Previous   |    Next

doi:10.2204/iodp.proc.344.101.2013

Principal results

Site U1381 summary

Background and objectives

Fundamental to sampling and quantifying the material comprising the seismogenic zone of a convergent margin is an understanding of the nature of the sediment and oceanic crust entering the seismogenic zone and the hydrologic and the thermal state of the oceanic crust. Site U1381 serves as a reference site on the subducting aseismic Cocos Ridge.

Site U1381 is ~4.5 km seaward of the deformation front offshore Osa Peninsula and Caño Island, Costa Rica (Fig. F6). This site was chosen for multiple reasons. First, a clear seismic record of the plate stratigraphy and basement is present at this site (Fig. F8). The seismic section shows a 100 m thick sediment section resting on reflective basement interpreted as Cocos Ridge igneous crust. The sedimentary section is composed of pelagic and hemipelagic sediment (Expedition 334 Scientists, 2012d). Secondly, it is located on a local basement high. Basement relief often focuses fluid flow, so data from this site are likely to document the vigor of fluid flow in this area. Thirdly, Site U1381 is far enough away from the frontal thrust that it is expected to be reasonably free from the influence of downslope debris flows from the convergent margin slope. Finally, Site U1381 is located on the same seismic line as Sites U1412, U1380, and U1379 (Fig. F2). Paleomagnetic data constrain the age of this portion of Cocos Ridge to 14 Ma (Barckhausen et al., 2001).

During CRISP Expedition 334 in 2011, Site U1381 was cored using the RCB because the primary objective was oceanic crust and time constraints precluded APC drilling through the sediment section (Expedition 334 Scientists, 2012d). However, a disadvantage of the RCB system is that recovered soft sediment is usually highly disturbed. Thus a primary goal for revisiting this site was to core the sediment section with the APC to obtain more pristine samples and better document the nature of the sediment entering the seismogenic zone.

Operations

After a 403 nmi transit from Panama, the vessel stabilized over Site U1381 at 0620 h on 26 October 2012 (all times in text are ship local time, which is UTC – 6). Hole U1381C was spudded at 2015 h on 26 October (8°25.7027′N, 84°9.4800′W, 2064.6 m water depth; Table T1). Advancement for Hole U1381C was 103.8 mbsf, with 103.5 m cored with the APC and 0.3 m cored with the extended core barrel (XCB). Recovery with the APC was 108.70 m (105%), and recovery with the XCB was 0.33 m (110%). The advanced piston corer temperature tool (APCT-3) was deployed on Cores 344-U1381C-3H, 5H, 6H, and 7H, and four good temperature curves were recorded. The FlexIT orientation tool was deployed on Cores 344-U1381C-1H through 9H with good results. After reaching basement, the APC was changed to the XCB and a single short core was cut to confirm and recover basement material. Hole U1381C ended at 1545 h on 27 October, and a total of 33.5 h was spent at Site U1381.

Principal results

The cored interval in Hole U1381C comprises five units (Fig. F9). The top of the hole contains hemipelagic sediment that is typical of an incoming plate location near a terrigenous source. The Unit I/II boundary is present in Core 344-U1381C-7H (55.93 mbsf) and is characterized by a change from greenish grayish silty clay with some nannofossils and terrigenous minerals to brownish grayish nannofossil-dominated ooze with abundant to common foraminifers and sections of abundant sponge spicules. This boundary was identified in smear slide and X-ray diffraction (XRD) data. The upper portion of Unit II is also characterized by a high abundance of tephra layers that reach 42 cm in thickness. Core description and smear slide data indicate a lithologic change in Core 344-U1381C-11H to finer grained nannofossil-rich clay with only some glass and feldspar components that marks the Unit II/III boundary (100.64 mbsf). The Unit III/IV boundary (103.20 mbsf) in Core 344-U1381C-12H again shows a drastic change in lithology to a pure clay/claystone. XRD data indicate mixing of smectite (probably saponite), pyrite, and to a lesser extent plagioclase. Sphalerite, calcite, and anhydrite are present as minor phases. Unit V starts at 103.55 mbsf in Core 344-U1381C-13X and consists of brecciated basalt. Alteration of the basaltic groundmass is slight to moderate, with smectite replacing interstitial glass and partially corroding plagioclase and clinopyroxene phenocrysts. Overall, this sediment reflects a sequence of hemipelagic siliceous ooze with terrigenous input, presumably remobilized from the slope sediment sequence, overlying sponge spicule–rich nannofossil calcareous ooze, pelagic calcareous ooze, and pelagic claystone. In addition, 84 tephra layers were recovered in Units I–IV.

