Adams, A., J. T. Germaine, P. B. Flemings, and R. J. Day-Stirrat, Stress induced permeability anisotropy of resedimented Boston blue clay, Geophys. Res. Lett., (submitted), 2012, #2454
Flemings, P. B., C. John, and J. H. Behrmann, Expedition 308 synthesis: Overpressure, comsolidation and slope stability on the continental slope of the Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2012, doi:10.2204/iodp.proc.308.215.2012, #2450 
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.
Foroozan, R., D. Elsworth, P. B. Flemings, F. Bilotti, and S. Muhuri, The role of permeability evolution in fault zones on the structural and hydro-mechanical characteristics of shortening basins, Marine Petroleum Geol., 2012, 143-151, 2012, doi:10.1016/j.marpetgeo.2011.08.007, #2395
We examine the role of basin-shortening on the development of structural compartments in passive margin basins. A coupled flow-deformation model is used to follow the evolution of an idealized prismatic basin during lateral shortening. This includes the deformation-induced generation (lateral compaction) and dissipation (hydraulic fracturing) of pore fluid pressures and the resulting natural evolution of an underlying décollement and subsidiary fault structures. This model is used to examine the influence of strata stiffnesses, strain softening, permeability-strain dependence, permeability contrast between layers, and deformation rate on the resulting basin structure and to infer fluid charge within these structures. For a geometry with a permeability contrast at the base of the basin a basal décollement forms as the basin initially shortens, excess pore pressures build from the impeded drainage and hydrofracturing releases fluid mass and resets effective stresses. As shortening continues, thrust faults form, nucleating at the décollement. Elevated pore pressures approaching the lithostat are localized at the hanging wall boundary of the faults. Faults extend to bound blocks that are vertically offset to yield graben-like structural highs and lows and evolve with distinctive surface topography and separate pore pressure signatures. Up-thrust blocks have elevated fluid pressures and reduced effective stresses at their core, and down-thrust blocks the converse. The development of increased permeability on localized fault structures is a necessary condition to yield this up-thrust and down-thrust geometry. In the anti-physical case where evolution of permeability with shear strain is artificially suppressed, pervasive shear develops throughout the basin depth as fluid pressures are stabilized everywhere to the lithostat. Correspondingly, permeability evolution with shear is an important, likely crucial, feedback in promoting localization.
Luo, G., M. A. Nikolinakou, P. B. Flemings, and M. R. Hudec, Geomechanical modeling of stresses adjacent to salt bodies: 1. Uncoupled models, AAPG Bull., 96, 43-64, 2012, doi:10.1306/04111110144, #2385
Nikolinakou, M. A., G. Luo, M. R. Hudec, and P. B. Flemings, Geomechanical modeling of stresses adjacent to salt bodies: 2. Poro-elasto-plasticity and coupled overpressures
, AAPG Bull., 96, 65-85, 2012, doi:10.1306/04111110143, #2387
Sawyer, D. E., P. B. Flemings, J. Buttles, and D. Mohrig, Mudflow transport behavior and deposit morphology: Role of shear stress to yield strength ratio in subaqueous experiments, Marine Geol., (in press), 2012, #2475
Bohn, C. W., P. B. Flemings, and R. L. Slingerland, Accommodation change during bypass across a late stage fan in the shallow Auger basin, in Application of Seismic Geomorphology Principles to Continental Slope and Base-of-Slope Systems: Case Studies from Seafloor and Near-Seafloor Analogues, edited by B. E. Prather, M. E. Deptuck, D. C. Mohrig, B. van Hoorn and R. B. Wynn, SEPM Spec. Publ. No. 99, 219-236, 2011, #2328
Apron 1 in the Shallow Auger Fan System records the transition from ponded deposition in the lobe complex to bypass at the top of the channel complex. The lobe complex, at the base of Apron 1, exhibits characteristics typical of ponded apron deposits: it onlaps the basin margin, exhibits a concentric isopach pattern, has a lobe geometry in amplitude extraction, and is composed of continuous seismic
Day-Stirrat, R. J., A. M. Schleicher, J. Schneider, P. B. Flemings, J. T. Germaine, and B. Van Der Pluijm, Preferred orientation of Phyllosilocates: Effects of composition and stress on resedimented mudstone microfabrics, J. Structural Geol., 33, 1347-1358, 2011, doi:10.1016/j.jsg.2011.06.007 , #2429
Development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material (Boston Blue Clay). Boston Blue Clay (BBC), an illitic glacio-marine clay composed of 57% clay-sized particles (<2 üm) was resedimented and step-loaded to a vertical effective stress of 0.1 MPa, 1 MPa, 6 MPa, and 10 MPa. These four samples show a small increase in preferred orientation of mica and chlorite with increasing vertical effective stress. Furthermore, pure BBC powder was admixed with silica (silt-sized quartz) in five different ratios of BBC:silica to form sediments with 57%, 52%, 48%, 44%, and 36% < 2 üm particles and were loaded to a maximum vertical effective stress of 2.4 MPa. Mica preferred orientation decreases significantly with decreasing clay content at one stress state. We used and compared three different techniques to characterize and quantify the preferred orientation of phyllosilicates: transmission-mode X-ray texture goniometry, reflection-mode X-ray texture goniometry, and grain and pore networks imaged using secondary and backscattered electron micrographs gathered on argon-ion-milled surfaces. Preferred orientation of the clay minerals shows good agreement between transmission-mode X-ray texture goniometry and reflection-mode X-ray texture goniometry. Mercury porosimetry further illuminates vertical effective stress and compositional controls on microfabric.
