History of Pore Pressure Build Up and Slope
Instability in Mud-Dominated Sediments
of Ursa Basin, Gulf of Mexico Continental Slope
R. Urgeles, J. Locat, D.E. Sawyer, P.B. Flemings, B. Dugan,
and N.T.T. Binh
Abstract The Ursa Basin, at ~1,000 m depth on the Gulf of Mexico continental slope,
contains numerous Mass Transport Deposits (MTDs) of Pleistocene to Holocene age.
IODP Expedition 308 drilled three sites through several of these MTDs and encompassing sediments. Logs, sedimentological and geotechnical data were collected
at these sites and are used in this study for input to basin numerical models. The
objective of this investigation was to understand how sedimentation history, margin
architecture and sediment properties couple to control pore pressure build-up and
slope instability at Ursa. Measurements of porosity and stress state indicate that fluid
overpressure is similar at the different sites (in the range of 0.5–0.7) despite elevated
differences in sedimentation rates. Modeling results indicate that this results from
pore pressure being transferred from regions of higher to lower overburden along
an underlying more permeable unit: the Blue Unit. Overpressure started to develop at
~53 ka, which induced a significant decrease in FoS from 45ka, especially where
overburden is lower.
Keywords Submarine landslides • pore pressure • basin modeling • slope instability
• scientific drilling
R. Urgeles ()
Institute of Marine Sciences, Spanish National Research Council (CSIC)
e-mail: urgeles@icm.csic.es
J. Locat
Université Laval, Dept. of Geology and Geological Engineering, Québec G1K 7P4, QC, Canada
D.E. Sawyer and P.B. Flemings
University of Texas at Austin, Jackson School of Geosciences, 1 University Station C1100,
Austin, TX 78712-0254, USA
B. Dugan
Rice University, Department of Earth Science, 6100 Main Street, Houston, Texas 77005, USA
N.T.T. Binh
Durham University, Department of Earth Sciences, Science Labs, Durham DH1 3LE, UK
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
179
180
1
R. Urgeles et al.
Introduction
Pleistocene sedimentation in the Gulf of Mexico from the Mississippi River is
characterized by rapid sedimentation upon a mobile salt substrate (Worrall and
Snelson 1989). Offshore Texas and Western Louisiana, individual slope minibasins
are surrounded by elevated salt highs (Pratson and Ryan 1994) producing a remarkable
hummocky topography. This morphology is obscured in front of the Mississippi
delta, where sedimentation has been very rapid, exceeding 25 m ky−1 (Expedition
308 Scientists 2005).
Ursa Basin (~150 km due south of New Orleans, Louisiana, USA) lies in ~1,000 m
of water (Fig. 1). From bottom to top, the Pleistocene to Holocene sedimentary
sequence in Ursa Basin consists of: (1) the lower Mississippi Canyon Blue Unit, a
late Pleistocene, sand-dominated, “ponded fan” (Winker and Booth 2000; Sawyer
et al., 2007), (2) a mud dominated channel-levee assemblage with dramatic alongstrike variations in thickness and (3) a mud drape deposited during the last ~20 ky
(Fig. 2; Flemings et al. 2006). The mudstone package, belonging to the eastern
Elevation
(m)
–500
–600
–700
28.4N
–800
–900
–1000
28.3N
–1100
–1200
–1300
28.2N
–1400
–1500
–1600
28.1N
–1700
–1800
–1900
28N
–2000
60°
27.9N
40°
20°
89.3W
89.2W
89.1W
89W
88.9W
0°
–140°
–100°
–60°
Fig. 1 Detailed swath bathymetry (US national archive for multibeam bathymetric data) shaded
relief of the Mars Ridge (eastern levee of the Mississippi Canyon) showing location of Ursa drill
sites U1322, U1324 and seismic line shown in Fig. 2. Profuse evidence of mass-wasting processes
is evident. Inset shows location
History of Pore Pressure and Slope Instability in Ursa Basin
1.4
181
U1324
1km
1.6
TWT (s)
U1322
1.8
2.0
2.2
NE
2.2 SW
1.4 U1324
1km
Hemipelagic
Drape
Subunit
Id
Seafloor
U1322
2.0
Slump Blocks
Unit I
Slope Failure 2
East Levee
Unit II
TWT (s)
Southwest Pass Canyon
1.8
Slope Failure 1B
Unit II
Unit I
Distal Le
vee
1.6
Slope Failure 1A
Top Blue
West Levee
Core
Top Blue
2.2
Ursa Canyon
Blue Unit
East Levee
Base Blue
2.2 SW
Base Blue
NE
Fig. 2 Top. Seismic cross-section (for location see Fig. 1). Bottom. Interpreted cross-section. The
