Flood geomorphology of Arthurs Rock Gulch, Colorado: paleoflood history

ELSEVIER

Geomorphology I I ( 1994)

1540

Flood geomorphology of Arthurs Rock Gulch, Colorado: paleoflood history Christopher F. Waythomas”, Robert D. Jarrettb “U.S. Geological Survey, 4230 Universiry Dr.. Suite 201, Anchorage, Alaska, USA hCJ.S. Geological

Suney.

(Received

MS 418, P.O. Box 25046, Denver Federal Center, Denver, CO, 80225. USA

February 3. 1993; revised January 19. 1994; accepted January 31, 1994)

Abstract Episodic late Quaternary flooding is recorded by bouldery deposits and slackwater sediments along Arthurs Rock Gulch, an ephemeral stream west of Fort Collins, Colorado. Flood deposits consist of individual granodiorite and pegmatite boulders, boulder bars, and coarse overbank sediment that rest on erosional terrace segments along the channel. We identified evidence for at least five floods in the lower two thirds of the 1.84 km2 drainage basin. Flood deposits are differentiated by their position above the active channel, weathering characteristics, degree of boulder burial by colluvium, amount of lichen cover, and position with respect to terrace and colluvial deposits. Age estimates for the Rood deposits are based on radiocarbon dating, tree-ring analyses, and relative-age criteria from four sites in the basin. At least two floods occurred in the last 300 years; a third flood is at least 5000 years old, but likely younger than 10,000 yr BP; and the two oldest floods occurred at least 40,000 years BP.

1. Introduction Recent large-magnitude flash floods in the Rocky Mountain Front Range foothills of Colorado, such as the 1976 Big Thompson flood (Costa, 1978; Shroba et al., 1979), have drawn attention to the geomorphic significance of flash floods in the foothills region. Flash floods like the Big Thompson flood are relatively rare events that transport large amounts of sediment, have substantial long-lasting effects on channel geometry, and pose significant hazards to humans (Jarrett and Costa, 1988). However, the timing of flash floods in the Colorado Front Range foothills is precisely known only for the period of instrumental and historic records, or roughly the last 50-75 years (Jarrett, 1987). With few exceptions (Costa, 1978; Waythomas and Jarrett, 1991; Jarrett and Waythomas, 1994) the paleoflood record of the region is unknown. Virtually every drain0169-555X/94/$07.00

0 1994 Elsevier Science B.V. All rights reserved

SSDlO169.555X(94)00004-8

age basin in the region contains stratigraphic and sedimentologic evidence of former flash floods (Bradley and Mears, 1980; Costa, 1983; Waythomas and Jarrett, unpubl. field data); however, a temporal framework for these floods is lacking. In this paper we (1) present evidence for multiple late Quaternary paleofloods in Arthurs Rock Gulch, a small ephemeral drainage basin in the Colorado Front Range foothills near Fort Collins, Colorado (Fig. 1); and (2) describe our approach to paleoflood retrodiction in this geomorphic setting. Arthurs Rock Gulch is a relatively small, ungaged catchment (drainage area= 1.84 km*) and contains abundant bouldery deposits that formed mainly by flash flooding. Similar basins are common in the Colorado Front Range foothills (Jarrett, 1987). Thus, the results of this study provide an example of the frequency and geomorphic effects of extreme flooding in a typical small Front Range foothills drainage basin.

CF. Wqthomas.

16

2. Regional geomorphology

R.D. Jurrett / Geornorphology

and flood hydrology

The Front Range of the Colorado Rocky Mountains is a major physiographic feature that significantly influences the flow of air masses, precipitation intensity and dlatribution, and flood runoff (Hansen et al., 1978). The primary cause of flash floods in the Colorado Front Range is summer thunderstorms that can be particularly xvere in the foothills and Piedmont (Fig. 1). The foot-

Fig.

1.Map

15--10

hills zone (McCain et al., 1979, p. 3) encompasses the region between about 1700 and 2400 m elevation and is almost entirely within the lower montane forest (Marr, 196 I ) . This area is characterized by an open forest of Ponderosa pine and Douglas fir with scattered shrubs, herbs and grasses (Marr, 196 I ) The lower part of the gulch near Horsetooth Reservoir is within the grassland zone (Marr, 196 1) Precambrian crystalline rocks, mainly granodiorite, pegmatite and assorted

showing location of study area in Larimer County and approximate

Front Range.

I I (1994)

boundaries of Colorado Piedmont, foothills

zone. and Colorado

C. F. Waythomas, R.D. Jarrett /Geomorphology

gneissic and schistose lithologies, are the dominant bedrock in the foothills zone (Tweto, 1979). Paleozoic sedimentary rocks lap onto the Precambrian core of the Front Range, forming prominent north-south trending hogbacks and strike valleys in the eastern foothills. Much of the foothills zone is below the limits of Pleistocene glaciation (Tweto, 1979). Exposed bedrock exhibits varying degrees of weathering. The resistant Precambrian rocks form rounded hills and level upland surfaces of moderate relief ( lo1000 m; Madole et al., 1987). In these areas, bedrock is exposed or is mantled by thin colluvium composed of grus. Colluvial deposits composed of weathered rock fragments are abundant on most hillslopes and typically form colluvial aprons and wedges that thicken downslope. Bouldery debris is transported to bottomland areas mainly by rockfall and debris-flow processes. These deposits are an important source of sediment for transport by flash floods. All areas below about 2300 m in the Colorado Front Range are susceptible to intense rainfall (Jarrett, 1987; Jarrett and Costa, 1988). The foothills zone between 1525 and 2135 m is particularly prone to flash-flood producing thunderstorms (Henz, 1974; Jarrett, 1987). The Fort Collins area averages about 60 thunderstorms per year, which is greater than most other instrumented foothills locations (Wirshborn, 1978). The southerly flow of moisture-laden air from the Gulf of Mexico, or from the subtropical Pacific Ocean (the summer monsoon), frequently initiates widespread heavy rainfall and intense local thunderstorm activity in the Colorado Front Range foothills and Piedmont (Hansen et al., 1978). In summer (June through mid-September) the orographic effect of the Front Range blocks the southerly flow of humid air. This affects the passage of frontal boundaries causing unstable meteorologic conditions and upslope winds that result in intense local convective thunderstorms or cloudbursts (Follensbee and Sawyer, 1948, p. 22; Wirshborn, 1978; Hansen et al., 1978). Streamflow and precipitation data, and the spatial distribution of paleoflood deposits in the Colorado Front Range, indicate that flood discharge decreases dramatically at higher elevation ( > 2300 m) (McCain and Jarrett, 1976; Jarrett, 1987, 1990). Above about 2300 m unit discharge typically is less than 1.1 m* s- ’ km-‘, whereas at lower elevations in the foothills, unit

1 I (1994) IS--IO

17

discharge commonly exceeds 44 m* s- ’ km-* (Jarrett, 1987; Jarrett and Costa, 1988).

