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doi:10.2204/iodp.proc.308.101.2006

Site results

Site U1319

Site U1319 is located on the southern flank of Brazos-Trinity Basin IV (Figs. F5, F6). The primary objective at this site was to establish a reference section of rock and fluid properties in a normally pressured basin. Secondary objectives included establishing an age model for Brazos-Trinity Basin IV and studying turbidite deposits. To achieve these objectives, Hole U1319A was continuously cored from the seafloor to a terminal depth (TD) of 157.5 mbsf. A MWD/LWD-dedicated hole (Hole U1319B) was then drilled to a TD of 180 mbsf.

Site U1319 is located at the southern edge of the basin. As a result, the cored succession of hemipelagic deposits and turbidites (Fig. F13) is condensed relative to that at the basin center. Nevertheless, a detailed record of turbidite deposition was recovered, which could be correlated to the basin center. Six lithostratigraphic units were recognized:

• Unit I (Holocene drape),
• Unit II (turbidites and debris flows),
• Unit III (hemipelagic sediments),
• Unit IV (distal turbidites),
• Unit V (hemipelagic sediments), and
• Unit VI (very distal turbidites mixed with hemipelagic sediments).

Unit V was deposited prior to formation of the Brazos-Trinity Basin IV, and all overlying sediments were deposited within it. Ash Layer Y8, a regional stratigraphic marker resulting from the Los Chocoyos (Guatemala) eruption and dated at 84 ka (Drexler et al., 1980; Mallarino et al., in press), was recovered in Unit III. Hemipelagic Units I and V are interpreted to have been deposited during eustatic highstands at the present and at 125 ka, respectively. Rare to abundant assemblages of well-preserved microfossils, spanning the late Pleistocene–Holocene period (marine oxygen isotope Stages [MIS] 1–6), were recovered. Tropical to subtropical species dominate the interglacial assemblages, whereas cool–temperate species are more common in assemblages from glacial intervals. The absence of reworked Cretaceous–Neogene nannofossils in the lower part of Hole U1319A (Units III and V) point to quiet open-marine environments. Moderately abundant benthic foraminifers in the upper ~30 m of the hole indicate a low-oxygen, high-nutrient environment.

Bulk density, measured both on the multisensor track (MST) and on discrete samples, increases with depth from ~1.3 to 2.0 g/cm³, reflecting normal compaction. Core resistivities, derived from the MST, increase with depth to ~60 mbsf and thereafter remain constant. Porosities decrease from initial values of ~80% to ~50% near the bottom of the hole (Fig. F13). Peak shear strengths increase with depth as a result of increasing vertical effective stress and sediment consolidation (Fig. F13). The trend is relatively smooth from the seafloor to ~80 mbsf; beneath 80 mbsf there is a sharp increase in undrained shear strength. The maximum peak strength recorded was 89 kPa.

Interstitial water chemistry shows large variations in alkalinity from 2.95 to 19.45 mM with a downhole concave profile and a rapid increase to a maxima at 15 mbsf. pH shows a similar concave depth profile, but with a maximum at ~30 mbsf. Sulfate concentrations in interstitial water show rapid downhole depletion with a sulfate/​methane interface (SMI) at 15 mbsf. The ionic concentrations of dissolved Mn²⁺ show a similar depletion trend as the sulfate concentration, whereas dissolved Ba, B, and Si show a concave-downward profile similar to those of alkalinity and pH. The sharp pore water chemistry changes at shallow subseafloor depths suggest rapid anaerobic degradation of organic matter through sequential redox reactions within the uppermost 15 m.

Average total organic carbon (TOC) content is relatively low for Gulf of Mexico sediments (0.75 wt%), but this value is consistent with the relatively low microbial biomass encountered (maximum cell density observed = 1 × 10⁶ cells/mL). Inorganic carbon concentrations are highly variable throughout the hole, ranging from 0.87 to 4.08 wt% (10.44 to 48.96 wt% CaCO₃). The average C/N ratio in the sediment was 3.77, suggesting either that algal material is the predominant source of organic matter or that the presence of inorganic nitrogen (such as ammonia) artificially lowers C/N ratios. The C/N maximum of 5.92 is coincident with the bottom of the sulfate reduction zone. The lack of any ethane (C₂) in headspace samples suggests that the relatively large quantities (as much as 11,310 ppmv) of methane (Fig. F13) detected are of biogenic, not thermogenic, origin.

