Site U1319 is located on the southern flank of Brazos-Trinity Basin #4 (Figs. F5, F6). The primary 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 establishing an age model for Brazos-Trinity Basin #4 and studying turbidite deposits. To achieve these objectives, Hole U1319A was continuously cored from the seafloor to a terminal depth of 157.5 mbsf. A MWD/LWD-dedicated hole (Hole U1319B) was then drilled to a terminal depth of 180 mbsf.
Site U1319 is located at the edge of the minibasin. 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 present, 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 the formation of the Brazos-Trinity minibasin, 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 PleistoceneHolocene period (marine oxygen isotope Stages [MIS] 16), were recovered. Tropical to subtropical species dominate the interglacial assemblages, whereas cooltemperate species are more common in assemblages from glacial intervals. The absence of reworked CretaceousNeogene 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 resistivity, derived from the MST, increases with depth to ~60 mbsf and thereafter remains constant. Porosities decrease from initial values near 80% to ~50% near the bottom of the hole (Fig. F13). Peak shear strength (Fig. F13) shows a general increase with depth as a result of increasing vertical effective stress and sediment consolidation. 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. Ratios between the peak and undrained shear strengths show that the clays encountered at Site U1319 have a low sensitivity.
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 its 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 at 15 mbsf. The ionic concentrations of dissolved Mn show a similar depletion trend as the sulfate concentration, whereas dissolved Ba, B, and Si show a concave downward profile similar to the 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 these values are consistent with the relatively low microbial biomass encountered (maximum cell density observed = 1 x 106 cells/mL). Inorganic carbon concentration is 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 as11,310 ppmv) of methane (Fig. F13) detected is 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 (LFV) of the bottom-hole assembly (BHA). 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 at 15.49 MPa and the shaft pressure at 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 (0157.5 mbsf). From the seafloor to 180 mbsf, the following trends were observed: gamma radiation increased from 45 to 75 gAPI, deep button resistivity increased from 0.6 to 1.8 m (Figs. F13), porosity decreased from 75% to 50%, and bulk density increased 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 the gamma ray has a step decrease (top of Unit III, foraminifer-rich clay), from 30.5 to 31.5 mbsf where gamma radiation increased (onset of fine lamina of sand, Unit IV), and from 78 to 93 mbsf, where bulk density decreased (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 the study of sediment compaction. The low thermal gradient (~20°C/km) was also striking. Finally, the ability to detect individual lithostratigraphic units within the uppermost 30 mbsf allowed us to date and describe these turbidite deposits.Site U1320
Site U1320 is located near the center of Brazos-Trinity Basin #4 (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 #4 and studying turbidite deposits. Hole U1320A was continuously cored and wireline logged to a terminal depth of 299.6 mbsf. A MWD/LWD-dedicated second hole (Hole U1320B) was then drilled to a terminal depth of 320 mbsf.
The lower part of the sedimentary succession (Fig. F14) is termed lithostratigraphic Unit V in Hole U1320A. This sedimentary sequence is dominated by clay with rare silt lamina often containing fragments of foraminifers. Most of the succession is intensely bioturbated. We interpret lithostratigraphic Unit V to represent 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 #4. Lithostratigraphic Unit III consists of a foraminifer-bearing light greenish gray clay that contains a 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 slump/debris flow deposits. 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 PleistoceneHolocene in MIS 16. Tropical to subtropical species dominate the interglacial assemblages, whereas cooltemperate 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 #4.
Results of physical property analysis at Site U1320 suggest that compaction-driven fluid expulsion was a main factor influencing the physical characteristics (Fig. F14) of the sediments. Lithostratigraphic Unit II is characterized by considerable scatter in porosity (36%71%) (Fig. F14). This is interpreted to result from variations in lithofacies, in particular the presence or absence of sandy intervals. Lithostratigraphic Unit V is characterized by a gradual decline in porosity with depth, reaching ~50% by ~299 mbsf. This porosity decrease drives increases in thermal conductivity, magnetic susceptibility, and resistivity. Bulk density increases 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 increases with depth from 1.1 to 1.3 W/mK.
Chemical pore water data in Hole U1320A suggest rapid anaerobic degradation of organic matter through sequential oxidation fronts within shallow depths, whereas chemical changes in deeper sections of the hole point to diagenetic processes and/or deep-seated fluid flow. Rapid change in interstitial water profiles occurs at shallow depths within the upper part of lithostratigraphic Unit II (uppermost 40 mbsf). The decrease in SO4, from approximately ambient seawater concentrations of 24.4 mM to a minimum of 0.5 mM at 21.5 mbsf, coincides with a concomitant increase in alkalinity from 4.77 to a maximum of 15.99 mM at 20 mbsf. Mn concentrations also decrease downhole to a minimum of 1.37 mM at 34.5 mbsf. Salinity and Ca, Mg, K, Li, and Sr decrease with depth to 40 mbsf. In lithostratigraphic Unit V, a significant increase in Ca and Sr corresponding with a decrease in Li concentration takes place. Ba has its maximum concentration between 120 and 180 mbsf (lithostratigraphic Units III and IV).
