Rapid Sedimentation , Overpressure , and Focused Fluid Flow , Gulf of Mexico Continental Margin

Expedition 308 of the Integrated Ocean Drilling Program (IODP) was the fi rst phase of a two-component project dedicated to studying overpressure and fl uid fl ow on the continental slope of the Gulf of Mexico. We examined how sedimentation, overpressure, fl uid fl ow, and deformation are coupled in a passive margin setting and investigated how extremely rapid deposition of fi ne-grained mud might lead to a rapid build-up of pore pressure in excess of hydrostatic (overpressure), underconsolidation, and sedimentary mass wasting. Our tests within the Ursa region, where sediment accumulated rapidly in the late Pleistocene, included the fi rst-ever in situ measurements of how physical properties, pressure, temperature, and pore fl uid compositions vary within low-permeability mudstones that overlie a permeable, overpressured aquifer, and we documented severe overpressure in the mudstones overlying the aquifer. We also drilled and logged three references sites in the BrazosTrinity Basin IV and documented hydrostatic pressure conditions and normal consolidation. Post-expedition studies will address how the generation and timing of overpressure control slope stability, seafl oor seeps, and large-scale crustal fl uid fl ow. The operations of Expedition 308 provide a foundation for future long-term in situ monitoring experiments in the aquifer and bounding mudstones. Introduction Rapid sedimentation at rates >1 mm·yr-1 generate overpressure in many sedimentary basins around the world (Rubey and Hubbert, 1959; Fertl, 1976). When low-permeability sediments are rapidly loaded, pore fl uids cannot escape, and the fl uids bear some of the overlying sediment load. In this situation a pore pressure exceeding the hydrostatic pressure (overpressure, P*) develops. Recent work has focused on the coupling of rapid sedimentation and stratigraphic architecture to produce twoand three-dimensional fl ow fi elds. If, for example, permeable sand is rapidly buried by low-permeability mud of laterally varying thickness (Fig. 1), fl uids fl ow sub-horizontally through the sand layer to regions of thin overburden before they are expelled into the overlying sediment. This creates characteristic distributions of sediment properties, fl uid pressure, effective stress, temperature, and fl uid chemistry in the aquifers and the bounding mud (Fig. 1). This simple fl ow-focusing process can cause slope instability near the seafl oor (Dugan and Flemings, 2000; Flemings et al., 2002). In the deeper subsurface, overpressures created by focused fl ow can drive fl uids through low-permeability strata to vent Rapid Sedimentation, Overpressure, and Focused Fluid Flow, Gulf of Mexico Continental Margin by Jan H. Behrmann, Peter B. Flemings, Cédric M. John, and the IODP Expedition 308 Scientists doi:10.2204/iodp.sd.3.03.2006 1 2 3 0 5 4 Site U1324


