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INTRODUCTION
Arroyos are steep-walled, entrenched channel forms that are characteristic of many dryland streams (Graf, 1983). They represent end-member channel states that form when streams incise into fine-grained valley-fill alluvium (Bull, 1997). Stratigraphic evidence exposed in arroyo walls suggest these systems are dominated by floodplain aggradation interrupted by episodic arroyo entrenchment. Historic observations indicate that entrenchment occurs on the order of years to decades and can lead to loss of pro-ductive agricultural lands and infrastructure along alluvial corridors. Moreover, the lowering of the alluvial aquifer associated with arroyo entrenchment leads to loss of ripar-ian habitat and floodplain cienega environments (Webb and Leake, 2006).
Abrupt arroyo cutting in the southwestern U.S. at the turn of the last century is historically one of the most sig-nificant geomorphic events in the region (see Cooke and Reeves, 1976; Graf, 1983; Webb and others, 1991). Ques-tions regarding the causes of early twentieth-century arroyo entrenchment attracted many of the great geologists of the
time (e.g. K. Bryan, J.T. Hack, E. Antevs, etc.). Several of the first workers to study the “arroyo problem” were quick to attribute the onset of incision to changes in land use such as overgrazing (Bailey, 1935; Antevs, 1952), although Bry-an (1925) discussed other hypotheses regarding entrench-ment. Continued investigation of the alluvial stratigraphy in arroyo walls throughout the southwestern U.S. revealed that several arroyo cutting and filling events occurred dur-ing the Holocene without major human influence (e.g. Hack, 1942; Cooke and Reeves, 1976; Hereford, 2002). Thus, while many workers regard land-use changes to be a contributing factor in historic arroyo entrenchment, it is not considered the sole causative mechanism, as prehistoric ar-royo incision events have been widely recognized.
This research project, undertaken in the upper Esca-lante River drainage in south-central Utah (figure 1), takes advantage of recent advances in dating methods to better understand the timing of arroyo aggradation and entrench-ment episodes. One of the major impediments to examining hypotheses regarding the role of climate has been the in-ability to precisely resolve the chronostratigraphic record. While the Holocene alluvial stratigraphy of the upper Es-
CHRONOSTRATIGRAPHY OF HOLOCENE VALLEY-FILLALLUVIUM AND ARROYO CUT-FILL EVENTS IN THE
UPPER ESCALANTE RIVER, SOUTHERN UTAH
ABSTRACT
During the late 1800s to early 1900s, many fluvial systems in the southwestern United States incised into their alluvium, forming steep-walled arroyos and causing economic and environmental impacts to settlers. Many studies of arroyo systems have been conducted over the last century, and several hypotheses have been proposed regarding the conditions necessary for recurring arroyo entrenchment. However, most of these studies have relied on radiocarbon (14C) dating, limiting the temporal resolution of chronostratigraphic records due to limited sampling opportunities and reworking of older charcoal. Recent advances in optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) 14C dating allow for more highly resolved fluvial chronologies from these sensitive semi-arid river systems.
Research was conducted along the upper Escalante River in south-central Utah to develop a chronostrati-graphic record of Holocene cut-fill cycles. Field work focused on recognition and description of unconformity-bounded fluvial sequences in well exposed arroyo walls. Alluvial packages were dated using 18 14C samples and 20 OSL samples. Results suggest that arroyo cut and fill dynamics became an important agent of landscape evolution approximately 4.5 ka. Since that time, at least five aggradation/entrenchment events occurred, with evidence for aggradational packages separated by incision at approximately 4.4 to 4.2 ka, 2.9 to 2.5 ka, 1.8 to 1.6 ka, 1.0 to 0.8 ka, and during the historic period of arroyo entrenchment that commenced in A.D. 1909.
byAnne Hayden-Lesmeister1 and Tammy R. Rittenour2
1Southern Illinois University, Environmental Resources & Policy, Carbondale, IL2Utah State University, Department of Geology, Logan, UT
Hayden-Lesmeister, A., and Rittenour, T.R., 2014, Chronostratigraphy of Holocene valley-fill alluvium and arroyo cut-fill events in the upper Escalante River, south-ern Utah, in MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors, Geology of Utah’s Far South: Utah Geological Association Publication 43, p. 57–76.
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calante is the focus of this paper, Hayden (2011) examined climate versus intrabasin controls by comparing the upper Escalante record with proximal drainages and regional paleoclimate records.
Webb (1985) and Webb and Hasbargen (1997) sug-gested that arroyo entrenchment events occurred roughly every 500 years over the past 2000 years (approximately 2000, 1500, 1000, and 500 14C yr B.P., as well as historical-ly) in the upper Escalante River (table 1, figure 2). While these studies provide a temporal framework for the con-tinued study of arroyo dynamics, ages from this earlier re-search were poorly constrained due to the limited number of dated field exposures and the lower resolution of older radiocarbon ages. Alluvial records from tributaries of the lower Escalante may show broadly similar timing for in-cision (2500–1900 14C yr B.P. and 1000–300 14C yr B.P); however, these are also poorly constrained (Boison and Patton, 1985; Patton and Boison, 1986).
The main objective of the study was to reconstruct the fluvial history of the upper Escalante River by developing a robust chronostratigraphy of the inset Holocene cut-fill alluvial stratigraphy. Reconstruction was aided by surfi-cial mapping of the field area, so a second objective was to create geologic maps focusing on Quaternary surficial deposits present along the alluvial corridor (plates 1-4). Al-though it was not the main goal of the project, the surficial
geologic map will provide important information to land management agencies and property owners regarding pos-sible hazards in the area. This once-isolated region became part of the Grand Staircase-Escalante National Monument in A.D. 1996, and many people now visit the area for rec-reational purposes. The main transportation route through the area, Utah State Route 12, is located in close proximity to the arroyo and is vulnerable to damage from continued erosion of arroyo walls.
STUDY AREA
The Escalante River heads in the Kaiparowits and Aquarius Plateaus of south-central Utah on the Colorado Plateau, western United States. The Escalante River has a drainage area of 5244 square kilometers (figure 1). The study area is centered on the main trunk stream of the Es-calante River located upstream of the town of Escalante, Utah. This alluvial reach is referred to as Upper Valley Creek until its confluence with North Creek; the stream is then referred to as the Escalante River (plate 2). To sim-plify the river names, the study area is hereafter referred to as the upper Escalante River. The upper Escalante River flows northeast and east through generally flat-lying Cre-taceous Straight Cliffs and Kaiparowits Formations com-
Figure 1. The Escalante River is a tributary to the Colorado River and is located within the Colorado Plateau physiographic province (shaded DEM). The headwaters study area is outlined and includes the alluvial portion of the river before it transitions to a bedrock-dominated stream downstream of Escalante, Utah.
MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors
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Outcrop Fill Unit 1
14C age 2 Weighted mean age 3
2σ age range 4 (cal ka B.P. 2010)
WH1 V 500 ± 45 0.60 0.55 - 0.69 WH1 V 580 ± 55 0.65 0.58 - 0.66 WH1 V 640 ± 60 0.67 0.60 - 0.74 WH1 III 1945 ± 65 1.95 1.78 - 2.10 WH1 III 2250 ± 50 2.30 2.21 - 2.41 WH1 III 2605 ± 80 2.73 2.43 - 2.93 WH2 V 470 ± 60 0.56 0.38 - 0.69 WH2 V 420 ± 90 0.50 0.35 - 0.63 WH2 V 470 ± 120 0.53 0.22 - 0.73 WH2 V 530 ± 60 0.62 0.56 - 0.71 WH2 V 620 ± 110 0.67 0.40 - 0.85 WH2 III 1750 ± 65 1.73 1.59 - 1.88 WH3 V 190 ± 90 0.26 0.06 - 0.49 WH3 V 570 ± 120 0.63 0.38 - 0.79 WH3 IV 1090 ± 100 1.08 0.85 - 1.32 WH3 IV 1640 ± 90 1.60 1.40 - 1.83 WH3 IV 1590 ± 110 1.56 1.35 - 1.78 1 Fill units were interpreted and assigned based on figures and text in Webb and Hasbargen (1997) 2 Radiocarbon age presented in Webb and Hasbargen (1997) 3 Maximum probability of 2-sigma range calculated using weighted mean; ages reported in calendar ka B.P. (2010) 4 Calibrated using IntCal09 (Reimer and others, 2009); 2-sigma range is shown in calendar ka B.P. (2010)
Table 1. Summary of 14C samples from previous work in the upper Escalante River.
CHRONOSTRATIGRAPHY OF HOLOCENE VALLEY-FILL ALLUVIUM AND ARROYO CUT-FILL EVENTS IN THE UPPER ESCALANTE RIVER, SOUTHERN UTAH – Hayden-Lesmeister, A., and Rittenour, T.
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Figure 2. Main outcrops studied by Webb and Hasbargen (1997) along the upper Escalante River to provide age control. Approximate locations are shown on inset map of study area. Modified from Webb and Hasbargen (1997). Fill units were assigned for comparison to this study.
MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors
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posed mostly of sandstones and mudstones (figure 3). Bed-rock structural features in the field area include the broad Laramide folds of the Upper Valley Anticline, the Alvey Syncline, and the Escalante Anticline. Downstream of the study area, the river crosses into the underlying Mesozoic strata and incises into sandstone bedrock of the Jurassic Glen Canyon Group, forming an approximately 90-kilom-eter-long reach of entrenched bedrock meanders before en-tering Lake Powell, a reservoir of the Colorado River.
The headwaters of the Escalante River (upstream of Escalante, Utah) drain about 570 square kilometers (figure 1). Elevations in this region range from 1729–3286 meters above sea level, with an average basin elevation of approx-imately 2393 meters above sea level. The upper Escalante River corridor is characterized by a broad alluvial valley (approximately 120–900 meters wide between bedrock walls) that is dissected by an approximately 35-kilometer-
long, steep-walled arroyo. Arroyo walls range from <5–15 meters in height. The lower portions of tributary valleys are similarly filled with alluvium and contain smaller arroyos connected to the main channel.
Sediment color and lithology indicate sourcing from Cretaceous and Tertiary bedrock in the catchment. Addi-tionally, large dissected Pleistocene deposits in the head-waters (map unit QTgr, Quaternary–Tertiary gravel, plate 4) are composed of reworked gravels (possibly sourced from Cretaceous conglomerates), and locally supply abun-dant gravel-sized sediment to the system. Except for mi-nor higher Pleistocene deposits, the upper Escalante River valley is dominated by one major geomorphic terrace unit (Qat1, Quaternary terrace level 1, lowest terrace) that is as-sociated with the pre-1909 floodplain surface (plates 1–4). There are subtle differences in heights and surface geomor-phology of Qat1 that may be associated with pre-arroyo
Figure 3. Simplified geologic map of the Escalante River watershed. The study area is underlain primarily by Cretaceous formations with some minor Tertiary strata in the headwaters.
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channels and floodplain levels. The alluvium underlying the Qat1 map unit is Holocene and records multiple arroyo cut-fill events.
As documented by Webb and Baker (1987), some aggradation of the modern channel has occurred since the historic period of arroyo cutting. These aggradational sur-faces (combined as map unit Qal1) consist of low terraces within the modern arroyo that are infrequently inundated by floods in the current climate and flow regime. This is indicated by vegetation communities on these surfaces that do not tolerate frequent flooding, such as pinyon, juniper, and sagebrush.
With the exception of one major knickpoint near the valley margin and a man-made diversion, the modern stream channel rests on alluvium, and the depth to bedrock is unknown. One well-log obtained from a landowner on Upper Valley Creek indicates that in some locations the al-luvium is up to 54 meters below the pre-arroyo floodplain (Qat1) (see figure 4 for well location).
The upper Escalante River began incising to form the modern arroyo following an extreme high-flow event that occurred from August 31 to September 1, 1909 (Webb and Baker, 1987). This event marked the beginning of a period of unusually large floods that lasted into the A.D. 1930s, causing further headward and lateral erosion of the arroyo until about A.D. 1940 (Webb, 1985). This arroyo cutting episode caused significant environmental and economic damage in the area of the newly established town of Esca-lante, Utah (Webb, 1985; Webb and Baker, 1987). While historical accounts documented this incision event, less is known about past arroyo cut-fill events within the upper Escalante.
METHODS
Field work was performed during A.D. 2009–2010 and included surficial mapping, stratigraphic descriptions, and radiocarbon (14C) and optically stimulated lumines-cence (OSL) sample collection. The selected map area en-compassed 146 square kilometers and covered parts of four USGS 7.5" quadrangles – Upper Valley Creek, Canaan Creek, Wide Hollow Reservoir, and Escalante. Detailed mapping of the area as part of a U.S. Geological Survey EDMAP project was the first step towards reconstruction of the arroyo history of the upper Escalante. The map area followed the alluvial corridor and focused on Quaternary surficial deposits (with an emphasis on alluvial deposits) at a 1:24,000 scale (plates 1-4). This was accomplished by mapping in the field with the aid of topographic maps and aerial photographs. Identified map units were digitized us-ing ArcGIS.
Initial field reconnaissance of arroyo-wall exposures along the upper Escalante River channel revealed numer-ous sites for detailed investigation. Study sites were select-ed based on recognition of unconformity-bounded fluvial
sequences or the presence of a thick sequence of aggrada-tional packages. A total of eleven sites (labeled A–J and Z) were selected for age control and/or detailed stratigraphic study. Outcrops A–J are located (from up to downstream) along the exposed arroyo walls, while Outcrop Z is des-ignated separately because it represents higher and older deposits exposed within a roadcut (figure 4).
The stratigraphy and sedimentology of each site were described and photo-documented, and sedimentary context was recorded for all OSL and 14C samples collected. De-tailed outcrop panels were recorded at sites A, B, C, D, E, F, H, and I and included descriptions of the following: con-tacts (e.g. unconformities, important marker horizons), bed thickness and geometry, grain size and sorting, sediment color, sedimentary structures, and indicators of weakly de-veloped soils. Representative stratigraphic sections were constructed from each of the outcrop panels. This paper provides the alluvial stratigraphy of outcrops A–C, F, and H; full descriptions from all study sites are available in Hayden (2011).