3. Study area The Arthurs Rock Gulch drainage basin (Fig. 2) is subdivided into three sub-basins (upper, middle and lower), based on general geomorphic characteristics. The upper part of the basin ranges in elevation from 1980 to 2100 m and is underlain by Boulder Creek Granodiorite of Precambrian age (Fig. 2; Braddock et al., 1989). In the upper part of the basin the active channel is straight and not more than 1 m wide. The active channel gradient is about 0.11 m/m. Fluvial deposits in the active channel are limited to a few longitudinal bars l-2 m in length that are composed of sand- and granule-sized sediment. Some bouldery rockfall deposits are found on hillslopes but are not common in or near the active channel. Vegetation in this part of the basin consists of open Ponderosa pine and Douglas fir forest with an herb and grass understory. Scattered shrubs are present along the active channel. The middle part of the basin (Fig. 2) ranges in elevation from 1680 to 1980 m. The channel is incised in bedrock and colluvial deposits, and the gradient is steep (0.17 m/m). Bedrock consists of Boulder Creek Granodiorite and discordant pegmatite dikes that locally crop out along the channel. One of these pegmatite dikes (hereafter called the “pegmatite weir”) extends across the channel of Arthurs Rock Gulch forming a prominent constriction (Fig. 3). Catastrophic paleofloods breached the pegmatite weir, deposited granodiorite boulders on its surface, and cut a narrow, deep notch in the bedrock. Downstream from the pegmatite weir, the channel is confined to a narrow bedrock canyon ( ‘ ‘Arthurs Rock canyon”, Fig. 2), where bouldery flood and rockfall deposits are common. Vegetation in the middle part of the basin consists of open forest to scattered stands of Ponderosa pine with an understory of grasses and low shrubs. A few cottonwood trees are present along the active channel but are restricted to isolated groups. The lower part of the basin is downstream from the foothills zone where the channel debouches from Arthurs Rock canyon. The channel is incised in colluvial deposits of Quaternary age and weathered sedi-

C. F. Waythomas,

18

1050

R. D. Jarrett

/ Geomorphology

(1994)

11

15-40 105"10'15'

11'

Horsetooth Reservoir (low water level)

35’

upper basin

te

middle basin

1

Site

2

34’

I

\

\

1 .O Miles

0.5

Site 5

I

I 1.O Kilometers

0.5 Contour interval

= 180 meters

EXPLANATION SURFICIAL

BEDROCK

Es4

Flood deposits

I

Sandstone

m

Debris-flow deposits

m

Pegmatite

cl

Colluvial deposits on hillslopes mantling undifferentiated bedrock

I?§z!

Granodiorite

Colluvial deposits on hillslopes mantling sandstone and conglomerate

l

Site location

m

5

x 1800

A A

Fig. 2. Generalized

DEPOSITS

hogbacks

Spot elevation,

in meters

Pegmatite blocks displaced by rockfall

geologic map of the Arthurs Rock Gulch drainage basin showing the location of study sites and important flood deposits.

mentary rocks of Paleozoic age (Fig. 2). Erosional i strath) terraces and prominent ooulder bars and berms are common along the channel. Near Horsetooth Reservoir the channel lies within a deep V-shaped water

qav cut into north-south trending sandstone hogbacks. Vegetation in this part of the basin consists mainly of prairie grasses, yucca, prickly pear cactus, and low shrubs. Cottonwood trees are common along the active

C.F. Waythomas. R.D. JarrettIGeomorphology

11 (1994) 15-40

19

Fig. 3. Photograph of pegmatite dike (“pegmatite weir”) that crosses channel of Arthurs Rock Gulch, site 5 (outlined by dashed line). Arrow locates granodiorite flood boulders (FBw) on pegmatite weir. Dotted lines indicate paleochannel. Active channel crosses the pegmatite weir to the left (south) of FBw flood boulders.

channel and scattered stands of Ponderosa pine are present in a narrow zone along the channel where it exits Arthurs Rock canyon. 4. Paleoflood evidence Imbricate, well-sorted, and rounded bouldery deposits, longitudinal boulder bars and berms, isolated “erratic” flood boulders, and localized concentrations of coarse overbank (slackwater) sediment are the primary depositional evidence for paleofloods in Arthurs Rock Gulch (Fig. 4). Flood deposits like these are loosely termed slackwater deposits because they form during the waning or slackening phase of floods in areas of backwater or diminished flow (Baker et al., 1979, 1983; Kochel and Baker, 1982, 1988). The highest limit of a slackwater deposit is a close approximation of the minimum peak flood stage and thus can be used as a paleostage indicator (PSI). In this study we use the boulder surface of the highest boulder per different age deposit as a minimum estimate of the paleostage of the flood flow that formed the deposit. In a compan-

ion paper (Jarrett and Waythomas, 1994) we use such paleostage data to estimate paleodischarge and flow competence at four sites in the lower and middle parts of the basin. To facilitate geomorphic mapping and correlation of flood deposits among sites, flood-boulder deposits were sequentially designated FBO through FB4. FBO deposits are oldest and are found at the highest elevations above the active channel, whereas FB4 deposits are youngest and are found closest to the active channel. Data were collected at four sites in the basin to assess the age, weathering characteristics, sedimentology, and general geomorphic relations among deposits. Samples for radiocarbon dating were collected (Table 1) and a variety of relative-age criteria (Table 2; Burke and Birkeland, 1979; McFadden et al., 1989) were used to infer age relations among flood deposits. Elevation differences of PSI’s at each site facilitate distinguishing the different flood deposits. Fluvial geomorphic features and flood deposits were mapped at about 1: 1000 scale, and 5 to 7 cross sections were surveyed at each site.

20

C. F. Waythomas, R. D. Jarrert / Ceomorphology I I ( 1994) 15-40

C. F. Waythomas, R. D. Jarrett / Geomorphoiogy

1 I (1994) 1.5-40

Fig. 4. (a) FB2 boulder bar, site I. View is downstream (northeast) toward inlet to Horsetooth Reservoir. (b) FB 1 longitudinal (am3~) on right bank, site 4. View is toward the south. (c) lmbricated flood boulders near the mouth of Arthurs Rock canyon.

4.1. Relative-age

criteria

Relative-age data collected at each site includes micro-pitting, grain relief, lichen cover, boulder burial, and surface boulder weathering (Table 2). Data were collected from 25-50 boulders in each deposit. Micropits are surface depressions that are < 1 cm diameter and typically less than 0.5 cm deep. Such features are common if present on more than 75% of the boulder surface, rare if present on less than 10% of the boulder surface, and incipient if the pits are small but aerially abundant. Grain relief is the amount of surface relief produced by weathering of mineral grains. We estimated the percentage of quartz and feldspar grains in relief on individual flood boulders and examined 2550 boulders per deposit. Data are reported as an estimate for the entire deposit (Table 2). Lichen cover is the estimated percentage of lichen growing on each flood boulder. Data are reported as an average based on examination of 25-50 boulders. Boulder burial refers to the percentage of the total boulder surface exposed above ground. Boulder burial is related to the addition of new sediment at the site (colluvium, eolian,

21

boulder ,bar

slope wash), boulder size reduction by weathering, and the original relief across the surface of the flood deposit. In general, the amount of boulder burial increases with time. Data are reported as an estimate for the entire deposit. Surface boulder weathering is the proportion of fresh, partially weathered and weathered granodiorite boulders in a flood deposit. Fresh boulders are those that exhibit less than 5% mineral grains in relief. Weathered boulders are those that exhibit more than 90% mineral grains in relief and show abundant surface pitting and spalling. Partially-weathered boulders are those that possess characteristics intermediate between weathered and fresh boulders. The weathering of flood boulders and lichen colonization of boulder surfaces is complicated by the effects of forest and range fires that likely happened in the region during the late Quaternary (Birkeland, 1984, pp. 63-65). Fire is an important factor in rock weathering because thermal expansion of the rock may cause complete or partial spalling of the boulder surface (Blackwelder, 1927; Bierman and Gillespie, 1991). If the outer part of the boulder spalls after a fire the boulder weathering “clock” is essentially reset and the

