THE ALLUVIAL RECORDS OF BUCKSKIN WASH, JONATHAN E. HARVEY , JOEL L. PEDERSON , AND TAMMY M. RITTENOUR Department of Geology, Utah State University, Logan, UT [email protected] [email protected] [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 . 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 basin of south-central Utah that features a broad alluvial reach draining into a constricted bedrock . 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 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 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.

INTRODUCTION past climate changes and stream response and shed light on how future climatic changes may af- Dryland populations are often inherently fect these sensitive systems and the populations intertwined with ephemeral streams that carry they support. floodwaters from catchments to trunk drainages. One of the best ways to understand the paleo- For example, floodwater farming has sustained hydrology of a dryland stream is to study the allu- both prehistoric and historic populations in the vial deposits preserved along it. Indeed, workers southwestern U.S. (Bryan, 1929). Many mod- have utilized alluvial records in the southwest- ern settlements continue to depend on seasonal ern U.S. for nearly a century (e.g. Bryan, 1925). streamflow for crop production and the grazing of Most commonly, workers examine and interpret livestock. This important connection of civiliza- the stratigraphy of valley-fill alluvium exposed in tion to environment has motivated great interest in cutbanks along modern streams. These records the paleohydrology of these streams. Paleohydro- often record cycles of cutting and filling over dec- logic investigations can reveal linkages between ades to millennia (e.g. Bailey, 1935). However, in

Geology of South-Central Utah, Stephanie M. Carney, David E. Tabet, and Cari L. Johnson, editors, Utah Geological Association Publication 39, 2010. J.E. Harvey, J.L. Pederson, and T.M. Rittenour UGA 39 recent decades, a newer approach has emerged— Paleoflood Hydrology of Bedrock Canyons paleoflood hydrology of bedrock canyons (Patton and others, 1979). There are important distinc- The second approach, paleoflood hydrology, tions between these two approaches, and they has emerged in only the last few decades. In this often lead to contrasting interpretations regard- approach, workers study sequences of slackwater ing the history of a particular stream. Here we flood deposits in order to characterize the pre-in- describe the important disparities between these strumental flood history of a stream (Patton and two approaches to studying alluvial records and others, 1979; Kochel and Baker, 1982). Workers reconcile them in a single drainage in the western often estimate paleodischarges through a mode- . ling exercise that requires estimation of the water surface profile during the flood and the cross-sec- Arroyo Cutting and Filling Cycles tional geometry at the time of emplacement (Webb and Jarrett, 2002). In order to minimize the latter In the first approach, workers study the uncertainty, paleoflood hydrologists generally fo- stratigraphy of valley-fill deposits exposed along cus their studies on slackwater deposits in bed- streams running through broad (> 100-m-wide) rock canyons where lateral channel boundaries alluvial valleys. In this setting, alluvial stratigra- are relatively stable. Importantly, it is commonly phies often record cycles of aggradation and deg- assumed in these settings that aggradation or deg- radation throughout the Holocene (Hack, 1942; radation of the channel bed is negligible over the Haynes, 1968; Hall, 1977). The magnitude of time period of interest. these changes in streambed elevation can reach up The southwestern U.S. has been the epicenter to 30 m in any particular event and can occur over of studies of this type, in part due to the abun- decadal to millennial timescales. The ages of past dance of bedrock canyons with preserved slack- arroyo cutting and filling cycles have primarily water deposits. Like with arroyo cutting-and- been constrained with radiocarbon ages within filling cycles, the ages of particular flood deposits aggradational packages and associations with cul- have been constrained mostly with radiocarbon tural material of known age. The result of these dating. In some cases, additional age constraints studies is a series of stream-specific chronologies have been provided through ring counts on buried of arroyo cutting and filling in the southwestern trees, cultural material caught in flood deposits U.S. For relatively recent reviews of arroyo cut- (e.g. post-settlement fence posts), and short-lived ting and filling cycles in the U.S. Southwest, see isotopes like post-bomb 137Cs (Ely and Webb, Cooke and Reeves (1976), Graf (1983), and Her- 1992). The first regional compilation of paleoflood eford (2002). studies was published by Ely (1997), who identi- Those particular arroyo-cutting or valley-fill- fied several episodes of ‘clustering’ of large floods ing episodes that have been detected and correlat- throughout the Holocene. These clusters were at- ed in many streams across a region are, especially tributed to centennial- to millennial-scale changes recently, interpreted as manifestations of climate in the frequency and magnitude of El Niño events. changes (Knox, 1983; Karlstrom, 1988; Hereford, Such a connection could be very important with 2002). A range of hypotheses have been suggest- regard to how streams in the Southwest might ad- ed regarding the specific mechanisms that link climate change to stream behavior. One frequent- just to changing climate in the future and perhaps ly-cited hypothesis is that arroyos are cut during what climate cycles could have driven arroyo cut- episodes of frequent, high-intensity flooding and ting and filling cycles throughout the Holocene. filled during periods of relatively infrequent and/ Disparity Between Approaches or low-magnitude flooding (Webb, 1985; Webb and others, 1991; Hereford, 2002). One way to test Both the study of arroyo cutting and filling this hypothesis would be to compare a stream’s cycles in broad alluvial valleys and the study of flood history to its cut-and-fill history. slackwater flood deposits in bedrock canyons have

