origin of overpressure and slope failure in the ursa...

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 mud-rich 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. StigallDepartment of Earth Science, Rice University, 6100 Main Street, MS 126, Houston, TX 77005 USAe-mail: [email protected]

D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences, 167Advances in Natural and Technological Hazards Research, Vol 28,© Springer Science + Business Media B.V. 2010

168 B. Dugan and J. Stigall

1 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 conti-nental 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 rela-tively 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

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

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).


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 cre-ated 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).

1

1zx

f w

kkP P P Tg

t x x z z t t

ffb fa

f⎡ ⎤ ⎡ ⎤∂ ∂ ∂ ∂ ∂ ∂ ∂⎛ ⎞= + − − +⎜ ⎟⎢ ⎥ ⎢ ⎥⎝ ⎠∂ ∂ ∂ ∂ ∂ − ∂ ∂⎣ ⎦ ⎣ ⎦

rm m

(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], r

w 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 vol-ume 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 deposi-tion 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 depos-ited 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.


Consolidation of sediments is modeled as an exponential relation between porosity (f) and vertical effective stress (s

v’ [Pa]) (e.g., Rubey and Hubbert 1959).

'v

o irre bsf f f−= + (2)

fo is a reference porosity [m3/m3] and f

irr is the irreducible porosity [m3/m3], such

that fo plus f

irr 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 (b

ul) is 10% of bulk compressibility (b

ul = 0.1b). We use data from

consolidation experiments (Long 2007; Long et al. 2008) to constrain fo, f

irr, and b

(Equation 2) for mud and silt (Table 1). For sand, we assume a low bulk compress-ibility 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-of-strain consolidation experiments. We follow the approach of Mello et al. (1994),

log( )xk A Bf= + (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, k

x/k

z [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-medium-grained 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 calcu-late the factor of safety (FS) using an infinite slope approximation (e.g., Lambe and Whitman 1969; Dugan and Flemings 2002).

2 * '' [( )cos ]tan

( )sin cosv w f

v w

c gz PFS

gz

s r q fs r q q

+ − −=

− (4)

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 (s

v [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, f

irr, and b define the consolidation

parameters (Eq. 2). A and B define the horizontal permeability (kx) function (Eq. 3) and k

x/k

z

defines the horizontal-vertical permeability ratio (anisotropy).

Lithology fo[m3/m3] f

irr[m3/m3] b[1/Pa] A [log(m2)] B [log(m2)] k

x/k

z

Mud 0.4 0.2 3.5 × 10−7 9.00 −20.16 100Silt 0.4 0.2 3.5 × 10−7 9.19 −17.77 100Sand 0.3 0.1 5 × 10−8 0.00 −12.00 1


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 fail-ures in the Ursa region. This simple assessment can identify potentially unstable regions that may require more advanced failure analysis, such as deep-seated rota-tional 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.


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 varia-tions 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 con-tinues 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 over-pressure 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.


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 obser-vation at Site U1322 (Fig. 3b). This pressure low may be related to pressure dissipa-tion 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 sedimenta-tion 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 tran-sients 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


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 prop-erties 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 seis-mic 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 fail-ure region (ten Brink et al. 2009). Preliminary models suggest that M

w ∼5 earth-

quakes 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.

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).


5 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 perme-able sand and silt has also controlled the magnitude and distribution of overpres-sure. 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 sedimenta-tion rates (average 3.6 mm/y) at Site U1322 created overpressures reaching approxi-mately 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 stabil-ity 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 sedi-mentation 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 sedi-mentation 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 channel-levee 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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