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INTRODUCTION
Arroyos are an end-member geomorphic state of semi-arid catchments with high sediment yields and are char-acterized by steep-walled channels entrenched into fine-grained valley-fill alluvium (e.g. Bryan, 1925; Bull, 1997). Historic observations suggest initiation of arroyo cutting from approximately A.D. 1880–1920 was accomplished by frequent, large-magnitude flood events (e.g. Bryan, 1925; Hack, 1942; Webb and others, 1991; Hereford, 2002). Ear-ly hypotheses for the cause of this historic arroyo cutting include land mismanagement and overgrazing following pioneer settlement (Bailey, 1935; Thornthwaite and others, 1942; Antevs, 1952; Patton and Boison, 1986). However, evidence for prehistoric cut-fill events exposed in arroyo walls suggests non-human related causes. Current hypoth-eses for prehistoric arroyo-cutting have been centered on autogenic geomorphic adjustments (Schumm and Hadley, 1957; Patton and Schumm, 1981; Patton and Boison, 1986; Tucker and others, 2006) and climate change to wetter or drier conditions (Antevs, 1952; Karlstrom, 1988; Hereford, 2002; Mann and Meltzer, 2007).
This study examines the Holocene alluvial stratig-raphy of Kitchen Corral Wash (KCW), a tributary of the Paria River in southern Utah. Currently, KCW is in an incised state, and stratigraphic evidence indicates that it
has experienced multiple periods of prehistoric arroyo en-trenchment and channel aggradation. The purpose of this study is to construct a detailed chronostratigraphy of KCW by expanding and updating the alluvial chronologies de-veloped by Hereford (2002) and Harvey and others (2011). This is achieved by combining detailed sedimentologic de-scriptions and stratigraphic relationships at 11 study sites, where buttress unconformities separate alluvial fills, with age control derived from accelerator mass spectrometry (AMS) radiocarbon dating of charcoal and optically stimu-lated luminescence (OSL) dating of quartz sand.
Background
KCW is a tributary of the Paria River located in Kane County, Utah, approximately 45 kilometers east of the town of Kanab. It is the main trunk stream of a drainage that as-sumes several names from its headwaters to its confluence with the Paria River (e.g. Park Wash, Deer Springs Wash, Kitchen Corral Wash, Kaibab Gulch, and Buckskin Gulch). However, for the purpose of this study the name KCW will be used for the Park Wash (PW) and Deer Spring Wash (DSW) reaches from the base of the White Cliffs and the main KCW alluvial valley that extends from the base of the Vermillion Cliffs to the intersection with Kaibab Gulch at U.S. Highway no. 89 (figure 1). From its headwaters in
HOLOCENE ALLUVIAL STRATIGRAPHY OFKITCHEN CORRAL WASH, SOUTHERN UTAH
ABSTRACT
Kitchen Corral Wash (KCW), a tributary of the Paria River in southern Utah, has experienced episodes of historic and pre-historic (Holocene) arroyo cutting and filling. During the most recent arroyo-cutting event (about A.D. 1880–1920), KCW and other regional drainages were entrenched 5–30 meters into their fine-grained alluvial valley fills. While previous studies have attempted to constrain the timing of arroyo cut-fill events in KCW, poor age control has limited the results. In order to better understand the timing of arroyo cutting events, this study updates and improves the arroyo cut-fill chronostratigraphy from KCW by using alluvial stratigraphic descriptions and age control from optically stimulated luminescence and accelerator mass spectrometry radiocarbon dating. Results are based on 11 study sites, each exposing a number of unconformity-bounded alluvial packages in the arroyo-wall stratigraphy, and suggest at least six arroyo cut-fill episodes over the last approximately 7000 years. From oldest to youngest, the six episodes of alluvial fill aggradation are: an older fill from Hereford (2002), Qf1, Qf2, Qf3, Qf4, and Qf5. Although not discussed here, these chronostratigraphic results were used by Huff (2013) to test hypotheses related to climatic forcing of arroyo dynamics by comparing the chronology from KCW to regional alluvial chro-nologies and paleoclimate records.
byWilliam M. Huff1 and Tammy R. Rittenour1,2
1Utah State University Department of Geology, Logan, UT; [email protected] Luminescence Laboratory, Logan, UT; [email protected]
Huff, W.M., and Rittenour, T.R., 2014, Holocene alluvial stratigraphy of Kitchen Cor-ral Wash, southern Utah, in MacLean, J.S., Biek, R.F., and Huntoon, J.E., edi-tors, Geology of Utah’s Far South: Utah Geological Association Publication 43, p. 77–96.
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UGA Publication 43 (2014)—Geology of Utah's Far South78
the Paunsaugunt Plateau to the head of Kaibab Gulch, the total drainage area of KCW is 511 square kilometers and ranges in elevation from about 2500 meters to just below 1800 meters above sea level. KCW flows approximately north to south and occupies a continuous arroyo that is en-trenched approximately 2–12 meters within its fine-grained alluvial valley fill. The total reach-length of the study area (combining PW, DSW, and KCW) is about 28 kilometers.
KCW heads in Tertiary (upper Paleocene to mid-dle Eocene) Claron Formation sandstones, siltstones, and mudstone of the Paunsaugunt Plateau (figure 2). The Clar-on Formation forms the Pink Cliffs that are exposed as the uppermost unit of the Grand Staircase physiographic prov-ince of the Colorado Plateau. From here the drainages con-tinue incising through Cretaceous Kaiparowits Formation, Wahweap Formation, and Straight Cliffs Formation sedi-ments as the lower Pink Cliffs change to the upper Gray Cliffs. The Kaiparowits Formation lies unconformably be-neath the Claron Formation and is composed of subarkose sandstone. The interbedded mudstone to sandstone of the Wahweap Formation overlies the Straight Cliffs Formation, which is made of cliff-forming sandstones, slope-forming mudstones, and coal interbeds (Doelling and others, 2000).
The drainages continue through resistant Dakota Formation sandstone at the base of the Gray Cliffs until transitioning into Late and Middle Jurassic Entrada Sandstone and Car-mel Formation sediments. Below these two sedimentary units, the White Cliffs of the Jurassic Navajo Sandstone are characterized by high-angle eolian crossbedding and are primarily composed of white to tan-colored fine- and medium-grained sand (Doelling and others, 2000). Exiting the White Cliffs, the drainage headwaters converge into PW and DSW and incise through Jurassic Kayenta For-mation and the upper Vermillion Cliffs. The confluence of PW and DSW form the main channel of KCW, which exits through the Moenave Formation at the base of the Vermil-lion Cliffs and then crosses Triassic Chinle and Moenkopi Formations. The Chinle Formation is primarily composed of interbedded mudstones, sandstone, and conglomerates and also contains fossilized wood of the Petrified Forest Member. Finally, after exiting the disconformity-bounded Moenkopi Formation, KCW is renamed Buckskin Gulch as it narrows and becomes entrenched within Permian lime-stones and sandstones of the Kaibab Formation.