The biostratigraphy of Hole U1381C was determined from calcareous nannofossils and radiolarians identified in core catcher samples from Cores 344-U1381C-1H through 11H. Biostratigraphic zones indicate that the base of Unit I is early Pleistocene (~1–2 Ma). Unit II represents the middle Miocene (~11–13 Ma), and based on nannofossil and radiolarian biostratigraphy, a ~9–11 m.y. hiatus separates Units I and II.

Benthic foraminifers are present throughout the cores, ranging from 1% to 5% of the total 125 µm sample size. Preservation is good, with little evidence of dissolution or breakage. Benthic foraminiferal assemblages from Unit I are substantially different than those in Unit II, indicating large paleoenvironmental changes related to organic carbon flux and bottom water ventilation. Of particular interest is the substantial presence of representatives of the Stilostomella extinction group in Sample 344-U1381C-6H-CC. These assemblages provide an alternative biostratigraphic datum to the calcareous nannofossils.

Thirty-five measurements of bedding were taken principally from tephra layers in Hole U1381C. Bedding is generally subhorizontal to gently dipping. In Unit III, near the bottom of the sedimentary succession, we observed a few deformation bands and mineral-filled extensional and shear fractures, which have a normal displacement component.

We collected and processed 28 whole-round (WR) core samples for interstitial water analyses and an additional 3 WR samples for helium analyses. In all cases, enough fluid was collected for shipboard and shore-based requests with 10 cm long WRs. Similar to Expedition 334 data from Hole U1381A, there is little variation in salinity, chloride, sodium, and potassium. Diagenesis of organic matter in the upper sediment is indicated by increases in alkalinity, ammonium, and phosphate and consumption of sulfate, which reaches a minimum value of 11 mM at ~40 mbsf. A concomitant calcium decrease indicates that carbonate precipitation is driven by the increase in alkalinity. The sulfate profile shows a reversal below ~40 mbsf, with a steady increase in concentration with depth. This observation is similar to that previously reported for Expedition 334 Hole U1381A and for the incoming sediment section offshore the Nicoya Peninsula. Overall, the concentration-depth profiles of Ca, SO₄, Li, Mn, and possibly Si indicate diffusional interaction with an altered seawater fluid in the oceanic basement.

We collected and analyzed 29 headspace samples for organic geochemistry analyses. Except for elevated values of hydrocarbons in the uppermost sample (at 1.5 mbsf), which has been attributed to drilling contamination, methane occurs only in trace amounts and no heavier hydrocarbons were detected.

Inorganic carbon distribution increases with depth in Hole U1381C, consistent with a change in lithology from the silty clay sediment of Unit I to the foraminiferal carbonate ooze of Unit II. The total organic carbon (TOC) concentration throughout the entire sediment section ranges from 1 to 2 wt%. The total nitrogen (TN) concentration ranges between 0.05 and 0.20 wt%.

Physical property measurements show high porosities (Unit I = 75%; Unit II = 78%) and low bulk densities (Unit I = 1.4 g/cm³; Unit II = 1.3 g/cm³), with little evidence for compaction. Magnetic susceptibility values stay fairly constant with depth but have intermittent peaks in Unit I and Unit II that correlate with tephra layers. Natural gamma radiation (NGR) gradually increases with depth and is highly variable around the Unit I/II boundary. Electrical conductivity values appear generally consistent with porosity trends. P-wave velocity measurements indicate slightly higher values in Unit II (1540 m/s) than in Unit I (1520 m/s), despite elevated porosities in Unit II, and P-wave velocity values are highest in Unit III (1620 m/s). Vane shear and penetrometer measurements show strength values generally increase downhole throughout Units I and II. Thermal conductivity results are inversely correlated with porosity. Downhole temperature measurements are consistent with those obtained previously on Expedition 334 and suggest a thermal gradient of 231°C/km. The calculated heat flow of 185 mW/m² is higher than that predicted for 15 Ma crust, suggesting significant fluid flow in the underlying ocean crust.

Downhole variations in the natural remanent magnetization (NRM) intensity for archive-half cores correlate well with lithology in Hole U1381C. Paleomagnetic measurements indicate that the silty clay of Unit I (0–55.93 mbsf) has a mean NRM intensity on the order of 10^–2 A/m, whereas the foraminifer-rich ooze in Unit II has a lower NRM intensity of 10^–2 to 10^–3 A/m. A number of higher NRM peaks appear in both Units I and II and can be tied directly to the presence of tephra layers. The measured NRM declinations of different cores are scattered, but upon correction with the FlexIT orientation data, declinations become close to magnetic north, indicating the remanence is of geomagnetic origin. Magnetic properties obtained from the archive section halves were confirmed by discrete sample measurements.