20 The Alluvial Records of Buckskin Wash, Utah been demonstrated throughout the southwestern eral stream featuring a broad alluvial valley that U.S. and drylands throughout the world. Though drains into a bedrock slot canyon. The two end- they are both focused on the interpretation of member reaches feature classic examples of both Holocene alluvial deposits, these approaches dif- record types, both of which have been the sub- fer in fundamental ways (Harvey, 2009). ject of previous research efforts demonstrating One aspect of the disconnect between these the two end-member approaches. We build upon two approaches lies in those reaches that are not these previous efforts with detailed stratigraphy, clearly distinguishable as a ‘broad alluvial valley’ sedimentology, and a multi-pronged geochronol- or a ‘constricted bedrock canyon’. In these reach- ogy. This unprecedented temporal resolution and es, it may not be clear which approach is appro- sedimentological detail allows comparison of the priate. For example, O’Connor and others (1994) timing of paleoflood deposition to arroyo cutting interpreted a stack of deposits along the Colorado and filling cycles upstream, as well as the proc- River upstream of Grand Canyon as a series of esses governing sediment storage and transfer in slackwater paleoflood deposits, and used their either setting. landscape positions to reconstruct discharges of the floods that emplaced them. Just downstream, STUDY AREA Hereford and others (1996) and Tainer (2010) in- terpreted alluvial deposits in similar landscape Physiography positions as terrace remnants from an episode of Buckskin Wash is a major tributary of the aggradation that occurred over a similar times- Paria River in south-central Utah (figure 1). It cale. This latter interpretation suggests that the is composed of two major tributaries: Kitchen deposits were not emplaced by an anomalously Corral Wash (DA = 987 km2) and Coyote Wash large flood, but that they are floodplain deposits (DA = 267 km2). After crossing the Laramide- from when the river was riding at a higher grade age East Kaibab Monocline, Kitchen Corral Wash in the past. Hence, in this case, the two approach- becomes increasingly constricted between walls es lead to fundamentally different paleohydrolog- of the Jurassic Navajo . Shortly after ic interpretations. plunging into the slot canyon proper, Coyote Wash A second component of the disconnect be- conflues from the west via the short Wire Pass slot tween approaches is the unclear temporal relation canyon. The Buckskin slot, in the Paria Canyon- between arroyo-fill and paleoflood deposits. If -ar Vermillion Cliffs wilderness area, continues for royo cutting is driven by episodes of anomalous ~16 km before meeting the Paria River (elevation flooding as hypothesized by Webb (1985) and ~1270 m). The Paria River goes on to meet the Hereford (2002), one might expect paleoflood dep- just downstream of Glen Canyon osition to be broadly anti-correlated with arroyo- Dam, where it provides a critical source of sand filling. Hereford (2002) describes valley alluvia- and silt to the sediment-starved Colorado River in tion from ~ A.D. 1400 to ~ A.D. 1880 followed Grand Canyon. by arroyo cutting from ~ A.D. 1880 to A.D. 1910 Kitchen Corral Wash heads in a series of across the Colorado Plateau. Ely’s (1997) regional steep gullies eroding into the Pink Cliffs at the chronology of paleoflood deposits is binned into southern end of Bryce Canyon National Park (ele- 200-yr intervals. Due to the very different resolu- vation ~2800 m). It then drains the broad mesas of tions of these regional records, direct comparison the Grey, White, Vermillion, and Chocolate cliffs is not appropriate. We argue that this question can of Grand Staircase-Escalante National Monument. best be addressed by studying both records within Composed of Triassic to Eocene sedimentary a single stream, as demonstrated by Webb (1985). rocks, these colorful ‘steps’ are dissected north- Here we test the hypothesis that paleoflood dipping plateaus with abrupt southern escarp- slackwater deposits are broadly anticorrelated to ments that give the area its namesake (Doelling arroyo-fill deposits in Buckskin Wash, an ephem- and Davis, 1989). These lithologies provide abun-

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

Figure 1. Terrain map of study area. Inset images show representative valley geometries. dant silt- to sand-sized sediment to the drainage. evation. Weather stations at Lees Ferry, AZ (elev. Where Kitchen Corral Wash dissects the Kaibab 978 m), Kanab, UT (1494 m), and Bryce Canyon, Uplift, harder Permian limestone and UT (2413 m) span the elevational range of the wa- contribute large boulders to the channel. In the tershed and report mean annual temperatures of slot canyon downstream, these boulders form sig- 62.9º F, 54.6º F, and 41.5º F, respectively. Mean nificant knickpoints where they are wedged be- annual precipitation values for the same sites tween the walls of the narrow sandstone canyon. are 168 mm, 381 mm, and 419 mm, respectively (Western Regional Climate Center, available at Climate http://www.wrcc.dri.edu/). Most flood-producing Climate in the watershed is semiarid, though precipitation can be categorized into three sea- there is considerable variation as a function of el- sonal storm regimes: late winter mid-latitude