22

Table

C. F. Waythomas. R. D. Jarrett / Geornorpholqy

I I ( 1994) 15-40

I

Relative-age L.)cation

data from granodiorite Micro-pitting

and deposit

flood boulders, Arthurs Rock Gulch Grain

Lichen

Boulder

relief

cover

burial

(%o)

(%o)

(%)

Surface boulder weathering Fresh

Partly

(%)

Lithology”

Weathered

-

nh

(%10)

100

GR

63

47

PG

38

28

ss

33

25

GR

31

36

PG

56

64

weathered Site 1, FBl

Site I, FBI

100

Common

100

Common

0

0

25

~25

0

0

0

0

100

ss Site I, FB2

<5

Incipient

Site I. FB4

None

Site 3, FBO

Common,

0 100

0

0

95

0

0

0

31

25

PC

30

24

ss _

62 _

51 _

GR

?I

45

PC

26

55

GR

49

69

PG

22

31

GR

44

65

PG

24

35

5

GR

0

0

100

0

0

75

70

0

0

100

spalling, incipient macro-pits Site 3, FBI Site 3, FB2

Site 4. FBO Sltc 4. FBO

80

Rare

95

Common

100

Common Incipient,

site 4.

85

Uncommon

rare

~25

50

60 5

5

70 73

s5 51

<5

0

16 86

0 0 90

84 14

0 0 IO

0 0

100 100 0

active

PG

19

35

GR

35

65

PG

50

13

CR

19

27

GR

S8

56

PG

46

44

GR

5

100

PG

0

0

12

63

channel Site 5,

FBw

Abundant

100

90

NA

0

0

100

Site 5, FBI

Common

100

70

61

0

0

100

GR PG

7

37

Site 5, FB3

Common

100

65

38

0

0

100

GR

21

84

PG

4

16

Site 5, FB3

None

<25

5

5

60

40

0

GR

47

12

PG

18

28

Site 5, ac‘tlve

None

0

0

0

99

1

0

GR

25

63

PG

15

37

channel “(iR = granodiortie,PC = pegmatite,SS = sandstone. “Number of samples.

subsequently developed rock weathering features record the time elapsed since the last episode of fire. Thus the differences in weathering between “old” and “young” flood boulders may not be as great as they otherwise would be if fires did not occur. Similarly, the

amount of lichen cover on flood boulders records the interval of time since the last fire. Thus, our rock weathering and lichen cover data provide only minimum age estimates.

C.F. Wa~thomas, R.D. Jarretr/

Geornorpholo~y

I I (1994) 1540

23

Table 2 Radiocarbon

dates from alluvial

deposits, Arthurs Rock Gulch

Lab no.

Sample No., location

and flash-flood

Reported age

Calibrated

material dated

I l/14/89-4 I, Unit

Site

age”

(C-14 yr BP)”

and

yr BP

yr AD

GX-I.5640

46Sz!z 160

644.517.3

14

GX-I.5641

240f

500,297.O

14SO. 1653, I955

GX-15643

4320f225

S289.4868.4566

3340,2919,26

GX-16693

21.5 *7s

311,289.O

1639,1661.

1955

GX- 16692

260 k 7.5

432,302.O

1518.1648,

1953

GX- 16694

430 + 75

53

1419.1441,

1493

GX- 16695

99.6 & 2.3%

1306.1433,

1636

C

wood 11/14/89-5

I, Unit

Site

170

C

wood 11/14/89-II Site

I, Unit

17’

A

charcoal 11/14/90-Cl4-I Site 4, FB3 wood 11/14/90-C14-2 Site 4, FB3 wood I1 /20/90-C14-2

I, 509,457

Site 5. FB3 wood

I I /20/90-C14-3 Site S, FB3

of 1950 c-14

wood

activity

“BP ages reported with respect to year AD 1950. “Dates calibrated

using method of Stuiver and Reimer ( 1987). reported as

[ - I CT (age)

+

1CT].

‘BC ages

4.2. Process identi$cation

and clastfabric

In steep mountain catchments like Arthurs Rock Gulch, coarse bouldery sediment is delivered to the valley bottom by several processes. Flash floods, debris flows, debris avalanches, and rockfalls are all important geomorphic processes in the Colorado Front Range foothills (Costa and Jarrett, 198 1) . Deposits produced by the last three processes are probably the primary sources of sediment for flash floods. In the western part of the foothills zone (in the sub-alpine ecotone) , above 2100 to 2300 m, rainfall-induced flash floods are less common than they are below this elevation and rainfall, discharge, and paleoflood data indicate no substantial

flooding above 2300 m (Jarrett, 1990). However, debris-flow deposits are recognized throughout the foothills zone, and commonly are found below the elevation limit for flash floods in the Colorado Front Range. Thus, in a paleoflood investigation it is important to differentiate between deposits formed by flash flooding and deposits formed by debris-flow or other mass-movement processes because paleoflood reconstructions would be erroneous if made on deposits other than water-flood deposits (Costa and Jarrett, 1981). Criteria for differentiating between debris-flow and water-flood deposits are described by Costa and Jarrett ( 198 1) and Costa ( 1988). Such criteria are most useful for deposits that are fresh ( < 100-200 years old) and

24

C. F. Waythomas, R.D. Jarrett / Geornorphology

well-exposed. However, with the passage of time, both debris-flow and flash-flood deposits may be modified by the combined effects of weathering, colluviation, and removal of interstitial sediment. As a result, the physical characteristics of the deposits become more alike. Criteria especially applicable to older deposits al-e needed. In this study we attempt to differentiate flash-flood from debris-flow and other mass-wasting deposits using the clast fabric of the deposit. Clast fabric data from deposits of known origin are compared with clast fabric data from Arthurs Rock Gulch flash-flood deposits. Data on clast fabric were obtained by measuring the azimuthal orientation and dip of the A-B plane (A-axis= long axis, Basis = intermediate axis) of disc-shaped cobbles and bcjulders. Twenty-five random measurements were made on each deposit during a traverse along the deposit axis. Clast-orientation data were analyzed by plotting the poles normal to A-B planes on a lower hemisphere equal-area projection (Schmidt projection) from which eigenvalues were derived by computer (Woodcock, 1977). Three normalized eigenvalues (Sl, S2, S3) provide a measure of the degree of clustering of the poles to A-B planes about their associated eigenvectors (VI, V2, V3). The eigenvectors indicate the predominant orientation of the p( )les to A-B planes. The eigenvalue Sl is a measure of the degree to which poles to planes cluster about the eigenvector VI and is equivalent to the mean vector (Mark, 1973; Woodcock, 1977). Eigenvector V3 is normal to VI, and S3 is inversely proportional to Sl (Woodcock, 1977; Lawson, 1979). If the deposit exhibits a high degree of preferred orientation the poles to planes will be parallel or tightly clustered and Sl will approach 1.O, whereas S3 will approach 0. Furthermore, the natural logarithm of the ratio 51 /S3 also indicates the degree of preferred orientation and values 01‘ I .O indicate random or uniform distributions and greater values ( >4) indicate increasingly stronger degrees of preferred orientation (Woodcock, 1977). Methods for graphical representation of eigenvalues are discussed by Woodcock (1977). We use a twoaxis plot of the eigenvalue data to illustrate fabric differences for debris-flow deposits, flash-flood deposits, and deposits from Arthurs Rock Gulch. The plot provides a method for investigating the origin of deposits whose mode of formation is unknown or uncertain.

