Origin of Overpressure and Slope Failure
in the Ursa Region, Northern Gulf of Mexico
B. Dugan and J. Stigall
Abstract Sedimentation-fluid flow-stability models predict the evolution of
overpressure (pressure above hydrostatic) and stability of sediments in the Ursa
region, northern Gulf of Mexico (IODP Expedition 308). Two-dimensional
models across the Ursa region from 65–0 ka simulate overpressure up to
2 MPa in the shallow subsurface at Site U1324 and near 1 MPa at Site U1322,
which is consistent with measurements. Overpressure history is controlled
by sedimentation rate and distribution. Today overpressure is decreasing, but
past high sedimentation periods were important overpressure sources. Infinite
slope stability analyses indicate the slope is stable. Overpressure increase and
stability decrease is most prevalent along the boundary between mud-rich mass
transport deposits and silt-rich channel-levee deposits between Sites U1323 and
U1322. This is controlled by high overpressure driving fluids laterally along
permeable sediments to where overburden is thin. Even with high sedimentation
rates at Ursa and lateral flow, unstable conditions are not simulated. Seismic
and core observations, however, document recurring slope failures in the mudrich deposits. We conclude that sedimentation-driven overpressure and lateral
transfer of pressure created a marginally stable slope, but additional driving
forces, such as extremely high sediment inputs or seismic accelerations created
by salt tectonic-related earthquakes were required to initiate failure.
Keywords Mass transport deposit • overpressure • slope stability • Gulf of
Mexico • basin modeling • IODP 308
B. Dugan () and J. Stigall
Department of Earth Science, Rice University, 6100 Main Street,
MS 126, Houston, TX 77005 USA
e-mail: dugan@rice.edu
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
167
168
1
B. Dugan and J. Stigall
Introduction
Mass transport deposits (MTDs) are the sedimentary record of marine sediments
that have been remobilized, transported downslope, and redeposited. These records
occur worldwide, can be created by a variety of mechanisms including turbidity
currents, debris flows, submarine landslides, or creep (Hampton et al. 1996), and
occur in a range of sizes and a variety of slopes (McAdoo et al. 2000). The type and
size of slope failure and resulting MTD are partly dependent on the initial
sedimentary conditions (e.g., slope, porosity, lithology) and external forces (e.g.,
overpressure, earthquake shaking).
The Ursa region is an area of active hydrocarbon exploration and is known to
have MTDs and shallow overpressure (pressure above hydrostatic) (Ostermeier
et al. 2000). Integrated Ocean Drilling Program (IODP) Expedition 308 used
the region as a hydrogeology study area to investigate the origin of shallow
overpressure and its influence on slope failure. The region served as a direct test
of the flow focusing hypothesis that lateral fluid flow in permeable layers is
controlled by rapid, asymmetric loading by low permeability sediments and this
could create low effective stress and slope instability where overburden is thin
(Dugan and Flemings 2000).
We present two-dimensional, forward models of the Ursa region that couple
sedimentation, overpressure generation, fluid flow, and slope stability. The models
use documented lithologic, hydrologic, and strength properties. We use models to
study sedimentation histories and explore how they affect overpressure generation
and stability at Ursa. We conclude that flow focusing affects overpressure, but
additional pressure sources or seismic accelerations are required to initiate failures
in the Ursa region.
2
Ursa Region
The Ursa region is located 210 km offshore New Orleans, LA (USA) on the continental slope in the northern Gulf of Mexico (Fig. 1). This region is on the eastern
flank of Mississippi canyon, has shallow overpressures, slope failures, and low
tectonic activity (e.g., Winker and Booth 2000; Ostermeier et al. 2000). This region
has extensive seismic, log, and core data from industry-related drilling, shallow
hazard studies, and IODP Expedition 308. Based on continuous, seafloor-parallel
seismic reflections from 0–30 m below seafloor (mbsf) and age control at Sites
U1322 and U1324 (Flemings et al. 2006), we interpret the seafloor has been relatively stable over the last 10 ka.