Liu, X. L., and P. B. Flemings, Capillary effects on hydrate stability in marine sediments, J. Geophys. Res., 116, B07102, 2011, 1 citation, doi:10.1029/2010JB008143, #2384
We study the three-phase (Liquid + Gas + Hydrate) stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases. The capillary pressure, a measure of the pressure difference across a curved phase interface, exerts a key control on the methane solubility in Liquid + Hydrate (L + H) and Liquid + Gas (L + G) systems. By calculating the L + H and L + G solubilities as a function of water depth (pressure) and pore size (interface curvature), we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three-phase zone. The top of the three-phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases. The thickness of the three-phase zone increases as the distribution of pore sizes widens. The top of the three-phase zone can overlie or underlie the bulk three-phase equilibrium depth. At Blake Ridge, we predict that the three-phase zone is 27.7 m thick and that the top of the three-phase zone lies 13 m above the predicted bulk equilibrium depth. This reconciles the observation of the bottom-simulating reflector (BSR) at Blake Ridge that is shallower than the predicted bulk equilibrium depth. In contrast, at Hydrate Ridge where water depth is shallower, we predict that the three-phase zone is 20.4 m thick and that the top of the three-phase zone lies 0.7 m below the predicted bulk equilibrium depth. Our model, which predicts an upward shift in the top of free gas occurrence with increasing water depth (pressure), is compatible with worldwide observations that the BSR is systematically shifted upward relative to the bulk equilibrium depth as water depth (pressure) is increased.
Long, H., P. B. Flemings, J. T. Germaine, and D. M. Saffer, Consolidation and overpressure near the seafloor in the Ursa Basin, deepwater Gulf of Mexico, Earth Planet. Sci. Lett., 305, 11-20, 2011, 1 citation, doi:10.1016/j.epsl.2011.02.007, #2353
In mudstones of the Ursa Basin, Gulf of Mexico, the volume of voids to solids, or void ratio, ranges from 4 (porosity = 80%) at the seafloor to 0.6 (porosity = 37%) at 600 m below seafloor. This seven-fold change in void ratio can only be described by a compaction model that includes greater sediment stiffening with stress than has commonly been used in geotechnical or geological approaches. Through uniaxial consolidation experiments, we show that specific volume (v = 1 + void ratio) declines as an exponential function of effective stress. We use this relationship to successfully predict in-situ overpressures at Integrated Ocean Drilling Program (IODP) Sites U1322 and U1324. This technique can be used around the world to describe sediment compaction and predict pore pressure in the first 1000 m below seafloor. Rapid sediment consolidation near the seafloor provides the fluid source that generates overpressure despite the fact that these sediments have high permeability. Ultimately sediment consolidation is a first order control on when and at what depth overpressure will be generated in the subsurface. This in turn will impact whether submarine landslides are expected and the regional gradient of continental margins.
Moore, J. C., R. Urgeles, M. Conin, P. B. Flemings, and G. J. Iturrino, Threshold of borehole failure: Breaking in before breaking out, Mississippi fan, Gulf of Mexico, Geosphere, 7, 684-693, 2011, 1 citation, doi:10.1130/GES00613.1, #2386
Resistivity images from Integrated Ocean Drilling Program (IODP) Site U1322 on the Mississippi fan (Gulf of Mexico) show borehole failure as (1) low-resistivity bands interpreted as breakouts and (2) high-resistivity bands. Both features occur as opposing pairs on opposite sides of the borehole, and have similar azimuthal orientations and widths. Failures occur at depths of 90-216 m in sediments very rich in expansive (smectite-illite) clays of 40%-50% porosity that are younger than 65 ka. The low-resistivity breakouts resemble similar features in other IODP boreholes from southwest Japan and offshore Oregon. The high-resistivity features are unknown in other boreholes. Estimates of stress magnitudes based on the overburden stress and the extensional tectonic environment in the Gulf of Mexico predict that the borehole was at failure. Experiments were conducted on cores with lithologies equivalent to those of the borehole failure localities from IODP Site U1322 and adjacent Site U1324. These experiments suggest an elastic-plastic deformation with strains of 10%-15% before reaching a plastic yielding. In the experiments, strain softening during plastic deformation ranges from 0% to 20%. Physically the experimental samples show a combination of lateral bulging and discrete conjugate shears. These experiments suggest that the resistive areas in the borehole are an initial state of bulging, or extrusion, into the borehole. We call these extrusive failures "breakins" to distinguish them from traditional breakouts. Extrusion into borehole decreases the amount of conductive borehole fluid between the bulging sediment and the resistivity tool, increasing the resistivity signal. The high residual strength of the sediment prevents disaggregation and spalling. Where spalling has developed, breakouts occur. This analysis is the first documentation of this incipient stage of borehole failure.