Soil and unconformity-bounded alluvial packages in the study outcrops were dated using both 14C and OSL dat-ing methods in order to better constrain the timing of ar-royo cut-and-fill events along the upper Escalante. A total of 18 14C samples and 20 OSL samples were collected from 11 outcrops (figure 4).
Radiocarbon Dating
In order to minimize problems associated with re-working of older charcoal, preference was given to un-burned organic material or charcoal obtained from in-situ burn surfaces; however, such material was not readily ob-tainable at most outcrops. Charcoal-rich lenses were fairly common at key outcrops. Although clearly water-transport-ed (thereby redeposited), large angular pieces were select-ed where available as they indicate minimal transport and deposition shortly after a fire within the catchment. Finally, in some locations, isolated pieces of charcoal or a single piece extracted from a bulk sample was collected, although these were the least desirable. Sample material and context were recorded for each sample collected (table 2).
Fifteen of the 18 14C samples were pre-treated and analyzed at the University of Arizona Accelerator Mass Spectrometry (AMS) Laboratory. The remaining three were sent to Beta Analytic. All 14C ages were calibrated using the IntCal09 calibration curve at the 2-sigma con-fidence level (Reimer and others, 2009) and are reported in calendar years B.P. (2010) to allow direct comparisons with OSL ages. The maximum probability and asymmetric 2-sigma error is reported on figures and is used for discus-sion in the text (Telford and others, 2004).
OSL Dating
OSL provides an age estimate for the last time sedi-
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Figure 4. Location of sites selected for age control and stratigraphic description.
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Outcrop Sample No. Lab No. Fill Unit 1 Depth (m) Material/ Context 2
δ13C 14C age 3 weighted mean 4 (cal ka
B.P. 2010)
Error (+/-) 5
A 14C-A1 AA87515 IV 1.0–1.5a C / B -24 1215 ± 36 1.20 0.12 / 0.08 A 14C-A2 AA87514 II 5.0–6.0a C / B -22 2880 ± 39 3.07 0.15 / 0.13 A 14C-A3 Beta 281705 II 7.0 C / CRL -23 3570 ± 40 3.97 0.17 / 0.18
B 14C-B1 AA87513 V 0.7 C / CRL -27 152 ± 35 0.21 0.13 / 0.15 B 14C-B2 Beta 261342 V 4.4 C / CRL -25 670 ± 40 0.68 0.06 / 0.07 B 14C-B3 AA87512 II 6.2 C / SP -23 3627 ± 40 4.01 0.13 / 0.11
C 14C-C1 AA87503 II 4.9 C / SP -25 3440 ± 48 3.77 0.12 / 0.13 C 14C-C2 AA87502 I 5.5 C / SP -23 4053 ± 40 4.61 0.25 / 0.13
D 14C-D1 AA87504 IV 4.0 C / H -14.0 1494 ± 37 1.44 0.13 / 0.08 D 14C-D2 AA87508 III 4.6 C / CRL -22 1940 ± 43 1.95 0.10 / 0.15 D 14C-D3 AA87506 III 4.7 C / SP -22.0 2022 ± 37 2.04 0.13 / 0.09
E 14C-E1 AA87507 V 2.6 C / CRL -20 173 ± 35 0.22 0.14 / 0.16
F 14C-F1 AA87505 V 2.5 C / CRL -27 357 ± 35 0.47 0.09 / 0.10
H 14C-H1 Beta 281704 V 1.0 O / P -22 40 ± 40 modern H 14C-H2 AA87501 V 1.5 C / SP -27 >49900 H 14C-H3 AA87509 III 3.6 C / CRL -22 2170 ± 38 2.26 0.11 / 0.14 I 14C-I1 AA87510 V 4.3 C / CRL -27 >49900 I 14C-I2 AA87511 V 4.9 C / CRL -22 1121 ± 36 1.09 0.14 / 0.09
1 Fill units refer to distinct Holocene aggradational packages in the study area denoted unit I (oldest) to unit V (youngest). 2 C = charcoal; O = organic material; CRL = charcoal-rich lens; SP = isolated single piece; H = hearth or burn horizon; B = single piece from a bulk sample from unit of interest (sediments were collected for gastropods); P - pine cone 3 δ13C corrected conventional age 4 Maximum probability of 2-sigma range calculated using weighted mean; ages reported in cal ka B.P. (2010) rounded to the nearest decade. Calculated using a spreadsheet provided by G. Meyer, University of New Mexico. 5 Calibrated using IntCal09 (Reimer and others, 2009); 2-sigma range is shown in cal ka B.P. (2010) a A range is given because a single piece from a bulk sample from the unit of interest was dated and exact depth is unknown.
Table 2. Radiocarbon samples – information and ages.
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ment (in this case, quartz sand) was exposed to sunlight (Huntley and others, 1985). After deposition and burial, de-fects in the quartz crystal lattice trap electrons produced by ionizing radiation from radioisotopes of potassium, rubid-ium, thorium, and uranium in the surrounding sediments and from exposure to incoming cosmic radiation. These traps are emptied, or zeroed, when exposed to light during sediment transport, a process called bleaching. In the lab, the natural luminescence signal acquired during burial is compared to the luminescence produced after exposure to known doses of radiation. The OSL age is calculated by di-viding the equivalent dose (De) by the environmental dose rate.
The OSL dating method assumes that the grains be-ing analyzed were completely reset (bleached) by sunlight prior to burial, which may not always be the case in fluvial settings (e.g. Wallinga and others, 2001; Jain and others, 2004; Rittenour, 2008). Transport distances in small river systems are short, limiting sunlight exposure. Moreover, addition of older sediments stored in arroyo walls is likely in the upper Escalante. Further, flows in semi-arid river systems are often turbid, which reduces sunlight penetra-
tion into the water column. Such conditions can lead to in-complete resetting of the luminescence signal (incomplete bleaching) of grains in a sample, which can cause an over-estimate of the calculated age (Wallinga and others, 2001; Murray and Olley, 2002; Jain and others, 2004).
All OSL samples were prepared and analyzed at the Utah State University Luminescence Laboratory (table 3). Because incomplete bleaching was expected to be an issue in this system (see Summa-Nelson and Rittenour, 2012), an effort was made to identify deposits containing sediments most likely to have experienced sufficient sunlight expo-sure prior to deposition. For example, deposits exhibiting sedimentary structures indicative of rapid flow and high sediment concentration were avoided. Bioturbated units (those displaying massive bedding or numerous root traces and/or animal burrows) were also avoided because biotur-bation causes the mixing of different-aged grain popula-tions. As suspected, most Holocene-age OSL samples dis-played evidence for incomplete bleaching, and therefore most OSL ages were calculated using the minimum-age model (MAM) of Galbraith and others (1999) to determine an accurate depositional age. Additionally, single-grain
Table 3. OSL samples – information and ages.
