ROLE OF PORE-FLUID PRESSURE AND SLOPE ANGLE IN TRIGGERING
SUBMARINE MASS MOVEMENTS: NATURAL EXAMPLES AND PILOT
EXPERIMENTAL MODELS
B. C. VENDEVILLE
Bureau of Economic Geology, The University of Texas at Austin, Box X, Austin, TX
78713-8924, USA
V. GAULLIER
Laboratoire de Sédimentologie Marine, Université de Perpignan, 52, avenue de
Villeneuve, 66860 Perpignan, France
Abstract
We illustrate and compare several natural examples of recent submarine mass
movements along the NW Mediterranean margin and discuss the respective role of the
main parameters controlling slope failures, including slope angle and high pore-fluid
pressure. We then present results of pilot experiments testing how an increase in porefluid pressure can trigger spontaneous downslope gliding of sediments along the
margin’s slope.
Keywords: Submarine mass movements, pore-fluid pressure, slope, physical modelling
1. Introduction
Slope instabilities are major sediment-transport processes contributing to the
construction of the continental slope and rise. Their role is particularly important in
submarine deep-sea fans, where mass-movement generated by cohesive flows (debris
flows) are commonly interbedded with sediments transported by non-cohesive flows,
such as turbidity currents. The initiation of such flows and the distribution of associated
deposits mostly depend on (1) changes in internal physical parameters (e.g., rheological
properties, liquefaction, diagenesis, differential compaction, fluid expulsion) and (2)
external parameters, such as seismicity, tectonic setting, slope parameters (slope
geometry, changes in slope gradient) or differential sediment loading.
In this article, we first describe and compare several natural examples of recent
submarine mass movements along the Mediterranean margins and try to identify the
respective role of the main parameters controlling slope failure. Second, we briefly list
the various parameters that can favour or trigger the development of slope instabilities.
Third, we present results of a series of pilot physical experiments that tested the
influence of the slope angle and the pore-fluid pressure on the initiation of slope failure
and subsequent mass movement of sediments located on a continental passive margin.
2. Natural examples of submarine mass movements in the NW Mediterranean
Numerous mass movement deposits, whose geometry, size, age and nature vary, have
shaped the continental slope and rise of the Mediterranean basin. Huge debris flows are
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Figure 1. A. Transparent echo-character map from 3.5 kHz echogram profiler illustrating the distribution of
main recent mass-movement deposits in the NW Mediterranean. 1: Partially transparent echo character. 2:
Wholly transparent echo character 3: Buried, transparent echo character (modified from Gaullier and
Bellaiche, 1998). B. Examples of recent mass-flow deposits along the inner wall of the Petit-Rhône canyon
(right-hand side) and in the thalweg (Gaullier, 1993). C. Isopach map (m s-1. TWT) from high-resolution
seismic data of a huge, partly buried debris flow located on the western levee of the Rhone deep-sea fan
(Gaullier et al., 1998). The total volume of displaced sediments is about 217km3.
Mass movements in nature and experiment
139
especially present on the east and west sides of the main channel of the Rhone deep-sea
fan (Gaullier et al., 1998) and in the west and central provinces of the Nile deep-sea fan
(Loncke et al., 2002). Smaller, gravity-driven sediment slumps exist in the entire
Mediterranean region, on the shelf break, the canyons inner walls or on the interfluve
areas. Sub-bottom echoes recorded on 3.5 kHz echo-sounders in the NW Mediterranean
have imaged the distribution of areas having transparent echo characters (Fig. 1A,
Damuth et al., 1980; Gaullier and Bellaiche, 1998) corresponding to mass-movement
deposits of various ages also observed on multi-beam bathymetric and high-resolution
seismic data (Droz, 1983; Coutellier, 1985; Droz and Bellaiche, 1985, Droz et al.,
2001). The youngest and largest mass-movement deposit forms a 170km-wide and
160m-thick, lens-shaped tongue located east of the main channel of the Rhône deep-sea
fan (Figure 1A; Bellaiche et al., 1990). This feature is a large debris flow resulting from
the destabilization of the east levee of the Petit-Rhône canyon and of the inter-canyon
area between the Petit-Rhône and the Grand-Rhône canyons (Droz, 1983), and its
provenance area is marked by arcuate scarps on the continental slope (Droz, 1983, Droz
and Bellaiche, 1985, Bellaiche et al., 1990). This area displays another large
allochtonous body, the western debris flow, located west of the Rhône deep-sea fan's
main channel (Fig. 1C). This unit, either surficial (Fig. 1A) or buried (interbedded
within the sedimentary column), is seismically transparent, and has an estimated volume
of 217 km3. It is sealed by silty-sandy strata dated from 5 Ky, indicating a setting period
between 21 and 5 Ky years.
