The Nonglacial Surﬁ cial Geology of the Henrys Fork, Uinta ... and Pederson 2005 UGA.pdf · The Nonglacial Surﬁ cial Geology of the Henrys Fork, Uinta Mountains, Utah and Wyoming

1

INTRODUCTION

This research is the result of the fi rst mapping to focus on the nonglacial Quaternary stratigraphy of the northeastern Uinta Mountains and what it can reveal about the landscape history, climatic, and tectonic forcing mechanisms. Although the classic works of Hansen (e.g., Hansen 1965, 1986) thoroughly examine the tectonic history of the eastern Uinta Mountains and document drainage pattern changes within the region, they pay relatively little attention to the meaning of the Quaternary record and focus on tectonics, rather than climate fl uctuation, as the

signifi cant agent of landscape change. Consequently, the sequence of events and the manner in which climatic and tectonic forcing have infl uenced the landscape evolution of the eastern Uinta Mountains are still poorly understood. The Quaternary record is a logical starting point for understanding the longer Cenozoic history since much is known about Quaternary climatic and tectonic events. Additionally, relatively young Quaternary deposits are more likely to be better preserved and in their original landscape positions. The studies that have been done on the Quaternary record in the Uinta Mountains have focused on

The Nonglacial Surfi cial Geology of the Henrys Fork, Uinta Mountains, Utah and Wyoming

Ron Counts*and Joel Pederson†

ABSTRACT

Surfi cial deposits below the glacial termini of the Henrys Fork drainage have been mapped at 1:24,000 scale to develop a nonglacial Quaternary stratigraphic framework for the northeastern Uinta Mountains. This study area spans from Pleistocene glacial moraines, approximately 6 km south of the Utah-Wyoming border, to the termination of Henrys Fork at Flaming Gorge Reservoir near Manila, Utah. The Henrys Fork nonglacial stratigraphy contains nine distinct mainstem gravels, six piedmont gravels, and landslide deposits. Gravels on the Henrys Fork are grouped as older, high remnant gravels that cannot be directly linked to glacial units and younger gravels that can be traced from glacial till, through outwash plains, to stream-valley gravels with terraces formed upon them. Henrys Fork gravels are clast-supported, cobble gravel derived from the Uinta Mountain Group and Paleozoic limestone units. Near moraines, gravels are thicker but they quickly thin downstream and lie on planar bedrock straths, and so form strath terraces that converge downstream. No absolute age control currently exists for any of the Henrys Fork gravels or terraces. Henrys Fork terraces Qag2 and Qag3 can be correlated to relatively well-dated Wind River terraces and tentative incision rates for the Henrys Fork are estimated at 80-110 m/my over the late Pleistocene. These rates are similar to rates estimated for the Green River on the north slope of the Uintas in western Browns Park, but are signifi cantly less than reported rates in other central Rocky Mountain ranges and are two to three times lower than incision rates, estimated without direct age control, for the south fl ank of the Uinta Mountains. Extrapolating a linear incision-rate suggests that the oldest gravels on the Henrys Fork were deposited in the early Pleistocene.

*Kentucky Geological Survey, Henderson, KY [email protected]†Department of Geology, Utah State University, Logan, UT 84321

Counts, R., Pederson, J., 2005, The nonglacial surficial geology of the Henrys Fork, Uinta Mountains, Utah and Wyoming, in Dehler, C.M., Pederson, J.L., Sprinkel, D.A., and Kowallis, B.J., editors, Uinta Mountain geology: Utah Geological Association Publication 33, p. x-x.

glacial deposits (e.g., Atwood, 1909; Bradley, 1936; Richmond, 1965; Carrera, 1980; Bryant, 1992; Munroe, 2001; Laabs, 2004). Though the late Pleistocene glacial history of the Uintas is relatively well studied (Munroe, this volume; Laabs and Carson, this volume), less is known about older glaciations due to poor preservation of older glacial deposits. In other central Rocky Mountain ranges, insights into older glaciations were gained by studying stream terraces with glaciofl uvial origins (e.g., Reheis and others, 1991; Hancock, 1999). Likewise, the key to understanding older glaciations in the Uintas may lie in the nonglacial Quaternary record downstream of glaciated valleys. Only two previous studies of the nonglacial Quaternary stratigraphy of the Uinta Mountains exist (Osborn, 1973; Nelson and Osborn, 1991), both on the south fl ank of the Uintas, and thus our study was motivated by the need to understand all aspects of the longer-term landscape evolution.