Outcrop USU No. Fill unit Depth (m) a
Analysis method b
Dose rate c (Gy/ka)
De ± 2 s.e. (Gy) d
OSL age ± 1 s.e. (ka)
Model
A USU-754 IV 1.3 SA 2.63 ± 0.14 3.13 ± 1.25 1.19 ± 0.26 MAM-3 A USU-753 II 4.8 SA 2.72 ± 0.15 9.44 ± 2.44 3.47 ± 0.71 MAM-3 A USU-755 II 6.2 SA 2.89 ± 0.15 11.39 ± 2.58 3.94 ± 0.80 MAM-4 A USU-814 - 9.0 SA 2.71 ± 0.15 204 ± 31 75 ± 14 mean
B USU-473 V 4.5 SG 2.19 ± 0.12 1.63 ± 0.31 0.74 ± 0.11 MAM-3 B USU-815 V 6.5 SG 1.89 ± 0.10 1.56 ± 0.37 0.82 ± 0.12 MAM-4 B USU-472 II 5.0 SG 1.65 ± 0.09 6.65 ± 1.73 4.04 ± 0.62 MAM-4
D USU-608 IV 3.5 SG 2.30 ± 0.12 3.11 ± 0.76 1.35 ± 0.28 MAM-4 D USU-607 III 4.5 SA 1.80 ± 0.10 4.77 ± 1.47 2.65 ± 0.54 MAM-3
E USU-702 V 1.5 SG 1.54 ± 0.09 0.40 ± 0.13 0.26 ± 0.05 MAM-3 E USU-706 V 2.4 SG 2.21 ± 0.12 0.38 ± 0.23 0.17 ± 0.11 MAM-4 E USU-705 V 3.1 SG 1.99 ± 0.11 1.48 ± 0.42 0.74 ± 0.15 MAM-4
F USU-700 V 2.2 SG 2.20 ± 0.12 0.80 ± 0.76 0.36 ± 0.12 MAM-4 F USU-701 IV 3.6 SA 2.20 ± 0.12 2.70 ± 0.95 1.23 ± 0.27 MAM-3
H USU-606 V 1.8 SG 1.84 ± 0.10 1.79 ± 0.47 0.98 ± 0.23 MAM-3 H USU-707 III 3.6 SG 2.09 ± 0.12 4.70 ± 0.30 2.25 ± 0.24 CAM I USU-756 V 1.4 SG 1.81 ± 0.10 0.45 ± 0.28 0.25 ± 0.08 MAM-4 I USU-604 V 3.0 SA 2.11 ± 0.11 0.99 ± 0.50 0.47 ± 0.13 MAM-4 I USU-603 V 4.6 SA 1.69 ± 0.09 1.89 ± 0.83 1.12 ± 0.27 MAM-3
Z USU-698 QTgr 9.6 SA 2.52 ± 0.13 179.07 ± 8.88 71.05 ± 7.38 CAM
a Indicates depth below current geomorphic surface (Qat1 for all sites A–J).
b SA: small aliquot (1-mm mask for all except USU 698); SG: single-grain.
c Dose rate conversion factors from Guérin and others (2011).
d Age analysis using the single-aliquot regenerative-dose procedure of Murray and Wintle (2000) on purified quartz sand between 90-150m. De and ages
calculated using the minimum age model (MAM) or central age model (CAM) of Galbraith and others (1999). Excel macros written by Sebastién Huot (UQAM).
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dating was used to obtain the age of 11 of the 20 OSL sam-ples. Single-grain analysis was generally used for samples that were very young (<1 ka) or showed significant posi-tive skew (indicative of incomplete bleaching) in the multi-grain small-aliquot data (Bailey and Arnold, 2006).
DATA PRESENTATION
Stratigraphy
Eight of the 11 outcrops (A–F, H, and I) were selected for detailed stratigraphic description (see Hayden, 2011). Given the rather uniform character of sediment in the wa-tershed (generally gray and tan silts and sands derived from Cretaceous bedrock), sediment packages of different ages were not always easy to distinguish. The eight outcrops were selected because they included either unconformable depositional sequences and soils or because they contained recognizable buttress unconformities that could help con-strain the timing of distinct aggradational packages. Five outcrops that best illustrate these relations (A, B, C, F, and H) are highlighted here. Five aggradational packages, denoted units I (oldest) – V (youngest), were identified based on bounding unconformities, soil development, geo-chronology, and sedimentary characteristics.
Within these units, five major depositional environ-ments (facies associations) were identified and named roughly following the terminology introduced by Harvey and others (2011) and refined in Huff (2013). These in-clude: 1) channel bottom deposits (CB) (both mainstem and tributary), which consist of imbricated gravel, granules and cross-bedded generally medium to coarse sand. These de-posits were generally lenticular and of limited lateral extent (2–10 meters). 2) Channel margin deposits (CM), which include small side-channel deposits and/or multiple cen-timeter-to-decimeter-scale tabular ripple-laminated sand with overlying mud drapes indicating deposition by non-channelized flow. The interpreted overbank flood deposits are often laterally continuous (up to 10s of meters). 3) The channel margin deposits of Harvey and others (2011) were modified to include a separate classification for slackwater deposits in floodplain lakes/marshes, denoted as slackwa-ter channel margin deposits (CMs). These consist of finer-grained, horizontally bedded silt and clay that often showed evidence of relatively slow aggradation as evidenced by the bioturbation/disruption of initial beds or laminae. These de-posits are commonly associated with snail assemblages and redoximorphic features indicative of a high water table for at least some portion of the year. These deposits were also laterally continuous (up to tens of meters). 4) Soils, eolian deposits and colluvial facies were grouped into the valley surface facies association (VS) of Harvey and others (2011). 5) Massive colluvial wedge deposits mantling paleo-arroyo walls and buttress unconformities. Massive deposits were also found filling former channels in the stratigraphy. Fol-
lowing Huff (2013), these were identified as valley-surface colluvial wedge deposits (VSc). Preservation of deposits was variable; several sites exhibited intense bioturbation and weak soil development, indicating periods of tempo-rary stability of geomorphic surfaces.
Outcrop A
Outcrop A (figure 5) is the farthest upstream study site and is located near the valley edge at the base of one of the Pleistocene gravels (QTgr) (figure 4; plate 4). This ap-proximately 10-meter-tall arroyo wall is characterized by three units separated by erosional unconformities and soil development. The oldest sediments at the base of Outcrop A (>3 meters thick) consist of four distinct beds. Basal de-posits consist of pebble-cobble channel gravel (CB facies association) deposits overlain by gravelly matrix-supported mass-movement deposits (VSc facies association). These in turn are conformably overlain by up to 2 meters of finer-grained sediment consisting of well-indurated fine sandy silt and occasional mud drapes. Dry colors (Gley 1 7/10Y matrix with abundant redox depletions [mottles] of 10YR 7/6) along with gastropods indicate deposition in a marshy or slackwater setting (CMs facies association). Weak soil development features (redox depletions, common calcite nodules, bioturbation) at the top of the oldest package indi-cate a period of floodplain stability and non-deposition. A preliminary OSL age estimate collected from the pebble-cobble gravel at the base of this unit (USU-814) produced an unexpectedly old age (approximately 75 ka), suggesting these gravels may be sourced from an older deposit, pos-sibly the QTgr (table 3).