22 The Alluvial Records of Buckskin Wash, Utah cyclones, summer monsoonal thunderstorms, the alluvial valleys in Kitchen Corral Wash and and early fall dissipating tropical cyclones (Ely, Coyote Wash (Euler and others, 1979). A handful 1992). Differences between the sites illustrate a of field studies related to prehistoric occupation of strong orographic amplification of precipitation the area are the subject of the Grand Staircase- (figure 2). Vegetation varies accordingly, with a Escalante National Monument visitor’s center in mixed conifer forest in the highlands grading into Kanab, UT. A to-scale exhibit at this visitor’s Pinyon/Juniper and sagebrush steppe in the low- center is a reproduction of an excavation that took lands and floodplains. place along Kitchen Corral Wash in the 1980s. This region of the Colorado Plateau was aban- Cultural Legacy doned by burgeoning Puebloan societies around This part of the Colorado Plateau has a rich A.D. 1200. The cause of this rapid disappear- cultural heritage. Artifacts related to the Virgin ance of cultural activity is debated, though severe and Kayenta Anasazi occupation of the area are drought and prehistoric arroyo cutting have been commonly preserved on bedrock benches above 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.

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

Euler and others, 1979). This event is valuable ing into the wash and onto the surface of the inset for establishing alluvial chronologies, as surfaces floodplain. containing Puebloan artifacts must be older than A.D. 1200. Constricted Reach Quaternary Deposits Downstream of the East Kaibab Monocline, Buckskin Wash is increasingly constricted be- Alluvial Reach tween walls of Navajo sandstone and accommo- Though the alluvial reach of the watershed is dation space is reduced accordingly. Between the quite extensive, we focus on the reach of Kitchen Buckskin Gulch trailhead and the start of the slot Corral Wash downstream of the Vermillion Cliffs. canyon, valley walls range between ~15 and ~200 Here, the Holocene valley fill is inset tens of me- m wide (figure 4). A single, prominent terrace ters below gravelly terraces that likely date to the fills the valley bottom. The wash is entrenched Pleistocene. These higher terraces are especially 4 to 5 m below this terrace surface. An active, prominent where US Hwy 89 crosses Kitchen Tamarisk-supporting floodplain correlative to that Corral Wash (figure 3). The alluvial valley is gen- found upstream lines the channel through this erally several hundred meters wide in this study reach. reach. About 5.5 km downstream from the Buck- The modern wash is entrenched ~4 to 10 m skin Gulch trailhead on House Rock Valley Rd, below the relatively flat surface of the alluvial val- Buckskin Wash enters the first slot canyon reach. ley. This entrenchment took place between A.D. At this point, the character of the deposits along 1883 and 1910, an event referred to hereafter as the wash changes considerably – significant allu- historic arroyo cutting (Bailey, 1935; Webb, 1985). vial deposits are found only in backwater areas This event was manifested in many streams upstream of severe constrictions, alcoves in bed- throughout the Southwest, providing support to rock walls, and at tributary confluences. These the hypothesis that arroyo cutting and filling cy- deposits usually reach a height of 10 to 12 m above cles are climate-driven. the modern wash, and are actively slumping into Modern arroyo walls reveal beautiful expo- the channel. The two upstream-most outcrops sures of valley-fill alluvium that can be traced (BG-B and BG-C) were studied by Ely (1992) as along Kitchen Corral Wash for hundreds of meters. part of her regional paleoflood chronology: one at These exposures were first studied by Hereford the Coyote Wash/Buckskin Wash confluence, and (2002), who concluded that the historic arroyo cut- another about 1.5 km upstream. These two sites ting was preceded by ~500 years of aggradation of were revisited as part of this study. the “settlement alluvium”. This episode was man- ifested in several regional streams, and correlative METHODS deposits are known elsewhere in the Colorado Plateau as the Naha Alluvium (Hack, 1942) and We studied six outcrops: three in the alluvial post-Bonito alluvium (Hall, 1977). This alluvia- reaches, two in the constricted reach, and one in tion was itself preceded by an arroyo cutting event a somewhat transitional reach between the two that occurred around A.D. 1200. Known as pre- end-member reaches (figure 1). At each site, we historic arroyo cutting, it has also been described identified, mapped onto photographic panels, and in other streams in the region (Hereford, 2002). described the sedimentology of individual-event Inset into the arroyo is a 1- to 2-m-high beds, soil horizons, and unconformities. Ten litho- floodplain deposited since ~ A.D. 1940 (Hereford, facies were defined to assist in sedimentological 1986; Graf and others, 1991). This floodplain lo- descriptions, which could then be grouped into cally supports dense Tamarisk thickets and was broader facies associations that relate suites of last overtopped by a flood in August 2008. In facies to particular depositional environments (Har- many places the arroyo walls are actively slump- vey, 2009).

24 The Alluvial Records of Buckskin Wash, Utah

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).

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

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).