The modern channel of KCW in the study area is typi-fied by steep, arroyo walls ranging from 12 to <5 meters in
VCKCW
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Elevation (m)High : 2869
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KCWKaibab Gulch at Highway-89
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Hereford (2002) study sites
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Figure 1. Location map of KCW and surrounding drainages in southern Utah.
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height, which were produced by the most recent arroyo cut-ting event between A.D. 1880 and 1920 (Hereford, 2002). The active channel is 20–100 meters wide and is inset into Holocene alluvium that lies within a broad (>1 kilometers wide) Quaternary alluvial valley bounded by Pleistocene-aged terraces that are mantled with cobble- to gravel-sized alluvium. Although mainly an alluvial channel, in places KCW narrows to several meters and is surrounded by bed-rock outcrops where it transitions from weak lithologic for-mations (e.g. Chinle Formation) to a more resistant lithol-ogy (e.g. Moenkopi Formation) and exposes >6-meter-tall knickpoints in PW and at the confluence of PW and DSW (figure 2).
Structural features within and surrounding the KCW catchment include folds initiated from the Sevier orogeny, early-Tertiary faults formed during the Laramide Orogeny, and faulting formed during Basin and Range extension starting around 15 Ma (Doelling and others, 2000). The most significant features include the Paunsaugunt Fault and East Kaibab monocline. The Paunsaugunt Fault is a high-angle normal fault that extends from northern Ari-zona through central Utah (Doelling and others, 2000) and
crosses KCW near the channel head of DSW. The East Kai-bab monocline, expressed as the Cockscomb near KCW, is a prominent, northeast-trending feature that is locally faulted. Neither of these features crosses the project study area, and no Quaternary activity has been reported along the several small-scale faults that exist within the catch-ment of the KCW.
Semi-arid conditions and ongoing arroyo cut-fill ac-tivity in KCW have limited the effect of soil-formation processes within alluvial deposits resulting in widespread but discernible entisols. Within the KCW study area, these soils may occur on top of the youngest alluvial terraces or as buried soils inset between alluvial deposits. These in-cipient soils formed in valley-fill alluvium and can be used as stratigraphic markers of a hiatus in sedimentation.
METHODS
Stratigraphy and Sedimentologic Descriptions
This study explores the stratigraphic cut-fill rela-tionships, sedimentology, and ages of alluvial packages
Kd (Dakota and Cedar
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(A-A’) Headwaters of Park Wash to Intersection at Kaibab Gulch
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Figure 2. Longitudinal profile of KCW from the headwaters of Park Wash to the intersection of Kaibab Gulch (A-A') and from the head-waters of Deer Springs Wash to the confluence with the main KCW trunk stream (B-B'). Three bedrock knickpoints in Park Wash and KCW range from 6–10 meters in height.
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exposed along arroyo walls in KCW. Approximately 28 kilometers of the KCW alluvial channel, starting at the in-tersection of U.S. Highway no. 89 and including PW and DSW, were traversed in A.D. 2011 and 2012 in order to identify outcrops with the best expressed arroyo cut-fill ar-chitecture. Each outcrop chosen for sampling and detailed description was typically >5 meters in height, displayed at least one near-vertical buttress unconformity within the stratigraphy, and had a colluvial wedge deposit preserved in the stratigraphy at the base of the paleoarroyo wall. These criteria were chosen in order to reduce the odds of selecting an exposure with deposits only recording chan-nel-migration. A total of 11 study sites were identified (fig-ure 1), which included three arroyo-wall outcrop exposures in DSW (KCW-A, KCW-C, and KCW-E), four exposures in PW (KCW-B, KCW-D, KCW-F, and KCW-G), and four exposures in the main-stem KCW stream (KCW-H, KCW-I, KCW-J, and KCW-K) (Huff, 2013). This paper presents detailed sedimentologic and stratigraphic descriptions of five study sites (KCW-E, -G, -H, -I, and -J).
The arroyo wall stratigraphy was separated into un-conformity-bounded stratigraphic units. Sedimentologic descriptions of alluvial fills from each outcrop were com-piled by noting bed thickness and geometry, Munsell color, grain size and sedimentary structures, grading and sorting, bioturbation or evidence of soil development, and character of the basal contact. Key depositional facies (table 1) and facies associations were then identified following Harvey and others (2011). See Huff (2013) for detailed descrip-tions of all study sites.
Geochronology
Age control for KCW alluvial deposits was obtained using AMS radiocarbon and OSL dating. Using these dat-ing techniques in tandem not only allowed for greater sam-pling opportunities in the field, but also provided a means for comparing the ages of alluvial deposits and reconciling aberrant results. In general, samples were collected from near the base and top of unconformity-bounded alluvial fills.
AMS Radiocarbon Dating
Thirty-seven samples for radiocarbon dating were collected from KCW. Two additional radiocarbon samples used in this study were collected by Harvey and others (2011), and three were collected by Hereford (2002). Char-coal was targeted for sampling because of its abundance within each of the aggradational fill packages, allowing for selective instead of opportunistic sampling. Due to poten-tial age inaccuracies from charcoal redeposition, targeted material typically consisted of large concentrations of char-coal or thin lenses of charcoal separating alluvial deposits. In a few instances, isolated (detrital) charcoal fragments were targeted where sediment for OSL dating was poor and no other sources of charcoal or woody debris were avail-able. However, detrital charcoal was avoided if present in massive deposits and only collected if sedimentary struc-tures were evident. Once a charcoal-rich layer was target-ed, attention was paid to the size and angularity of the char-coal in order to reduce the odds of collecting redeposited material.
Table 1. Key depositional facies described in text.
Facies Codea Sedimentary Structures Sedimentology Flow RegimeDepositional
InterpretationSl Low-angle crossbeds Sand, very fine to coarse with variable
amounts of gravels and pebblesLower-Upper Scour-fill, transverse or
linguoid bedformsSt Trough crossbeds Sand, very fine to coarse with variable
amounts of gravels and pebblesLower-Upper Bedform migration
Sh Horizontally/Planar bedded Sand, very fine to coarse with variable amounts of gravels and pebbles
Upper Plane bed flow
Sr Ripple crossbeds Sand, very fine to coarse with variable amounts of gravels and pebbles
Lower Supercritical climbing ripples
Sm Massive Sand, very fine to coarse with variable amounts of gravels and pebbles
Upper Inner channel flood, overbank flood, levee
depositFl Isolated , thinnly laminated or
dessicated bedsClay, Silt Lower Overbank or waning
flood depositFsmv Interbedded massive and thin
laminationsVariegated clay, silt, very fine sand Lower Backswamp, marsh, or
cienga depositP Incipient soil, bioturbation Clay, silt, very fine sand often containing
roots and burrowsSoil formation along a
stable surfaceGh Horizontally bedded or massive Clast or matrix supported gravels and
pebblesUpper Lag deposits
Gt Crossbedded Clast or matrix supported gravels and pebbles
Upper Channel Fills
aList of the most common sedimentary facies evident within the KCW study area. Facies codes closely follow those describedin Miall (2000). OSL and radiocarbon samples were primarily targeted within the lithofacies highlighted in gray.
