± 69 501 ± 73 New Home Bench NHB-02 739 ± 112 645 ± 99 632 ± 97 638 ± 98 671 ± 104 NHB-03 489 ± 72 430 ± 65 422 ± 63 426 ± 64 446 ± 67 NHB-04 532 ± 77 469 ± 69 460 ± 68 465 ± 69 486 ± 72 NHB-05 417 ± 66 364 ± 59 357 ± 57 360 ± 58 379 ± 61 NHB-06 529 ± 76 468 ± 69 459 ± 67 464 ± 68 484 ± 71 NHB-07 572 ± 82 502 ± 74 493 ± 72 498 ± 73 521 ± 77 Durffey Mesa DM-01 883 ± 131 777 ± 117 764 ± 115 771 ± 116 801 ± 121 DM-02 384 ± 57 333 ± 50 327 ± 49 329 ± 49 345 ± 52 DM-03 1087 ± 163 949 ± 144 932 ± 141 940 ± 143 983 ± 150 DM-04 875 ± 131 770 ± 116 756 ± 114 764 ± 115 795 ± 121 Deer Creek Bench DCB-01 408 ± 63 358 ± 56 351 ± 55 354 ± 56 370 ± 59 DCB-02 337 ± 52 294 ± 46 289 ± 45 291 ± 46 303 ± 48 DCB-03 1547 ± 265 1350 ± 232 1324 ± 227 1339 ± 230 1395 ± 241 DCB-04 862 ± 127 763 ± 114 748 ± 112 756 ± 113 785 ± 118

900 r = – 0.07 BMB 900 r = 0.73 NHB

800 800

700 700

600 600

500 500 He exposure age (ka) He exposure age (ka) 3 3 400 400

300 300 0.4 0.5 0.6 0.7 0.8 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Boulder height (m) Boulder height (m)

1200 1800 r = 0.73 DM r = 0.98 DCB 1100 1600 1000 1400 900 800 1200 700 1000 600 800 500 He exposure age (ka) He exposure age (ka) 3 3 600 400 300 400

200 200 0.3 0.4 0.5 0.6 0.7 0.8 0.1 0.3 0.5 0.7 0.9 Boulder height (m) Boulder height (m)

Figure 11. Relationship between cosmogenic 3He exposure ages and boulder height for the deposits we sampled. The r values are the correlation coefﬁ cients between exposure age and boulder height determined for each surface. BMB—Black Mesa Bench; NHB—New Home Bench; DM—Durffey Mesa; DCB—Deer Creek Bench.

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TABLE 6. INCISION RATE ESTIMATES Depth of Oldest Maximum incision exposure age incision rate Surface Drainage (m) (ka) (m Ma–1) Black Mesa Bench Pine Creek 228 706 ± 105 323 New Home Bench Calf Creek 206 671 ± 104 307 Dry Gulch 142 671 ± 104 212 Durffey Mesa W side, unnamed n/d 983 ± 150 — E side, unnamed n/d 983 ± 150 — Deer Creek Bench Deer Creek 210 1395 ± 241 151 Note: n/d—not determined.

2400 A A′ approximate top of Navajo Ss

2200

BMB relief between deposit straths 2000 ~100 m

NHB

Elevation (m) incision below deposit straths 1800 200–250 m

V.E. = 16× 1600 0 5 10 15 20 Distance (km)

2100 B DM B′

relief between approximate top of Navajo Ss deposit straths 70–80 m 2000 NHB

DCB

1900

incision below deposit straths Elevation (m) 150–250 m 1800

V.E. = 13× 1700 02 4 68 Distance (km)

Figure 12. Topographic proﬁ les from A to A′ and B to B′ in Figure 2. The stratigraphic top of the Navajo Sandstone is shown with a dashed line as estimated from the structure contours in Weir and Beard (1990a, 1900b) and Williams et al. (1990). Ss—Sandstone; V.E.—vertical exaggeration.

