® .Geomorphology 28 1999 201­221
Alluvial fan and catchment initiation by rock avalanching,
Owens Valley, California
Terence C. Blair )
Blair & Associates, 1949 Hardscrabble Place, Boulder, CO 80303, USA
Received 3 July 1998; received in revised form 29 October 1998; accepted 4 November 1998
Abstract
The North Long John alluvial fan of the Inyo Mountains piedmont, Owens Valley, CA, was catastrophically initiated by
® .a prehistoric early Holocene? rock avalanche. This avalanche resulted from the collapse and disintegration of the central
part of a 1.1=2.0 km range front bedrock facet comprising the divide between the catchments of two large, adjoining
alluvial fans. Failure rapidly produced and transferred ;25 million m3
of new sediment to the piedmont, where it was
deposited in the trough between two coalesced fans. These deposits are of unstratified, angular, muddy, bouldery and cobbly
pebble gravel present in a U-shaped form. This form consists of lateral levees 10 to 60 m tall that lead 1.6 km from the range
front to a 108q m high distal snout. The avalanche sediment comprise the initial platform for, and still dominate the nascent
North Long John fan. Rock avalanche failure also created a 100q m deep concave scar in the range front that now serves as
the fan catchment. The slopes of this catchment are mantled by colluvial sediment generated from intense shear along the
avalanche detachment surface. Thus, the avalanche served as an alluvial fan `starter kit' by building the fan platform, and by
creating a range front catchment lined with sediment. Because of this unique and recent origin, the North Long John fan and
its catchment are in an early stage of development, lacking features such as old surfaces and a well-developed drainage net
typical of the older, adjoining alluvial fans and catchments. Post-avalanche fan activity has been dominated by the deposition
of ;4 million m3
of sediment by debris flows. These flows, including one in 1984, were instigated by the concentration of
flashy thunderstorm precipitation across the colluvium of the nascent catchment. Debris flows have built a conical tract that
extends 1 km from the fan apex, and partially buries the older avalanche deposits. The debris flow tract consists of muddy,
® .pebbly, cobbly, boulder gravel in levees 50­200 cm tall in the steep 12­168 upper segment, and boulder-free lobes in the
® .lower-sloping 58 distal segment. This tract terminates in a `pond' behind the avalanche snout, where recessional debris
flow pebbly mud has been trapped. This pond eventually filled and was breached by a deep channel that now transects the
snout of the rock avalanche. A second debris flow tract 300 m in radius has prograded outward from the end of the
avalanche bypass channel. The facies of this tract are identical to those of the proximal fan. An abundance of colluvial
sediment mantling the steep slopes of the catchment indicates that the North Long John fan will continue to enlarge by
thunderstorm-induced debris flows. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: alluvial fans; rock avalanches; Owens Valley, California
)
Tel: q1-303-499-6005; Fax: q1-303-499-6087; E-mail: tcblair@aol.com
0169-555Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
® .PII: S0169-555X 98 00111-1
( )T.C. BlairrGeomorphology 28 1999 201­221202
1. Introduction
Alluvial fans and fan catchments are common in
tectonically active regions because of the develop-
ment and maintenance of steep, fault-bounded range
fronts with several kilometers of relief. Fan catch-
ments typically originate and, thus, are focused on
geological discontinuities in the uplifted block, such
as faults, fracture zones, and geological contacts
® .Blair and McPherson, 1994a,b . Catchments arise in
these naturally predisposed sites because of higher
rates of physical and chemical weathering promoted
by tectonic fracturing and water transfer. Erosion of
sediment from these zones creates depressions that
naturally focus surface drainage, in turn promoting
more weathering and erosion, synergistically enlarg-
ing the fan catchment. Though rates of development
may vary, these processes through time produce
multiple, coeval catchments with matching alluvial
fans that line the transverse margins of linear range
®fronts, such as those along extensional basins Fig. 1;
.Wallace, 1978; Bull, 1984 . The fans are the deposi-
tional sites for sediment eroded from the catchment,
and transferred to the range front through the highest
order channel of the drainage net called the feeder
channel. The commonly ;0.5 to 3.0 km wide di-
vides between the catchments comprise triangular
® .bedrock facets at the range front Fig. 1 . The form
of the triangular facets results from the erosional
sloping of the sides of the catchment towards the
respective feeder channels. The relief of both the
catchments and facets will increase through time by
®continued fault offset and downcutting Wallace,
.1978; Bull, 1984 .
The progressive development of a range front,
especially increasing relief, may result in the instiga-
tion of a fan and fan catchment within the confines
of a bedrock facet between two mature fan catch-
ment systems. Such nascent `facet fan' and catch-
ment systems possess geomorphic characteristics dif-
ferent than the adjoining older fan systems, with
important implications for diachroneity between fans
and fan surfaces, piedmont evolution, and potential
geologic hazards. The purpose of this paper is to
illustrate the existence of this poorly documented
phenomenon by presenting a case study of a bedrock
facet fan and catchment initiated about 10 Ka BP by
a catastrophic rock avalanche. This feature, called
the North Long John fan, is located within the
mature Inyo Mountains piedmont of Owens Valley
® .near Lone Pine, CA Fig. 2 . The methods of study
® .included: 1 describing and mapping the form and
facies of the deposits using an enlarged aerial photo-
® .graph base, 2 measuring surface slopes with a
® .clinometer, 3 analyzing the grain size of five rep-
® .resentative samples, 4 constructing topographic
Fig. 1. Idealized fault-bounded range front and alluvial fan piedmont. Fan catchments are divided at the range front by bedrock triangular
facets.
( )T.C. BlairrGeomorphology 28 1999 201­221 203
® . ® .Fig. 2. A Geological features of Owens Valley, CA. B Map of southern Owens Valley, including the location of the ends of cross-section
X
® . ® . X
x­x of C , and the North Long John fan study area covered in Fig. 3. C Structural cross section x­x between Mount Whitney in the
® .Sierra Nevada through the study area to New York Butte in the Inyo Mountains after Pakiser et al., 1964 .
profiles and drainage nets using field data and
X
®U.S. Geological Survey 7.5 quadrangles scales
. ® .1:24,000; contour intervals10 m , and 5 compar-
ing these characteristics with the older adjoining
fans. Textural terminology used in this paper follows
® .Folk 1974 as modified by Blair and McPherson
® .1999 .
2. Setting
Owens Valley is a modern north­northwest trend-
ing rift 15 to 25 km wide and 150 km long located
along the western margin of the Basin and Range
® .province in California Fig. 2 . The basin is enclosed
at both ends by Quaternary volcanoes, including
Long Valley in the north and the Coso complex in
the south. Owens Valley is precipitously bounded on
the west, with 3200 m relief, by the Sierra Nevada,
and on the east by the Inyo Mountains, with 2200 m
® .of relief Fig. 2 . These margins are delineated by
the active Sierra Nevada normal fault on the west,
with )3500 m of throw, and by an equally promi-
nent normal fault along the Inyo Mountains to the
® .east Pakiser et al., 1964 . Additionally, a major
oblique-slip fault called the Owens Valley fault cuts
longitudinally down the basin along the east side of
® .the Alabama Hills Fig. 2; Pakiser, 1960 . Both
® . ® .dextral strike slip 11 m and dip slip 2 m occurred
( )T.C. BlairrGeomorphology 28 1999 201­221204
Fig. 3. Location map of the Inyo Mountain piedmont in the study area, including a detailed plan of the alluvial fans and the catchment
® . ® . ® .drainage nets see Fig. 2 for location . Drainage nets are based on the stream-order methodology of Horton 1945 and Strahler 1964 , and
X
® .use of 7.5 U.S. Geological Survey topographic maps scale: 1:24,000; contour interval: 10 m .
along this fault on 26 March 1872 during one of the
® .largest M ;8.3 historic earthquakes in the USA
® .Lubetkin and Clark, 1988 . Basin development re-
sults from subsidence of the Owens Valley hanging-
wall between this Owens Valley fault and the Inyo
® .range front fault Fig. 2 . Gravity data indicate that
the sedimentary fill thickens to 2500 m at the Inyo
®front, and outlines an east-tilted half graben Pakiser
.et al., 1964 .