The average TOC content (0.53 wt%) is consistent with the concentrations observed in Hole U1319A 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, N, C/N, and H 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 the methane. The calculated sulfate/methane interface (SMI) depth is 22 mbsf. The inverse correlation between sulfate and methane gradients suggests local methanogenesis; however, the low microbial biomass (1 x 106 cells/mL) cannot support the 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 deployment resulted in a slight bend to the needle probe, 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, 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 (Fig. F14), and porosity logs from downhole logging operations delineate a series of interbedded sand and mud facies that correspond to lithostratigraphic Unit II. Porosity decreases with depth from 87% to ~45% at a total depth of 297 mbsf, indicating compaction and potential fluid expulsion throughout the entire drilled interval. LWD resistivity images of the borehole show apparent breakouts at the bottom of the hole with an east-west orientation.
All primary and secondary drilling objectives were accomplished at Site U1320. Drilling results, together with those from Site U1319, provided key information on the space-time evolution of sedimentary and geochemical systems in Brazos-Trinity Basin #4 and on the range of variation for physical properties for this basin.Site U1321
Site U1321 is located on the southern portion of Brazos-Trinity Basin #4 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 to better correlate lithostratigraphic units and individual sand layers across the south-southwest margin of Brazos-Trinity Basin #4 and to document the lateral change in petrophysical properties (Fig. F15) of the fan units above Reflector R40 (Fig. F6, F15). The LWD data indicate a shallow series of interbedded sand and mud facies that correspond to lithostratigraphic Unit II in Hole U1320B. Porosity from logging data (Fig. F15) decreases with depth from 80% to 45% at ~34 mbsf, indicating rapid compaction and potential fluid expulsion in shallow sections. Most of the units identified in the logs seem to be thinning with respect to the units identified in Hole U1320B. Resistivity images of the borehole show apparent breakouts at the bottom of the hole with an east-west orientation, similar to what was observed at Site U1320. These breakouts indicate a north-south maximum horizontal stress direction. The resistivity images are also characterized by 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 permits bed by bed correlation between Sites U1319 and U1320 (see discussion in "Synthesis of Brazos-Trinity Basin #4 Geology"), which is critical for the study of sandy turbidites.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 permeable Blue Unit (Fig. F9). The principal objectives of drilling Site U1322 were to document rock physical properties at the location of minimal overburden in Ursa Basin, 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 sediment accumulation rates at this location. The ultimate goal of drilling in Ursa Basin was to explore fluid flow and fluid pressures in an overpressured basin.
Hole U1322A was the first dedicated MWD/LWD hole in Ursa Basin. It was decided to utilize MWD/LWD before coring because acquisition of real time pressure and lithology data were needed to know whether shallow water flow was occurring during drilling. The approach established for IODP that MWD/LWD as a viable tool to monitor pressure and lithology in a drill hole. Drilling in Hole U1322A advanced at an average rate of penetration (ROP) of 30 m/h to a depth of 200 mbsf. Below this, to prevent any communication with the Blue Unit, drilling continued at a reduced ROP of 20 m/h to the target depth of 238 mbsf. The MWD/LWD operation in Hole U1322A reached 238 mbsf without encountering any major sand units. Overall, 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 occasionally 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 the division of the lithostratigraphic column (Fig. F16) encountered in Hole U1322B into two lithostratigraphic units (Units I and II) and the further division of lithostratigraphic Unit I into Subunits IaId. The constructed synthetic seismogram for Hole U1322A demonstrates that the correlation between logging data and 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 GVR electrical images obtained in this hole reflect the occurrence of undisturbed sediments but also of 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 exposed by the drilling process.
Based on visual description of the cores in Hole U1322B, the 234.5 m sediment succession (Fig. F16) was divided into two lithostratigraphic units (Units I and II). The total depth of this succession ties closely to seismic Reflection 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 (Subunits IaId) based on the occurrence of intervals of deformed sediment. Lithostratigraphic Unit II is characterized by alternating intervals of meter-scale deformed and coherently laminated clay and mud. The deformed intervals are composed of brownish and greenish gray mud yielding intervals 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.