The middle sediment package is unconformably in-set into the older unit below and has a total thickness of approximately 6 meters (figure 5). Basal deposits include imbricated, well-rounded gravels and planar to cross-bed-ded sand (CB facies association). These channel deposits are capped by approximately 1.5 meters of centimeter- to decimeter-scale planar-bedded medium sand separated by millimeter-scale silty sand lamina. These tabular CM facies sand units show minimal evidence for bioturbation, indi-cating fairly rapid deposition by unconfined flows. Overly-ing this are two distinct deposits consisting of alternating millimeter-scale layers of very fine silty sand and darker, more organic-rich silt (CMs facies association). These lay-ers contain abundant rooting with redoximorphic concentra-tions, indicating quiet shallow-water deposition. The finer-grained deposits are separated by approximately 1 meter of medium-grained sand with small gravel lenses at the base, suggesting lateral channel shift. The uppermost unit in this intermediate package consists of faintly bedded to massive sand with a possible weak A horizon at the top as indicated by darkened sediments. Two 14C and two OSL samples were collected from this intermediate soil and unconformity-bound unit (tables 2 and 3). Results are in stratigraphic order within error and suggest aggradation between 4 and 3 ka.
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The youngest package of deposits in Outcrop A con-sists of approximately 1 meter of tabular centimeter- to decimeter-scale fine to medium cross-bedded sand (CM facies association) capped by thin silty layers and the mod-ern soil. OSL and 14C results are similar for this package and indicate deposition at approximately 1.2 ka (tables 2 and 3).
Outcrop B
Outcrop B is approximately 7 meters in height and exhibits a major buttress unconformity separating two packages (figure 6). The older alluvial package consists of packets of alternating fine-grained (clay to silt) CMs de-posits and sand-dominated CM deposits. The fine-grained deposits (approximately 1 meter thick) are characterized by tabular, weakly bedded clay and silt that exhibit blocky structure and vertical cracking. Gastropods are common throughout both these CMs deposits. Sandy deposits (<1–1.5 meters thick) consist of fine to medium sand with some coarse sand/pebble gravel lenses. Weak bedding was vis-ible in some packages, although many sand bodies exhibit-ed massive structure and rooting, indicating that deposition
was not occurring at a constant rate. Small lenses of imbri-cated gravel and bedded sand indicate channel aggradation (CB facies association). Radiocarbon and OSL results from the base of the oldest alluvial package at Outcrop B suggest deposition approximately 4 ka (figure 6, tables 2 and 3).
Following aggradation of the older package at Out-crop B, the system incised at least 4.6 meters as indicated by clear truncation of layers and mantling of the paleo-arroyo wall with a colluvial wedge (VSc facies). The younger sed-iment package is inset into and fills this paleo-arroyo chan-nel. Sedimentary beds are generally tabular, although they become broadly lenticular at the paleo-arroyo wall bound-ary. Overall, sediments of the main body of the younger package are more sand-dominated (CM facies) than the sub-adjacent older sediments, although there are two rela-tively thin beds of fine-grained clay/silt (CMs facies) with-in the paleo-arroyo fill. Basal trough cross-bedded, fine to coarse sand includes common mud rip-up clasts, indicating channel deposition (CB facies).
The upper part of the younger package consists of more than 2 meters of sediments that cap the older depos-its and inset channel fill, and consists of tabular decimeter-scale planar bedded sand interbedded with thin silt-rich
Figure 5. Outcrop A photograph and stratigraphic section with summary of all ages. Samples were collected over a greater distance, but units are laterally traceable, so all samples are displayed on one photo for simplicity. Explanation will be used for all subsequent outcrop figures.
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layers, indicating deposition through a series of unconfined sheet-like flood events. A faint A horizon, indicated by a darkening of sediments, is evident at the top of both the older fill and the inset portion of the younger fill, which may indicate a local hiatus in deposition prior to deposition of the upper 2 meters of CM sediment at Outcrop B. OSL and radiocarbon ages from the inset fill suggest deposition from approximately 0.8 to 0.2 ka.
Outcrop C
Outcrop C (figure 7) is approximately 7 meters in height and consists of three sediment packages separated by erosional unconformities. The oldest sedimentary unit (>1.5 meters thick) at this outcrop consists of massive tan sand with burrows, indicating weak soil development and surface stability following deposition. Radiocarbon age control suggests this is one of the oldest deposits within the upper Escalante River at 4.6 ka (14C-C2, table 2).
Intermediate-aged deposits are slightly erosional into the oldest unit and consist of up to approximately 2 me-ters of fine grained (silty clay) deposits (CMs facies) that display centimeter-scale bedding. Extensive redoximor-phic concentrations (matrix color Gley 1 7/5G with 10R 7/4 mottles, dry), rooting, and common gastropods indicate
quiet shallow water deposition. Radiocarbon age results suggest deposition at 3.8 ka (14C-C1, table 2).
The upper 3–4 meters of this outcrop consists mostly of massive sand. Imbricated gravels and weakly preserved planar bedding in a channel form at the base of this up-per unit suggest channel incision (CB facies) followed by subsequent aggradation (CM facies). No age control was obtained from this upper depositional unit.
Outcrop F
Outcrop F (figure 8) is approximately 3.8 meters in height and consists of two alluvial packages. The older unit consists of at least 1 meter of faintly planar bedded fine to medium sand (CM facies) capped by a heavily bioturbated package of massive silty very fine sand (up to 0.5 meter) with some burrows, indicating a period of surface stability. An OSL analysis (USU-701) of this unit produced an age of 1.2 ka (table 3).
The younger unit is erosionally inset into the older de-posits. It consists of imbricated channel gravels (CB facies) at the base and massive to faintly bedded sand with small gravel lenses above (CM). The channel-filling sediments of the younger unit interfinger laterally with channel marginal deposits, with evidence for bioturbation increasing with
Figure 6. Outcrop B photograph with geochronology summary and representative stratigraphic sections. See figure 5 for symbol expla-nations.
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Figure 7. Photograph showing a portion of Outcrop C and representative stratigraphic section. The two samples were collected over a larger lateral distance but approximate stratigraphic positions are indicated on the photo. See figure 5 for symbol explanations.
Figure 8. Outcrop F photograph and geochronology with stratigraphic section from the channel margin portion of the outcrop. See figure 5 for symbol explanations.
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distance from the channel form. Radiocarbon and OSL age control suggests that the inset unit was deposited 0.5-0.4 ka (14C-F1 and USU-700, tables 2 and 3).
Outcrop H
Outcrop H (figure 9) is a combination of two nearby but separate outcrop exposures. The oldest units are pre-served on the downstream end of the exposure, and the youngest units are preserved at the upstream portion. At this outcrop, the oldest alluvial unit consists of planar bed-ded fine to medium sand (2.5Y 5/3, CM facies) that transi-tions upward into more massive sand with only faint bed-ding preserved. The massive sand, slightly lighter color (2.5Y 6/3), and more intense rooting in the upper portion of this unit indicate soil development.