26 The Alluvial Records of Buckskin Wash, Utah

A diverse geochronology provided age con- osition in a channel-bottom (CB) environment. trol. Optically-stimulated luminescence (OSL) Irregular upper and lower contacts and lenticular ages were analyzed according to the single aliq- bed geometries suggest frequent scour and refill- uot regenerative dose (SAR) protocol described ing, consistent with deposition on the channel bot- by Murray and Wintle (2003). Accelerated Mass tom. These deposits are generally overlain by a Spectrometry (AMS) Radiocarbon ages were con- thick sequence of tabular, laminated medium to verted to calendar years according to INTCAL04 coarse sand beds. The upper surfaces of these (Reimer and others, 2004). Other age constraints beds become more bioturbated upward, with were provided by association with cultural arti- multiple buried soils toward the top of each pack- facts; ring counts on buried trees; and detection age. Composing the majority of packages I to of the short-lived isotope 137Cs, which marks the III, these deposits are part of the channel margin start of nuclear testing around A.D. 1950 (Ely and (CM) facies association. A modern analog for this Webb, 1992). This multi-pronged geochronologi- depositional environment is the broad, vegetated cal approach provides the necessary resolution to floodplain that is beginning to fill the modern -ar understand the complex geomorphic relations be- royo bottom. These CM deposits are overlain by tween the alluvial and constricted reaches of the thin caps of bioturbated thin-bedded silts and fine drainage. sands. These beds are associated with deposition on the valley surface (VS), where slopewash and RESULTS eolian inflation and deflation produce a complex suite of deposits that is heavily rooted and bur- Kitchen Corral Wash Valley Fills rowed. The modern analog for this environment is the broad, sagebrush-covered valley surface that Stratigraphy and Sedimentology is dissected by the arroyo system. Four alluvial packages bound by sharp un- Package IV is clearly different in appear- conformities are present in Kitchen Corral Wash. ance. The dominance of the reddish brown silty The best exposure of these packages is found fine to medium sand suggests a more local tribu- at study site KCW-A. Thus, the following de- tary draining the Vermillion Cliffs. Directly ad- scription is based on the stratigraphy exposed jacent to the unconformity between packages III there. Site KCW-A is located on a nearly verti- and IV is a sequence of channel-shaped deposits cal west-facing cutbank that is actively eroding that range from massive pebbly gravel to massive into the modern channel (figure 5). The arroyo in silty sand. Occasional lenses of yellowish-brown this reach is about 10 m deep and 30 m wide. A sand are interbedded within the dominantly red- ~2-m-high floodplain occupies much of the arroyo dish brown package. These channel-shaped de- bottom, pinning the channel against the cutbank. posits transition laterally into a series of tabular, Package I is stratigraphically lowest. Package II laminated fine to medium sands. This pattern overlies and almost overtops a steep paleobank continues up-section to the top of the outcrop. that truncates most of package I. Similarly, pack- Packages I to III record three cycles of arroyo age III truncates and overtops package II by ~1.2 cutting followed by progressive filling and over- m. Package IV fills in a paleochannel that trun- topping of paleoarroyos. Though the CB deposits cates the entirety of the outcrop and locally over- are only preserved in the lower portions of each lies package III. package, the CM deposits clearly record aggrada- Packages I to III contain a similar sequence tion of the arroyo bottom system. The VS deposits of depositional units. The lower portions of each represent hundreds of years of deposition on the are dominated by massive to imbricated, ma- valley surface during this period of entrenchment trix- to clast-supported gravels interbedded with and slow aggradation. Package IV records the trough-crossbedded coarse sand. These facies infilling of a tributary arroyo. Most of the units record high flow velocities and are related to dep- in Package IV record flow events in the tributary

27 J.E. Harvey, J.L. Pederson, and T.M. Rittenour UGA 39 C ages. 14 Stratigraphic panel of study site KCW-A. Light lines are contacts, bold lines are unconformities, and vertical hachures represent verticalhachures unconformities, and are lines boldcontacts, are lines LightKCW-A. site study of Stratigraphicpanel Figure 5.Figure soil horizons. Circles are OSL ages, triangles are

28 The Alluvial Records of Buckskin Wash, Utah itself, whereas those yellowish-brown sandy inter- to 0.8 ka. A radiocarbon sample of detrital twigs beds record the inundation of the tributary arroyo from the base of package IV yields an age of 660 by mainstem flood events. The vertically-stacked to 540 cal yr B.P. geometry of the package suggests that there was relatively little lateral channel migration during Buckskin Gulch Slackwater Deposits aggradation, as opposed to the mainstem arroyo- Stratigraphy and Sedimentology fills in packages I to III. There are many mod- ern analogs for this tributary arroyo system in the Site BG-B is located at the head of a steeper landscape. Since these tributary arroyo systems channel reach where the walls of Navajo Sand- are graded to the mainstem, the presence of chan- stone converge to constrict the channel into a slot nel gravels ~7 m above the modern wash is clear canyon (figure 4). Just upstream of the site, the evidence that Kitchen Corral Wash experienced stream flows through an initial, shorter bedrock aggradation of a similar scale during the deposi- notch where undulating walls are 6 to 10 m apart. tion of package IV. The studied outcrop is located on the west face of a 10- to 12-m-high fill terrace in an expansion be- Geochronology tween this initial slot and the first severe, continu- A sample of detrital charcoal from near the ous constriction of Buckskin Gulch ~100 m down- base of package I yields an age of 2340 to 2130 cal stream. The terrace surface is roughly concordant yr B.P., while an OSL sample 1 m above it gives with the upward broadening of the constricting an age of ~2.5 to 2.8 ka (tables 1 and 2). Another walls downstream, suggesting that its height is re- OSL sample at the base of package II gives an age lated to a upward limit of the backwater effect of of ~0.9 to 2.3 ka. Potsherds found eroding from the constriction (Ely, 1992). Correlative deposits the buried surface at the top of package II suggest drape the bedrock topography throughout the ex- that it was occupied during Pueblo II time, ~1.2 pansion.

