fine-grained debris flows in coarse-grained alluvial …€¦ · the fountain and cutler formations...
TRANSCRIPT
Journal of Sedimentary Research, 2017, v. 87, 763–779
Research Article
DOI: http://dx.doi.org/10.2110/jsr.2017.45
FINE-GRAINED DEBRIS FLOWS IN COARSE-GRAINED ALLUVIAL SYSTEMS:
PALEOENVIRONMENTAL IMPLICATIONS FOR THE LATE PALEOZOIC
FOUNTAIN AND CUTLER FORMATIONS, COLORADO, U.S.A.
DUSTIN E. SWEET
Department of Geosciences, Texas Tech University, 125 Science Building, Lubbock, Texas 79409, U.S.A.
e-mail: [email protected]
ABSTRACT: The Fountain and Cutler formations are coarse-clastic alluvial wedges that mantled ancestral RockyMountain uplifts. Muddy granule sandstone (MGSF) is a volumetrically important facies to both systems. The facies ismassive and unsorted with grain-size distributions that range from clay to granule. These deposits are bestcharacterized as cohesive fine-grained debris flows. Yet, the MGSF rarely contains clasts . 10 mm, although otherintercalated alluvial facies commonly contain cobbles and small boulders.
Competence modeling was undertaken to assess the amount of water needed to account for the observed coarsestfraction of the MGSF. These results indicate that flows would need inflation with water by one-third to two-thirds,depending on clay mineralogy and the range of clay in the MGSF. This paper proposes that flows began as debrisflows but underwent flow transformation through incorporation of water during flow in the paleohighlands. Dilationof the flow reduced competence, and each flow lost the coarser than 10 mm fraction. Flows would rheologically stiffenupon reaching the unconfined alluvial surface, ultimately behaving as cohesive fine-grained debris flows. The MGSFat both study areas does not fit into end-member facies models of alluvial fans. Depositional systems elsewhere thatexhibit similar facies may want to consider flow transformation and the associated environmental implications wheninferring the depositional setting.
INTRODUCTION
Sedimentation in alluvial settings forms a spectrum of transport styles
between two end members from grain-by-grain movement in dilute stream
flow to mass-wasting gravity events (Beverage and Culbertson 1964;
Coates 1977; Varnes 1978; Smith 1986; Smith and Lowe 1991; Benvenuti
and Martini 2002). The variety of sediment-transport styles between these
two end members largely reflects the sediment-to-water ratio of the flow
(e.g., Lawson 1982; Pierson and Costa 1987). Understanding where the
sedimentology of an alluvial deposit fits on this sediment-to-water
spectrum is necessary for correct interpretation of environmental setting,
though the challenge lies in estimating the amount of water during the flow.
Common sedimentologic characteristics that bear on the sediment-to-water
ratio are type and presence of sedimentary structures, and range and
sorting of grain-size distribution incorporated in the flow.
The Fountain and Cutler formations record coarse-grained alluvial
sedimentation deposited adjacent to Precambrian-cored uplifts and are
hallmark signatures of the ancestral Rocky Mountains orogeny (Mallory
1972; Rascoe and Baars 1972; McKee 1975). Depositional environment
and associated climatic interpretations vary for these two units.
Depositional interpretations for the Fountain Formation include coalescing
alluvial fans, fan deltas, and braid plains (e.g., Howard 1966; Suttner et al.
1984; Maples and Suttner 1990; Blair and McPherson 1994; Sweet and
Soreghan 2010a; Sweet and Soreghan 2012; Hogan and Sutton 2014;
Sweet et al. 2015). Climatic interpretations during deposition of the
Fountain Formation range from a warm-humid climate (Wahlstrom 1948;
Hubert 1960; Mack and Suttner 1977), a warm-arid climate (Raup 1966;
Walker 1967), a progressively drier climate up through the section (Suttner
and Dutta 1986; Dutta and Suttner 1986; Sweet and Soreghan 2010a;
Sweet et al. 2015), to even an intermittent cold-wet climate (Sweet and
Soreghan 2008; Sweet and Soreghan 2010b). Depositional interpretations
for the Cutler Formation also conflict and include arid alluvial fan (Walker
1967; Werner 1974; Mack et al. 1979; Mack and Rasmussen 1984; Dutta
and Suttner 1986; Suttner and Dutta 1986), humid or seasonally wet
alluvial fan (Campbell 1979, 1980; Tidwell 1988; Dubiel et al. 1996;
Huntoon et al. 2014), and a cold-wet proglacial fan (Soreghan et al. 2009;
Soreghan et al. 2014; Keiser et al. 2015). Thus, using process
sedimentology to assess the sediment-to-water ratio for particular facies
may highlight those climate and depositional-environment interpretations
that agree with the flow processes.
In the Cutler and Fountain stratigraphic record, an enigmatic facies
occurs and is characterized as unsorted, massive, clay- to gravel-size
deposit that is relatively tabular along exposures and interpreted as a
cohesive fine-grained debris flow (Soreghan et al. 2009; Sweet and
Soreghan 2010a). In a vacuum, this facies interpretation is not remarkable;
however, the facies is commonly juxtaposed with deposits that have much
coarser clasts, up to boulders. Yet, curiously, this facies lacks the coarse-
grained fraction so abundant throughout these two formations. This paper
attempts to assess the sediment-to-water ratio during the depositional
events responsible for the fine-grained debris flows, which will bear on the
amount of water available and ultimately on the depositional and climatic
setting.
Published Online: August 2017Copyright � 2017, SEPM (Society for Sedimentary Geology) 1527-1404/17/087-763/$03.00
GEOLOGIC SETTING
The ancestral Rocky Mountains are the product of late Paleozoic
intracratonic deformation that uplifted Precambrian-cored basement blocks
along high-angle faults (e.g., Kluth and Coney 1981). Arkosic sediments
shed from the block uplifts were deposited in adjacent basins as thick,
wedged-shaped packages (e.g., Mallory 1972; McKee 1975; Kluth and
Coney 1981; Ye et al. 1996). The Woodland Park trough and the Paradox
basin comprise two of these ancestral Rocky Mountain basins and
accumulated sediments that compose the Fountain and Cutler formations,
respectively (Figs. 1, 2). During deposition of the lower and middle part of
the Fountain Formation in the Middle Pennsylvanian (e.g., Sweet and
Soreghan 2010a), the Woodland Park trough resided within a few degrees
of the equator (Domeier et al 2012; Domeier and Torsvik 2014). The Cutler
Formation at Gateway, Colorado is poorly age constrained with recovered
sparse flora that range from Middle Pennsylvanian to Permian (Huntoon et
al. 2014). Lithostratigraphic correlation to more distal and better dated
units suggests the most proximal undivided Cutler Formation exposed near
the Gateway study site is likely early Permian (e.g., Barbeau 2003), which
would place the Gateway study area around 108 north of the equator
(Torsvik et al. 2012; Domeier and Torsvik 2014). Across both study areas,
easterlies associated with zonal atmospheric circulation predominated
during the Middle Pennsylvanian, but monsoonal circulation with
westerlies and easterlies originated in the Early Permian (Parrish and
Peterson 1988; Soreghan et al. 2002).
Depositional and Climate Setting for the
Woodland Park Trough Study Site
The Fountain Formation is a first-cycle, arkosic sandstone and
conglomerate derived from Precambrian rocks exposed during ancestral
Rocky Mountain uplift (e.g., Hubert 1960; Mallory 1972). In the
Woodland Park trough, sediment was shed from the Ute Pass block across
the Ute Pass fault (Fig. 3). Sedimentologic, stratigraphic, and structural
data indicate that the Ute Pass fault was active during deposition of the
lower and middle parts of the Fountain Formation, but had likely ceased by
deposition of the upper portion of the unit (Sweet and Soreghan 2010a). At
Manitou Springs, Colorado, the lower and middle portions of the Fountain
Formation record ~ 573 meters of fan-delta deposition as indicated by
alluvial facies that rapidly fine away from the Ute Pass fault, exhibit semi-
radial paleocurrents, and distally intercalate with marine strata (Fig. 3;
Suttner et al. 1984; Maples and Suttner 1990; Sweet and Soreghan 2010a;
Sweet and Soreghan 2012). The alluvial sequences are largely character-
ized by two facies groups: 1) scour-and-fill sandstone and granule to
cobble conglomerates inferred as stream deposits that exhibit rare
channelization, and 2) massive, very poorly sorted, muddy-granule
sandstone inferred as cohesive fine-grained debris flows (Sweet and
Soreghan 2010a). This latter facies is the facies studied at this locality.
