STRUCTURAL CHARACTERISTICS OF COHESIVE GRAVITY-FLOW
DEPOSITS, AND A SEDIMENTOLOGICAL APPROACH ON THEIR FLOW
MECHANISMS
E.K. TRIPSANAS, W.R. BRYANT, D.B. PRIOR
Texas A&M University, Department of Oceanography, College Station, Texas, USA
Abstract
Abundant mass-transport deposits characterize the seafloor of the northwestern
continental slope and rise of Gulf of Mexico. Most of the deposits reveal a cohesive
nature, consisting mainly of a silty-clay matrix with dominant to rare mud-clasts. Five
types of cohesive gravity-flow deposits have been distinguished, based on their
structural characteristics, nature of the mud matrix, and mud-clast percentage. Each type
of the cohesive gravity-flow deposits implies different rheological behavior for the
flows from which they originate, depending on their viscosity, mud-clast abundance,
and external factors (e.g. hydroplaning), thus revealing that these deposits have resulted
from a wide range of different types of flows.
Keywords: Cohesive gravity-flow, debris-flow, mud-flow, self-lubricating layers
1. Introduction
Debris-flows are gravity-driven flows of highly concentrated sediment/water mixtures,
displaying diverse properties, behaviors, and depositional characteristics (Sohn, 2000).
The geometry and internal structure of debris-flow deposits are primarily determined by
the type of flow (Bingham vs. inertial grain flow, hydroplaning vs. nonhydroplaning
flows, etc.), nature (cohesive/noncohesive), concentration and grain-size of the sediment
particles, and the topography of the area in which they evolve and are eventually
deposited (Iverson, 1997; Sohn, 2000; Mulder and Alexander, 2001). Deposition of
debris-flows is caused by “freezing” of the flow when the applied shear stress falls
below a threshold value (yield stress) that leads to en masse deposition, initiated at the
front of the flow (Prior et al., 1984; Mulder and Cochonat, 1996; Papatheodorou and
Ferentinos, 1996; Elverhøi et al., 1997; Huang and Garcia, 1999). However, many
researchers argue that based on field observations and experimental data, it is more
probable that debris-flow deposits result from an amalgamation of deposits in
successive surges, occurring during a single event (Masson et al., 1993; Iverson, 1997;
Major, 1997; Sohn, 2000).
Subaerial and submarine debris-flows display very different characteristics and flow
mechanisms due to their propagation in different medias, the latter being able to flow
faster and for longer distances than the former (Mohrig et al., 1998, 1999; Mulder and
Alexander, 2001). Although extensive studies have been conducted on recent submarine
mass-transport deposits through seismic data and sediment core collection and analysis,
little is know about their internal structure that can be highly variable even in flows
consisting of similar sediments.
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The highly unstable sediments of the northwest continental slope of Gulf of Mexico
provide an excellent opportunity for the study of mass-transport deposits. This paper
presents the results of detailed sedimentological analysis on long sediment cores from
the continental slope (Bryant Canyon area), and rise (eastern Sigsbee Escarpment) of the
Gulf of Mexico (Fig. 1), in an attempt to provide a better insight into the structure and
flow properties of mass-transports.
Figure 1. Geomorphologic map of the northwest continental slope of Gulf of Mexico displaying the locations
of Bryant Canyon and eastern Sigsbee Escarpment areas.
2. Mass-transport deposits
Severe halokinetic processes interacting with sedimentological processes (mainly during
low sea-level stands) have resulted in the development of massive and numerous
sediment failures on the northwestern continental slope of Gulf of Mexico (Bryant et al.,
1990; Lee, 1990; Tripsanas et al., this volume). Fine-grained sediments of the area have
contributed to the cohesive nature of the mass-transport deposits, observed in our
sediment cores (Tripsanas et al., 2001). According to the existing literature, mud-flow
deposits are differentiated from debris-flow deposits by having less than 5% gravel by
volume and a ratio of mud to sand of more than 1:1 (Mulder and Alexander, 2001).
However, based on this distinction all the cohesive gravity-flow deposits of this study
would have been considered as mud-flows. This division leads to confusion considering
their differentiation and the determination of the rheological behavior of the flows that
led to their deposition. For this reason, we have redefined debris-flow deposits in this
study as those consisting of more than 5% per volume of mud-clasts. Seven types of
cohesive mass-transport deposits have been distinguished in the cores of this study, and
are described in detail in the following paragraphs.