Characteristic remanent magnetization (ChRM) declinations and inclinations from discrete measurements were used to define magnetic polarity sequences for the oriented core section in Hole U1381C. At a low-latitude area such as Site U1381, a near 180° shift in declination in the cores is a more reliable sign of a polarity transition than a change in the magnetic inclination. For the upper part of Unit I, both pass-through and discrete sample measurements show signs of dominantly normal polarity of ChRM. An additional constraint is provided by an ash layer at ~25 mbsf, which is inferred to be the Ar-Ar dated (320 ka) Tiribí Tuff ash layer (Pérez et al., 2006). Thus, we interpret that the sediment from 0 to 49 mbsf was deposited within the Brunhes Chron (<0.78 Ma). In Sections 344-U1381C-6H-5 through 6H-7, we see dominantly reversed polarity in the section-half measurements. Discrete samples from this interval also show negative inclinations, and corrected declinations show a ~180° shift, consistent with the magnetization acquired in a reversed field. Thus, we tentatively conclude that the Brunhes/Matuyama boundary is at 49 mbsf in Section 344-U1381C-6H-3.

Site U1380 summary

Background and objectives

Site U1380 is located on the middle slope of the Costa Rica margin along seismic Line BGR99-7 (Fig. F6). Interplate earthquakes and geodetic measurements indicate that this site is seaward of the updip extent of seismicity and that the plate interface is not locked (Fig. F3). At Site U1380, the margin consists of a 550 m thick section of slope sediment overlying upper plate framework rock (Fig. F10). Hole U1380A was drilled during Expedition 334 to ~480 mbsf, but because of poor drilling conditions it was terminated prior to reaching the framework rock (Expedition 334 Scientists, 2012c). The primary goals for revisiting this site were to determine the nature, composition, and physical properties of the upper plate framework rock; to understand the nature of the landward-dipping seismic reflectors; and to estimate the state of stress. Seismic reflection Line BGR99-7 shows that this site is above the seaward edge of one of the high-amplitude reflectors.

Operations

After an 11.4 nmi transit from Site U1381, the vessel stabilized over Site U1380 at 1715 h on 27 October 2012. Hole U1380B was spudded at 0118 h on 28 October (8°35.9952′N, 84°4.3908′W, 502.7 m water depth; Table T1). The purpose of this hole was to conduct a jet-in test with a 14¾ inch tricone bit to verify the length of 16 inch casing that could be jetted into the formation with a reentry cone.

Hole U1380C was spudded at 1145 h on 29 October (8°35.9879′N, 84°4.3918′W, 502.7 m water depth; Table T1). The operations plan for this hole included a reentry system with two strings of casing to an undetermined depth, followed by RCB coring to 800 mbsf. The reentry cone and 16 inch casing string were installed to 48 mbsf. A 14¾ inch bit was used to drill the hole to 438 mbsf. The initial plan was to run 10¾ inch casing string to 430 mbsf. After drilling the 14¾ inch hole, it became impossible to keep the hole open to run casing to 430 mbsf. A new casing length of 405 m was selected to try to bridge a problematic zone from 337 to 396 mbsf. The hole was reamed and cleaned repeatedly prior to running casing. Eventually, the problematic zone was cemented and redrilled. After several days of hole cleaning, the 405 m long 10¾ inch casing string was run into the hole. With the end of the casing 18 m above the desired depth, we were unable to advance the casing any further and were forced to pull the casing back to the surface. To avoid disassembling the entire casing string, we secured the casing string to the moonpool doors and tripped back in the hole with a 9⅞ inch tricone bit and a 14¾ inch underreamer. We then attempted to ream the hole back to bottom. During this process, 391 m of casing parted just below sea level and followed the drill string down into the hole. To determine the condition of the casing string inside the hole, we switched to a coring bottom-hole assembly (BHA) with an RCB bit and drilled down carefully from 336 to 438 mbsf. RCB coring commenced at 1745 h on 7 November and continued without interruption until 0400 h on 10 November to a total depth of 800 mbsf. Nonmagnetic RCB core barrels were used starting with Core 344-U1380C-11R. In an effort to improve recovery we started cutting half cores with Core 344-U1380C-25R. The total cored interval in Hole U1380C was 362.0 m, with 202.4 m recovered, for an overall recovery of 56%.

The hole was prepared for logging, the bit was dropped on the seafloor, and the triple combination (triple combo)–Ultrasonic Borehole Imager (UBI) logging string was rigged up. During the first logging attempt, the logging tools were unable to advance past 398 mbsf, which coincided with the estimated depth of the casing shoe. The tools were pulled from the hole, and the drill pipe was advanced to 467 mbsf without difficulty. The drill pipe was pulled back to 394 mbsf, and a reconfigured triple combo string without the UBI was run into the hole at 2310 h. This time the logging tools made it to 458 mbsf before encountering an obstruction. A review of the logs showed that the casing had slipped further down. After retrieving the logging tools, a BHA with a used RCB bit was deployed, the hole was washed down to 781 mbsf, the hole was conditioned for logging, and the bit was dropped in the bottom of the hole. The triple combo logging tool string was rigged up for the third time and run into the hole at 1310 h on 13 November. However, the logging tools encountered a similar obstruction at 462 mbsf. After several attempts to get the tools to reenter the open hole, logging operations were terminated. The hole was plugged with a 10 bbl cement plug, and the acoustic beacon was recovered. The drill string was pulled from the hole, clearing the rig floor at 0055 h on 14 November and ending Hole U1380C operations. A total of 17.3 days were spent on Site U1380.