Despite numerous petrographic, paleontologic, and sedimentologic
studies of the Fountain Formation along the Colorado Front Range,
evidence cited for climatic conditions during Fountain deposition are
conflicting. Early studies suggested a warm-humid climate, citing an
inferred lateritic paleosol, local coaly layers, and scattered plant fragments
(Wahlstrom 1948; Hubert 1960). In the lower part of the Fountain
Formation and Glen Eyrie Member, Mack and Suttner (1977) inferred
tropical conditions, arguing that compositional maturity of these strata
exceeds that of Holocene sand in Front Range fans. More recent work,
however, has indicated that most of the compositional maturity could be
explained by physical destruction of feldspars in beach settings of the fan
delta (Kairo et al. 1993). Other authors preferred a warm-arid climate
interpretation, citing the interstitial clay composition and hematite content
(Raup 1966; Walker 1967). In an attempt to assess the conditions during
crystallization of authigenic clays, Dutta and Suttner (1986) suggested that
decreasing kaolinite upward in the Fountain Formation reflects a change
from warm-humid to warm semiarid conditions. Data used in these
interpretations predominantly bear on relative humidity, but consensus of
water availability is not apparent. Based on the presence of inferred
glacially induced microtextures exhibited on the surfaces of quartz grains
recovered from alluvial deposits, Sweet and Soreghan (2010b) proposed
that the Fountain Formation could record proglacial deposition from
upland valley glaciers. This climatic interpretation bears largely on
temperature, but proglacial systems are also commonly characterized by
facies that demonstrate abundant water availability (e.g., Boothroyd and
Ashley 1975; Boothroyd and Nummedal 1978; Lawson 1982; Maizels
1993, 1997; Marren 2002).
Depositional and Climate Setting for the Gateway Study Site
The Cutler Formation records first-cycle deposition from sediments
derived from the Precambrian-cored Uncompahgre uplift (e.g., Rascoe and
Baars 1972). Near Gateway, Colorado, the Cutler Formation rests on
Precambrian rocks of the Uncompahgre Plateau, forming a buttress
unconformity (Fig. 4; Cater 1955; Moore et al. 2008; Soreghan et al. 2009;
Soreghan et al. 2012). The modern Uncompahgre Plateau comprised part
of the larger Uncompahgre Uplift of the ancestral Rocky Mountains.
Sediments were shed across the Uncompahgre Thrust into the Paradox
basin as the Uncompahgre Uplift was rising (e.g., Frahme and Vaughn
1983); however, by the time the strata exposed near Gateway, Colorado,
were deposited, the Uncompahgre Thrust was inactive and buried by up to
1 km of sediment (Cater 1955; Soreghan et al. 2012). Here, the Cutler
Formation is undifferentiated and consists largely of alluvial deposits (e.g.,
Campbell 1980; Mack and Rasmussen 1984; Soreghan et al. 2009). The
lowermost stratigraphic record exposed directly adjacent to the Uncom-
pahgre Uplift has been interpreted as proglacial lake deposition based in
part upon sediment gravity flows that display characteristics most
consistent with subaqueous deposition, outsized clasts encased in mud
that are compatible with a dropstone interpretation, and up to a 308
depositional dip accordant with a Gilbert-type foreset geometry (Moore et
al. 2008; Soreghan et al. 2009; Soreghan et al. 2014). This proximal-
proglacial–lacustrine interpretation is still under debate (e.g., Huntoon et
al. 2014); however, the deposits that occur distally beyond the debated
lacustrine section are consistently inferred to be coarse-grained alluvial and
fluvial deposits (Campbell 1980; Mack and Rasmussen 1984; Soreghan et
al. 2009; Huntoon et al. 2014). In this unequivocally alluvial part of the
section, unsorted and massive muddy, pebble conglomerate deposits occur.
This facies is the focus of research at this locality.
Many studies of the proximal undifferentiated Cutler Formation argue
that the climatic state during deposition was ever-warm or arid to semiarid.
These interpretations are: 1) based on climate-ambiguous data, such as
sandstone petrography (Werner 1974; Mack et al. 1979; Suttner and Dutta
1986); 2) derived from comparison of facies models of modern arid
alluvial fans (Mack and Rasmussen 1984); or 3) inferred from presence of
interstitial hematite or neoformed clay mineralogy (Walker 1967; Dutta
and Suttner 1986). Moreover, warm-arid interpretations are often coupled
with expected rain-shadow effects under low-latitude zonal atmospheric
circulation (e.g., Mack et al. 1979; Mack and Rasmussen 1984). Other
workers utilize sedimentology or fossilized flora with expected seasonality
under monsoonal atmospheric circulation to invoke a warm-humid climate
(Campbell 1979, 1980; Huntoon et al. 2014) or warm but seasonally wet
climate (Dubiel et al. 1996). Conversely, other workers have suggested that
the Cutler Formation records at least periodic proglacial sedimentation
under a cool and wet climate (e.g., Soreghan et al. 2009). A proglacial
interpretation is based largely on sedimentologic data, including process-
oriented facies interpretations (Soreghan et al. 2009), quartz-grain
microtextures (Soreghan et al. 2008; Keiser et al. 2015), and age
relationships establishing the antiquity of a paleo-canyon that fed the
D.E. SWEET764 J S R
Cutler fan (Soreghan et al. 2007; Soreghan et al. 2014; Soreghan et al.
2015).
METHODS OF GRAIN-SIZE ANALYSIS
Samples of the Fountain and Cutler formations were collected from the
sections measured by Sweet and Soreghan (2010a) and Soreghan et al.
(2009), respectively (Supplemental Data 1, 2, see Supplemental Material).
Facies sampled are exclusively muddy granule sandstone (Sm-g) from the
Fountain Formation (Sweet and Soreghan 2010a) and massive pebble
diamictite from the Cutler Formation (Soreghan et al. 2009). Fresh, fist-
sized samples were collected by scraping away loose, weathered exterior
commonly down to a depth of 3 cm. Very rare, floating-outsized clasts
(small cobbles) locally occurred in sampled horizons and were noted in
size but were not included in sampling as these clasts would
uncharacteristically influence the grain-size distribution of our fist-sized
samples.
Iron oxide and/or hematite-stained clays commonly form the cement in
the Fountain Formation (e.g., Hubert 1960). Disaggregation of samples
employed the citrate–bicarbonate–dithionite (CBD; Janitsky 1986) method,
which selectively removes iron oxide into the solution. The solution was
decanted and remaining material was captured for analysis of grain-size
distribution. Because of the lithological similarity of the Cutler and
Fountain formations, the methodology was successfully employed for both
units.
Once disaggregated, samples were sieved into coarse-grained (. 250
lm) and fine-grained (, 250 lm) fractions. The coarse-grained fraction
was split into 4000 lm, 2000 lm, 1000 lm, 500 lm, and 250 lm bins,
then weighed. The fine-grained fraction was sonicated for 10 minutes in a
dispersant solution (Calgont) and analyzed for grain-size distribution with
a Beckman-Coulter LS-13330 laser particle analyzer. Each sample was
then sonicated for another 10 minutes and reassessed by laser particle
analyzer. The grain-size distributions from the 10-minute and the 20-
minute sonicated samples were compared to assess any further disaggre-
FIG. 1.—Late Paleozoic tectonic elements of
the greater ancestral Rocky Mountain. The most
proximal clastic-wedge deposits in the study areas
(red rectangles) are alluvial systems that grade to a
shoreline. In the Pennsylvanian, the white areas of
the map were predominantly marine; however, the
epeiric seas were much reduced in the early
Permian. Compiled from Lindsey et al. (1986),
Hoy and Ridgway (2002), Sweet and Soreghan
(2010a), and Baltz and Myers (1999). Location of
equators estimated from Peterson (1988). WPT,
Woodland Park Trough.
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 765
gation of clay particles associated with the extra sonication time. Most of
the second-run histograms were similar, and thus the sample was
considered disaggregated. If the second-run histogram exhibited a
noticeable increase in percent clay, the sample was sonicated for another
10 minutes. With the exception of two samples that were taken from well-
developed paleosol horizons, all histograms demonstrated very little
change after 20 minutes of sonication. Grain-size distributions were
finalized by normalizing the fine-grained fraction to the overall weight of
the initial sample. For example, the fine-grained fraction was converted to
weight percent by multiplying by the percent volume data from the laser
particle with the weight of the fine-grained fraction of the entire sample.
The coarse- and fine-grained fractions were then combined into a single
histogram.