SETTING

The Henrys Fork drains ~1,400 km2, starting in a compound cirque basin in the high Uintas and

terminating at Flaming Gorge Reservoir near Manila, UT. The study area discussed here encompasses the Henrys Fork drainage from glacial termini, just south of Lonetree, WY, to Flaming Gorge Reservoir on the north fl ank of the Uinta Mountains (fi gure 1 and Plate 1). This area includes the lower reaches of Beaver Creek and Burnt Fork, which are Henrys Fork tributaries that were glaciated. The Henrys Fork is a major tributary of the Green River, but their confl uence is now inundated by Flaming Gorge Reservoir. Physiographically, the Henrys Fork fl ows from the foothills of the Uinta Mountains onto the margin of the Green River Basin (fi gure 1). The upper Henrys Fork dissects glacial till and has a meandering channel that generally fl ows north. The steep foothills near glacial termini are thickly vegetated with lodgepole pine and aspen (fi gure 2a). Further from the foothills and away from glacial moraines, the landscape transitions to an open, sagebrush steppe used by ranchers for cattle grazing (fi gure 2b). This unglaciated section of the Henrys Fork valley begins ~6 km south of the Utah-Wyoming border. Near Lonetree, WY, the Henrys Fork abruptly turns to the east. Approximately 18

2

The Nonglacial Surfi cial Geology of the Henrys Fork, Uinta Mountains, Utah and Wyoming R. Counts and J. Pederson

Figure 1. Hillshaded digital elevation model of the lower Henrys Fork drainage showing mapped, nonglacial surficial deposits. Qal in streams and tributaries colored black, all other surficial deposits are white.

km downstream of Lonetree, the Henrys Fork valley narrows to ~0.4 km wide, on average, and meanders through a broad, grassy fl oodplain that lies between bedrock valley walls. Both the modern fl oodplain and a broad terrace 20 m above the Henrys Fork are fl ood irrigated and used for grazing cattle and cultivating hay.

PREVIOUS QUATERNARY RESEARCH

W.W. Atwood published the fi rst detailed study of glaciations in the Wasatch and Uinta Mountains in 1909 and recognized two distinct deposits that he assigned to an “earlier glacial epoch” and a “later glacial epoch”. Bradley (1936) focused his research

on the geomorphology of the northern slopes of the Uintas and identifi ed deposits of three glacial advances, the Little Dry, the Blacks Fork, and the Smiths Fork glaciations, expanding the work of Atwood. Several unpublished theses and dissertations examine glacial deposits in various northern Uinta Mountain basins (e.g., Schoenfeld, 1969; Grogger, 1974; Barnhardt, 1973; Douglass, 2000), refi ning the glacial chronology of Richmond (1965) by identifying additional Blacks Fork, Smiths Fork, and Neoglacial advances in specifi c basins. Munroe (2001) mapped the surfi cial geology in all the glaciated valleys in the northern Uinta Mountains and was able to defi ne age constraints for late-Wisconsinan ice advances using radiocarbon-dated lake cores. On the southern slopes of the Uinta Mountains, Osborn (1973) used relative dating techniques on moraines and especially downstream outwash terraces and concluded there were eight glacial episodes, some hypothetically pre-dating the Quaternary. Nelson and Osborn (1991) expanded this work and placed at least 12 terraces into six relative age groups. Laabs and others, (2003) constrain the last glacial maximum in the southern Uintas from 15-19 ka using cosmogenic-exposure dating of boulders on moraines. Most recently, Laabs (2004) mapped the surfi cial geology of the glaciated valleys of the southern Uinta Mountains and refi ned the timing of the last glacial maximum to 17.6 +/- 1.1 ka. The late Pleistocene glacial history of the northern Uintas is relatively well understood, however much less is known about older glaciations. Soils research in the Uinta Mountains is still relatively unexplored frontier, especially in the lower, unglaciated reaches of stream valleys. Boettinger and Zimmer (1993) and Zimmer (1996) examined pedogenesis on quartzite-rich moraine crests in the Smiths Fork drainage (immediately east of the Henrys Fork) and defi ned a chronosequence. Bockheim and Koerner (1997) identifi ed a thin, widespread silt layer in the eastern Uintas and interpreted it as a loess mantle. Douglass (2000) used soil-profi le development indices on moraines in the Beaver Creek drainage to identify a glacial advance between the Smiths Fork and Blacks Fork advances. Most recently, the U.S. Department of Agriculture published a detailed soil survey of the lower Henrys Fork region (U.S. Department of Agriculture, 2004).