Nikolinakou, M. A., G. Luo, M. R. Hudec, and P. B. Flemings, Geomechanical modeling of stresses and pore pressures in mudstones adjacent to salt bodies, in Amer. Rock Mechanics Assoc. 45th Ann. Meeting,, extended abstracts, ARMA 11-271, 2011, #2428
Schneider, J., P. B. Flemings, R. J. Day-Stirrat, and J. T. Germaine, Insights into pore-scale controls on mudstone permeability through resedimentation experiments, Geology, 39, 1011-1014, 2011, doi:10.1130/G32475.1, #2383
At a given porosity, mudstone permeability increases by an order of magnitude for clay contents ranging from 57% to 36% (<2 õm). This increase in vertical permeability results from a dual-porosity system that develops through three mechanisms: (1) silt bridging preserves large pore throats, (2) stress bridges inhibit clay particle alignment, and (3) local clay particle compression within stress bridges alters pore throat size distribution. Uniaxial consolidation experiments on resedimented clay-silt mixtures illuminate how permeability varies as a function of clay fraction during burial. Backscattered electron microscope images show that silty mixtures have larger pore throats and fewer aligned clay particles than do more clay-rich mixtures. We describe the permeability of clay-silt mixtures with a geometric mean model. Our method provides a promising framework for modeling of mudstone permeability as a function of clay fraction and porosity. How permeability and consolidation evolve during burial affects the ability of mudstones to seal CO2 and hydrocarbons in the subsurface, how mudstones behave as gas reservoirs, and under what conditions mudstones will be overpressured. Dual-porosity systems have fundamentally different transient flow and solute transport behaviors.
Smith, A. J., P. B. Flemings, and P. M. Fulton, Observations and models of heat and salt generation in deepwater Gulf of Mexico vents, in Proc. 7th Int. Conf. Gas Hydrates (ICGH 2011), Edinburgh, Scotland, Paper 639, 12 pp., 2011, #2439
Song, I., D. M. Saffer, and P. B. Flemings, Mechanical characterization of slope sediments: Constraints on in situ stress and pore pressure near the tip of the megasplay fault in the Nankai accretionary complex, Geochem., Geophys., Geosyst., 12, Q0AD17, 2011, doi:10.1029/2011GC003556, #2388
Huuse, M., P. Van Rensbergen, C. A.-L. Jackson, P. B. Flemings, R. J. Davies, and R. J. Dixon, Subsurface sediment remobilization and fluid flow in sedimentary basins: Preface, Basin Res., 22, 341, 2010, 3 citations, doi:10.1111/j.1365-2117.2010.00487.x, #2276
Huuse, M., C. A.-L. Jackson, P. Van Rensbergen, R. J. Davies, P. B. Flemings, and R. J. Dixon, Subsurface sediment remobilization and fluid flow in sedimentary basins: An overview
, Basin Res., 22, 342-360, 2010, 3 citations, doi:10.1111/j.1365-2117.2010.00488.x, #2277
Subsurface sediment remobilization and fluid flow processes and their products are increasingly being recognized as significant dynamic components of sedimentary basins. The geological structures formed by these processes have traditionally been grouped into mud volcano systems, fluid flow pipes and sandstone intrusion complexes. But the boundaries between these groups are not always distinct because there can be similarities in their geometries and the causal geological processes. For instance, the process model for both mud and sand remobilization and injection involves a source of fluid that can be separate from the source of sediment, and diapirism is now largely discarded as a deformation mechanism for both lithologies. Both mud and sand form dykes and sills in the subsurface and extrusive edifices when intersecting the sediment surface, although the relative proportions of intrusive and extrusive components are very different, with mud volcano systems being largely extrusive and sand injectite systems being mainly intrusive. Focused fluid flow pipes may transfer fluids over hundreds of metres of vertical section for millions of years and may develop into mud volcano feeder systems under conditions of sufficiently voluminous and rapid fluid ascent associated with deeper focus points and overpressured aquifers. Both mud and sand remobilization is facilitated by overpressure and generally will be activated by an external trigger such as an earthquake, although some mud volcano systems may be driven by the re-charge dynamics of their fluid source. Future research should aim to provide spatio-temporal ââ¬Ëinjectiteââ¬â¢ stratigraphies to help constrain sediment remobilization processes in their basinal context and identify and study outcrop analogues of mud volcano feeders and pipes, which are virtually unknown at present. Further data-driven research would be significantly boosted by numerical and analogue process modelling to constrain the mechanics of deep subsurface sediment remobilization as these processes can not be readily observed, unlike many conventional sediment transport phenomena.