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

# aliquots Lab Equivalent Rd Age ± σ Age Range Site Depth (m) accepted Position Number Dose (Gy) (Gy/kyr) (ka) (ka) (analyzed)

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

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

14 Calibrated 2σ Sample Depth C Age (yr Site Lab Number Material age range (yr Position Number (m) BP) 1 BP)

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)

29 J.E. Harvey, J.L. Pederson, and T.M. Rittenour UGA 39 C ages, and 14 Cs was detected. BlackCs was triangle from is correlative unit across from the wash studied outcrop. 137 Cs samples, closed where 137 Stratigraphic panel of study site BG-B. A) Upstream portion of outcrop. B) Downstream continuation of outcrop. Light lines are Figure 6. contacts, bold lines are unconformities, and vertical hachures represent soil horizons. Circlessquares are are OSL ages, triangles are

30 The Alluvial Records of Buckskin Wash, Utah

Five stratigraphic packages are present, sepa- ring counts described below. Thus, package I is rated by buttress unconformities (figure 6). Pack- a complex sequence of deposition that may have age I contains 15 depositional units and forms been interrupted by longer periods of nondeposi- the core of the deposit. Package II is inset into tion. and overtops package I by ~2 m and contains 20 Package II overlies a wedge of sandy hills- units. Package III consists of 6 units and nearly lope colluvium along the unconformity that trun- overtops package II. Packages IV and V continue cates package I. Its basal units are four medium the pattern of filling in void space above down- beds of well-cemented, faintly-laminated to mas- stream-dipping unconformities with 9 and 6 units, sive, yellowish-brown sands that pinch out against respectively, though they are much smaller. The the bounding surface between I and II. These are deposits in the fill terrace here were first studied overlain by a voluminous, 2-m-thick bed of yel- by Ely (1992), who gave a brief description and lowish-brown sand with ripple cross-bedding and provided some radiocarbon age control. floating pebbles and granules that fills much of the Sedimentologically, package I is a series of void left by the erosion of Package I. A series of medium to thick beds of laminated, silty medium 14 downstream-thickening beds of reddish brown to coarse sand. Beds are mostly tabular and lat- and light yellowish brown laminated to ripple- erally continuous. The sandy texture of these cross-bedded sand compose the upper 4 m of the facies records deposition by high-energy events, deposit, overtopping package I and burying the though their tabular geometry does not support a junipers that had germinated on its surface. Con- channel-bottom setting. They probably represent tacts between all units are smooth and free of bio- a channel-margin environment, preserved here on turbation, indicating that the entire package was the inside of a ninety-degree bend in the chan- deposited rather quickly and that successive units nel. Three significant hiatuses are preserved as were passively laid over existing deposits. weakly-developed entisols or heavily bioturbated We note here that package II is the most vol- horizons within this package. The first is about 3 uminous of packages preserved at this particular m from the base of package I. Weak A and Btk expansion. Additionally, a correlative package is horizons are present, as well as abundant rhizo- present at several expansions farther downstream. liths, burrows, and root casts. The second is a bio- Most notably, site BG-C, which is located at the turbated zone atop an irregularly-shaped deposit confluence of Wire Pass and Buckskin Gulch (fig- that appears to be an eolian wedge deposited over ure 1), contains several exposures of this same the first. It is less developed and preserved than package. Hence, it appears to be the most domi- the first, yet features a reddish-brown stain and nant series of slackwater deposits in the Buckskin infilled root traces and burrows. The third, best- Gulch slot canyon. developed soil is found along the upper surface of Packages III-V are similar to package II, each the 2 m-thick unit of massive, medium to coarse consisting of a series of medium to thick beds of sand capping package I. This represents exposure laminated to ripple crossbedded, silty medium for a relatively significant period of time, as it is to coarse sands. Again, no bioturbated horizons heavily bioturbated, contains abundant rhizoliths, or buried soils are present, suggesting rapid em- and has a strong reddish-brown stain, likely a re- placement. No channel-bottom deposits are pre- sult of incorporation of slopewash from nearby served in the studied outcrop. Most units in each bedrock hillslopes into the unit via infiltration and package feature a reverse then normal grading, translocation processes. Correlation by stratig- probably recording deposition during both the ris- raphy and landscape position across the wash to ing and falling limb of flood events in a channel- other exposures indicates that this marker surface margin setting. We interpret this environment as had numerous junipers germinated on it that are an eddy that formed downstream of the existing now partially buried (figure 7). Two of these bur- terrace, which is supported by variable paleocur- ied trees are still living and are the target of tree- rents in package II.

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

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.