Sedimentologically, southern Owens Valley is
characterized by basin-marginal alluvial fans of the
Sierra Nevada and Inyo piedmonts, and on the basin
floor by the axial Owens River and Owens Lake
® .system Fig. 2 . The alluvial fans have longer radii
® .Fig. 4. Features of the North Long John fan and catchment. A Overview of the 2200 m high Inyo Mountain range front and piedmont,
® . ® .including the North Long John fan apex at middle arrow , and the adjoining New York Butte apex at left arrow and Long Canyon fans
® . ® . ® .apex at right arrow . Two prominent triangular facets are present, one north A and one south B of the New York Butte catchment. The
® .North Long John fan catchment is developed within the latter facet. Also identified are the frontal snout of the rock avalanche s , and a
® . ® . ® .fault-backed bench along the south side of the catchment b . B Close up of the colluvium-mantled catchment c , downdropped bedrock
® . ® . ® . ® . ® .facet bench b , fan apex arrow , proximal debris flow tract below arrow , distal debris flow tract d , and the 108 m high frontal snout s
® . ® . ® .of the rock avalanche. Also marked are the New York Butte N and Long Canyon L drainages around the triangular facet. C Aerial
® . ® . ® . ® .photograph with features marked, including the apex a , proximal p and distal d debris flow tracts, rock avalanche snout s , lateral
® . ® . ® .levees l , back-avalanche pond n , sinuous channel through the snout c , and the sharp cut through older New York Butte fan deposits
® . ® .arrows . The margin of the U-shaped rock avalanche is outlined. True width of photograph is 1.6 km. D Upslope view of the proximal
® . ® . ® .debris flow tract p . Light-colored deposits arrow are from a 22 August 1984 debris flow. Also visible is the avalanche scar c and the
® . ® . ® . ® .fan apex a . E Upslope view towards the rock avalanche snout s showing the distal debris flow tract, including its apex a at the end of
the snout-bypass channel.
( )T.C. BlairrGeomorphology 28 1999 201­221 205
® .5­8 km and lesser thickness in the Sierra pied-
® .mont, and shorter radii typically -2.5 km and
probably greater thickness in the Inyo piedmont. The
basin floor is dominated by the 0.228 sloping longitu-
dinal Owens River, which flows from its headwaters
in the high Sierra Nevada and Long Valley volcano
southward 100 km to its present terminus in Owens
® .Lake Fig. 2 . Owens Lake is now a flat evaporitic
playa formed by damming of the river by the Coso
Range at the south end of the valley. Sedimentation
is taking place in this valley under a high desert
climate resulting from its position in the rain shadow
of the Sierra Nevada. The valley floor is vegetated
by alkaline scrub, including sage, grass, saltbush,
( )T.C. BlairrGeomorphology 28 1999 201­221206
yucca, and cacti, whereas juniper and pinyon pine
®thrive in the adjoining mountains Hollett et al.,
.1991 . Annual precipitation varies with elevation
from 10 to 20 cm, falling as rain and snow during
the cooler months, and as local thunderstorms during
® .the summer months Hollett et al., 1991 .
3. Morphologic overview of the North Long John
fan and catchment
3.1. Catchment characteristics
The North Long John fan is located in the Inyo
Mountains piedmont between the larger New York
® .Butte and Long John Canyon fans Fig. 3 . The
North Long John catchment is established in the
prominent 2.0 km wide and 1.2 km tall triangular
bedrock facet between the drainages of the adjoining
® .fans Fig. 4A . This catchment is a spoon-like con-
cavity about 500 m in width, 50 to 100 m in depth,
® .and with 1065 m of relief Fig. 4B . It has an area of
2.0 km2
, and a poorly developed drainage net char-
®acterized by two second-order feeder channels Fig.
.3 . The upper and medial parts of the catchment
consist of 28 to 328 dipping slopes that are thinly
® .mantled by colluvium Fig. 4B . The 188 sloping
lower catchment contains a thicker colluvial mantle,
and has a rougher surface due to the presence of
first-order rills. The catchment is bounded laterally
by intact remnants of the bedrock facet. These rem-
nants comprise 70 to 100 m thick benches down-
dropped 450 m from the facet crest along a sheared
® .detachment surface Fig. 4A . This surface is parallel
to, and constitutes the northern extension of a normal
fault along which Long Canyon is focused. Creta-
ceous granophyric aplite underlies the North Long
John catchment, whereas Paleozoic sedimentary
rocks and Mesozoic andesite underlie the adjoining
New York Butte and Long Canyon fan catchments
® .Ross, 1967; Streitz and Stinson, 1974 .
3.2. Fan morphology and proinces
The North Long John fan has an area of 0.83
km2
, relief of 270 m, and extends radially for 1750
m from the range front at an average slope of 8.88
® .Figs. 3 and 5 . Rather than having a smooth slope,
the North Long John fan has an irregular composite
morphology arising partly by its constricted position
between the larger adjoining fans, but mostly be-
cause it consists of three provinces that have unique
depositional origins. These provinces constitute a
large rock avalanche that separates proximal and
® .distal debris flow tracts Fig. 4C­E . The rock
avalanche tract has a U-shaped planview pattern 480
m across at the fan apex, and expanding to 640 m
® .across at the terminus Fig. 4C and Fig. 5 . Paired
lateral levees 30 to 50 m wide extend from the Inyo
Mountain footwall for 950 m to the rock avalanche
snout. This snout sharply terminates 1560 m from
the range front. The height of the avalanche deposits
above the adjoining fan surfaces is variable, increas-
ing from 8 to 15 m in the proximal levees to as much
as 108 m distally, resulting in a segmented longitudi-
®nal profile and a wedge-shaped radial geometry Fig.
.6 . Besides a distal increase in thickness, this wedge
shape also results from a lessening of surface slope
® .from 158 in the proximal zone to 58 distally Fig. 5 .
Cross-avalanche profiles consist of a double hump in
the proximal leveed area, changing to a steep-sided
® .blocky pattern in the distal snout Fig. 6 . Not in-
cluding the inner courtyard above the snout and
between the levees, this avalanche has an area of
;500,000 m2
, most present in the distal levees and
snout. A sediment volume of ;25 million m3
is
determined for the avalanche by multiplying the area
by thicknesses determined from field measurements
® .Fig. 5 . Correcting for an assumed average porosity
of 25%, this volume of rock avalanche sediment
accounts for about 86% of the present volume of the
fan catchment.
Rather than comprising continuous ridges, the
paired avalanche levees consist of three definable
segments 250­500 m in length, the most proximal
two having a discontinuous relationship, and the
® .distal two an overlapping relationship Fig. 5 . The
three segments are delineated by differences in height
and by discernible sharp boundaries. All three have
surface slopes that decrease from 9­158 proximally
® .to 2­78 distally Fig. 7A . The lower two segments
of the paired levees and the distal snout are separated
by prominent, nearly vertical shear contacts that
accommodated the downslope transfer of the
® .avalanche Fig. 7B . The progressive downslope off-
set of the more distal levees and snout from the
( )T.C. BlairrGeomorphology 28 1999 201­221 207
® . ® .Fig. 5. A Components of the North Long John fan, and magnitude and direction of the slope of the surface. B Study stations, sample
sites, measured thickness sites, and modal orientations of the long axes of elongate coarse clasts at given stations. Avalanche deposits in
both maps are stippled.