CHARACTERIZATION OF THE MUDDY GRANULE SANDSTONE FACIES
Although similar in character, the facies analyzed in this paper were
given different names during the Cutler Formation study versus the
Fountain Formation study. For clarity, the facies nomenclature is
standardized and uses the terminology of Sweet and Soreghan (2010a),
muddy granule sandstone rather than massive pebble diamictite. Each of
the 18 samples disaggregated for grain-size analysis spans a minimum of
14 u bins (Fig. 5) with average mean of 3.88 u and 3.22 u for the Fountain
and Cutler samples, respectively (Tables 1, 2). Data that characterize the
muddy granule sandstone facies at each study site is presented separately
below.
Woodland Park Trough Study Site
The Fountain Formation at the Woodland Park trough study site is
approximately 920 m thick and contains three informal members—lower,
middle, and upper—separated by internal tectonic unconformities (Sweet
and Soreghan 2010a). The lower and middle members are genetically
related and are inferred to record fan-delta deposition; however, the upper
member is best characterized as a braid-plain system and postdates late
Paleozoic movement on the Ute Pass fault (Sweet and Soreghan 2010a).
MGSF samples were collected from the lower and middle members of the
Fountain Formation from a composite stratigraphic section located within 1
to 2 km of the faulted contact with Precambrian basement (Fig. 3,
Supplemental Material 1). This composite stratigraphic section is
composed of cyclic alluvial–marine strata. The lower member represents
the distal portions of the alluvial environment within the larger fan-delta
system, whereas the middle member represents a mid-fan position (Fig.
3B; Sweet and Soreghan 2012). Moreover, since the Fountain Formation is
eastward dipping yet depositional dip was northward (Suttner et al. 1984),
sampling along this section covered an east–west transect across the paleo-
fan delta.
The muddy granule sandstone facies (MGSF) constitutes ~ 27% of the
lower 573 meters of the Fountain Formation (Sweet and Soreghan 2010a).
FIG. 2.—Stratigraphic chart showing ages of
the Fountain and Cutler formations in the
respective study areas. Vertical black lines denote
hiatuses. Manitou Springs stratigraphy is adopted
from Sweet et al. (2015). Gateway stratigraphy is
adopted from Soreghan et al. (2009), and age of
the proximal Cutler Formation is here unknown
and is estimated by correlation to better-dated
units in the Paradox basin. Time scale is from
Gradstein et al. (2012).
D.E. SWEET766 J S R
Individual intervals of the MGSF range up to 4 meters; however, careful
lateral inspection indicates that intervals this thick are likely a series of
stacked beds, commonly 1–2 meters thick, separated by discontinuous and
thin lenses composed of granule conglomerate and moderately sorted
coarse sandstone. Individual beds are massive and demonstrate no apparent
vertical or lateral sorting (Fig. 6). Basal contacts are abrupt but mantle
underlying beds rather than scour (Fig. 6B). The best exposures in the area
are along Highway 24 and trend essentially east–west, which is along
depositional strike (Suttner et al. 1984; Sweet and Soreghan 2012). Beds of
the MGSF are laterally continuous along the extent of these roadcuts,
which range up to ~ 185 meters. Beds are also continuous along
depositional dip, at least throughout a single exposure. Underlying and
overlying strata commonly contain boulders and cobbles, but with the
exception of very rare outsized small cobbles, the MGSF lacks this coarse-
tail fraction that is so common to other facies (Fig. 6A–D). In thin section,
sand-size grains are unsorted, angular to subangular with no preferred
orientation (Fig. 6E). Feldspathic grains exhibit minimal chemical
alteration. Mud matrix, commonly clay minerals, surrounds the sand-size
grains such that grains appear to float in the two-dimensional, thin-section
plane. Long-axis alignment of the mud grains, predominantly clay and
micaceous minerals, is not random but rather wraps around existing sand-
FIG. 3.—A) Early to Middle Pennsylvanian paleogeography of Woodland Park Trough area. Modified from Sweet and Soreghan (2010a). B) Schematic cross section
illustrating the north-to-south stratigraphic relationships of the Fountain Formation in the study area shown in Part A. Modified from Suttner et al. (1984).
FIG. 4.—A) Early Permian paleogeographic map of the Uncompahgre and proximal alluvial and fluvial system. Line a–a0 parallels West Creek near the town of Gateway,
Colorado. B) Stratigraphic schematic cross section at the time of deposition of the MGSF-bearing strata. More proximal strata are currently eroded but were assumed to have
been similar to the exposed MGSF bearing strata. The heavier boldface line in the center of the section indicates the time surface at the end of lacustrine deposition. Cross
section is adapted from Soreghan et al. (2009).
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 767
size grains or exhibits a weak undulose fabric through the larger pockets of
mud (Fig. 6F).
Nine samples of the MGSF of the Fountain Formation were
disaggregated for grain-size analysis. In addition, two well-developed
vertisol horizons, indicated by slickensides and wedge-shaped peds (Mack
et al. 1993), were collected for grain-size comparison. The mean average
grain size of the MGSF samples is very fine sand (3.44 u; ~ 0.09 mm), but
the range spans 14 to 15 u classes (Fig. 7). Using the Folk and Ward
(1957) method, the average sorting is 3.07 u as indicated by one standard
deviation (r1) of the distribution and the average skewness is 0.21 u.
These values equate to very poorly sorted and fine-tail-skewed histograms,
respectively. The fine-tail-skewed nature of the distribution is apparent by
the abrupt drop in weight percent between 1 and 10 mm (Fig. 7).
Gateway Colorado Study Site
The Cutler Formation at the Gateway Colorado study site is
approximately 970 m thick and forms a buttress unconformity with the
underlying Precambrian basement (Cater 1955; Moore et al. 2008;
Soreghan et al. 2009). Here, the Cutler Formation is composed of five
facies associations that correspond to distance away from the buttress
unconformity. The most distal facies associations (i.e., facies association 4
and 5 of Soreghan et al. (2009); Supplemental Data 2) represent
unequivocal alluvial strata, whereas depositional-environment interpreta-
tions for the more proximal facies associations are debated as discussed
earlier. The MGSF from the Cutler Formation studied in this paper occur
from 2.5 to 6 km of the exposed buttress unconformity and represents
deposits that are in unequivocal alluvial strata. In contrast to the Fountain
system, the Cutler Formation currently dips in the direction of the inferred
depositional dip (Mack and Rasmussen 1984; Soreghan et al. 2009); thus,
the sampling transect does not provide an across-fan representation of the
alluvial strata. Overall, the MGSF is volumetrically much less abundant in
the Cutler Formation than in the Fountain Formation.
Where exposed, the MGSF in the Cutler Formation mantles underlying
strata, but the upper contact is commonly scoured by subsequent stream-
flow facies (Fig. 8A–C). Scouring also inhibits assessment of the original
lateral continuity of the facies. Individual intervals range up to 3 meters
thick and typically contain abundant pebbles and granules that float in a
matrix of sand and mud. Clast orientation in the matrix is random, and
beds are universally structureless. In thin section, the facies is similarly
unsorted, with clay through very coarse sand grains juxtaposed in the same
field of view (Fig. 8D–F). Grains are also angular to subangular, display no
preferred orientation, and demonstrate minimal feldspar alteration. Platy
phyllosilicate grains are warped around larger quartz and feldspar grains
(Fig. 8F).
Five samples were collected from the unequivocal alluvial strata for
grain-size analysis. In addition, two samples from the debated lacustrine
interval were collected for comparison. The mean average grain size of the
MGSF samples is fine sand (3.44 u; ~ 0.12 mm). The range of grain sizes
present in the distribution span 14 to 15 u (clay to gravel) classes (Fig. 9).
Using the same methodology as applied to the Fountain Formation
samples, the average sorting is very poorly sorted or 3.07 u. In contrast to
the distributions in the Fountain Formation, the skewness is within the
nearly symmetrical range (0.09 u), but visual inspection of the histograms
indicates that the skewed direction is fine-tailed with a steep drop in weight
percent around 10 mm.
Comparison of Grain-Size Distributions with Other Facies
A concern with grain-size analysis of ancient deposits is diagenetic
alteration of the original grain-size distribution. Clay minerals created
during pedogenesis are of particular concern in the Fountain Formation
because numerous paleosols occur in the alluvial strata (Sweet and
Soreghan 2010a). Two of those paleosol horizons were assessed for grain-
size analysis. Although the range of grain sizes in the distribution is similar
to that of the MGSF, the mean (5.88 u; 0.02 mm) is much finer and the
distribution is coarse-tail skewed (Fig. 7). The clay grain-size fraction
averages 27 weight percent and the silt-size fraction averages 44 weight
percent. In stark contrast, the MGSF samples averaged 10 weight percent
clay-size fraction and 30 weight percent silt-size fraction.
Fluvial facies in the Fountain Formation exhibit a much coarser fraction
than the MGSF (Fig. 6). Sweet and Soreghan (2010a) provided grain-size
FIG. 5.—Combined grain-size distributions of the MGSF in the Cutler and Fountain formations.