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Sixteen of the 37 samples collected were sent to the UC Irvine Keck AMS Laboratory for analysis, and an ad-ditional 17 samples were pretreated by the lead author at the University of Arizona NSF-AMS Laboratory in Tuc-son and later analyzed by the AMS lab staff. One of the 37 radiocarbon samples was not analyzed because it did not survive pretreatment chemistry and three other samples were deemed unnecessary for analysis. Sample ages were converted from radiocarbon years to calendar years B.P.2010
using Calib 6.0 and the IntCal09 dataset (Reimer and oth-ers, 2009). Results are reported in B.P.2010 as a weighted mean of the calibrated ages (Telford and others, 2004) with an asymmetric 2-sigma error (table 2). Reporting as cal kyr B.P.2010 allows for a direct comparison to OSL ages.
Optically Stimulated Luminescence (OSL) Dating
OSL dating provides an age-estimate of the last time sediment was exposed to light prior to burial (Huntley and others, 1985). During transport, quartz or feldspar grains are exposed to sunlight (or heat) and their luminescence signal is reset (bleached) when electron charges stored in mineral defects are released (Aitken, 1998). Following deposition, defects in the crystal lattice begin to accumulate electrons produced by ionizing radiation from surrounding sediments and from cosmic rays, which cause the luminescence signal to grow over time. In this study, the dose-rate surrounding each sample was calculated using ICP-MS analysis of K, U, Th, and Rb content and conversion factors of Guerin and others (2011). Single-grains of quartz sand were ana-lyzed using the single-aliquot regenerative technique of Murray and Wintle (2000) and equivalent dose (De) values were calculated using the minimum-age-model (MAM) of Galbraith and others (1999).
OSL dating was used in concert with AMS radiocar-bon dating in this study because samples can be collected from any sandy deposit not otherwise containing charcoal or organic materials. However, as with radiocarbon dating, OSL dating has its own set of problems that can lead to erro-neous ages. In fluvial environments, such as KCW, the most challenging problem to overcome is incomplete resetting of the luminescence signal, commonly referred to as partial bleaching (see reviews by Wallinga, 2002; Rittenour, 2008). Partial bleaching may occur for a number of reasons in dry-land fluvial systems due to flashy flow and high-magnitude discharge events, limited transparency of the water column, and/or rapid deposition. In fact, a previous study from KCW (Harvey and others, 2011) and studies in nearby drainages (Hayden, 2011; Summa-Nelson and Rittenour, 2012) have identified partial bleaching as a significant problem. Partial bleaching can lead to significant overdispersion and high positive skew of De values and might contribute to signifi-cant age overestimations (e.g. Wallinga and others, 2001; Murray and Olley, 2002; Rittenour, 2008).
To reduce the possibility of sampling partially bleach-ed sediments, OSL sample collection was based on guide-
lines described in Huff (2013) and Summa-Nelson and Rittenour (2012). Special attention was paid to the sedi-mentology and stratigraphy of alluvial fills targeted for sample collection. First, sampling preference was given to thin (<0.4 meters) ripple-cross laminated sand beds, which were indicated by Summa-Nelson and Rittenour (2012) to have more likely been exposed to adequate sunlight. Beds displaying sedimentary structures indicative of rapid depo-sition and/or high sediment concentrations were avoided because they were likely exposed to limited sunlight prior to burial. Additionally, bioturbated beds and soils were avoided because of the possibility of sediment mixing.
Twenty-nine OSL samples (two from Harvey and oth-ers, 2011) were collected from the 11 study sites in KCW and processed at the USU Luminescence Lab (table 3). Samples were wet-sieved to specific grains size fractions (150–250 microns or 180–250 microns) and pretreated with 10% hydrochloric acid to dissolve carbonates and chlorine bleach to remove organics. Heavy minerals were separated using sodium polytungstate (2.7 g/cm³), and then targeted sediments were treated with concentrated hydroflouric acid for 90 minutes to remove feldspars and etch quartz grains and re-sieved at 75 microns to remove partially dissolved grains.
DATA PRESENTATION
Stratigraphy and Sedimentology of KCW
The alluvial stratigraphy of KCW is characterized by very fine- to coarse-grained sand deposited as either broad-ly tabular or lenticular beds. Colors are primarily 10YR (yellowish-brown), 2.5YR (red), and 5R (reddish-brown). These distinct colors are a result of sediment being derived from upstream sources, as opposed to more local bedrock surrounding the study reaches, and allowed individual beds and units to be more easily distinguished. The yellowish-brown sand units are fine- to medium-grained, well-sort-ed, sub- to well-rounded, and composed of frosted quartz grains predominately derived from the eolian Jurassic Navajo Formation upstream of the field area (figure 2). The red to reddish-brown sand units are poorly to moderately sorted and composed of very fine- to medium-grained, sub-angular to sub-rounded sands with variable amounts of clay and silt. These are predominately derived from mud-dier bedrock lithologies proximal to the study area includ-ing the Jurrasic Kayenta and Moenave Formations and the Triassic Chinle and Moenkopi Formations. In agreement with Harvey (2009), the yellowish-brown beds are consid-ered mainstem sediments because of their upstream Navajo Formation source whereas the red and reddish-brown sand were likely sourced from smaller tributaries of the main-stem.
Arroyo wall exposures vary in height. Those located directly downstream of a knickpoint are the tallest (about
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Table 2. Summary of radiocarbon information and ages.
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8–12 meters), and sites located directly upstream of a knickpoint and in the downstream reach of KCW are the shortest (about 2–5 meters). The tallest arroyo-wall expo-sures were the most helpful in reconstructing the arroyo cut-fill stratigraphy of KCW and identifying the sedimen-tary facies in alluvial fills.
A total of ten sedimentary facies were identified in KCW and are primarily defined by their grain size and sed-imentary structures. Three depositional facies associations, similar to those described by Harvey and others (2011), have been identified and include channel-bottom, channel-margin, and valley-surface. Additionally, two new subsets were identified and include channel-margin slackwater and valley-surface colluvial wedge facies associations. See Huff (2013) for more detail.
Depositional Facies
Detailed stratigraphic and sedimentologic descrip-tions were made at each arroyo wall exposure prior to sam-pling for radiocarbon and OSL dating. These descriptions were used to extract and compile a list of the most frequent-ly observed depositional facies comprising KCW alluvial fills. Ten depositional facies (table 1) have been identified and have been given a facies code primarily based upon grain size and sedimentary textures or structures closely following those described in Miall (2000).
The most common facies assemblage in KCW con-sists of trough crossbedded (St), ripple crossbedded (Sr), low-angle (<15º) crossbedded (Sl), and horizontally bed-ded (Sh) very fine- to medium-grained sand. Deposits with these characteristics are generally about 20–50 centimeters thick, display tabular or broadly lenticular geometries, and are representative of medium- to low-energy depositional environments (table 1).