( )T.C. BlairrGeomorphology 28 1999 201­221208
Fig. 6. Topographic profiles of the North Long John fan based on field measurements and contour maps. The rock avalanche deposits are
shaded gray, debris flows are stippled, bedrock is cross-hatchured, the recessional pond has a diagonal pattern, and adjoining fan deposits
have a gravel pattern.
inside margin of the upslope levee segments outlines
® .a telescoped pattern between these elements Fig. 5 .
The 2­58 surface slope of the snout is the lowest of
the avalanche elements. This low slope abruptly
changes at a brink point at the snout terminus, where
the avalanche front descends as much as 108 m at a
® .288 slope Fig. 4B . An ephemeral pond with a
nearly flat, muddy surface 150 m long and 150 m
wide is present immediately upslope from the snout
and between the avalanche levees, produced by
® . ® .Fig. 7. A Upslope view of the boulder-studded top of the northern levee, with the New York Butte fan to the north a at apex . The long
® . ® . ® . ® .axes of the clasts have a slope-parallel alignment e.g., arrows . B Downslope view of the second 2 and third 3 levee segments along
® .the north margin of the rock avalanche. Sheared contacts, along which the third levee segment and snout deposits s were offset downslope,
® . ® .are marked with arrows. C Light-colored muddy sediment of the back-snout `pond' p deposited by recessional flows from the proximal
® .debris flow tract. The head of the channel c that transects the avalanche snout is visible. Rock avalanche levees laterally bound this zone.
® . ® .D Downslope view of the U-shaped planform of the rock avalanche, including the proximal end of the third levee segments arrows , the
® . ® . ® . ® .distal snout s , and the breached channel b through the snout. In the background are the town of Lone Pine L , Alabama Hills A fronted
® . ® .by the Owens Valley fault, and the 3200 m high Sierra Nevada S . E Vertical cut 5 m high of monolithologic megabreccia of the rock
® .avalanche snout. The coarsest clasts are irregularly concentrated towards the top. F Close-up view of typical rock avalanche megabreccia
texture consisting of unstratified, unsorted, clast-supported, very angular, muddy, sandy, cobbly, fine to coarse pebble gravel. The darkened
zone above the 10 cm scale bar is a Bt soil horizon.
( )T.C. BlairrGeomorphology 28 1999 201­221 209
® .damming of the snout and the levees Fig. 7C . The
pond and avalanche snout are now breached by a
prominent, sinuous channel extending from the pond
®to the southwest end of the avalanche Figs. 5 and
.7D . This 45 m deep, V-shaped channel is 5 m
across at its bed and 20­50 m across at the snout
surface. The channel walls typically slope inward
35­408, whereas the channel bed slopes 98 west-
ward.
The remainder of the North Long John fan con-
sists of the two debris flow tracts. The largest one is
developed on the proximal fan between the rock
® .avalanche levees Fig. 4D and Fig. 5 . This tract is
300 m across and extends about 1 km from the fan
( )T.C. BlairrGeomorphology 28 1999 201­221210
apex to the back-snout pond. It has an overall conical
morphology that slopes between 12 to 168 proxi-
mally and ;58 distally, before merging with the
® .nearly flat surface of the back-snout pond Fig. 6A .
The surface of this tract is rough where 1­4 m deep
gullies are present, and smooth where a mantle of
varnished angular boulders and cobbles is developed
® .Fig. 7D . The distal debris flow tract is 150 m
across and 300 m long, commencing where the
snout-bypass channel intersects the front of the rock
® .avalanche Fig. 4E and Fig. 5 . Like the proximal
tract, this distal tract has a conical form with a
surface slope that slightly decreases distally from 9
to 78. Overall, it is smoother than the proximal tract,
with a slightly rilled surface of cobbles and boulders.
4. Rock avalanche facies and emplacement history
4.1. Rock aalanche facies
The rock avalanche deposits were described at 26
stations sited to cover all zones, including 10 in the
northern levee, 10 in the southern levee, and six in
® .the snout Fig. 5 . These deposits, whether of the
levees or snout, dominantly consist of unstratified
and unsorted, slightly bouldery, muddy, cobbly, fine
® .to coarse pebble gravel Fig. 7E­F . The clasts are
monolithologic, consisting of tan to pinkish gray
aplite identical to the bedrock underlying the catch-
ment. The deposits mostly are clast-supported, and
vary from tightly interlocking with abundant inter-
clast long contacts, to loosely interlocking with tan-
gential and floating grain contacts. These clasts have
a diagnostic angular to very angular outline and
® .dominantly are 2 to 8 cm across Fig. 7F . The
pebble-dominated, clast-supported texture becomes
matrix-supported in the lower exposures of the
avalanche snout. Particle size analysis of the -1.6
® .cm -y4f fraction was conducted on two sam-
ples of rock avalanche sediment, including NLJ-02
from an upper levee site, and NLJ-03 collected from
® .a lower snout exposure Fig. 5 . The data show these
samples are extremely poorly sorted, and contain a
full spectrum of sediment grades that include, on
average, 58% pebbles and granules, 28% sand, 12%
® .silt, and 2% clay Fig. 8; Table 1 . The clast-sup-
ported sample from the upper avalanche has a
gravel­sand­mud ratio of 64­26­10 and the
® .matrix-supported lower one 55­30­15 Fig. 8 .
Boulders and coarse cobbles overall are less com-
mon in the rock avalanche deposits than are pebbles
and fine cobbles, and they lack a homogeneous
distribution. Coarse cobbles and boulders are most
abundant in the upper and outer parts of the deposits,
where the sediment texture transitionally changes to
an unstratified, angular to very angular, muddy, peb-
® .bly, cobble boulder gravel Fig. 7E . Boulders com-
®monly stud the surface of the avalanche levees Fig.
.7A . These clasts typically have an equant to bladed
shape, and internally display crackle-breccia and jig-
saw-breccia fabrics.
This coarsest fraction of the avalanche deposits
was evaluated in greater detail by measurement of
® .maximum clast size intermediate axis, d , and theI
orientation of the long axes of the 10 largest elongate
clasts, exclusive of those that have undergone post-
avalanche movement. The d values range from 38I
® .to 381 cm fine to very coarse boulders without any
® .radial trend Fig. 9 . For example, the largest clast in
®the north margin of the rock avalanche d of 300I
.cm is present in the second levee segment, which is
notably coarser overall than the proximal and distal
levees at this margin. In contrast, the largest clasts
® .along the southern margin d to 381 cm , are in theI
® .distal snout Fig. 9 . Unlike maximum clast size
data, the a-axis orientations of elongate cobbles and
boulders are organized, displaying a moderately de-
veloped fabric at most of the study stations. The long
axes of elongate clasts at the top and outer margins
of the levee deposits are preferentially aligned either
parallel to the trend of the levees or, more com-
monly, at about a 308 angle to this trend, with the
inside end of the clasts consistently turned downs-
® .lope Fig. 5 . A preferred elongate clast alignment
also characterizes the snout deposits, with the modal
trend generally oriented perpendicular to slope, par-
® .alleling the snout front Fig. 5 .