sand-prone Blue Unit has been incised by a channel-levee complex and then overlain by a thick and
heavily slumped hemipelagic mudstone wedge that thickens to the west (left). The Blue Unit sands
are correlated to a distinct seismic facies. The thickness of the hemipelagic mudstone above the Blue
Unit does not change significantly in the north-south direction. Major lithostratigraphic units identified during IODP Exp. 308 are labeled for correlation with Figs. 3–5. Seismic section reproduced
with permission of Shell Exploration and Production Company (After Flemings et al. 2006)
margin of the larger channel-levee system of the Mississippi Canyon, has Mass
Transport Deposits (MTDs) that record failure of the margin during the Pleistocene
(Figs. 1 and 2; Flemings et al., 2006; Sawyer et al., 2007). Holocene failures have
also been mapped all along the Gulf of Mexico, with the largest ones originating
from Ursa Basin (Fig. 1; see also McAdoo et al. 2000). Ursa Basin is of economic
interest because of its prolific oilfields, which are currently being exploited in
the nearby tension leg platforms of Mars and Ursa. Geohazard characterization
is therefore essential for safe offshore activities. For the purpose of this paper, we
use data from two boreholes (Figs. 1–3) acquired during IODP expedition 308
182
R. Urgeles et al.
Fig. 3 Stripe of seismic profile with major reflectors labeled and lithologic log (MTDs in red,
non-failed sediments in green) for drill sites U1324 and U1322 (see also Expedition 308 Scientists
2005). Logs of physical properties correspond from left to right to: (1) Median grain size (red line)
and grain size fraction abundance; (2) Undrained shear strength as determined from shipboard
motorized vane tests and trends for Cu•(g’z)−1 ratios of 0.1, 0.3 and 0.5; (3) Plastic limit (red),
liquid limit (cyan) and shipboard measured water content (blue) and (4) Overpressure determined
from Skempton’s (1957) eq. (blue line), porosity vs. stress relationships (green line with dots),
preconsolidation pressures measured in oedometer tests, DVTPP (blue inverted triangles and T2P
(green inverted triangles) piezometers and 1-D modeling results (red line)
History of Pore Pressure and Slope Instability in Ursa Basin
183
(Expedition 308 Scientists 2005, Flemings et al. 2006), including Logging While
Drilling (LWD), measurements of physical properties and geotechnical analysis
performed on whole round samples.
The aim of this paper is to document how pressure, stress, and geology couple
to control fluid migration on Ursa Basin, illuminate the controls on slope stability
and understand the timing of sedimentation and slumping.
1.1
Methods
To accomplish the objectives stated above a laboratory testing program was established, which included grain size analysis using a Coulter LS100 laser diffractometer and Atterberg Limits determined using a British Fall cone device (Feng et al.
2001), isotropically consolidated undrained (CIU) triaxial tests, and uniaxial incremental loading tests (see also Urgeles et al. 2007). The data resulting from these
tests are here discussed together with IODP Expedition 308 Shipboard data
(Expedition 308 Scientists 2005; Flemings et al. 2006), including visual core
descriptions, moisture and density data, vane shear and pocket penetrometer
strength determinations and in-situ pore pressure measurements using the DVTPP
and T2P piezoprobes (see Flemings et al. 2008; Long et al. 2008 for further details
on these measurements). In this paper, pore pressure is most often described in
terms of overpressure (l), defined as:
l = (P – Ph)/(sv -Ph)
(1)
where P is pore pressure Ph is hydrostatic pressure and sv is lithostatic or total
stress. A value of 0 implies hydrostatic conditions, a value of 1 means pore pressure
equals the lithostatic stress.
2-D margin simulations along the transect of Fig. 2 were carried using the Finite
Element Software “BASIN” (Bitzer et al. 1996, 1999), which allows for stratigraphic, tectonic, hydrodynamic and thermal evolution to be modeled. In “BASIN”
compaction and fluid flow are coupled through the consolidation equation and
the nonlinear form of the equation of state for porosity, allowing non-equilibrium
compaction and overpressuring to be calculated.