I I (1994)

15-40

This differentiation is needed for subsequent assessment of discharge estimates from coarse particles.

5. Flood geomorphology

and chronology

5. I. Site 1 Site 1 is located near the mouth of Arthurs Rock Gulch at Horsetooth Reservoir, in the valley between two north-south trending hogbacks (Fig. 2). Granodiorite and pegmatite boulders record at least four paleofloods (Fig. 5a). Flood deposits are differentiated by their maximum height above the active channel, radiocarbon ages, and by their weathering characteristics. The oldest flood deposits (FB 1, Fig. 5a) are about 6 m above the active channel (Fig. 6). FBI deposits are clast-supported and lack a fine-grained ( < 3 mm) matrix. All FBI granodiorite boulders are weathered and show considerable surface pitting and grain relief (Fig. 7; Table 1). FB2 deposits are limited to one locality near the base of the sandstone hogback on the east side of Site 1 (Figs. 4a, 5a) and form a crescent-shaped boulder bar about 1.7 m above the active channel. FB2 deposits are little weathered in comparison to older FBl deposits (Fig. 7; Table 2). However, the weathering characteristics of FB2 boulders probably do not reflect the true relative age of the deposit because most of Site 1 is beneath the waters of Horsetooth Reservoir at least a month or more each year (Fig. 5a). Episodic submergence and wave action on FB2 deposits has removed overlying sediment and weathering products, and FB2 granodiorite boulders appear fresher than other FB2 granodiorite boulders in Arthurs Rock Gulch. It appears that FB2 deposits were buried by fine-grained alluvium after a brief( ?) period of subaerial exposure and weathering and were recently exhumed by wave action at the margin of Horsetooth Reservoir. FB2 deposits are stratigraphically and topographically beneath radiocarbondated alluvium and FB3 deposits (Fig. 8). FB3 flood sediments (small boulders, cobbles and gravel) are preserved on the upstream end of an alluvial terrace (T3, Fig. 5a) along the right (south) bank of Site 1. These sediments were examined, but no weathering data were collected because granodiorite cobbles are absent. The stratigraphy of the T3 terrace deposits was examined in a hand dug trench that exposed about

Fig. 5a. Sketch map of site 1 showing indicated.

b

l/

flood boulders.

30 Meters

of reseruoir

Approximate high-water limit

(a)SITE 1

surficial deposits.

cross-section

locations,

F

Granodiorite

m

10

30

Pegmatite

jJ?YJ

20

Bedrock (undifferentiated)

40

50

section, and representative

cross-section

with pakostages

DISTANCE FROM LEFT BANK, IN METERS

0

Terrace

location

m

L....‘....‘....‘....‘....‘....

measured

25

@J

Cross-section

Colluvial deposits

Large flood boulders

11111111111111~,, Fluvial scarp q1111

~1

0

0 00 0

Concentration

@j of flood boulders

Boundary between terrace tread and riser

--+--+

EXPLANATION

C. F. Waythomas, R. D. Jarrett / Geomorphology

26

I I (1994) 15--/O

lb1 SITE 3

I

0 cn 5

92

n

1’1’1

I

n

1

30 Meters

I’

SITE 3, CROSS SECTION 2

iii

2

90

2 0

86

z !i

84

Ei I

82 0

20

40

60

80

100

DISTANCE FROM LEFT BANK, IN METERS

Fig. 5b. Sketch map of site 3 showing fluvial geomorphic Explanation of symbols given in Fig. 5a.

features, flood deposits, cross-section

locations,

and representative

cross section,

C.F. Waythomas. R.D. Jarrett/

(d SITE4

Geotnorphology

/FBoL do

11 (1994)

27

1540

o

O

0 NORTH

FBO /

0

;

104 _

Li

HI02

I' I' 1'1' I ’ SITE 4 CROSS SECTION 3

I

v

I

n

I'

FRO paleostage

-

30 Meters

I'

_....

z rioo

-

2 a

96

-

Y 0

96

-

2 I’

94

-

: I

92

I.

0

I

40 DISTANCE

Fig. 5c. Sketch map of site 4 showing fluvial geomorphic Explanation of symbols given in Fig. 5a.

I

I

I

I

80

I

I

I I

120

I

I

I

I

I

I

160

I

200

FROM LEFT BANK, IN METERS

features, flood deposits, cross-section

locations,

and representative

cross section.

C. F. Waythomas. R. D. Jarrett / Geomorphology

2x

I 0

6

i 1 (1994) 1540

I meters

g

160

P !j

159

z

158

2 L? Lj

154

-SITE 5 CROSS-SECTION

153

5

L~~~~I~~~~I~~~~I~~~~I~~~lI 20

30

DISTANCE

40

50

60

70

FROM LEFT BANK, IN METERS

Fig. 5d. Sketch map of site 5 showing location of flood deposits, cross-section locations, and representative given in Fig. 5a. FBw identifies granodiorite flood boulders on the pegmatite weir.

cross-section.

Explanation

of symbols

C. F. Waythomas, R.D. Jarrett / Geomorphology 11 (1994) 1540

DISTANCE

UPSTREAM

FROM SITE 1, IN METERS

Fig. 6. Maximum height of paleostage indicators (PSI’s) above the lowest point in the active channel versus distance upstream from site I.

z

60

uw

40

$

20

z i?

0

c7 FBO

t

I

FBO I 2

FBl

FE2

FE3

FB4

AC

I

I

I

1

I

FBl

FB2

FB3

FB4

AC

I

I

I

I

29

1.7 m of alluvium. Three alluvial units appear in the trench (Fig. 8). Unit A consists of massive orangebrown silty sand to sandy silt. Wood fragments collected from unit A (about 1.6 m depth) yielded a radiocarbon age of ca. 4868 yr BP * (GX- 15643, Table 2). Unit B is about 1.4 m thick and consists of massive, slightly organic, silty sand and sandy silt with a few granules and pebbles. Detrital wood and charcoal samples from unit B yielded radiocarbon ages of ca. 297 yr BP (1.1 m depth; GX-15641; Table 2), and ca. 517 yr BP (25 cm depth; GX- 15640; Table 2). The radiocarbon ages indicate that a period of aggradation in the lower part of Arthurs Rock Gulch was in progress by ca. 4800 yr BP. A nonconformity of about 4500 yr BP is denoted by an abrupt erosional contact separating units A and B. Unit B is evidence for an interval of stream alluviation that commenced at least 300 years ago that we interpret as reworked colluvial sediment formed by gully erosion of colluvial deposits upstream from Site 5. Numerous relict gullies that expose silty organic sediment are present in this part of the basin. The lack of coarse sediment in unit B indicates that it probably did not form by flash flooding. Unit C consists of well-sorted pebbly gravel composed of > 95% pegmatite clasts. Unit C is equivalent to FB3 and records at least one episode of overbank flood deposition that occurred sometime after 297 yr BP. FB4 deposits are present along the active channel margin and consist of small clusters of imbricate boulders and cobbles (Fig. 5a). FB4 deposits are unweathered and are about 60 cm lower than FB3 deposits (Fig. 6; Table 2).

j

I

5.2. Site 2

0

FBO (oldest)

FBl

FB2 RELATIVE

FB3 AGE

FB4

AC

Site 2 is located in the lower part of the basin about 0.5 km upstream from Site 1 (Fig. 2). Bouldery flood deposits of different ages are present at this site, but were not studied in detail. The channel lacks a straight reach suitable for making hydraulic computations and the PSI’s are located only on the outside bend of the channel.