The second most common facies assemblage through-out KCW alluvial fills is represented by very fine- to coarse-grained structureless sands, which often have variable amounts of clays and silts. These sand units range from a few centimeters to tens of centimeters thick that are broadly tabular and fall into three sedimentary facies classifica-tions: massive sands (Sm), thinly laminated or desiccated clays and silty-sands (Fl), variegated clay, silt, and very-fine sand interbeds (Fsmv). Massive sand deposits are typified by moderately to well-sorted, fine- to coarse-grained sand likely deposited during high energy and high sediment yield events (upper flow regimes). Individual beds of thinly lami-nated (<10 centimeters) or dessicated clays and silty-sands are often interbedded with very fine- to coarse-sand facies (Sh, St, Sr, Sl) and are deposited as a high flow event begins to wane within the channel. Thinly bedded (<5 centimeters), commonly variegated, clayey-silt to very fine-grained and laminated sand deposits are typically over 30 centimeters thick and are one of the most recognizable facies through-out KCW. Fsmv facies deposits are often laterally traceable for tens of meters and were likely deposited in slackwater
settings. Incipient soils have been grouped separately and are commonly distinguishable as highly bioturbated or bur-ied entisols or inceptisols. The protofacies of an incipient soil may be one of any previously mentioned facies textures (e.g. Sl, St, Sm, Fsmv).
The least common facies assemblage includes matrix- or clast-supported pebble-gravels that are commonly im-bricated and horizontally bedded (Gh) or crossbedded (Gt). These facies are most commonly seen in basal deposits that generally occur as lenticular beds extending only a few me-ters. They are commonly seen within or underlying other lenticular deposits of Sh, St, Sl, Sr, or Sm sand.
Facies Associations
The channel-bottom facies association is character-ized by basal stratum and includes thalweg or other axi-al channel deposits. These deposits are relatively coarse and include horizontally bedded or crossbedded, matrix or clast-supported pebble-gravel and medium- to coarse-grained sand. They usually have a lenticular geometry, and because they are deposited during high-energy scour-fill events in the active channel they may be imbricated.
As with Harvey and others (2011), the channel-mar-gin facies association is the most abundant in the arroyo wall stratigraphy and is primarily composed of tabular to broadly lenticular, laterally extensive, very fine- to coarse-grained sand that is deposited adjacent to the main channel thalweg or as unconfined sheet-flows filling paleoarroyo channels and adjacent flood plains. Upper flow regime sed-iments may be deposited as St, Sr, or Sm deposits, whereas low flow regime sediments may be Sl or Sh. Massive sand deposits are generally very fine- to medium-grained sands, but may also contain variable amounts of clay, silt, or even coarse-grained sands depending on the source of the allu-vium. In addition, thick sand deposits may be capped with thinly bedded silty sand that was deposited during a wan-ing flow. Because these channel-margin deposits are usu-ally associated with vertical accretion, they tend to display broadly tabular bedding geometries with thicknesses that most often range from thin (<20 centimeters) to sub-meter scale.
Within the channel-margin but distal to axial channel flows and sediment sources, deposits may be composed of thin, very fine-grained laminated sands and Fsmv beds or as very fine to fine-grained Sm beds. These beds are interpreted to have been deposited in quiet water settings and are generally identifiable as channel-margin slack-water deposits. These slackwater deposits are a subdivi-sion of the original channel-margin facies association of Harvey and others (2011) and display a broadly tabular geometry that may be continuous for tens of meters. As channel-margin slackwater deposits are not part of the ac-tive channel, they are often subject to bioturbation or soil formation and may sometimes appear as structureless Sm deposits or contain incipient soils that include roots and
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organic-rich forms of these deposits may be similar to the cienega environments observed in many regional stream prior to historic arroyo cutting.
The valley-surface facies association consists of non-fluvial deposits that are either hillslope or eolian sediments deposited outside of the channel-margin or, more com-monly, as incipient soils where the valley floodplain was stable for an extended period. Incipient soils are devoid of sedimentary structures due to considerable bioturbation and are composed of variable amount of clays, silts, and sands. Hillslope sediments are generally characterized as Sm beds resulting from scour-fill deposition at the valley surface, while eolian sediments are primarily fine-grained and typically cap older incipient soils.
The valley-surface facies association of Harvey and others (2011) has been subdivided to identify valley-sur-face colluvial deposits covering or deposited at the base of buttress unconformities separating paleoarroyo walls. These deposits are primarily composed of clays, silts, and very fine- to coarse-grained sands that have no sedimentary structures and contain gravel- to cobble-sized rip-up clasts and blocks of cohesive older sediment. These deposits are commonly identified as colluvial wedges that are generally derived from older alluvium and deposited following in-cision. These deposits are similar to modern mass failure deposits at the base and slopes of arroyo-wall exposures. The presence of these valley-surface colluvial deposits at the toe of an arroyo is a good indication that the arroyo wall face was stable for a significant period. This helps discern unconformities developed due to arroyo entrench-ment from those formed during contemporaneous channel migration.
Geochronology
A total of 32 AMS radiocarbon (plus two compiled from Harvey and others [2011] and three from Hereford [2002]) and 10 OSL ages were used to build an alluvial chronostratigraphy for this study. Nearly all of the radio-carbon samples returned stratigraphically consistent ages (table 2). However, eight radiocarbon ages revealed evi-dence of charcoal redeposition or the analysis of Creta-ceous coal (Huff, 2013). Additionally, every OSL sample showed evidence of partial bleaching, which necessitated using single-grain dating and MAM statistical analysis of Galbraith and others (1999) to calculate De values (Huff, 2013). In general, radiocarbon and OSL ages suggest that KCW unconformity-bound alluvial fills are middle to late Holocene in age, ranging from approximately 7.3 to 0.2 ka.
Chronostratigraphic Observations and Interpretations
Identification of alluvial fill packages was prima-rily based on crosscutting relationships and the presence of buried soils, and secondarily based on stratigraphically
consistent ages from AMS radiocarbon and OSL results. Five Holocene-aged aggradational fill packages from this study and one older unit dated and described by Hereford (2002) have been identified in the KCW study area. This is more than the three aggradational packages originally iden-tified by Hereford (2002) and the four aggradational pack-ages identified by Harvey and others (2011). Based on a recalibrated radiocarbon age from Hereford (2002), the oldest fill package began to aggrade prior to 7.30 cal kyr B.P.2010, whereas the youngest fill package stopped ag-grading prior to the approximately A.D. 1880–1920 arroyo entrenchment. The following sections describe stratigraph-ic and sedimentologic observations and deposit ages from select study sites in KCW. Descriptions of all study sites can be found in Huff (2013).