2.1 THIN NORMALLY GRADED LAYERED DEBRIS-FLOW DEPOSITS
These deposits are characterized by small thickness (less than 1 m thick), and consist of
a uniform and soft mud-matrix with small, soft mud-clasts (usually 1 to 5 mm in
diameter), organized in faint, normally graded layers/bands (Fig. 2a). The mud-clasts
appear to be more abundant at the base of the layers/bands. The normally graded
layers/bands are interpreted as deposition of successive surges, during a single debris-
Structural characteristics of cohesive gravity-flow deposits
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flow event. In addition, the organization of the clasts in normally graded layers, and
their abundance at the base of the layers reveals that: 1) these flows were of low
viscosity, allowing for a slow settling of the clasts, and 2) the mud-clasts were not only
transported as floating clasts in a cohesive matrix, but probably in a bedload mode as
well. Large-scale debris-flow experiments support the idea of bedload clast
transportation by several successive surges, occurring during a single flow event (Major,
1997). The limited occurrence of those deposits in the sediment cores of this study
reveals that are probably local scale events, occurring as thin debris-flow carpets.
Figure 2. Image displaying X-ray radiographs (negative) and photographs of: a) a typical thin normally graded
layered debris-flow, and b) a typical mud-flow (MF) and mud-matrix dominated debris-flow (MDF) deposit.
Note the deformation of the mud-clasts in the lower part (below 426 cm) of the MDF. Bubble-like structures
and pillars in image b are interpreted as dewatering structures. HS= hemipelagic sediments, SL= slump
deposit, LBZ= laminated basal zone.
2.2 MUD-FLOW AND MUD-MATRIX DOMINATED DEBRIS-FLOW DEPOSITS
Mud-flow and mud-matrix dominated debris-flow deposits are examined together in this
section, because both of them originate from cohesive gravity-flows with similar
rheological properties. Both deposits range in thickness from a few decimeters to
several meters and consist of a chaotic mud-matrix, with small to large floated mudclasts (more than 5% per volume) in the mud-matrix dominated debris-flow deposits
(Fig. 2b). Lineations and convolute laminae are present in the mud matrix throughout
the entire thickness of the deposits, and are more pronounced in their lower parts. The
mud clasts in the mud-matrix dominated debris-flow deposits appear to be generally
ungraded. In many cases, the mud clasts tend to become fewer and more reworked
(shear-elongated) in the lower parts of the deposits. The most interesting features of
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these deposits is the presence of a basal zone (5 to 20 cm thick), that is characterized by
strong lenticular laminations with evidence of “necking” or boudinage, and shearelongated mud-clasts parallel to the lamination. Similar features have been observed by
Thorton (1984) and were interpreted as zones of layer by layer shear, produced at the
base of the flows where the strongest shearing is exhibited (Hampton, 1975). All of the
above observations indicate that remolding and shearing occurred throughout the entire
thickness of the mud-flows and mud-matrix dominated debris flows (the applied shear
stress was exceeding the yield stress of the bulk material of the flows throughout their
entire thickness). However, the shear laminated basal zone and the intensification of
lineations and deformation of the mud-clasts in the lower parts of the deposits indicate
that the applied shear stress and shear rates were highest at the base of the flows and
decreased gradually upwards in the flows (Iverson et al., 1997). Convolute laminations,
fault-like surfaces, thrust faults, microfaults, and imbricated slices are interpreted as
structures developed during the “freezing” of the flows, thus implying that the
solidification of the flows was initiated at their fronts and margins, and was later
transmitted backwards and internally into their main bodies (Prior et al., 1984; Huang
and García, 1999; Major and Iverson, 1999). Dewatering structures are common in these
deposits and mainly occur as bubble-like and pillar structures.
2.3 LAYERED MUD-MATRIX DOMINATED DEBRIS-FLOW DEPOSITS
Layered mud-matrix debris-flow deposits are similar to the mud-matrix dominated
debris-flow deposits described above, with the difference that they consist of successive
inverse graded layers (more than 1 m thick) characterized by: 1) an upper zone of a mud
matrix with abundant and large mud clasts, and 2) a lower zone of a dominant mud
matrix with much fewer and smaller mud clasts. The contacts between the zones, and
layers are gradational and/or uncertain. This information reveals that these deposits have
most probably resulted from the deposition of successive surges, occurring in a single
flow event, rather than the sudden “freezing” of the entire flow, as in the typical mudmatrix dominated debris flows. The surging of these flows may be related to the
hydroplaning of fast moving debris flows. Mohrig et al. (1998) stated that hydroplaning
causes the fronts of debris flows to accelerate away from their bodies to the point of
completely detaching from the bodies, thus increasing head velocity, producing surging,
and leading to the formation of arcuate ridges and amalgamated deposits (Laberg and
Vorren, 2000).