4.2. Reconstructed depositional history
The textures, fabric, monolithologic composition,
and large-scale U-shaped lateral levee and snout
morphology of the North Long John deposits are
consistent with the products of a single large rock
avalanche based on the similarity of these character-
( )T.C. BlairrGeomorphology 28 1999 201­221 211
® . ® .Fig. 8. A Cumulative curves based on grain-size analysis of two samples from the rock avalanche facies NLJ-02 and -03 and three
® .samples of post-avalanche debris flows NLJ-04, -05, and -06 . Sample sites are located in Fig. 5, and size-analysis data are summarized in
® . ® .Table 1. B and C Histograms of rock avalanche samples NLJ-02 and -03. D and E Histograms of debris flow samples NLJ-04 and -06.
® . ® . ® .The mud content finer than 4f in these samples is greater relative to the rock avalanche samples. F Gravel­sand­mud G-S-M ternary
® . ® .plot, and, G normalized sand­silt­clay S-Z-C ternary plot of the grain-size data from the five analyzed samples. Symbols are the same
® .as those used in A .
®istics with those of historical rock avalanches e.g.,
.Hadley, 1964; Hsu, 1975; Gates, 1987 , and with¨
documented pristine deposits of prehistoric rock
®avalanches e.g., Burchfiel, 1966; Shreve, 1968,
.1987; Fauque and Strecker, 1988 . The position of
the proximal ends of the North Long John rock
avalanche levees at the Inyo Mountain range front,
and extending downslope from either side of the
prominent range front bedrock scar, indicates that the
avalanche was instigated by the catastrophic collapse
( )T.C. BlairrGeomorphology 28 1999 201­221212
Table 1
Summary of grain-size data, North Long John fan samples
FeaturerSmp NLJ-02 NLJ-03 NLJ-04 NLJ-05 NLJ-06
Facies OTRAV LWRAV PDFLV PDFLB DDFLB
® .Mode f y3.63 y3.63 y3.63 3.88 y3.13
® .Sorting f 3.05 3.47 4.37 3.66 4.22
G-S-M 64­26­10 55­30­15 40­28­32 19­47­34 35­30­35
Nr S-Z-C 72­25­3 67­26­7 46­39­15 58­35­7 46­41­13
Grade Weight percent of total sample
m pebble 31.2 22.4 19.2 8.6 13.8
f pebble 19.5 18.4 12.3 5.7 11.4
granule 13.0 14.1 8.4 4.3 10.0
vc sand 7.4 9.3 6.8 5.5 6.0
c sand 6.1 7.4 5.5 7.7 4.6
m sand 4.6 5.4 5.2 8.5 3.8
f sand 3.3 3.7 3.5 9.0 5.0
vf sand 4.8 4.5 6.9 16.5 10.2
c silt 2.2 2.4 6.8 10.3 8.0
m silt 2.3 2.7 5.8 6.7 6.5
f silt 2.2 2.9 5.4 5.4 5.3
vf silt 2.2 3.9 5.3 6.4 6.9
clay 1.2 2.9 8.9 5.4 8.5
Smp--sample.
Facies: OTRAV--outer rock avalanche; LWRAV--lower rock avalanche; PDFLV--proximal debris flow levee; PDFLB--proximal debris
flow lobe; DDFLB--distal debris flow lobe.
® . ® .Sorting is Inclusive Graphic Standard Deviation S of Folk 1974 .I
G-S-M: gravel­sand­mud wt.%.
Nr S-Z-C: normalized sand­silt­clay wt.%.
of the bedrock slab that previously occupied this scar
® .Fig. 4A . This failure probably was promoted by
weathering fatigue of the slab because of progressive
fracturing, chemical weathering, and syntectonic en-
hancement of relief. The actual failure event possibly
was triggered by coseismic ground motion from a
major earthquake along one of the three faults pass-
ing though this part of Owens Valley, as suggested
by the known propensity of earthquakes to instigate
® .rock avalanches Keefer, 1984, 1994 , the historic
record of earthquakes as large as Ms8.3 in this part
of Owens Valley, and the documented longer-term
record of large earthquakes in Owens Valley ob-
®tained from the study of fault scarps Lubetkin and
.Clark, 1988 .
Upon failure, the frontal plate of the bedrock
triangular facet moved as a rockslide down the 328
sloping detachment surface still exposed in the upper
® .scar Fig. 4A­B . The central part of this rockslide
underwent continued failure by disaggregation, and
was extensively disintegrated upon impacting the
proximal piedmont at the base of the range front
® .Fig. 10A . Extensive disintegration combined with
momentum caused this rock avalanche to transform
to an inertial grain flow upon reaching the piedmont.
By analogy with avalanches that have been observed,
® .such as the one at Elm, Switzerland Hsu, 1975 , the¨
North Long John rock avalanche probably pro-
® .gressed rapidly 50­100 kmrh downslope upon
flow transformation, depositing the entire ;25 mil-
lion m3
of gravelly deposits as far as 1560 m from
the range front during a few to tens of seconds. This
short period included four phases of deposition indi-
cated by the telescoped pattern between the three
® .levee segments and the snout Fig. 10B .
The surface of the rock avalanche deposits has
been modified since emplacement by secondary pro-
® .cesses cf., Blair and McPherson, 1994a . Rilling
from overland flow, caused by direct rainfall on the
avalanche deposits, is most prevalent along the steep
( )T.C. BlairrGeomorphology 28 1999 201­221 213
® .Fig. 9. Maximum clast size d for the northern and southern margins of the rock avalanche, and of the debris flow tracts plotted againstI
radial position from the fan apex. No trends are apparent.
® .Fig. 10. Depositional history of North Long John fan shown by paleogeographic frames. A Fan initiation by rockslide failure of the range
front bedrock facet. The central part of the slide disintegrates, and transforms to an inertial flow upon impact with the piedmont. The sole of
® .the mass is sheared at the base of the range front, and the levee segments are deposited along the bedrock lining the outer scar. B Rock
avalanche deposition is completed seconds later with the deposition of two additional levee segments and the snout 1.56 km from the range
® .front. C Post-avalanche clast-rich debris flow levee and lobe deposition, and recessional flood `pond' deposition, take place on the
® .proximal fan. D Breaching of the avalanche snout subsequently occurs from overtopping, and the distal debris flow tract is initiated.
( )T.C. BlairrGeomorphology 28 1999 201­221214
® .margins of the distal levees and snout Fig. 4B .
Overland flow from the catchment is responsible for
the channeled breach through the snout deposits.
Winnowing from rainsplash, overland flow, and wind
erosion on the top of the avalanche deposits have
produced a pebbly desert pavement layer one or a
few clasts thick. Oxidation of clasts exposed at the
surface have contributed to the development of a
light to dark brown varnish. Accumulations of
wind-blown sand also are present, especially on the
snout of the rock avalanche where a 5 cm thick Av
soil horizon is developed beneath the desert pave-
ment. Translocation of finer sediment and iron ox-
ides has resulted in the production of a 30 cm thick
® .Bt soil horizon below the Av horizon Fig. 7F .
Desert plants cover about 30% of the avalanche
surface, including xerophytes such as cattle spinach,
desert holly, burroweed, creosote, and various grasses
and cacti. Rodent burrows are locally common at the
base of some of these plants, especially where they
coincide with eolian deposits of sand.