The middle alluvial unit scours into the oldest and is characterized by a large deposit of imbricated, rounded channel gravels (CB facies) topped by planar to ripple-lam-inated fine to coarse sand with granule-pebble lenses (CM facies). This package is overlain by mostly massive fine sand that is likely part of the middle unit as well, although the age of these beds was not determined because the 14C sample collected here was composed of coal and was radio-carbon dead (14C-H2, table 2).
The upstream portion of the streambank exposes an inset deposit at the mouth of a tributary fan. This inset pack-age of channel gravels and fine to coarse sand (CB facies) is topped by up to 1.3 meters of well-preserved planar to trough bedded medium sand (CM facies) that probably rep-resent one flood event. Another slightly younger or coeval
package of sediment is inset into this deposit. Radiocarbon analysis of a pinecone collected from the older of the two inset deposits produced a modern age (14C-H1, table 2).
Outcrop Z
Outcrop Z is a road cut into QTgr deposits along Utah State Route 12 (figure 4). Exposed deposits are composed of pebble to cobble gravel with a carbonate-rich soil devel-oped into the surface. An OSL sample collected from with-in the deposit produced an age of 71 ka (USU-698, table 3).
Chronostratigraphy
The Holocene alluvial chronostratigraphy of the upper Escalante was developed using stratigraphic relationships and age control from all studied outcrops, although the out-crops discussed above (A – C, F and H) are highlighted in this section given their importance in the identification of cut-fill events. Five aggradational packages, denoted units I (oldest) – V (youngest), were identified based on bounding unconformities, soil development, age control, and sedi-mentary characteristics.
Unit I
Evidence for the oldest Holocene fill in the upper Es-calante (Unit I) was only identified at two locations (Out-crops A and C) (figures 5 and 7). The onset of deposition for this unit is not known, although 14C results from Out-crop C indicate deposition was ongoing by 4.6 cal ka B.P. OSL analyses on what are interpreted to be channel gravels
Figure 9. A. Outcrop H illustration with geochronology of upstream portion of outcrop. B. Outcrop H illustration of downstream portion of outcrop. C. Photograph showing downstream portion of outcrop. See figure 5 for symbol explanations.
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below Unit I deposits at Outcrop A produced a preliminary age estimate of 75 ± 14 ka (USU-814, table 3), but the sedi-ments sampled are interpreted to have been reworked from Pleistocene gravel deposits immediately upslope and be-hind this outcrop (QTgr map unit, plate 4).
The unit I deposits at Outcrop C consist of up to 1.8 meters of massive sandy soil, and a 14C sample from the top of this unit returned an age of 4.61 cal ka B.P. As the top of the unit I sediments were eroded in places, the termination of aggradation likely occurred later. Outcrop A (figure 5) suggests that at least 3 meters of probable unit I sediments aggraded prior to approximately 4 ka based on an age of 3.97 cal ka B.P. (Beta 281705, 14C-A3, table 2) that was obtained from the base of the overlying sedi-ments. Field stratigraphic evidence suggests that the river may have been below the modern level at the onset of unit I deposition given that the base of unit I sediments are not visible at either location where they are present.
Unit I sediments at both Outcrops A and C showed signs of soil development. Outcrop A (figure 5) displayed significant mottling, calcite nodules, and a slight darken-ing towards the top, indicating a weakly developed A ho-rizon. Unit I sediments at Outcrop C consisted of massive/blended sand with common burrowing. The heights of these weakly developed soils and upper deposits of unit I at both sites were only 2–3 meters above the present ar-royo bottom. These observations suggest that the preserved extents of deposits approximate the top of unit I sediments, and that aggradation heights were lower during this period than during more recent fill episodes. However, it should be noted that some erosion and truncation of the tops of the deposits were observed at both sites.
Unit II
The second oldest aggradational package is present at three of the study outcrops (A, B, and C). Age control obtained indicates that aggradation of this package started approximately 4 ka and continued until at least approxi-mately 2.9 ka. Charcoal samples were collected from the base of unit II sediments at Outcrops A and C. At Outcrop A (figure 5), a 14C sample was collected from channel-margin sands (CM facies) that grade laterally to channel gravels (CB facies) that are inset into unit I, indicating that these sediments date the approximate onset of unit II aggrada-tion. Radiocarbon age results of 3.97 cal ka B.P. (Beta 281705, 14C-A3, table 2) are consistent with an age of 3.77
cal ka B.P. (AA87503, 14C-C1, table 2) obtained from basal unit II sediments at Outcrop C (figure 7).
Although the base of unit II sediments was not ob-served at Outcrop B, the sediments are consistent both in timing and in sedimentary context with those found at Out-crops A and C. Unit II sediments at Outcrops A and C com-monly contain fine-grained cienega-type deposits (CMs facies), and this was observed at Outcrop B as well (figure 6). A 14C date obtained from near the base of Outcrop B has
an age of 4.01 cal ka B.P. (AA87512, 14C-B3, table 2) and a stratigraphically higher OSL sample has an age of 4.04 ± 0.62 ka (USU-472, table 3). At least 2 meters of sed-iments aggraded after deposition of USU-472, indicating that deposition continued until a period of surface stability that produced a weak A horizon.
A well dated sequence at Outcrop A indicates that unit II aggradation was still ongoing by approximately 3 ka. Two OSL samples and a 14C sample returned ages within error that suggest continual aggradation from 3.94 ± 0.80 ka (USU-755) through 3.47 ± 0.71 ka (USU-753) (table 3). A 14C sample from a cienega-type deposit (CMs facies) be-tween USU-755 and USU-753 returned a slightly younger but stratigraphically consistent age of 3.07 cal ka B.P. (AA87514, 14C-A2, table 2). Given that the upper 3 meters of unit II was not dated at Outcrop A, it is clear that aggra-dation continued beyond 3 ka, but the termination of unit II deposition is not well-constrained. Consistent with Outcrop B, a possible weakly developed A horizon at the top of unit II indicates a period of surface stability after deposition of this package.
Unit III
Unit III sediments are present at two outcrops in the study area, D and H (figure 9). Age control suggests that aggradation occurred from approximately 2.5 ka to at least 2 ka. At both locations unit III sediments are located at the base of outcrops. Sediments from this unit are generally uniform and consist mainly of CB/CM sandy facies with common soil development features.
At Outcrop H (figure 9), an OSL sample and 14C sam-ple were taken side-by-side, and yielded consistent ages of 2.25 ± 0.24 (USU-707, table 3) and 2.26 cal ka B.P., (AA87509, 14C-H3, table 2), respectively. Outcrop H also shows evidence of soil development in the upper 0.5–1.0 meter including massive sand, common rooting and CaCO3 accumulations. At Outcrop D, ages of 2.04
cal ka B.P. (AA87506, 14C-D3, table 2), 1.95 cal ka B.P. (AA87508, 14C-D2, table 2) and an OSL age of 2.65 ± 0.54 ka (USU-607, table 3) were obtained at the base of the outcrop (see Hayden, 2011). The base of unit III is not exposed, so it is not possible to say with certainty when aggradation commenced. However, soil development features at the top of this unit (massive sand with floating granules/pebbles, burrows, common fine CaCO3 stringers) indicate a period of stability around 2 ka.
