THE ALLUVIAL RECORDS OF BUCKSKIN WASH, UTAH · JONATHAN E. HARVEY , JOEL L. PEDERSON , AND TAMMY M. RITTENOUR Department of Geology, Utah State University, Logan, UT [email protected]

INTRODUCTION

Dryland populations are often inherently intertwined with ephemeral streams that carry floodwaters from catchments to trunk drainages. For example, floodwater farming has sustained both prehistoric and historic populations in the southwestern U.S. (Bryan, 1929). Many mod-ern settlements continue to depend on seasonal streamflow for crop production and the grazing of livestock. This important connection of civiliza-tion to environment has motivated great interest in the paleohydrology of these streams. Paleohydro-logic investigations can reveal linkages between

past climate changes and stream response and shed light on how future climatic changes may af-fect these sensitive systems and the populations they support.

One of the best ways to understand the paleo-hydrology of a dryland stream is to study the allu-vial deposits preserved along it. Indeed, workers have utilized alluvial records in the southwest-ern U.S. for nearly a century (e.g. Bryan, 1925). Most commonly, workers examine and interpret the stratigraphy of valley-fill alluvium exposed in cutbanks along modern streams. These records often record cycles of cutting and filling over dec-ades to millennia (e.g. Bailey, 1935). However, in

THE ALLUVIAL RECORDS OFBUCKSKIN WASH, UTAH

Geology of South-Central Utah, Stephanie M. Carney, David E. Tabet, and Cari L. Johnson, editors,Utah Geological Association Publication 39, 2010.

JONATHAN E. HARVEY , JOEL L. PEDERSON , AND TAMMY M. RITTENOURDepartment of Geology, Utah State University, Logan, UT

[email protected]@usu.edu

[email protected]

ABSTRACT

Paleohydrologic records are important for the study of past, present, and future rela-tions among streams, climate, and humans in drylands. Alluvial deposits are often the best paleohydrologic record available. Two main approaches to studying dryland alluvial records are 1) the study of valley fills exposed along streams in broad alluvial valleys and 2) the study of slackwater paleoflood deposits in constricted bedrock canyons. Despite often be-ing demonstrated on different reaches of the same streams, these two approaches can lead to contrasting paleohydrologic interpretations. We reconcile these two approaches and record types in Buckskin Wash, an ephemeral stream in the Paria River basin of south-central Utah that features a broad alluvial reach draining into a constricted bedrock canyon. We report a new chronostratigraphy supported by detailed sedimentology and diverse geochronology. The alluvial-reach deposits preserve at least four cycles of arroyo cutting and filling since ~3 ka. The majority of slackwater flood deposits in the slot canyon appear to be correlated to historic arroyo cutting (~ A.D 1880 to A.D. 1910) in the alluvial reach upstream. We argue that constricted reach deposits do indeed relate to arroyo cutting upstream, but that they reflect a sedimentary, not hydrologic, signal. Large-scale transfer of sediment from alluvial valleys during arroyo cutting temporarily enhanced preservation of alluvial deposits in the bedrock canyon downstream via altered stage-discharge relationships. Thus the bulk of the slackwater deposits in Buckskin Gulch are a function of upstream geomorphic changes rather than simply a record of flood frequency and magnitude. This result has important implications for those workers who rely on similar slackwater deposits to extend the flood history of a stream.

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J.E. Harvey, J.L. Pederson, and T.M. Rittenour UGA 39

recent decades, a newer approach has emerged—paleoflood hydrology of bedrock canyons (Patton and others, 1979). There are important distinc-tions between these two approaches, and they often lead to contrasting interpretations regard-ing the history of a particular stream. Here we describe the important disparities between these two approaches to studying alluvial records and reconcile them in a single drainage in the western Colorado Plateau.

Arroyo Cutting and Filling Cycles

In the first approach, workers study the stratigraphy of valley-fill deposits exposed along streams running through broad (> 100-m-wide) alluvial valleys. In this setting, alluvial stratigra-phies often record cycles of aggradation and deg-radation throughout the Holocene (Hack, 1942; Haynes, 1968; Hall, 1977). The magnitude of these changes in streambed elevation can reach up to 30 m in any particular event and can occur over decadal to millennial timescales. The ages of past arroyo cutting and filling cycles have primarily been constrained with radiocarbon ages within aggradational packages and associations with cul-tural material of known age. The result of these studies is a series of stream-specific chronologies of arroyo cutting and filling in the southwestern U.S. For relatively recent reviews of arroyo cut-ting and filling cycles in the U.S. Southwest, see Cooke and Reeves (1976), Graf (1983), and Her-eford (2002).