The age of the North Long John rock avalanche
deposition is uncertain, but several lines of evidence
point to the earliest Holocene. The history of the
complex post-avalanche pond and breached snout,
the presence of gullies, desert pavement, a soil, and
the partial burial by subsequent debris flows suggest
that the avalanche event occurred many thousands of
years ago. These and other characteristics of the
North Long John avalanche deposits are similar to,
or are less well developed than those of the nearby
Blackhawk rock avalanche of Lucerne Valley, CA
® .Shreve, 1968, 1987 . The Blackhawk event oc-
curred about 20 Ka BP based on carbon-14 age
determinations from snail shells collected from a
back-snout pond. Unlike the North Long John
avalanche, the Blackhawk avalanche has been ce-
mented at the surface by calcite. Older avalanche
deposits on the Blackhawk Canyon fan are notably
better cemented, which indicates an increasing de-
® .gree of cementation with age Shreve, 1968, 1987 .
The lack of any cements, and the lighter desert
varnish and less developed desert pavement, suggest
that the North Long John avalanche was deposited
much more recently than 20 Ka. Given these older
and younger age constraints, the time of deposition
of the North Long John rock avalanche is estimated
at about 10 Ka BP.
The rock avalanche and subsequent debris flow
deposits account for all of the volume of the fan
catchment, indicating that a catastrophic avalanche
initiated the North Long John fan and fan catchment
® .system. This relatively recent ;10 Ka age of
initiation contrasts with the probable onset of the
adjoining New York Butte and Long Canyon fans
several million years earlier, during the nascent stage
of Owens Valley development. Prior to the North
Long John rock avalanche, the triangular bedrock
facet that shed the avalanche was largely intact, and
lacked a fan at its base. This pre-avalanche state
likely was similar to the present state of the next
triangular facet to the north, between the New York
® .Butte and French Spring fan Fig. 4A . Emplacement
of the North Long John rock avalanche established
the initial platform for this fan between the larger,
previously coalesced New York Butte and Long
Canyon fans. This event forced the feeder channel of
the Long Canyon fan to lengthen 1 km to bypass the
® .rock avalanche platform Figs. 3 and 4C . The Long
Canyon fan apex also was repositioned 1 km downs-
lope, and 123 m lower in elevation.
5. Post-avalanche fan evolution
About 4 million m3
of debris flow sediment accu-
mulated on the North Long John fan since the rock
avalanche, including deposits from the most recent
® .debris flow of 22 August 1984 Fig. 4C . This 1984
event resulted from colluvial slope failure instigated
by strong, flashy thunderstorm precipitation in the
catchment. This thunderstorm also triggered debris
flows on other fans of the Inyo piedmont, including
®the Dolomite fan Fig. 3; Blair and McPherson,
.1998 . Post-avalanche debris flow deposition on the
North Long John fan dominantly has occurred in the
proximal tract, where ;3.7 million m3
are found,
compared to about 0.3 million m3
of sediment for
® .the distal tract Fig. 4B­E . These deposits were
described at 11 stations across the fan to characterize
® .their facies and constituent forms Fig. 5 .
5.1. Forms and facies of the debris flow tracts
Three debris flow facies are differentiated based
on textures, bedding, and forms. The first consists of
unstratified, sandy, muddy, pebbly, cobbly, coarse to
fine boulder gravel present in paired, radially aligned,
( )T.C. BlairrGeomorphology 28 1999 201­221 215
straight to sinuous levees 50 to 200 cm high and 50
® .to 100 cm wide Fig. 11A­B . Clast concentration is
high but inhomogeneous in these levees, with the
most clast-rich part present along the outer margins,
and the more matrix-rich sediment along the insides.
Elongate boulders and cobbles display a preferred
orientation either parallel or slightly oblique to both
the direction of slope and the trend of the levees
® .Fig. 11B . This levee facies is well developed on
the proximal fan outward from the apex, between the
proximal rock avalanche levees. Radial slopes in this
®tract slightly lessen distally from 168 to 128 Fig.
. ®6A . The bowed-upward composite cross profile Fig.
.6D has significant roughness because of 1­4 m of
®local relief caused by the distribution of levees Fig.
.11A . This facies also is present, though less well
developed, on the 98 sloping upper end of the distal
debris flow tract.
The second debris flow facies consists of clast-rich
and clast- to matrix-supported, slightly bouldery,
sandy, muddy, cobbly, fine to very coarse pebble
gravel. These deposits are present in lobate forms
10­50 cm high, 3­10 m across, and extending many
® .tens of meters in a radial trend Fig. 11C­E . They
comprise stacked beds of similar dimensions when
® .viewed in vertical cuts Fig. 11F . The most clast-rich
and coarsest part of this facies fringe the lobe mar-
gins, where interlocked fine boulders and cobbles are
® .concentrated Fig. 11C . Elongate clasts in these
outer zones have a preferred alignment parallel to the
lobe margin, including a radial alignment along the
sides and a slope-perpendicular alignment along the
snouts. The inner zones are more pebble dominated
® .and matrix-supported Fig. 11D . Cuts reveal that the
coarsest fraction in the lobe centers tend to be con-
® .centrated near the tops Fig. 11E . The clast-rich
outer margins of many of these lobes lack matrix,
although matrix-sized sediment commonly is found
nearby, drained outward from the lobes. Matrix-free,
imbricated pebbles and cobbles also are present in
® .shallow rills through or upon these lobes Fig. 11E .
This debris flow lobe facies is widely developed
downslope from the levee-dominated zone on the
® .proximal and distal fan tracts Fig. 5 . The boundary
between the levees and lobes on the proximal fan is
demarcated by a prominent slope inflection from 128
® .to 58. This lobe facies has a steeper 7­88 surface
® .slope on the distal debris flow tract Fig. 4E .
The third debris flow facies consists of matrix-
supported, pebbly sandy mud and pebbly muddy
sand deposited downslope from the proximal clast-
®rich lobe zone in the back-snout `pond' Figs. 5 and
.7C . Where exposed, this facies consists of interbed-
ded clast-poor units 5 to 20 cm thick. They contrast
with the adjoining lobe facies by the presence of
well-stratified clast-poor muddy beds, and by con-
taining mudcracked silt layers. These beds are nearly
horizontal, dipping downslope at -18.
To further characterize the debris flow tracts,
® .maximum clast size d was determined at 11 studyI
stations, and grain-size analysis was conducted on
three samples. Maximum clast size varies from 33 to
442 cm, although most clasts are -150 cm across
® .Fig. 9 . The single largest clast, a fine-grained block
with a d of 442 cm, is present within a levee nearI
the fan apex. Except for this clast, and the absence of
boulders in the ponded area, the maximum clast size
in the debris flow deposits is similar, irrespective of
® .radial distance from the apex Fig. 9 . Boulder con-
centration, however, is significantly higher in the
levee zones than in the lobe zones. The maximum
values of clast size in the debris flows rival those of
the rock avalanche facies, but show no correlation to
® . ® .them Fig. 9 . The finer fraction -1.6 cm or y4f
of the debris flows was characterized by grain-size
analysis of three samples representative of each fa-
cies, including NLJ-04 from a levee of the upper
tract, NLJ-05 from the clast-poor deposits of the
back-snout `pond,' and NLJ-06 from a lobe deposit
® .of the distal fan Fig. 5 . The proximal clast-rich
levee and distal clast-rich lobe samples have a simi-
lar distribution of grain sizes, with a wide span of
grades that average 28% pebbles, 9% granules, 29%
® .sand, 25% silt, and 9% clay Fig. 8; Table 1 . Both
of these samples are unsorted, with Folk Inclusive
® .Graphic Standard Deviation S values of 4.37f andI
4.22f. The clast-poor `pond' sample contains less
gravel and more sand than the clast-rich samples but
® .still is extremely poorly sorted S s3.66f , con-I
taining 14% pebbles, 4% granules, 47% sand, 29%
® .silt, and 6% clay Fig. 8; Table 1 .