Rapid
sedimentation at rates >1 mm•yr -1 generate overpressure in many sedimentary basins around the world (Rubey and Hubbert, 1959;Fertl, 1976).When low-permeability sediments are rapidly loaded, pore fl uids cannot escape, and the fl uids bear some of the overlying sediment load.In this situation a pore pressure exceeding the hydrostatic pressure (overpressure, P*) develops.
Recent work has focused on the coupling of rapid sedimentation and stratigraphic architecture to produce two-and three-dimensional fl ow fi elds.If, for example, permeable sand is rapidly buried by low-permeability mud of laterally varying thickness (Fig. 1), fl uids fl ow sub-horizontally through the sand layer to regions of thin overburden before they are expelled into the overlying sediment.This creates characteristic distributions of sediment properties, fl uid pressure, effective stress, temperature, and fl uid chemistry in the aquifers and the bounding mud (Fig. 1).This simple fl ow-focusing process can cause slope instability near the seafl oor (Dugan and Flemings, 2000;Flemings et al., 2002).In the deeper subsurface, overpressures created by focused fl ow can drive fl uids through low-permeability strata to vent Rapid Sedimentation, Overpressure, and Focused Fluid Flow, Gulf of Mexico Continental Margin Sedimentation rate decreases from left to right, resulting in fi nal wedge-shaped geometry.Rapid sedimentation generates overpressure (P*; color contours) that is greatest on the left (red) side of the picture.Flow is driven laterally (left to right) along the aquifer and is expelled at the toe of the slope where the aquifer ends (white arrows).Vertical effective stress (black contours) is lowest on the right side of the picture.[B] Predicted overpressure profi les where overburden is thick (Site U1324) and thin (Site U1322).(1) Overpressure at Site U1322 is slightly greater than at Site U1324 for equivalent depths.
(2) Vertical effective stress (σv´) is much lower at Site U1322 than at Site U1324.(3) Pore pressures (P) equal overburden stress (σv) at Site U1322.(4) Slope failure is predicted by infinite slope analysis near Site U1322 for FS < 1. FS relates failure-driving stress to available shear strength for shallow failures.
[C] Drawing of how fl ow focusing drives slope instability.
time in the history of scientifi c ocean drilling that a hole was drilled exclusively for logging purposes.
Integration of core, downhole measurement, and seismic data enabled a detailed lithostratigraphic (Fig. 4) and physicochemical characterization of the basin and dating of key surfaces.The data provide the basis to estimate sediment fl uxes over time across a source-to-sink system.A 175-m-thick sequence of sand-rich turbidite fans, masstransport deposits, and hemipelagic sediment has accumulated within the last ~120,000 years in Brazos Trinity Basin IV.Pre-fan deposits form a conformable sequence dominated by terrigenous clays transported by dilute turbidity currents and river plumes.The two sequences are separated by a thin (~1 m) layer of microfossil-rich clay, interpreted to represent the sea-level highstand during Marine Isotope Stage 5e.The basin infi ll is marked by two main pulses of mass-gravity fl ow deposition, separated by a pause in turbidity current activity.The pause lasted as much as 45 ky, starting at ~90 ka.The microfossil-rich clays in this interval contain the Los Chocoyos ash layer (Y8), a physical correlation marker across Basin IV and adjacent basins.The lower part of the basin contains mostly thin-bedded, muddy turbidites.The upper part contains muddy slump deposits derived from the basin margin and sand-rich turbidites that form several packets of very fi ne to fi ne sand beds that are ultimately at the seafl oor (Boehm and Moore, 2002;Davies et al., 2002;Seldon and Flemings, 2005).This is a potentially important mechanism for the transfer of fl uids from the solid earth to the hydrosphere and the atmosphere.

Brazos-Trinity Reference Sites
Brazos-Trinity Basin IV is located 200 km due south of Galveston, Texas (U.S.A.) in ~1400 m of water (Fig. 2).As one of a chain of fi ve basins that are separated by interbasinal highs, it is a classic area for analysis of turbidite depositional environments, and it is a modern analog to describe the formation of deep-water turbidite reservoir deposits (e.g., Badalini et al., 2000;Winker, 1996;Winker and Booth, 2000).The primary data used to evaluate the borehole locations comprise a high-resolution, twodimensional (2-D) seismic survey conducted by Shell Exploration and Production Company to image the turbidite stratigraphy.The three drilling sites are shown on dip seismic Line 3020 (Fig. 3).Site U1320 is located where the turbidite deposits are thickest, whereas Site U1319 lies along the southern fl ank of Basin IV where turbidite deposits are more condensed.Site U1321 was a loggingwhile-drilling and measurementwhile-drilling (LWD/MWD) site and was not cored.This is the fi rst   R40; this was not anticipated but is explained by rapid sediment loading precluding fl uid drainage and consolidation of muds of the basin interior.