Those particular arroyo-cutting or valley-fill-ing episodes that have been detected and correlat-ed in many streams across a region are, especially recently, interpreted as manifestations of climate changes (Knox, 1983; Karlstrom, 1988; Hereford, 2002). A range of hypotheses have been suggest-ed regarding the specific mechanisms that link climate change to stream behavior. One frequent-ly-cited hypothesis is that arroyos are cut during episodes of frequent, high-intensity flooding and filled during periods of relatively infrequent and/or low-magnitude flooding (Webb, 1985; Webb and others, 1991; Hereford, 2002). One way to test this hypothesis would be to compare a stream’s flood history to its cut-and-fill history.

Paleoflood Hydrology of Bedrock Canyons

The second approach, paleoflood hydrology, has emerged in only the last few decades. In this approach, workers study sequences of slackwater flood deposits in order to characterize the pre-in-strumental flood history of a stream (Patton and others, 1979; Kochel and Baker, 1982). Workers often estimate paleodischarges through a mode-ling exercise that requires estimation of the water surface profile during the flood and the cross-sec-tional geometry at the time of emplacement (Webb and Jarrett, 2002). In order to minimize the latter uncertainty, paleoflood hydrologists generally fo-cus their studies on slackwater deposits in bed-rock canyons where lateral channel boundaries are relatively stable. Importantly, it is commonly assumed in these settings that aggradation or deg-radation of the channel bed is negligible over the time period of interest.

The southwestern U.S. has been the epicenter of studies of this type, in part due to the abun-dance of bedrock canyons with preserved slack-water deposits. Like with arroyo cutting-and-filling cycles, the ages of particular flood deposits have been constrained mostly with radiocarbon dating. In some cases, additional age constraints have been provided through ring counts on buried trees, cultural material caught in flood deposits (e.g. post-settlement fence posts), and short-lived isotopes like post-bomb 137Cs (Ely and Webb, 1992). The first regional compilation of paleoflood studies was published by Ely (1997), who identi-fied several episodes of ‘clustering’ of large floods throughout the Holocene. These clusters were at-tributed to centennial- to millennial-scale changes in the frequency and magnitude of El Niño events. Such a connection could be very important with regard to how streams in the Southwest might ad-just to changing climate in the future and perhaps what climate cycles could have driven arroyo cut-ting and filling cycles throughout the Holocene.

Disparity Between Approaches

Both the study of arroyo cutting and filling cycles in broad alluvial valleys and the study of slackwater flood deposits in bedrock canyons have

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The Alluvial Records of Buckskin Wash, Utah

been demonstrated throughout the southwestern U.S. and drylands throughout the world. Though they are both focused on the interpretation of Holocene alluvial deposits, these approaches dif-fer in fundamental ways (Harvey, 2009).

One aspect of the disconnect between these two approaches lies in those reaches that are not clearly distinguishable as a ‘broad alluvial valley’ or a ‘constricted bedrock canyon’. In these reach-es, it may not be clear which approach is appro-priate. For example, O’Connor and others (1994) interpreted a stack of deposits along the Colorado River upstream of Grand Canyon as a series of slackwater paleoflood deposits, and used their landscape positions to reconstruct discharges of the floods that emplaced them. Just downstream, Hereford and others (1996) and Tainer (2010) in-terpreted alluvial deposits in similar landscape positions as terrace remnants from an episode of aggradation that occurred over a similar times-cale. This latter interpretation suggests that the deposits were not emplaced by an anomalously large flood, but that they are floodplain deposits from when the river was riding at a higher grade in the past. Hence, in this case, the two approach-es lead to fundamentally different paleohydrolog-ic interpretations.

A second component of the disconnect be-tween approaches is the unclear temporal relation between arroyo-fill and paleoflood deposits. If ar-royo cutting is driven by episodes of anomalous flooding as hypothesized by Webb (1985) and Hereford (2002), one might expect paleoflood dep-osition to be broadly anti-correlated with arroyo-filling. Hereford (2002) describes valley alluvia-tion from ~ A.D. 1400 to ~ A.D. 1880 followed by arroyo cutting from ~ A.D. 1880 to A.D. 1910 across the Colorado Plateau. Ely’s (1997) regional chronology of paleoflood deposits is binned into 200-yr intervals. Due to the very different resolu-tions of these regional records, direct comparison is not appropriate. We argue that this question can best be addressed by studying both records within a single stream, as demonstrated by Webb (1985).

Here we test the hypothesis that paleoflood slackwater deposits are broadly anticorrelated to arroyo-fill deposits in Buckskin Wash, an ephem-

eral stream featuring a broad alluvial valley that drains into a bedrock slot canyon. The two end-member reaches feature classic examples of both record types, both of which have been the sub-ject of previous research efforts demonstrating the two end-member approaches. We build upon these previous efforts with detailed stratigraphy, sedimentology, and a multi-pronged geochronol-ogy. This unprecedented temporal resolution and sedimentological detail allows comparison of the timing of paleoflood deposition to arroyo cutting and filling cycles upstream, as well as the proc-esses governing sediment storage and transfer in either setting.

