- Email: [email protected]
Evidence for Holocene stability of steep slopes, northern Peruvian Andes, based on soils and radiocarbon dates
CATENA
vol. 20, p. 1-12
Cremlingen 1993 ]
E v i d e n c e for H o l o c e n e S t a b i l i t y of S t e e p S l o p e s , Northern Peruvian Andes, B a s e d on Soils and R a d i o c a r b o n D a t e s D.C. M i l l e r , P.W. B i r k e l a n d & D.T. R o d b e l l
Summary Radiocarbon dating and soil relationships indicate that landscapes in highaltitude glaciated valleys of the northern Peruvian Andes have been remarkably stable during the Holocene. Radiocarbon dates show that deglaciation was underway by 12 ka, and that slopes and alluvial fans at the bases of slopes were essentially stabilized by at least 8 ka. The soils consist of fine-grained loessial A horizons overlying Bw horizons in gravelly till or alluvial-fan gravel. Following deglaciation, widespread gullying took place in till on the steep (maxinlum angle: 37 ° ) sideslopes of most valleys; the eroded material was deposited as fans at the bases of the slopes. Loess was then deposited as a fairly uniform blanket across most elements of the landscape. Soil formation began during or following loess deposition, and because soil-profile morphology is sufficiently similar at most sites, soil formation has been a dominant process during much of the Holocene. This remarkable stability, especially for such steep slopes, is attributed to a combination of tight packing of the till, permeability of ISSN 0341-8162 @1993 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0341--8162/93/5011851/US$ 2.00 + 0.25
CATt~NA
An
Inter¢lisclplilL~ry
Journal
of SOIL
the capping loess, rapid revegetation following ice retreat, and roots from the present grassland vegetation and possibly former forests.
1
Introduction
Soils can be used to indicate the relative stability of slopes. For example, well-developed soils on steep slopes suggest that the slope has not been eroding. Soils on the alluvia] fans at the bases of slopes also can be used to indicate relative slope stability. Welldeveloped surface soils and the lack of buried soils in the fan deposits suggest long-term stability of the sideslopes; in contrast, a stack of buried soils and weakly-developed surface soils demonstrate episodic deposition at the bases of slopes due to erosion of the slopes (Burns & Tonkin 1982). In addition, cumulic (overthickened) soil horizons (A or B) in the surface soils of fan deposits suggest that the downslope transfer of materials is relatively slow compared to the rate of soil formation (Nikiforoff 1949, Birkeland 1984). Here we use soil development in various parts of the landscape to assess slope stability in the high Andes of northern Peru. The area was extensively glaciated during the last major glaciation, so all landforms and soils stud-
$CIENCI~--HYDROLO(;Y--
GI~OMORPHOLOGY
2
Miller, Birkeland & Rodbell
and granite in Quishuar Valley; granite and quartzite in Manachaque Valley; and chiefly granite in Chigualen Valley (Rodbell 1991). Climatic data are not available but have been estimated by Young (1990), from which we make further estimations. Chochos Valley receives warm, moist Atlantic air masses moving upward from the Amazon Basin. Mean annual precipitation is estimated to be 2-3 m, with up to one-third occurring as mist. Precipitation decreases to the west; Quishuar and Manachaque Valleys are estimated to receive about 1.5 m and Chigualen Valley less than 1 m of mean annual precipitation. Mean annual temperature is poorly known, but it is probably be2 L o c a t i o n and soil tween 6 and 10°C (Rios et al. 1982). f o r m i n g factors Grasses, herbs, and shrubs comprise the paramo vegetation of the study area The study area consists of four valleys (Weberbauer 1945, Tosi 1960). The located along an alphle climatic tran- western edge of the Amazon rainforest sect in the Cordillera Oriental (7-8 ° S) reaches the lower Chochos Valley and of northern Peru, approximately 400 km the upper end of Manachaque Valley, inland of the coast (fig. 1). Chochos and is represented by isolated tree isValley lies within Rio Abiseo National lands. The rainforest may have exPark and drains eastward from a local tended west of the present paramodivide to the Huallaga River in the Ama- forest boundary in the past as the zon Basin. Quishuar Valley and Man- present boundary is artificially mainachaque Valley are in the central part of tained by cattle grazing and periodic the study area and Chigualen Valley is burning (Young 1990). All soils studied to the southwest of them; all drain west- are presently vegetated by grasses and ward to the Rio Marafion in the Amazon herbs. Basin. Age control is provided by radiocarThe soil-forming factors (Jenny 1980) bon dates from Manachaque and Choof the region are as follows. Par- chos Valleys. Birkeland et al. (1989a) ent material is till on the sideslopes, and Rodbell (1991) used radiocarbon and mostly alluvium makes up the al- dates to conclude that deglaciation comluvial fans at the bases of the slopes. menced before 12 ka and that little or Bedrock, and therefore till composition, no ice remained after 10.3 ka. Two ravaries from valley to valley. The litho- diocarbon dates suggest that slopes have logic composition of stream gravel indi- been stable for at least the last 8 ka cates that bedrock is mostly quartzite (tab. 1). Radiocarbon dates on a buried in Chochos Valley; quartzite, rhyolite, A horizon high on a steep sideslope in
ied post-date that glaciation (Birkeland et al. 1989a). Valley floors lie at approximately 3400-3800 m altitude and are surrounded by peaks approaching 4500 m. At present, glaciers are absent. Little detailed work has been done on the soils of the region (Drosdoff et al. 1960, Beck & Bramao 1968, Thomas & Winterhalder 1976, Wilcox et al. 1988). However, a recent study of soil-catena relations on end moraines in the area (Miller & Birkeland in press) showed consistent trends in most soil properties with slope position, and concluded that end moraine slopes had been stable since about the time of moraine deposition.
CATI~NA
An
Inteldisclpliltaly
Journal
of SOIL
S(711DNCID- HYDROLOGY
GEOMORPHOLOGY
Holocene slope stability, N. Peruvian Andes
3
77°30,w 77°35 ' W
77020 . W
7°50'S
main divide Ioc~ divide stream rnor~ne crest (usua]tymultJp/e) ]
Huar~ ~
montaneIorest
21621- 1:25.003air photographs 21628 (SE-NW)providing coverageof Ihe demarcated area. • towns Z~ stuW site 4= mdlooaKoonsemple$oc~Jizy
'6~d 7 ~
0
5
1Okra I
Fig. 1: Map of study area showing sites for detailed catena studies, radiocarbon sample localities, and main moraines. Valleys studied are, from north to south," Quishuar (Q), Chochos (CC), Manachaque (M), and Chigualen (C). CATBNA--An Interdigciplin~ry Journal of
SOIL
SCIENCE--HYDROLOGY--GBOMORPI-IOLOGY
4
Miller, Birkeland & Rodbell
Laboratory number
Radiocarbon date yr B.P.
Location and remarks
GX-14357
8435 ± 220
Till-mantled long side slope of Manachaque Valley. Slope is 36 °, and site is about 75 m above the valley floor. Material dated is desseminated organic matter from a buried A horizon at 76-85 cm depth. Colluvium/till boundary is at 120 cm. Treated with hot dilute NaOH; insolubles analyzed.
GX-14357-H
8240± 225
GX-14358
8290 ~ 235
Same sample as GX-14357, but analysis is for NaOH-soluble fraction. Material dated is desseminated organic matter from the lowermost of 3 buried A horizons (143-149 cm depth) in a fan deposit at the base of till-mantled side slope, Chochos valley. Treated with hot dilute NaOH; insolubles analyzed.
GX-14358-H
8330 ± 225
Same sample as GX-14358, but analysis is for the NaOH-soluble fraction.
Tab. 1: Radiocarbon dates for slope and fan deposits in the study area.
M a n a c h a q u e Valley (fig. 1; GX-14357 a n d 14357-H) i n d i c a t e t h a t t h a t site is s t a b l e a n d has not been e r o d i n g since a b o u t 8 ks. T h e lowermost of 3 b u r i e d A horizons in a fan at the base of a sideslope in Chochos Valley (fig. 1) also d a t e s n e a r 8 ka (GX-14358 and 14358-H). The d e v e l o p m e n t of the overlying soils suggests t h a t fan d e p o s i t i o n p r o b a b l y did not continue into the l a t t e r half of the Holocene. For reasons to be p r e s e n t e d later, we favor the i n t e r p r e t a t i o n t h a t m o s t slope a d j u s t m e n t and fan formation was c o m p l e t e by a b o u t 8 ks, and this is an a p p r o x i m a t e m i n i m u m age for m a n y of the soils discussed here. Of interest for r a d i o c a r b o n d a t i n g b o t h in the a r e a and in general, d a t e s on b o t h the N a O H - s o l u b l e and N a O H - i n s o l u b l e fractions at b o t h sites are not significantly different (tab. 1). T o p o g r a p h i c s e t t i n g as r e g a r d s sampling s t r a t e g y varies with area. In Chochos, Q u i s h u a r , and Chigualen Valleys, c a t e n a s were s t u d i e d on valley sideslopes. A l t h o u g h p o s t - d e p o s i t i o n a l
CATBNA--An
Interdisciplinary
Jou[n~[
gulleying of the sideslopes varies f r o m m i n o r to deep (up to a b o u t 3 m), sites s t u d i e d had m i n o r gulleying. Each slope c a t e n a was d i v i d e d into three morphologically d i s t i n c t s e g m e n t s ( R u h e & Walker 1969): s u m m i t (SU); b a c k s l o p e (BS), the relatively s t r a i g h t , steep m i d dle section of the slope; and the concave footslope (FS). P i t s e x c a v a t e d at each slope s e g m e n t were along a line est i m a t e d to best reflect the p a t h of sedim e n t and w a t e r m o v e m e n t (surface a n d throughflow) downslopes. This s a m pling s t r a t e g y followed t h a t of S w a n s o n (1985), B e r r y (1987), a n d B i r k e l a n d & Burke (1988). In c o n t r a s t , a reconnaissance survey of soils was m a d e t h r o u g h out M a n a c h a q u e .