5.2. Reconstructed depositional processes
The debris flows of the North Long John fan
originated through the destabilization of colluvial
( )T.C. BlairrGeomorphology 28 1999 201­221216
® .Fig. 11. Post-avalanche debris flow deposits on the North Long John fan. A Overview towards the fan apex of sinuous debris flow levees 1
® . ® .to 2 m high. The rubbly, matrix-poor outer levee o contrasts with the more matrix-rich inner zone i . Also marked is an older winnowed
® . ® .mantle m . B View of a proximal fan levee with abundant cobbles and boulders displaying a preferred radial a-axis alignment. Also
® . ® .visible are the varnished mantle m on older debris flow deposits. C End of the 50 cm high margin of a clast-rich lobe on the proximal fan
® . ® .at fieldbook, arrow . Interlocked angular cobbles are concentrated along this lobe margin. D Cut 1 m tall through the center of a
® .proximal-fan lobe displaying an unorganized, matrix-supported, muddy, fine to coarse pebble gravel texture. E Vertical cut 1 m tall of the
central part of a thick lobe of the distal debris flow tract revealing a clast- to matrix-supported, clast-rich, angular, muddy, cobbly, fine to
® .coarse pebble gravel. The coarsest clasts are concentrated near the top. F Vertical cut 1.5 m tall of the distal debris flow tract containing at
least five stacked beds of debris flows 10 to 20 cm thick. They consist of clast- to matrix-supported, clast-rich, angular, muddy, sandy,
cobbly, fine to coarse pebble gravel divided by finer-grained clast-poor debris flows.
( )T.C. BlairrGeomorphology 28 1999 201­221 217
sediment mantling the rock avalanche scar that now
® .serves as the nascent fan catchment Fig. 4A . As
exemplified by the 22 August 1984 event, these
colluvial slopes are transformed into debris flows by
the rapid addition of thunderstorm precipitation in
® .the catchment Fig. 4C . The North Long John fan
catchment is the sole origin of these debris flows
based on the source direction indicated by the coni-
cal shape of the debris flow tracts, and on the direct
match in composition between the nearly monolitho-
logic aplite clasts and the aplite bedrock unique to
this catchment.
The texture, fabric, and radial orientation of the
levees of the debris flow tracts are characteristics
consistent with features observed to form from the
passage of clast-rich and boulder-bearing surges of
®debris flows Sharp, 1942; Sharp and Nobles, 1953;
.Johnson, 1970 . The levees form as the outer mar-
gins of a passing front of a lobate surge, and undergo
deposition after being sheared from the surge be-
®cause of frictional drag along the sides Sharp and
.Nobles, 1953; Johnson, 1984; Pierson, 1986 . The
radial fabric of the clast long axes originates as these
coarse clasts first are lifted by differential buoyancy
to the top of the moving flow, then conveyed to the
front by the greater velocity of the upper flow tread
resulting from frictional drag along the base,
and then finally pushed laterally aside in a snow-
plow-like fashion by the mass of the debris flow
®Johnson, 1970; Pierson, 1980; Blair and McPher-
.son, 1998 .
The distinctive form, unsorted gravelly texture,
and the fabric of the lobes that are present down-
slope from the leveed zones are typical of debris
®flow lobe deposits Sharp and Nobles, 1953; Broscoe
.and Thomson, 1969 . The concentration of the most
clast-rich part of these lobes along the outer margins
indicates that selective sorting of the more buoyant
coarse clasts was taking place, first towards the lobe
®front, and then to the side margin e.g., Blackwelder,
.1928; Sharp and Nobles, 1953; Pierson, 1981 . A
key difference between the lobes and levees, besides
morphology, is that the lobes overall lack boulder-
sized clasts, whereas the levees are boulder-rich. The
selective depletion of the boulder fraction from the
moving debris flows causes a loss of lateral support,
®allowing them to expand into lobes Blair and
.McPherson, 1998 . This transition is reflected geo-
morphically on the North Long John fan by a change
® .in surface slope from 128 to 58 Fig. 5 .
Another feature observed in the clast-rich debris
flow facies of the North Long John fan is outward
drainage of the matrix from the coarse margins
immediately upon levee or lobe deposition. This
process initiates the boulder and cobble mantles
® .common on the upper fan Fig. 7D . Matrix draining
indicates that either this matrix had low cohesive
strength, or that the clasts were not adequately inter-
locked to effectively contain the fines. Interlocking
may be difficult for the North Long John fan debris
flows because the clasts, all derived from the
avalanche scar, are angular and typically equant in
shape. The softness of the matrix also denotes that it
has low cohesive strength, especially when compared
®to debris flows on other Inyo piedmont fans Blair
.and McPherson, 1998 . This contrast in matrix cohe-
sion may result from differences in the weathering
products of the catchments because of variable
bedrock lithology. Probably a more important factor
is that the North Long John matrix formed cataclasti-
cally during the rock avalanche, rather than through
chemical weathering typical of the neighboring fan
catchments. This cataclastic origin has produced a
suite of clay-sized sediment consisting of minute
fragments of the minerals making up the aplite,
instead of a collection of clay minerals produced
through chemical alterations, and possessing clay
mineral properties such as adhesion and crystal ani-
sotropy.
The more mud-dominated, clast-poor, debris flow
facies in the back-snout `pond' of the North Long
John fan is interpreted as the trapped, less viscous,
clast-poor phase of debris flows, as well as the
deposits of sediment-rich flows of recessional water
that have winnowed the debris flows. Observations
of debris flows on fans indicate that a clast-poor
debris flow stage followed by recessional flood wa-
®ter flow are common Sharp and Nobles, 1953;
Pierson, 1981, 1986; Johnson, 1984; Blair and
.McPherson, 1998 . Recessional water flows result
from continued drainage from the catchment of over-
land flow not significantly charged with sediment.
The key affect of recessional water flow is the
selective fine-fraction winnowing of the poorly sorted
debris flow material, and the movement of this sedi-
ment into the back-snout pond. This recessional flow
( )T.C. BlairrGeomorphology 28 1999 201­221218
facies is developed in the proximal part of the North
Long John fan, instead of on the distal fan, because
of the damming effect of the rock avalanche de-
posits.
® .The grain-size data of the -1.6 cm -y4f
fraction provide some important insight to the rela-
tionship between the North Long John rock avalanche
and the subsequent debris flows. This fraction in the
debris flows is generally similar to that of the rock
avalanche, including an unsorted distribution of size
® .grades Fig. 8 . The key differences are that the
samples from the rock avalanche contain a higher
® .percentage of gravel 60% vs. 20 to 40% and have
® .significantly less mud 12% vs. 33%; Fig. 8B­F .
These differences are represented in the cumulative
curves by a well-shaped arcuate distribution of sam-
ples from the rock avalanche, and an irregular linear
pattern for the samples form the debris flows, which
especially diverge from the rock avalanche pattern in
® .the mud fraction Fig. 8A . The cumulative curve in
® .the mud fraction finer than 4f for the debris flow
samples have an identical plot, regardless of facies
® .Fig. 8A . Given that the debris flow deposits are
derived either from the cataclastic material accumu-
lated along the avalanche scar or, in the case of the
distal fan by partial reworking of the avalanche snout
deposits, the overall similarities in the size suite is
expected. The enrichment in mud in the debris flow
samples, however, shows that this fraction was selec-
tively concentrated by the debris flow process. This
enrichment in the mud fraction probably is indicative
of the critical mud concentration needed for the
initiation of a debris flow given the overall texture of
the cataclastic colluvium.