In addition, numerous mass-gravity flows are common in the unstable areas of the
continental slope and in canyon thalwegs, such as the slide illustrated in Figure 1B and
located in the upper part of the Petit-Rhône canyon, at the base of the canyon's NE
flank. On the continental rise, transparent bodies also exist at the base of scarps (e.g.,
associated with salt diapirs) subjected to avalanche processes.
In the study area, combined high sedimentation rates, steep slopes, sea-floor
deformation related to salt tectonics, and fluid overpressure have made siliciclastic
sediments unstable and promoted formation of slumps, slides and debris flows whose
distribution is widespread in both space and time. Overall, mass-flow deposits in the
NW Mediterranean cover larger areas and are thicker than in other basins, a
characteristic of the vigorous sediment remobilization that started as early as Pliocene
times and increased during the Quaternary, especially during glacial-eustatic low-stands.
From a hydrocarbon-exploration perspective, distinguishing domains of slumps, slides
and debris flows from those of classic turbidites, has direct implications on reservoir
geometry. Turbidity currents commonly generate continuous, sheetlike sand bodies. In
contrast, slumps, slides and debris flows typically lead to discontinuous and
compartmentalised sand reservoirs (Shanmugam et al., 1995).
3. Parameters contributing to development of slope instabilities
Parameters that can favour or trigger slope instabilities include the angle of the slope
margin, the basement architecture, sediment loading, seismic activity, rapid sea-level
changes, sea currents, and high pore-fluid pressure.
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In this article, we focus on the role of fluid pressure. High pore-fluid pressure can be
generated by several processes acting either combined or in isolation (Mukerji et al,
2002). The two most common mechanisms are hydrocarbon generation and
disequilibrium compaction. Hydrocarbon generation increases the fluid volume while
compaction and diagenesis can decrease permeability and porosity, which therefore can
lead to an increase in pore-fluid pressure (Mukerji et al., 2002). Because the
hydrocarbon-generating window depends on the depth of burial, fluid pressure depends
on the rate at which the strata are loaded under newly deposited sediments. Clay
dehydration and diagenesis, another depth-sensitive process, can also lead to
overpressure (Dutta, 1984). Finally, where sediments have a high porosity and low
permeability, it is possible that changes in the balance between lithostatic stress and
pore-fluid pressure, for example associated with loading or unloading of the strata due
to rapid sediment accumulation or sea-level change, may cause temporary overpressure.
The mechanical effect of high fluid pressure is to greatly weaken the strata, thereby
allowing the overlying rocks to move over considerable distances under the effect of
gravity alone (Hubbert and Rubey, 1959). In the following section, we illustrate the
effect of this effect using experiments designed to replicate this process in the
geological setting of slope instabilities developing along passive continental margins.
4. Deformation associated with slope and fluid overpressure: Experimental
examples
The goal of our experimental work was modest and consisted of testing whether a brittle
overburden (representing sediments resting on a continental slope) could glide
spontaneously if the underlying layers were subjected to high pore-fluid pressure. We
conducted two suites of experiments using the same basic set up (Fig. 2). The models
comprised a thick (i.e., 5-10 cm) layer of high-porosity, high permeability sand,
representing the overpressured basal layer in nature, overlain by a thinner (0.5-1 cm)
sealing layer made of either wet clay (a plastic material) or viscous silicone polymer. An
upper sand layer (representing the overburden sediments) whose thickness varied from 2
to 8 cm between experiments overlay the sealing layer. The basal sand layer was
connected to two pipes allowing application and monitoring of the pore-fluid pressure
within this layer. In some models, the pore fluid was air as in experiments by Cobbold
and Castro (1999) and Cobbold et al (2001). In others, the pore fluid was water. In all
experiments, the model was tilted by a few degrees (3-6 degrees), then the fluid pressure
in the basal layer was progressively increased until the sealing layer and the upper sand
layer started to glide downslope, indicating that the increase in pore-fluid had
effectively weakened the basal sand layer enough so that it behaved as a decollement.
Figure 2. Set ups for experiments of fluid-assisted downslope gliding. A: models using air as pore fluid. B:
models using water as pore fluid.