Core, log, and seismic data have been used to characterize in situ overpressure
(Flemings et al. 2008), bulk physical properties (Long et al. 2008; Flemings et al.
2006), and distribution and lithology of channel-levee deposits and MTDs (Sawyer
et al. 2007). Direct pressure measurements and laboratory experiments show that
overpressure starts near the seafloor and generally increases to the top of the Blue
Origin of Overpressure and Slope Failure in the Ursa Region
169
Fig. 1 (a) Bathymetric map locating Ursa in the northern Gulf of Mexico. (b) Bathymetric map of the
Ursa region. (c) Interpreted cross-section based on seismic data from the Ursa region (Flemings et al.
2006). Mud indicates hemipelagic mud and MTD indicates mass transport deposits that are mud-rich.
Blue unit is interbedded sand (yellow) and hemipelagic mud. Ursa Canyon and Southwest Pass Canyon
are channel-levee deposits associated with silt. Surface between base of MTDs and channel-levee
deposits (red line) is surface along which factor of safety is characterized (Section 4.2; Fig. 4).
unit (Flemings et al. 2008; Dugan and Germaine 2008). These overpressures range
from 50–70% of the hydrostatic effective stress. For slope failure to occur due to
overpressure alone, overpressure must exceed 93% of the hydrostatic effective
stress (Flemings et al. 2008).
High sedimentation rates are the primary source of overpressure in the Ursa
region. When deposition of high porosity, low permeability mud occurs at rates
greater than ∼1 mm/y, overpressure generation is common (e.g., Gibson 1959). In
the Ursa region, sedimentation rates were consistently above 1 mm/y and exceeded
170
B. Dugan and J. Stigall
10 mm/y during some periods (Fig. 2). At Site U1324, the average sedimentation
rate over the last 65 ka is 9.3 mm/y; at Site U1322 the average sedimentation rate
is 3.6 mm/y (Flemings et al. 2006). This difference in sedimentation rate has created an asymmetric, mud-dominated sediment wedge overlying the sand of the
Blue unit (Fig. 1c).
MTDs within the Ursa region have recurred through the burial history of the
Blue unit (e.g., Urgeles et al. 2007). MTDs appear to be confined to the mud-rich
sediments that overlie the silt-rich channel-levee deposits (Fig. 1c). The mud
between Sites U1323 and U1322 are dominated by MTDs from the top of the
channel-levee deposits to 85 mbsf (meters below sea floor). This includes one large
MTD that extends across the entire Ursa region from 100–160 mbsf (Site U1324)
to 100–195 mbsf (Site U1323) to 85–125 mbsf (Site U1322) (Fig. 1c). This MTD is
underlain by channel-levee sediments at Sites U1324 and U1323 and stacked
MTDs at Site U1322 (Fig. 1c).
3
Basin Modeling
We use a two-dimensional, sedimentation-fluid flow model to simulate pressure,
consolidation, stability, and temperature evolution of the Ursa basin. The system is
modeled using Basin2 (Bethke et al. 1993, 1988) (Eq. 1).
fb f
∂P ∂ ⎡ k x ∂P ⎤
∂ ⎡ k z ⎛ ∂P
1 ∂f
∂T
⎞⎤
=
+
− rw g⎟ ⎥ −
+ fa
⎜
⎠ ⎦ 1 − f ∂t
∂t ∂x ⎢⎣ m ∂x ⎥⎦ ∂z ⎢⎣ m ⎝ ∂z
∂t
(1)
f is porosity [m3/m3], bf is fluid compressibility [1/Pa], P is fluid pressure [Pa], t is
time [s], x is horizontal distance [m], z is vertical distance [m], kx is horizontal
permeability [m2], kz is vertical permeability [m2], rw is fluid density [kg/m3], m is
water dynamic viscosity [Pa·s], g is acceleration due to gravity [m/s2], a is water
thermal expansion coefficient [1/oC], and T is temperature [oC]. Equation (1)
describes changes in fluid pressure due to horizontal/vertical Darcy flow, pore volume changes, and thermal expansion of water. All simulations have no fluid flow
lateral/basal boundaries and a hydrostatic seafloor. The east and west boundaries
are located >5 km from Sites U1324 and U1322 to minimize boundary effects. Each
simulation begins with a 750-m-thick, hydrostatic mud layer. The Blue unit is
deposited on this unit. This is followed by erosion of Ursa canyon and deposition
of the Ursa channel-levee complex. Next we erode the Southwest Pass Canyon and
subsequently deposit its channel-levee complex (Fig. 1c). The last stage is deposition of MTDs and hemipelagic mud (Fig. 1c).