D.E. SWEET768 J S R
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Founta
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MS
Pi-
8.5
MG
SF
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.7
%3.2
%12.8
%17.6
%15.8
%11.2
%11.7
%7.2
%5.8
%4.1
%3.4
%2.9
%1.9
%1.5
%0.2
%0.0
%
MS
Pi-
15
MG
SF
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%3.6
%11.1
%13.4
%12.5
%12.1
%9.3
%8.4
%8.5
%6.7
%5.3
%4.2
%2.6
%2.0
%0.2
%0.0
%
MS
Pii
-59
MG
SF
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%1.1
%4.3
%9.2
%15.6
%6.7
%8.8
%8.9
%10.2
%9.8
%9.4
%7.8
%4.3
%2.8
%0.8
%0.0
%
MS
Pii
i-69.5
MG
SF
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%6.0
%1.9
%9.0
%13.4
%17.1
%8.7
%7.4
%6.5
%7.1
%6.5
%5.9
%4.6
%2.4
%1.6
%1.3
%0.4
%
MS
Pv-1
6M
GS
F0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.2
%2.4
%8.7
%15.3
%14.3
%11.6
%10.7
%8.8
%8.9
%6.5
%4.9
%3.8
%2.3
%1.6
%0.2
%0.0
%
MS
Pv-2
1.5
MG
SF
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%1.6
%5.4
%11.2
%18.3
%13.6
%14.3
%9.9
%7.9
%5.6
%4.6
%3.7
%2.2
%1.5
%0.2
%0.0
%
MS
Pvi-
5M
GS
F0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%1.2
%1.9
%4.8
%9.6
%12.9
%10.3
%11.8
%10.8
%10.7
%8.5
%6.7
%5.3
%3.1
%2.1
%0.2
%0.0
%
MS
Pvi-
41
MG
SF
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.8
%2.4
%6.0
%9.4
%12.7
%16.8
%11.1
%8.5
%9.0
%7.3
%5.9
%4.9
%3.0
%2.0
%0.2
%0.0
%
MS
Pvi-
72.5
MG
SF
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%2.2
%2.9
%6.1
%10.8
%20.4
%6.4
%9.4
%9.3
%9.1
%7.1
%6.1
%5.1
%3.0
%2.0
%0.2
%0.0
%
MS
Pii
i-15.5
Pal
eoso
l0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.6
%1.7
%3.2
%5.6
%6.1
%8.4
%8.0
%9.2
%11.2
%14.2
%14.9
%9.7
%6.7
%0.7
%0.0
%
MS
Pii
i-62
Pal
eoso
l0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.6
%1.7
%3.7
%7.2
%9.3
%10.0
%8.7
%9.9
%12.2
%13.9
%11.9
%6.0
%3.7
%0.9
%0.2
%
CO
G_upper
Flu
via
l16.0
%7.0
%20.0
%12.0
%13.0
%1.0
%0.3
%2.0
%5.7
%7.3
%6.5
%5.5
%3.8
%1.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%
Sch
ool
Flu
via
l5.0
%7.0
%9.0
%15.0
%34.0
%1.7
%0.6
%3.5
%6.2
%5.3
%4.9
%4.5
%2.6
%0.7
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%
MS
Pii
-52
Flu
via
l0.0
%17.0
%19.0
%20.0
%20.0
%1.8
%0.6
%3.6
%3.6
%4.2
%3.9
%4.1
%1.8
%0.4
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%
MS
Pii
i-53
Flu
via
l0.0
%0.0
%0.0
%3.0
%7.0
%12.0
%26.0
%6.6
%12.4
%11.1
%9.2
%9.9
%1.4
%0.4
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%
GO
TG
-ZF
luvia
l0.0
%3.0
%0.0
%5.0
%23.0
%0.9
%0.3
%1.7
%2.5
%5.6
%20.0
%27.7
%8.3
%2.1
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%
Cutl
erF
orm
atio
n
CU
TS
ix-1
MG
SF
allu
via
l0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%1.6
%4.6
%8.6
%10.5
%14.5
%7.8
%10.7
%10.6
%9.3
%6.2
%5.3
%4.9
%3.1
%2.2
%0.2
%0.0
%
CU
Tix
-30
MG
SF
allu
via
l0.0
%0.0
%0.0
%0.0
%0.0
%6.0
%12.8
%13.9
%9.8
%9.3
%11.8
%9.1
%10.9
%5.4
%3.5
%2.3
%2.0
%1.6
%0.9
%0.6
%0.1
%0.0
%
CU
Tx-1
MG
SF
allu
via
l0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%1.2
%5.8
%8.1
%9.3
%11.6
%11.4
%14.2
%12.8
%9.2
%5.2
%3.9
%3.4
%2.3
%1.6
%0.2
%0.0
%
CU
Tx-7
1.5
BM
GS
Fal
luvia
l0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%2.2
%3.7
%5.6
%9.8
%12.4
%6.5
%14.3
%17.3
%11.9
%6.1
%3.9
%3.0
%1.9
%1.3
%0.1
%0.0
%
CU
Txi-
4.6
MG
SF
allu
via
l0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%2.6
%6.5
%9.8
%12.8
%13.1
%9.5
%8.0
%7.0
%7.5
%6.5
%5.6
%4.9
%3.4
%2.6
%0.3
%0.0
%
CU
Tvi.
5-1
72
MG
SF
Lac
ust
rine
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.5
%0.6
%1.3
%2.5
%7.3
%16.2
%20.8
%15.8
%12.7
%7.8
%5.4
%4.2
%2.8
%2.0
%0.2
%0.0
%
CU
TS
vii
i-76
MG
SF
Lac
ust
rine
0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%0.0
%1.1
%1.5
%3.0
%8.5
%13.1
%23.4
%17.3
%12.2
%7.1
%5.0
%3.8
%2.3
%1.6
%0.2
%0.0
%
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 769
distribution data from a variety of fluvial facies. Those data are averaged
and compared with the MGSF data in this study (Fig. 7). The average mean
grain size of the fluvial facies is 16 mm, which equates to . 55% of the
fluvial-facies grain-size distribution is coarser than the coarsest fraction of
the MGSF.
In the Cutler Formation, the MGSF occurs in alluvial facies as well as a
proposed lacustrine section (Soreghan et al. 2009). Grain-size distributions
of the MGSF recovered from the proposed lacustrine interval demonstrate
sorting and skewness statistics similar to those from the alluvial strata
(Table 2); however, visual inspection of the histogram indicates that the
lacustrine samples are much more symmetrical and have finer median
grain size than the samples from alluvial strata (Fig. 9).
Soreghan et al. (2009) report grain-size data for hyperconcentrated flood
and traction flows in fluvial facies of the Cutler Formation; however, the
data ranges only from –2 u to 4 u and accounts for the finest 80% of the
distribution (Fig. 9). Yet, those data demonstrate that ~ 20% of the clasts in
the fluvial deposits are . 4 mm, indicating that the Cutler alluvial
depositional system contains an abundance of clasts larger than the
coarsest 1% of the average MGSF distribution.
INTERPRETATION OF FLOW PROCESS
The MGSF at both localities is very poorly sorted, comprising grains
ranging from less than a micron to about 10 millimeters. Such a variety of
grain sizes is not compatible in a flow where each grain is transported
independently of the other grains, such as a dilute stream flow. Rather, the
flow process must have moved the grains en masse with a cohesive matrix
strength as indicated by the relative proportion of clay-size material (e.g.,
Pierson and Costa 1987; Smith and Lowe 1991). One caveat to this
interpretation is the relative abundance of detrital versus diagenetic clay.
The clay-size content of the MGSF averages ~ 10 and 8 weight percent for
the Fountain and Cutler formations, respectively. The clay-size content of
the Fountain Formation paleosols averages ~ 27 weight percent. Paleosol
histograms are coarse-skewed versus the fine-skewed character of the
MGSF. Samples from paleosol horizons in the Cutler Formation were not
assessed. Thus, in strata that exhibit substantial pedogenesis the clay
content is substantially elevated relative to the MGSF samples. Presumably,
the increase in pedogenically produced clay would also shift the skewness
of the histogram. In thin section, clay minerals wrap around siliciclastic
sand grains and form distinct, connected, and aligned layers, producing a
matrix that the larger grains commonly float within (Figs. 6, 8). These
observations indicate that clay material was present during the flow and
supported the larger grains. The preferred interpretation of flow process
that accounts for the wide variety of grain sizes, the internal clay-mineral
alignment, and internally massive character that characterize the MGSF
deposits is best characterized as a cohesive fine-grained debris flow
(Middleton and Hampton 1973; Hampton 1975; Lowe 1979).