Mass movements in nature and experiment
141
4.1. EXPERIMENTS USING AIR AS PORE FLUID
In these experiments, all layers remained dry throughout the entire model evolution. For
obvious practical reasons, we used viscous silicone, rather than wet clay, as a seal
between dry sand layers. Several layers of coloured sand representing the uppermost
sediment layers overlay the silicone seal. After tilting the model by a few degrees, the
air pressure within the basal sand layer was gradually raised until the upper half of the
model (i.e., the silicone seal and upper sand layer) glided downslope. During gliding,
shortening was accommodated downslope by thrust faults and folds, whereas upslope
extension was accommodated by normal faults (Fig. 3). It is worth noting that gliding
was a very rapid process that typically lasted between a few seconds and 2-3 minutes,
suggesting that deformation occurred by shear within the lower sand layer, rather than
by shear of the viscous silicone layer. There have been many previous experiments in
which a viscous silicone layer was used as an analogue for a weak evaporitic
decollement (e.g., Vendeville, 1987). In such models, gliding took place by simple shear
of the viscous decollement, a viscosity- and time-dependent process that typically lasted
a few hours. In our new models, gliding occurred within a few seconds because the
motion took place by shearing the overpressured, lower sand layer, rather than the
viscous seal. As gliding progressed, stretching of the silicone seal and overlying upper
sand layer in the extensional, upslope area eventually broke the seal, allowing fluid
escape from below, reducing the pore-fluid pressure in the basal layer, and ending
deformation.
Figure 3. Overhead photograph of an air-pressurised model after deformation. The silicone seal and upper
sand cover glided downslope (right-hand side), forming thrust faults and buckle folds downslope and normal
faults upslope. Surface strains were recorded by deformation of a grid of 1.5cm x 1.5cm square passive
markers. Thin arrows are displacement vectors in selected areas.
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4.2. EXPERIMENTS USING WATER AS PORE FLUID
The main difference with the above experiments is the role of lateral gradients in porefluid pressure and lithostatic stress. Unlike air, whose density is negligible, water has a
density of about half or a third of that of the model materials (dry sand, silicone, or wet
clay). Consequently, a lateral change in the height of water column (∆h in Fig. 2) can
affect the mechanics of the system. In this set of models, the pores of the lower sand
layer were saturated with water. A thin (0.5 to 1 cm) of wet clay or viscous silicone
polymer was deposited on top of the lower sand layer and acted as a seal. The seal was
overlain by an upper sand layer (the cover). Then the model was immersed in water that
filled the pores of the upper sand layer. Under reasonably low deformation rates (i.e., <
1 m/s), the rheological properties of water-saturated sand are identical to those of dry
sand (air-saturated). The model was tilted slowly by a few degrees, causing the water
depth to vary from about 5cm upslope to more than 20cm downslope (Fig. 2B). The
water column in the lower sand layer was higher upslope than downslope, resulting in a
downslope increase in pore pressure. However, this lateral pressure gradient was exactly
compensated for by a similar downslope increase in water column above the upper sand
layer. The effects of increase in lithostatic stress caused by the increase in water depth
and downslope increase in fluid pressure in the lower sand layer effectively cancelled
out one another. The pore-fluid pressure in the lower sand layer then was gradually
increased until the model spontaneously deformed by downslope gliding.
Figure 4. Details of cross sections in water-pressurised models illustrating a fluid-escape structure triggered by
local extension of a wet-clay seal and upper sand layer. Extension broke the clay seal, allowing the pore-fluid
in the basal sand layer to escape upward. Because a black dye was mixed with the pore fluid, darker patches
on the section have recorded the part of the upper sand layer affected by fluid escape.
Mass movements in nature and experiment
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Results varied depending on the type of sealing layer used in the experiment. In models
using a wet-clay seal, only local slides occurred, presumably because although the
downslope increase in water depth did not affect the mechanical properties of the upper
sand layer, it increased the differential stress required to deform the impermeable clay
layer, making it effectively stronger. Where local slides occurred, both clay and upper
sand layers failed locally in extension, thereby breaking the clay seal and providing
conduits for the overpressured fluid to escape upward through local vents (Fig. 4).
Deformation of models using a viscous silicone seal was similar to that of models using
air as a pore fluid. The entire slab glided rapidly downslope, causing downslope
shortening and upslope extension.
5. Conclusions and perspectives
The experiments presented in this article represent the first step in simulating the
initiation of slope instabilities triggered by high pore-fluid pressure. Results clearly
indicate that an increase in fluid pressure can effectively trigger instantaneous gliding of
sediment strata downslope. In the experiments, the gliding cover deformed as a coherent
slab, partly because inertial forces were negligible, thus preventing turbulent flow. In
nature, following the initial disruption caused by fluid overpressure, such detached units
may evolve into coherent or turbulent instabilities travelling dowmslope over greater
distances.
6. Acknowledgements
This work was funded by the ongoing French research project "Gravity Instabilities", a
part of the GDR "Marges". The experimental work was conducted at the Applied
Geodynamics Laboratory, Bureau of Economic Geology, funded by the consortium of
following companies: Anadarko, BHP Petroleum (Americas), BP, Burlington, ChevronTexaco, Conoco, ENI-Agip, Enterprise, ExxonMobil, Marathon, PanCanadian,
Petrobras, ConocoPhillips, Shell, TotalFinaElf, Unocal-Spirit, and Woodside.
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