KCW-E Observations
KCW-E is located in the DSW tributary of the main KCW study area along a meander bend approximately 9.5 kilometers upstream of U.S. Highway no. 89 and Kaibab Gulch (figure 1). This north-facing site is 6.9 meters tall, and its cut-fill architecture displays two alluvial fill pack-ages that are separated by a buttress unconformity (figure 3). The older alluvial fill is at least 4.5 meters thick and is composed of 2.5YR to 10YR colored, broadly lenticular and tabular, St, Sl, and Fl sands (figure 3). An upper St bed shows evidence of soil formation and is capped with an approximately 1-meter-thick Sm bed that also shows evi-dence of an incipient soil. A radiocarbon sample (14C-11) collected near the base of this fill produced an age of 1.38 ± 0.02 cal kyr B.P.2010 and an OSL sample (USU-1192) col-lected about 1.1 meters below the surface produced an age of 1.14 ± 0.19 ka (figure 3; table 2).
The younger 6.9-meter thick inset fill contains a col-luvial wedge draped across the buttress unconformity. This colluvial wedge is on-lapped and draped by Sr, Sl, St, and Fl beds that are 10–30 centimeters thick, basally lenticular and transitioning to broadly tabular near the top of the de-posit. These are capped with a sub-meter-thick slackwat-er deposit showing evidence of soil formation (figure 3). An erosional surface extends across the top of this Fsmv deposit and is overlain by the uppermost deposit of the younger fill. This deposit is about 2.5 meters thick and con-tains broadly tabular interbeds of Sl, St, and Fl. A radiocar-bon sample (14C-12) collected about 1.5 meters above the modern channel grade produced an age of 0.87 cal kyr B.P.2010 and another 14C sample from the base of the up-permost channel-margin deposit returned an age of 0.2 ± 0.14 cal kyr B.P.2010 (table 2). An OSL sample (USU-1191) collected from the middle of the younger channel fill was not analyzed.
KCW-E Interpretations
Age control and stratigraphic evidence at study site
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Figure 3. Study site KCW-E showing a 6.9-meter-tall arroyo wall with two alluvial packages (Qf4, Qf5) separated by an erosional surface with: A. Radiocarbon and OSL age results, and B. Stratigraphic facies and depositional facies associations.
KCW-E suggest the presence of two alluvial fills and indi-cate that the channel incised to or below its current grade prior to 1.38 ± 0.02 cal kyr B.P.2010 (figure 3). This was fol-lowed by aggradation until a brief hiatus in sedimentation shortly after 1.14 ± 0.19 ka, as indicated by an OSL sample age and buried soil. Deposition resumed at this location for some amount of time until aggradation was interrupted by another episode of entrenchment prior to 0.87 cal kyr B.P.2010. Aggradation of the younger alluvial fill persisted until at least 5 meters of alluvium had been deposited, at which time slackwater sedimentation ensued at the chan-nel margin and soil formation followed prior to 0.2 ± 0.14 cal kyr B.P.2010. This was followed by localized erosion and channel-margin sheetflow deposition prior to historic chan-nel cutting.
KCW-G Observations
Study site KCW-G is located approximately 12 kil-ometers upstream of U.S. Highway no. 89 and Kaibab Gulch, and nearly 3 kilometers upstream of a 6-meter-tall bedrock knickpoint (knickpoint 3) (figure 1 and 2). This site is a 5-meter-high west-facing arroyo wall that shows evi-dence of four alluvial fill packages separated by soils and erosional unconformities (figure 4). The oldest fill is com-posed of tabular St and Sl beds overlain by a 1-meter-thick Sm deposit, and an approximately 65-centimeter-thick, red buried soil that is highly bioturbated and has no visible sedimentary structures (figure 4). A 14C sample (14C-21) was collected near the base of this fill and returned an age of 3.73 ± 0.04 cal kyr B.P.2010 (table 2).
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Figure 4. Study site KCW-G showing a 5-meter-tall arroyo wall with four alluvial packages (Qf1,Qf3, Qf4, Qf5) separated by two erosional surfaces with: A. Radiocarbon and OSL age results, and B. Stratigraphic facies and depositional facies associations.
Overlying the oldest fill units is a tabular Sl bed and overlying variegated silts and sands (Fsmv) and Sm de-posit with evidence of an incipient soil (figure 4). An OSL sample (USU-1181) was collected from the Sl bed (unit 3) of this intermediate fill and produced an age of 2.35 ± 0.41 ka (table 3). Onlapped against these beds is a younger fill composed of basal St and Sl interbeds and is capped by an 80-centimeter-thick Fsmv deposit that also shows evidence of soil formation. Radiocarbon samples 14C-17 (1.18 cal kyr B.P.2010) and 14C-20 (not analyzed) were collected from the base of this unit (table 2).
The youngest fill is inset against a buttress uncon-formity and is composed of a thin colluvial wedge that is onlapped by alternating beds of Sl, Sm, Sh, St and Fsmv and two interbedded units of Fsmv (figure 4). A radiocar-
bon sample 14C-18 (0.68 cal kyr B.P.2010) was col-lected from the base of this fill (table 2), and an OSL sample (USU-1186) was collected from the middle of this package but was not analyzed. The uppermost slackwater bed in the youngest fill indicates about 20 centimeters of soil formation across an erosional upper contact with an overlying valley-surface deposit (figure 4).
KCW-G Interpretations
The stratigraphy at site KCW-G, suggests at least 4 alluvial fills with aggradation of the oldest fill beginning sometime after 3.73 ± 0.04 cal kyr B.P.2010 following a pe-riod of entrenchment. Evidence of a massive buried soil suggests a long period of surface stability nearly 3 meters
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Figure 5. Study site KCW-H showing a 10.7-meter-tall arroyo will with three alluvial packages (Qf1, Qf4, Qf5) separated by erosional surfaces with: A. Radiocarbon and OSL age results, and B. Stratigraphic facies and depositional facies associations.
above the modern channel bottom that was eventually fol-lowed by an episode of aggradation that capped these older deposits by 2.35 ± 0.41 ka (figure 4). Aggradation of this intermediate fill was interrupted by a hiatus in sedimenta-tion and subsequent channel incision prior to 1.18 cal kyr B.P.2010. Aggradation of this younger fill continued with channel-margin and slackwater deposition until sometime prior 0.68 cal kyr B.P.2010 when aggradation ceased and the stream entrenched to its modern grade, producing a buttress unconformity (figure 4). Ensuing aggradation of the youngest fill continually shifted from channel-margin to slackwater deposition until a final hiatus in sedimenta-tion at 4.9 meters above the modern channel. This was fol-lowed by a period of soil formation and sheetflood deposi-tion prior to historic arroyo cutting.