Alternate flow processes that reportedly can produce unsorted and
structureless deposits, such as traction carpets (e.g., Todd 1989; Sohn
1997) or hyperconcentrated flows (e.g., Scott et al. 1995; Smith 1986), are
TABLE 2.—Parameters and statistics of the grain-size data.
Sample Name Facies
Stratigraphic
position(1)
(in meters)
in phi units in mm
phi
95
phi
84
phi
50
phi
16
phi
5 mean skewness
sorting
(1r) D95 D84 D50 D16 D5
Fountain Formation
MSPi-8.5 MGSF 8.5 -0.90 -0.05 2 5.60 8.45 2.52 0.33 2.83 1.87 1.04 0.25 0.021 0.003
MSPi-15 MGSF 15.0 -0.09 0.10 2.75 6.70 8.95 3.18 0.28 3.02 1.06 0.93 0.15 0.010 0.002
MSPii-59 MGSF 69.0 -0.10 1.10 4.50 7.98 9.65 4.53 0.03 3.20 1.07 0.47 0.04 0.004 0.001
MSPiii-69.5 MGSF 158.5 -2.20 -0.10 2.20 7.05 9.30 3.05 0.30 3.53 4.59 1.07 0.22 0.008 0.002
MSPv-16 MGSF 281.0 -0.65 0.33 2.78 6.45 8.70 3.18 0.23 2.95 1.57 0.80 0.15 0.011 0.002
MSPv-21.5 MGSF 286.5 -0.30 0.80 2.98 6.25 8.65 3.34 0.23 2.72 1.23 0.57 0.13 0.013 0.002
MSPvi-5 MGSF 360.0 -0.65 0.85 3.75 7.20 9.05 3.93 0.09 3.06 1.57 0.55 0.07 0.007 0.002
MSPvi-41 MGSF 396.0 -0.68 0.75 3.13 6.98 9.00 3.62 0.22 3.02 1.60 0.59 0.11 0.008 0.002
MSPvi-72.5 MGSF 427.5 -1.05 0.50 3.15 7.25 9.05 3.63 0.19 3.22 2.07 0.71 0.11 0.007 0.002
Average -0.74 0.48 3.03 6.83 8.98 3.44 0.21 3.06 1.85 0.75 0.14 0.010 0.002
MSPiii-15.5 Paleosol 104.5 0.88 2.83 6.68 9.08 10.28 6.20 -0.23 2.99 0.54 0.14 0.01 0.002 0.001
MSPiii-62 Paleosol 151.0 0.80 2.30 5.90 8.50 10 5.57 -0.13 2.94 0.57 0.20 0.02 0.003 0.001
Average 0.84 2.57 6.29 8.79 10.14 5.88 -0.18 2.97 0.56 0.17 0.01 0.002 0.001
COG_upper Fluvial n/a -7.60 -6.90 -4.30 2.10 4.00 -3.03 0.43 4.01 194 119 19.70 0.23 0.06
School Fluvial n/a -7.00 -5.50 -3.60 1.40 3.60 -2.57 0.40 3.33 128 45 12.13 0.38 0.08
MSPii-52 Fluvial 62.0 -6.60 -6.10 -4.30 0.50 3.30 -3.30 0.49 3.15 97 69 19.70 0.71 0.10
MSPiii-53 Fluvial 142.0 -3.80 -2.50 -0.90 2.60 3.60 -0.27 0.29 2.40 14 6 1.87 0.16 0.08
GOTG-Z Fluvial n/a -4.30 -3.60 2.50 3.80 4.50 0.90 -0.60 3.18 20 12 0.18 0.07 0.04
Cutler Formation
Average -5.86 -4.92 -2.12 2.08 3.80 -1.65 0.20 3.21 90.53 50.21 10.71 0.31 0.07
CUTSix-1 MGSF alluvial 581.0 -1.23 0.10 2.23 6.93 9.10 3.09 0.35 3.27 2.35 0.93 0.21 0.01 0.002
CUTix-30 MGSF alluvial 610.0 -3.10 -2.35 0.80 4.05 7.10 0.83 0.13 3.15 8.57 5.10 0.57 0.06 0.007
CUTx-1 MGSF alluvial 561.0 -1.30 0.10 3.20 6.05 8.70 3.12 0.03 3.00 2.46 0.93 0.11 0.02 0.002
CUTx-71.5B MGSF alluvial 631.5 -1.20 0.50 3.70 6.23 8.40 3.48 -0.07 2.89 2.30 0.71 0.08 0.01 0.003
CUTxi-4.6 MGSF alluvial 705.0 -1.60 -0.28 2.50 7.10 9.30 3.11 0.25 3.50 3.03 1.21 0.18 0.01 0.002
Average -1.69 -0.39 2.49 6.07 8.52 2.72 0.14 3.16 3.74 1.78 0.23 0.02 0.003
CUTvi.5-172 MGSF Lacustrine 427.0 1.00 2.25 4.05 6.75 8.95 4.35 0.22 2.33 0.50 0.21 0.06 0.01 0.00
CUTSviii-76 MGSF Lacustrine 515.0 0.90 2.25 3.95 6.55 8.70 4.25 0.21 2.26 0.54 0.21 0.06 0.01 0.00
Average 0.95 2.25 4.00 6.65 8.83 4.30 0.21 2.29 0.52 0.21 0.063 0.010 0.002
(1) Stratigraphic position is the reported value from the composite sections of Sweet and Soreghan (2010a) for the Fountain Formation and Soreghan et al. (2009) for the
Cutler Formation.
D.E. SWEET770 J S R
FIG. 6.—Images of the MGSF at the Fountain Formation study area. A) Photograph of contact between a bed of the MGSF at base and the overlying clast-supported cobble
conglomerate. Hammer is approximately 32 cm long. B) Photograph demonstrating the lateral continuity and nonerosional character of the basal contact of the MGSF. White
arrows indicate basal surface of the MGSF bed. Note the small cobbles located right at the base of the bed. Hammer is approximately 32 cm long. C) Close-up photograph of
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 771
untenable mechanisms because those deposits are predominantly or
entirely sand and gravel. Thus, to invoke these interpretations, the mud
and especially clay component would need to be added during diagenesis,
and be absent from the flow, which is unlikely given the observations listed
above.
Observations of modern cohesive debris flows with ~ 5–10 weight
percent clay indicate entrainment of 25–50% gravel material and
commonly carry boulders . 0.5 m in diameter (Blair and McPherson
1998; Berti et al. 1999). Granule and pebble clasts are relatively common
in the MGSF, with ~ 4 and ~ 12 weight percent of the grain-size
distribution between 2 mm and 10 mm in the Fountain and Cutler samples,
respectively. Clasts . 10 mm are extremely rare in samples from both
localities, even though the intercalated fluvial facies at both localities
exhibit an abundance of grains . 10 mm. Invoking a presorted source of
material for each fine-grained debris flow event is unlikely because that
interpretation mandates that stream channels would pull from an alternate
unsorted source. Another potential explanation is that the competence of
the flow changed (i.e., dilation flow transformation), which allowed the
larger clasts to drop out of the flow during the event.
Sediment flows can change behavior if the sediment-to-water ratio
changes through loss or gain of either parameter during flow. During an
individual event, a spectrum of flows is possible depending on the
availability of water and easily erodible sediment. For example, Scott et al.
(1995) proposed that on Mount Rainer a flow can begin as a dilute stream
flow and steadily incorporate sediment to achieve first a hyperconcentrated
flood flow, then a debris flow. Downslope freezing of those debris flows
resulted in dewatering, providing the fluid for a subsequent fine-grained
debris flow. Applying this concept to the Fountain and Cutler depositional
systems fails because it implies that proximal settings would record a
similar number of flow events that would classify as unsorted, cohesive
debris flows. The Fountain Formation has a few proximal unsorted, debris-
flow deposits (Suttner et al. 1984), but these deposits are much less
common than the volume of MGSF in the unit. The Cutler Formation does
have a significant component of proximal debris flows, but they are best
categorized as noncohesive (Schultz 1984) and may have been subaqueous
(Soreghan et al. 2009). An alternative process of flow transformation
involves incorporation of water, which inflates the flow volume and
reduces competence (Hampton 1975; Scott et al. 1995). Applying this
concept implies that coarse material drops out of the flow as water is
incorporated, and thus, deposits should have the coarser fraction in
proximal deposits and coarse clasts should be more prevalent along the
bases of the beds. In the Fountain Formation, the coarsest clasts typically
congregate toward the bases of beds (Fig. 6B). Thus, this process of flow
transformation may apply; if so, then the amount of water necessary to
inflate flows to enable deposition of larger (. 10 mm) clasts can be
modeled.