STUDY AREA

Physiography

Buckskin Wash is a major tributary of the Paria River in south-central Utah (figure 1). It is composed of two major tributaries: Kitchen Corral Wash (DA = 987 km2) and Coyote Wash (DA = 267 km2). After crossing the Laramide-age East Kaibab Monocline, Kitchen Corral Wash becomes increasingly constricted between walls of the Jurassic Navajo Sandstone. Shortly after plunging into the slot canyon proper, Coyote Wash conflues from the west via the short Wire Pass slot canyon. The Buckskin slot, in the Paria Canyon-Vermillion Cliffs wilderness area, continues for ~16 km before meeting the Paria River (elevation ~1270 m). The Paria River goes on to meet the Colorado River just downstream of Glen Canyon Dam, where it provides a critical source of sand and silt to the sediment-starved Colorado River in Grand Canyon.

Kitchen Corral Wash heads in a series of steep gullies eroding into the Pink Cliffs at the southern end of Bryce Canyon National Park (ele-vation ~2800 m). It then drains the broad mesas of the Grey, White, Vermillion, and Chocolate cliffs of Grand Staircase-Escalante National Monument. Composed of Triassic to Eocene sedimentary rocks, these colorful ‘steps’ are dissected north-dipping plateaus with abrupt southern escarp-ments that give the area its namesake (Doelling and Davis, 1989). These lithologies provide abun-

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dant silt- to sand-sized sediment to the drainage. Where Kitchen Corral Wash dissects the Kaibab Uplift, harder Permian limestone and sandstones contribute large boulders to the channel. In the slot canyon downstream, these boulders form sig-nificant knickpoints where they are wedged be-tween the walls of the narrow sandstone canyon.

Climate

Climate in the watershed is semiarid, though there is considerable variation as a function of el-

evation. Weather stations at Lees Ferry, AZ (elev. 978 m), Kanab, UT (1494 m), and Bryce Canyon, UT (2413 m) span the elevational range of the wa-tershed and report mean annual temperatures of 62.9º F, 54.6º F, and 41.5º F, respectively. Mean annual precipitation values for the same sites are 168 mm, 381 mm, and 419 mm, respectively (Western Regional Climate Center, available at http://www.wrcc.dri.edu/). Most flood-producing precipitation can be categorized into three sea-sonal storm regimes: late winter mid-latitude

Figure 1. Terrain map of study area. Inset images show representative valley geometries.

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cyclones, summer monsoonal thunderstorms, and early fall dissipating tropical cyclones (Ely, 1992). Differences between the sites illustrate a strong orographic amplification of precipitation (figure 2). Vegetation varies accordingly, with a mixed conifer forest in the highlands grading into Pinyon/Juniper and sagebrush steppe in the low-lands and floodplains.

Cultural Legacy

This part of the Colorado Plateau has a rich cultural heritage. Artifacts related to the Virgin and Kayenta Anasazi occupation of the area are commonly preserved on bedrock benches above

the alluvial valleys in Kitchen Corral Wash and Coyote Wash (Euler and others, 1979). A handful of field studies related to prehistoric occupation of the area are the subject of the Grand Staircase-Escalante National Monument visitor’s center in Kanab, UT. A to-scale exhibit at this visitor’s center is a reproduction of an excavation that took place along Kitchen Corral Wash in the 1980s. This region of the Colorado Plateau was aban-doned by burgeoning Puebloan societies around A.D. 1200. The cause of this rapid disappear-ance of cultural activity is debated, though severe drought and prehistoric arroyo cutting have been suggested as contributing factors (Bryan, 1929;

Figure 2. Seasonal patterns of precipitation and temperature at three sites spanning the elevational gradient of the Buckskin watershed.

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Euler and others, 1979). This event is valuable for establishing alluvial chronologies, as surfaces containing Puebloan artifacts must be older than A.D. 1200.

Quaternary Deposits

Alluvial ReachThough the alluvial reach of the watershed is

quite extensive, we focus on the reach of Kitchen Corral Wash downstream of the Vermillion Cliffs. Here, the Holocene valley fill is inset tens of me-ters below gravelly terraces that likely date to the Pleistocene. These higher terraces are especially prominent where US Hwy 89 crosses Kitchen Corral Wash (figure 3). The alluvial valley is gen-erally several hundred meters wide in this study reach.

The modern wash is entrenched ~4 to 10 m below the relatively flat surface of the alluvial val-ley. This entrenchment took place between A.D. 1883 and 1910, an event referred to hereafter as historic arroyo cutting (Bailey, 1935; Webb, 1985). This event was manifested in many streams throughout the Southwest, providing support to the hypothesis that arroyo cutting and filling cy-cles are climate-driven.