32 The Alluvial Records of Buckskin Wash, Utah

Geochronology and at site BG-C. A diverse geochronology at this site provides relatively detailed age control, especially for the DISCUSSION younger portion of the record. A sample of detri- Interpretations of Channel Change tal charcoal from just below the lowest paleosol in package I yields an age of 1820 to 1600 cal yr B.P. The sedimentology and stratigraphy of the A similar sample taken six units above this gives alluvial deposits in the constricted and alluvial an age of 1280 to 1070 cal yr B.P. An OSL sample reaches of the watershed provide an interesting taken between the two returns an age of ~1.4 to perspective on the processes of deposition and 2.0 ka, consistent with the radiocarbon results. A erosion that operate in each setting. In the allu- radiocarbon sample of tree litter found on the bur- vial reach, channel-bottom and channel-margin ied hillslope between packages I and II constrains deposits are preserved up to 9 m above the mod- the age of package II to less than 290 cal yr B.P. ern channel and record past episodes of aggrada- Additional OSL samples were taken from the base tion of the streambed during the filling of paleoar- and middle of package II. Though the samples royos. The best record of such an aggradational clearly date to the late Holocene ages, initial re- episode can be seen in package IV as the progres- sults revealed that they were poorly bleached dur- sive filling of an approximately stationary tribu- ing transport and therefore return unreliably older tary channel with interfingered channel-bottom ages (table 1). Our radiocarbon samples and those and channel-margin deposits. Thus, the majority from Ely (1992) consistently argue that package of the alluvial deposits in Kitchen Corral Wash II is younger than 290 cal yr B.P. A living ju- are the result of aggradation and (sometimes) lat- niper tree buried by the uppermost 4 to 5 units eral migration of the channel bed. In contrast, of package II is exposed near the head of a gully the constricted reach deposits preserve no distinct on stream right (figure 7). A tree-ring count on a evidence of channel aggradation. core collected ~2 to 3 m above the tree’s germina- tion horizon places a minimum age of ~85 years Timescales of Deposition on the tree, a maximum age for overlying units. A second living, buried Juniper on top of package II The abundance of bioturbated units and soil yields a tree-ring count of ~110 years. These data horizons in the alluvial reach deposits suggest that suggest that the upper portion of package II and arroyo filling is a long, slow process involving in- the whole of packages III, IV, and V were, con- cremental lateral and vertical accretion of channel servatively, deposited after A.D. 1850. margins within the arroyo. This several-hundred Finally, five units at site BG-B were analyzed year period of entrenchment is enough for soil for the presence of post-bomb 137Cs (figure 6). No formation on the abandoned valley surface. In 137Cs was detected in packages II or III. A mini- contrast, we know from historical evidence (e.g. mal amount (0.0092 ± 5% cps) was detected in Bryan, 1925; Webb, 1985) that the most recent ar- the middle of package IV, whereas a significant royo cutting took only one or two decades. Thus, amount (0.053 ± 5% cps) was detected in a unit the arroyo cutting and filling cycles in the alluvial near the top of package IV. This suggests that reach of the drainage can be characterized by cen- bomb testing occurred sometime near the end of tennial-scale episodes of aggradation punctuated deposition of package IV. Hence, all stratigraphi- by decadal-scale episodes of incision. cally older units were deposited before around In contrast, in the constricted reach, the allu- A.D. 1950. These data, combined with the tree- vial sequence appears to have been emplaced rap- ring data, argue that packages II, III, and IV were idly, especially in the more voluminous packages rapidly deposited between ~150 and 50 years ago. II-V where smooth contacts between beds and the This conclusion is supported by the presence of absence of bioturbation suggests that very little deep, unfilled gullies bisecting the terraces here time passed between preserved events. At site

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

BG-B, the buried soil atop package I and support- that they are a function of upstream geomorphic ing geochronology indicate that there was ~900 changes: specifically, they record pulses of sedi- years of nondeposition between the deposition of mentation associated with the large-scale excava- Packages I and II. Following this extended depo- tion of valley fills during arroyo cutting upstream. sitional hiatus, the 41 flood deposits of Packages Our alternative interpretation is supported by II-V were emplaced within a period of <100 years. different lines of evidence. Geochronology con- The temporal pattern of decadal-scale episodes servatively constrains the deposition of the bulk of rapid deposition separated by centennial-scale of the deposits in the constricted reach to between hiatuses is nearly opposite to that of the alluvial ~ A.D. 1850 and A.D. 1950, while incision of the reach (figure 8). alluvial valleys upstream took place between ~A.D. 1880 and A.D. 1910. Enormous amounts of Paleohydrologic Interpretations sediment were carried downstream during arroyo cutting (Webb and others, 1991). Once initiated, Under the standard paradigm of paleoflood arroyo headcuts likely migrated upstream during hydrology, one might interpret the deposits at site every flow event, with each migration contribut- BG-B as recording a cluster of 15 large floods be- ing a new pulse of sedimentation. This extraor- tween ~2 and 1 ka followed by a ~900-year ab- dinary sediment loading could have repeatedly sence of flooding between ~1 ka and 0.15 ka, and overwhelmed the backwaters in the Buckskin slot then 41 floods in rapid succession between 0.15 ka canyon, causing temporary aggradation or at least and 0.05 ka. This interpretation is very difficult the local deposition of great amounts of sediment to explain hydroclimatically, and such a drastic in channel margins. Because of the small width- shift in hydrology has not been described in any to-depth ratio of the slot canyon, the temporary regional record. Thus, we argue that the slackwa- storage of sediment along channel margins would ter deposits at site BG-B do not simply record the have altered the local stage-discharge relationship, paleoflood history of the stream. Rather, we argue 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.