2
Results
In Ursa basin, Site U1324 (on the upslope part) was drilled to more than 600 mbsf
and site U1322 (downslope) to about 250 mbsf (Fig. 2). The sediments drilled at
these sites included several MTDs (5 at Site U1324 and 9 at U1322). On seismic
reflection profiles the thicker MTDs are characterized by discontinuous and/or
transparent to low amplitude reflections. MTDs show increased resistivity and density
in logs, presumably due to transport-induced compaction, while folds, some
with half-wavelengths of a meter or more, are apparent both in cores and logs.
184
R. Urgeles et al.
Despite differences in visual aspect and physical properties the sediment composition
shows no major differences between MTDs and non-failed deposits. Sediment
grain size analyses indicate that at both sites the sediments are made of ~30% clay
and ~70% silt, the mean grain size is about 4 microns, and these values remain
fairly constant with depth (Fig. 3). The sediment plastic limit is at about 35%, while
the liquid limit decreases from 70% to 55% in the upper 10 m of sediment column
and then becomes relatively constant around the latter value (Fig. 3). The sediment
water content moves from values at or higher than the liquid limit for the upper
20 mbsf and then gradually decreases to values close to the plastic limit at 100–
125 mbsf. From that depth downhole the water content sticks to the plastic limit
(IL =0%). On the Casagrande plot, samples from both sites plot on a line parallel
and above the A-line identifying the sediment as clays of high plasticity.
The vane shear and pocket penetrometer data show similar values of undrained
shear strength at equivalent depth for both drill sites, ranging from a few kPa near the
seafloor to about 250 kPa at about 600 mbsf (Fig. 3). CIU triaxial tests (see Urgeles
et al. 2007) were carried out on samples obtained from MTDs and non-failed
deposits at Sites U1324 and U1322. Prior to shearing the samples were isotropically
consolidated. Some of the samples were brought into the normally consolidated
state others remained in the overconsolidated state prior to shearing. However, on
the stress path plot the whole set of tests show relatively consistent results with a
friction angle of 28° and little cohesion of around 7 kPa.
Using consolidation theory, overpressure was estimated from pre-consolidation
pressures determined from incremental loading consolidation tests, and measurements
of pore pressure using the DVTPP and T2P pressure probes (Fig. 3; see also Flemings
et al. 2006, 2008; Long et al. 2008). Despite significant scatter, results appear to
show overpressures in the range of 0.6 to 0.8, suggesting that non-equilibrium
consolidation occurs in Ursa Basin (Fig. 3).
The consolidation tests results also provide parameters that can be used in basin
and interstitial fluid flow modeling. These parameters include the initial porosity,
hydraulic conductivity and specific storage (see Table 1 and Fig. 4). Consolidation
tests were performed in sediments of the Southwest Pass Canyon Formation and
hemipelagic sediments above (corresponding to Lithostratigraphic Units I and II in
the cores; see Fig. 2 for equivalence). Unfortunately, no samples could be retrieved
Table 1 Parameters for basin and fluid flow modeling
Upper
SW Pass
Recent Canyon
(%)
Fm. (%)
Sand
0
Silt
0
Hemipelagic 100
drape
SW Pass
0
Canyon Fm.
Lower
SW Pass
Blue
Canyon Ursa Canyon unit
Fm. (%) Fm. (%)
(%)
Initial
Initial
specific Grain
Hydraulic
porosity storage density conductivity
(%)
(m−1)
(kg/m3) (m/s)
0
0
0
0
20
0
10 (Core 40)
30 (Core 30)
0 (Core 40)
70
20
0
60
70
80
0.001
0.004
0.09
2,650
2,650
2,650
1 × 10−8
5 × 10−8
3 × 10−9
100
80
60 (Core 30)
10
75
0.07
2,650
3 × 10−9
History of Pore Pressure and Slope Instability in Ursa Basin
185
Fig. 4 Permeability and specific storage vs. effective stress derived from incremental loading
consolidation experiments used to derive the set of initial parameters for 2-D basin modeling
within “BASIN”. For location of experiments see Fig. 3
from the Blue Unit and central sandier part of the Ursa Canyon, as drilling within
these formations implied a high risk for shallow water flow sands. Parameters for
Sand and Silt which, according to industry well-log data (Sawyer et al. 2007) are a
significant constituent of the Blue Unit and core of the Ursa Canyon are taken from
the literature (Table 1; Reed et al. 2002).