3
Methods
Soil field p r o p e r t i e s were d e s c r i b e d using the m e t h o d s and horizon n o m e n c l a t u r e of Soil S u r v e y Staff (1975) a n d Birkel a n d (1984). T h e d i f f e r e n t i a t i o n between
of S O I L S C I I D N C B - - H Y D R O L O G Y - - G B O M O R P H O L O G Y
Holocene slope stability, N. Peruvian Andes
Bw and Cox horizons is based on color and clay content. Because B and C horizons have hues of 10YR, horizon delineation is based on differences in value and chroma (tab. 2). Also some Bw horizons have a slight clay increase of <3%. Laboratory analyses were selected for their ability to detect major pedological processes in profiles as well as soil differences that may exist along the climatic gradient. Particle-size distribution with depth should reflect parent material differences, and sedimentation and pedological processes. Pedogenic iron reflects mainly pedological processes. Organic carbon distribution with depth will indicate cumulative, noncumulative, or pachic development of organic-enriched horizons, which can be used to determine relations between soil formation and slope stability. Laboratory methods used are the standard ones performed on the <2m m fraction of the soils (Singer & Janitzky 1986): particle-size distribution by sieve and pipette, and readily oxidizable organic carbon by the Walkley-Black method (Allison 1965) not multiplied by any factor to predict total organic carbon. Two iron extracts are used: oxalate-extractable iron (Feo) and citrate-bicarbonatedithionite-extractable iron (Fed).
3.1
Reconaissance soil-geomorphic s t u d i e s in Manachaque
Valley
Prior to the detailed soil-catena studies in the three surrounding valleys, reconnaissance field studies were undertaken in Manachaque Valley. There were three goals to this work: 1. help define a relative-age chronology for the many moraines mapped
CATENA--An
Inter,lisciplina.ry
Je, urn~,l of S O I L
5
in the valley (Birkeland et al. 1989a and fig. 1); 2. establish the relation between soil development and various elements of the glaciated landscape (valley sideslopes and fans at the bases of slopes); and 3. determine if soil chronosequences exist in the fan deposits. Similar development of soils on moraines throughout the valley suggests the same general age for all, one that is likely equivalent to the last major glaciation throughout the Andes (ca. 2515 ka, reviewed in Rodbell 1991). Soils at the summits of moraine crests are similar A / 2 B w / 2 C o x profiles with the fine-grained A horizons formed from eolian materials, and the deeper coarsetextured gravelly horizons formed from till (Birkeland et al. 1989b; field descriptions of all soils are in Birkeland et al. 1987; English translations of both articles are available from the authors). In addition, tile degree of clast weathering is similar on all moraines, and the most highly-weathered clasts are in the A horizons. A soil from what m a y be one of the older late Pleistocene moraines in the area is found in the continuous rainforest in Rio Abiseo National Park, just east of Manachaque Valley, and does not show significantly greater development than soils from the younger moraines in the same drainage, or in Manachaque Valley. Profile development index values (Harden 1982) range from 0.12-0.31, average 0.24, and show no systematic variation downvalley. From this we conclude that all of the moraines, including those furthest up-valley close to the valley heads, zLre of one general age group, which we tentatively assign to the last
SCI~NCE--HYDROLOGY--GEOMORPHOLOGY
Valley
Choehos
Valley 0-32 32-165 0-27 27-34 34-165
Valley 0-25 25-39 39-100 0-42 42-53 53-79 79-150 0-54 54-72 72-112
Qulshuar SU/A 2Cox BS/A 2Bt 2Cox
Chlgualen SU/A 2Bt 2Cox BS/A 2Bwi 2Bw2 2Cox FS/A 2Bw 2Cox
E T T E T T T E T T
E T E T T
E T Z T T T E T T
Parent Mat. 1
2/1 4/4 2/1 3/2 4/4
2/1 5/6 2/1 3/3 4/3 5/8 2/1 4/4 4/4 2/1 6/3 3/1 4/3 7/3
3/1 6/6 3/1 5/3 6/3 6/6 3/1 6/4 6/4
7.5YR 3/0 10YR 7 / 2 10YR 7 / 3 10YR 3/1 10YR 5/3 10YR 5 / 4 10YR 6 / 3 10YR 3/1 10YR 5/3 10YR 7 / 3
10YR 10YR 10YR 10YR 10YR
10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR
Color Dry
7.5YR 2/0 10YR 4 ~3 1 0 Y R 4 ~3 1 0 Y R 2 ~1 1 0 Y R 4 f3 1 0 Y R 4 14 10YR 4 13 10YR 2 I1 1 0 Y R 4 13 1 0 Y R 4 t4
10YR 10YR 10YR 10YR 10YR
10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR
Moist
2,c,sbk 2,m-c,sbk 2,c-vc,sbk 2,m-c,sbk sg,f,sbk sg,f,sbk sg,f,sbk 2,f-m,sbk 2,c,abk 1,f-m,sbk
sg,f, abk 2,f-c,abk sg,f,abk 1,m-c,abk 2,c,abk
sg,f,sbk sg,f-m,gr sg,f,sbk 2,m,abk 2,m,abk 2,m,abk sg,f,sbk 2,f,sbk 2,f-m,abk
Structure
<10 50 50 10 75 >75 >75 25 75 75
10 50 <10 <10 75
10 50-75 10 25 25 75 10 50 75
Est'd Gravel %
L SL SL L LS S S SCL LS S
CL SL CL SL SL
CL SL SiC C CL L C SL SL
Texture
s,w a,i -c,s a,w a,s -a,s c,w --
s,w -s,w s.w --
a,s -c,w c,s c,s -c,s g,s --
Horizon Boundary
-3,mk,pf,cobr 2,mk,cobr -1,mk,pf,cobr 1,n,br 1,n,br -3,mk,cobr 1,n,br
-3,mk,cobr -2,n,br 3,mk,cobr
---3,mk,cobr 3,mk,cobr 3,mk,cobr -3,mk,cobr 3,mk,cobr
Clay Films
Tab. 2: Field descriptions for sideslope catena soils, northern Peruvian Andes.
1 E = eolian material, T = till or reworked till 2 1 = u n w e a t h e r e d , 2 = slightly w e a t h e r e d , 3 = highly w e a t h e r e d
0-27 27-40 0-51 51-67 67-75 75-105 0-33 33-50 50-94
2Cox BS/A 2Btl 2Bt2 2Cox FS/A 2Bt 2Cox
SU/A
Depth (cm)
Position/ Horizon
(50%), 3 (50%) (50%), 2 (50%) (80%), 2 (20%)
(90%), 3 (10%)
(90%), 2 (10%)
2 (80%), 3 (20%) 2 1 3 2 1-2 1 2 (25%), 3 (75%) 1-2 1-2
3 2 (50%), 3 (50%) 3 2 2
3 3 2 2 2 2 1 1 1
Stone Weathering 2
O
c~
?