Two deployments of the T2P were completed in Hole U1319A. The first deployment was at 1388 meters below sea level (41.6 m above seafloor) and provided a successful pressure test that demonstrated the tool could successfully pass through the lockable float valve of the bottom-hole assembly. Measured pressure (13.76 MPa) was slightly below hydrostatic (13.94 MPa), and the recorded water temperature was 4.9°C. A second T2P deployment at 80.5 mbsf recorded 1 m of penetration into the sediment. After 30 min, the tip pressure was 15.49 MPa and the shaft pressure 15.95 MPa; hydrostatic pressure was 15.19 MPa and formation temperature was 7.3°C. Because of the nonvertical penetration of the T2P into the sediment, the tip of the tool was bent.

MWD/LWD operations were completed in Hole U1319B to 180 mbsf with data coverage by all MWD/LWD tools over the interval cored in Hole U1319A (0–157.5 mbsf). From the seafloor to 180 mbsf, the following trends were observed:

• gamma radiation increases from 45 to 75 gAPI,
• deep button resistivity increases from 0.6 to 1.8 Ωm (Fig. F13),
• porosity decreases from 75% to 50%, and
• bulk density increases from 1.4 to 2.0 g/cm³.

These data suggest a normal compaction trend in the clay-rich section of Site U1319 (Fig. F13). Deviations from this trend occur at 25 mbsf where gamma radiation shows a step decrease (top of Unit III, foraminifer-rich clay), at 30.5–31.5 mbsf where gamma radiation increases (onset of fine laminae of sand, Unit IV), and at 78–93 mbsf where bulk density decreases (consistent with physical properties observed in the cores).

Drilling objectives at Site U1319 were fully accomplished. The almost continuous coring, lithostratigraphic, biostratigraphic, and logging records make Site U1319 an important reference location for study of sediment compaction. The low thermal gradient (~20°C/km) was striking. Finally, the ability to detect individual lithostratigraphic units within the uppermost 30 mbsf allowed us to date, describe, and correlate these turbidite deposits to the other Brazos-Trinity Basin IV sites.

Site U1320

Site U1320 is located near the center of Brazos-Trinity Basin IV (Fig. F5). The main drilling objective at this site was to establish a reference section to determine the rock and fluid properties in a normally pressured basin. Secondary objectives included improving the age model for Brazos-Trinity Basin IV and studying turbidite deposits. Hole U1320A was continuously cored and wireline logged to a TD of 299.6 mbsf. A MWD/LWD-dedicated second hole (Hole U1320B) was then drilled to a TD of 320 mbsf.

The lower part of the sedimentary succession in Hole U1320A (Fig. F14) (lithostratigraphic Unit V) is dominated by clay with rare silt laminae often containing fragments of foraminifers. Most of the succession is intensely bioturbated. We interpret lithostratigraphic Unit V as hemipelagic sedimentation with a high influx of siliciclastic material derived from either river plumes and/or very low density turbidity currents. Above lithostratigraphic Unit V, Unit IV is dominated by clay and represents the initial pulse of turbidite influx into the Brazos-Trinity Basin IV. Lithostratigraphic Unit III consists of a foraminifer- and nannofossil-bearing light greenish gray clay that contains volcanic ash Layer Y8, the product of the Los Chocoyos (Guatemala) eruption dated at 84 ka (Drexler et al., 1980; Mallarino et al., in press). Lithostratigraphic Unit II represents the main phase of basin filling and is defined by a 135 m thick succession of both sandy and muddy turbidites and muddy MTDs. Lithostratigraphic Unit I consists of a thin veneer of Holocene hemipelagic sediments. The overall basin fill succession shows a general upward increase in proportion of sand and thickness of turbidite packages (Fig. F14).