Modern arroyo walls reveal beautiful expo-sures of valley-fill alluvium that can be traced along Kitchen Corral Wash for hundreds of meters. These exposures were first studied by Hereford (2002), who concluded that the historic arroyo cut-ting was preceded by ~500 years of aggradation of the “settlement alluvium”. This episode was man-ifested in several regional streams, and correlative deposits are known elsewhere in the Colorado Plateau as the Naha Alluvium (Hack, 1942) and post-Bonito alluvium (Hall, 1977). This alluvia-tion was itself preceded by an arroyo cutting event that occurred around A.D. 1200. Known as pre-historic arroyo cutting, it has also been described in other streams in the region (Hereford, 2002).

Inset into the arroyo is a 1- to 2-m-high floodplain deposited since ~ A.D. 1940 (Hereford, 1986; Graf and others, 1991). This floodplain lo-cally supports dense Tamarisk thickets and was last overtopped by a flood in August 2008. In many places the arroyo walls are actively slump-

ing into the wash and onto the surface of the inset floodplain.

Constricted Reach

Downstream of the East Kaibab Monocline, Buckskin Wash is increasingly constricted be-tween walls of Navajo sandstone and accommo-dation space is reduced accordingly. Between the Buckskin Gulch trailhead and the start of the slot canyon, valley walls range between ~15 and ~200 m wide (figure 4). A single, prominent terrace fills the valley bottom. The wash is entrenched 4 to 5 m below this terrace surface. An active, Tamarisk-supporting floodplain correlative to that found upstream lines the channel through this reach.

About 5.5 km downstream from the Buck-skin Gulch trailhead on House Rock Valley Rd, Buckskin Wash enters the first slot canyon reach. At this point, the character of the deposits along the wash changes considerably – significant allu-vial deposits are found only in backwater areas upstream of severe constrictions, alcoves in bed-rock walls, and at tributary confluences. These deposits usually reach a height of 10 to 12 m above the modern wash, and are actively slumping into the channel. The two upstream-most outcrops (BG-B and BG-C) were studied by Ely (1992) as part of her regional paleoflood chronology: one at the Coyote Wash/Buckskin Wash confluence, and another about 1.5 km upstream. These two sites were revisited as part of this study.

METHODS

We studied six outcrops: three in the alluvial reaches, two in the constricted reach, and one in a somewhat transitional reach between the two end-member reaches (figure 1). At each site, we identified, mapped onto photographic panels, and described the sedimentology of individual-event beds, soil horizons, and unconformities. Ten litho-facies were defined to assist in sedimentological descriptions, which could then be grouped into broader facies associations that relate suites of facies to particular depositional environments (Har-vey, 2009).

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Figure 3. Aerial view of Kitchen Corral Wash study reach. The Holocene alluvial valley is several hun-dred meters wide and inset tens of meters below gravelly Pleistocene strath and fill terraces. Base photo courtesy Utah Automated Geographic Reference Center (http://gis.utah.gov).

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Figure 4. Aerial view of Buckskin Gulch study reach. The stream enters from the north, entering the narrows of Buckskin Gulch just downstream of site BG-B. Coyote Wash enters from the west, entering a narrow canyon reach (Wire Pass) just before the confluence with Buckskin Gulch. Base photo courtesy Utah Automated Geographic Reference Center (http://gis.utah.gov).

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A diverse geochronology provided age con-trol. Optically-stimulated luminescence (OSL) ages were analyzed according to the single aliq-uot regenerative dose (SAR) protocol described by Murray and Wintle (2003). Accelerated Mass Spectrometry (AMS) Radiocarbon ages were con-verted to calendar years according to INTCAL04 (Reimer and others, 2004). Other age constraints were provided by association with cultural arti-facts; ring counts on buried trees; and detection of the short-lived isotope 137Cs, which marks the start of nuclear testing around A.D. 1950 (Ely and Webb, 1992). This multi-pronged geochronologi-cal approach provides the necessary resolution to understand the complex geomorphic relations be-tween the alluvial and constricted reaches of the drainage.

RESULTS

Kitchen Corral Wash Valley Fills

Stratigraphy and Sedimentology

Four alluvial packages bound by sharp un-conformities are present in Kitchen Corral Wash. The best exposure of these packages is found at study site KCW-A. Thus, the following de-scription is based on the stratigraphy exposed there. Site KCW-A is located on a nearly verti-cal west-facing cutbank that is actively eroding into the modern channel (figure 5). The arroyo in this reach is about 10 m deep and 30 m wide. A ~2-m-high floodplain occupies much of the arroyo bottom, pinning the channel against the cutbank. Package I is stratigraphically lowest. Package II overlies and almost overtops a steep paleobank that truncates most of package I. Similarly, pack-age III truncates and overtops package II by ~1.2 m. Package IV fills in a paleochannel that trun-cates the entirety of the outcrop and locally over-lies package III.