Holocene slope stability, N. Peruvian Andes
Pleistocene glaciation. Soils on the long, gullied valley sideslopes, many of which are >30 °, and on the fans at the bases of the slopes, have a profile development similar to those on the moraines, and in places the soils on the slopes and fans have a degree of development greater than those on the moraine crests. This greater degree of development is shown either by color or by clay accumulation. The one soil analyzed from a fan has high silt and clay content in the eolian A horizon and pronounced Fed accumulation in the B horizon (Birkeland et al. 1989b). Some B horizons on slopes or fans seem to have sufficient field-estimated clay accumulation to be Bt horizons. The eolianderived A horizons have a similar thickness at most sites. A concerted effort was made to identify soil chronosequences in the fan deposits. Although several were identified (Birkeland et al. 1987), these deposits make up a small proportion of the landscape. In conclusion, the similar A / B w / C o x profiles and nearly uniform loess thicknesses on flat moraine crests, steep moraine slopes, and fan deposits in Manachaque Valley suggest long-term landscape stability. Pedogenic processes have dominated over geomorphic processes for much of the time since deglaciation. In order to assess the stability in more detail, it was decided to undertake detailed sideslope catena studies along a climatic gradient in adjacent valleys, the subject of the next section. 3.2
Results of catena studies
The soils in the three other valleys also consist of a thin surface layer of eo-
(~,ATE;NA--An Inte1~ilscipll]Lary Jourl~i
7
lian material that makes up the A horizon, overlying till or alluvial-fan gravel (tab. 2). An eolian origin for the surface layer is supported by grain sizes much finer than those of the underlying coarsegrained till or fan deposit, textures of loam or finer, clear modes in the siltand clay-size fractions, and low (generally 10% or less) gravel content (tab. 2 and 3; fig. 2). This surface material is considered to be loess (Burns & Tonkin 1982, Pye 1987, Betzer et al. 1988). In places, relatively high sand content in loess suggests it became mixed into surface horizons during subsequent pedological processes and bio- and pedoturbations (Burns & Tonkin 1982); so it is ternled mixed loess. Clasts in loess are likely due to loessial deposition engulfing surface clasts, as most are concentrated at the base of the loess (lower A horizon). Till was recognized in the field by a high volume of gravel (generally 75°/o or more) and sandy textures (tab. 2 and 3). In places, till has such a densely packed matrix that it forms vertical outcrops and is retained on steep slopes (37 ° maximum). Both the thickness and low gravel content of the loess (tab. 2) suggest that most of the catena slopes have not undergone much erosion since the time of significant loess accumulation. Only the Chigualen catena shows downslope thickening and increase in gravel content of mixed loess A horizons from SU to FS positions. These data suggest that if slope adjustments took place, they preceeded the main episode of loess deposition. Oxidizable organic carbon follows norreal trends of decreasing amounts with depth, and does not display significant accumulation in the downslope soils (fig. 2). Furthermore, buried A hori-
of S O I L S C I E N C I ~ - - H Y D R O L O G Y - - - G B O M O R P H O L O G Y
8
Miller, Birkeland & Rodbell
Chochos Valley []
2Bt [l]]/2Bw
SU A.
Qulshuar Valley
s*
BS
~:~ lOO~
m
SU
SU
14"
.sZ_"
[
FS
Chlgualen Valley
BS
2~A x - - 36" - - ~ ' ~I--,~
i
SU --*~ - ~ m
(1 8z)
B.
(4.11)
(2.22)
% ~ 1 6 1 10 20
~ 1()O 0
150
fl
< (153)
(20 2)
c.
oE
J
(8 31
I ,$cj
,
9)
,~4
tL (,1.02.045)
(3 12,052)
1).
',
i,
044, 018 ~ " ~ 0
%~Fe ~ , , ~ 2 56,118 ) 2 3 4
" I'
38 0 20
lOO4 1501
13 (2,53, 086)
(0 66. 035)
(0 47 0 27)
Fig, 2: Schematic diagrams of sideslope catena soils. Vertical hillslope and soil profile scales are exaggerated relative to the horizontal scale. Values in parentheses at the top of each diagram are the profile weighted mean percents. A. Variation in soil m o r p h o l o g y in each catena. All horizons d e s i g n a t e d "2" are f o r m e d in till, hillslope colluvium, or alluvial fan gravel. B. D i s t r i b u t i o n of oxidizable organic c a r b o n (O.C.) with d e p t h a n d slope position. C. D i s t r i b u t i o n of silt (dashed line) a n d clay (solid line) with d e p t h a n d slope position. Values in p a r e n t h e s e s are for clay. D. D i s t r i b u t i o n of Feo ( d a s h e d line) a n d Fed (solid line) with d e p t h a n d slope position. T h e two values in p a r e n t h e s e s above t h e d i a g r a m s are for Fed a n d Feo, respectively. Values in p a r e n t h e s e s below t h e d i a g r a m s are c a t e n a weighted m e a n values for Fed a n d Feo, respectively ( m e t h o d for calculation in Birkeland & B u r k e 1988). (",ATIBNA--An Inteld. lsclplin~.ry Journal of SOIL SCIIgN¢ZE--HYDROLOGY---G~OMORPHO]LOGy
Holocene slope stability, N. Peruvian Andes
Position/ Horizon
Choehos
S a n d Fraction, % V. C o a r s e Very - Fine Fine 2 0 0 0 - l()0/l 100-50/1
Particle-Size, 2 nun, % Total Total Total Sand Silt Clay 2000
50/~
<50-2~
9
42/~
Silt Fraction, % Medium-
Coarse <50-201/
Fine <20-5~
Very
Fine
Mz <2 mm
<5-2/1
Valley I
SU/A 2Cox
38.6 58.3
28.6 24.0
32.8 17.7
35.8 52.2
2.8 6.1
4.7 9.1
15.2 10.3
8.8 4.7
BS/A 2Btl 2B12 2Cox FS/A 2Bt 2Cox
11.4 20.6 25.8 48.5 20.9 54.0 74.8
43.6 37.7 42.8 29.0 36.3 29.2 17.2
45.0 41.8 31.4 22.5 42.8 16.8 8.0
8.6 15.1 19.7 42.0 16.3 45.2 70.3
2.8 5.4 6.2 6.5 4.6 8.8 4.6
7.3 9.0 12.9 11.4 7.3 10.4 6.0
18.5 16.7 18.5 11.5 17.0 13.2 7.5
17.8 11.1 11.4 6.1 12.0 5.7 3.0
40.1 24.0 31.9 27.4 25.9
28.9 12.1 32.0 18.2 12.7
26.9 55.3 30.8 46.9 53.5
4.1 8.7 5.4 7.5 7.9
! !
2.5 9.4 6.4 7.6 10.0
32.3 10.2 17.5 14.6 10.8
5.2 4.4 8.0 5.2 5.1
0.13 0.35 0.15 0.30 0.34
38.1 28.5 23.8 37.3 10.1 3.7 7.2 21.5 11.9 9.4
24.3 9.0 5,0 20.1 6.7 5.2 2.8 22.7 5.1 3.0
31.3 52.5 60.2 38.5 78.4 89.2 85.8 50.6 76.2 83.9
6.3 10.0 10.9 4.2 4.8 1.9 4.2 5.2 6.8 3.8
[ i
11.2 12.0 12.7 5.0 4.2 2.0 4.1 2.9 5.1 3.4
16.9 12.5 8.0 21.9 4.5 1.4 2.5 11.1 5.1 4.6
10.1 4.0 3.2 10.4 1.2 0.3 0.6 7.5 1.8 1.3
0.16 0.32 0.37 0.22 0.49 0.58 0.54 0.32 0.50 0.56
Quishuar SU/A 2Cox BS/A 2Bt 2Cox
ChigualeD
SU/A 2Bt 2Cox BS/A 2Bwl 2Bw2 2Cox FS/A 2Bw 2Cox
1
0.19 0.32 0.01 0.04 0.07 0.27 0.04 0.28 0.44
Valley 31.0 63.9 36.2 54.4 61.2
Valley 37.0 62.5 71.1 42.7 83.2 91.1 90.0 55.8 83.0 87.7
:
Tab. 3: Particle-size distribution data for the less than 2 mm fraction of horizons m
sideslope eatenas, northern Peruvian Andes. Mean particle-size (Mz) is calculated from the graphic mean of cumulative curve plots for each horizon (Folk 197~). zons are not c o m m o n downslope. Both of these relations attest to the relative stability of the slopes above the fans. Few trends in grain size with slope position are apparent (fig. 2, tab. 3). Silt and clay contents are high in the A horizons and low in the underlying horizons. Furthermore, there is little parent material sorting and few trends in horizon mean particle size on catena slopes or at the bases of slopes in spite of their steepness. One trend is a drcrease in profile weighted mean % clay in a transect from the wetter (Chochos) to the drier (Chigualen) valley. This might reflect greater clay-size dust input in Cho-
CATENA--An
chos Valley because the parent material there is resistant quartzite, combined with greater weathering of loess particles in the wetter valley. Iron trends are variable (fig. 2). Fed either increases or decreases from the A to the B horizon, and profile weighted mean Fed either decreases (wetter valley), remains the same, or increases (drier valley) downslope. I11 general, trends in Feo follow those in Fed. As before, at least for Chochos Valley, the Fe probably comes mainly from the weathering of loess as there should be little Fe in the parent material. Although they do not follow the predictable trends of
I~tterdlsc[pli~tary Jout'n~] of SOIL S C I E N C E
HYDROLOGY
GI~OMORPHOLOGY
Miller, Birkeland & Rodbell
10
This stability is remarkable for long slopes that reach a m a x i m u m angle of 37 ° and can be attributed to the following factors: (1) the tight packing of the till that allows for individual outcrops to be near vertical, (2) the structure and permeability of the loess that tends to inhibit overland water flow (Ollier 1976, Pye 1987) and (3) the dense root system of present-day grasses as well as those of the possible former forests.
the end-moraine catenas, the Fe profiles support the contention that pedogenesis is dominant over erosion and deposition on these slopes and fans. In contrast to the relatively consistent catena trends for soils formed on non-gullied end moraines in the same valleys (Miller & Birkeland in press), few predictable and consistent catena trends are present in these gullied sideslope catenas. Perhaps the common presence of gulleys on the sideslope catenas fouled obtaining consistent trends because some of the pedogenic waters were routed into the gulleys rather than to the next soil site downslope.