MODELING FLOW COMPETENCE
The competence of debris flows has long been known as a force balance
between the weight of the largest clast counteracted by matrix strength,
also commonly referred to as cohesion and buoyancy (e.g., Johnson 1970).
The matrix strength of a flow is related to a network of flocculated clay
particles, where the strength of the network decreases or increases as a
function of the relative abundance of water and clay, respectively
(Hampton 1972). Thus, the competence of debris flows as a function of
matrix strength is predominantly controlled by percent water in the flow if
the mineralogy and volume of the clay remains constant (Hampton 1975).
An upward buoyant force also supports grains floating in a clay–water
mixture. This force is a function of the density of the fluid and the
concentration of coarse material that imposes a load on the fluid, which
increases the vertical pressure gradient of the fluid above the effects of fluid
density alone (Hampton 1979). The buoyancy from the non-matrix load is
the weight of the grains acting on the fluid and increasing pore pressure in
the flow, such that the pressure below grains is higher than above grains.
The relative percentage of buoyancy to matrix strength as a support
mechanism increases with grain size because the surface area affected by
the pressure gradient is larger (Pierson 1981). This relationship between
grain concentration, matrix strength, and buoyancy is demonstrated by the
following relationship (Hampton 1979):
Dd=Dm ¼ 1= 1 � Cgð Þ ð1Þ
where Dd is the competence of the flow as a function of both the buoyant
FIG. 7.—Various grain-size distributions from the Fountain Formation reported as
A) weight-percent histograms and B) percent-finer-than distributions. Black lines,
MGSF; gray lines, well-developed paleosol; red lines, average fluvial facies (data
from Sweet and Soreghan 2010a).
the bed in Part B. D) Photograph of a fresh surface of the MGSF demonstrating the lack of sorting and massive character of the facies. Each black-and-white rectangle on the
scale is 1 cm thick. E) Photomicrograph at 43 magnification in cross-polarized light demonstrating the unsorted character of the MGSF. Scale bar in bottom right corner is 1
mm. F) Photomicrograph at 203 magnification in cross-polarized light. Note the alignment of the long platy phyllosilicate minerals. Scale bar in bottom right corner is 200 lm.
D.E. SWEET772 J S R
FIG. 8.—Photographs and photomicrographs of the Cutler Formation at the Gateway, Colorado, study site. A) Stratigraphic succession of alluvial facies demonstrating
interbedded MGSF (reddish-brown beds) and stream-flow facies (pink). Three white arrows show location of Parts B and C. B) Close-up of MGSF heavily scoured by
overlying stream-flow facies. White arrows denote the base of the MGSF deposit. C) Close-up demonstrating the abrupt and relatively planar basal contact of MGSF denoted
by the white arrows. D–F) a series of photomicrographs demonstrating the poor sorting and immature texture of the MGSF at a variety of scales. All three images are taken at
the same place in the same thin section.
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 773
force and matrix strength, Dm is the competence of the flow as a function of
matrix strength alone, and Cg is the volume percent concentration of the
grains coarser than clay size. Equation 1 shows that the competence
becomes increasingly dependent on matrix strength rather than buoyancy
as the non-matrix grain concentration approaches zero. The full
competence of a debris flow related to buoyancy and matrix strength can
be calculated if Dm and Cg can be assessed. Note that for increasing grain
concentration values, the potential for interaction between grains also
increases, which could result in other support mechanisms such as grain-
to-grain contact and dispersive pressure (Hampton 1972, 1979; Rodine and
Johnson 1976; Lowe 1979; Pierson 1981). These alternative mechanisms
are not assessed in this model.
Modeling the maximum clast size supported by a clay–water mixture, or
Dm, was experimentally derived by Hampton (1975) Equation 2:
Dm ¼ 8:8k=gðqs � qf Þ ð2Þ
where Dm is the competence of the flow, k is the yield strength of the
matrix, g is the acceleration of gravity, qs is the density of the sediment
(i.e., 2.65 g/cm3), and qf is the density of the fluid matrix. Other densities
utilized in the model are 2.72 g/cm3 (kaolinite), 2.63 g/cm3 (montmoril-
lonite), and 1.03 g/cm3 (water; Wada and Wada 1977).
Matrix yield strength varies depending on clay mineralogy. Studies on
the part of the Fountain Formation that contains the MGSF indicate a
predominate kaolinite mineralogy (Dutta and Suttner 1986; Sweet and
Soreghan 2008); however, the Cutler Formation chiefly contains chlorite
and smectite clays (Dutta and Suttner 1986). Therefore, the experimentally
derived relationships between matrix strength and weight-percent water
developed for montmorillonite and kaolinite (Hampton 1975) can be used
for the Cutler and Fountain systems, respectively.
The matrix strength of a clay–water mixture will decrease if the clay
volume remains unchanged while the flow dilates due to addition of water
(Hampton 1975). The models presented here allow the clay:water ratio to
vary within a static interstitial volume. The output is the competence of a
flow as a function of the clay:water ratio. Estimation of competence for the
MGSF in this study can be calculated from the dry-clay weight percent
obtained from the grain-size distribution. Competence related solely to
matrix strength (Dm) can be calculated as a function of the matrix strength
in the interstitial space of the flow by using Equation 2.
The volume percent concentration of the non-clay fraction (Cg) is
calculated by converting the dry weight percent of the non-clay fraction to
volume percent based on the relative proportions of dry weight percent
clay, dry weight percent non-clay fraction, and water. For example, Cg will
decrease if the flow inflates due to addition of water. Finally, using
Equation 1 the total competence can be modeled as a function of varying
clay:water ratio in the interstitial space and the associated grain
concentration.
The Fountain Formation Case: Modeling Competence with Kaolinite
Three different volumes of interstitial space were modeled where the
clay:water ratio varied in that space (Fig. 10). The models presented here
relate competence in terms of dry-clay weight percent, rather than
clay:water ratio during the flow, for easier comparison with the measured
grain-size distribution. Each model demonstrates a larger separation in
competence related to matrix strength (Dm) than the total competence (Dd)
for lower than higher clay values. This is because at low clay:water ratios
the matrix strength is weak, but also the concentration of coarse grains
remains high, and accordingly the buoyancy force is stronger. At very high
clay values, the matrix strength is correspondingly stronger, but the coarse-
grain fraction must diminish and thus the buoyancy force weakens. This
relationship only amplifies as the interstitial space changes from 30% to
60% of the flow volume. For example, in the 60% interstitial volume
model at about 30 dry weight percent clay the ratio of Dd to Dm is nearly
one, indicating that matrix strength is the driving support mechanism.
Hampton (1979) originally demonstrated these Dd–Dm relationships.
The Fountain Formation averages ~ 10 weight percent clay in the
MGSF and ranges from ~ 7 to 15%. Assuming an entire kaolinite-based
slurry, the total modeled competence (Dd) at a clay:water ratio consistent
with the average measured clay content is approximately 30 cm, 2.4 cm,
and 0.5 cm for the 30%, 45%, and 60% interstitial volume models,
respectively (Fig. 10). For the highest measured clay value (i.e., ~ 16%),
the respective models indicate competence values of 328 cm, 13 cm, and
1.5 cm. These results indicate that a flow with only 30% interstitial volume
and the average weight percent of clay measured in the MGSF should have
the competence to carry the coarsest clasts observed in the entire system.
Yet, the average coarsest 5%, or D95, is , 2 mm indicating that the
interstitial volume of the flow must have been considerably larger. Using an
FIG. 9.—Various grain-size distributions from the Cutler Formation reported as A)
weight-percent histograms and B) percent-finer-than distributions. Black lines,
MGSF; gray lines; inferred lacustrine facies (see text for details); red line, fluvial
facies (solid red line is data from Soreghan et al. 2009; dashed red line is inferred in
the diagram from histogram shape and field observations of maximum clast size).
D.E. SWEET774 J S R
interstitial volume of 60%, the competence aligns much closer to the
observed grain-size characteristics of the MGSF.