Unit IV
There is evidence for the fourth aggradation event at three locations within the study area, (Outcrops A, D, and F). Age control indicates that aggradation of this package occurred at approximately 1.6 ka and continued until at ap-proximately 1 ka. At Outcrop A (figure 5), the onset of unit IV aggradation is inferred from a change in sedimentary
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characteristics. A continuous aggradation packet of unit II sediments (consisting of alternating sand beds and cienega-type deposits, CMs facies) is capped by a weak A horizon, indicating the likely end of unit II deposition. OSL and 14C ages obtained from the overlying unit IV sediments result in consistent ages of 1.19 ± 0.26 ka (USU-754, table 3) and 1.20 cal ka B.P. (AA87515, 14C-A1, table 2) (figure 5). Unit IV correlative sediments from Outcrop F produced a similar small-aliquot OSL age of 1.23 ± 0.27 ka (USU-701, table 3).
Unit V
Unit V is the most abundant alluvial fill package found in the study area, and is exposed at seven of the ten study outcrops (B, D, E–I). These well-dated sediments suggest that the most recent period of aggradation initiated approxi-mately 0.8 ka and continued until historic arroyo cutting in 1909 A.D.
Outcrop B (figure 6), which displays a large channel scour cutting at least 4.7 meters into older unit II depos-its, represents the best example of unit V. An OSL sam-ple was collected from the base of this large channel scour and produced an age of 0.82 ± 0.12 ka (USU-815, table 3). A 14C sample collected from a charcoal-rich sand lens ap-proximately 2 meters above USU-815 returned an age of 0.68 cal ka B.P. (Beta 261342, 14C-B2, table 2), and an OSL sample collected from ripple-laminated sand ap-proximately 1 meter above this has an age of 0.74 ± 0.11 (USU-473, table 3). Approximately 1 meter of aggradation occurred after deposition of USU-473, and a possible weak A horizon indicates a period of stability prior to the up-per 2 meters of unit V deposition at this site, which pro-duced a radiocarbon age of 0.21 cal ka B.P. (14C-B1, AA87513, table 2).
At Outcrop F (figure 8), unit V channel gravels are inset approximately 1.5 meters into the underlying unit IV sediments. A radiocarbon sample and an OSL sample were collected from planar bedded fine sands near the channel margin and produced stratigraphically consistent ages of 0.47 cal ka B.P. (AA87505, 14C-F1, table 2) and 0.36 ± 0.12 ka (USU-700, table 3), respectively.
At Outcrop H (figure 9), unit V sediments are ero-sional into underlying unit III sediments. A thick suite of channel gravels (CB facies) fine upward into planar bed-ded fine upper to coarse sand (CM facies). An OSL sample was taken from well-bedded CB sand capping the channel gravels and returned an age of 0.98 ± 0.23 ka (USU-606, table 3). This well-bedded sand is capped by approximately 2 meters of mostly massive sand. A 14C date obtained from these massive sands was radiocarbon dead and therefore interpreted to be coal.
Throughout the study area, it was observed that a number of exposures were capped by 1–3 meters of sheet-flood-type CM deposits exhibiting well-preserved sedimen-tary structures and weak surface soils indicative of recent
deposition. These younger unit V deposits are well illustrat-ed at Outcrop B and consist of ≥2 meters of centimeter-to-decimeter-scale bedded sand with thin silty interbeds. A 14C sample was collected from near the top of unit V sediments at Outcrop B (figure 6) and returned an age of 0.21 cal ka B.P. (AA87513, 14C-B1, table 2). Similar deposits near the top of Outcrop I produced a similar OSL age of 0.25 ± 0.08 ka (USU-756, table 3).
Older Deposits (QTgr)
The QTgr map unit comprises a prominent map unit in the upper Escalante River catchment (plate 4). These large dissected Pleistocene deposits are composed of tens of meters of well-rounded, imbricated gravels of varying lithology that have been reworked from Cretaceous to Ter-tiary formations in the study area, and appear to be graded to a base level that is below the pre-1909 floodplain. Two OSL samples were collected from cobble to pebble gravels in the catchment. The similarity of the OSL age results sug-gest that similar-aged QTgr deposits are found at Outcrop A and Z. These results suggest regional gravelly alluvial fan deposition 70-75 ka (USU-814 and USU-698, table 3).
DISCUSSION
Interpretations of the aggradation and entrenchment events recorded in the alluvial chronostratigraphy are pre-sented in figure 10. It is possible there were more cut-fill events than described here, especially in the older por-tion of the record as older sequences are less likely to be preserved. Alternatively, it is possible that we have over-interpreted the stratigraphy and fewer cut-fill events have occurred. For example, some identified buttress uncon-formities may not represent system-wide entrenchment but instead lateral channel shift. Preservation of paleo-arroyo channels and good examples of buttress unconformities are rare, and in some locations other indicators of incision such as soil formation were used to infer arroyo cutting. How-ever, it is noted that other conditions such as local stability with no net incision or aggradation could also result in soil development.
While efforts were made to collect the most reliable samples for 14C and OSL dating, some samples may be pro-ducing age overestimates due to incomplete resetting of the luminescence signal, redeposition of charcoal from an older deposit, or the influence of charcoal originating from heartwood or ancient wood on the landscape (e.g. Schiffer, 1986). Partial bleaching of luminescence samples was ameliorated by use of single-grain dating and a minimum-age model to calculate the OSL age (e.g. Bailey and Ar-nold, 2006). However, some OSL ages may still be over-estimated and one sample (USU-607, Outcrop D) produced a stratigraphic inversion that is not within error of 14C and other OSL results (see figure 10). Based on the possibility
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Figure 10. Graphical representation of geochronology obtained for the upper Escalante River. The geochronology, along with stratig-raphy and geomorphic relations, resulted in the identification of five distinct aggradational events since the middle Holocene. Samples are color-coded to match the depositional unit to which they belong, and white areas indicate time periods of incision (or stability/non-deposition).
of age overestimates in OSL samples and redeposition of older charcoal, the timing of initiation of aggradation was based on multiple lines of evidence including coherence of ages within depositional packages, presence of stratigraph-ic unconformities and soils, and youngest age results from subjacent fill packages.
Chronostratigraphic results suggest that there have been at least five arroyo cut-fill episodes since the middle Holocene. OSL and 14C age control suggests that aggrada-tion occurred prior to 4.6 ka (unit I), ~4.2–2.9 ka (unit II), ~2.5–1.8 ka (unit III), 1.6–1.0 ka (unit IV) and 0.8–0.1 ka (unit V) (figure 10). Unconformity and soil-based evidence suggests entrenchment occurred at ~4.4–4.2 ka, ~2.9–2.5 ka, ~1.8–1.6 ka, ~1.0–0.8 ka, and during the historic arroyo entrenchment 0.1 ka (commencing A.D. 1909).