4
5. Although the region has been inhabited by humans for about the past 8 ka (W.B. Church, written communication, 1990), and cattle grazing and burning are contemporary practices, the landscape shows no obvious widespread geomorphic response to these impacts.
Conclusions
The field and laboratory data and radiocarbon dates for the area argue for longterm landscape stability for the area since initial deglaciation. The sequence of events seems to have been:
6. Finally, when one is predicting the sources of solid load for rivers draining the high glaciated Andes (see review in Stallard 1986), perhaps much of it comes from the steep, Vshaped river valleys downstream of the flat-floored, U-shaped valleys of the kind reported on here.
1. Deglaciation was probably relatively rapid after 12 ka, as shown by similar soils on all moraines. 2. Sideslopes of the valley, mantled by till, were extensively gullied, with the eroded materials deposited as fans at the bases of the slopes. 3. A thin loess mantle was deposited across most elements of the landscape, including the sideslopes of the gulleys. We have no data on the duration of loess deposition. 4. Soil development took place during and followillg loess deposition, and soil formation has been dominant over erosional and depositional processes for much of post-glacial time, over much of the landscape.
CATENA
An
Interdisciplin~.ry
Journe,[
Acknowledgements This research was funded by the Graduate School, the Department of Geological Sciences, and the Rio Abiseo National Park Research Project of the University of Colorado, Boulder. The radiocarbon dates were funded through a grant from the University of Colorado Council on Research and Creative Work. We thank Susanne Birkeland, Jos~ Abram Escobedo and Carmelo Marreros who assisted with the field work.
of S O I L
SCIENCE--HYDROLOGY---GEOMORPltOLOGY
Holocene slope stability, N. Peruvian Andes
References A L L I S O N , L.E. (1965): Organic carbon. In: C.A. Black (ed.), Methods of Soil Analysis. Amerian Society of Agronomy, Monograph Series 9, 1367-1378. B E E K , K.J. & B R A M A O , D.L. (1968): Nature and geography of South American soils. In: E.J. Fittkau, J. Illies, H. Klinge, G.H. Schwabe & H. Sioli (eds.), Biogeography and Ecology in South America. W. Funk Press, The Netherlands, 82-112. B E R R Y , M . E . (1987): Morphological and chemical characteristics of soil catenas on Pinedale and Bull Lake moraine slopes in the Salmon River mountains, Idaho. Quaternary Research 28, 210-225. BETZER, P.R., CARDER, K.L., DUCE, R.A., MERRILL, J.T., TIND A L E , N . W . , U E M A T S U , M., C O S TELLO, D.K., Y O U N G , R.W., FEELY, R . A . , B R E L A N D , J . A . , B E R N S T E I N , R . E . & G R L O , A . M . (1988): Long-range transport of giant aerosol mineral particles. Nature 336, 568-571. B I R K E L A N D , P . W . (1984): Soils and Geomorphology. Oxford University Press, New York, 372 pp. BIRKELAND, P.W. & BURKE, R.M. (1988): Soil catena chronosequences on eastern Sierra Nevada moraines, California, U.S.A. Arctic and Alpine Research 20, 473484.
11
DROSDOFF, ZAMORA,
M., C.
QUEVEDO, F. (1960): Soils
& of
Peru. Transactions of the 7th International Congress of Soil Science 4, 97-104. F O L K , R.L. (1974): Petrology of Sedimentary Rocks. Hemphill Publishing, Austin, Texas. 182 pp. H A R D E N , J . W . (1982): A quantitative index of soil development from field descriptions: Examples from a chronosequence in central California. Geoderma 28, 1-28. J E N N Y , H. (1980): The Soil Resource. Springer-Verlag, New York, 377 pp. M I L L E R , D . C . & B i r k e l a n d , P . W . (in p r e s s ) : Soil catena variation along an alpine climatic transect, northern Peruvian Andes. Geoderma. N I K I F O R O F F , C . C . (1949): Weathering and soil evolution. Soil Science 67, 219-223. O L L I E R , C . D . (1976): Catenas in different climates. In: E. Derbyshire (ed.), Geomorphology and Climate. Wiley and Sons, New York, 137-169. P Y E , K. (1987): Aeolian dust and dust deposits. Academic Press, London, 334 pp. R O D B E L L , D . T . (1991): Late Quaternary glaciation and climatic change in the northern Peruvian Andes. Ph.D. dissertation, University of Colorado, Boulder, Colorado.
B I R K E L A N D , P . W . , R O D B E L L , D.T., SHORT, S.K. & YOUNG, K.R. (1987): Estudios paleoambientales y de suelos estudios sobre la vegetaeion del Parque National Rio Abiseo. Unpublished report for Ministerio de Agricultura, Peru. 37 p.
RIOS, M.A., PONCE, C.F., VASQ U E Z , P. & T O V A R , A. (1982): Planificacion para el establicimiento de unidades de conservacion en el bosque nublado del noreste dei Peru. Informe Final Proyecto WWF-IUCN/1972, Lima.
B I R K E L A N D , P . W . , R O D B E L L , D.T. & S H O R T , S . K . (1989a): Radiocarbon dates on deglaciation, Cordillera Central, northern Peruvian Andes. Quaternary Research 32, 111-113.
R U H E , R . V . & W A L K E R , P . H . (1968): Hillslope models and soil formation. I. Open systems. Transactions of the 9th International Congress of Soil Science 4, 551-560.
B I R K E L A N D , P . W . , R O D B E L L , D.T., MILLER, D.M. & SHORT, S.K. (1989b): Investigaciones geologicas en el Parque Nacional Rio Abiseo, San Martin. Boletin de Lima 64, 55-64. B U R N S , S . F . & T O N K I N , P . J . (1982): Soil-geomorphic models and the spatial distribution and development of alpine soils. In: C.E. Thorn (ed.), Space and Time in Geomorphology. Allen and Unwin, London, 2543.
CATI~NA--Axt
Interdlscip]i3~ary
Jnurn,~l
~,f S O I L
S I N G E R , M . J . & J A N I T Z K Y , P. (eds.) (1986): Field and laboratory procedures used in a soil chronosequence study. U.S. Geological Survey Bulletin 1648, 49 pp. S O I L S U R V E Y S T A F F (1975): Soil Taxonomy. U.S. Department of Agriculture, Soil Conservation Service, Handbook 436, 753 pp. S T A L L A R D , R . F . (1986): Weathering and erosion in the humid tropics. In: A. Lerman & M. Maybeck (eds.), Physical and
SCIENCE--HYDROLOGY
GEOMOR.
PHOLOGY
Miller, Birkeland & Rodbell
12
Chemical Methods in Geochemical Cycles. Klumer Academic Press, Dordrecht, The Netherlands, 225-246. S W A N S O N , D . K . (1985): Soil eatenas on Pinedale and Bull Lake moraines, Willow Lake, Wind River Mountains, Wyoming. CATENA 12, 329-342. THOMAS, R.B. & WINTERHALDER, B.P. (1976): Physical and biotic environment of southern highland Peru. In: P.T. Baker & M.A. Little (eds.), Man in the Andes: a Multidisciplinary Study of HighAltitude Quechua. Dowden, Hutchinson & Ross, Stroudsberg, Pennsylvania, 21-59. TOSI, J.A., Jr. (1960): Zonas de vida natural en el Peru. Instituto Interamerieano de Cienas Agricolas de la OEA. Zona Andina Boletin Technico 5, Lima. WILCOX, B.P., ALLEN, B.L. & B R Y A N T , F . C . (1988): Description and classification of soils of the high elevation grassland of central Peru. Geoderma 42, 7994. Y O U N G , K . R . (1990): Biogeography and ecology of a timberline forest in north-central Peru. Ph.D. dissertation, University of Colorado, Boulder, Colorado.
A d d r e s s e s of a u t h o r s : D.C. Miller
F.M. Stoller Corp. 5700 Flatiron Parkway Boulder, Colorado 80301 U.S.A. P r o f . Dr. P . W . B i r k e l a n d Department of Geological Sciences University of Colorado Boulder, Colorado, 80309-0250 U.S.A. Dr. D.T. Rodbell U.S. Geological Survey, MS 96g Denver, Colorado 80225-0046 U.S.A.