We model the Ursa region (Fig. 1c) with three lithologies: mud; silt; and sand.
Sand interbedded with mud is used to create the permeable Blue unit. Silt is deposited to create the Ursa Canyon and Southwest Pass Canyon channel-levee systems.
These systems are capped with mud for the MTDs and hemipelagic mud. Sediments
are deposited following the age constraints to recreate the observed basin geometry.
We independently define the physical properties of each lithology.
Origin of Overpressure and Slope Failure in the Ursa Region
171
Consolidation of sediments is modeled as an exponential relation between
porosity (f) and vertical effective stress (sv’ [Pa]) (e.g., Rubey and Hubbert 1959).
f = fo e − bs v + firr
'
(2)
fo is a reference porosity [m3/m3] and firr is the irreducible porosity [m3/m3], such
that fo plus firr is the depositional porosity. b is the bulk sediment compressibility
[1/Pa]. Only a portion of primary consolidation is elastic, so we assume unloading
compressibility (bul) is 10% of bulk compressibility (bul = 0.1b). We use data from
consolidation experiments (Long 2007; Long et al. 2008) to constrain fo, firr, and b
(Equation 2) for mud and silt (Table 1). For sand, we assume a low bulk compressibility to maintain a near-constant porosity reflecting the fine-to-medium sand at
Ursa (Table 1).
Intrinsic permeability for mud and silt is based on data from constant-rate-ofstrain consolidation experiments. We follow the approach of Mello et al. (1994),
log(k x ) = Af + B
(3)
Schneider et al. (2008) characterized the empirical coefficients A [log(m2)] and B
[log(m2)] for mud and silt from the Ursa region (Table 1). We assume mud and silt
have permeability anisotropy (horizontal/vertical, kx/kz [m2/m2]) of 100 based on
the permeability-layering model of Schneider et al. (2008). No permeability data
exist for sand from Ursa, therefore we assign a constant, isotropic permeability of
1 × 10−12 m2 for the sand (Table 1). This is representative of a fine-to-mediumgrained sand (e.g., Freeze and Cherry 1979), which is consistent with the sand in
the Blue unit.
For a simple assessment of slope stability evolution in the Ursa basin, we calculate the factor of safety (FS) using an infinite slope approximation (e.g., Lambe and
Whitman 1969; Dugan and Flemings 2002).
FS =
c '+ [(s v − rw gz ) cos2 q − P * ]tan ff'
(4)
(s v − rw gz )sin q cos q
Factor of safety (FS) > 1 represents a stable slope and FS ≤ 1 represents an unstable
slope. Sediment cohesion for effective stress (c’ [Pa]), total vertical stress (sv [Pa]),
hydrostatic pore pressure (rwgz [Pa]), seafloor slope angle (q[°]), angle of internal
friction for effective stress (ff ’ [°]), and overpressure (P* [Pa]) are required to
Table 1 Sediment properties used in Ursa region models. fo, firr, and b define the consolidation
parameters (Eq. 2). A and B define the horizontal permeability (kx) function (Eq. 3) and kx/kz
defines the horizontal-vertical permeability ratio (anisotropy).