KCW-H Observations
The KCW-H study site is located approximately 7.5 kilometers upstream of Kaibab Gulch and U.S. Highway no. 89 and less than 1 kilometer downstream from knick-point 3 in the main trunk channel of KCW (figures 1 and 2). It is a mostly north-facing, 10.7-meter-tall arroyo wall that wraps around a meander bend whose sediments have been partially deposited along outcropping bedrock of Triassic Chinle Formation (figure 5). The arroyo-wall stratigraphy at KCW-H shows evidence of at least three alluvial fills that are separated by erosional unconformities. An addi-tional 4-meter-tall fill between the oldest and intermediate appears to be a separate alluvial fill. However, it contains discontinuous, lenticular beds and is considered to be part
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of the intermediate fill. This outcrop predominantly con-tains 10- to 50-centimeter-thick beds that are composed of 10YR, 7.5YR, and 2.5YR hued sediments, suggesting a range of sediment sources (figure 5). The oldest alluvial fill consists of several tabular alternating Sl, Sh, and Sm beds (figure 5). A radiocarbon sample (14C-26) was collected from the base of this alluvial fill and yielded an age of 4.14
cal kyr B.P.2010 (table 2), and an OSL sample (USU-1101) was extracted 1 meter above this and produced an age of 3.76 ± 0.61 ka (table 3). Another radiocarbon sam-ple (14C-22) was collected about 2 meters above this OSL sample and yielded an age of 3.82 cal kyr B.P.2010. The uppermost portion of this oldest alluvial package is com-posed of a thick (2.2 meters) massive sand deposit that is now highly bioturbated and shows clear evidence of soil formation. This soil appears to be similar to that observed in the oldest fill at KCW-G.
An intermediate fill inset against the oldest fill pack-age contains an erosional contact within the general stratig-raphy (figure 5). Basal channel-bottom deposits upstream of the erosional contact contain lenticular Gh beds overlain by St and Sl beds and overlying 10- to 20-centimeter-thick, Sl and Sm interbeds, of which radiocarbon sample 14C-24 (1.08 cal kyr B.P.2010) and OSL sample USU-1102 (not analyzed) were collected. Downstream of the erosional surface is a 5-meter-thick package of channel-margin St, Sm, Sh, and Sl interbeds that are overlain by a slackwater deposit and massive bioturbated sand (figure 5). An OSL sample (USU-1103) was collected approximately 4 meters above the modern channel grade and produced an age of 1.23 ± 0.16 ka. Compared to the radiocarbon ages from the same fill, it appears that USU-1103 is producing an age overestimation.
Buttressed against this intermediate fill is a small col-luvial wedge within the youngest fill that is onlapped by a sequence of channel deposits containing broadly lenticular beds Gt, Sh, Sl, St, and Sr, and Fl. A radiocarbon sample (14C-25) was collected from the colluvial wedge and yield-ed an age of 2.28 cal kyr B.P.2010, while a 14C sample (14C-27) collected outside of the colluvial wedge yielded an age of 0.83 cal kyr B.P.2010 (figure 5; tables 1 and 2). An OSL sample (USU-1104) was collected 1 meter be-low the valley surface but was not analyzed.
KCW-H Interpretations
Stratigraphic evidence and age control suggest this outcrop exposes three alluvial fill packages (figure 5). En-trenchment at or below the modern channel grade oc-curred prior to 4.14 cal kyr B.P.2010 and was followed by aggradation in the channel until sometime after 3.82
cal kyr B.P.2010. A pause in sedimentation was caused by another incision event and allowed pedogenesis to occur along the stable surface of the older alluvial fill. The in-termediate fill began aggrading prior to 1.08 cal kyr B.P.2010 and filled to a level similar in height as the older fill.
A steep buttress unconformity suggests an arroyo-cutting event lowered the channel to near its modern position prior to 0.83 cal kyr B.P.2010 and was followed by at least 10.7 meters of aggradation prior to historic arroyo cutting.
KCW-I Observations
Study site KCW-I, previously studied by Harvey and others (2011), contains a nearly a 10-meters-tall arroyo wall that faces the west and is located just over 5.5 kil-ometers upstream from Kaibab Gulch and U.S. Highway no. 89 (figures 2 and 6). Arroyo wall stratigraphy shows four alluvial fill packages separated by three buttress un-conformities (figure 6). Beds in this outcrop are general-ly 10- to 50-centimeters thick and mostly contain 10YR and 2.5YR hued sediments. The oldest fill is composed of tabular interbeds of Sm, St, and Sl that are capped by a thick Fsmv bed that shows signs of soil formation. An OSL (USU-531) and radiocarbon (14C-39) samples were pre-viously collected from this deposit by Harvey and others (2011) and returned ages of 2.28 ± 0.30 ka and 2.3 cal kyr B.P.2010, respectively (figure 6; tables 1 and 2). The next younger alluvial fill is about 7 meters thick and is com-posed of basal lenticular Gh and St beds that are overlain by broadly tabular Sh, St, Sm and Sl beds and capped by a thick, red Sm bed. This Sm bed shows evidence of in-cipient soil formation and has an erosional base in places (figure 6). A radiocarbon sample (14C-29) was collected near the base of the fill and returned an age of >46 cal kyr B.P.2010 and an OSL sample (USU-530) collected by Har-vey and others (2011) from an inset channel-bottom de-posit returned a single-grain age of 1.31 ± 0.17 ka (table 3). Based on fill stratigraphy and a comparison to other OSL and radiocarbon ages at KCW-I, it appears that USU-530 is producing and age underestimation. Another OSL sample (USU-1025) was collected from a slightly higher position but was not analyzed.
Buttressed against the intermediate fill is a younger fill package composed of basal Gh beds overlain with tabu-lar channel-margin (e.g. Sl, Fl, Sl, Sr, Sm) and slackwater (e.g. Fsmv) beds (figure 6). Capping this fill package is an approximately 1-meter-thick incipient soil. A radiocarbon sample near the base of this fill yielded an age of 1.72 ± 0.05 cal kyr B.P.2010 (table 2). The youngest alluvial fill is at least 9 meters thick, overtops all other alluvial fills, and is composed of lenses of Gh and tabular beds of Sl, Sh, and Fl. These beds onlap a thick colluvial wedge that drapes the buttress unconformity of the intermediate fill (figure 6). A radiocarbon sample (14C-38) was collected by Harvey and others (2011) from the middle of this fill and yielded an age of 0.66 cal kyr B.P.2010 (table 2). Additional observa-tions of KCW-I are described in Harvey and others (2011) under the name KCW-A.
KCW-I Interpretations
Based on stratigraphic relationships and OSL and
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Figure 6. Study site KCW-I showing a 9 m tall arroyo wall displaying four alluvial packages (Qf2, Qf3, Qf4, Qf5) separated by erosional surfaces with: A. Radiocarbon and OSL age results, and B. Stratigraphic facies and depositional facies associations. This site was previously visited by Harvey and others (2011) and has been reinterpreted in this study.