(youngest)

Fig. 7. Selected plots of relative-age data (Table 2) used to differentiate bouldery flood deposits in Arthurs Rock Gulch. AC is active channel.

*All radiocarbon dates given in text are calibrated to the dendrochronologic timescale according to the method of Quiver and Reimer (1987).

C. F. Waythomas, R.D. Jarrett / Geomorphology 11 (1994) INO

30

STRATIGRAPHY

UNITS

DESCRIPTION

AND RADIOCARBON

DATES

Light brown, pebbly gravel, well sorted, clast

Dark brown, silty sand jo sandy silt with disseminated organic detritus, charcoal and wood. Matrix-supported, granule- and pebble-sized clasts

240 f 170 yr. BP (GX-15641)

VERTICAL SCALE, IN CENTIMETERS 0 ---------

Unconformity? Orange-brown, sandy silt with minor granules and pebbles, and few wood fragments and charcoal, thickness unknown. Unit may overlie FB2. 4,320 f 225 yr. BP (GX-15643)

Fig. 8. Measured section and radiocarbon

5.3. Site 3 Site 3 covers a reach of channel about 90 m long, and is located about 0.8 km upstream from Horsetooth Reservoir (Fig. 2). Erosional terraces mantled by flood boulders are inset within a low-relief, eastward-sloping colluvial apron that mantles the underlying bedrock (Fig. 5b). FBO boulders at Site 3 (and Site 4) are the highest and oldest flood deposits in the lower part of the basin. The boulders lie on an old alluvial surface (TO, Fig. 5b) that is cut into the colluvial apron. FBO deposits are easily differentiated from all younger deposits by their position above the active channel and weathering characteristics (Fig. 7; Table 2). FB 1 deposits are present on the outer portion of the 1‘1 terrace at or just above the break in slope defining the terrace riser about 0.9 to 1 m below the FBO deposits (Fig. 5b). Natural exposures through the Tl terrace show up to 1 m of coarse gravel and boulders on top of fine-grained colluvial deposits and bedrock. A mature soil with a thickBt horizon has formed on the Tl terrace alluvium and underlying colluvial deposits. The Bt horizon is 20 cm to more than 1 m thick, exhibits 7.5YR ?I4 colors, and contains 13-36% clay relative to one sample of oxidized alluvium containing about 3% clay ( Table 3). Because Arthurs Rock Gulch is located in a state park, there were limited opportunities to exca\,ate deep trenches to obtain soil data from the C-horizon. In some locations the alluvial deposits are thin and

ages, site

I. Location of section given in Fig. 5a.

the B-horizon extends to bedrock. Although data from unweathered parent material is lacking the amount of clay and color of the B-horizon of soils developed on the Tl terrace indicates that the terrace is at least of Holocene age or older, but probably not older than middle Pleistocene age. Soil chrono-stratigraphic studies in the Colorado Front Range indicate that surficial deposits with soil B-horizons comparable to that described above are probably at least 100,000 years old (Shroba and Birkeland, 1983; Birkeland, 1984). However, the lack of data on C-horizon characteristics for the soils developed on the Tl terrace limit age estimates. The weathering characteristics of both FBO and FB 1 deposits are similar to those recorded for erratic boulders on late Pleistocene (Pinedale) moraines in the Colorado Rocky Mountains (Birkeland, 1973, 1984; Madole, 1976): As discussed previously, spalling of boulder surfaces may occur following a forest fire, and the degree of boulder weathering could record the time since the last episode of spalling induced by fire. Although FBO and FBI deposits may be correlative with ca. 40,000 year old Pinedale age erratic boulders, the degree of weathering on FBO and FBl boulders may not indicate the time since deposition because of the possible effects of fire. Thus, FBO and FB 1 deposits could be older than 40,000 yr BP. FB2 deposits consist of clusters of imbricate granodiorite and pegmatite clasts that are 1 to 2.8 m above

C. F. Waythomas. R.D. Jarrett / Geomorphology

31

1 I (1994) 1540

Table 3 Field- and particle size data for soils developed in terrace deposits and colluvium L,ocation

Horizon thickness

Soil horizon” or substrate

Munsell” color (moist)

(cm)

PercentC

Parent material

Sand

Silt

Clay

Tl terrace, Site 3

o-13 13-35 35-250 > 250

A/Cox 2Bt 2cox bedrock

IOYR 412 1.5YR 314 7.5YR 3.514 _

12 76 93 _

21 II 3 _

7 13 3 _

Colluvium Alluvium Alluvium Fountain Fm.

Tl terrace. Site 3

o-3 3-20 2040 >40

A Btl Bt2 bedrock

1OYR 412 1OYR 313 7.5YR 313

36 30 23 _

49 45 42

15 26 36

Colluvium Alluvium Alluvium Fountain Fm.

T I terrace, Site 4

O-2.5 2.5-10 I O-20 20-120 > 120

A 2Btl 2Bt2 3Bt covered

IOYR 7.5YR 1.5YR 7.5YR _

59 43 63 46

25 28 18 31 _

16 29 20 24 _

Colluvium Alluvium Alluvium Colluvium _

T3 terrace, Site 4

O-10 IO-30 >30

A A2 Cn

IOYR 412 IOYR 212 IOYR 212

83 60 96

12 21 3

5 12

Alluvium Alluvium Alluvium

o-5 5-30 30-70 >70

A A/Bt Bt2 covered

IOYR 2/l IOYR 313 IOYR 314 _

47 48 25 _

36 35 3-l

17 17 38

Colluvial slope, Site 5

3/3 314 314 314

I

Colluvium Colluvium Colluvium

“Soil horizon nomenclature from Soil Survey Staff ( 1975). “Munsell Soil Color Chart, Munsell Company, 1954. ‘Less than 2 mm fraction.

the active channel. FB2 deposits are found mainly along the break in slope between the terrace tread and riser (Fig. 5b) and form distinct boulder bars whose long dimension is generally parallel to the active channel. FB2 deposits are considerably less weathered than FB 1 or FBO deposits (Fig. 7; Table 2). FB3 flood deposits are recognized at Site 3, but are limited in extent (Fig. 5b). These deposits are found about 1 m above the active channel (Fig. 6). FB3 deposits are little weathered (Fig. 7; Table 2). FB4 deposits are the youngest deposits recognized at Site 3. FB4 deposits are about 15 cm above the active channel and define the channel margin (Fig. 5b). FB4 deposits consist of unweathered, little-modified concentrations of rounded to subrounded, imbricate pegmatite and granodiorite boulders. Matrix sediment is rarely preserved and probably has been eroded by moderate floods incapable of mobilizing the coarsest FB4 boulder fraction.