2.4 CLAST-DOMINATED DEBRIS-FLOW DEPOSITS
These deposits are characterized by the great abundance and dominance of mud clasts.
Their thickness ranges from a few decimeters to several meters. They consist of three
discrete zones (Fig. 3a, and 3b): 1) an upper plug-zone of interlocked well-rounded to
rounded mud clasts that usually preserve their primary syn-sedimentary structures, 2) a
middle remolded zone of chaotic mud matrix with more deformed and smaller mudclasts (usually they appear to be inverse graded and more deformed downcore), and 3) a
thin lenticular to parallel laminated basal zone, with evidence of “necking” and
boudinage, and may even contain a few shear-elongated mud-clasts. The upper plugzone is the thickest one, and usually occupies more than the half of the thickness of the
debris-flow deposit. Clast-dominated debris-flow deposits have most probably resulted
Structural characteristics of cohesive gravity-flow deposits
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from the deposition of Bingham flows, consisting of an upper plug zone in which the
yield stress was not exceeded, and an underlying zone where the shear stress exceeded
the yield stress (Johnson, 1970; Iverson et al., 1997). However, the presence of mud
clasts in the upper zone reveals that either episodically, or at least during the first stages
of the flow evolution, intense shearing was distributed throughout the entire thickness of
the flow, with an upward shear-rate decrease.
2.5 SLUMP DEPOSITS
Slump deposits were identified in the sediment cores as accumulated blocks,
characterized by a distorted/convoluted internal structure, which are separated from
each other by highly inclined to horizontal fault-like surfaces. Dewatering structures are
very common in these deposits and occur as dish, bubble-like, and pillar structures.
2.6 SHEAR-LAMINATED, AND CONVOLUTED THIN LAYERS
These deposits have a thickness ranging from a few centimeters up to 20 cm thick, and
are characterized by structures indicating intense shearing (Fig. 3c, and 3d). Shearlaminated thin layers consist of finely lenticular laminated mud with evidence of
“necking” or boudinage, and highly deformed and shear-elongated mud-clasts parallel
to the lamination. They usually occur as isolated layers interbeded in relatively
undisturbed hemipelagic sediments, without being connected with large overlaying
cohesive gravity-flow deposits. Their top and basal contacts are sharp and/or erosional.
Convoluted thin layers consist of highly convoluted/distorted mud, with siltier (silty
mud) bases that display thin silty laminae. Their top and basal contacts are sharp and/or
erosional. They usually occur as successive zones below thick (more than 1 m thick)
cohesive gravity–flow deposits. A possible explanation of the shear-laminated and
convoluted thin layers is that they may represent deformed seafloor sediments, caused
by the over-riding of gravity flows. However, in that case it would have been expected
that the distortion of the sediments would decrease with depth; that is not the case in
these layers. In addition the sharp bases of these layers, combined with the strong
shearing that they have been subjected to, reveal that they were parts of the cohesive
gravity-flows, rather than just deformed sea-floor sediments.
Cambell (1989), through numerical modelling of slumps/debris flows, proposed the
development of self-lubricating layers (thin basal layers of highly agitated particles of
low-concentration) over which debris-flows may run for long distances with little or no
erosion and distortion of the underlying seafloor sediments (Laberg and Vorren, 2000).
In addition, Gee at al. (1999) explained the long run out of Saharan debris-flow by the
presence of an undrained liquefied muddy volcaniclastic sand basal layer, produced by
the mobilization of volcaniclastic sediments by the passing over of the Saharan debrisflow. The interpretation that we propose for the convoluted and shear-laminated thin
layers is that they resulted from the depositional “freezing” of an almost liquefied, selflubricating, basal layer (active layer) of highly agitated and sheared sediments with high
pore pressures. The impermeable nature of the cohesive gravity flow deposits, observed
in our cores, supports the existence of such basal lubricating/active layers, by sustaining
their high pore pressures (undrained conditions). The generation of these layers is most
probably related to: 1) water entrained at the base of the flows, due to a possible
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hydroplaning at the head of the flows (Mohrig et al., 1998, 1999; Mulder and
Alexander, 2001), 2) liquefaction and mobilization of seafloor sediments with high
water-content (Elverhøi et al., 1997; Gee et al., 1999), 3) a preferential rheological
behavior of debris/mud-flows in order to compensate for the strong shearing at their
bases (Cambell, 1989; Iverson and LaHusen, 1989; Nemec, 1990), or 4) a combination
of the above. In any case, the presence of these layers at the bases of cohesive-gravity
flows would cause significant reduction of bottom drag and allow them to travel for
long distances. Their deposition is most probably caused by the significant reduction of
the shear stress and pore pressures, as they are left behind (due to their smaller
velocities compared to the rest of flow), while a new active/lubricating layer is produced
(regenerated) at the frontal part of the cohesive gravity-flows.