2005 Utah Geological Association Publication 33

3

Dehler, Pederson, Sprinkel, and Kowallis, editors

Figure 2. (a) Interface between Smiths Fork outwash and till on the Henrys Fork; view facing south. (b) View looking SE over sagebrush-covered plains at the base of the Uinta Mountains. The Henrys Fork in the middle ground has a multiple-thread channel incised in older outwash and flowing right to left.

METHODS

The Quaternary stratigraphy of approximately 70 km of stream corridor along the Henrys Fork was mapped at a 1:24,000 scale. Mapping was performed on U.S. Geological Survey 7.5-degree topographic maps and augmented by analyses of aerial photographs. Field maps were compiled in a Geographic Information System through on-screen digitization using digital topographic maps, digital 1-m orthophotoquads (DOQs), and 10-m digital elevation models (DEMs) as base maps. Individual units were distinguished and correlated through fi eld inspections of sedimentology, composition, landscape position, surface characteristics such as vegetation and topography, and the degree of soil development where outcrops were available. Cross sections were surveyed with a total station, and unit thicknesses were measured using a hand level. Clast counts were performed by stretching a tape measure across outcrops and sampling at regular intervals (Wohl and others, 1996) or by random walks on terraces (Wolman, 1954). Long profi les for terraces were created using elevations extracted from U.S. Geological Survey 10-m DEMs.

HENRYS FORK STRATIGRAPHY

Glacial deposits, mainstem and tributary stream deposits, and piedmont deposits of small drainages and hillslopes form the Quaternary stratigraphy of the Henrys Fork drainage. The stratigraphy of the study area includes four glacial till deposits, ten mainstem gravels and gravel remnants, and six piedmont gravels. Other than some landslide deposits, mappable colluvium is absent from the study area. The only numeric age controls for surfi cial deposits in the Henrys Fork region are radiocarbon dates of Holocene cirque lake deposits in several catchments south of the study area (Munroe, 2001). In the descriptions that follow, relative ages and correlations were assigned to deposits based upon landscape position, surface morphology, and clast lithologies. Alluvial gravels that grade to glacial deposits were assigned the same age as the glacial deposits, and similarly, piedmont gravels were correlated by the mainstem alluvial gravel to which they are graded. More detailed descriptions of

surfi cial deposits in the Henrys Fork study area can be found in Counts (2005). Complete mapping of the Henrys Fork study area appears in Plate 1 (cd only).

Stream – Outwash Gravels

Gravel deposits along the Henrys Fork can be grouped into two categories: 1) older, high gravels that cannot be directly linked to glacial units; and 2) younger gravels that can be traced from moraines, through outwash plains, and fi nally to stream-valley gravels with terraces formed upon them (Plate 1). A suite of the older gravels is preserved near McKinnon, WY, which aids in establishing their positions relative to one another. However, older gravels are relatively few in number and often isolated, so their identifi cation and correlation are based upon their clast lithologies and elevations above the modern valley fl oor. Clasts of all Henrys Fork gravels generally include Paleozoic limestone (Madison and Morgan formations), Tertiary conglomerates (Wasatch Formation), various Mesozoic sandstones units, and clasts from the Neoproterozoic Uinta Mountain Group (UMG), which includes white to brownish quartz and maroon orthoquartzite. Clasts in all the gravels range from pebble-sized to tens of centimeters in diameter.