Overpressured Ursa Basin Sites
Ursa Basin lies approximately 150 km south-southeast of New Orleans, Louisiana (U.S.A.) in ~1000 m of water (Fig. 2).The region is of economic interest because of its prolifi c oilfi elds that lie >4000 meters below the seafl oor (mbsf; e.g.Mahaffi e, 1994).We were interested in the sediments from 0 to 1000 mbsf (e.g., Pratson and Ryan, 1994;Pulham, 1993).Four three-dimensional (3-D) seismic data sets were used to constrain stratigraphy within Ursa Basin.The highresolution surveys were conducted by Shell Exploration and Production Company for the purpose of shallow hazards analysis.Figure 5 shows the seismic transect along which Sites U1322, U1323, and U1324 were drilled.The sanddominated Mississippi Canyon Blue Unit (Fig. 5) is a late-Pleistocene "ponded fan" deposited in a broad topographic low.The Blue Unit is overlain by a mud-dominated leveechannel assemblage.The most spectacular feature is the 5-25 m thick (Fig. 4).Rates of basin subsidence, sea-level change, and average sediment accumulation or erosion are of similar magnitude in this system, resulting in a complex interaction between (a) sea level changes and deltaic dynamics, which affect the delivery of sediment in the source area, (b) salt tectonics, which affects the basin confi guration and topographic gradients over time, and (c) the dynamic interaction between turbidity currents and the underlying topography.
The three drilling sites in the Brazos-Trinity Basin IV provided evidence of progressive and punctual downhole changes in physical properties as measured onboard the JOIDES Resolution, especially at Site U1319.Basin in-fi ll sediments (down to refl ector R40) and underlying hemipelagic drape sediments (below refl ector R40) are essentially non-slumped, normally consolidated muds that provide an excellent reference for materials found within the basin (Site U1320) and at Ursa Basin (Sites U1322 and U1324).The sediment porosity at Site U1319 decreases exponentially downhole from 80% to 50%.In comparison, Site U1320 has a section of underconsolidated muds and clays below Refl ector  sand-cored levee-channel of the Ursa Canyon, overlain by the muddy eastern levee deposits of the Southwest Pass Canyon and a hemipelagic drape cover.The mudstone package lying above the Blue Unit has numerous detachment surfaces that record slumping and mass transport deposits.
The Ursa Basin sites provided a west-east transect that tested the fl ow-focusing model of differential loading on a permeable aquifer.Overburden was drilled and sampled to 608 m depth at Site U1324 (thick overburden) and to 234 m at Site U1322 (thin overburden).We used a penetrometer (Fig. 6) to measure overpressure below 100 m at both sites.Normalized overpressure of approximately 0.6 was determined at the base of each site (i.e., the pore pressure lies 60% of the way between hydrostatic pressure and lithostatic pressure).The temperature gradient is 18°C•km -1 at Site U1324 and 26°C•km -1 at Site U1322.Thermal con ductivities at the two locations are similar (1.15-1.2W•m -1 •K -1 ), implying a vertical conductive heat fl ow of ~22 mW•m -2 at Site U1324 versus ~30 mW•m -2 at Site U1322.
Sedimentation has accumulated more rapidly at Site U1324 (10 mm•y -1 ) than at Site U1322 (3.8 mm•y -1 ).In spite of the almost three-fold difference in sedimentation rate, the similar overpressure gradients present at these two sites imply a component of lateral fl ow between them.This lateral fl ow drives fl uids from Site U1324 toward Site U1322, increases the pressure at Site U1322 relative to a system with only vertical fl uid migration, and decreases pressure at Site U1324 relative to a system with only vertical migration.The [B] Interpreted cross-section A-A'.Sand-prone Blue Unit has been incised by channel-levee complex and then overlain by thick and heavily slumped hemipelagic mudstone wedge that thickens westward (left).Blue Unit sands correlate to distinct seismic facies.Thickness of hemipelagic mudstone above Blue Unit does not change signifi cantly in north-south direction.Seismic reproduced with permission of Shell Exploration and Production Company.
Blue Unit, composed of interbedded sheet sands and mudstones, is interpreted to facilitate the lateral transfer of fl uids from Site U1324 to Site U1322, making the regional pressure fi eld diverge from a simple one-dimensional, compaction system.Core, log, and seismic interpretations document numerous scales of slumping, faulting, and softsediment deformation with increased occurrence at Site U1322.This deformation is consistent with predictions of the fl ow-focusing model.The dramatic difference in sedimentation rates between the sites also helps to explain the difference in observed thermal gradients.These results suggest that at the basin scale, this type of lateral fl uid fl ow may be the prime factor for the distribution and occurrence of cold seeps, mud volcanoes, and repeated submarine landslides generating major mass transport deposits.