5.4. Site 4

Site 4 is located about 0.96 km upstream from Horsetooth Reservoir and about 500 m downstream from the mouth of Arthurs Rock canyon (Fig. 2). Four alluvial terraces and five flood boulder deposits are recognized at Site 4 (Fig. 5~). The terraces are similar to those recognized at Site 3 and the upper two terraces (TO, Tl, Fig. 5c) can be traced continuously from Site 3 to Site 4. The lower terraces and flood deposits (T2, T3, FB2, FB3, Fig. 5c) are present at about the same relative elevations above the active channel as are equivalent deposits at Site 3 (Fig. 6). Although FBO deposits are close to the mountain front, we interpret them as fluvial deposits rather than colluvial or debris-flow deposits, because ( 1) FBO deposits are inset within the colluvial slope rather than being part of it, indicating fluvial erosion of the colluvium, (2) a source for granodiorite boulders found in

32

C. F. Waythomas, R.D. Jarrett / Geomorphology

FBO deposits is lacking in the foothills west of TO along the north and south sides of Arthurs Rock Gulch (Fig. 2) and no granodiorite boulders were observed protruding from the colluvium upslope from FBO deposits, and (3) clast-fabric measurements made on FBO deposits indicate a strong preferred orientation, with clast B-axes oriented normal to the channel and a modal i mbrication angle of about 50” suggesting fluvial transport. Coarse bouldery flood debris on the T 1 terrace (FB 1, Fig. 5c) records at least one flood. Boulders in FBl deposits exhibit considerable weathering (Table 1) . Natural exposures into Tl reveal up to 60 cm of clastsupported gravel and boulders, resting on pebbly, matrix-supported colluvium. A thick soil is developed 1~1the terrace gravel and colluvial deposits. This soil IS characterized by a well-developed red-brown 7.5YR 3/4) Bt horizon (Table 3). The T2 terrace is about 2 m above the active channel, but is not continuous between Sites 3 and 4. FB2 depostts consist of concentrations of granodiorite and pegrnatite boulders that form linear bar-like features along rhe margin of the T2 terrace (Fig. 5~). The largest boulders (up to 1.5 m, B-axis diameter) are present ~)nly along the break in slope between the terrace tread and riser. Smaller boulders are present along the middle .md back edge of the terrace; however, some of these boulders may be reworked from FB 1 deposits. Weathcring data were not collected from FB2 deposits at Site 1 because equivalent deposits were better preserved at Site 3. Paired FB3 boulder bars are recognized at both the upstream and downstream ends of Site 4 (Fig. 5~). Small wood fragments (twigs, stems) collected from Ihe contact zone between two large FB3 boulders vtelded radiocarbon ages of 289 yr BP and 302 yr BP (1Table 2). These dates provide maximum limiting ages for the FB3 flood; however, the dated wood samples were small twigs and stems that were probably < 10 years old before being incorporated into the flood debris. Thus, these dates may actually closely date the FB3 event to about 300 yr BP ( 1655 AD). Several large granodiorite and pegmatite boulders are present within and immediately adjacent to the active channel. These deposits are mapped as FB4 deposits but are not associated with a terrace. FB4 deposits form imbricate pockets of coarse gravel and cobbles, and lack weathering or lichen cover (Table

11 (1994) 15-40

2). Some of the FB4 sediments reworked from older flood deposits.

were

probably

5.5. Site 5 Fluvial geomorphic features and flood deposits at Site 5 are the only deposits in the foothills zone of Arthurs Rock Gulch that were studied in detail. Most deposits are similar to those observed at Sites 3 and 4. The oldest flood deposits are the granodiorite boulders on the pegmatite weir (FBw) about 10 m above the active channel (Figs. 5d, 6). Some FBw boulders are imbricate, whereas others are present along the margins of a paleochannel that extends over the top of the pegmatite weir. The paleochannel forms a shallow, rectangular notch in the pegmatite (Fig. 3) and exhibits a smooth, polished surface. FBw boulders have B-axis diameters of 0.4 to 1.5 m, are well rounded, and are considerably weathered (Table 1) . Other granodiorite flood boulders are found on the downstream (east) side of the pegmatite weir and are additional evidence for paleoflood transport of boulders over the pegmatite weir. The age of the flood(s) that deposited the FBw boulders is unknown. FB 1 deposits are preserved above the margins of the present channel at the base of a colluvial apron that slopes toward the channel axis (Fig. 5d). The boulders are likely reworked by colluvial processes but retain a crude linear alignment that is generally parallel to the active channel (Fig. 5d). FB2 and FB3 deposits are within 2 m of the active channel and are inset within the colluvial slopes that flank Site 5 (Fig. 5d). FB2 deposits are recognized at only one locality and are about 1 mn above FB3 deposits (Figs. 5d, 6). FB3 deposits are longitudinal bars composed of cobbles and boulders, with strong particle fabric, but relatively low imbrication angles. Some FB3 deposits are weathered as much as FB2 and FB 1 deposits in the lower part of the basin, and are differentiated from FB 1 and FB2 deposits at Site 5 because they are found at a lower elevation, The fabric and weathering characteristics of FB3 deposits indicate that these deposits may have formed by debris-flow processes and consist of reworked flood sediments (Figs. 7, 10). Two wood samples collected from FB3 deposits gave disparate radiocarbon ages. One of the samples yielded a radiocarbon age of ca. 509 yr BP (GX- 16694, Table 2) providing a maximum limiting age for the

C. F. Waythomas. R. D. Jarrett / Geomnrpholo.gy

Table 4 Clast fabric data from fluvial and mass-wasting Deposit

Location

Arthurs FB I, FB2, FB3, FB4, FB2. FB3.

Rock Gulch, Colorado Site 4 Site 4 Site 4 Site 3 Site I Site 5

33

I I (I 994) I S-40

deposits Number of samples”

Normalized eigenvalues SI

S2

s3

Eigenvector trend and plunge VI

0.151 0.239 0.239 0.214 0.203 0.315

0.078 0.078 0.143 0.123 0.089 0.148

28/SO 8158 46156 29142 48149 33152

0.367

0.075 (0.028)

Ref.”

Flash flood Flash flood Flash flood Flash flood Flash flood Debris flow?

I I 1 I

0.771 0.684 0.617 0.663 0.707 O.S36

Mt. Hekla. Iceland

Debris flow

2

0.558

West Grain valley, England West Grain valley, England

Debris torrent Debris flow

4 I

0.606 (0.103) 0,s I4

0.307 0.347

0.087 (0.026) 0.139

Long Gulch, Colorado

Flash flood

I

0.769

0.132

0.099

130/43

Turkey Creek, Colorado Bar I Bar2 Bar 3

Flash flood Flash flood Flash flood

I I I

0.692 0.7 IS 0.638

0.228 0.161 0.240

0.078 0.123 0.122

22152 88148 77144

III Ill ItI

Tucker Gulch, Colorado Site 2 Site I. bar I

Flash flood? Flash flood?

1

0.600 0.646

0.28 I 0.261

0.1 I8 0.092

49148 90/s I

III III

Nigel Pass, Canadian Rocky Mts.