Lithology
Mud
Silt
Sand
fo[m3/m3]
0.4
0.4
0.3
firr[m3/m3]
0.2
0.2
0.1
b[1/Pa]
−7
3.5 × 10
3.5 × 10−7
5 × 10−8
A [log(m2)]
B [log(m2)]
kx/kz
9.00
9.19
0.00
−20.16
−17.77
−12.00
100
100
1
172
B. Dugan and J. Stigall
assess stability. Total vertical stress, hydrostatic pore pressure, and overpressure
are calculated in the sedimentation-consolidation model (Equation 1). We assume
cohesion is 0 and angle of internal friction is 26° for effective stress conditions
based on shear strength experiments on sediments from the Ursa region (Dugan
and Germaine 2009). We use a constant seafloor slope angle of 2° based on the
regional seafloor gradient.
The infinite slope analysis is appropriate for addressing stability of slopes where
failure can be considered in one dimension, such that the failure is thin, parallel to
the seafloor and edge effects can be neglected wide. This approximation provides
a reliable stability calculation for regional, thin slope failures similar to some failures in the Ursa region. This simple assessment can identify potentially unstable
regions that may require more advanced failure analysis, such as deep-seated rotational failures (e.g., Bishop 1955). Thus our analysis provides an initial stability
calculation and identifies regions that warrant detailed stability calculations.
4
Results and Discussion
We completed a suite of models for the Ursa region with various sedimentation
histories that recreate the time-averaged sedimentation rates for Sites U1324 and
U1322 and produce the basin architecture documented by seismic and log data.
We present details of two sedimentation histories (Fig. 2) that capture the general
behavior of the system as well as the important details of sedimentation history.
Fig. 2 Sedimentation rates for Sites U1324 (a) and U1322 (b). Solid lines (Observations = Obs.)
are from biostratigraphic markers (Flemings et al. 2006). Dashed (Model 1 = Mod. 1) and dotted
(Model 2 = Mod. 2) lines are two modeled sedimentation histories at Sites U1324 and U1322 used
to recreate the observed basin architecture (Fig. 1) while maintaining the time-averaged rates.
Model rates match observations before 55 ka.
Origin of Overpressure and Slope Failure in the Ursa Region
173
We focus on high sedimentation from 37–16 ka because this period has the best age
constraints and corresponds with MTDs in the Ursa region. Sedimentation rate variations during this period likely changed due to changes in sediment discharge from
rivers and channel switching/migration. For this study, we have not evaluated the
high sedimentation period near 50 ka at Site U1324, which is poorly constrained
and is difficult to correlate across the basin. The transient basin models provide
estimates of fluid pressure, effective stress, and stability throughout the evolution
of the basin. We present two aspects of the basin model results: (1) modern-day
overpressure and (2) evolution of basin stability.
The Ursa region has overpressure that begins near the seafloor and extends to the
Blue unit (Fig. 3) (Flemings et al. 2008; Dugan and Germaine 2008). Site U1324
has overpressure that increases to 1 MPa by 200 mbsf; below this overpressure is
approximately constant to the top of the Blue unit (Fig. 3a). For our basin models,
we predict the modern-day overpressure increases to 1 MPa by 200 mbsf, but continues to increase to a maximum <2 MPa at 400 mbsf, and then is constant to the
top of the Blue unit (Fig. 3a).
At Site U1322, overpressure increases to ∼1MPa by 200 mbsf, however the
deepest pressure measurement shows an abrupt pressure decrease (Fig. 3b). This
pressure decrease may reflect pressure loss due to hydrocarbon exploration in the
region over the last 15 years (Flemings et al. 2008). Sedimentation Model 1 and
Model 2 (Fig. 2) both match the observed pressures at Site U1322 well with overpressure reaching 1–1.25 MPa at 250 mbsf, just above the Blue unit. We are unable
to recreate the pressure decrease just above the Blue unit.
Overall our basin model pressure predictions are consistent with the measured
pressures at Ursa (Fig. 3). Even though Model 2 has higher sedimentation rates than
Fig. 3 Measured and simulated overpressure at IODP Sites U1324 (a) and U1322 (b).