Figure 3. Image displaying X-ray radiographs and photographs of: a) a typical clast-dominated debris-flow
deposit (CDF) that is overlain on a sheared mud-matrix dominated debris flow deposit (MDF), b) the lower
part of a 3.1 m CDF, c) successive convoluted thin layers (CTL) under a transitional slump/debris-flow
deposit, and d) shear-laminated thin layers (SLTL) in relatively undisturbed hemipelagic sediments (HS). Note
that even though the hemipelagic sediments are eroded at the base of the SLTL, the preservation of intact
burrows indicates that they have not been sheared/deformed, by passing cohesive gravity flows. UPZ= upper
plug zone, MRZ= middle remolded zone, LBZ= laminated basal zone, SL= slump deposit, DF= debris-flow
deposit, LDF= normally graded layered debris-flow deposit.
According to the above interpretation, the presence of the siltier mud bases with silty
laminae, in the convoluted thin layers, can be justified by the elutriation of the finer
particles by escaping pore fluids. The successive convoluted layers below the
debris/mud flows are interpreted as: 1) imbricated slices, generated during the sudden
“freezing” of the frontal parts of the flows, and 2) deposition of the basal selflubricating/active layers of successive over-riding cohesive gravity-flows.
3. Conclusions
Five types of cohesive gravity-flow deposits have been recognized in the sediment cores
of this study: 1) thin normally graded layered debris-flow, 2) mud-flow, 3) mud-matrix
dominated debris-flow, 4) layered mud-matrix dominated debris-flow and 5) clast-
Structural characteristics of cohesive gravity-flow deposits
135
dominated debris-flow deposits. The first type is characterized by its relatively small
thickness (less than 1 m), a mud matrix with small (less than 0.5 cm) and soft mud
clasts, and a faint layering. The mud clasts reveal a normal grading and become more
abundant towards the base of each layer. That reveals that their deposition resulted from
several successive surges/pulses, developed in a single flow event, rather than the
sudden “freezing” of the whole flow. The main difference between mud-flow and mudmatrix dominated debris-flow deposits is the presence of small to large mud-clasts in the
latter (more than 5 % per volume). Both deposits consist of a chaotic mud-matrix
(revealing shearing throughout the entire flow), and a basal shear laminated zone, where
the strongest shearing of the flow was exhibited. Convolute laminations, fault-like
surfaces, thrust faults, microfaults, and imbricated slices are interpreted as structures
occurring during the “freezing” of the flows, whereas microfaults and folds could have
also been caused by adjustments (back-sliding) of the deposits. Layered mud-matrix
dominated debris-flow deposits consist of successive layers of chaotic mud-matrix with
inversely graded mud-clasts, and have probably been resulted from the deposition of
surging debris-flows. Clast-dominated debris-flow deposits consist of three zones: a) an
upper plug-zone characterized by large interlocked clasts, b) a mid-zone of highly
deformed, inversely graded clasts, floating in a mud-matrix, and c) a lower shear
laminated zone. The structure of the last type of cohesive gravity-flow deposits indicate
that they represent deposition of typical Bingham flows, consisting of an upper plugzone in which the yield stress is not exceeded and an underlain shearing zone, where the
shear stress exceeded the yield strength of the sediments.
Shear-laminated and convoluted thin layers (5- to 20 cm) with sharp bases, displayed as
successive layers at the base of mud/debris flow deposits, or as isolated depositional
units interbedded in hemipelagic sediments, are as interesting as they are enigmatic.
They are interpreted as basal self-lubricating (active) layers, of highly agitated, sheared
sediments with high pore pressures, over which debris/mud-flows were able to travel for
very long distances.
4. Acknowledgments
This project was sponsored by a National Science Foundation grant to Texas A&M (No.
BES-9530370) with supplemental support from Chevron, Mobil, Texaco, Phillips,
Marathon, and MARSCO Inc.
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