Early Holocene deposits are notably absent in the study area. The oldest deposit (unit I) is dated to 4.61
cal ka B.P. (14C-C2, table 2 and figure 7). The basal stratigraphic position of unit I at outcrops A and C (the top of unit I is only 2–3 meters above the modern channel in both locations) suggests that the river had a lower grade at about 4.6 ka. Thus, it is possible that older deposits may be buried beneath the alluvium. Alternatively, older depos-its may have once existed but are no longer preserved in the system. Finally, it is possible that a significant package of early Holocene sediment was not deposited in the study reach.
The onset of aggradation of unit 1 is not well con-strained, and only one age was obtained for this unit (Outcrop C, figure 7). However, incision following unit 1 aggradation is fairly well constrained. At Outcrop C, unit II sediments are only slightly erosional into unit I, but there is a clear difference in sedimentary characteristics. Unit I is capped by a sandy soil, whereas unit II sediments at this outcrop indicate deposition in a marshy slackwater setting (CMs facies). Outcrop A provides further evidence for the first incision event approximately 4.2–4.4 ka (figure 5). An age of 3.97 cal ka B.P. (14C-A3) was obtained from basal unit II channel-marginal deposits (CM facies) associ-ated with a channel scour into the underlying deposits and indicates that the approximate onset of unit II aggradation is being dated. When used in conjunction with slightly old-er unit II deposits present at Outcrop B (figure 6), the prob-able incision event is placed at approximately 4.4–4.2 ka.
While initiation of unit II deposits is fairly well con-strained, the cessation of unit II deposition was not well dated. However, there is an approximately 600-year break in the geochronologic record after deposition of 14C-A2 (the youngest unit II sample) and before deposition of 14C-H3, the oldest reliable sample from unit III. While within error of 14C-H3, USU-607 is stratigraphically reversed and is likely producing an age overestimate due to partial bleaching. Where present, the topmost unit II sediments indicate a period of surface stability as evidenced by the
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presence of soil development. This suggests that the second incision event occurred after 3.0 ka but before deposition of the oldest unit III sediments at approximately 2.5 ka. At both outcrops where they occur, unit III sediments are lo-cated at the base of the arroyo wall whereas the top of unit II sediments are generally 4.5–8 meters above the modern channel, further evidence that the system had incised.
Age control suggests that aggradation of unit II oc-curred over a longer duration than any other alluvial fill deposited over the last 5000 years (approximately 1500 years as compared to ≤ 700 years for units III–V). Although this aggradation episode is two times longer than any other identified fill, the distribution of age control across several outcrops, without any clear evidence of incision, supports this interpretation.
Unit III is present only at Outcrops D and H, and age control suggests that deposition spanned at least 500 years (~2.5–1.8 ka). The third entrenchment event (~1.8–1.6 ka) is indicated by an erosional unconformity at Outcrop H (fig-ure 9) and by a gap in the chronology of approximately 0.6 ka (figure 10). Further evidence of entrenchment prior to deposition of unit IV comes from soil development associ-ated with the samples providing terminal ages for unit III deposition at Outcrops D and H (figure 9). Based on this record, unit III aggradation concluded approximately 1.8 ka.
Unit IV deposits have been identified at Outcrops A, D and F. Age control from these sites suggests aggradation between 1.6–1.0 ka. Following entrenchment at approxi-mately 1.0–0.8 ka, unit V aggradation commenced and continued until just prior to historic arroyo cutting.
At Outcrop H, unit V deposits are dominated by chan-nel gravels. The base of these channel gravels extend below the modern channel. The location of these gravels below the top of unit III further strengthen the argument that the system incised prior to unit V deposition. Similarly, at Out-crop B (figure 6) a large buttress unconformity separates unit V deposits inset into older unit II sediments.
Chronostratigraphic observations indicate that the relatively young (≤ 0.25 ka) upper portion of unit V is composed of laterally extensive sandy sheetflood deposits (CM) whereas the older deposits are characterized by a mix of CB and CM deposits. The transition to sheetflood-domi-nated deposits may indicate that the system was reaching a threshold. For example, the system may have re-aggraded to the point that overbank flows were no longer confined to the paleoarroyo boundaries.
Although sediment packages from units I–III contain some channel gravels, thick channel gravel deposits are much more common in unit IV and V sediments, indicating a possible change in hydrologic regime at approximately 1.5 ka. Larger clast sizes point to increased sediment trans-port capacity and are similar in size to modern channel gravels. However, younger deposits are generally found in the downstream reach of the study area, so this difference in
grain size may be related to location within the watershed. The increased regularity of arroyo cut-fill events
around 2.5 ka fits reasonably with the timing of incision that was summarized by Webb and Hasbargen (1997). They found evidence for five possible incision events at approximately 2000 14C yr B.P., 1500 14C yr B.P., 1000 14C yr B.P., 500 14C yr B.P., and during the historic period of arroyo incision. This study found evidence for three inci-sion events over the past 2.5 ka instead of the five reported by Webb and Hasbargen (1997). Webb and Hasbargen’s (1997) evidence for the incision event around 500 14C yr B.P. appears to come from one outcrop (WH2 in figure 2); however, given the height of the outcrop it is possible that this site represents lateral channel shift as opposed to true system-wide entrenchment. The lack of clear evidence of incision in any of the newly studied outcrops led to our interpretation that the system likely continued to aggrade from approximately 0.8 ka–0.1 ka. Prior to 2.5 ka, periods of arroyo aggradation and entrenchment were apparently longer in duration but started to occur with more regular-ity at 2.5 ka, with a spacing of approximately 0.6–0.7 ka between entrenchment events.
CONCLUSIONS
One of the main objectives of this study was to estab-lish a chronostratigraphy for the Holocene alluvial depos-its of the upper Escalante River watershed in south-cen-tral Utah. Ages obtained indicate that arroyo cut-and-fill events became an important agent of landscape evolution following approximately 4.5 ka. Since that time, at least five aggradation and entrenchment episodes have occurred, with evidence for five distinct aggradational packages and incision occurring at approximately 4.4–4.2 ka, 2.9–2.5 ka, 1.8–1.6 ka, 1.0–0.8 ka, and during the historic period of ar-royo entrenchment 0.1 ka.
ACKNOWLEDGMENTS
The authors thank Michelle S. Nelson and William Huff for comments on a previous version of this manu-script. Field work was partially supported by the Geologi-cal Society of America, SEPM (Society for Sedimentary Geology), the Association for Women Geoscientists–Salt Lake Chapter, and the Utah State University Department of Geology. Mapping was made possible by USGS EDMAP grant award number G09AC00141 to T. Rittenour. Support was also provided by NSF-1057192 ARROYO-CAREER grant to T. Rittenour. Radiocarbon preparation and analy-ses were generously provided through the University of Arizona AMS Laboratory Student Internship Program, and OSL ages were completed at Utah State University’s Lumi-nescence Laboratory.
MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors
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