Expedition 308 synthesis: overpressure, consolidation, and slope stability on the continental slope of the Gulf of Mexico

Abstract

Integrated Ocean Drilling Program Expedition 308 quantified the coupling between sedimentation, consolidation, overpressure, fluid flow, and slope instability in continental margin settings. We summarize and synthesize peer-reviewed hydrogeologic studies published since the end of Expedition 308 that focus on Expedition 308 sites drilled in Ursa Basin: Sites U1322, U1323, and U1324. There is a rich stratigraphic complexity in the Ursa Basin, deepwater Gulf of Mexico. The sandstone-prone Blue Unit is a permeable aquifer overlain by mudstone-prone leveed-channel deposition. Multiple mass transport deposits, each densified relative to surrounding material, are present. Eight coeval surfaces mapped across the drilling transect span ~70 ka to present. Sedimentation rates were >10 mm/y at Site U1324. In situ penetrometer measurements document severe overpressures that begin at or near the seafloor and extend to the base of the drilled holes. Uniaxial consolidation experiments quantified the compression behavior of the sediments and provided an approach to predict pressure from porosity. Laboratory analysis of permeability and compressibility document that the coefficient of consolidation (hydraulic diffusivity) of Ursa Basin mudstones is 10 -8 m 2 /s over the effective stresses encountered during drilling at Ursa Basin (0.1 to 5 MPa). This low and constant value for the coefficient of consolidation is responsible for the presence of high overpressure very near the seafloor. Although permeability is very high, the sediments are also highly compressible. Forward modeling of Ursa Basin sediments shows that overpressure near the seafloor is reasonable given the rock properties and sedimentation rates that are observed. Modeling also illustrates that flow is driven laterally along an underlying aquifer, the Blue Unit. Soon after Blue Unit deposition, lateral flow induced submarine landslides. Later in the evolution of this system, overpressure may have preconditioned the slope to failure by earthquakes of magnitude 5.0.

What causes large submarine landslides on low gradient (<2°) continental slopes with slow (∼0.15 m/kyr) sediment accumulation?

“…Considering a slope of uniform material, permeability be must be less than 10 −11 m/s at the seafloor and 10 −13 m/s at 500 m depth. This is 500 times lower than the values measured for hemipelagic mud [ Flemings et al , ]. Thus, such low permeabilities seem unrealistic.…”

“…This decline is prescribed in the Modified Cam Clay model ( ) with e 0 =3.0 at unity (1 kPa). Permeability depends on porosity following the relationship for hemipelagic mud from Flemings et al [] (original equation converted to give k in m/s instead of m 2 for seawater at ∼15°C): …”

“…Permeability, k , is a function of porosity, n . We use the porosity‐permeability relationship given by Flemings et al [] for hemipelagic mud (equation ). A higher horizontal ( k x ) than vertical permeability ( k z ) with an anisotropy ratio of 10 is applied, which has been measured, for example, on mudstone samples from the Gulf of Mexico [ Reece et al , ].…”

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“…Prior well-log based studies from IODP and ODP drilling in the Northern Gulf of Mexico (Shipp et al, 2004;Sawyer, 2007;Sawyer et al, 2009;Dugan, 2012;Flemings et al, 2012) and…”

A two-dimensional layer-averaged numerical model for turbidity currents

“…We did not find any data for porosity values in the study area. As a reference value reported by Flemings et al (2012) at the Ursa Basin, porosity declines from 80 to 40% with increasing depth below seabed; and it remains approximately 40% when the depth is greater than 100 m. We have run a test case by setting p = 0.4 and keeping the other parameters the same as in case 8. The results (not shown here) are slightly different from the one with p = 0.5; so we let p = 0.5 in the present study.…”