HIGH-LEVEL GRAVELS

The oldest gravels identifi ed on the Henrys Fork: Qag9, Qag8, Qag7, Qag6, and Qag5, are generally clast-supported, moderately- to-well sorted, subangular- to-well rounded, coarse gravels composed primarily of limestone and maroon UMG clasts. Yet there are distinct compositional differences between them. Qag9 and Qag8 contain greater percentages of limestone clasts than the other gravels, and limestone clasts are generally larger than the UMG clasts. An exception is near Antelope Wash (Plate 1), where Qag8 contains large, maroon UMG boulders with medial axes measuring up to 35 cm. Qag7 and Qag6 contain greater percentages of maroon UMG than limestone, and the UMG clasts are coarser. Qag7 commonly contains UMG boulders measuring ~0.3 m in diameter. Near the terminus of the Henrys Fork at Flaming Gorge Reservoir, the percentage of brown

4


quartzite clasts in Qag6 increases considerably at the expense of limestone. This compositional change in Qag6 may refl ect mixing of Henrys Fork and Green River gravel. The Qag5 gravel is a severely eroded, small remnant gravel identifi ed in only one area near Antelope Wash. Roughly half of the Qag5 gravel is composed of maroon UMG clasts, and the other half is tan, light gray, brown, and greenish-white quartzite clasts. The geometry and landscape positions of the high-level gravels vary considerably. Qag9 lies unconformably upon a strath cut in tilted shale and siltstone, is up to 15 m thick, and its surface forms a terrace that lies approximately 124 m above the Henrys Fork valley fl oor (fi gure 3). Three remnants of the Qag8 gravel were identifi ed on the Henrys Fork: one near McKinnon, WY, and two 16 km farther downstream on the western side of Antelope Wash (Plate 1). The Qag8 gravel is ~7 m thick, overlies a strath cut in the Tertiary Bridger Formation (Tbr), and its terrace lies approximately 115 m above the modern Henrys Fork valley fl oor (fi gure 3). The thickest observed Qag7 deposit measures ~15 m, but some UMG and carbonate clasts on its surface have pedogenic carbonate coatings, indicating the surface is somewhat exhumed and the gravel was originally thicker. The Qag7 terrace lies ~97 m above the modern valley fl oor. The thickest preserved Qag6 remnant measures ~14 m, lies on a Tbr bedrock strath, and stands ~75 m above the Henrys Fork valley fl oor. What remains of the Qag5 gravel lies 58 m above the modern Henrys Fork fl oodplain, but its

actual thickness is unknown and the elevation of the original surface may have been considerably higher. There are only two mappable, older piedmont gravels. Qagp7 is matrix- to clast-supported pebble gravel composed primarily of angular sandstone pebbles in a matrix of fi ne silt and sand. Qagp5 is clast-supported, angular, poorly sorted, pebble gravel composed of limestone and chert clasts. Qagp7, the oldest, lies in a single location near the terminus of Henrys Fork at Flaming Gorge Reservoir. It has a smooth surface that grades to the Qag7 terrace, but it has been dissected by Cottonwood Creek, a small Henrys Fork tributary that fl ows in a strike valley. The Qagp7 gravel is separated from its source area, an upland area that appears to have once existed above what is now Lucerne Valley in Manila, UT. Qagp5 gravel was identifi ed in only two locations: near the Henrys Fork terminus at Flaming Gorge Reservoir, and on the southwestern slope of Cedar Mountain across from Lonetree, WY. This piedmont gravel grades to approximately 60 m above the modern fl oodplain, a landscape position that falls between the surfaces of Qag6 and Qag3. Since virtually no Qag5 gravel is preserved along the Henrys Fork, the inference that it is Qagp5 is based only upon landscape position.