(2ATE:NA---An
Inte,disclplinary
J,:,ur31~l ,,f S O I L
Sc~IEN(~E
HYDROLOGY
GI~OMORPHOLOCY
vol. 20, p. 1-12
Cremlingen 1993 ]
E v i d e n c e for H o l o c e n e S t a b i l i t y of S t e e p S l o p e s , Northern Peruvian Andes, B a s e d on Soils and R a d i o c a r b o n D a t e s D.C. M i l l e r , P.W. B i r k e l a n d & D.T. R o d b e l l
Summary Radiocarbon dating and soil relationships indicate that landscapes in highaltitude glaciated valleys of the northern Peruvian Andes have been remarkably stable during the Holocene. Radiocarbon dates show that deglaciation was underway by 12 ka, and that slopes and alluvial fans at the bases of slopes were essentially stabilized by at least 8 ka. The soils consist of fine-grained loessial A horizons overlying Bw horizons in gravelly till or alluvial-fan gravel. Following deglaciation, widespread gullying took place in till on the steep (maxinlum angle: 37 ° ) sideslopes of most valleys; the eroded material was deposited as fans at the bases of the slopes. Loess was then deposited as a fairly uniform blanket across most elements of the landscape. Soil formation began during or following loess deposition, and because soil-profile morphology is sufficiently similar at most sites, soil formation has been a dominant process during much of the Holocene. This remarkable stability, especially for such steep slopes, is attributed to a combination of tight packing of the till, permeability of ISSN 0341-8162 @1993 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0341--8162/93/5011851/US$ 2.00 + 0.25
CATt~NA
An
Inter¢lisclplilL~ry
Journal
of SOIL
the capping loess, rapid revegetation following ice retreat, and roots from the present grassland vegetation and possibly former forests.
1
Introduction
Soils can be used to indicate the relative stability of slopes. For example, well-developed soils on steep slopes suggest that the slope has not been eroding. Soils on the alluvia] fans at the bases of slopes also can be used to indicate relative slope stability. Welldeveloped surface soils and the lack of buried soils in the fan deposits suggest long-term stability of the sideslopes; in contrast, a stack of buried soils and weakly-developed surface soils demonstrate episodic deposition at the bases of slopes due to erosion of the slopes (Burns & Tonkin 1982). In addition, cumulic (overthickened) soil horizons (A or B) in the surface soils of fan deposits suggest that the downslope transfer of materials is relatively slow compared to the rate of soil formation (Nikiforoff 1949, Birkeland 1984). Here we use soil development in various parts of the landscape to assess slope stability in the high Andes of northern Peru. The area was extensively glaciated during the last major glaciation, so all landforms and soils stud-
$CIENCI~--HYDROLO(;Y--
GI~OMORPHOLOGY
2
Miller, Birkeland & Rodbell
and granite in Quishuar Valley; granite and quartzite in Manachaque Valley; and chiefly granite in Chigualen Valley (Rodbell 1991). Climatic data are not available but have been estimated by Young (1990), from which we make further estimations. Chochos Valley receives warm, moist Atlantic air masses moving upward from the Amazon Basin. Mean annual precipitation is estimated to be 2-3 m, with up to one-third occurring as mist. Precipitation decreases to the west; Quishuar and Manachaque Valleys are estimated to receive about 1.5 m and Chigualen Valley less than 1 m of mean annual precipitation. Mean annual temperature is poorly known, but it is probably be2 L o c a t i o n and soil tween 6 and 10°C (Rios et al. 1982). f o r m i n g factors Grasses, herbs, and shrubs comprise the paramo vegetation of the study area The study area consists of four valleys (Weberbauer 1945, Tosi 1960). The located along an alphle climatic tran- western edge of the Amazon rainforest sect in the Cordillera Oriental (7-8 ° S) reaches the lower Chochos Valley and of northern Peru, approximately 400 km the upper end of Manachaque Valley, inland of the coast (fig. 1). Chochos and is represented by isolated tree isValley lies within Rio Abiseo National lands. The rainforest may have exPark and drains eastward from a local tended west of the present paramodivide to the Huallaga River in the Ama- forest boundary in the past as the zon Basin. Quishuar Valley and Man- present boundary is artificially mainachaque Valley are in the central part of tained by cattle grazing and periodic the study area and Chigualen Valley is burning (Young 1990). All soils studied to the southwest of them; all drain west- are presently vegetated by grasses and ward to the Rio Marafion in the Amazon herbs. Basin. Age control is provided by radiocarThe soil-forming factors (Jenny 1980) bon dates from Manachaque and Choof the region are as follows. Par- chos Valleys. Birkeland et al. (1989a) ent material is till on the sideslopes, and Rodbell (1991) used radiocarbon and mostly alluvium makes up the al- dates to conclude that deglaciation comluvial fans at the bases of the slopes. menced before 12 ka and that little or Bedrock, and therefore till composition, no ice remained after 10.3 ka. Two ravaries from valley to valley. The litho- diocarbon dates suggest that slopes have logic composition of stream gravel indi- been stable for at least the last 8 ka cates that bedrock is mostly quartzite (tab. 1). Radiocarbon dates on a buried in Chochos Valley; quartzite, rhyolite, A horizon high on a steep sideslope in
ied post-date that glaciation (Birkeland et al. 1989a). Valley floors lie at approximately 3400-3800 m altitude and are surrounded by peaks approaching 4500 m. At present, glaciers are absent. Little detailed work has been done on the soils of the region (Drosdoff et al. 1960, Beck & Bramao 1968, Thomas & Winterhalder 1976, Wilcox et al. 1988). However, a recent study of soil-catena relations on end moraines in the area (Miller & Birkeland in press) showed consistent trends in most soil properties with slope position, and concluded that end moraine slopes had been stable since about the time of moraine deposition.
CATI~NA
An
Inteldisclpliltaly
Journal
of SOIL
S(711DNCID- HYDROLOGY
GEOMORPHOLOGY
Holocene slope stability, N. Peruvian Andes
3
77°30,w 77°35 ' W
77020 . W
7°50'S
main divide Ioc~ divide stream rnor~ne crest (usua]tymultJp/e) ]
Huar~ ~
montaneIorest
21621- 1:25.003air photographs 21628 (SE-NW)providing coverageof Ihe demarcated area. • towns Z~ stuW site 4= mdlooaKoonsemple$oc~Jizy
'6~d 7 ~
0
5
1Okra I
Fig. 1: Map of study area showing sites for detailed catena studies, radiocarbon sample localities, and main moraines. Valleys studied are, from north to south," Quishuar (Q), Chochos (CC), Manachaque (M), and Chigualen (C). CATBNA--An Interdigciplin~ry Journal of
SOIL
SCIENCE--HYDROLOGY--GBOMORPI-IOLOGY
4
Miller, Birkeland & Rodbell
Laboratory number
Radiocarbon date yr B.P.
Location and remarks
GX-14357
8435 ± 220
Till-mantled long side slope of Manachaque Valley. Slope is 36 °, and site is about 75 m above the valley floor. Material dated is desseminated organic matter from a buried A horizon at 76-85 cm depth. Colluvium/till boundary is at 120 cm. Treated with hot dilute NaOH; insolubles analyzed.
GX-14357-H
8240± 225
GX-14358
8290 ~ 235
Same sample as GX-14357, but analysis is for NaOH-soluble fraction. Material dated is desseminated organic matter from the lowermost of 3 buried A horizons (143-149 cm depth) in a fan deposit at the base of till-mantled side slope, Chochos valley. Treated with hot dilute NaOH; insolubles analyzed.
GX-14358-H
8330 ± 225
Same sample as GX-14358, but analysis is for the NaOH-soluble fraction.
Tab. 1: Radiocarbon dates for slope and fan deposits in the study area.
M a n a c h a q u e Valley (fig. 1; GX-14357 a n d 14357-H) i n d i c a t e t h a t t h a t site is s t a b l e a n d has not been e r o d i n g since a b o u t 8 ks. T h e lowermost of 3 b u r i e d A horizons in a fan at the base of a sideslope in Chochos Valley (fig. 1) also d a t e s n e a r 8 ka (GX-14358 and 14358-H). The d e v e l o p m e n t of the overlying soils suggests t h a t fan d e p o s i t i o n p r o b a b l y did not continue into the l a t t e r half of the Holocene. For reasons to be p r e s e n t e d later, we favor the i n t e r p r e t a t i o n t h a t m o s t slope a d j u s t m e n t and fan formation was c o m p l e t e by a b o u t 8 ks, and this is an a p p r o x i m a t e m i n i m u m age for m a n y of the soils discussed here. Of interest for r a d i o c a r b o n d a t i n g b o t h in the a r e a and in general, d a t e s on b o t h the N a O H - s o l u b l e and N a O H - i n s o l u b l e fractions at b o t h sites are not significantly different (tab. 1). T o p o g r a p h i c s e t t i n g as r e g a r d s sampling s t r a t e g y varies with area. In Chochos, Q u i s h u a r , and Chigualen Valleys, c a t e n a s were s t u d i e d on valley sideslopes. A l t h o u g h p o s t - d e p o s i t i o n a l
CATBNA--An
Interdisciplinary
Jou[n~[
gulleying of the sideslopes varies f r o m m i n o r to deep (up to a b o u t 3 m), sites s t u d i e d had m i n o r gulleying. Each slope c a t e n a was d i v i d e d into three morphologically d i s t i n c t s e g m e n t s ( R u h e & Walker 1969): s u m m i t (SU); b a c k s l o p e (BS), the relatively s t r a i g h t , steep m i d dle section of the slope; and the concave footslope (FS). P i t s e x c a v a t e d at each slope s e g m e n t were along a line est i m a t e d to best reflect the p a t h of sedim e n t and w a t e r m o v e m e n t (surface a n d throughflow) downslopes. This s a m pling s t r a t e g y followed t h a t of S w a n s o n (1985), B e r r y (1987), a n d B i r k e l a n d & Burke (1988). In c o n t r a s t , a reconnaissance survey of soils was m a d e t h r o u g h out M a n a c h a q u e .