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Table 3. Single-grain quartz OSL ages.
radiocarbon results, four identified alluvial packages are present in KCW-I (figure 6). It appears that entrenchment at this site to or below its modern depth occurred prior to 2.30 cal kyr B.P.2010. Ensuing aggradation of the old-est alluvial fill was briefly interrupted by a hiatus in sedi-mentation as indicated by a buried soil (figure 6). A second episode of entrenchment after 2.28 ± 0.30 ka is identified by a buttress unconformity and was followed by deposi-tion of the second oldest alluvial fill and incipient soil form ation. Following a third prehistoric incision event, the sec-ond youngest fill started to aggrade prior to 1.72 cal kyr B.P.2010 and continued until overtopping the oldest fill at 7.5 meters above the modern channel grade. A buried soil at the top of this fill suggests another long hiatus in sedimentation until a final prehistoric arroyo cutting event prior to 0.66
cal kyr B.P.2010, which was followed by the vertical ac-cretion of the youngest fill and historic arroyo cutting event.
KCW-J Observations
Site KCW-J is located approximately 5 kilometers up-stream from U.S Highway no. 89 and Kaibab Gulch and has an exposed arroyo wall rising between 5 meters and 6.6 meters above the modern alluvial channel (figures 1 and 7). As one of the longest exposures in this study, the cut-fill architecture of KCW-J expands well over 50 meters of a meander bend in a north-south direction. Unconformable bounding buttresses suggest three aggradational fill pack-
ages (figure 7). The oldest alluvial fill contains channel-margin deposits that are primarily composed of 10YR and 2.5YR hued, and broadly tabular St, Sl, and Sm sands that are capped by a thick Fsmv deposit (figure 7). A Sm deposit filled with 30- to 50-centimeter-thick, blocky and tabular pebbles and cobbles with no evidence of sorting is inset below the Fsmv deposit and does not conform to the sur-rounding facies. Radiocarbon and OSL samples (tables 1 and 2) were collected near the base of this fill and yield single-grain ages of 1.70 ± 0.19 ka (USU-1028) and 1.58 ± 0.25 ka (USU-1027) and a 14C age of 1.70 cal kyr B.P.2010 (14C-31). Additionally, a radiocarbon sample from the upper Fsmv deposit produced an age of 1.85 c a l kyr B.P.2010. The next younger inset alluvial fill consists of both broadly tabular and lenticular Sl, St, and Sh beds that are capped by a 0.8-meter-thick slackwater Fsmv de-posit (figure 7). An OSL sample (USU-1026) and two ra-diocarbon samples (14C-30, 14C-33) were collected from the middle of this alluvial fill and returned ages of 1.29 ± 0.25 ka, >31.7 cal kyr, and 2.52 cal kyr B.P.2010, re-spectively (tables 1 and 2). The youngest alluvial deposit is erosionally inset in places but largely drapes over the two older fill packages and is characterized by three highly bio-turbated, tabular beds with incipient soils. All beds show some evidence of low-angle cross-bedding at their base but are primarily structureless due to bioturbation. OSL sample USU-1029 was collected 1.5 meters below the valley sur-face and yielded an age of 0.97 ± 0.19 ka (table 3).
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Figure 7. Study site KCW-J showing a 5.0–6.6-meter-tall arroyo will displaying three alluvial packages (Qf3, Qf4, Qf5/Qf5’) separated by erosional surfaces with: A. Radiocarbon and OSL age results, and B. Stratigraphic facies and depositional facies associations.
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Fsmv
Sl
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Sl
Sl
SlSl
SmSm
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StSt Sl
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USU-10261.29 ± 0.25
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SlSl Sl
SlSl
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14C-321.85
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14C-30>31.7 3
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14C-311.70
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Radiocarbon (cal kyr BP2010)OSL (cal ka)
USU-10281.70 ± 0.19
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KCW-J Interpretations
The stratigraphy and age control from site KCW-J suggest that three Holocene alluvial fills separated by soils and buttress unconformities are preserved at this site. Prior to 1.70 ± 0.19 ka, KCW was at a grade similar to the modern channel. Aggradation of the oldest fill began proximal to the channel until the thalweg of the channel migrated past this site and left an inset channel scour that was later infilled by overbank and slackwater deposits. The tabular pebbles and cobbles in the Sm deposit are far from any bedrock source and may have been placed due to anthropogenic activity. Alternative interpretations of the stratigraphy in the oldest fill package may suggest two in-dividually aged alluvial packages, but current age control suggests deposits on either side of the channel scour are contemporaneous. Age control from the next younger allu-vial fill suggests aggradation was underway by 1.29 ± 0.25 ka. Sometime after 1.29 ± 0.25 ka the channel began to de-posit tabular sheetflood beds at this site, with several lapses in sedimentation as indicated by the presence of a buried soil horizon (figure 7). Age control for these upper capping deposits is limited due to a slight OSL age overestimation (USU-1029), but their stratigraphic position suggests they are some of the youngest alluvium and were deposited prior to historic arroyo cutting.
DISCUSSION
Development of Holocene Chronostratigraphy
Arroyo-wall chronostratigraphic relationships observed at eleven study sites in KCW indicate that at least six epi-sodes of middle to late Holocene aggradation repeatedly refilled the arroyo system to varying levels above the mod-ern channel floor, following five episodes of prehistoric arroyo entrenchment (figure 8). The identification of these aggradation episodes was primarily based on stratigraphic evidence from the study sites and secondarily from 14C and OSL age control. From oldest to youngest, the six episodes of alluvial aggradation are: an older fill (approximately 7.3–4.75 ka) identified by Hereford (2002), and alluvial fills identified here as Qf1, Qf2, Qf3, Qf4, and Qf5. The older fill of Hereford (2002) is not described in this study because the exposure was not seen during field research and may no longer be preserved.
The oldest alluvial fill, Qf1, is evident at sites KCW-G and KCW-H and has an age range of about 4.35–3.4 ka. This alluvial fill package has a unique thick red soil devel-oped at its surface that is generally about 3.0–8.5 meters above the modern channel floor. The older fill of Hereford (2002) has been identified as a separate aggradation epi-sode from Qf1 because age results from these two fills do not appear to overlap (see tables 2 and 3). However, it is possible the Qf1 alluvial fill is representative of continued aggradation of the older fill of Hereford (2002). Alluvial fill
Qf2 is located at sites KCW-C, KCW-F, and KCW-I and has an age range of about 3.2–2.25 ka. The preserved sur-face of the Qf2 ranges from about 5.0–7.5 meters above the modern channel and is characterized by weakly developed soils. Alluvial fill Qf3 is located at sites KCW-A, KCW-D, KCW-I, and KCW-J and has an age range of about 2.15–1.45 ka. The preserved surface of this alluvial fill ranges from about 4.5–7.5 meters above the modern channel. The Qf4 alluvial fill is exposed at sites KCW-B, KCW-C, KCW-E, KCW-G, KCW-H, KCW-I, KCW-J, and KCW-K and has an age ranging from about 1.3–0.8 ka. Preserved surface heights of this alluvial fill package ranges from ap-proximately 2.0–9.0 meters above the modern channel. The youngest alluvial fill, Qf5, is evident at every site in KCW and has an age range of about 0.7–0.12 ka. This alluvial fill is commonly the thickest or capping deposit in the study area and reaches a maximum height of about 11.0 meters above the modern channel floor.