LATE PLEISTOCENE GRAVELS

The Qag4 gravel is the oldest gravel that can be directly linked to a glacial advance, but is only found in two tributaries to the Henrys Fork. Qag4


5


'

Figure 3. Surveyed topographic profile and cross section of the Henrys Fork valley northwest of McKinnon, WY (see Fig. 1) showing the relative landscape position of the high-level and the younger Henrys Fork gravels. Dashed lines are inferred contacts. View is looking downstream.

grades to pre-Blacks Fork till of Douglass (2000), and though a single Qag4 deposit exists on the broad Beaver Creek outwash plain, it forms nearly half of the Burnt Fork outwash plain immediately to the east. This poorly sorted, boulder- to pebble-gravel is composed primarily of UMG clasts, but it contains minor limestone, chert, and brown to tan quartzite clasts. The Qag4 gravel in the lower Burnt Fork valley was mapped as Bull Lake outwash (Qag3) by Schoenfeld (1969) based primarily on landscape position. Mapping shows that terraces Schoenfeld assumed were of Smiths Fork age can be traced to till that was recently identifi ed as Blacks Fork (Douglass, 2000). On the west fork of Beaver Creek, the Qag4 terrace stands ~20 m above Qag3 outwash (fi gure 4). Qagp4 gravel, like Qag4 gravel, is uncommon in the Henrys Fork drainage basin and was identifi ed in one location near the southern edge of Hickey Mountain. This piedmont gravel is ~6 m thick, overlies Tbr, and grades to a position slightly higher than till deposited during Qag3 time. It is clast-supported, angular, poorly sorted gravel composed chiefl y of limestone and chert clasts, but it does contain well rounded limestone and UMG clasts derived from landslide blocks of the Bishop Conglomerate found immediately upslope. The most prevalent deposit in the Henrys Fork study area is the Qag3 gravel. The Qag3 terrace can

be traced up the Henrys Fork drainage to a Blacks Fork moraine, which hypothetically was deposited during marine-isotope stage (MIS) 6 (Douglass, 2000). It forms a broad terrace on the southern side of the Henrys Fork that stands ~20 m above the modern alluvial valley near Lonetree, WY. Near Flaming Gorge Reservoir, the surface of Qag3 is ~15 m above the fl oodplain, thus the Qag3 terrace converges with the modern valley profi le. Likewise, Qag3 is thickest in the upstream sections near till and thins downstream. Near the moraines, Qag3 can be >10 m thick and is typically matrix-to clast-supported, very poorly sorted, boulder- to cobble-gravel (fi gure 5a), whereas downstream, Qag3 is 3-5 m thick, clast-supported, well rounded, imbricated, moderately-to well-sorted cobble gravel on an exposed bedrock strath (fi gure 5b). Limited clast count data for the Qag3 gravel along Beaver Creek and the Henrys Fork indicate it fi nes downstream, but the clast size increases locally at the Burnt Fork confl uence (table 1). Soils developed on the Qag3 gravel, in both the thicker outwash deposits near moraines and the thinner deposits downstream, exhibit a strong calcic (Bk) horizon nearly 2 m thick, and cobbles have stage II+ to stage III carbonate morphology (Birkeland, 1999). Qagp3 gravels, similar to its mainstem equivalents, are the most prevalent piedmont gravels in the study area. The Qagp3 gravel is 2-8

6


Figure 4. Valley-longitudinal profile of the modern Henrys Fork and local elevation data for terraces Qag3, Qag6, Qag7, Qag8, and Qag9 between Beaver Creek confluence and Flaming Gorge Reservoir. Profile elevations were extracted from U.S. Geological Survey 10-m digital elevation models.

? ????