3
Methods
Soil field p r o p e r t i e s were d e s c r i b e d using the m e t h o d s and horizon n o m e n c l a t u r e of Soil S u r v e y Staff (1975) a n d Birkel a n d (1984). T h e d i f f e r e n t i a t i o n between
of S O I L S C I I D N C B - - H Y D R O L O G Y - - G B O M O R P H O L O G Y
Holocene slope stability, N. Peruvian Andes
Bw and Cox horizons is based on color and clay content. Because B and C horizons have hues of 10YR, horizon delineation is based on differences in value and chroma (tab. 2). Also some Bw horizons have a slight clay increase of <3%. Laboratory analyses were selected for their ability to detect major pedological processes in profiles as well as soil differences that may exist along the climatic gradient. Particle-size distribution with depth should reflect parent material differences, and sedimentation and pedological processes. Pedogenic iron reflects mainly pedological processes. Organic carbon distribution with depth will indicate cumulative, noncumulative, or pachic development of organic-enriched horizons, which can be used to determine relations between soil formation and slope stability. Laboratory methods used are the standard ones performed on the <2m m fraction of the soils (Singer & Janitzky 1986): particle-size distribution by sieve and pipette, and readily oxidizable organic carbon by the Walkley-Black method (Allison 1965) not multiplied by any factor to predict total organic carbon. Two iron extracts are used: oxalate-extractable iron (Feo) and citrate-bicarbonatedithionite-extractable iron (Fed).
3.1
Reconaissance soil-geomorphic s t u d i e s in Manachaque
Valley
Prior to the detailed soil-catena studies in the three surrounding valleys, reconnaissance field studies were undertaken in Manachaque Valley. There were three goals to this work: 1. help define a relative-age chronology for the many moraines mapped
CATENA--An
Inter,lisciplina.ry
Je, urn~,l of S O I L
5
in the valley (Birkeland et al. 1989a and fig. 1); 2. establish the relation between soil development and various elements of the glaciated landscape (valley sideslopes and fans at the bases of slopes); and 3. determine if soil chronosequences exist in the fan deposits. Similar development of soils on moraines throughout the valley suggests the same general age for all, one that is likely equivalent to the last major glaciation throughout the Andes (ca. 2515 ka, reviewed in Rodbell 1991). Soils at the summits of moraine crests are similar A / 2 B w / 2 C o x profiles with the fine-grained A horizons formed from eolian materials, and the deeper coarsetextured gravelly horizons formed from till (Birkeland et al. 1989b; field descriptions of all soils are in Birkeland et al. 1987; English translations of both articles are available from the authors). In addition, tile degree of clast weathering is similar on all moraines, and the most highly-weathered clasts are in the A horizons. A soil from what m a y be one of the older late Pleistocene moraines in the area is found in the continuous rainforest in Rio Abiseo National Park, just east of Manachaque Valley, and does not show significantly greater development than soils from the younger moraines in the same drainage, or in Manachaque Valley. Profile development index values (Harden 1982) range from 0.12-0.31, average 0.24, and show no systematic variation downvalley. From this we conclude that all of the moraines, including those furthest up-valley close to the valley heads, zLre of one general age group, which we tentatively assign to the last
SCI~NCE--HYDROLOGY--GEOMORPHOLOGY
Valley
Choehos
Valley 0-32 32-165 0-27 27-34 34-165
Valley 0-25 25-39 39-100 0-42 42-53 53-79 79-150 0-54 54-72 72-112
Qulshuar SU/A 2Cox BS/A 2Bt 2Cox
Chlgualen SU/A 2Bt 2Cox BS/A 2Bwi 2Bw2 2Cox FS/A 2Bw 2Cox
E T T E T T T E T T
E T E T T
E T Z T T T E T T
Parent Mat. 1
2/1 4/4 2/1 3/2 4/4
2/1 5/6 2/1 3/3 4/3 5/8 2/1 4/4 4/4 2/1 6/3 3/1 4/3 7/3
3/1 6/6 3/1 5/3 6/3 6/6 3/1 6/4 6/4
7.5YR 3/0 10YR 7 / 2 10YR 7 / 3 10YR 3/1 10YR 5/3 10YR 5 / 4 10YR 6 / 3 10YR 3/1 10YR 5/3 10YR 7 / 3
10YR 10YR 10YR 10YR 10YR
10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR
Color Dry
7.5YR 2/0 10YR 4 ~3 1 0 Y R 4 ~3 1 0 Y R 2 ~1 1 0 Y R 4 f3 1 0 Y R 4 14 10YR 4 13 10YR 2 I1 1 0 Y R 4 13 1 0 Y R 4 t4
10YR 10YR 10YR 10YR 10YR
10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR
Moist
2,c,sbk 2,m-c,sbk 2,c-vc,sbk 2,m-c,sbk sg,f,sbk sg,f,sbk sg,f,sbk 2,f-m,sbk 2,c,abk 1,f-m,sbk
sg,f, abk 2,f-c,abk sg,f,abk 1,m-c,abk 2,c,abk
sg,f,sbk sg,f-m,gr sg,f,sbk 2,m,abk 2,m,abk 2,m,abk sg,f,sbk 2,f,sbk 2,f-m,abk
Structure
<10 50 50 10 75 >75 >75 25 75 75
10 50 <10 <10 75
10 50-75 10 25 25 75 10 50 75
Est'd Gravel %
L SL SL L LS S S SCL LS S
CL SL CL SL SL
CL SL SiC C CL L C SL SL
Texture
s,w a,i -c,s a,w a,s -a,s c,w --
s,w -s,w s.w --
a,s -c,w c,s c,s -c,s g,s --
Horizon Boundary
-3,mk,pf,cobr 2,mk,cobr -1,mk,pf,cobr 1,n,br 1,n,br -3,mk,cobr 1,n,br
-3,mk,cobr -2,n,br 3,mk,cobr
---3,mk,cobr 3,mk,cobr 3,mk,cobr -3,mk,cobr 3,mk,cobr
Clay Films
Tab. 2: Field descriptions for sideslope catena soils, northern Peruvian Andes.
1 E = eolian material, T = till or reworked till 2 1 = u n w e a t h e r e d , 2 = slightly w e a t h e r e d , 3 = highly w e a t h e r e d
0-27 27-40 0-51 51-67 67-75 75-105 0-33 33-50 50-94
2Cox BS/A 2Btl 2Bt2 2Cox FS/A 2Bt 2Cox
SU/A
Depth (cm)
Position/ Horizon
(50%), 3 (50%) (50%), 2 (50%) (80%), 2 (20%)
(90%), 3 (10%)
(90%), 2 (10%)
2 (80%), 3 (20%) 2 1 3 2 1-2 1 2 (25%), 3 (75%) 1-2 1-2
3 2 (50%), 3 (50%) 3 2 2
3 3 2 2 2 2 1 1 1
Stone Weathering 2
O
c~
?