Observations (Obs., solid circles) are based on penetrometer data (Flemings et al. 2008). Model 1
(Mod. 1) and Model 2 (Mod. 2) are the simulated overpressure profiles based on two separate
sedimentation histories shown in Fig. 2.
174
B. Dugan and J. Stigall
Model 1 until 20 ka, both models are good matches to the shallow data at Sites
U1322 and U1324 (Fig. 3). While the modeled pressure histories are different, the
lower sedimentation rates from 20 ka to present (Model 2) or from 16 ka to present
(Model 1) and the relatively high porosity and high permeability of the shallow
sediments facilitate dissipation of fluid pressure to the observed conditions.
Therefore low sedimentation rates have helped increase effective stress along the
slope and modern, low sedimentation rates will allow this process to continue.
At Site U1324, we overpredict pressures below 200 mbsf (Fig. 3a). This may be
attributed to drainage by higher permeability sand layers interbedded in the silt of
the channel/levee systems below 200 mbsf. Thin sand layers were documented on
IODP Expedition 308 (Flemings et al. 2006) however we neglected these layers in
our model because they are at a scale finer than our model layers. If these layers are
interconnected, they could explain the observations. Future models could test the
importance of thin sand layers. We also overpredict the deepest pressure observation at Site U1322 (Fig. 3b). This pressure low may be related to pressure dissipation in mud due to pressure depletion in sands of the Blue unit from industry wells.
We did not simulate drilling-related pressure depletion so were not able to recreate
this decrease.
We track the stability of the Ursa region (Equation 4) to understand the relation
between sediment loading, fluid flow, and slope failure. We present general stability
results along the boundary between the MTDs and the channel-levee deposits (red
surface, Fig. 1c). We focus on this surface because it is the surface upon which
MTDs started to form in the Ursa region. For all Ursa simulations we observe three
key stability results: (1) decreased slope stability during periods of high sedimentation with minimum stability near the end of a rapid sedimentation period; (2)
decreased stability east of Site U1323 and west of Site U1322; and (3) FS > 1 for
a range of sediment properties and sedimentation rates constrained by Ursa data.
The simulated decrease in FS associated with sediment loading is controlled by
increases in overpressure generation and lateral flow along the Blue unit. While the
sand in the Blue unit allows rapid pressure equilibration, the silt that comprises the
channel-levee deposits is also important to the pressure distribution and stability.
The top of the silt-rich channel-levee deposits is important to basin stability because
the mud-prone sediments above it are deposited rapidly; overpressure generation is
rapid, but overburden is thin so the total stress is low. These features combine to
lower FS (Equation 4). This suggests that lithologic contrasts and pressure transients are key features to controlling slope failure in the Ursa region. Similar
decreased stability due to pressure transients has been interpreted for subaquatic
slopes of Lake Lucerne based on observations and measurements (Stegmann et al.
2007). Hubbert and Rubey (1959) also suggested pressure transients were critical
for decreasing stability and initiating slip on faults.
Based on our models, we predict the lowest FS exists east of Site U1323 and
extends toward Site U1322. This is controlled by lateral flow along permeable sand
and silt from areas of thick sediment loading to regions of thin overburden.
Qualitatively this is consistent with more MTDs between Sites U1323 and U1322
Origin of Overpressure and Slope Failure in the Ursa Region
175
Fig. 4 Factor of safety (FS) along the MTD basal boundary (red surface Fig. 1c) at 24, 20, 16,
and 0 ka. Dashed lines locate Sites U1324, U1323, and U1322 (Fig. 1). (a) Predicted FS for Model
1 (Fig. 2) shows stable conditions (FS > 4) with minor temporal variation. (b) Predicted FS for
Model 2 (Fig. 2) shows FS decrease toward 1.5; stability increases from 20 to 0 ka. FS scales differ
between (a) and (b).
in the Ursa region (Fig. 1c). Quantitatively, our models did not simulate unstable
conditions (FS < 1); we only predicted marginally stable conditions with FS ∼1.5
during rapid overpressure generation (Fig. 4b). Additional decreases in FS could be
induced by additional pressure sources. Potential pressure sources could be short
periods of higher sedimentation that we have not simulated based on limited age
data or increased pore pressure from seismic loading (Biscontin et al. 2004).