The Cutler Formation Case: Modeling Competence with
Montmorillonite
Clay mineralogy in the Cutler Formation is reported as predominantly
smectite (Dutta and Suttner 1986). Matrix strength data exists for
montmorillonite, a member of the smectite group, and thus is an
appropriate proxy for this model. Three different volumes of interstitial
space were modeled similar to the kaolinite case above, but substituting the
matrix-strength relationship (k values for Equation 1) derived from various
montmorillonite-to-water ratios (Hampton 1975). MGSF samples of the
Cutler Formation average ~ 7.5 weight percent clay and range from ~ 3 to
11%. At a clay:water ratio consistent with 7.5 dry weight percent clay, the
total competence (Dd) for the 60%, 70%, and 80% interstitial volume
models is 226 cm, 5 cm, and 0.3 cm, respectively (Fig. 10). Fluvial facies
in the Cutler Formation indicate that 20% of the grains in these facies
exceed 4 mm (Fig. 9). Field observations indicate that some beds contain
boulders nearly a meter in size, and clasts around 30 cm are relatively
common. The average D95 of the MGSF is ~ 3.7 mm. For deposits that
have values near the measured minimum clay fraction, around 60%
interstitial space models the observed competence well, whereas interstitial
space around 80% is necessary to account for the maximum clay values
FIG. 10.—Competence models derived from matrix-strength and fluid-buoyancy clast-support mechanisms. Results are presented in terms of competence (vertical axis) and
dry weight percent clay (horizontal axis). Red dotted trend indicates competence related to both buoyancy and matrix strength combined, or Dd in text. Dotted blue trends
indicate competence related only to matrix strength, or Dm in text. Clasts that have diameters that fall above these dotted trends are larger than the theoretical competence
related to these clast-support mechanisms alone. Green horizontal line is the D95 percent-finer grain size for the Fountain Formation (left-hand side, kaolin model) and the
Cutler Formation (right-hand side, montmorillonite model). Dark gray shaded region represents the range of grain sizes entrainable within the full range of the measured clay
fraction in the grain-size distribution, and dashed vertical black line represents the average clay fraction, for the Fountain (left side) and Cutler (right side) samples.
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 775
measured (Fig. 10). If the clay mineralogy was a mixture of kaolinite and
montmorillonite, then the interstitial volume needed to model the observed
competence would likely have been lower due to the lower matrix strength
of kaolinite.
Notes on Competence-Model Assumptions
The above competence models assume the following: 1) competence is
predominantly a function of fluid buoyancy and matrix strength; 2) each
flow is a closed system with respect to the clay fraction of the grain-size
distribution; 3) clay grain-size fraction is entirely either kaolin or
montmorillonite clay minerals; and 4) matrix strength is not a function
of the silt grain-size fraction. Implications of each assumption are
discussed below.
In flows with a relatively high grain concentration, grain-to-grain
interactions may increase competence beyond the theoretical effects of
fluid buoyancy and matrix strength alone (Rodine and Johnson 1976;
Hampton 1979; Pierson 1981). In experiments using grain-size distribu-
tions similar to the MGSF in this paper (i.e., clay to granules), the effects of
grain-to-grain interactions increased competence beyond the theoretical
magnitude of buoyancy and matrix strength combined at around two-thirds
water volume and 20 to 30 percent non-clay grain concentration (Hampton
1979). Moreover, in that experimental data, an increase in water volume
leads to a larger departure from the theoretical competence value,
presumably because the increase in water reduced matrix strength allowing
grains to interact more readily. If grain-to-grain contact did provide
significant clast support during the flows responsible for the MGSF, then
the water volume of each flow would need to be increased to reduce the
competence resulting from buoyancy and matrix strength, which in turn
would inflate the competence related to the grain-to-grain contact
mechanism. The end result of this feedback would likely result in a flow
that was able to winnow fines and produce at least graded bedding, which
are not observed in the MGSF. The first assumption appears valid because
grain-to-grain contact as a significant support mechanism would either
have resulted in a higher competence than is observed or lead to
winnowing and graded bedding.
If flows responsible for the MGSF in this study were able to incorporate
clay during the event, then matrix strength and subsequent competence of
the flows would increase. This process would result in flows becoming
increasingly more cohesive with the ability to entrain larger and larger
clasts, at odds with the observed character of the deposits. Thus, the second
assumption appears to be valid.
Matrix strength results predominantly from internal friction of the grains
with a component of electrochemical bonding between clay minerals.
Internal friction is the result of a strong network of interlocking grains that
resist shear (Rodine and Johnson 1976), which is most commonly
attributed to granular-shaped grains larger than clay size (Rowe 1962).
Studies that measure or model the effect on competence from diluting the
clay grain-size fraction with non-phyllosilicate minerals are not known to
the author. The competence models presented here impose an interstitial
space, which inflates flow volume such that internal friction can be
overcome and internal shear is possible. Suppose that the flow volumes
were not inflated as the models impose; then the competence of each flow
should have carried a much coarser grain size than is observed. Therefore,
to account for the much finer observed grain-size distribution in the MGSF,
adding granular-shaped silicate-mineral grains to a clay-sized phyllosilicate
matrix would need to greatly reduce competence. This seems unlikely
given that with tighter grain packing the internal friction of the granular
load increases and results in particle interlocking and rigid flow (Rowe
1962; Rodine and Johnson 1976). Moreover, in studies that have attempted
to address the mineralogy of the clay fraction in the Cutler and Fountain
formations, various phyllosilicates were the predominant mineral (Dutta
and Suttner 1986; Sweet and Soreghan 2008). For these reasons, I argue
that the third assumption is valid and also the fourth assumption, given that
increasing matrix strength from the silt fraction would exacerbate the
issues stated above.
IMPLICATIONS FOR THE FOUNTAIN AND CUTLER DEPOSITIONAL SYSTEMS
Competence modeling suggests that for the MGSF exhibited in the
Fountain Formation, interstitial volume of the flow was likely near 55% to
account for the observed grain-size distribution. The model assumes that
the clay fraction is constant during a flow event, and thus subtracting the
amount of clay from the interstitial space should yield the water volume of
each flow, which is around 30 to 40 percent. Applying the same rationale to
the MGSF in the Cutler Formation, the amount of water in the flows likely
ranged from 57 to 70 percent. Thus, water volume in the flows probably
ranged from around one-third to two-thirds depending on amount and type
of clay mineralogy. This ratio of sediment to water straddles the border of
hyperconcentrated flood flows to debris flows on classification charts
(Pierson and Costa 1987; Smith and Lowe 1991), yet as previously stated
the depositional product is more indicative of an en masse freezing debris
flow. The latter suggests that, just before deposition, grain interlocking had
occurred and internal fluid pore-pressure had diminished such that the flow
was a relatively rigid body with presumably basal shear only, in other
words, a cohesive debris flow. The alternative that the fine-grained debris
flows were initiated with the grain-size distributions reported herein is
untenable because a mechanism that would presort the debris flow material
into less than 10 mm grains before the flow began, while also keeping the
material entrained in normal stream flow unsorted, is hard to envision.
Therefore, during flow initiation or at some point when the flow was still
confined to the highland tributary system, water must have been
incorporated to account for the observed grain-size distribution.
The strata housing the MGSF for both the Cutler and Fountain
formations were deposited within a few kilometers of the Precambrian
uplifts that sourced the sedimentary material. However, the Cutler system
was a much larger fan system (Figs. 3, 4) indicating that the deposits record
a more proximal setting than the Fountain system. Despite this, both
systems record the MGSF at least one kilometer from the suspected
paleohighlands front, indicating a minimum unconfined flow distance.
This paper proposes that flow events were initiated in these highlands as
water-saturated flows and desaturation took place after the flow became
unconfined on the alluvial surface. In the case of the Fountain Formation,
the MGSF is the most abundant facies, comprising nearly 27% of the lower
573 m of the unit (Sweet and Soreghan 2010a). Furthermore, the MGSF
persists throughout the stratigraphic interval, indicating that the process
responsible for each debris-flow event was a pervasive and important
component of the depositional system. Scouring of MGSF deposits by
overlying stream-flow facies is much more common in the Cutler
Formation (Figs. 6, 8), which may be related to higher energy and/or
reduced accommodation in the proximal fan.
The arguments above indicate that the environmental processes
operating in the Fountain Formation depositional system must have been
able to combine kaolinite with relatively unweathered sandy material and
consistently have readily available water. The alluvial surface of the
Fountain Formation graded to the shoreline of a broad epeiric sea situated
east of the Ute Pass Uplift (Fig. 3; Suttner et al. 1984; Maples and Suttner
1990; Sweet and Soreghan 2012), a paleogeographic setting which should
have resulted in high precipitation under zonal atmospheric circulation.
Abundant precipitation would have likely resulted in highly weathered soil
profiles, vegetation, and available water. The kaolinite mineralogy is
consistent with more intense chemical weathering (Dutta and Suttner
1986), but the abundant minimally weathered feldspar (Fig. 6E; Hubert
1960; Suttner and Dutta 1986; Sweet and Soreghan 2010a) is inconsistent
with high chemical weathering. Moreover, abundant vegetation associated
with a humid climate would likely reduce the sediment load delivered to
D.E. SWEET776 J S R
the alluvial realm (e.g., Leeder et al. 1998). Slope failure that involved a
weathered profile and the underlying minimally altered basement was
invoked to account for mixed low and high chemical-weathering signals in
rivers draining plutonic rocks in Costa Rica (Joo et al. 2016). This seems
like a viable mechanism to account for minimally altered feldspar and
kaolinite in the MGSF deposits in the Fountain Formation.