34 The Alluvial Records of Buckskin Wash, Utah may not have been particularly large. Thus, we tional processes are distinct between the argue that the slot canyon deposits are not simply alluvial and constricted reaches of Buck- a record of paleoflooding, but also of geomorphic skin Wash, and that the alluvial deposits changes upstream. stored in either reach are fundamentally The results of this study suggest that slack- different paleohydrologic records. water flood deposits in areas that may be subject to the extraordinary sediment loading during inci- 2) Deposition across the two end-mem- sion of alluvial valleys upstream may be unsuita- ber reach types has been broadly anti- ble hydrologic records. If such deposits are inter- correlated during the late Holocene. preted as accurate records of paleoflood frequency There have been at least four cycles of arroyo cutting and filling in Kitchen Cor- and magnitude, they might lead to an incorrect ral Wash since ~3.0 ka. The youngest interpretation that there were very many anoma- arroyo-fill package was deposited from lously large flood events during arroyo cutting ~0.7 ka to 0.15 ka and was incised dur- and an extreme dearth of floods between arroyo ing historic arroyo cutting between ~ cutting events. It is possible that this phenomenon A.D. 1880 and A.D. 1910. The slack- has been overlooked in previous paleoflood stud- water deposits in the Buckskin Gulch ies. slot canyon were deposited in at least The influence of temporary sediment stor- two phases: one from ~2.0 to 1.0 ka and age on paleoflood slackwater deposits can be another, conservatively, between ~ A.D. minimized. If possible, one should avoid work- 1850 and A.D. 1950. ing downstream of broad, alluvial reaches. Oth- erwise, one should avoid backwaters upstream of 3) The majority of the deposits in the tight constrictions and instead use only slackwater Buckskin Gulch slot canyon record an sequences in alcoves or areas less subject to pond- episode of enhanced preservation during ing of water during floods. In any case, results arroyo cutting upstream. Extraordinary should be interpreted cautiously, and attention sediment loads associated with upstream migration of arroyo headcuts during suc- should be directed to the sensitivity of the channel cessive floods led to temporary sediment cross section to episodes of sediment storage. storage and changed stage-discharge re- Our interpretations would be strengthened if lations in the canyon, serving to rapidly the older package in the constricted reach could emplace and preserve dozens of floods of be linked to earlier episodes of arroyo cutting up- uncertain discharge. Thus, the slackwa- stream. At this point, the ages of arroyo cutting ter deposits at the studied sites in Buck- events preceding A.D. 1200 are imprecise, as are skin Gulch are not a reliable record of the ages of the deposits in package I of the con- paleoflood frequency and discharge. It is stricted reach. It would also be beneficial to per- possible that other published paleoflood form similar studies in analogous settings in the records may have been affected by this southwestern U.S. Do slackwater deposits pre- signal from upstream arroyo cutting and served in bedrock canyons downstream of a broad should be re-visited. alluvial reach with an arroyo always record the arroyo-cutting signal, or is the case of Buckskin AKNOWLEDGMENTS Wash an anomaly? This study was related to the first author’s CONCLUSIONS Masters Thesis at Utah State University. It was supported by a research grant from the Geological 1) Through careful analysis of stratigra- Society of America, the Arthur D. Howard Award phy and sedimentology at several study from the QGG Division of GSA, and a scholar- sites, we have confirmed that deposi- ship from the Utah State University Department