Debris flow

84

0.606 (0.079)

0.291 (0.059)

0.103 (0.041)

1x1

Allagan Alps, W. Germany

Debris flow

9

0.607 (0.039)

0.277 (0.039)

0.1 16 (0.043)

161

“Minumum of 25 measurements per sample. “References: I I I This paper. 121 Geirsdottir ‘Average value. ‘lone standard deviation.

I I

1

(1988).

(0.037)”

131 Carling (1987a).

[4] Carling (1987b).

151 Owens (1973).

161 Rappol (1983).

deposit. The radiocarbon activity of the second sample (GX- 16695, Table 1) is 99.6 + 2.3% of the 1950 standard 14C activity. This sample was probably deposited in the FB3 boulder matrix by a recent flood and thus provides a minimum limiting age for FB3. Large boulders along the active channel margin are considered to be FB4 deposits (Fig. 5d). These boulders were probably reworked from older deposits. FB4 deposits show little to no weathering (Fig. 7; Table 1). 5.6. Clust fabric and deposit origin Eigenvalue data from clast fabric measurements of water-flood deposits in the Colorado Front Range (including Arthurs Rock Gulch) are listed in Table 4. Fabric data from debris-flow deposits are included for

In (5263)

Fig. 9. Ratio plot of normalized eigenvalues (S I, S2, S3) for clastfabric data showing differences between water-flood deposits and debris-flow hyperconcentrated-flow( ?) deposits. Data sources listed in Table 4.

C. F. Waythomas, R.D. Jarrett / Geomorphology

11 (1994)

15-40

B 100

RS. PROXIMAL

75 5 B 3 0 L

50

25

0 0

20

DIP ANGLE,

40

60

80

0

20

40

DIP ANGLE,

IN DEGREES

60

80

IN DEGREES

0

20

40

60

60

DIP ANGLE,

0

20

40

60

80

IN DEGREES

“l6CO Ftg.

10. Frequency

I O\(6),

and (B)

histograms

debris-flow

of imbrication

angles

of cla.stsfrom (A) glacigenicmass-flowdeposits (modifiedfrom

deposits in the Scottish Highlands

(modified

Dowdeswell

and Sharp,

from Innes. 1986). RB. is right bank, LB. is left bank, proximal

is

pl-oxirnal to channel, distal is distal from channel. N is number of samples.

comparison. Data from Table 4 are plotted as a ratio plot of normalized eigenvalues (Fig. 9). The relation implied by Fig. IO is not suitable for prediction (via regression analysis) because theX-Y variables indicate a spurious relation (Chayes, 1949, 1971) and considerable scatter. However, the plot illustrates some distlnct differences in deposit clast fabric that may be related to the processes that formed the deposits. Clasts in fluvial deposits that form in high-energy environmcnts tend to have a high degree of preferred orientation (Rust, 1972; Kauffman and Ritter, 1981). E.igenvalue ratios associated with these deposits will plot in the upper left corner of Fig. 9. Clasts in debrisflow and hyperconcentrated-flow deposits show less preferred orientation. Eigenvalue ratios from those deposits should plot in the middle to lower right part csf Fig. 9. In nature, it is likely that a continuum of elgenvaluc ratios is Possible and that this is controlled by the hydrodynamic properties and composition of the sediment-fluid mixture. Thus, other criteria (see e.g. (‘osta, 1988)) in addition to clast fabric data, must be tJied to identify the depositional process of bouldery &posits in upland catchments like Arthurs Rock Gulch. The data shown in Fig. 9 indicate that debris-flow and hyperconccntrated-flow(?) deposits have In Sl / J’i values that are < 1. I, and In S2/S3 values > 0.7. In contrast, water-flood deposits have In Sl /S2 values that

range from 0.95 to 1.8 and In S2/S3 values that range from about 0.2 to 0.8. Eigenvalues derived from fabric measurements on FB3 deposits at Site 5 and FB2 deposits at Site 4 plot in or near the domain of hyperconcentrated-flow ( ?) and debris-flow deposits, indicating that these deposits may not have formed by flash flooding. Other Colorado Front Range flash-flood deposits from Tucker Gulch near Golden, Colorado and Turkey Creek near Morrison, Colorado, also have clast fabrics and associated eigenvalues that are more like hyperconcentrated-flow( ?) and debris-flow deposits than water-flood deposits (Fig. 9). These deposits require further study before their specific origin can be adequately evaluated. Another component of the clast fabric that may be useful for differentiating debris-flow from flash-flood deposits is the dip, or imbrication angle of the clast AB plane. Frequency histograms of imbrication angles from debris-flow and glacigenic mass-wasting deposits (sediment-flows, ice-slopecolluvium; Fig. 10) are typically unimodal and negatively skewed (modal class about 15-20”). The frequency distribution of imbrication angles associated with flash-flood deposits, in contrast, is unimodal and symmetric (modal class about N-55”; Fig. 11) . The frequency distribution of imbrication angles associated with the FB3 deposit at Site 5 and the FB2 deposit at Site 4 are negatively

C. F. Waythomas, R.D. Jarrett / Geornorphology I I (1994) 1540

Colorado

Y 0 2 LL

Front Range

10

5

0 0

10

20

30

40

IMBRICATION

Fig.

I I, Frequency

histogram

50

ANGLE,

of imbrication

the A-B planes of clasts from flash-flood Front Range foothills

60

and Arthurs

70

80

90

IN DEGREES

angles associated with deposits in the Colorado

Rock Gulch.

skewed (mode about 18”) and further indicates that these deposits probably did not form by water-floods.

We interpret at least five paleofloods from the distribution of granodiorite boulders, erosional terraces, and slackwater deposits in Arthurs Rock Gulch. Relative-age criteria, stratigraphic relations, and the results of radiocarbon dating and tree coring are used to estimate the chronology of flooding and late Quaternary fluvial-geomorphic history of the basin (Fig. 12). Because the weathering characteristics of FBw, FBO, and FB 1 deposits are similar, these deposits may have been deposited by three or fewer closely-spaced floods. The age of the flood(s) that deposited FBw, FBO, and FBI -is estimated by comparison of boulder weathering characteristics with montane glacial deposits of known age in Colorado. These comparisons suggest that FBw, FBO, and FB 1 flood boulders may correlate with deposits formed during the Pinedale glaciation (Birkeland, 1973. 1984; Madole, 1976; Porter et al., 1983). FBw, FBO, and FBI granodiorite food boulders lack the extreme weathering found on erratics of the penultimate Bull Lake glaciation that dates to ca. 130,000 yr BP (Madole, 1976; Shroba et al., 1983; Madole et al., 1984) and thus the FBw, FBO, and FBl paleofloods may have happened after this time. However, if forest fires swept the area then the weathering of surface boulders would have been effected by thermal expansion and spalling thus complicating attempts to use rock weathering as a chronologic tool. At present, Sites 3

35

and 4 are below the lower treeline and only scattered cottonwood trees are present along the active channel. For fire to have been a factor in rock weathering, lower treeline at Sites 3 and 4 would have to have been depressed at least 10 to 15 m from its present lower limit. The altitudinal position of the upper treeline in the Colorado Front Range has fluctuated vertically over of hundreds of meters during the late Pleistocene and Holocene (Elias, 1985; Short, 1985; Short and Elias, 1987). However, little is known about the position of the lower treeline during this same time period, although Markgraf and Scott ( 1981) suggest that in south central Colorado, montane forests occupied present day sagebrush shrublands in lowland areas from the late Pleistocene until about 4000 yr BP. Even considering the effects of fire, it is unlikely that FBw, FBO, and FB 1 deposits are younger than the Pinedale glaciation because none of the granodiorite flood boulders are weathered less than Pinedale erratics. We did not find any fluvial deposits older than TO and FBO in the lower part of the basin. FBO boulders Event Stratigraphy 0

and Radiocarbon

Dates

FB4 (> 1941AD?)