Site U1320 yielded rare to abundant assemblages of calcareous microfossils spanning the late Pleistocene–Holocene in MIS 1–6. Tropical–subtropical species dominate the interglacial assemblages, whereas cool–temperate species are more common in assemblages from glacial intervals. Intervals deposited during MIS 5 show no reworked nannofossils, indicating a quiet open-marine environment during sea level highstands. Frequent small thin-shelled benthic species of Bolivina and Bulimina are found in the lower part of Hole U1320A, suggesting that low-oxygen, nutrient-rich bottom conditions prevailed during MIS 6 in Brazos-Trinity Basin IV.

Lithostratigraphic Units I–III are characterized by considerable scatter in porosity values (36%–71%) (Fig. F14). This is interpreted to result from variations in lithofacies, in particular the presence or absence of sandy intervals. Lithostratigraphic Units IV and V are characterized by a gradual decline in porosity with depth. This porosity decrease drives increases in thermal conductivity, magnetic susceptibility, and resistivity. Bulk density values increase with depth from 1.4 g/cm³ at the seafloor to 2.0 g/cm³ at 273 mbsf. Grain density variations are small (between 2.6 and 2.8 g/cm³). Thermal conductivity values increase with depth from 1.1 to 1.3 W/(m·K).

Pore water chemistry data in Hole U1320A suggest that rapid anaerobic degradation of organic matter occurred through sequential oxidation fronts at shallow depths. However, chemical changes in deeper sections of the hole point to diagenetic processes and/or deep-seated fluid flow. Rapid changes in interstitial water profiles occur at shallow depths within the upper part of lithostratigraphic Unit II (uppermost 40 mbsf). The decrease in SO₄^2– from approximately ambient seawater concentration of 24.4 mM to a minimum of 0.5 mM at 21.5 mbsf coincides with an increase in alkalinity from 4.77 to a maximum of 15.99 mM at 20 mbsf. Mn²⁺ concentrations also decrease downhole to a minimum of 1.37 mM at 34.5 mbsf. Salinity and Ca²⁺, Mg²⁺, K⁺, Li⁺, and Sr²⁺ decrease with depth to 40 mbsf. In lithostratigraphic Unit V, significant increases in Ca²⁺ and Sr²⁺ correspond with a decrease in Li⁺ concentrations. Ba²⁺ has a maximum concentration between 120 and 180 mbsf (lithostratigraphic Units III and IV).

The average TOC content (0.53 wt%) is consistent with concentrations observed in Hole U1320A and is estimated to be either primarily derived from algal material (average C/N = 4.21) or to contain a substantial amount of inorganic (bound) nitrogen that lowered the C/N ratio. Trends in total inorganic carbon, TOC, total nitrogen (TN), C/N, and total hydrogen (TH) data clearly correlate with seismic reflector surfaces R10 and R20. The highest concentration of methane (57,714.2 ppm) is observed at 122 mbsf (Fig. F14). Methane to ethane ratios (C₁/C₂) are very high, suggesting a biogenic origin for methane. The calculated SMI depth is 22 mbsf. The inverse correlation between sulfate and methane gradients suggests local methanogenesis; however, the low microbial biomass (1 × 10⁶ cells/mL) cannot support in situ production of large amounts of methane.

Two deployments of the T2P were completed in Hole U1320A. The first deployment was at 126.3 mbsf (below Core 308-U1320A-15X) and the second deployment was at 213.0 mbsf (below Core 308-U1320A-24X). Both deployments used the tapered needle probe. The first deployment was completed with the drill bit ~1 m from the bottom of the hole and used the drill string to push the T2P into the formation. The needle probe was bent slightly during this deployment, and the pressure transducer did not record data. The second deployment was also completed with the drill bit 1 m above the bottom of the hole, but instead of using the pressure of the drill string, the tool string weight was used to insert the probe into the formation. All transducers performed well during this deployment, and the T2P was retrieved without damage. Both deployments recorded pressures that were slightly below hydrostatic. The temperature gradient between the two deployments was 20°C/km.

From the seafloor to 177 mbsf, resistivity, gamma ray, and porosity logs from downhole logging operations delineate a series of interbedded sand and mud facies that correspond to lithostratigraphic Unit II (Fig. F14). Porosities decrease with depth from 87% to ~45% at a TD of 297 mbsf. LWD resistivity images of the borehole show apparent east-west-oriented breakouts at the bottom of the hole.