Holocene slope stability, N. Peruvian Andes
Pleistocene glaciation. Soils on the long, gullied valley sideslopes, many of which are >30 °, and on the fans at the bases of the slopes, have a profile development similar to those on the moraines, and in places the soils on the slopes and fans have a degree of development greater than those on the moraine crests. This greater degree of development is shown either by color or by clay accumulation. The one soil analyzed from a fan has high silt and clay content in the eolian A horizon and pronounced Fed accumulation in the B horizon (Birkeland et al. 1989b). Some B horizons on slopes or fans seem to have sufficient field-estimated clay accumulation to be Bt horizons. The eolianderived A horizons have a similar thickness at most sites. A concerted effort was made to identify soil chronosequences in the fan deposits. Although several were identified (Birkeland et al. 1987), these deposits make up a small proportion of the landscape. In conclusion, the similar A / B w / C o x profiles and nearly uniform loess thicknesses on flat moraine crests, steep moraine slopes, and fan deposits in Manachaque Valley suggest long-term landscape stability. Pedogenic processes have dominated over geomorphic processes for much of the time since deglaciation. In order to assess the stability in more detail, it was decided to undertake detailed sideslope catena studies along a climatic gradient in adjacent valleys, the subject of the next section. 3.2
Results of catena studies
The soils in the three other valleys also consist of a thin surface layer of eo-
(~,ATE;NA--An Inte1~ilscipll]Lary Jourl~i
7
lian material that makes up the A horizon, overlying till or alluvial-fan gravel (tab. 2). An eolian origin for the surface layer is supported by grain sizes much finer than those of the underlying coarsegrained till or fan deposit, textures of loam or finer, clear modes in the siltand clay-size fractions, and low (generally 10% or less) gravel content (tab. 2 and 3; fig. 2). This surface material is considered to be loess (Burns & Tonkin 1982, Pye 1987, Betzer et al. 1988). In places, relatively high sand content in loess suggests it became mixed into surface horizons during subsequent pedological processes and bio- and pedoturbations (Burns & Tonkin 1982); so it is ternled mixed loess. Clasts in loess are likely due to loessial deposition engulfing surface clasts, as most are concentrated at the base of the loess (lower A horizon). Till was recognized in the field by a high volume of gravel (generally 75°/o or more) and sandy textures (tab. 2 and 3). In places, till has such a densely packed matrix that it forms vertical outcrops and is retained on steep slopes (37 ° maximum). Both the thickness and low gravel content of the loess (tab. 2) suggest that most of the catena slopes have not undergone much erosion since the time of significant loess accumulation. Only the Chigualen catena shows downslope thickening and increase in gravel content of mixed loess A horizons from SU to FS positions. These data suggest that if slope adjustments took place, they preceeded the main episode of loess deposition. Oxidizable organic carbon follows norreal trends of decreasing amounts with depth, and does not display significant accumulation in the downslope soils (fig. 2). Furthermore, buried A hori-
of S O I L S C I E N C I ~ - - H Y D R O L O G Y - - - G B O M O R P H O L O G Y
8
Miller, Birkeland & Rodbell
Chochos Valley []
2Bt [l]]/2Bw
SU A.
Qulshuar Valley
s*
BS
~:~ lOO~
m
SU
SU
14"
.sZ_"
[
FS
Chlgualen Valley
BS
2~A x - - 36" - - ~ ' ~I--,~
i
SU --*~ - ~ m
(1 8z)
B.
(4.11)
(2.22)
% ~ 1 6 1 10 20
~ 1()O 0
150
fl
< (153)
(20 2)
c.
oE
J
(8 31
I ,$cj
,
9)
,~4
tL (,1.02.045)
(3 12,052)
1).
',
i,
044, 018 ~ " ~ 0
%~Fe ~ , , ~ 2 56,118 ) 2 3 4
" I'
38 0 20
lOO4 1501
13 (2,53, 086)
(0 66. 035)
(0 47 0 27)
Fig, 2: Schematic diagrams of sideslope catena soils. Vertical hillslope and soil profile scales are exaggerated relative to the horizontal scale. Values in parentheses at the top of each diagram are the profile weighted mean percents. A. Variation in soil m o r p h o l o g y in each catena. All horizons d e s i g n a t e d "2" are f o r m e d in till, hillslope colluvium, or alluvial fan gravel. B. D i s t r i b u t i o n of oxidizable organic c a r b o n (O.C.) with d e p t h a n d slope position. C. D i s t r i b u t i o n of silt (dashed line) a n d clay (solid line) with d e p t h a n d slope position. Values in p a r e n t h e s e s are for clay. D. D i s t r i b u t i o n of Feo ( d a s h e d line) a n d Fed (solid line) with d e p t h a n d slope position. T h e two values in p a r e n t h e s e s above t h e d i a g r a m s are for Fed a n d Feo, respectively. Values in p a r e n t h e s e s below t h e d i a g r a m s are c a t e n a weighted m e a n values for Fed a n d Feo, respectively ( m e t h o d for calculation in Birkeland & B u r k e 1988). (",ATIBNA--An Inteld. lsclplin~.ry Journal of SOIL SCIIgN¢ZE--HYDROLOGY---G~OMORPHO]LOGy
Holocene slope stability, N. Peruvian Andes
Position/ Horizon
Choehos
S a n d Fraction, % V. C o a r s e Very - Fine Fine 2 0 0 0 - l()0/l 100-50/1
Particle-Size, 2 nun, % Total Total Total Sand Silt Clay 2000
50/~
<50-2~
9
42/~
Silt Fraction, % Medium-
Coarse <50-201/
Fine <20-5~
Very
Fine
Mz <2 mm
<5-2/1
Valley I
SU/A 2Cox
38.6 58.3
28.6 24.0
32.8 17.7
35.8 52.2
2.8 6.1
4.7 9.1
15.2 10.3
8.8 4.7
BS/A 2Btl 2B12 2Cox FS/A 2Bt 2Cox
11.4 20.6 25.8 48.5 20.9 54.0 74.8
43.6 37.7 42.8 29.0 36.3 29.2 17.2
45.0 41.8 31.4 22.5 42.8 16.8 8.0
8.6 15.1 19.7 42.0 16.3 45.2 70.3
2.8 5.4 6.2 6.5 4.6 8.8 4.6
7.3 9.0 12.9 11.4 7.3 10.4 6.0
18.5 16.7 18.5 11.5 17.0 13.2 7.5
17.8 11.1 11.4 6.1 12.0 5.7 3.0
40.1 24.0 31.9 27.4 25.9
28.9 12.1 32.0 18.2 12.7
26.9 55.3 30.8 46.9 53.5
4.1 8.7 5.4 7.5 7.9
! !
2.5 9.4 6.4 7.6 10.0
32.3 10.2 17.5 14.6 10.8
5.2 4.4 8.0 5.2 5.1
0.13 0.35 0.15 0.30 0.34
38.1 28.5 23.8 37.3 10.1 3.7 7.2 21.5 11.9 9.4
24.3 9.0 5,0 20.1 6.7 5.2 2.8 22.7 5.1 3.0
31.3 52.5 60.2 38.5 78.4 89.2 85.8 50.6 76.2 83.9
6.3 10.0 10.9 4.2 4.8 1.9 4.2 5.2 6.8 3.8
[ i
11.2 12.0 12.7 5.0 4.2 2.0 4.1 2.9 5.1 3.4
16.9 12.5 8.0 21.9 4.5 1.4 2.5 11.1 5.1 4.6
10.1 4.0 3.2 10.4 1.2 0.3 0.6 7.5 1.8 1.3
0.16 0.32 0.37 0.22 0.49 0.58 0.54 0.32 0.50 0.56
Quishuar SU/A 2Cox BS/A 2Bt 2Cox
ChigualeD
SU/A 2Bt 2Cox BS/A 2Bwl 2Bw2 2Cox FS/A 2Bw 2Cox
1
0.19 0.32 0.01 0.04 0.07 0.27 0.04 0.28 0.44
Valley 31.0 63.9 36.2 54.4 61.2
Valley 37.0 62.5 71.1 42.7 83.2 91.1 90.0 55.8 83.0 87.7
:
Tab. 3: Particle-size distribution data for the less than 2 mm fraction of horizons m
sideslope eatenas, northern Peruvian Andes. Mean particle-size (Mz) is calculated from the graphic mean of cumulative curve plots for each horizon (Folk 197~). zons are not c o m m o n downslope. Both of these relations attest to the relative stability of the slopes above the fans. Few trends in grain size with slope position are apparent (fig. 2, tab. 3). Silt and clay contents are high in the A horizons and low in the underlying horizons. Furthermore, there is little parent material sorting and few trends in horizon mean particle size on catena slopes or at the bases of slopes in spite of their steepness. One trend is a drcrease in profile weighted mean % clay in a transect from the wetter (Chochos) to the drier (Chigualen) valley. This might reflect greater clay-size dust input in Cho-
CATENA--An
chos Valley because the parent material there is resistant quartzite, combined with greater weathering of loess particles in the wetter valley. Iron trends are variable (fig. 2). Fed either increases or decreases from the A to the B horizon, and profile weighted mean Fed either decreases (wetter valley), remains the same, or increases (drier valley) downslope. I11 general, trends in Feo follow those in Fed. As before, at least for Chochos Valley, the Fe probably comes mainly from the weathering of loess as there should be little Fe in the parent material. Although they do not follow the predictable trends of
I~tterdlsc[pli~tary Jout'n~] of SOIL S C I E N C E
HYDROLOGY
GI~OMORPHOLOGY
Miller, Birkeland & Rodbell
10
This stability is remarkable for long slopes that reach a m a x i m u m angle of 37 ° and can be attributed to the following factors: (1) the tight packing of the till that allows for individual outcrops to be near vertical, (2) the structure and permeability of the loess that tends to inhibit overland water flow (Ollier 1976, Pye 1987) and (3) the dense root system of present-day grasses as well as those of the possible former forests.
the end-moraine catenas, the Fe profiles support the contention that pedogenesis is dominant over erosion and deposition on these slopes and fans. In contrast to the relatively consistent catena trends for soils formed on non-gullied end moraines in the same valleys (Miller & Birkeland in press), few predictable and consistent catena trends are present in these gullied sideslope catenas. Perhaps the common presence of gulleys on the sideslope catenas fouled obtaining consistent trends because some of the pedogenic waters were routed into the gulleys rather than to the next soil site downslope.