Flow transformations of debris flows have been both experimentally and
empirically observed (Johnson 1970; Hampton 1972). These observations
have led to a model, termed surface transformation, where debris flows are
diluted commonly through entry of water beneath the nose of the flow
(Fisher 1983). Applying this model to either the Fountain or the Cutler
depositional systems requires that abundant water is available on the
alluvial surfaces at all times; yet, both alluvial units lack the sedimentology
consistent with permanent lower-flow-regime systems. Rather the deposits
have characteristics more consistent with events that indicate flows laden
with sediment such as unchannelized flows, scour-and-fill structures, and
hyperconcentrated flows (Mack and Rasmussen 1984; Suttner et al. 1984;
Soreghan et al. 2009; Sweet and Soreghan 2010a). However, the highlands
may have had permanent flowing streams housed in granite valleys, which
could have provided the source of water that facilitated flow expansion.
Accordingly, one tenable scenario to account for the MGSF is as follows.
Slope failure, involving the weathered profile and minimally altered
granite, initiated a debris flow. Upon reaching the local valley flow within
the highlands, the debris flow intersected relatively clean water.
Incorporation of water into the flow resulted in flow expansion and the
beginning of slurries or high-density hyperconcentrated flows. During this
time, flows are posited to have had competencies consistent with the grain
size of the depositional product. Upon exiting the mountain front and
entering the alluvial plain, the flow was no longer confined and began to
lose water and rheologically stiffen, ultimately resulting in the MGSF
deposits. Incorporation of water during flows leading to flow expansion
and dilution is a common flow-transformation process observed in modern
debris flows derived from Mount Rainer (Scott et al. 1995).
An alternative process that could have produced abundant water and
sediment is alpine glaciation in the equatorial highlands (Soreghan et al.
2008; Sweet and Soreghan 2010b). Elsewhere, Atokan (late Bashkirian to
early Moscovian) to Desmoinesian (latest Moscovian) strata record at least
periodic glaciation throughout most of Gondwana (Fielding et al. 2008)
and a colder isotopic signature recorded in low-paleolatitude recovered
brachiopods (e.g., Frank et al. 2008; Giles 2012)—all suggesting that this
period was a globally cool period potentially conducive to upland
equatorial glaciation (Soreghan et al. 2015). Glacial erosion of Precam-
brian basement during intervals of glacial growth provides a mechanism to
account for relatively unweathered feldspathic grains, but it is hard to
reconcile with a kaolinite weathering profile. However, even if glaciers
periodically occupied substantially higher elevations, tropical weathering
conditions likely existed at lower elevations or during global glacial
minima, thus sourcing the kaolinite. Advance and retreat of alpine glaciers
provides a mechanism to mix low and high chemically weathered products.
Moreover, glacial melt could provide the water indicated by the
competence modeling. Modern subaerial sediment flows emanating near
glacier snouts is an important process and range from matrix-strength-
supported flows to more liquefied flows (i.e., types II and III of Lawson
1982), which commonly produce grain-size distributions similar to this
study. Moreover, the stratigraphic section in the Fountain Formation is
highly cyclical, denoted by intercalated marine and alluvial deposits, which
has been related to glacioeustatic rise and fall (Sweet and Soreghan 2012).
Interestingly, the alluvial strata containing the MGSF in the Fountain
Formation are repeatedly sandwiched between intervals of marine strata,
indicating that the alluvial strata are deposited during relative-sea-level
lowstands (Sweet and Soreghan 2012), which is the expected relationship
if global glacial periods lowered sea level and produced equatorial upland
glaciers.
IMPLICATIONS FOR ALLUVIAL-FAN FACIES MODELS
The Fountain Formation is a fairly well-documented fan-delta
depositional system (Suttner et al. 1984; Maples and Suttner 1990; Kairo
et al. 1993; Sweet and Soreghan 2010a; Sweet and Soreghan 2012). The
alluvial portions of fan deltas are characterized by alluvial fans that
prograde into a standing body of water; as such, the subaerial component
behaves similarly to alluvial fans (McPherson et al. 1987). Quaternary
alluvial fans are extremely variable, from large, river-dominated mega-
systems to smaller, fault-controlled systems. Despite this wide variability,
alluvial-fan facies models designed for stratigraphic records consist largely
of two endmembers, debris-flow-dominated and sheetflood-flow-dominat-
ed (Bull 1972; Blair and McPherson 1994). Debris-flow-dominated
alluvial fans commonly exhibit deposits with cobbles and boulders floating
in a sand–mud matrix (Blair and McPherson 1998; Berti et al. 1999),
whereas sheetflood-flow deposits are often crudely stratified and lack any
mud component (e.g., Blair and McPherson 1994). The MGSF are the
most dominant deposits in the distal alluvial strata of the Fountain
Formation, yet the character of this facies does not resemble either of the
two endmember-specific facies. Rather, the MGSF is best characterized as
a fine-grained debris-flow deposit. The intercalation with much coarser
deposits implies that the flows responsible for the MGSF deposits suggests
flow transformation related to water inflation. The argument presented
herein is most compatible with a humid fan-delta depositional model for
the lower and middle part of the Fountain Formation. The upper part of the
Fountain Formation contains calcic paleosols and is interbedded with
eolian deposits, lacks the MGSF, and is gradational with overlying eolian
Permian units. Relationships that are all consistent with the idea that the
upper part of the Fountain Formation unconformably overlies the older
Fountain strata and represents a drastic tectonic and climatic regime change
(Sweet and Soreghan 2010a; Sweet et al. 2015).
Both the Cutler and the Fountain depositional systems have been
inferred as proglacial deposition based on evidence other than presented in
this paper. Some modern ice-proximal subaerial flows can have grain-size
distributions similar to the MGSF reported herein (Lawson 1982). Systems
elsewhere that exhibit similar deposits to the MGSF reported may record
the proglacial realm, especially if corroborating facies are consistent with
such an interpretation.
CONCLUSIONS
Sedimentology of the MGSF contained in the Fountain and Cutler
stratigraphic sections is most consistent with a flow that underwent en
masse transport and freezing, such that these deposits are best classified as
cohesive fine-grained debris flows. However, the lack of cobbles and
boulders in the MGSF that are common to other intercalated facies
suggests that a cohesive-debris-flow interpretation must not fully
characterize the entire flow process since coarse-grained material should
be entrained and carried during each flow event. Results of competence
modeling indicate that a kaolinite-based slurry would need to be inflated
with water by 40 to 50% to account for the observed coarsest clasts
observed in the MGSF. Using a montmorillonite-based slurry, flows would
have been inflated with even more water, 60 to 70%, to account for the
observed grain-size distribution. Thus, abundant and persistent water must
have been available. Two tenable, yet not exclusive of each other, scenarios
are possible to account for the MGSF. The first is that repeated slope
failure involving a highly weathered profile and relatively unweathered
crystalline rock produced cohesive debris flows. Flow transformation of
cohesive debris flows into non-cohesive debris flows, slurries, and/or
hyperconcentrated flows allowed the coarse fraction to settle out of the
flow. Upon reaching the alluvial surface at the mountain front, the flow
became unconfined and dewatered, ultimately resulting in the fine-grained
debris flow deposit. The second scenario invokes upland glaciers. Advance
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 777
of glaciers provided a means to mix relatively chemically unweathered
feldspar and kaolinite from weathering profiles at lower elevations. Glacial
melt during retreat further provides a mechanism to dilute cohesive flows.
In both the Fountain and the Cutler formations, fine-grained debris flows
were an important product not accounted for in classic alluvial-fan facies
models, which might reflect the low-latitude and potentially glacial
inference of these depositional systems.
SUPPLEMENTAL MATERIAL
Stratigraphic columns for the Fountain and Cutler formations are available
from JSR’s Data Archive: http://sepm.org/pages.aspx?pageid¼229.
ACKNOWLEDGMENTS
The author would like to thank L. Soreghan for access to samples from the
Cutler Formation and for numerous manuscript discussions, C. Findlay and H.
Baird for lab assistance during disaggregation for grain-size analysis, and J.
Browning for producing the thin sections in the Fountain Formation. Reviews
by R. Langford, D. Le Heron, and an anonymous reviewer greatly improved
clarity and flow of the manuscript.
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Received 10 January 2017; accepted 1 June 2017.
FINE-GRAINED DEBRIS FLOWS IN A COARSE-GRAINED ALLUVIAL SYSTEMJ S R 779