Principal results

Hole U1380C was cored to investigate the lithostratigraphy and structural geology of the lower portions of the middle slope sequence and the uppermost portions of the basement, as interpreted in multichannel seismic reflection data. These data are summarized in Fig. F11. This hole deepens Site U1380, drilled during Expedition 334 (Expedition 334 Scientists, 2012c), and is a complementary hole to nearby Site U1378 where only the upper 539.9 m of the slope sediment was drilled during Expedition 334.

The majority of the sedimentary sequence consists of clayey siltstone (~59%) and silty claystone (~25%) with common interlayered centimeter- to decimeter-sized beds of fine- to medium-grained sandstone (~15%). The siltstone sedimentary sequence is disrupted by coarse-grained shell-rich sandstones, two conglomerates (~1%), and 25 tuff layers (<1%). Three lithostratigraphic units were distinguished based on lithologic compositional changes.

Unit I extends from 438 to 552.72 mbsf (Sections 344-U1380C-2R-1 to 13R-7, 55 cm) and is characterized by massive dark greenish gray silty clay and three sandstone-rich horizons with centimeter- to decimeter-sized sandy layers. These sandy layers are cemented with calcite and increase in abundance, thickness, and grain size with depth. Matrix components comprise lithic fragments (sedimentary and magmatic), feldspar, glass shards, and amphiboles. Biogenic components include rare nannofossils, diatoms, and foraminifers.

Unit II extends from 552.72 to 771.62 mbsf (Sections 344-U1380C-13R-7, 55 cm, to 47R-3, 106 cm) and is visually defined by a relatively sharp lithologic change into a greenish gray clayey siltstone with intercalated sandstone and conglomerate layers. Unit II is further divided into two subunits based on compositional and depositional variations. Subunit IIA is characterized by well-lithified siltstones and poorly to weakly lithified sand beds that contain abundant to common shell fragments. The matrix also contains abundant lithic fragments, feldspar, and zeolites, but the main mineralogical variation is the higher abundance of amphibole in this subunit. Low recovery hinders the precise measurement of the thickness of Subunit IIA (~11.38 m), but we assume that the missing material is composed of the same poorly lithified sand as the recovered sections. The lithologic boundary between Subunits IIA and IIB (123.24 m thick) in Core 344-U1380C-15R (564.10 mbsf) was not recovered. Subunit IIB is very dark greenish gray clayey siltstone characterized by two fining-upward sequences of centimeter- to decimeter-thick, medium- to coarse-grained sandstones and fine conglomerates. Matrix and sandstone components are predominantly terrigenous lithic fragments (magmatic more common than sedimentary), feldspar, glass, and rare heavy minerals. Biogenic material is mostly absent, and none was observed toward the bottom of the unit. Highly altered tuff layers in the lower parts of Cores 344-U1380C-30R (688.04 mbsf) and 47R (770.56 mbsf) are characterized by color changes from the dark greenish gray clayey siltstone to a reddish brown calcareous siltstone and by variations in the matrix mineralogy.

Both subunits of Unit II are characterized by normally graded sandstone beds that are often laminated, with abundant sapropel fragments that form several centimeter-thick conspicuous horizons between the laminae. The conglomerates are very thickly bedded and contain poorly sorted, pebble-sized, matrix-supported lithic fragments. Upper contacts between fine- and coarse-grained sediment, when observed, are mostly gradational, whereas the lower contacts are erosional and commonly contain rip-up clasts, load clasts, and sand lenses. In general, Unit II is more indurated and lithified than Unit I.

Unit III (Cores 344-U1380C-47R through 52R) is a 29.44 m thick, fine-grained silty claystone with rare but thick (as thick as 1.20 m) intercalated fine- to coarse-grained sandstone. The matrix contains mostly terrigenous material dominated by lithic fragments and feldspar but is nearly devoid of biogenic material.

Unit I is a continuation of sediment cored in Hole U1380A during Expedition 334 (Expedition 334 Scientists, 2012c) and correlates well with Unit II from Hole U1378B (Expedition 334 Scientists, 2012a). The sedimentary succession recovered from Hole U1380C likely reflects an alternating, terrestrially sourced, turbiditic upper slope (Units I and III) to shelf (Unit II) sequence, eventually influenced by deltaic-derived sediments. The dynamic depositional environment is manifested by varying grain sizes that range up to pebble-sized conglomerates, especially in Unit II, and the cyclical increase in sandstone abundance throughout the sedimentary sequence.