2540

2560

2580

2600

2620

2640

2660

2680

2700

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Distance (meters)

2540

2560

2580

2600

2620

2640

2660

2680

2700

Qag4

Qag3

Qag1 Qal/Qag0

VE = 5x

Unmapped Till

B B'northwest southeast

Wes

t F

ork

of

Bea

ver

Cre

ek

elev

atio

m (m

eter

s)

m thick, clast-to matrix-supported, angular, poorly sorted, pebble-gravel composed primarily of angular limestone pebbles with minor rounded pebbles and small cobbles of various lithologies. Where the Henrys Fork valley is wide, Qagp3 piedmont gravels are capped by long, nearly continuous surfaces that sweep down hillsides to the Henrys Fork valley. Where the valley is narrow, Qagp3 gravels form an apron of sediment that buries Qag3 on the alluvial valley edge. Qagp3 is generally underlain by Tbr, but within the Henrys Fork valley it progrades out over Qag3. However, near the terminus of Henrys Fork, where Cottonwood Creek dissects Qagp3, Qagp3 interfingers with Qag3. These observations indicate Qagp3 deposition was contemporaneous with Qag3, but also continued after Qag3 deposition ceased. Qag2 gravel is well rounded, moderately- to well-sorted, imbricated, cobble gravel deposited on a planar bedrock strath. It is preserved in only one area immediately upstream from Burnt Fork Creek and cannot be directly linked to any glacial deposits, but it is potentially important for interpreting the glacial history of the Henrys Fork. The Qag2 terrace lies 5 m below the Qag3 terrace and 12 m above the Qag1 terrace (figure 6), indicating it was deposited sometime between the Blacks Fork and Smiths Fork glaciations. Qag2 may correlate to the MIS 4 glacial advance identified by Douglass (2000), which occurred between the Blacks Fork and Smiths Fork glaciations. An alternative hypothesis is that Qag2 correlates to a late Blacks Fork advance, supported by the landscape position


7


Figure 5. (a) Qag3 gravel on the outwash plain between the Henrys Fork and Beaver Creek is at least 8 m thick and has a well developed calcic soil horizon. (b) Thin Qag3 gravel remnant overlying a planar strath cut on the Tbr. View is looking downstream, ~2 km above the Burnt Fork Creek confluence.

Location nmaroon UMG limestone quartzite† sandstone mean of

all clasts (n)% cm* % cm % cm % cm

Qag3, site 1a 102 64 7.4 6 4.1 23 6.8 7 5.6 7.0

Qag3, site 2 b 101 46 8.2 9 4.0 31 4.5 14 4.1 6.2

Qag3, site 3 c 100 42 11.0 23 3.3 32 4.7 3 5.7 6.1

Qag3††, site 4 d 116 63 9.4 20 4.5 7 6.9 10 6.1 7.9

Qag2 e 109 55 7.1 11 2.3 30 4.0 4 4.8 5.5

Table 1. Characteristics of the Qag2 and Qag3 gravels. *mean clast diameter of given lithology, †quartzite from the UMG and non-UMG sources, ††sample site is downstream of Burnt Fork confluence, perturbing the trend of downstream fining of clasts

a Hickey gravel pit: UTM 569548.0 E, 4540455.5 N d Wyoming Hwy 1: UTM 588268.5 E, 4544741.5 N b Earl Hanks: UTM 582374.5 E, 4543148.5 N e Hanks Homestead: UTM 582964.0 E, 4543114.5 Nc Hanks Homestead: UTM 582950.0 E, 4543043.5 N

of the Qag2 terrace, which lies only 4 m below the Qag3 terrace. Only two Qagp2 piedmont gravel remnants were identifi ed along the Henrys Fork, both along the base of Hickey Mountain. These are < 3 m thick, underlain by Tbr, and grade to a position above Qag1 and below Qag3. They are poorly sorted, matrix-to clast-supported pebble-gravels composed of angular limestone pebbles and minor rounded limestone and UMG clasts from the Bishop Conglomerate on Hickey Mountain. Qag1 gravels are clast-supported, imbricated, well-rounded, cobble-gravels composed of maroon UMG and brown to tan quartzite clasts. Qag1 is typically overlain by 10-50 cm of fi ne sand and silt, and the Qag1 terrace is vegetated with grasses, sagebrush, and willow trees. Qag1 gravels are inset within the Qag3 and Qag2 gravels and can be traced upstream to Smiths Fork till, where Qag1 is at least 10 m thick (the base was not observed). Near moraines Qag1 rapidly thins downstream, forming a broad, outwash plain where Henrys Fork and Beaver Creek coalesce. Approximately 5 km downstream from Lonetree, Qag1 converges with the fl oodplain and it is no longer exposed along the main Henrys Fork valley. The modern Henrys Fork and the west and middle forks of Beaver Creek fl ow in shallow, multiple-threaded channels across the