3
Discussion
As shown above, overpressure estimates and measurements performed in various
ways, indicate that an overpressure of 0.6–0.8 is present in Ursa Basin (Fig. 3; see
Flemings et al. 2006; Long et al. 2008). Despite the significant scatter observed in
these estimates and measurements, all data suggest that similar overpressure exists
at Sites U1324 and U1322. 1-D modeling results however indicate that given the
sedimentation rates at both sites, which are much higher at Site U1324 than at U1322,
there should be a significantly higher overpressure at Site U1324 (Fig. 3). Mean sedimentation rates at Site U1324 are 9.6 m/ky with peaks exceeding 25 m/ky, while at
site U1322 mean sedimentation rates are 3.5 m/ky with peaks at 16 m/ky (Expedition
308 Scientists 2005). According to 1-D modeling results, these sedimentation rates
imply that overpressure should be around 0.8 at Site U1324 and only around 0.2 at
U1322 (Fig. 3). Using the 2-D modeling software “BASIN” the margin stratigraphic
evolution and the resulting interstitial fluid flow pattern and overpressure generation
can be better understood (Fig. 5). For this experiment initial and boundary conditions
are: (1) no flow occurs at the basement and model sides, (2) initial conditions are
hydrostatic, (3) no subsidence occurs and (4) the initial (decompacted) thickness of
the various formations can be estimated from van Hinte’s (1978) equation:
Fig. 5 Margin stratigraphic and hydrodynamic modeling with “BASIN” at final simulated present-day
conditions. (a) Margin stratigraphy (red MTDs) according to seismic profile depicted in Fig. 2.
(b) Fractional porosity. (c) Log hydraulic conductivity (m/s). (d) Excess pore pressure (kPa). (e)
Overpressure (l)
History of Pore Pressure and Slope Instability in Ursa Basin
T0 =
(1 − Φ N )TN
(1 − Φ 0 )
187
(2)
where jN, TN are the present-day porosity and thickness (assuming vp: 1,600 m/s;
Flemings et al. 2005) and j0, T0 are the original porosity and thickness. MTDs
removed/added little overburden because the failed masses did not evacuate the
failing zone and therefore have little to no effect in both 1-D and 2-D simulations.
The margin stratigraphic evolution is modeled since deposition of the Blue Unit,
roughly 100 ka ago, along the seismic line shown in Fig. 2. Margin modeling is performed using a set of initial sediment types and their corresponding hydraulic conductivity and specific storage shown in Table 1. Each stratigraphic unit is made of a
mixture of the different sediment types (Table 1), which result in averaged initial
physical properties (see Bitzer et al. 1999 for further details). The “BASIN” simulations show that, as it should be expected, porosity and hydraulic conductivity decrease
with time and depth along the section (Fig. 5b, c). However, high hydraulic conductivities ~10−8.5 m/s remain in the sandier Blue Unit and in the core of the Ursa Canyon
after the whole sedimentary package has been deposited. The hydraulic conductivities
on the muddy sediment wedge above the Blue Unit are between 1 to almost 3 orders
of magnitude lower (Fig. 5c). The modeled porosity and hydraulic conductivities
result in fluid flow from West to East along the Blue Unit and Ursa Canyon and a
vertically upward migration on the overlying muddy formations. The model also
shows that pore pressures above hydrostatic started to develop at ~53 ka with onset of
the Southwest Pass Canyon sedimentation, which deposited more clayey material
(Fig. 5d). Due to the higher sedimentation rates excess pore pressure develops further
near Site U1324, where the overburden is thicker. The more permeable nature of the
lower Blue Unit allows fluid to flow laterally from Site U1324 to U1322 inducing
higher excess pore pressures at this Site than should be expected given the sedimentation
rate at this location (Fig. 5). This effect is better seen in terms of overpressure (l).
Figure 5e clearly shows similar, or sometimes higher, overpressure at Site U1322
compared to Site U1324. The 2-D simulation results agree better with the estimated
and observed overpressures at both Sites. It also shows that the higher overpressures
concentrate within the depth range of 100 to 200–350 mbsf.
Overpressure build up in Ursa Basin has probably played an important role in
slope failure generation. Major additional controls in slope stability in Ursa Basin
include variations in slope angle due to depositional processes and salt tectonics.