Deformation structures increase gradually in Cores 344-U1380C-2R through 13R. Normal faults are present throughout the interval, with reverse faults increasing downhole. Deformation culminates in Cores 344-U1380C-11R through 13R with the development of pronounced foliation. Bedding is commonly steeply dipping. Deformation is moderate in Cores 344-U1380C-14R through 45R, and there is no foliation. Bedding remains steep until Core 344-U1380C-31R, and from Core 32R downhole it is subhorizontal to gently dipping. In the deeper cores there is an increase of discrete brittle shear zones.

The biostratigraphy of Hole U1380C was determined by nannofossils because core catcher samples were barren of radiolarians. Four biostratigraphic zones were identified. The interval from Sample 344-U1380C-2R-CC to 4R-CC is assigned to nannofossil Zones NN19–NN21. The second interval, between Samples 344-U1380C-5R-CC and 8R-CC, is assigned to Zone NN19. The third zone, NN18, is defined by the first downhole occurrence of Discoaster brouweri, which appears in Sample 344-U1380C-9R-CC. The fourth biostratigraphic zone encompasses Samples 344-U1380C-51R-CC through 52R-CC (791–797 mbsf) and is assigned to Zones NN15–NN17, based on the last occurrence (LO) of Discoaster pentaradiatus, the first occurrence (FO) of Pseudoemiliania lacunosa in Sample 344-U1380C-51R-CC, and the FO of Discoaster asymmetricus in Sample 344-U1380C-52R-CC. Therefore, the oldest sediment found at this hole is older than 2.4 Ma.

Benthic foraminifers were studied in 44 of the 52 core catcher samples collected. Benthic foraminifers vary from few (Samples 344-U1380C-2R-CC through 9R-CC) to present (Samples 344-U1380C-31R-CC through 52R-CC), and preservation ranges from moderate where abundance is higher to poor where foraminifers are present or rare. Overall, benthic foraminifer assemblages are characterized by species generally associated with organic carbon–rich environments and low bottom water oxygenation such as Uvigerina peregrina, Epistominella smithi, Brizalina spp., and Hansenisca altiformis. Downhole changes in benthic foraminifer assemblages are relatively subtle.

The most significant geochemical finding is the presence of fluid flow through a shear zone that extends from ~480 to 550 mbsf, at the boundary between lithostratigraphic Units I and II. The Cl, Li, and hydrocarbon data indicate that this fluid originated from a source depth where temperatures are >90°C. Below this horizon there is a marked increase in C₁/C₂₊ ratio toward a more biogenic signature of the gases, which is consistent with pore water data (Sr and Li) that indicate lower temperature volcanic tephra alteration reactions within the sediment of lithostratigraphic Unit II. Both the inorganic and organic carbon concentrations of the sediment are distinctly higher in lithostratigraphic Unit I than in the sediment underlying the 550 mbsf unconformity.

Within Unit I, magnetic susceptibility is generally low, NGR increases with depth to values of 38 cps, and bulk density and porosity appear to follow a steady compaction trend that is consistent with that observed in shallower sediment from Site U1378, located 1 km from Site U1380. P-wave velocity, thermal conductivity, and sediment compressive strength increase with depth.

Most of the physical properties show sharp changes at the Unit I/II boundary, near the major seismic reflector at 550 mbsf. Below this boundary, magnetic susceptibility becomes more variable and NGR counts decrease. Between 550 and 555 mbsf, bulk density increases from ~2.0 to 2.1 g/cm³, porosity decreases from 43% to 35%, and thermal conductivity, P-wave velocity, and sediment strength increase.

A second major change in physical properties occurs at ~700 mbsf and is identified by a strong positive spike in magnetic susceptibility, a positive spike in P-wave velocity, and a slight negative excursion in NGR. In addition, bulk densities peak at 2.2 g/cm³ and porosities reach their lowest values of 25% to 26%. This horizon could be correlated to a strong landward-dipping reflector at ~700 mbsf. Near the Unit II/III boundary (771.62 mbsf), there is a notable change in magnetic susceptibility, P-wave velocity, and thermal conductivity.

NRM measurements were made on all archive-half cores and on 60 discrete samples taken from the working halves in Hole U1380C. Archive-half cores were subjected to alternating field (AF) demagnetization up to 30 mT and discrete samples to stepwise thermal and AF demagnetization up to 475°C and 120 mT, respectively, in order to establish a reliable magnetostratigraphy at this site and to observe the magnetic properties of each lithology in the lithostratigraphic units recovered.