Qag1 outwash plain, which retains subdued bar-and-swale topography of the braided meltwater channels. Qag1 reappears temporarily at the confl uence of the Henrys Fork and Burnt Fork, but it rapidly merges downstream with the modern fl oodplain. Active alluvium within channels and on the Henrys Fork fl oodplain compose Qal. Characteristically, Qal deposits are silt and fi ne sand underlain by imbricated, well-rounded, well-sorted, pebble-cobble gravel that is chiefl y composed of maroon UMG clasts. In Counts (2005), a higher level of the fl oodplain was distinguished as Qag0 because it potentially represents an early, abandoned Holocene fl oodplain (fi gure 7). Here it is grouped with Qal. Sagebrush and willow trees grow naturally on the Qal surface, but in many places along the lower Henrys Fork, ranchers have removed the natural vegetation and created pastures and fi elds that are fl ood-irrigated for cultivating hay and grazing livestock. Gravel bars and gravel banks are characteristic of the modern Henrys Fork, and it is a mainly a meandering, single-threaded channel, yet sand is rare or absent in the modern system. The Henrys Fork fl ows directly on bedrock for short distances in several places between Lonetree and Burnt Fork, but it is mostly an alluvial stream. The sinuosity of the Henrys Fork noticeably increases ~6 km

8


Figure 6. Relative positioning of Qag3, Qag2, and Qag1 gravel underlying the fi eld in foreground. The Qag3 and Qag2 terraces respectively lie 16 m and 11 m above the Henrys Fork fl oodplain. The Qag2 gravel could be an early Smiths Fork-equivalent gravel deposited during marine-isotope stage 4 (MIS 4), or a late Blacks Fork-equivalent gravel deposited in Bull Lake time (MIS 6). View is from the Burnt Fork valley looking west, just above its confl uence with the Henrys Fork.

downstream from the Burnt Fork confl uence. There is no signifi cant channel gradient change here, but the bedrock lithology does change from Tbr to the Tertiary Green River formation in this vicinity (Love and Christiansen, 1985). Qagp1 and Qagp0 gravels are found throughout the Henrys Fork study area grading to Qag1 gravels and Qal, respectively. The texture and composition of Qagp1 and Qagp0 gravels are essentially the same as Qagp3 gravels. Distinguishing between Qagp1 and Qagp0 is sometimes diffi cult because Qag1 converges with the modern fl oodplain. Qagp0 deposits can be inset within Qagp1 deposits, but more typically they form small, thin, fan-shaped

deposits that overlie Qagp1 and grade to Qal surfaces.

Landslides

Landslides in the Henrys Fork study area occur along the base of Hickey and Cedar Mountains where erosion by the paleo-Henrys Fork has over-steepened slopes of Tertiary shale overlain by the Bishop Conglomerate. Landslide blocks form hummocky topography that can be mistaken for glacial till, and UMG clasts are abundant in both types of deposits. All observed landslides appeared to be stabilized, and their ages are unknown.