Seismic acceleration due to local earthquakes can also contribute to destabilization
of low angle, overpressured slopes (Strasser et al. 2007; ten Brink et al. 2009).
Earthquakes in the Gulf of Mexico are neither frequent nor large, however, Mw
4.6–5.8 earthquakes were observed in 2006 (Gangopadhyay and Sen 2008). These
small earthquakes, potentially driven by stress differences due to mechanical properties of salt and basin sediments (Gangopadhyay and Sen 2008), could create
seismic accelerations that decrease the marginally stable (FS < 1.5) Ursa sediments
during periods of rapid sedimentation and rapid overpressure generation. The seismic accelerations necessary to decrease stability can be defined as a function of the
magnitude of the earthquake and the proximity of the earthquake to the slope failure region (ten Brink et al. 2009). Preliminary models suggest that Mw ∼5 earthquakes located less than 100 km from the Ursa region are necessary to initiate
failure. Further research on the magnitude and distribution of Gulf of Mexico
earthquakes is required to quantify the exact distance–magnitude relationships for
earthquake-induced failure in the Ursa region.
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B. Dugan and J. Stigall
Conclusions
Sedimentation-fluid flow models of the Ursa region predict that overpressure
begins near the seafloor and increases to the top of the Blue unit. The magnitude of
overpressure has varied through time, increasing rapidly when sedimentation
increases and decreasing when sedimentation decreases. Lateral flow in the permeable sand and silt has also controlled the magnitude and distribution of overpressure. In the modern environment overpressure is decreasing because sedimentation
rates are low. Over the past 65 ka high sedimentation rates at Site U1324 (average
9.3 mm/y) created overpressures that approach 2 MPa whereas the lower sedimentation rates (average 3.6 mm/y) at Site U1322 created overpressures reaching approximately 1MPa. This overpressure gradient from Site U1324 to U1322 drives lateral
fluid flow through permeable sand and silt in the Blue unit and the channel-levee
deposits. Without the flow focusing effect, Site U1322 would have lower pressures
than are observed and predicted.
The evolution of overpressure and flow focusing is a dominant control on stability of the Ursa region. From our infinite slope analysis, we predict stable conditions
(FS > 1) from 65–0 ka. The minimum FS predicted (1.5) is controlled by the sedimentation history and lateral flow in permeable sand and silt. Without overpressure
FS would exceed 12 at Ursa, thus overpressure provides a significant FS decrease.
FS minima occur during periods with the highest sedimentation rates, which result
in the greatest overpressure generation rate. FS is lowest on the east side of the Ursa
region between Sites U1323 and U1322. These low FS values occur in a region of
low sedimentation rates for the Ursa region, but they are influenced by higher sedimentation rates on the west side of the region and lateral transfer of fluids from the
west (Site U1324) to the east through permeable layers. The silts in the channellevee deposits play a key role in flow focusing and we predict that the boundary
between the channel-levee silt and the overlying mud is consistently a low stability
surface. This is consistent with the regionally extensive MTDs in the Ursa region
occurring immediately above this boundary. The results of our simulations indicate
that additional driving forces are required to initiate slope failure. These additional
driving forces could be pressure generation or seismic accelerations. Pressure
sources could be from extremely high sediment inputs beyond our age resolution.
Seismic accelerations, created by salt tectonic earthquakes, could destabilize the
slope through horizontal accelerations and overpressure generation.
Acknowledgments This research used data provided by the Integrated Ocean Drilling Program.
Constructive comments by A. Bradshaw and M. Strasser strengthened this paper.
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