Several relatively well defined polarity intervals have been identified in downhole magnetostratigraphic records in spite of some samples showing unstable and ambiguous magnetization. Based on biostratigraphic data, we were able to tentatively correlate certain parts of the magnetic polarity interval recorded in the sediment with the geomagnetic polarity timescale. The Matuyama/Gauss Chron boundary (2.581 Ma) is tentatively placed at ~770 mbsf, based on both discrete sample and pass-through results.

Site U1412 summary

Background and objectives

Site U1412 (proposed Site CRIS-9A, located on seismic Line BGR99-7) targeted the frontal sedimentary prism at the base of the slope near the toe, the main subduction thrust, the underthrust sedimentary section beneath the subduction thrust, and the upper oceanic crust (Fig. F6). The scientific objectives were to (1) characterize the frontal sedimentary prism by documenting its lithology and age; (2) sample the décollement at a shallow depth to define the deformation style, stress, fault friction, and behavior; (3) characterize any diagenetic processes, microstructures, and potential sealing/healing processes; (4) characterize the physical properties of the subducting incoming and frontal prism sediments that are thought to mix with eroded upper plate material; and (5) determine the chemistry of pore waters within the wedge, décollement, and underthrust sections and characterize the fluid pathways.

Operations

After a 7.0 nmi transit from Site U1380, the vessel stabilized over Site U1412 at 0200 h on 14 November 2012. The original operations plan for Site U1412 called for two holes, an APC/XCB hole to ~500 mbsf and an RCB hole to 980 mbsf. Eventually, four holes were cored at this site. Coring conditions at this site were extremely difficult with below average recovery, and all four cored holes were abandoned short of the targeted depths because of poor hole conditions. Hole U1412A (8°29.3294′N, 84°7.6686′W, 1921 m water depth; Table T1) was spudded at 1520 h on 14 November. This was an APC/XCB drilled hole to 200.3 mbsf. APCT-3 formation temperature measurements were taken with Cores 344-U1412A-3H, 5H, 7H, and 8H. The FlexIT orientation tool was deployed with Cores 344-U1412A-1H through 15H. Core 344-U1412A-25X reached 200.3 mbsf before the hole became too unstable to continue coring. The vessel was offset 350 m in a south-southwest direction in an attempt to find a more favorable coring location. Hole U1412B (8°29.1599′N, 84°7.7512′W, 1965 m water depth; Table T1) was spudded at 2350 h on 16 November. This hole was drilled without recovery to 149.1 mbsf, and Core 344-U1412B-20X reached 304.3 mbsf before deteriorating hole conditions forced us to stop coring. Logging with the triple combo tool string was attempted on 20 November, but the tools could not exit the BHA and the drill string became stuck. After freeing the drill string, the logging tools were laid out and the hole was abandoned. Because this hole did not reach the depth objective, a third hole was attempted 50 m to the northeast. Hole U1412C (8°29.1700′N, 84°7.7467′W, 1965 m water depth; Table T1) was spudded at 0730 h on 21 November. This hole was drilled without recovery to 300 mbsf, and Core 344-U1412C-10R reached 387 mbsf. After several hours spent trying to clean the hole and getting stuck, the decision was made to abandon Hole U1412C and proceed to Site U1413. After coring objectives were completed at Site U1413, the vessel returned to Site U1412. Hole U1412D (8°29.1402′N, 84°7.7793′W, 1973 m water depth; Table T1) was spudded at 0850 h on 1 December. This hole was drilled without recovery to 350.4 mbsf. We were only able to recover Cores 344-U1412D-2R and 3R before the hole collapsed.

A total of 55 cores were recovered at this site: 16 APC cores, 28 XCB cores, and 11 RCB cores. The APC cored interval was 114.9 m, with 115.5 m recovered (101%), the XCB cored interval was 240.6 m, with 82.8 m recovered (34%), and the RCB cored interval was 105.8, with 48.8 m recovered (46%). The overall recovery for Site U1412 was 54%. The total time spent on Site U1412 was 277.5 h or 11.56 days (60.25 h in Hole U1412A, 104.5 h in Hole U1412B, 71.25 h in Hole U1412C, and 41.5 h in Hole U1412D).

Principal results

Site U1412 was drilled to investigate the lithostratigraphy and structural geology of the slope-toe sequence and the uppermost portions of the basement, as interpreted in multichannel seismic reflection data (Fig. F12). The primary goal of this site was to penetrate the décollement and investigate the fluid flow regime within the sediment as well as the oceanic crust.