Preliminary biostratigraphic data from nannofossils and planktonic foraminifers indicate that the sediment sequence recovered at Site U1322 was deposited over the last 60 k.y., more specifically during MIS 14. Sedimentation rates of ~1 to 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 mass flow deposits. Distinctive cyclic patterns were observed in the distribution of nannoplankton and foraminifers, indicating periodic influx of sediments from the Mississippi River associated with turbidity currents in Ursa Basin. Persistent low-oxygenated "stress" environments due to rapid sediment loading encouraged the proliferation of infaunal benthic foraminifers. A deltaic benthic foraminifer assemblage from the interval between 185 and 234 mbsf is similar to those existing today along the shelf edge of the Mississippi delta, suggesting a period of strong gravity flow due to levee overspills or slope failures at ~60 ka.
Variations in physical properties correlate well 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 down to the bottom of the borehole. In slumped intervals, the porosity is lower than in nonslumped intervals: the maximum porosity decrease in the slumps is 5 porosity units. The undrained shear strength shows a more linear increase with depth (Fig. F16). A shift of 50 kPa at 125 mbsf is observed at the base of lithostratigraphic Subunit Id, a 30 m thick slump deposit. Smaller shifts are also observed at the base of each slump deposit. Slumps typically have higher undrained shear strengths than nonslumped deposits.
Chemical composition of the interstitial waters at Site U1322 shows large variations in the top 100 mbsf, in particular around the boundary between seismic Reflectors S10 and S20. Alkalinity, pH, concentrations of Ca, Sr, Li and B ions, and ammonium show concave depth profiles with maxima centered at the depth around Reflector S10. Above Reflector S10, salinity and sulfate concentrations are constantly high, and they show a rapid decrease between Reflectors S10 and S20 associated with the decrease of several other elements such as Ca, Mg, B, Li, and Sr. At Site U1322, the SMI is very deep at 74 mbsf, which corresponds to a rapid increase in methane concentration. Above the SMI, only minor amounts of methane (several ppmv) were detected. The highest concentration of methane (29,53651,001 ppmv) was 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 a biogenic origin of the methane, which could come from in situ microbial activities or hydrogeologically driven migration. Considering the low abundance of subsurface microbes, in situ methanogenesis should also be low. Hence, the anomalously high concentrations in the middle part of lithostratigraphic Unit I are inferred to be associated with fluid flow from beneath the above-mentioned strata.
A maximum cell density of 4.0 x 105 cells/mL was observed at 2.9 mbsf in Hole U1322B. Microbial abundance decreased with depth below the cell enumeration confidence limit of 1.0 x 104 cells/mL at 74.5 mbsf. The extremely low cellular biomass at Site U1322 is consistent with microbial abundance levels at Site U1324. An important 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 #4. This phenomenon is as yet unexplained.
In Hole U1322C, there was one high-quality DVTPP deployment at 236 mbsf and one high-quality T2P deployment at 150 mbsf. These provided us with a reasonable record of in situ temperature and pressure for Site U1322. Most of the other deployments recorded subhydrostatic pressures immediately after the drill string was raised. 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. This decision was also motivated by the fact that our revised authorized depth of penetration at Site U1323 (174 mbsf) meant that the interval of major scientific interest could not be penetrated. 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 to be sampled for geotechnical analysis and then processed through the onboard laboratories.
All of the objectives set for Site U1322 are considered fulfilled. The principal result was that we acquired a good data set of formation pressures and temperatures for this site that can now be compared to Site U1324. T2P and DVTPP measurements at Site U1322 provided critical data for understanding overpressure and associated flow in Ursa Basin. Dissipation curves at Site U1322 (seven measurements) document overpressure at 50 mbsf and continuing to the bottom. The temperature gradient at Site U1322 is 26.4°C/km (13 measurements from 42 to 238 mbsf).Site U1323
Site U1323 is located in Ursa Basin between Sites U1322 and U1324 (Figs. F8, F11). Site U1323 has a sediment cover above the permeable Blue Unit intermediate in thickness between those of Site U1322 and U1324. The objectives of drilling 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 sediment accumulation rates at this location. Site U1323 was logged using MWD/LWD and was not cored. This was because an overpressured sand was encountered during MWD/LWD at a relatively shallow depth. Ultimately, we chose to spend the remaining time making downhole measurements at Site U1322 rather than return to Site U1322 for coring.
MWD/LWD at Site U1323 proceeded at an average ROP of 30 m/h, but borehole diameters were typically >24 cm to a depth of 204 mbsf, where an overpressured sand was encountered. The silty sand layer, ~3 m thick, was detected at 204 mbsf and simultaneously a jump in pressure of 150 psi (~1 MPa) over the background drilling pressure in the APWD log was observed. A residual backpressure of 150 psi was also observed by the driller when he shut down the mud pumps. At 242 mbsf, a rapid drop in gamma radiation, suggestive of a second sand interval, was observed. 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. Site U1323 was the JOIDES Resolution's first experience with riserless drilling using weighted mud. It was a valuable learning exercise and everyone came away with confidence in the ability to handle downhole pressures in a routine fashion. We confirmed that we can carefully monitor shallow flows, take appropriate action to control the flow, drill ahead under appropriate conditions, and provide accurate real-time downhole information.