Packages I to III contain a similar sequence of depositional units. The lower portions of each are dominated by massive to imbricated, ma-trix- to clast-supported gravels interbedded with trough-crossbedded coarse sand. These facies record high flow velocities and are related to dep-

osition in a channel-bottom (CB) environment. Irregular upper and lower contacts and lenticular bed geometries suggest frequent scour and refill-ing, consistent with deposition on the channel bot-tom. These deposits are generally overlain by a thick sequence of tabular, laminated medium to coarse sand beds. The upper surfaces of these beds become more bioturbated upward, with multiple buried soils toward the top of each pack-age. Composing the majority of packages I to III, these deposits are part of the channel margin (CM) facies association. A modern analog for this depositional environment is the broad, vegetated floodplain that is beginning to fill the modern ar-royo bottom. These CM deposits are overlain by thin caps of bioturbated thin-bedded silts and fine sands. These beds are associated with deposition on the valley surface (VS), where slopewash and eolian inflation and deflation produce a complex suite of deposits that is heavily rooted and bur-rowed. The modern analog for this environment is the broad, sagebrush-covered valley surface that is dissected by the arroyo system.

Package IV is clearly different in appear-ance. The dominance of the reddish brown silty fine to medium sand suggests a more local tribu-tary draining the Vermillion Cliffs. Directly ad-jacent to the unconformity between packages III and IV is a sequence of channel-shaped deposits that range from massive pebbly gravel to massive silty sand. Occasional lenses of yellowish-brown sand are interbedded within the dominantly red-dish brown package. These channel-shaped de-posits transition laterally into a series of tabular, laminated fine to medium sands. This pattern continues up-section to the top of the outcrop.

Packages I to III record three cycles of arroyo cutting followed by progressive filling and over-topping of paleoarroyos. Though the CB deposits are only preserved in the lower portions of each package, the CM deposits clearly record aggrada-tion of the arroyo bottom system. The VS deposits represent hundreds of years of deposition on the valley surface during this period of entrenchment and slow aggradation. Package IV records the infilling of a tributary arroyo. Most of the units in Package IV record flow events in the tributary

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itself, whereas those yellowish-brown sandy inter-beds record the inundation of the tributary arroyo by mainstem flood events. The vertically-stacked geometry of the package suggests that there was relatively little lateral channel migration during aggradation, as opposed to the mainstem arroyo-fills in packages I to III. There are many mod-ern analogs for this tributary arroyo system in the landscape. Since these tributary arroyo systems are graded to the mainstem, the presence of chan-nel gravels ~7 m above the modern wash is clear evidence that Kitchen Corral Wash experienced aggradation of a similar scale during the deposi-tion of package IV.

Geochronology

A sample of detrital charcoal from near the base of package I yields an age of 2340 to 2130 cal yr B.P., while an OSL sample 1 m above it gives an age of ~2.5 to 2.8 ka (tables 1 and 2). Another OSL sample at the base of package II gives an age of ~0.9 to 2.3 ka. Potsherds found eroding from the buried surface at the top of package II suggest that it was occupied during Pueblo II time, ~1.2

to 0.8 ka. A radiocarbon sample of detrital twigs from the base of package IV yields an age of 660 to 540 cal yr B.P.

Buckskin Gulch Slackwater Deposits

Stratigraphy and Sedimentology

Site BG-B is located at the head of a steeper channel reach where the walls of Navajo Sand-stone converge to constrict the channel into a slot canyon (figure 4). Just upstream of the site, the stream flows through an initial, shorter bedrock notch where undulating walls are 6 to 10 m apart. The studied outcrop is located on the west face of a 10- to 12-m-high fill terrace in an expansion be-tween this initial slot and the first severe, continu-ous constriction of Buckskin Gulch ~100 m down-stream. The terrace surface is roughly concordant with the upward broadening of the constricting walls downstream, suggesting that its height is re-lated to a upward limit of the backwater effect of the constriction (Ely, 1992). Correlative deposits drape the bedrock topography throughout the ex-pansion.

Site LabNumber Depth (m)

# aliquots accepted(analyzed)

Equivalent Dose (Gy)

Rd(Gy/kyr)

Age ± σ(ka)

Age Range (ka) Position

KCW-A1 USU 530 8.0 30 (45) 2.43 ± 1.07 1.56 ± 0.07 1.56 ± 0.69 0.9 – 2.3 Base of II

KCW-A1 USU 531 4.0 34 (41) 5.93 ± 0.15 2.29 ± 0.10 2.64 ± 0.18 2.5 – 2.8 Middle of I

BG-B2 USU 523 6.0 15 - 1.73 ± 0.08 - - Middle of II

BG-B2 USU 522 9.0 15 - 1.76 ± 0.08 - - Base of II

BG-B1 USU 521 5.5 24 (35) 3.87 ± 0.71 2.29 ± 0.10 1.69 ± 0.33 1.4 – 2.0 Middle of I 1age calculated using minimum age model (Galbraith and others, 1999) using Excel spreadsheet created by Sebastian Hoot. 2sample was poorly bleached and was determined to be unsuitable for OSL analysis