35 J.E. Harvey, J.L. Pederson, and T.M. Rittenour UGA 39 of Geology. Support for OSL and radiocarbon Graf, J. B., Webb, R. H., and Hereford, R., 1991, Re- age determination provided by the USU Lumines- lation of sediment load and flood-plain forma- cence Laboratory. tion to climatic variability, Paria River drainage basin, Utah and : Geological Society of America Bulletin, v. 103, p. 1405-1415. REFERENCES Hack, J. T., 1942, The changing physical environment Bailey, R. W., 1935, Epicycles of erosion in the valleys of the Hope Indians of Arizona: Peabody Muse- of the Colorado Plateau province: Journal of Ge- um Papers, v. 25, no. 1., 85 p., 12 plates. ology, v. 43, p. 337-355. Hall, S.A., 1977, Late Quaternary sedimentation and Bryan, K., 1925, Date of channel trenching (arroyo- paleoecologic history of Chaco Canyon, New cutting) in the arid Southwest: Science, v. 62, p. Mexico: Geological Society of America Bulletin, 338-344. v. 88, 1593–1618. Bryan, K., 1929, Floodwater farming: Geographical Harvey, J. E., 2009, Reconciling Holocene alluvial Review, v. 19, p. 444-456. records in Buckskin Wash, southern Utah: Lo- gan, Utah State University, M.S. thesis, 135 p. Cooke, R.U., and Reeves, R.W., 1976, Arroyos and Haynes, C. V. Jr., 1968, Geochronology of late-Qua- environmental change: Oxford, Clarendon Press, ternary alluvium, in Morrison, R. B., and Wright, 213 p. H. E., editors, Means of correlation of Quater- Doelling, H. H., and Davis, D. D., 1989, The geology nary successions: Salt Lake City, University of of Kane County, Utah: Utah Geological and Min- Utah Press, p. 591-631. eral Survey Bulletin 124, 192 p., 8 plates. Hereford, R., 1986, Modern alluvial history of the Par- Ely, L. L., 1992, Large Floods in the southwestern ia River drainage basin: Quaternary Research, v. United States in relation to Late-Holocene cli- 25, p. 293-311. matic variations: Tucson, University of Arizona, Hereford, R., 2002, Valley-fill alluviation during the Ph.D. dissertation, 326 p. Little Ice Age (ca. A.D. 1400-1880), Paria Riv- Ely, L. L., 1997, Response of extreme floods in the er basin and southern Colorado Plateau, United south-western United States to climatic varia- States: Geological Society of America Bulletin, tions in the late Holocene: Geomorphology, v. 19, v. 114, p. 1550-1563. p. 175-201. Hereford, R., Thompson, K. S., Burke, K. J., and Fair- Ely, L. L., and Webb, R. H., 1992, Accuracy of post- ley, H. C., 1996, Tributary debris fans and the late bomb 137Cs and 14C in dating fluvial deposits: Holocene alluvial chronology of the Colorado Quaternary Research, v. 38, p. 196-204. River, eastern Grand Canyon, Arizona: Geologi- cal Society of America Bulletin, v. 108, p. 3-19. Euler, R. C., Gumerman, G. J., Karlstrom, T. N. V., Dean, J. S., and Hevly, R. H., 1979, The Colorado Karlstrom, T. N. V., 1988, Alluvial chronology and Plateaus – Cultural Dynamics and Paleoenviron- hydrologic change of Black Mesa and nearby re- ment: Science, v. 205, p. 1089 - 1101. gions, in Gumerman, G. J., editor, The Anasazi in a changing environment: Cambridge, Cam- Galbraith, R. F., Roberts, R. G., Laslett, G. M., Yosh- bridge University Press, p. 45-91. ida, G. M., and Olley, J. M., 1999, Optical dat- ing of single and multiple grains of quartz from Knox, J. C., 1983, Response of river systems to Jinmium rock shelter, northern – Part Holocene climates, in Wright, H. E., Jr., editor, 1 – experimental design and statistical models: Late Quaternary environments of the United Archaeometry, v. 41, p. 339-364. States, Volume 2, The Holocene: Minneapolis, University of Minnesota Press, p. 26-41. Graf, W. L., 1983, The arroyo problem—Paleohy- Kochel, R. C., and Baker, V. R., 1982, Paleoflood Hy- drology and paleohydraulics in the short term, drology: Science, v. 215, p. 353-362 in Gregory, K.G., editor, Background to paleo- hydrology: New York, John Wiley and Sons, p. Murray, A. S., and Wintle, A. G., 2003, The single 279-302. aliquot regenerative dose protocol: Potential for

36 The Alluvial Records of Buckskin Wash, Utah

improvements in reliability: Radiation Measure- 2004, IntCal04 terrestrial radiocarbon age cali- ments, v. 37, p. 377-381. bration, 26–0 ka BP: Radiocarbon, v. 46, p. 1029- 1058. O’Connor, J. E., Ely, L. L., Wohl, E. E., Stevens, L. E., Melis, T. S., Kale, V. S., and Baker, V. R., 1994, A Tainer, E., 2010, High-resolution chronostratigraphy 4500-year record of large floods on the Colorado of archeological sites in Grand Canyon: Logan, River in the Grand Canyon, Arizona: The Jour- Utah State University, M.S. thesis, 182 p. nal of Geology, v. 102, p. 1-9. Webb, R.H., 1985, Late Holocene flooding on the Es- Patton, P. C., Baker, V. R., and Kochel, R. C., 1979, calante River, south-central Utah: Tucson, Uni- Slackwater deposits -- A geomorphic technique versity of Arizona, Ph.D. dissertation, 204 p. for the interpretation of fluvial paleohydrology,in Rhodes, D. D., and Williams, G. P., editors, Ad- Webb, R. H., and Jarrett, R. D., 2002, One-Dimen- justments of the fluvial system: Dubuque, Kend- sional estimation techniques for discharges of all-Hunt, p. 225-252. paleofloods and historical floods, in House, P.K., Webb, R.H., Baker, V.R., and Levish, D.R., edsi- Reimer, P. J., Baillie, M. G. L., Bard, E., Bayliss, A., tors, Ancient Floods and Modern Hazards: Prin- Beck, J. W., Bertrand, C. J. H., Blackwell, P. G., ciples and Applications of Paleoflood Hydrology: Buck, C. E., Burr, G.S., Cutler, K. B., Damon, American Geophysical Union Water Science and P.E., Edwards, R. L., Fairbanks, R. G., Friedrich, Application Series, v. 5, p. 111-125. M., Guilderson, T. P., Hogg, A. G., Hughen, K. A., Kromer, B., McCormac, F. G., Manning, S. Webb, R. H., Smith, S. S., and McCord, V. A. S., 1991, W., Ramsey, C. B., Reimer, R.W., Remmele, S., Historic channel change of Kanab Creek, south- Southon, J. R., Stuiver, M., Talamo, S., Taylor, F. ern Utah and northern Arizona: Grand Canyon W., van der Plicht, J., and Weyhenmeyer, C. E., Natural History Association Monograph 9, 91 p.

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