Lin, W., M.-L. Doan, J. C. Moore, L. McNeil, T. B. Byrne, T. Ito, D. M. Saffer, M. Conin, M. Kinoshita, Y. Sanada, K. T. Moe, E. Araki, H. J. Tobin, D. Boutt, Y. Kano, N. W. Hayman, P. B. Flemings, G. J. Huftile, D. Cukur, C. Buret, A. M. Schleicher, M. Efimenko, K. Kawabata, D. M. Buchs, S. Jiang, K. Kameo, K. Horiguchi, T. Wiersberg, A. Kopf, K. Kitada, N. Eguchi, S. Toczko, K. Takahashi, and Y. Kido, Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009, Geophys. Res. Lett., 37, L13303, 2010, 10 citations, doi:10.1029/2010GL043158, #2275
A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.
Reilly, M. J., and P. B. Flemings, Deep pore pressures and seafloor venting in the Auger Basin, Gulf of Mexico, Basin Res., 22, 380-397, 2010, 2 citations, doi:10.1111/j.1365-2117.2010.00481.x, #2274
Pore fluid overpressures in four reservoir sandstones in the Auger Basin, deepwater Gulf of Mexico, are similar across the basin, suggesting that these sandstones are hydraulically connected over distances of 420 km. Small overpressure gradients within them suggest upward flow rates between 1 and 20mm/year. At the crest of these sandstones, pore pressure equals or exceeds the least principal stress, and we interpret that high fluid pressure is fracturing the caprock and driving flow vertically. A well drilled into the crest of the Auger sandstones confirmed the presence of extreme overpressures that converge on both the least principal stress and the overburden stress. Above these zones,
Urgeles, R., J. Locat, D. E. Sawyer, P. B. Flemings, B. E. Dugan, and N. T. T. Binh, History of pore pressure build up and slope instability in mud-dominated sediments of Ursa Basin, Gulf of Mexico continental slope, in Submarine Mass Movements and Their Consequences, edited by D. C. Mosher, L. Moscardelli, R. C. Shipp, J. D. Chaytor, C. P. D. Baxter, H. J. Lee, and R. Urgeles, Advances in Natural and Technological Hazards Research, 28, 179-190, 2010, 1 citation, doi:10.1007/978-90-481-3071-9_15, #2268
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.
Liu, X. L., and P. B. Flemings, Dynamic response of oceanic hydrates to sea level drop, Geophys. Res. Lett., 36, L17308, 2009, doi:10.1029/2009GL039821, #2171
During sea-level drop, water and gas pressures within oceanic hydrate systems can exceed the total vertical stress and this can drive slope failure and gas venting. We investigate this behavior with a multi-phase fluid and heat flow model. During sea-level drop, fluid pressures drop much less than the total stress due to both the high gas compressibility and hydrate dissociation. In permeable sediments, hydrate dissociation, water expulsion and gas mobility combine to induce underpressure and downward water flow from the seafloor. This study provides a causal mechanism for slope failure and fluid exchange that occur in hydrate systems during sea-level fall.
Moore, J. C., G. J. Iturrino, P. B. Flemings, and D. E. Sawyer, Data report: Stress orientations from borehole breakouts, IODP Expedition 308, Ursa area, Mississippi Fan, Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2009, doi:10.2204/iodp.proc.308.212.2009, #2172
Borehole failures are a conspicuous feature of the logging-while-drilling resistivity images at Integrated Ocean Drilling Program Sites U1322 and U1324. Failures appear as irregular zones of low resistivity on opposite sides of the well bore (resembling traditional breakouts) and also as zones of high resistivity flanked by narrower low-resistivity intervals. The failures show a consistent eastâwest trend at both sites and with depth in each. The inferred SHmin directions are 85°â265° and 91°â271°, respectively, at Sites U1322 and U1324. SHmax at Sites U1322 and U1324 is oriented subparallel to the overall southerly slope of the Gulf of Mexico slope in this region. At Site U1322, SHmin is perpendicular to en echelon extensional fractures along the margin of a submarine landslide.
Nelson, H. M., P. B. Flemings, J. T. Germaine, and B. E. Dugan, Data report: Radiography and X-ray CT imaging of whole core from IODP Expedition 308, Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2009, doi:10.2204/iodp.proc.308.213.2009, #2174
We completed 151 radiographs and 12 X-ray computed tomography (CT) scans from whole cores taken during Integrated Ocean Drilling Program (IODP) Expedition 308. In Brazos-Trinity Basin IV, 29 radiographs and 1 X-ray CT scan were taken at Sites U1319 and U1320. In Ursa Basin, 122 radiographs and 11 X-ray CT scans were taken at Sites U1322 and U1324. These radiographs and X-ray CT scans were completed to select undisturbed portions of the core for experiments and to qualitatively assess the presence of inclusions and variation in the whole-core soil samples from the expedition. The imaging was performed at three locations: Pennsylvania State University, Massachusetts Institute of Technology, and Rice University.