Site Sample Number Lab Number Depth

(m) Material 14C Age (yr

BP) Calibrated 2σ age range (yr

BP)1 Position

KCW-A RCKCW2 Beta - 256838 6.8 twigs 610 ± 40 540 - 660 Middle of IV

KCW-A RCKCW4 Beta - 256840 6.3 charcoal 2220 ± 40 2130 - 2340 Middle of I

BG-B RCBG3 Beta - 256834 3.5 tree litter 150 ± 40 0 - 290 > Base of II

BG-B RCBG6 Beta - 256836 5.5 charcoal 1250 ± 40 1070 - 1280 Middle of I

BG-B RCBG5 Beta - 256835 8.2 charcoal 1780 ± 40 1600 - 1820 Base of I 1radiocarbon ages calibrated using INTCAL04 (Reimer and others, 2004)

Table 1. Summary of optically-stimulated luminescence (OSL) ages from this study.

Table 2. Summary of AMS-radiocarbon ages from this study.

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Five stratigraphic packages are present, sepa-rated by buttress unconformities (figure 6). Pack-age I contains 15 depositional units and forms the core of the deposit. Package II is inset into and overtops package I by ~2 m and contains 20 units. Package III consists of 6 units and nearly overtops package II. Packages IV and V continue the pattern of filling in void space above down-stream-dipping unconformities with 9 and 6 units, respectively, though they are much smaller. The deposits in the fill terrace here were first studied by Ely (1992), who gave a brief description and provided some radiocarbon age control.

Sedimentologically, package I is a series of medium to thick beds of laminated, silty medium to coarse sand. Beds are mostly tabular and lat-erally continuous. The sandy texture of these facies records deposition by high-energy events, though their tabular geometry does not support a channel-bottom setting. They probably represent a channel-margin environment, preserved here on the inside of a ninety-degree bend in the chan-nel. Three significant hiatuses are preserved as weakly-developed entisols or heavily bioturbated horizons within this package. The first is about 3 m from the base of package I. Weak A and Btk horizons are present, as well as abundant rhizo-liths, burrows, and root casts. The second is a bio-turbated zone atop an irregularly-shaped deposit that appears to be an eolian wedge deposited over the first. It is less developed and preserved than the first, yet features a reddish-brown stain and infilled root traces and burrows. The third, best-developed soil is found along the upper surface of the 2 m-thick unit of massive, medium to coarse sand capping package I. This represents exposure for a relatively significant period of time, as it is heavily bioturbated, contains abundant rhizoliths, and has a strong reddish-brown stain, likely a re-sult of incorporation of slopewash from nearby bedrock hillslopes into the unit via infiltration and translocation processes. Correlation by stratig-raphy and landscape position across the wash to other exposures indicates that this marker surface had numerous junipers germinated on it that are now partially buried (figure 7). Two of these bur-ied trees are still living and are the target of tree-

ring counts described below. Thus, package I is a complex sequence of deposition that may have been interrupted by longer periods of nondeposi-tion.

Package II overlies a wedge of sandy hills-lope colluvium along the unconformity that trun-cates package I. Its basal units are four medium beds of well-cemented, faintly-laminated to mas-sive, yellowish-brown sands that pinch out against the bounding surface between I and II. These are overlain by a voluminous, 2-m-thick bed of yel-lowish-brown sand with ripple cross-bedding and floating pebbles and granules that fills much of the void left by the erosion of Package I. A series of 14 downstream-thickening beds of reddish brown and light yellowish brown laminated to ripple-cross-bedded sand compose the upper 4 m of the deposit, overtopping package I and burying the junipers that had germinated on its surface. Con-tacts between all units are smooth and free of bio-turbation, indicating that the entire package was deposited rather quickly and that successive units were passively laid over existing deposits.

We note here that package II is the most vol-uminous of packages preserved at this particular expansion. Additionally, a correlative package is present at several expansions farther downstream. Most notably, site BG-C, which is located at the confluence of Wire Pass and Buckskin Gulch (fig-ure 1), contains several exposures of this same package. Hence, it appears to be the most domi-nant series of slackwater deposits in the Buckskin Gulch slot canyon.

Packages III-V are similar to package II, each consisting of a series of medium to thick beds of laminated to ripple crossbedded, silty medium to coarse sands. Again, no bioturbated horizons or buried soils are present, suggesting rapid em-placement. No channel-bottom deposits are pre-served in the studied outcrop. Most units in each package feature a reverse then normal grading, probably recording deposition during both the ris-ing and falling limb of flood events in a channel-margin setting. We interpret this environment as an eddy that formed downstream of the existing terrace, which is supported by variable paleocur-rents in package II.