Presently the regional slope in Ursa Basin does not exceeds 2°, and local slopes
rarely exceed 4°. The Gulf of Mexico is an area where large seismic ground
motions are not probable (Petersen et al. 2008). No large earthquakes have been
reported recently with the exception of two 5 < Mw < 6 earthquakes (Preliminary
Determination of Earthquakes (PDE) Catalog 1973–present). Recent studies, suggest however that these are a result of shallow slippage (most probably large-scale
gravitational sliding) rather than deep-seated tectonic processes (Nettles 2007,
Dewey and Dellinger 2008), and therefore we will not consider seismic ground
motions as potential triggering mechanism.
The main characteristics of MTDs in Ursa Basin are: (a) they occur on a more or
less uniform slope, (b) the failure planes are subparallel to the seafloor and (c) the
188
R. Urgeles et al.
length of the failure surfaces is large compared to the failure thickness (Figs. 1 and 2).
Therefore, it is considered that an “infinite slope” approach provides a first approximation to the margin stability. Urgeles et al. (2007) show the results of a drained
slope stability evaluation in Ursa Basin using the parameters identified from the
CIU triaxial tests (c = 7 kPa, F = 28°). Urgeles et al. (2007) show that for the range
of overpressures observed in Ursa Basin and the present regional slope gradient, the
slope can be considered safe. For failure to occur overpressure values close to 0.9
are needed, or slope gradients need to approach angles between 7.5 an 11° depending
on the overpressure. Using undrained shear strengths the slope appears in the stable
to metastable conditions (Urgeles et al. 2007).
Using the output from the overpressure simulations it is also possible to investigate
the margin stability conditions during the last 100 ky. Figure 6a shows that the margin
remained relatively stable with a Factor of Safety (FoS) ~6 until ~45 ka, and then an
overall decrease in margin stability occurred. Despite several oscillations, the
Fig. 6 (a) Ursa Basin FoS at Sites U1324 and U1322 using the output from overpressure (l)
simulations performed with “BASIN”. (b) Depth to the potential failure surface (depth of lowest
FoS) at Sites U1324 and U1322
History of Pore Pressure and Slope Instability in Ursa Basin
189
margin’s FoS (Fig. 6a) has remained lower at the site of lower overburden (Site U1322)
until about 20 ka, in agreement with core evidence showing that most failures are
found at this Site. From 20 ka onwards the FoS became similar for both sites. Using the
output from the overpressure simulations it is also possible to investigate the depth
at which failure could most likely have occurred. Figure 6b indicates that the reduction
in the margin’s FoS was accompanied by an increase in thickness of the potentially
unstable sedimentary package. It is also found that the potentially unstable sediment
package is thinner at Site U1322 than at Site U1324 in agreement with the thicker
MTD found at the latter site (Fig. 6b).
4
Conclusions
Physical property data and geotechnical measurements indicate that sediments from
Ursa Basin are largely overpressured, inducing effective stresses that are only 25%
of those that would exist under hydrostatic conditions. At Sites U1324 and U1322
similar overpressures are found despite large differences in sedimentation rates.
Modeling simulations show that this most probably results from excess pore pressure
being laterally transferred through the Blue Unit from places of high to low overburden.
The highest overpressures concentrate in the depth range between ~50 and 250–
350 mbsf. 2-D modeling results indicate that pore pressure started to build up with
onset of deposition of the South West Pass Canyon Formation ~53 ka ago.
Basic regional slope stability analysis suggests that the slope is stable under current
conditions. For the slope to fail it is necessary that the overpressure exceeds a value of ~0.9
or the regional slope steepens above 7°. Using the output from the margin stratigraphic
and hydrodynamic evolution it is found that the margin’s fluid flow pattern induced
lower stability at the foot of the slope where sedimentation rates are lower. The margin
stability reduced significantly from ~45 ka onwards. The decrease in margin’s stability
was accompanied by increased thickness of the potential failure package.
Acknowledgments This research used samples and data provided by the Integrated Ocean
Drilling Program (IODP). Research was funded by the Spanish “Ministerio de Educación y
Ciencia” (grant CGL2005-24154-E) and University of Barcelona through a “Promotion and
Intensification of the Research Activity” grant (56552-EK00A-790 01). Helpful reviews were
provided by E. Doyle and V. Lykousis.
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