100 200 Lz % :

FB3 (289 yr BP

300 400 500

2 0 i

2,000 1,000

z::

4,000 3,000

302 yr B.P j

-Gully erosion in upper basin -Alluviation in lower basin (Site 1, Unit B) (297yrs.P., 517yr B.P.) ‘Debris-flow

deposition,

Episodic alluviation, 5

”

m 10,000

>100,000?

FE2

!

~.~.“~.~~_~~~.

& I, ,.‘r,+,obt

Fl d

Site 1 (Un it A)

(4,868 yr B P )

5,000

z

g

Site 5

(509 yr B.P )

:‘R

A?

,

4

i Colluviation

and soil development?

. ... -7 . ..

m:yt_ ::::::::::: ?

Fig. 12. Summary of geomorphic

Colluviation

and soil development

events inferred from exposed sur-

ticial deposits in Arthurs Rock Gulch. Radiocarbon

dates on the right

are calibrated according to the method of Stuiver and Reimer ( 1987).

C. F. Waythomas, R.D. Jarrett / Geomorphology

(4

TO-FBO Phase

04

Tl-FBI

Phase

I I (1994) 15-40

C. F. Waythomas. R.D. Jarrett / Geonorphology I I (1994) 15-40

(d

T2-FB2 Phase

Fig. 13. Schematic fluvial-geomorphic reconstructions FBO phase, (b) TI-FBI phase, (c) T2-FB2 phase.

of sites 3 and 4, showing the distribution

were deposited by a flash flood(s) sometime after formation of the TO terrace (Fig. 13a). After the formation of TO and deposition of FBO, the channel was incised and the Tl terrace was formed (Fig. 13b). FB 1 deposits are found only along the outer margins of Tl (Fig. 13b) and are not part of the thin gravel wedge that forms the terrace deposit. Because FBI boulders are on top of Tl terrace gravels at Sites 3 and 4, we believe the deposition of FB 1 postdates the formation of Tl . Continued incision and erosion of Tl terrace deposits after the FB 1 flood and subsequent alluvial deposition resulted in the formation of the T2 terrace (Fig. 13~). FB2 flood boulders were deposited on the T2 terrace as coarse overbank deposits. Overbank (slackwater?) sediments consisting of silty fine sand cap the T2 terrace and indicate episodic vertical accretion, probably by smaller floods. These deposits are tentatively correlated with the organic-rich silty alluvial fill underlying FB3 deposits at Site 1 (Unit B, Fig. 8) because they occupy similar stratigraphic positions within the alluvial sequence.

of terrace and flood-boulder

deposits. (a) TO-

Further incision and lateral erosion after deposition of FB2 was limited by the armoring effect of FB2 flood boulders which were probably too large for transport by post-FB2 floods. Thus, the T3 terrace and FB3 sediments probably formed only where FB2 deposits could be removed by fluvial erosion. Stratigraphic relations at Site 1 indicate that a flood (FB3?) occurred sometime after deposition of unit B which is at least 297 years old (Fig. 8). We mapped the flood deposits (Unit C, Fig. 8) as FB3 deposits (Fig. 5a) on the basis of their position above units A and B which, in turn, we think overlie FB2 deposits at Site 1. FB3 deposits at Site 4 are ca. 295 years old, indicating that FB3 deposits at Site 1 must have formed soon after unit B was deposited. Radiocarbon dates on unit B at Site 1 are stratigraphically inverted; however, the dates are statistically equivalent since they overlap at 1 standard deviation. Because we interpret unit B as reworked colluvium formed by gully erosion in the upper part of the basin, probably upstream from Site 5, the dates may reflect the sequential erosion and redeposition of colluvial sediment.

3X

C. F. Waythomas, R.D. Jarrett / Geomorpholqy

The T4 terrace and FB4 flood boulders represent the final phases of significant fluvial activity in Arthurs Rock Gulch, and provide evidence for limited channel incision and gravel deposition. FB4 flood boulders are evidence for a minor flood that occurred sometime after ff)rmation of the T4 terrace. The oldest trees growing (In FB4 deposits range in age from 50 to 80 years old. Thus, the FB4 flood must be at least 50 years old but 1~:~sthan 297 years old.

I I (1994)

1540

are similar to the weathering of surface erratics on early-middle Holocene moraines above treeline in the Colorado Front Range (Benedict, 198 1; Dowdeswell, 1982). The stratigraphic position and weathering of FB2 deposits indicates that they probably formed between 5000 and 10,000 yr BP. FB3 deposits are radiocarbon dated to about 300 yr. BP. FB4 deposits are at least 50 years old, but not older than about 300 yr BP.

Acknowledgements 6. Conclusions We interpret the majority of coarse bouldery deposits found at elevations up to 10 m above the modern channel of Arthurs Rock Gulch as flood deposits. Other similar deposits are probably debris-flow deposits. Flash floods in Arthurs Rock Gulch capable of transporting large granodiorite boulders occurred at least f1j.e times during the late Quaternary. We have derived a preliminary flood chronology based on relative-age c rrteria, radiocarbon dating, tree-coring, and stratigraphic relations (Fig. 12). Relative-age estimates for FBw, FBO, and FBI deposits indicate that these deposits may be close in age provided the relative-age criteria is suitable for indicating differences in age among these deposits. However, the morphostratigraphic position of FB 1 deposits (Sites 3,4,5) indicates that they are younger than FBw and FBO. Age estimates for FBw, FBO, and FB 1 are based on tentative correlation with erratic boulders on Pinedale moraines. These correlations suggest that the FBw, FBO, and FB 1 flood boulders are at least 40,000 yr. old. Furthermore, FBO and FB 1 flood boulders (Sites 3, 4) overlie oxidized TO and Tl terrace gravel and deeply-weathered colluvial deposits with mature soils that have thick Bt horizons (Table 3). This degree of soil development suggests that the colluvium, and possibly the overlying TO and Tl terrace deposits as well, may be at least 100,000 years old and possibly older (Shroba and Birkeland, 1983). FBw granodiorite boulders at Site 5 cannot be diftzrentiated from FBO deposits on the basis of surface weathering features. It is not known if FBw and FBO deposits formed during the same or different floods. FB2 deposits at Site 1 are stratigraphically beneath alluvium dated to ca. 4500 yr BP. FB2 deposits t excluding Site 1) show weathering characteristics that

Funding for this study was provided by the U.S. Bureau of Reclamation and the U.S. Geological Survey. We thank the staff of Lory State Park for permission to conduct this study within the confines of the park. A. Duran assisted with preparation of the figures. We acknowledge the thoughtful comments and discussion provided by G.P. Williams, W.R. Osterkamp, J.D. Smith, G. Leavesley, P.A. Carling, J.E. Costa, R.H. Webb, D.S. Kaufman, and two anonymous reviewers,

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