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Figure 7. Buried juniper trees at study site BG-B. Living trees on top left and bottom were cored, whereas dead tree on top right could not be.

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Geochronology

A diverse geochronology at this site provides relatively detailed age control, especially for the younger portion of the record. A sample of detri-tal charcoal from just below the lowest paleosol in package I yields an age of 1820 to 1600 cal yr B.P. A similar sample taken six units above this gives an age of 1280 to 1070 cal yr B.P. An OSL sample taken between the two returns an age of ~1.4 to 2.0 ka, consistent with the radiocarbon results. A radiocarbon sample of tree litter found on the bur-ied hillslope between packages I and II constrains the age of package II to less than 290 cal yr B.P. Additional OSL samples were taken from the base and middle of package II. Though the samples clearly date to the late Holocene ages, initial re-sults revealed that they were poorly bleached dur-ing transport and therefore return unreliably older ages (table 1). Our radiocarbon samples and those from Ely (1992) consistently argue that package II is younger than 290 cal yr B.P. A living ju-niper tree buried by the uppermost 4 to 5 units of package II is exposed near the head of a gully on stream right (figure 7). A tree-ring count on a core collected ~2 to 3 m above the tree’s germina-tion horizon places a minimum age of ~85 years on the tree, a maximum age for overlying units. A second living, buried Juniper on top of package II yields a tree-ring count of ~110 years. These data suggest that the upper portion of package II and the whole of packages III, IV, and V were, con-servatively, deposited after A.D. 1850.

Finally, five units at site BG-B were analyzed for the presence of post-bomb 137Cs (figure 6). No 137Cs was detected in packages II or III. A mini-mal amount (0.0092 ± 5% cps) was detected in the middle of package IV, whereas a significant amount (0.053 ± 5% cps) was detected in a unit near the top of package IV. This suggests that bomb testing occurred sometime near the end of deposition of package IV. Hence, all stratigraphi-cally older units were deposited before around A.D. 1950. These data, combined with the tree-ring data, argue that packages II, III, and IV were rapidly deposited between ~150 and 50 years ago. This conclusion is supported by the presence of deep, unfilled gullies bisecting the terraces here

and at site BG-C.

DISCUSSION

Interpretations of Channel Change

The sedimentology and stratigraphy of the alluvial deposits in the constricted and alluvial reaches of the watershed provide an interesting perspective on the processes of deposition and erosion that operate in each setting. In the allu-vial reach, channel-bottom and channel-margin deposits are preserved up to 9 m above the mod-ern channel and record past episodes of aggrada-tion of the streambed during the filling of paleoar-royos. The best record of such an aggradational episode can be seen in package IV as the progres-sive filling of an approximately stationary tribu-tary channel with interfingered channel-bottom and channel-margin deposits. Thus, the majority of the alluvial deposits in Kitchen Corral Wash are the result of aggradation and (sometimes) lat-eral migration of the channel bed. In contrast, the constricted reach deposits preserve no distinct evidence of channel aggradation.

Timescales of Deposition

The abundance of bioturbated units and soil horizons in the alluvial reach deposits suggest that arroyo filling is a long, slow process involving in-cremental lateral and vertical accretion of channel margins within the arroyo. This several-hundred year period of entrenchment is enough for soil formation on the abandoned valley surface. In contrast, we know from historical evidence (e.g. Bryan, 1925; Webb, 1985) that the most recent ar-royo cutting took only one or two decades. Thus, the arroyo cutting and filling cycles in the alluvial reach of the drainage can be characterized by cen-tennial-scale episodes of aggradation punctuated by decadal-scale episodes of incision.

In contrast, in the constricted reach, the allu-vial sequence appears to have been emplaced rap-idly, especially in the more voluminous packages II-V where smooth contacts between beds and the absence of bioturbation suggests that very little time passed between preserved events. At site

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BG-B, the buried soil atop package I and support-ing geochronology indicate that there was ~900 years of nondeposition between the deposition of Packages I and II. Following this extended depo-sitional hiatus, the 41 flood deposits of Packages II-V were emplaced within a period of <100 years. The temporal pattern of decadal-scale episodes of rapid deposition separated by centennial-scale hiatuses is nearly opposite to that of the alluvial reach (figure 8).

Paleohydrologic Interpretations

Under the standard paradigm of paleoflood hydrology, one might interpret the deposits at site BG-B as recording a cluster of 15 large floods be-tween ~2 and 1 ka followed by a ~900-year ab-sence of flooding between ~1 ka and 0.15 ka, and then 41 floods in rapid succession between 0.15 ka and 0.05 ka. This interpretation is very difficult to explain hydroclimatically, and such a drastic shift in hydrology has not been described in any regional record. Thus, we argue that the slackwa-ter deposits at site BG-B do not simply record the paleoflood history of the stream. Rather, we argue

that they are a function of upstream geomorphic changes: specifically, they record pulses of sedi-mentation associated with the large-scale excava-tion of valley fills during arroyo cutting upstream.