The surface of the North Long John debris flow
deposits has been modified by secondary processes.
Foremost amongst these is the fine-fraction winnow-
ing of sediment by overland flows, aided by rain-
splash and wind erosion, to produce a matrix-free,
surface gravel lag. This winnowing process occurs
during recessional water flow of a debris flow event,
and during rainfall-triggered overland flows. The
most widespread product of this winnowing process
is the development of a boulder­cobble mantle or
® .pavement one or more clasts thick Fig. 7D , a
feature also noted on the surface of the nearby
®Trollheim fan in Deep Springs Valley, CA Blair and
.McPherson, 1992 . The presence of drained matrix
from the lobes of North Long John fans suggests that
the development of this mantle is initiated by matrix
drainage, and then advanced by subsequent sec-
ondary water winnowing. Overland flow from the
fan catchment also is responsible for the develop-
ment of the incised channel through the rock
avalanche snout that leads to the debris flow tract of
the distal fan. Other prominent secondary processes
on the debris flow surfaces include oxidation of
clasts, and the growth of desert bushes and grasses.
5.3. Reconstructed post-aalanche depositional his-
tory
The proximal debris flow tract post-dates deposi-
tion of the rock avalanche, given that it is derived
from colluvium mantling the avalanche scar, and that
®it has partially buried avalanche deposits Fig. 4A­
.B . Deposition of this tract commenced upon the
development of the colluvial mantle in the nascent
catchment, which accumulated as rockfall of the
loose bedrock fragments granulated along the
avalanche scar. These colluvial particles were cata-
clastically formed during the rock avalanche through
pulverization as the frontal bedrock facet slid along
the detachment surface. Observations from historical
®events indicate that such rockfall colluvium `after-
.fall' accumulates within weeks to months of the
® .event Hadley, 1978; Blair and McPherson, 1994b .
The transformation of this colluvial material in the
North Long John fan catchment to a debris flow
awaited favorable flashy precipitation in the catch-
ment, precipitation known in Owens Valley to result
®from summer thunderstorms Beaty, 1970; Blair and
.McPherson, 1998 . The infrequency of such precipi-
tation in a given catchment dictates that the proximal
North Long John fan debris flow tract aggraded
sporadically through time from rare storms, with
secondary surface processes affecting sedimentation
between storms. Debris flows instigated since the
avalanche event have deposited as much as 20 m of
clast-rich levees and lobes in this proximal fan tract,
whereas the more mobile, recessional clast-poor de-
bris flows and water flows accumulated within the
;6 m deep trap behind the rock avalanche snout
® .Fig. 10C . This trap eventually filled to the level of
® .the snout Fig. 7C­D .
( )T.C. BlairrGeomorphology 28 1999 201­221 219
Deposition of debris flows on the distal fan tract
commenced upon breaching of the rock avalanche
snout by a channel that connects the proximal and
® .distal debris flow tracts Fig. 10D . This breach
likely occurred once the back-snout pond had filled
with recessional debris flow material from the proxi-
mal tract, after which time flows overtopped the
snout. Once initiated, downcutting of the sinuous
snout-bypass channel probably was rapid because of
the uncemented, homogeneous texture of the
avalanche material in the snout. This theory of
downcutting by overtopping is supported by the loca-
tion of this incised channel at the topographically
® .lowest point through the snout Fig. 4B and Fig. 7C .
With the development of this avalanche-bypass
channel, debris flows could proceed from the fan
apex to the distal debris flow tract, as did the 22
® .August 1984 debris flow Fig. 4C . Some of the
debris flow deposits of the distal tract also were
derived from the avalanche snout during incision of
the bypass channel. The lack of darkly varnished
pavements on the debris flow deposits of the distal
®tract, in contrast to the proximal tract cf. Fig. 3E
.and Fig. 7D , supports a more recent age of develop-
ment of the distal tract. This relative age also is
suggested by the low volume of sediment found on
the distal tract.
The abundance of colluvium upon the steep slopes
of the North Long John fan catchment indicates that
debris flows will continue to be instigated given
favorable input of flashy thunderstorm precipitation.
Aggradation of the proximal tract will lead to
progradation of the debris flow levee and lobe facies
belts, and to further burial of the rock avalanche
deposits. Breaching of the avalanche snout allows
future clast-rich lobes and recessional stage deposi-
tion to continue reaching and aggrading the distal
tract, thereby lengthening the fan radius.
6. Comparison with adjoining mature piedmont
fans and fan catchments
The North Long John fan and catchment have
morphometric characteristics that significantly differ
from the adjoining New York Butte and Long Canyon
systems. The New York Butte and Long Canyon fan
catchments have respective areas of 9.9 and 19.5
km2
, relief of 1713 to 1911 m, and well-developed,
dense drainage nets of fourth- and fifth-order feeder
® .channels Figs. 3 and 4A . These attributes contrast
with the 2.0 km2
area, 1065 m relief, and second-
order feeder channel of the North Long John catch-
'® .ment. Relief ratios catchment reliefr area are 0.54
and 0.43, respectively, for the New York Butte and
Long Canyon fan catchments, vs. 0.75 for the North
Long John fan catchment.
The 0.8 km2
area of the North Long John fan is
notably smaller than the 2.0 and 4.3 km2
areas of the
adjoining New York Butte and Long Canyon fans,
®which also have significantly longer radii 2.4 and
.2.8 km long, respectively than the 1.7 km long
radius of the North Long John fan. The average
slope of 8.88 for the North Long John fan is steeper
than the 6 to 78 average slopes of the adjoining fans.
The slope of the North Long John fan also differs
from its relatively smooth-surfaced conical neighbors
by having a more irregular composite morphology
resulting from the presence of rock avalanche de-
posits and two debris flow tracts. The adjoining fans
are smoother because they are continuously under-
lain by clast-rich debris flow lobes, and because they
contain surfaces of well-developed and deeply var-
nished desert pavement. These differences between
the North Long John fan vs. the New York Butte and
Long Canyon fans, and between the respective catch-
ments, are the result of the nascent stage of develop-
®ment of the North Long John system Stage 1 of
.Blair and McPherson, 1994a compared to the older
and more mature development of the adjoining fans
of the Inyo Mountains piedmont.
7. Implications
The key implication of the study of the North
Long John fan is that not all fans of a particular
piedmont are of the same longevity or stage of
development. This is especially true of fans devel-
oped from the range front bedrock triangular facets
dividing more mature fan catchments. Instability of
these triangular bedrock facets with time promotes
failure, perhaps by the initiation of rock avalanches,
that leave colluvium-lined scars ideal for subsequent
fan depositional events. This study also points out
( )T.C. BlairrGeomorphology 28 1999 201­221220
that the geological hazards on the piedmont beneath
®triangular bedrock facets rockfalls, rockslides, and
.rock avalanches contrast with the dominantly debris
flow or water-flood hazards of the more mature fans.
Differences in the relative ages of adjoining fans also
lead to complications for the common practice of
mapping relative-aged surfaces on piedmonts. This
study demonstrates that complications arise from
differences in the timing of fan initiation, and with
the degree of diachroneity of fan provinces.
The North Long John fan case study also has
implications for the stratigraphic record of pied-
monts. Differentiating the deposits of ancient pied-
monts are complicated because a new fan can be
initiated much later than other fans. The stratigraphic
signature of this phenomenon for the North Long
John fan would be the sharp accumulation of
megabrecciated rock avalanche deposits above the
coalesced tips of older and larger fans, followed by
debris flow deposits with especially angular clasts of
a monolithologic composition. The peculiar occur-
rence of recessional flood `pond' facies in the proxi-
mal fan area also would be an important indicator of
the presence of a rock avalanche barrier directly
downfan.