All primary and secondary drilling objectives were accomplished at Site U1320. Drilling results from this site, together with those from Site U1319, provide key information on the space-time evolution of sedimentary and geochemical systems in Brazos-Trinity Basin IV and on the range of variation for physical properties for this basin.

Site U1321

Site U1321 is located on the southern edge of Brazos-Trinity Basin IV within a section of basin turbidites underlain by a thicker section of hemipelagic mud (Figs. F5, F6). Hole U1321A was drilled as a dedicated MWD/LWD hole for correlation between other sites and to document the lateral change in petrophysical properties of the fan units above seismic Reflector R40 (Fig. F6, F15). The LWD data indicate a facies of interbedded sand and mud that corresponds to lithostratigraphic Unit II in Hole U1320B. Porosity values from logging data (Fig. F15) decrease with depth from 80% to 45% at ~34 mbsf. Most of the units identified in the logs seem to be thinning with respect to the lithostratigraphic units identified in Hole U1320B. Resistivity images of the borehole show apparent east-west-oriented breakouts at the bottom of the hole, similar to those observed at Site U1320. These breakouts indicate a north-south maximum horizontal stress direction. The resistivity images also show a series of thin alternating resistive and conductive laminations that may represent variations in silt content. Steep features at the bottom of the hole have been identified as potential slump deposits or faulted blocks.

MWD/LWD operations at Site U1321 permit bed-by-bed correlation between Sites U1319 and U1320 (see discussion in “Synthesis of Brazos-Trinity Basin IV geology”).

Site U1322

Site U1322 is the easternmost site drilled in Ursa Basin during Expedition 308 (Fig. F8). Of the three sites in Ursa Basin, Site U1322 has the thinnest sediment cover above the Blue Unit (Fig. F11). The principal objectives of drilling Site U1322 were to document rock physical properties at the location of thin overburden above the Blue Unit, measure in situ formation temperature and pressure, document geochemical composition of the pore water, and establish a preliminary age model leading to an estimate of sedimentation rates at this location.

Hole U1322A was the first dedicated MWD/LWD hole in Ursa Basin. MWD/LWD operations were performed before coring because real-time pressure and lithology data were needed to determine if shallow-water flows were occurring during drilling. The MWD/LWD operation in Hole U1322A reached 238 mbsf without encountering any sand units. Hole quality remained good (average diameter = 26.9 cm) for almost the entire borehole. Hole U1322A is characterized by relatively monotonous logging data, mostly indicating clay, mud, and rare silt (Fig. F16). Resistivity and gamma ray measurements show the highest variability and can be correlated to several units defined by visual observation of the cores (see below) and to seismic Reflectors S10 and S30. In particular, logging data support division of the lithostratigraphic column (Fig. F16) encountered in Hole U1322B into two lithostratigraphic units (Units I and II) and further division of lithostratigraphic Unit I into Subunits IA–ID. The synthetic seismogram for Hole U1322A demonstrates that the correlation between logging data and the high-resolution seismic matches only the uppermost 100 mbsf. Nevertheless, the overall quality of the time-depth model allows an approximate correlation of seismic reflections with observations in core and logging data. The resistivity images obtained reveal intervals of undisturbed sediments and contorted and faulted sediments. The most striking features are parallel east-west-orientated contours of analog resistivity that may represent breakouts indicating the direction of the minimal horizontal stress.

Based on visual description of the cores in Hole U1322B, the 234.5 m sediment succession was divided into two lithostratigraphic units (Fig. F16). The total depth of this succession ties closely to seismic Reflector S60-1322, and the boundary between lithostratigraphic Units I and II (125.8 mbsf) occurs just above the prominent seismic Reflector S30. Lithostratigraphic Unit I is dominated by clay locally interbedded with silt and is further divided into four subunits based on the occurrence of intervals composed of deformed sediment. Lithostratigraphic Unit II is characterized by alternating meter-scale intervals of deformed and coherently laminated clay and mud. The deformed mud intervals are composed of dipping beds, small-scale faults, recumbent folds, and mud clasts. Nine deformed intervals with thicknesses varying from 2 to 5 m were recognized based on the occurrence of undeformed mud layers at their base.