Our alternative interpretation is supported by different lines of evidence. Geochronology con-servatively constrains the deposition of the bulk of the deposits in the constricted reach to between ~ A.D. 1850 and A.D. 1950, while incision of the alluvial valleys upstream took place between ~A.D. 1880 and A.D. 1910. Enormous amounts of sediment were carried downstream during arroyo cutting (Webb and others, 1991). Once initiated, arroyo headcuts likely migrated upstream during every flow event, with each migration contribut-ing a new pulse of sedimentation. This extraor-dinary sediment loading could have repeatedly overwhelmed the backwaters in the Buckskin slot canyon, causing temporary aggradation or at least the local deposition of great amounts of sediment in channel margins. Because of the small width-to-depth ratio of the slot canyon, the temporary storage of sediment along channel margins would have altered the local stage-discharge relationship, serving to rapidly preserve a series of floods that

Figure 8. Summarized chronostratigraphy of the greater Buckskin drainage. A) Alluvial reaches, B) Constricted reach. Roman numerals signify relative sequence of packages in either reach.

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may not have been particularly large. Thus, we argue that the slot canyon deposits are not simply a record of paleoflooding, but also of geomorphic changes upstream.

The results of this study suggest that slack-water flood deposits in areas that may be subject to the extraordinary sediment loading during inci-sion of alluvial valleys upstream may be unsuita-ble hydrologic records. If such deposits are inter-preted as accurate records of paleoflood frequency and magnitude, they might lead to an incorrect interpretation that there were very many anoma-lously large flood events during arroyo cutting and an extreme dearth of floods between arroyo cutting events. It is possible that this phenomenon has been overlooked in previous paleoflood stud-ies.

The influence of temporary sediment stor-age on paleoflood slackwater deposits can be minimized. If possible, one should avoid work-ing downstream of broad, alluvial reaches. Oth-erwise, one should avoid backwaters upstream of tight constrictions and instead use only slackwater sequences in alcoves or areas less subject to pond-ing of water during floods. In any case, results should be interpreted cautiously, and attention should be directed to the sensitivity of the channel cross section to episodes of sediment storage.

Our interpretations would be strengthened if the older package in the constricted reach could be linked to earlier episodes of arroyo cutting up-stream. At this point, the ages of arroyo cutting events preceding A.D. 1200 are imprecise, as are the ages of the deposits in package I of the con-stricted reach. It would also be beneficial to per-form similar studies in analogous settings in the southwestern U.S. Do slackwater deposits pre-served in bedrock canyons downstream of a broad alluvial reach with an arroyo always record the arroyo-cutting signal, or is the case of Buckskin Wash an anomaly?

CONCLUSIONS

1) Through careful analysis of stratigra-phy and sedimentology at several study sites, we have confirmed that deposi-

tional processes are distinct between the alluvial and constricted reaches of Buck-skin Wash, and that the alluvial deposits stored in either reach are fundamentally different paleohydrologic records.

2) Deposition across the two end-mem-ber reach types has been broadly anti-correlated during the late Holocene. There have been at least four cycles of arroyo cutting and filling in Kitchen Cor-ral Wash since ~3.0 ka. The youngest arroyo-fill package was deposited from ~0.7 ka to 0.15 ka and was incised dur-ing historic arroyo cutting between ~ A.D. 1880 and A.D. 1910. The slack-water deposits in the Buckskin Gulch slot canyon were deposited in at least two phases: one from ~2.0 to 1.0 ka and another, conservatively, between ~ A.D. 1850 and A.D. 1950.

3) The majority of the deposits in the Buckskin Gulch slot canyon record an episode of enhanced preservation during arroyo cutting upstream. Extraordinary sediment loads associated with upstream migration of arroyo headcuts during suc-cessive floods led to temporary sediment storage and changed stage-discharge re-lations in the canyon, serving to rapidly emplace and preserve dozens of floods of uncertain discharge. Thus, the slackwa-ter deposits at the studied sites in Buck-skin Gulch are not a reliable record of paleoflood frequency and discharge. It is possible that other published paleoflood records may have been affected by this signal from upstream arroyo cutting and should be re-visited.

AKNOWLEDGMENTS

This study was related to the first author’s Masters Thesis at Utah State University. It was supported by a research grant from the Geological Society of America, the Arthur D. Howard Award from the QGG Division of GSA, and a scholar-ship from the Utah State University Department

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of Geology. Support for OSL and radiocarbon age determination provided by the USU Lumines-cence Laboratory.

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Documents

THE ALLUVIAL RECORDS OF BUCKSKIN WASH, UTAH · JONATHAN E. HARVEY , JOEL L. PEDERSON , AND TAMMY M. RITTENOUR Department of Geology, Utah State University, Logan, UT [email protected]