PATTERNS, CORRELATIONS, AND INCISION RATES

Nine mainstem stream terraces and associated gravels were identifi ed and mapped along the Henrys Fork study area (fi gure 8). Late Pleistocene gravels from the past two glacial advances are the most extensive, but the older six gravels provide valuable insight to the Quaternary history of this area. Though the gravels are thick immediately below moraines, they lie upon planar bedrock straths and the terraces associated with them are strath terraces. The younger gravels rapidly thin in the downstream direction and are thinner than most of the older gravels. This suggests that ice from older glaciations may have traveled farther down the valleys, delivering relatively coarse material to a given location. Clast composition of gravels


9


Figure 7. View from the Qag3 terrace looking across the Henrys Fork valley ~ 15 km downstream from Burnt Fork. At least two levels of Qal cover most of the valley floor, with the broad valley bottom rarely inundated. By this point down the drainage, Qag1 is below grade and underlies Qal. Qagp3 gravels grade from slopes of the bedrock cliffs to the Qag3 terrace. View is to the northeast.

Figure 8. Composite cross-valley profile of the Henrys Fork showing landscape position of major deposits identified in this research. Qag4 gravel was not identified along the Henrys Fork, but it exists along Beaver Creek and Burnt Fork.

distance (km)

2150

2190

2230

2270

Qag9Qag8

Qag7

Qag6

Qag5

Qag3

Qagp5

Qag2

Qag1

South North

Hen

rys

Fork

0 1.0 2.0 3.0 4.0 5.0 6.0

2150

2190

2230

2270

VE=5x

Tbr

TbrQagp3Qag4

Qal

elev

atio

n (m

)

changes through time. Paleozoic limestone clasts are numerous in older gravels and become progressively rarer in younger gravels, whereas brown, tan, and white quartzite clasts from the UMG are scarce in the oldest gravels and become increasingly prevalent in younger gravels. This shift suggests that older glaciers encountered more Paleozoic bedrock than more recent glaciers, which could refl ect unroofi ng by glacial erosion through the Pleistocene, or may imply older glaciers were larger and not confi ned to narrow valleys dissecting hogback ridges of Paleozoic rocks. Longitudinal profi les for older terraces must be cautiously regarded because deposits are few in number, scattered, and the amount of post-depositional erosion is uncertain. Thus terraces profi les are based on minimum elevations and correlations are tentative. The Qag9 terrace profi le appears to converge with the modern valley profi le, whereas the profi les of Qag8, Qag7, and Qag6 are roughly parallel to it (fi gure 9). As previously noted, Qag3 and Qag1 both converge downstream with the modern valley fl oor in the upper reaches of the Henrys Fork, and this is associated with coarser bedload near moraines and decreasing gravel

thicknesses downstream. In the lower reaches of the Henrys Fork, the Qag3 profi le is roughly parallel to the modern profi le, but at the confl uence of the glaciated Burnt Fork Creek, the profi le is locally convex up. The Henrys Fork fault crosses the Henrys Fork near Manila, UT, but there was no observed offset in any of the surfi cial deposits mapped, so the Qag3 convexity is probably due to local sediment loading and coarsening by the Burnt Fork, which infl ated the profi le (fi gure 9). Initial cosmogenic exposure ages indicate that the last glacial maximum (LGM) in the southern Uinta Mountains occurred 15-19 ka (Laabs and others, 2003). In the Wind River Mountains, the chronology of late Pleistocene glacial advances is well constrained between 16-25 ka (Gosse and others, 1995; Phillips and others, 1997; Chadwick and others, 1997; Hancock and others, 1999; Sharp and others, 2003). Although the LGM ice peaked somewhat later in the Uinta Mountains, perhaps due to the proximity of the Uintas to Lake Bonneville (Munroe and Mickelson, 2002), the LGM ended at nearly the same time in both mountain ranges. The timing of pre-LGM glaciations in the two ranges are likely also comparable, so the Wind River glacial

10


Figure 9. Composite cross-valley profile of the Henrys Fork showing landscape position of major deposits identified in this research. Qag4 gravel was not identified along the Henrys Fork, but it exists along Beaver Creek and Burnt Fork.

Documents

The Nonglacial Surﬁ cial Geology of the Henrys Fork, Uinta ... and Pederson 2005 UGA.pdf · The Nonglacial Surﬁ cial Geology of the Henrys Fork, Uinta Mountains, Utah and Wyoming