Nannofossil and planktonic foraminifer data indicate that the sediment sequence recovered at Site U1322 was deposited over the last 60 k.y., more specifically during MIS 1–4. It is difficult to constrain sedimentation rates because of the MTDs that compose much of this site. Sedimentation rates of ~1–2 m/k.y. were estimated for the intervals above 30 mbsf and between 125 and 185 mbsf. Between 30 and 125 mbsf and below 185 mbsf, sedimentation rates increased to 12 m/k.y. or possibly higher in the intervals of MTDs. Distinctive cyclic patterns were observed in the distribution of nannoplankton and foraminifers, indicating periodic influx of sediments from the Mississippi River. Persistent low-oxygenated “stress” environments due to rapid sedimentation allowed infaunal benthic foraminifers to thrive. A deltaic benthic foraminifer assemblage from the interval between 185 and 234 mbsf is similar to those that exist today along the shelf edge of the Mississippi Delta, suggesting a period of turbidity currents as levee overbank deposits.

Variations in physical properties correlate with lithostratigraphic units and seismic reflectors at Site U1322. The porosity profile (Fig. F16) shows a relatively rapid decrease from the seafloor to 30 mbsf and then a more gentle decrease to TD. Within MTDs, the porosity is lower relative to undeformed intervals: the maximum porosity offset in MTDs is ~5% relative to undeformed sediment.

The chemical composition of the interstitial waters at Site U1322 varies widely in the uppermost 100 mbsf, in particular around the boundary between seismic Reflectors S10 and S20. Alkalinity, pH, concentrations of Ca²⁺, Sr²⁺, Li⁺, and B³⁺ ions, and NH₄⁺ have concave depth profiles with maxima centered around seismic Reflector S10. Above seismic Reflector S10, salinity and sulfate concentrations are consistently high and decrease rapidly between seismic Reflectors S10 and S20 associated with a decrease in several other elements such as Ca²⁺, Mg²⁺, B³⁺, Li⁺, and Sr²⁺. At Site U1322, the SMI is very deep (74 mbsf) and corresponds to a rapid increase in methane concentrations. Above the SMI, only minor amounts of methane (several parts per million by volume) were detected. The highest concentrations of methane (29,536–51,001 ppmv) were observed between 75 and 129 mbsf (Fig. F16). Only trace amounts of ethane (<3.4 ppmv) and ethylene (<2.6 ppmv) were detected in a few headspace samples. No higher hydrocarbons were detected at Site U1322. The high C₁/C₂ ratios suggest biogenic origin of the methane, which could derive from in situ microbial activities or hydrogeologically driven migration.

A maximum cell density of 4.0 × 10⁵ cells/mL was observed at 2.9 mbsf in Hole U1322B. Microbial abundances decreased with depth below the cell enumeration confidence limit of 1.0 × 10⁴ cells/mL at 74.5 mbsf. The extremely low cellular biomass at Site U1322 is consistent with low microbial abundances at Site U1324. A surprising observation is that microbial biomass in Ursa Basin is an order of magnitude lower than cell densities observed at Sites U1319 and U1320 in Brazos-Trinity Basin IV.

At this site, there was one high-quality DVTPP deployment at 236 mbsf and one high-quality T2P deployment at 150 mbsf. These provided a reasonable record of in situ temperature and pressure for Site U1322. Most of the other deployments recorded subhydrostatic pressures. Based on these results (or lack thereof), we decided to spend the remaining 36 h of operation time drilling an additional geotechnical hole at this site. The purpose of the new Hole U1322D was to deploy the pressure and temperature probes and spot core after each deployment. The cores obtained were sampled for geotechnical analysis and then processed through the onboard laboratories.

All of the objectives set for Site U1322 were fulfilled. The principal result was that we acquired a good data set of formation pressures and temperatures that can be compared with those at Site U1324. T2P and DVTPP measurements at Site U1322 provided critical data for understanding overpressure and fluid flow in Ursa Basin. Pressure dissipation curves at Site U1322 (seven measurements) document overpressure starting at 50 mbsf and continuing to TD. The temperature gradient at Site U1322 is 26.2°C/km.