Complications involved in determining the timing of incision and deposition events in KCW were primarily due to problems inherent to the dating techniques. One of the primary problems with radiocarbon dating was mistakenly analyzing Cretaceous coal. Five radiocarbon samples re-turned near-infinite ages ranging from approximately 42.6–49.4 cal kyr B.P. (figure 8; table 2). Redeposition of char-coal was equally problematic and accounted for four age reversals at study sites KCW-D (14C-9), KCW-H (14C-25), and KCW-J (14C-33), and KCW-K (14C-35). Hence, constraining the timing of aggradation at each of these sites relied on OSL ages or stratigraphically consistent radiocar-bon ages. Moreover, a major assumption with radiocarbon dating in general is that sample deposition occurred shortly after the death of the organism dated. Hence, the radiocar-bon ages only provide a maximum age of deposition.
Partial bleaching or resetting of the luminescence sig-nal within sandy alluvium was the primary concern for OSL dating. The flashy flow and high sediment-discharge events indicated in the alluvial stratigraphy suggest that most sand grains may not have been sufficiently exposed to light be-fore being deposited. This was expected as indicated by previous OSL-related studies from dryland fluvial systems (e.g. Bailey and Arnold, 2006; Arnold and others, 2007; Summa-Nelson and Rittenour, 2012). Most of these prob-lems were reduced by using single-grain dating and MAM (e.g. Galbraith and others, 1999) to calculate the equivalent dose and resultant OSL age. Despite the use of singe-grain dating and MAM, OSL samples USU-1177 (KCW-B) and USU-1029 (KCW-J) show age inversions when compared to the arroyo wall stratigraphy at each site and other radio-carbon and OSL ages (figure 8; table 3).
In addition to problems with geochronologic dating techniques, preservation of alluvial fill packages varied throughout KCW. Only four sites (KCW-G-H) showed evidence for three or more alluvial fills in their arroyo wall exposures (figure 8). In general, the two youngest alluvial
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Figure 8. Chronology of aggradation events at KCW derived from detailed stratigraphic mapping, AMS radiocarbon (triangles) and OSL (circles) dating. Green = Qf5 fill, orange = Qf4 fill, blue = Qf3 fill, yellow = Qf2 fill, purple = Qf1 fill, Gray = Older fill of Hereford (2002). Space between fill packages represents a period of entrenchment.
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Indicates possible redeposition of charcoal, old wood, or other radiocarbon problems.
Indicates OSL age underestimations due to dose rate inaccuracies or overestimations due to partial bleaching.
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lder Fill”
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fill packages (Qf4, Qf5) were most commonly exposed in the study sites, so age reconstruction on the aggradation of these fills is the best constrained. Unsurprisingly, the oldest alluvial fill packages (Qf1,Qf2, and Qf3) were preserved at fewer study sites, so age constraints of these fills are lim-ited. Variability in preservation of alluvial fills at KCW was expected because the dynamic nature of the arroyo system removes the sedimentary record over time and limits the preservation and exposure of older deposits. Importantly, identification of the oldest alluvial fill (Qf1) was largely aided by the presence of a thick (>65 centimeters) buried soil capping this fill (figures 4 and 5).
Deposition of Qf5’alluvium
A number of arroyo wall exposures in KCW are capped with 1–3 meters of channel-margin and valley-sur-face, broadly tabular sheetflow deposits (e.g. St, Sl, Sm, Sr, Sh) that typically overlie a buried incipient soil. These deposits are commonly the capping units of alluvial fill Qf5 and are referred to as Qf5’ to separate these deposits from the main body of Qf5 fill. Downstream of knickpoint 1 in PW, Qf5’ deposits are absent at KCW-B, thicken to 2.3 me-ters at KCW-D, thin to 1.9 meters at KCW-F, and begin to pinch out at KCW-G (figure 8). In DSW these sheet-flow deposits first appear as a 2.4-meter-thick unit at KCW-C and thin to 1.3 meters at KCW-E (figure 3). In the main trunk channel of KCW, Qf5’ deposits are initially missing below knickpoint 3 at KCW-H and KCW-I, and then ap-pear at KCW-J as a 50-centimeter-thick, Sl sand deposit. A radiocarbon sample (14C-8) collected 40 centimeters be-low the Qf5’ deposit at site KCW-D returned an age of 0.57
cal kyr B.P.2010, and a radiocarbon sample (14C-20) col-lected above a buried soil in a Qf5’ deposit at site KCW-E returned a near modern age of 0.2 cal kyr B.P.2010, suggesting these deposits are younger than the Qf5 alluvial fills they overlie.
The proto-historic radiocarbon ages and minimal pedogenic development within the Qf5’ deposits suggest it is possible that a change in paleoflood hydrology or in-creased sediment supply caused renewed sheetflow deposi-tion just before historic channel incision. Additionally, bur-ied soils that appear to be separating Qf5’ alluvial deposits at KCW-J suggest these sheetflood episodes may have been discontinuous (figure 7). Whatever the depositional mecha-nism, it appears that Qf5’ is the youngest and highest fill deposit within the study area (figures 3–7).
CONCLUSIONS
A detailed chronstratigraphy of KCW was built us-ing stratigraphic relationships from arroyo wall exposures and age results from AMS radiocarbon and OSL dating. Meter-scale arroyo fill packages primarily consisted of sub-meter-scale beds of trough crossbedded (St) or ripple crossbedded (Sr) sand deposits in a channel-margin facies
association. While each dating method was subject to po-tential problems, combining the two methods allowed for increased sampling opportunities and crosschecking of re-sults to eliminate aberrant ages. Ultimately, this newly de-veloped chronostratigraphy identified at least six episodes of Holocene aggradation: an older fill (about 7.3–5.0 ka) of Hereford (2002) and five alluvial fill deposits ranging from approximately 4.35–3.4 ka, 3.2–2.25 ka, 2.15–1.45 ka, 1.3–0.8 ka, and 0.7–0.12 ka that are interrupted by periods of arroyo entrenchment.
ACKNOWLEDGMENTS
This research was funded by the National Science Foundation Arroyo-CAREER grant (NSF-105719) award-ed to Tammy Rittenour and by additional grants awarded to Will Huff from the Colorado Scientific Society, USU Lu-minescence Laboratory, and Utah State University Depart-ment of Geology. Field and lab support was provided by Michelle Summa-Nelson, Kerry Riley, and Ben Lariviere. Appreciation is given Michelle Summa-Nelson and Anne Hayden-Lesmeister for their editorial remarks. Additional thanks are due to the University of Arizona NSF-AMS Laboratory, UC-Irvine AMS Laboratory, and Beta-ana-lytic, Inc. for processing and analyzing radiocarbon sam-ples. Specifically, the authors thank the helpful staff at the University of Arizona NSF-AMS Laboratory for offering us the experience to process a number of our radiocarbon samples.
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