8. Conclusions
The North Long John alluvial fan was initiated by
a large rock avalanche triggered by the collapse of a
former bedrock triangular facet. The rock avalanche
provided the initial sediment of the nascent fan, and
left a large, spoon-shaped scar in the range front
stocked with cataclastically generated muddy and
gravelly colluvium. Thus, this rock avalanche served
as an alluvial fan `starter kit.' Thunderstorm-induced
debris flows generated from the catchment subse-
quent to rock avalanche emplacement have built
proximal and distal debris flow tracts that partially
bury the rock avalanche. This process will continue
because of the abundance of mud-bearing sediment
upon the steep catchment slopes, and the likely
continued occurrence of thunderstorms in Owens
Valley. The nascent stage of the North Long John
fan contrasts with the more mature, larger neighbor-
ing fans of the Inyo Mountains piedmont.
Acknowledgements
Two anonymous reviewers kindly provided cri-
tiques that improved this manuscript. Blair & Asso-
ciates is thanked for research support.
References
Beaty, C.B., 1970. Age and estimated rate of accumulation of an
alluvial fan White Mountains, California. American Journal of
Science 268, 50­77.
Blackwelder, E., 1928. Mudflow as a geologic agent in semi-arid
mountains. Geological Society of America Bulletin 39, 465­
484.
Blair, T.C., McPherson, J.G., 1992. The Trollheim alluvial fan
and facies model revisited. Geological Society of America
Bulletin 104, 762­769.
Blair, T.C., McPherson, J.G., 1994a. Alluvial fan processes and
® .forms. In: Abrahams, A.D., Parsons, A. Eds. , Geomorphol-
ogy of Desert Environments. Chapman & Hall, London, pp.
354­402.
Blair, T.C., McPherson, J.G., 1994b. Alluvial fans and their
natural distinction from rivers based on morphology, hydraulic
processes, sedimentary processes, and facies. Journal of Sedi-
mentary Research A 64, 451­490.
Blair, T.C., McPherson, J.G., 1998. Recent debris-flow processes
and resultant form and facies of the Dolomite alluvial fan
Owens Valley, California. Journal of Sedimentary Research
68, 800­818.
Blair, T.C., McPherson, J.G., 1999. Grain-size and textural classi-
fication of coarse sedimentary particles. Journal of Sedimen-
tary Research 69, 6­19.
Broscoe, A.J., Thomson, S., 1969. Observations on an alpine
mudflow, Steele Creek, Yukon. Canadian Journal of Earth
Sciences 6, 219­229.
Bull, W.B., 1984. Tectonic geomorphology. Journal of Geological
Education 32, 310­324.
Burchfiel, B.C., 1966. Tin Mountain landslide, southeastern Cali-
fornia, and the origin of megabreccia. Geological Society of
America Bulletin 77, 95­100.
Fauque, L., Strecker, M.R., 1988. Large rock avalanche deposits
® .Strurzstrome, sturzstroms at Sierra Aconquija, northern Sier-
ras Pampeanas, Argentina. Eclogae Geologicae Helvetiae 81,
579­592.
Folk, R.L., 1974. Petrology of Sedimentary Rocks. Hemphill
Publishing, Austin, TX.
Gates, W.B., 1987. The fabric of rockslide avalanche deposits.
Bulletin of the Association of Engineering Geologists 24,
389­402.
Hadley, J.B., 1964. Landslides and related phenomena accompa-
nying Hebgen Lake earthquake of August 17, 1959. U.S.
Geological Survey Professional Paper 435, 107­138.
Hadley, J.B., 1978. Madison Canyon rockslide, Montana, U.S.A.
® .In: Voight, B. Ed. , Rockslides and Avalanches, Vol. 1.
Elsevier, Amsterdam, pp. 167­196.
( )T.C. BlairrGeomorphology 28 1999 201­221 221
Hollett, K.J., Danskin, W.R., McCaffrey, W.H., Walti, C.L.,
1991. Geology and water resources of Owens Valley, Califor-
nia. U.S. Geological Survey Water-Supply Paper 2370-B, 77
pp.
Horton, R.E., 1945. Erosional development of streams and their
drainage basins; hydrophysical approach to quantitative mor-
phology. Geological Society of America Bulletin 56, 275­370.
® .Hsu, K.J., 1975. Catastrophic debris streams Sturzstroms gener-¨
ated by rockfalls. Geological Society of America Bulletin 86,
129­140.
Johnson, A.M., 1970. Formation of debris flow deposits. In:
® .Johnson, A.M. Ed. , Physical Processes in Geology. Freeman,
Cooper, San Francisco, pp. 433­448.
Johnson, A.M., 1984. Debris flow. In: Brunsden, D., Prior, D.B.
® .Eds. , Slope Instability. Wiley, New York, pp. 257­361.
Keefer, D.K., 1984. Rock avalanches caused by earthquakes:
source characteristics. Science 223, 1288­1290.
Keefer, D.K., 1994. The importance of earthquake-induced land-
slides to long-term slope erosion and slope-failure hazards in
seismically active regions. Geomorphology 10, 265­284.
Lubetkin, L.K.C., Clark, K.M., 1988. Late Quaternary activity
along the Lone Pine fault, eastern California. Geological Soci-
ety of America Bulletin 100, 755­766.
Pakiser, L.C., 1960. Transcurrent faulting and volcanism in Owens
Valley, California. Geological Society of America Bulletin 71,
153­160.
Pakiser, L.C., Kane, M.F., Jackson, W.H., 1964. Structural geol-
ogy and volcanism of Owens Valley region, California--A
geophysical study. U.S. Geological Survey Professional Paper
438, 65 pp.
Pierson, T.C., 1980. Erosion and deposition by debris flows at Mt.
Thomas, North Canterbury, New Zealand. Earth Surface Pro-
cesses 5, 227­247.
Pierson, T.C., 1981. Dominant particle support mechanisms in
debris flows at Mt. Thomas, New Zealand, and implications
for flow mobility. Sedimentology 28, 49­60.
Pierson, T.C., 1986. Flow behavior of channelized debris flows,
® .Mount St. Helens, Washington. In: Abrahams, A.D. Ed. ,
Hillslope Processes. Allen and Unwin, Boston, pp. 269­296.
Ross, D.C., 1967. Generalized geologic map of the Inyo Moun-
tains region, California. U.S. Geological Survey Map 1­506.
Sharp, R.P., 1942. Mudflow levees. Journal of Geomorphology 5,
222­227.
Sharp, R.P., Nobles, L.H., 1953. Mudflow of 1941 at Wright-
wood, southern California. Geological Society of America
Bulletin 64, 547­560.
Shreve, R.L., 1968. The Blackhawk landslide. Geological Society
of America Special Paper 108, 47 pp.
Shreve, R.L., 1987. Blackhawk landslide, southwestern San
Bernardino County, California. Geological Society of America
Centennial Field Guide Cordilleran Section, pp. 109­114.
Strahler, A.N., 1964. Quantitative geomorphology of drainage
® .basins and channel networks. In: Chen, V.T. Ed. , Handbook
of Applied Hydrology. McGraw-Hill, New York, pp. 40­74.
Streitz, R., Stinson, M.C., 1974. Geologic map of the 1:250,000
scale Death Valley sheet, California. California Division of
Mines and Geology, Sacramento.
Wallace, R.E., 1978. Geometry and rates of change of fault-gener-
ated range fronts, north-central Nevada. U.S. Geological Sur-
vey Journal of Research 5, 637­650.