4
5. Although the region has been inhabited by humans for about the past 8 ka (W.B. Church, written communication, 1990), and cattle grazing and burning are contemporary practices, the landscape shows no obvious widespread geomorphic response to these impacts.
Conclusions
The field and laboratory data and radiocarbon dates for the area argue for longterm landscape stability for the area since initial deglaciation. The sequence of events seems to have been:
6. Finally, when one is predicting the sources of solid load for rivers draining the high glaciated Andes (see review in Stallard 1986), perhaps much of it comes from the steep, Vshaped river valleys downstream of the flat-floored, U-shaped valleys of the kind reported on here.
1. Deglaciation was probably relatively rapid after 12 ka, as shown by similar soils on all moraines. 2. Sideslopes of the valley, mantled by till, were extensively gullied, with the eroded materials deposited as fans at the bases of the slopes. 3. A thin loess mantle was deposited across most elements of the landscape, including the sideslopes of the gulleys. We have no data on the duration of loess deposition. 4. Soil development took place during and followillg loess deposition, and soil formation has been dominant over erosional and depositional processes for much of post-glacial time, over much of the landscape.
CATENA
An
Interdisciplin~.ry
Journe,[
Acknowledgements This research was funded by the Graduate School, the Department of Geological Sciences, and the Rio Abiseo National Park Research Project of the University of Colorado, Boulder. The radiocarbon dates were funded through a grant from the University of Colorado Council on Research and Creative Work. We thank Susanne Birkeland, Jos~ Abram Escobedo and Carmelo Marreros who assisted with the field work.
of S O I L
SCIENCE--HYDROLOGY---GEOMORPltOLOGY
Holocene slope stability, N. Peruvian Andes
References A L L I S O N , L.E. (1965): Organic carbon. In: C.A. Black (ed.), Methods of Soil Analysis. Amerian Society of Agronomy, Monograph Series 9, 1367-1378. B E E K , K.J. & B R A M A O , D.L. (1968): Nature and geography of South American soils. In: E.J. Fittkau, J. Illies, H. Klinge, G.H. Schwabe & H. Sioli (eds.), Biogeography and Ecology in South America. W. Funk Press, The Netherlands, 82-112. B E R R Y , M . E . (1987): Morphological and chemical characteristics of soil catenas on Pinedale and Bull Lake moraine slopes in the Salmon River mountains, Idaho. Quaternary Research 28, 210-225. BETZER, P.R., CARDER, K.L., DUCE, R.A., MERRILL, J.T., TIND A L E , N . W . , U E M A T S U , M., C O S TELLO, D.K., Y O U N G , R.W., FEELY, R . A . , B R E L A N D , J . A . , B E R N S T E I N , R . E . & G R L O , A . M . (1988): Long-range transport of giant aerosol mineral particles. Nature 336, 568-571. B I R K E L A N D , P . W . (1984): Soils and Geomorphology. Oxford University Press, New York, 372 pp. BIRKELAND, P.W. & BURKE, R.M. (1988): Soil catena chronosequences on eastern Sierra Nevada moraines, California, U.S.A. Arctic and Alpine Research 20, 473484.
11
DROSDOFF, ZAMORA,
M., C.
QUEVEDO, F. (1960): Soils
& of
Peru. Transactions of the 7th International Congress of Soil Science 4, 97-104. F O L K , R.L. (1974): Petrology of Sedimentary Rocks. Hemphill Publishing, Austin, Texas. 182 pp. H A R D E N , J . W . (1982): A quantitative index of soil development from field descriptions: Examples from a chronosequence in central California. Geoderma 28, 1-28. J E N N Y , H. (1980): The Soil Resource. Springer-Verlag, New York, 377 pp. M I L L E R , D . C . & B i r k e l a n d , P . W . (in p r e s s ) : Soil catena variation along an alpine climatic transect, northern Peruvian Andes. Geoderma. N I K I F O R O F F , C . C . (1949): Weathering and soil evolution. Soil Science 67, 219-223. O L L I E R , C . D . (1976): Catenas in different climates. In: E. Derbyshire (ed.), Geomorphology and Climate. Wiley and Sons, New York, 137-169. P Y E , K. (1987): Aeolian dust and dust deposits. Academic Press, London, 334 pp. R O D B E L L , D . T . (1991): Late Quaternary glaciation and climatic change in the northern Peruvian Andes. Ph.D. dissertation, University of Colorado, Boulder, Colorado.
B I R K E L A N D , P . W . , R O D B E L L , D.T., SHORT, S.K. & YOUNG, K.R. (1987): Estudios paleoambientales y de suelos estudios sobre la vegetaeion del Parque National Rio Abiseo. Unpublished report for Ministerio de Agricultura, Peru. 37 p.
RIOS, M.A., PONCE, C.F., VASQ U E Z , P. & T O V A R , A. (1982): Planificacion para el establicimiento de unidades de conservacion en el bosque nublado del noreste dei Peru. Informe Final Proyecto WWF-IUCN/1972, Lima.
B I R K E L A N D , P . W . , R O D B E L L , D.T. & S H O R T , S . K . (1989a): Radiocarbon dates on deglaciation, Cordillera Central, northern Peruvian Andes. Quaternary Research 32, 111-113.
R U H E , R . V . & W A L K E R , P . H . (1968): Hillslope models and soil formation. I. Open systems. Transactions of the 9th International Congress of Soil Science 4, 551-560.
B I R K E L A N D , P . W . , R O D B E L L , D.T., MILLER, D.M. & SHORT, S.K. (1989b): Investigaciones geologicas en el Parque Nacional Rio Abiseo, San Martin. Boletin de Lima 64, 55-64. B U R N S , S . F . & T O N K I N , P . J . (1982): Soil-geomorphic models and the spatial distribution and development of alpine soils. In: C.E. Thorn (ed.), Space and Time in Geomorphology. Allen and Unwin, London, 2543.
CATI~NA--Axt
Interdlscip]i3~ary
Jnurn,~l
~,f S O I L
S I N G E R , M . J . & J A N I T Z K Y , P. (eds.) (1986): Field and laboratory procedures used in a soil chronosequence study. U.S. Geological Survey Bulletin 1648, 49 pp. S O I L S U R V E Y S T A F F (1975): Soil Taxonomy. U.S. Department of Agriculture, Soil Conservation Service, Handbook 436, 753 pp. S T A L L A R D , R . F . (1986): Weathering and erosion in the humid tropics. In: A. Lerman & M. Maybeck (eds.), Physical and
SCIENCE--HYDROLOGY
GEOMOR.
PHOLOGY
Miller, Birkeland & Rodbell
12
Chemical Methods in Geochemical Cycles. Klumer Academic Press, Dordrecht, The Netherlands, 225-246. S W A N S O N , D . K . (1985): Soil eatenas on Pinedale and Bull Lake moraines, Willow Lake, Wind River Mountains, Wyoming. CATENA 12, 329-342. THOMAS, R.B. & WINTERHALDER, B.P. (1976): Physical and biotic environment of southern highland Peru. In: P.T. Baker & M.A. Little (eds.), Man in the Andes: a Multidisciplinary Study of HighAltitude Quechua. Dowden, Hutchinson & Ross, Stroudsberg, Pennsylvania, 21-59. TOSI, J.A., Jr. (1960): Zonas de vida natural en el Peru. Instituto Interamerieano de Cienas Agricolas de la OEA. Zona Andina Boletin Technico 5, Lima. WILCOX, B.P., ALLEN, B.L. & B R Y A N T , F . C . (1988): Description and classification of soils of the high elevation grassland of central Peru. Geoderma 42, 7994. Y O U N G , K . R . (1990): Biogeography and ecology of a timberline forest in north-central Peru. Ph.D. dissertation, University of Colorado, Boulder, Colorado.
A d d r e s s e s of a u t h o r s : D.C. Miller
F.M. Stoller Corp. 5700 Flatiron Parkway Boulder, Colorado 80301 U.S.A. P r o f . Dr. P . W . B i r k e l a n d Department of Geological Sciences University of Colorado Boulder, Colorado, 80309-0250 U.S.A. Dr. D.T. Rodbell U.S. Geological Survey, MS 96g Denver, Colorado 80225-0046 U.S.A.
(2ATE:NA---An
Inte,disclplinary
J,:,ur31~l ,,f S O I L
Sc~IEN(~E
HYDROLOGY
GI~OMORPHOLOCY