Site U1323

Site U1323 is located between Sites U1322 and U1324 in Ursa Basin (Figs. F8, F11). The objectives of drilling at Site U1323 were to document rock physical properties at this location, measure in situ formation temperature and pressure, document geochemical composition of the pore water, and establish a preliminary age model leading to an estimate of sedimentation rates at this location. Site U1323 was logged using MWD/LWD and ultimately was not cored because an overpressured sand was encountered during MWD/LWD operations at a relatively shallow depth.

MWD/LWD operations at Site U1323 proceeded at an average penetration rate of 30 m/h, but borehole diameters were typically >24 cm to a depth of 204 mbsf, where an overpressured sand was encountered. This sand body, ~3 m thick, was detected at 204 mbsf on the gamma ray log, and a simultaneous jump in pressure of 150 psi (~1 MPa) over the background drilling pressure in the annular pressure-while-drilling log was observed. A residual backpressure of 150 psi was then observed by the driller when he shut down the mud pumps. At 242 mbsf, a rapid drop in the gamma ray log suggested the presence of another sand interval. At this point it was decided that to maximize the amount of science and conserve mud we should move to Site U1324 and plug and abandon Hole U1323A.

Logging and seismic data (Fig. F17) confirm that the upper 197 m interval is predominantly mud and clay rich, including two MTDs. Preliminary interpretation of the resistivity image data shows several highly deformed intervals confirming the original logging-seismic interpretation of several MTDs. These MTDs also display trademark characteristics of higher bulk density and resistivity compared with surrounding undeformed sediment.

Despite not coring at Site U1323, the high-quality logging data are valuable for analysis of the stratigraphic history of Ursa Basin. Drilling objectives for Site U1323 were thus achieved in three different ways: (1) overpressure was evidenced during MWD/LWD operations, (2) the novel IODP approach to “riserless-controlled drilling” proved efficient in controlling the flow, and (3) data obtained at Site U1323 provide information on the lateral continuity and stratal architecture of Ursa Basin.

Site U1324

Site U1324 is the westernmost site drilled in Ursa Basin during Expedition 308 (Fig. F8). Of the three sites in Ursa Basin, Site U1324 has the thickest overburden above the overpressured Blue Unit. The principal objectives of drilling Site U1324 were to document rock physical properties, measure in situ formation temperature and pressure, document geochemical composition of the pore water, and establish an age model to estimate sedimentation rates.

The stratigraphy of Hole U1324A (Fig. F18) was first divided into two main logging units. These units were further divided into several subunits based on comparisons with nearby core data from Hole U1324B and variations in the logging responses. The main regional seismic reflectors (S10–S50) can be identified in the logging data as significant variations in velocity, gamma radiation, and/or resistivity (Fig. F18). The LWD resistivity images show a large degree of deformation, especially in logging Unit II. These images show significant folds and variable dips ranging from shallow to relatively steep (>60°), suggesting a significant amount of deformation within MTDs. Tilted beds, folds, and faults are dominated by a general east-west strike.

The 612 m thick sedimentary succession overlying the Blue Unit at Site U1324 records the evolution of the eastern levee of the Southwest Pass Canyon channel-levee system (Fig. F18). Visual observation of the cores supported the division of the succession into two lithostratigraphic units. Lithostratigraphic Unit I is composed of clay and mud and contains two MTDs. Lithostratigraphic Unit II is composed of interbedded silt, sand, and mud and contains at least three MTDs. Prior to Expedition 308, seismic facies analysis suggested that acoustically semitransparent intervals in lithostratigraphic Unit I represent regional MTDs composed of faulted and contorted masses of mud and clay. However, close examination of the cores reveals that these MTDs contain levee clay and mud that are only mildly deformed and tilted and thus are interpreted to have not been transported very far from their original position.

Variations in physical properties correlate with lithostratigraphic units. The interbedded silt, sand, and mud and MTDs in lithostratigraphic Unit II are characterized by highly variable bulk density, porosity (Fig. F18), and peak shear strength. Physical properties show much less scatter in the uniform hemipelagic mud and clay in lithostratigraphic Unit I. MAD bulk densities are consistent with those measured by MST and LWD in lithostratigraphic Unit I. A porosity increase at 40 mbsf correlates with seismic Reflector S10. A decrease in resistivity and low thermal conductivity were also observed at that depth. A sharp porosity increase at ~160 mbsf is related to the silt layer above seismic Reflector S30 (Fig. F18), which may be significantly overpressured. This explanation is supported by the observed decrease in P-wave velocity, thermal conductivity, and undrained shear strength at this depth.