Lithostratigraphic interpretation of LWD and seismic data are still preliminary as a malfunctioning battery required onshore processing of the resistivity image data before a comprehensive interpretation could be performed. Logging and seismic data (Fig. F17) confirm that the upper 197 m interval is predominantly mud and clay rich, including two mass transport deposits. Preliminary interpretation of the resistivity image data that were sent back to the ship at the end of the expedition shows several highly deformed intervals confirming the original logging-seismic interpretation of the presence of several mass transport deposits. These mass transport deposits also displayed trademark characteristics of higher bulk density and resistivity compared with surrounding undeformed sediment and will provide great data for postcruise analysis of such deposits.
Despite not coring Site U1323, the high-quality logging data prove 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 the 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 sediment package above the permeable Blue Unit. The principal objectives of drilling Site U1324 were to document rock physical properties at the location of maximum overburden thickness in Ursa Basin, 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 sediment accumulation rates at this location.
The stratigraphy of Hole U1324A (Fig. F18) was first divided into two main units based on logging responses. 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 (S10S50) 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°). This suggests a significant amount of deformation most likely caused by mass transport events. These events seem to indicate a high degree of slope instability due to the presence of turbidites that have been dislocated, and in some instances, also folded. 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 lithologies into two lithostratigraphic units. Lithostratigraphic Unit I is composed of clay and mud and contains two mass transport deposits. Lithostratigraphic Unit II is composed of interbedded silt, sand, and mud and contains at least three mass flow deposits. Prior to Expedition 398, seismic correlation suggested that acoustically semitransparent intervals in lithostratigraphic Unit I represent regional mass flow deposits composed of faulted and contorted masses of mud and clay. However, close examination of the cores reveals that the mass flow deposits 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 well with lithostratigraphic units. The interbedded silt, sand, and mud and mass flow deposits 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 density is 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.
Preliminary biostratigraphic data from 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 14. 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 sediments from the Mississippi River associated with turbidity currents in Ursa Basin. The infauna-dominated benthic foraminifer assemblages also suggest a prevalence of low-oxygenated "stress" environments due to rapid sediment loading in the basin during the last glacial period.
Variation in interstitial water chemistry at Site U1324 is largest at shallow depths (<100 mbsf). Below this depth, only very limited changes are observed. Pronounced pore water chemical changes are particularly important from the seafloor to the S10 seismic reflector (~35 mbsf). Li, B, and Sr reach their maxima within this depth range, and Mn reaches its minimum at ~3540 mbsf. H4SiO4 and Fe reach their maxima between ~20 and 25 mbsf. Between 40 and 160 mbsf, salinity, Li, B, and Sr decrease; Ba, Fe, and NH4 increase; and Cl, Mn, and H4SiO4 are constant. The extremely high ammonium contents (up to 6820 conditions at this site compared with the sites drilled in Brazos-Trinity Basin #4. The general downhole increase in ammonium likely reflects enhanced organic degradation at greater depths. The vertical profile, especially the surficial maximum and minimum in dissolved Fe and Mn, are consistent with the hierarchy of redox reactions often observed in deep-marine sediments. The high Fe contents at shallow depths might reflect enhanced Fe reduction and/or greater availability of detrital Fe 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, Ca, Mg, Sr, B, and Li concentrations at ~35 mbsf. The causes for the very acidic (pH < 7.0) nature of the pore water found above 200 mbsf at Site U1324 are unclear and needs further investigation.
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 for the methane. 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 x 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. At this time, the predominance of clay-rich sediment at Site U1324 preventing fluid migration is the only reason thought to explain the low abundance of microbial communities.
In situ measurements made with the T2P and DVTPP documented fluid overpressure and a low thermal gradient at Site U1324. Successful fluid pressure measurements at 117, 300, 405, and 608 mbsf yielded values for * between 0.2 and 0.6 (* = ratio of overpressure to hydrostatic vertical stress). Eighteen temperature measurements constrain a geothermal gradient of about 19°C/km.
All of the objectives set for Site U1324 are considered fulfilled. The principal result is that we acquired a good data set of formation pressures and temperatures for this site. Once compared to a similar data set at Site U1322, this will allow testing of the model of overpressure on which the strategy of Expedition 308 was based. However, the fact that pore water pressures in Holes U1324B and U1324C lie between the lithostatic and hydrostatic pressure gradient is already a demonstration that Ursa Basin is overpressured. Moreover, results from lithostratigraphy and biostratigraphy have constrained unusually high sedimentation rates and the timing of mass flow deposition at this location. The data and observations made at Site U1324 are critical for logging-seismic integration, and for the understanding of geological processes leading to overpressure in Ursa Basin.
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