Three units were distinguished in the sedimentary rocks (Fig. F13). Unit I is predominantly dark greenish to greenish gray clay (33%) with small variations in grain size to silt (16%) and silty clay (17%) and contains eight tephra layers. This is terrigenous-influenced sediment. Unit I shows indications for gas expansion and gas hydrate dissociation. Unit II is light to dark grayish brown calcareous nannofossil ooze with varying amounts of diatoms and sponge spicules. Primary sedimentary structures could not be observed because of very strong drilling disturbance. Unit III is a sequence of grayish green clayey siltstone with minor interlayered sandstone and contains five dark grayish tephra layers. The Unit III matrix composition is dominated by terrigenous clay and contains nannofossils as well as radiolarian and diatom fragments. Hole U1412D sediment is slightly more clayey, highly bioturbated, and shows a decrease in the biogenic components.

The change in lithology at this site from terrigenous input to Miocene calcareous ooze to Pleistocene hemipelagic sediment likely reflects accretion of large slivers of the incoming plate.

Nannofossil biostratigraphy indicates that the upper portion of Hole U1412A is Zone NN19 (0.4–1.89 Ma). A major change from Pleistocene to Miocene is recorded in Core 344-U1412B-8X, consistent with the lithostratigraphic boundary between Units I and II (204.74 mbsf). In Hole U1412C, Samples 344-U1412C-2R-CC through 5R-CC are indicative of the middle Miocene. A major age inversion is recorded between Cores 344-U1412B-5R and 6R, with the reappearance of Pleistocene assemblages. Benthic foraminifers are common in the upper sections of Hole U1412A and include Uvigerina peregrina, Uvigerina auberiana, Cibicidoides pachyderma, Melonis affinis, and Cassidulina carinata. A substantial change in foraminiferal assemblages occurs starting with Core 344-U1412B-7X and is characterized by Globocassidulina subglobosa, Cibicidoides pachyderma var. bathyalis, Hansenisca altiformis, and Planulina renzi. The latter species last appeared in the middle Miocene, which agrees with the nannofossil and radiolarian biostratigraphy.

Hole U1412A has subhorizontal to gently dipping bedding at the very top of the drilled section. Steeply dipping normal faults are common in the lower part of the section. Hole U1412C has steeply dipping bedding, normal and reverse faults, and intervals of brecciated sediment, in some cases with fragments where striated and polished surfaces are developed. The fault zone with well-developed foliation located at 330 to 342 mbsf coincides with an age reversal from Miocene to Pleistocene. Such large accumulated displacement implies that the fault zone has been highly active and has experienced multiple events.

The upper sections of sediment reflect changes associated with organic matter diagenesis. The sulfate–methane transition zone (SMTZ) occurs at 14.73 mbsf, and high-resolution headspace samples were collected below this depth for shore-based geomicrobiology studies. The decrease in calcium and magnesium concentrations at the SMTZ depth reflects precipitation of authigenic carbonates. The presence of gas hydrate in the sediment was inferred from discrete chloride anomalies and excursions to low C₁/C₂₊ ratios between 60 and 85 mbsf, consistent with observations of mousse-like texture in sediment. We observe decreasing concentrations in alkalinity, boron, and magnesium, whereas calcium, barium, strontium, and lithium concentrations increase with depth. There is, however, not enough resolution in the sample coverage from lithostratigraphic Units II and III to delineate in situ diagenetic reactions or fluid migration pathways. In general, methane concentrations increase rapidly below 14.1 mbsf, consistent with the depth of the SMTZ at this site. The gas composition of the headspace and void gas is consistent with a biogenic methane source. However, an observed decrease to ~400 in the C₁/C₂₊ ratios of headspace and void gas samples and a few occurrences of propane in Unit III point to a mixture of biogenic gas and thermogenic hydrocarbons that migrated from depth.

Physical properties measured indicate that bulk density values increase and porosity values decrease in the upper 30 m. Both properties remain stable to the base of Unit I in Hole U1412A, with average bulk density and porosity values of 1.69 g/cm³ and 60%, respectively. Bulk density and porosity values in Unit III show considerable variability but suggest slightly more compaction (higher bulk density and lower porosity) than at the base of Unit I. In Unit I, background magnetic susceptibility is low, with some high excursions. NGR is relatively stable throughout Unit I and higher and more scattered within Unit III. Strength increases in the upper 30 m and then remains stable until Unit III, where it is higher and more scattered. Formation factor was measured in the uppermost 110 m, and values vary between 1.5 and 4. Thermal conductivity and P-wave measurements were rare and scattered because of gas disturbance. Measured P-wave velocities are low, varying between 1500 and 1600 m/s. Thermal conductivity ranges from 0.7 to 1.3 W/(m·K). A least-squares linear fit to the downhole temperature data yields a gradient of 114°C/km.