Sawyer, D. E., P. B. Flemings, B. E. Dugan, and J. T. Germaine, Retrogressive failures recorded in mass transport deposits in the Ursa Basin, Northern Gulf of Mexico, J. Geophys. Res., 114, B10102, 2009, 10 citations, doi:10.1029/2008JB006159, #2170
Clay-rich mass transport deposits (MTDs) in the Ursa Basin, Gulf of Mexico, record failures that mobilized along extensional failure planes and transformed into long runout flows. Failure proceeded retrogressively: scarp formation unloaded adjacent sediment causing extensional failure that drove successive scarp formation updip. This model is developed from three-dimensional seismic reflection data, core and log data from Integrated Ocean Drilling Project (IODP) Expedition 308, and triaxial shear experiments. MTDs are imaged seismically as low-amplitude zones above continuous, grooved, high-amplitude basal reflections and are characterized by two seismic facies. A Chaotic facies typifies the downdip interior, and a Discontinuous Stratified facies typifies the headwalls/sidewalls. The Chaotic facies contains discontinuous, high-amplitude reflections that correspond to flow-like features in amplitude maps: it has higher bulk density, resistivity, and shear strength, than bounding sediment. In contrast, the Discontinuous Stratified facies contains relatively dim reflections that abut against intact pinnacles of parallel-stratified reflections: it has only slightly higher bulk density, resistivity, and shear strength than bounding sediment, and deformation is limited. In both facies, densification is greatest at the base, resulting in a strong basal reflection. Undrained shear tests document strain weakening (sensitivity = 3). We estimate that failure at 30 meters below seafloor will occur when overpressure = 70% of the hydrostatic effective stress: under these conditions soil will liquefy and result in long runout flows.
Schneider, J., P. B. Flemings, B. E. Dugan, H. Long, and J. T. Germaine, Overpressure and consolidation near the seafloor of Brazos-Trinity Basin IV, northwest deepwater Gulf of Mexico, J. Geophys. Res., 114, B05102, 2009, 4 citations, doi:10.1029/2008JB005922, #2173
Pore water overpressures (u*) within mudstones beneath Brazos-Trinity Basin IV (deepwater Gulf of Mexico, offshore Texas) are greater than 70% of the hydrostatic vertical effective stress (σ′ vh ) [λ* = 0.7 = (u*/σ′ vh )]. These results are compatible with recent observations that suggest sedimentation rates in this region are rapid (6 mm/a). We compare the petrophysical properties and pore pressures within a 127-m-thick package of mudstone penetrated at two locations: Integrated Ocean Drilling Program (IODP) sites U1319 and U1320. Site U1319 is at the margin of Brazos-Trinity Basin IV, whereas Site U1320 lies at its center, beneath 180 m of turbidite fill. Experimentally derived preconsolidation stresses and an in situ pore pressure measurement record overpressure at Site U1319 and Site U1320 (λ* ∼ 0.2 to 0.8 and λ* ∼ 0.8, respectively). We use these data to define an average vertical effective stress gradient. Assuming that void ratio (e) is proportional to the log of vertical effective stress (σ′ v ), we predict pore pressures (u) throughout the mudstone at both sites using bulk density data. Overpressures are greater at Site U1320 due to rapid deposition of the overlying turbidites. However, a large fraction of the overpressure induced by the turbidite load applied at Site U1320 has dissipated by drainage into the overlying basin fill. High overpressures near the seafloor drive shallow fluid flow, reduce slope stability, and may explain large submarine landslides.
Flemings, P. B., H. Long, B. E. Dugan, J. T. Germaine, C. John, J. H. Behrmann, D. E. Sawyer, and the IODP Expedition 308 Scientists, Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Lousiana, Gulf of Mexico, Earth Planet. Sci. Lett., 269, 309-325, 2008, 9 citations, doi:10.1016/j.epsl.2007.12.005, #2034
Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70% and 60% of the hydrostatic effective stress () in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. High overpressures are present within low permeability mudstones where there have been multiple, very large, submarine landslides during the Pleistocene. Beneath 200 mbsf at Site U1324, pore pressures drop significantly: there are no submarine landslides in this mixture of mudstone, siltstone, and sandstone. The penetrometer measurements did not reach the in situ pressure at the end of the deployment. We used a soil model to determine that an extrapolation approach based on the inverse of square route of time () requires much less decay time to achieve a desirable accuracy than an inverse time (1/t) extrapolation. Expedition 308 examined how rapid and asymmetric sedimentation above a permeable aquifer drives lateral fluid flow, extreme pore pressures, and submarine landslides. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River. Reduced overpressure at depth at Site U1324 suggests lateral flow (drainage) whereas high overpressure at Site U1322 requires inflow from below: lateral flow in the underlying permeable aquifer provides one mechanism for these observations. High overpressure near the seafloor reduces slope stability and provides a mechanism for the large submarine landslides and low regional gradient (2ð) offshore from the Mississippi delta.