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gravels would still allow the northern tributaries suggest that if that knickzone is part of a transient topographic inversion or inversion of relief on a to transport smaller, reworked ande sitic clasts incision signal propagating up the Escalante variety of scales. Cosmogenic 3He exposure-age across the underlying bedrock. Since the ande- River from the main-stem Colorado system, dating of some of the largest boulders exposed sitic clasts are signifi cantly tougher than the local then incision rates upstream of the knickzone on the treads of four different deposits range sedimentary bedrock (McLelland et al., 2008), should be slower than those measured at several from 303 ± 48 to 1395 ± 241 ka. The tallest this should lead to increased or easier incision. sites downstream of the knickzones. Our inci- boulders exposed on the deposit surfaces tend This “tool” effect is the opposite of the boulder sion rates of 151–353 m Ma–1 are faster than the to yield the oldest exposure ages, suggesting armoring or “cover” effect previously mentioned, background incision rate of Cook et al. (2009) that boulder erosion and deposit erosion are and demonstrates the interesting dichotomy in of ~100 m Ma–1; however, they are maximum controlling the exposure-age populations and landscape response of areas with resistant litholo- estimates. This may indicate that a fragment indicating that even the oldest exposure ages gies that dominate the sediment load and weaker of the knickzone from Colorado River integra- from a given deposit are likely minimum age lithologies that make up the local bedrock when tion may have already passed up the Escalante estimates. Using the oldest exposure age from the amount and size of the resistant lithology in and its northern tributaries. In this hypotheti- each surface, we estimate maximum tributary the sediment system varies (Sklar and Dietrich, cal scenario, the majority of the knickzone is incision rates of 151–323 m Ma–1. Our incision 2001; Johnson et al., 2009). still actively incising and likely hung up on the rates are the same order of magnitude as other (2) Exposure of the Navajo Sandstone Escalante River convexity (Cook et al., 2009). regional incision rate estimates determined over allowed the northern Escalante tributaries to tap (5) Regional-scale mantle or epeirogenic the same time interval. However, if our incision into a major source of water. The Navajo Sand- based uplift may have caused faster incision. rate estimates are not an overestimate, then the stone exposed along the northern tributaries has Studies of Colorado Plateau mantle dynamics northern tributaries of the Escalante River may relatively high porosity and hydraulic conduc- have shown that faster incision rates are often be incising faster than other regional sites over tivity, and as demonstrated by the incredible associated with areas of lower-velocity mantle the past 0.6–1.4 Ma. iron mineralization in the area, has had consid- (Karlstrom et al., 2008, 2011, 2012; Crow et al., erable fl uid fl ow in the past (e.g., Beitler et al., 2011). Since the upper Escalante drainage basin ACKNOWLEDGMENTS 2003; Loope et al., 2011). Several of the most is near a marked transition in mantle P-wave We thank Alan Rigby, Suzanne Bethers, Will deeply incised tributaries in the area start out as velocities (fi g. 1 in Crow et al., 2011), the poten- Gallin , and Elsie Denton for help with fi eldwork. The springs (e.g., Calf Creek), and most tributaries tially faster incision rates that we determine may 2010 Western State College of Colorado Research in gain signifi cant discharge from groundwater be attributable to a more buoyant mantle at our Quaternary Geology class made most of the boulder (Wilberg and Stolp, 2005). The Navajo Sand- site, which is near the western edge of the Colo- and pebble count measurements. Kip Solomon and Alan Rigby assisted with He analyses at the Uni- stone is also heavily fractured and jointed (e.g., rado Plateau. Pederson et al. (2007) argue that versity of Utah Dissolved and Noble Gas Labora- Weir and Beard, 1990a, 1990b), and many of the the deep exhumation in the central Canyonlands tory. Doug Powell of the Grand Staircase Escalante northern tributaries are strongly controlled by region over the past ~6 Ma has led to >1 km National Monument helped with fi eldwork, logistics, the joint or fracture trends. Extreme fracturing of epeirogenic uplift of the central Plateau. Our and securing funding. The Grand Staircase Escalante and jointing, like that seen in the Navajo near research site is near the western margin of that National Monument, University of Utah, Colgate Uni- versity, and Western State College of Colorado pro- the Escalante monocline, signifi cantly weakens uplift and may have higher than expected inci- vided funding. Conversations with Bob Webb, Cassie the overall rock unit, thus making fl uvial incision rates in the past 0.6–1.5 Ma because of it. Fenton, Brenda Beitler Bowen, and John Dohrenwend sion easier (Molnar et al., 2007). infl uenced our thoughts on the incredible landscapes (3) Increased discharge due to melting of the CONCLUSIONS of the Grand Staircase Escalante National Monument (GSENM). Two anonymous reviewers and an Asso- Boulder Mountain ice cap at the end of the last ciate Editor of the Colorado River evolution themed two glacial cycles may have impacted the inci- The gravel deposits located high above the issue provided very helpful reviews and comments. sion history of the Escalante River northern northern tributaries of the Escalante River tributaries during the late Quaternary. Boulder are composed of andesitic boulders that were REFERENCES CITED Mountain had an ice cap with outlet glaciers derived from the southern margins of Aquarius Anderson, R.A., Repka, J.L., and Dick, G.S., 1996, Explicit during the LGM; however, it is unclear if it had Plateau and Boulder Mountain. 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