Nannofossils and planktonic foraminifer assemblages as well as magnetostratigraphy indicate that the sediment sequence recovered at Site U1324 was deposited over the last 60 k.y., more specifically during MIS 1–4. Sedimentation rates varied between 5 and >10 m/k.y. for lithostratigraphic Unit I in the interval above 365 mbsf of Hole U1324B, with possible sedimentation rate peaks of 12 m/k.y. or more in the intervals of mass flow. Between 365 and 608 mbsf, in lithostratigraphic Unit II, sedimentation rates appear to have been higher, perhaps in excess of 25 m/k.y. However, the low microfossil abundance and the relatively young age of the sediments render precise dating of this interval difficult. Distinctive cyclic patterns were observed in the distribution of nannoplankton and foraminifers, indicating periodic influx of turbidites sourced from the Mississippi River. The infauna-dominated benthic foraminifer assemblages also suggest a prevalence of low-oxygenated “stress” environments due to rapid sedimentation during the last glacial period.

Variations in interstitial water chemistry at Site U1324 are largest at shallow depths (<100 mbsf). Below this depth, limited changes are observed. Pore water chemical changes are particularly important from the seafloor to seismic Reflector S10 (~35 mbsf). Li⁺, B³⁺, and Sr²⁺ reach their maxima within this depth range, and Mn²⁺ reaches its minimum at ~35–40 mbsf. H₄SiO₄ and Fe²⁺ reach their maxima between ~20 and 25 mbsf. Between 40 and 160 mbsf, salinity, Li⁺, B³⁺, and Sr²⁺ decrease; Ba²⁺, Fe²⁺, and NH₄⁺ increase; and Cl^–, Mn²⁺, and H₄SiO₄ are constant. The extremely high ammonium contents (up to 6820 µM) in pore water are consistent with more reducing conditions at this site compared with the sites in Brazos-Trinity Basin IV. The downhole increase in ammonium likely reflects enhanced organic degradation at greater depths. The vertical profile, especially the surficial maximum and minimum in dissolved Fe²⁺ and Mn²⁺, are consistent with the hierarchy of redox reactions often observed in deep-marine sediments. The high Fe²⁺ contents at shallow depths might reflect enhanced Fe²⁺ reduction and/or greater availability of detrital Fe²⁺ oxides/​oxyhydroxides or simply Fe-rich clays. The pore water chemistry is probably dominated by dissolution processes rather than by organic matter degradation, which enhances alkalinity, Ca²⁺, Mg²⁺, Sr²⁺, B³⁺, and Li⁺ concentrations at ~35 mbsf. The causes for relatively acidic pore water (pH < 7.0) above 200 mbsf at Site U1324 are unclear.

Methane concentration (Fig. F18) increases dramatically in the middle section of lithostratigraphic Unit I (160 mbsf) but remains low in the rest of the hole. The predominant hydrocarbon found in Hole U1324B was methane, and the C₁/C₂ ratios were high, suggesting a biogenic origin. Therefore, we interpret the methane found at Site U1324 as resulting from in situ microbial activities or, alternatively, as having migrated from lateral locations. The microbial cell count at Site U1324 was low, with a maximum cell density of 2.0 × 10⁵ cells/mL at 2.8 mbsf. This is an extremely low and unexpected value considering the location and high sedimentation rate of this site. The predominance of clay-rich sediment at Site U1324 may prevent fluid migration and inhibit the development of microbial communities.

In situ measurements made with the T2P and DVTPP documented fluid overpressure and a low temperature gradient at Site U1324 relative to the gradient at Site U1327. Successful fluid pressure measurements at 117, 300, 405, and 608 mbsf yielded values for λ* between 0.2 and 0.6 (λ* = ratio of overpressure to vertical hydrostatic effective stress). Eighteen temperature measurements constrain a geothermal gradient of ~18.4°C/km.

All of the objectives for Site U1324 were fulfilled. The principal result is that we acquired a good data set of formation pressures and temperatures with which to compare Site U1322. Results from lithostratigraphy and biostratigraphy indicated very high sedimentation rates.

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