Flemings, P. B., H. Long, B. E. Dugan, J. T. Germaine, C. John, J. H. Behrmann, D. E. Sawyer, and the IODP Expedition 308 Scientists, Erratum to 'Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Lousiana, Gulf of Mexico', Earth Planet. Sci. Lett., 274, 269-283, 2008, 9 citations, doi:10.1016/j.epsl.2008.06.027, #2035
The publisher regrets that when the above article was printed, there were a series of errors in the text. The full correct article is printed on the following pages. We apologize for any inconvenience this may have caused. The Publisher. Available online 3 August 2008.
Gilhooly, W. P., S. A. Macko, and P. B. Flemings, Data report: Isotope compositions of sedimentary organic carbon and total nitrogen from Ursa basin (Sites U1322 dnU1324) and Brazos-Trinity basin #4 (Sites U1319 and U1320), deepwater Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.208.2008, #2033
Organic carbon and total nitrogen stable isotopes are reported for sediments drilled during Integrated Ocean Drilling Program Expedition 308. Brazos-Trinity Basin IV sediments exhibited a broad range in organic carbon 13C ranging between â27 and â20 (13C average = â24.1) compared to Ursa Basin sediments that are generally more depleted in 13C (13C average = â25.7). Bulk 15N values across all basins ranged from â2.7 to 8.2 (15N average = 3.7) and showed no obvious trend. The relative contribution of marine and terrestrial detrital material deposited within these sediments was inferred by comparing isotopic compositions to C/N values. The significant contribution of inorganic nitrogen (Nbound average 75%), as estimated from total organic carbon/total nitrogen plots, likely lowered the observed C/N values.
Long, H., P. B. Flemings, J. T. Germaine, D. M. Saffer, and B. E. Dugan, Data report: Consolidation characteristics of sediments from IODP Expedition 308, Ursa Basin, Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.203.2008, #2036
We conducted temperature and pore pressure measurements using the Davis-Villinger Temperature-Pressure Probe and the temperature/dual pressure probe penetrometers during Integrated Ocean Drilling Program Expedition 308. In Ursa Basin, 18 measurements were used to determine that the geothermal gradient at Site U1324 is bilinear. The temperature gradient is 18.6°C/km in lithostratigraphic Unit I and 16.7°C/km in Unit II. Based on nine measurements at Site U1322, the geothermal gradient is 21.9°C/km. In Brazos-Trinity Basin IV, the geothermal gradient at Site U1320 is 23.1°C/km. In Ursa Basin, significant overpressures (overpressure ratio = ~0.7) are observed in the sediments above ~200 meters below seafloor (mbsf) at Sites U1322 and U1324. At Site U1324, pore pressure decreases with increasing depth between 200 and 300 mbsf. Below 300 mbsf and within lithostratigraphic Unit II, overpressure is approximately constant (~1 MPa). Unit II is composed of silty claystone interbedded with beds of silt and very fine sand. In Brazos-Trinity Basin IV, only two penetrometer deployments were made and the data are inconclusive.
Long, H., P. B. Flemings, B. E. Dugan, J. T. Germaine, and D. Ferrell, Data report: Penetrometer measurements of in situ temperature and pressure, IODP Expedition 308, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.203.2008, #2038
We conducted temperature and pore pressure measurements using the Davis-Villinger Temperature-Pressure Probe and the temperature/dual pressure probe penetrometers during Integrated Ocean Drilling Program Expedition 308. In Ursa Basin, 18 measurements were used to determine that the geothermal gradient at Site U1324 is bilinear. The temperature gradient is 18.6°C/km in lithostratigraphic Unit I and 16.7°C/km in Unit II. Based on nine measurements at Site U1322, the geothermal gradient is 21.9°C/km. In Brazos-Trinity Basin IV, the geothermal gradient at Site U1320 is 23.1°C/km. In Ursa Basin, significant overpressures (overpressure ratio = ~0.7) are observed in the sediments above ~200 meters below seafloor (mbsf) at Sites U1322 and U1324. At Site U1324, pore pressure decreases with increasing depth between 200 and 300 mbsf. Below 300 mbsf and within lithostratigraphic Unit II, overpressure is approximately constant (~1 MPa). Unit II is composed of silty claystone interbedded with beds of silt and very fine sand. In Brazos-Trinity Basin IV, only two penetrometer deployments were made and the data are inconclusive.