Expectations and Achievements of IODP Expedition 308
A fundamental achievement of IODP Expedition 308 is that the overpressure profi le as a function of depth at Sites U1322 and U1324 in Ursa Basin could be directly measured.These measurements were diffi cult, and we experienced a high rate of failure; however, we ultimately acquired enough data to constrain the overpressure fi eld above the Blue Unit.Preliminary interpretations suggest that fl ow focusing is occurring in this basin and contributing to deformation and failure of sediments where overburden is thin.This is the fi rst time in the history of scientifi c ocean drilling that the spatial variation of the pressure fi eld has been documented at such resolution.We also acquired an extraordinary data set documenting a striking difference in temperature gradients between Sites U1322 and U1324.
We wanted to establish reference logging and core properties where overpressure is not present at a range of effective stresses in Brazos-Trinity Basin IV.Coring and logging were successful at all locations there, resulting in a highresolution recontruction of basin architecture and lithostratigraphy, in part below the level of individual lithostratigraphic subunits.
Our data on pore pressure, sediment properties, and overburden stress (Fig. 7) will provide a basis to assess the potential for slope failure, especially in Ursa Basin, and to estimate the conditions that drove previous slope failures.A major component of the ongoing post-expedition research is the integration of the stratigraphic geometry, physical properties, timing, and pressures associated with these mass-wasting processes.
IODP Expedition 308 monitored downhole pressure and lithology in real time using the MWD approach and for the fi rst time used weighted mud as a tool to drill and core Data from the ponded turbidite system in Brazos Trinity Basin IV and the channelized systems present in Ursa Basin are of great interest for further studies by academic and industry researchers.They will likely break new ground, especially in the fi eld of geotechnical and hydrogeological analysis of continental slopes along passive and active continental margins.We have also shown that in situ measurements of pore pressure in fi ne-grained sediments can be performed with overall success and that drilling into overpressured formations with riserless technology can be managed using heavy mud.Future drilling in a variety of settings might benefi t from the controlled use of weighted mud to stabilize the borehole.

Figure 1 .
Figure1.Flow-focusing model approximating conditions in Ursa Basin.[A]Low permeability sediments are rapidly deposited on a high permeability aquifer (outlined in white).Sedimentation rate decreases from left to right, resulting in fi nal wedge-shaped geometry.Rapid sedimentation generates overpressure (P*; color contours) that is greatest on the left (red) side of the picture.Flow is driven laterally (left to right) along the aquifer and is expelled at the toe of the slope where the aquifer ends (white arrows).Vertical effective stress (black contours) is lowest on the right side of the picture.[B] Predicted overpressure profi les where overburden is thick (Site U1324) and thin (Site U1322).(1) Overpressure at Site U1322 is slightly greater than at Site U1324 for equivalent depths.(2) Vertical effective stress (σv´) is much lower at Site U1322 than at Site U1324.(3) Pore pressures (P) equal overburden stress (σv) at Site U1322.(4) Slope failure is predicted by infinite slope analysis near Site U1322 for FS < 1. FS relates failure-driving stress to available shear strength for shallow failures.[C]Model parameters: low permeability mudstone kv < 5 × 10 -8 m 2 and kh < 5 × 10 -16 m 2 ;

Figure 2 .
Figure 2. Bathymetric image of continental slope of Gulf of Mexico.Drilling took place in Brazos-Trinity (reference sites) and Ursa (overpressured sites) regions.

Figure 4 .
Figure 4. Stratigraphic cross-section of Brazos-Trinity Basin IV.Data of core and well logs are at sea level.Stratigraphic ties between sites are based on seismic, lithology, log signature, and biostratigraphy.Each column contains depth scale, schematic lithology, gamma ray (GR) and resistivity logs (from LWD data), and depth of key seismic surfaces.In core recovery diagram, black = cored, red = overlap due to core expansion, and white = no recovery.mbsf = meters below sea fl oor.

Figure 5
Figure 5.[A] Seismic cross-section A-A' from Ursa Basin.[B] Interpreted cross-section A-A'.Sand-prone Blue Unit has been incised by channel-levee complex and then overlain by thick and heavily slumped hemipelagic mudstone wedge that thickens westward (left).Blue Unit sands correlate to distinct seismic facies.Thickness of hemipelagic mudstone above Blue Unit does not change signifi cantly in north-south direction.Seismic reproduced with permission of Shell Exploration and Production Company.