Sawyer, D. E., R. Jacoby, P. B. Flemings, and J. T. Germaine, Data report: Particle size analysis of sediments in the Ursa Basin, IODP Expedition 308 Site U1324 and U1322, northern Gulf of Mexico, Proc. Int. Ocean Drilling Prog., 308, 2008, doi:10.2204/iodp.proc.308.205.2008, #2037
We conducted particle size analyses on 340 samples from Integrated Ocean Drilling Program Expedition 308 Sites U1324 (246 samples) and U1322 (94 samples) in the Ursa Basin (Gulf of Mexico) and found two characteristic lithologies: silty clay and clayey silt. Silty clays are composed of ~60% (±10%) (by weight, ~40% silt-sized particles by weight, and <1% sand-sized particles by weight. Clayey silts are generally composed of ~30% clay-sized particles by weight, 65%â70% silt-sized particles by weight, and 0%â5% sand-sized particles by weight. Site U1322 is dominated by silty clays with little particle size variation throughout the cored interval (0â235 meters below seafloor [mbsf]). At Site U1324, both lithologies occur where the lowermost section (~360â608 mbsf) is dominated by clayey silt and the uppermost section (0â360 mbsf) is dominated by silty clay.
Sawyer, A. H., P. B. Flemings, D. Elsworth, and M. Kinoshita, Response of submarine hydrologic monitoring instruments to formation pressure changes: Theory and application to Nankai advanced CORKs, J. Geophys. Res., 113, B01102, 2008, 4 citations, doi:10.1029/2007JB005132, #2175
We describe the response of a compressible submarine hydrologic monitoring instrument to formation pressure changes in low-diffusivity rock. The measured pressure depends on the frequency of the pressure signal, the hydraulic diffusivity, and the wellbore storage. The Nankai advanced circulation obviation retrofit kits (ACORKs) (offshore Japan) record tide-induced formation pressure changes with small amplitudes (<10% of seafloor amplitudes) and large phase shifts (>25°). The pressure measurements occur in thick, homogeneous, compressible, low-permeability sediment, where in situ tidal pressure responses should approximate the seafloor tidal signal. A wellbore storage of 2 à 10−8 m3 Pa−1 can explain many of the observed tidal responses, given the hydraulic diffusivities of the monitored intervals. A reduced permeability around the wellbore of 1000-fold and a wellbore storage of 10−11 m3 Pa−1 can also reconcile the data. Our analysis suggests that ACORK screens in the Lower Shikoku Basin facies have a critical frequency on the order of 5 à 10−8 Hz (equivalent to a period of 250 days); higher-frequency formation pressure signals will be distorted in the pressure record. Within the Lower Shikoku Basin facies the time for this monitoring system to record 90% of an instantaneous pressure change is on the order of 10 d. We suggest that the ACORK instrument compliance contributes to, but does not fully explain, the small tidal amplitudes and large phase shifts recorded at the least permeable monitoring intervals.
Liu, X. L., and P. B. Flemings, Dynamic multiphase flow model of hydrate formation in marine sediments, J. Geophys. Res., 112, B03101, 2007, 30 citations, doi:10.1029/2005JB004227, #2176
We developed a multicomponent, multiphase, fluid and heat flow model to describe hydrate formation in marine sediments; the one- and two-dimensional model accounts for the dynamic effects of hydrate formation on salinity, temperature, pressure, and hydraulic properties. Free gas supplied from depth forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation: Salinity and hydrate concentration increase upward from the base of the regional hydrate stability zone (RHSZ) to the seafloor, and the base of the hydrate stability zone has significant topography. In fine-grained sediments, hydrate formation leads to rapid permeability reduction and capillary sealing to free gas. This traps gas and causes gas pressure to build up until it exceeds the overburden stress and drives gas through the RHSZ. Gas chimneys couple the free gas zone to the seafloor through high-salinity conduits that are maintained at the three-phase boundary by gas flow. As a result, significant amounts of gaseous methane can bypass the RHSZ, which implies a significantly smaller hydrate reservoir than previously envisioned. Hydrate within gas chimneys lies at the three-phase boundary, and thus small increases in temperature or decreases in pressure can immediately transport methane into the ocean. This type of hydrate deposit may be the most economical for producing energy because it has very high methane concentrations (S h > 70%), located near the seafloor, which lie on the three-phase boundary.
Long, H., P. B. Flemings, and J. T. Germaine, Interpreting in situ pressure and hydraulic properties with borehole penetrometers in ocean drilling: DVTPP and Piezoprobe deployments at southern Hydrate Ridge, offshore Oregon, J. Geophys. Res., 112, B04101, 2007, 2 citations, doi:10.1029/2005JB004165, #2177
Two borehole penetrometers, Fugro-McClelland's Piezoprobe and the Ocean Drilling Program's (ODP) DVTPP, were deployed 50 m below seafloor at Site 1244 on ODP Leg 204 to measure formation pressure at southern Hydrate Ridge, offshore Oregon. Pore pressure is interpreted to be hydrostatic and the sediment's coefficient of consolidation is interpreted to lie between 6.92 and 7.8 ÃÂÃÂÃÂÃÂÃÂÃÂâÃÂÃÂÃÂÃÂÃÂâÃÂââÃÂÃÂÃÂìÃÂââÃÂìÃÂà10−7 m2/s, which is in approximate agreement with laboratory measurements. The Piezoprobe pressure reaches 90% of dissipation 14 times sooner than the DVTPP. The observed and modeled pore pressure responses illustrate how penetrometer geometry impacts our ability to interpret in situ properties and demonstrate under what conditions these tools can be effectively used. Because of its narrow tip, the Piezoprobe disturbs a narrower zone than the DVTPP does. This generates a narrower zone of pressure increase around the piezoprobe, which dissipates much faster than the DVTPP. As consolidation proceeds, pressure dissipation of the Piezoprobe is retarded and forms a ÃÂÃÂÃÂÃÂÃÂÃÂÃÂâÃÂÃÂÃÂâÃÂââÃÂìÃÂ
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Sawyer, D. E., P. B. Flemings, R. C. Shipp, and C. D. Winker, Seismic geomorphology, lithology, and evolution of the Late Pleistocene Mars-Ursa turbidite region, Mississippi Canyon area, northern Gulf of Mexico, in Encyclopedia of Modern Coral Reefs, edited by D. Hopley, Springer-Verlag, AAPG Bull., 91, 215-234, 2007, 22 citations, doi:10.1306/08290605190, #2178
The interplay between sedimentation and erosion during the late Pleistocene in the Mars-Ursa region, northern Gulf of Mexico, resulted in a complex compartmentalized reservoir. Rapid deposition, directly downdip of the Mississippi River beginning about 70 k.y., quickly filled antecedent topography in the Mars-Ursa region with a thick accumulation of sand and mud called the blue unit. This permeable reservoir was rapidly and asymmetrically buried by thick, mud-rich levees of two channel-levee systems. Both systems plunged from north to south with a steeper gradient than the underlying blue unit. Rotated channel-margin slides present in both channel-levee systems rotated low-permeability, mud-rich levee deposits beneath the sand-rich channel fill. As a result of the channel-levee systems, the east-west hydraulic connectivity of the blue unit decreases progressively from north to south until its eastern and western halves become completely separated.
Trehu, A. M., P. B. Flemings, N. L. Bangs, J. Chevallier, E. Gracia, J. E. Johnson, C.-S. Liu, X. L. Liu, M. Riedel, and M. E. Torres, Feeding methane vents and gas hydrate deposits at south Hydrate ridge, Geophys. Res. Lett., 31, L23310, 2004, 31 citations, doi:10.1029/2004GL021286, #1783
Log and core data document gas saturations as high as 90% in a coarse-grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas-saturated bed is defined by a strong, negative-polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse-grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.
Mikada, H., M. Kinoshita, K. Becker, E. E. Davis, R. D. Meldrum, P. B. Flemings, S. P. S. Gulick, O. Matsubayashi, S. Morita, S. Goto, N. Misawa, K. Fujino, and M. Toizumi, Hydrogeological and geothermal studies around Nankai trough (KR02-10 Nankai Trough cruise report), JAMSTEC J. Deep Sea Res., 22, 125-171, 2003, #1661
Metzger, J. M., P. B. Flemings, N. Christie-Blick, G. S. Mountain, J. A. Austin, and S. Hesselbo, Late Miocene to Pleistocene sequences at the New Jersey outer continental shelf (ODP leg 174A, sites 1071 and 1072), Sedimentary Geol., 134, 149-180, 2000, 13 citations, doi:10.1016/S0037-0738(00)00018-X , #2258
2-D seismic, wireline log, and core data at ODP Leg 174A Sites 1071 and 1072 on the outer continental shelf of New Jersey reveal two major depositional sequences of late MioceneâPliocene and Pleistocene age. The late MioceneâPliocene sequence is a thick (not, vert, similar100 m) deepening-upward succession landward of the clinoform rollover and a shoaling-upward succession seaward of the clinoform rollover. The Pleistocene sequence deepens abruptly near its base, shoals upward, and then deepens again before it is truncated by its overlying unconformity. There is no onlap onto clinoforms (no lowstand wedge) in either sequence. Sequence stratigraphic analysis and a geometric depositional model are used to interpret that the unusually thick transgressive component of the late MioceneâPliocene sequence was formed by high-frequency eustatic cycles (1â2 m.y.) superimposed on a longer-term eustatic rise (not, vert, similar5 m.y.). This conclusion is supported by independent evidence of eustasy. The sequences of this study are correlated to sequences in the North Atlantic coastal plain and in the Great Bahama Bank. These sequences have very different architectures than underlying middle Miocene sequences, which contain thick lowstand wedge deposits, and are interpreted to have formed by high-frequency eustatic cycles superimposed on longer-term eustatic fall.