We measured the NRM of archive section halves and demagnetized them in an alternating field up to 40 mT. There are more significant variations in NRM intensity within lithostratigraphic Unit I than in Units II and III. We demagnetized 61 discrete samples to verify the section data. We recognized seven magnetozones in Hole U1412A and interpreted them to include the Brunhes Chron and the Jaramillo, Cobb Mountain, and Olduvai Subchrons. Our magnetostratigraphic data suggest extremely high sediment accumulation rates for Hole U1412A, similar to those found in Hole U1379C during Expedition 334. Paleomagnetic measurements of nine discrete samples from Hole U1412B display a change from normal to reversed polarity at ~167 mbsf. Changes in the inclination sign are also observed for the seven samples studied from Hole U1412C. Two discrete samples from Hole U1412D were measured for shipboard structural analyses. Based on the small number of samples, no magnetostratigraphic interpretations are available at this time.

Site U1413 summary

Background and objectives

Site U1413 (proposed Site CRIS-13B) targeted the upper slope of the Costa Rica margin (Fig. F6). This site is within the 3-D seismic data set along Line 2466 and crossing Line 4882 (Fig. F14). Interplate earthquake relocations (Fig. F3) and interpretation of geodetic measurements show that this site is located above the seismogenic zone (Bilek et al., 2003; LaFemina et al., 2009). The primary purpose of drilling Site U1413 was to determine the nature, composition, and physical properties of the slope sediment. This site is also designed as a “pilot hole” in preparation for proposed deeper CRISP Program B drilling at this location. Science objectives at Site U1413 included (1) documenting the lithology and physical properties, (2) determining the stress orientation of the margin above the seismogenic zone, and (3) constraining the fluid-flow regime. Documenting periods of subsidence and uplift provides important information about the process of tectonic erosion that characterizes the Costa Rica margin. One technique for estimating the relative motion of the margin is through stratigraphic correlations of multiple sites.

Operations

After a 15.0 nmi transit from Site U1412, the vessel stabilized over Site U1413 at 2330 h on 23 November 2012. The original operations plan for Site U1413 called for two holes: an APC/XCB hole to ~600 mbsf and an RCB hole to 1430 mbsf. Eventually, three holes were cored at this site. Hole U1413A (8°44.4593′N, 84°6.8095′W, 540 m water depth; Table T1) was spudded at 0635 h on 24 November. This was an APC/XCB hole to 189.1 mbsf. APCT-3 formation temperature measurements were taken with Cores 344-U1413A-3H, 5H, 6H, 7H, and 8H. The FlexIT orientation tool was deployed with Cores 344-U1413A-1H through 18H. Hole U1413B (8°44.4593′N, 84°6.7992′W, 540 m water depth; Table T1) was spudded at 0055 h on 26 November. This was a shallow hole to 25.6 mbsf drilled primarily for geochemical analyses. Hole U1413C (8°44.4482′N, 84°6.7993′W, 540 m water depth; Table T1) was spudded at 0940 h on 26 November. This hole was drilled without recovery to 178.0 mbsf, was cored with the RCB system to 582.2 mbsf, and was terminated to allow time to complete other expedition objectives. Following coring, three logging runs were conducted on 30 November with the triple combo, UBI, and Formation MicroScanner (FMS) tool strings.

A total of 71 cores were recovered at this site: 21 APC cores, 8 XCB cores, and 42 RCB cores. The APC cored interval was 166.2 m, with 171.2 m recovered (103%). The XCB cored interval was 48.5 m, with 43.7 m recovered (90%). The RCB cored interval was 404.2 m, with 313.9 m recovered (78%). The overall recovery at Site U1413 was 85%. The total time spent on Site U1413 was 170.25 h or 7.1 days (48.5 h in Hole U1413A, 5.75 h in Hole U1413B, and 116.0 h in Hole U1413C).

Principal results

Site U1413 was drilled to investigate the lithostratigraphy and structural geology of the upper slope sequence as a preliminary study for future deep riser drilling.

Three lithostratigraphic units (Fig. F15) were distinguished in the sediment of Holes U1413A–U1413C, with an overall abundance of silty clay to clay (32.1%), clayey silt to sandy silt (53.3%), silty sand to sand (14.4%), and tephra (0.2%). The 44.6 m thick Unit I recovered in Holes U1413A and U1413B is dominated by dark greenish gray silty clay with multiple centimeter-sized turbidite sequences of fine-sand laminae. A slump/slide event was identified in the uppermost ~3.5 m of this unit. The lithology changes into a brownish green chaotic mixture of silty clay and dark gray sands in the lowermost part of Core 344-U1413A-5H. Ten tephra layers were also identified in Unit I. Lithostratigraphic Unit II, starting at 44.60 mbsf, is characterized by a well-consolidated light greenish gray calcareous clayey silt(stone) with occasional variations to silty clay, as well as minor sand(stone) layers. Unit II exhibits moderate variability in the amount and extent of calcareous cementation with depth. In addition, this unit contains several horizons of lithified and reworked rounded carbonate mud clasts. Fourteen tephra layers were identified in Unit II. Near the bottom of Hole U1413A (Section 344-U1413A-20X-1), a chaotic layer of intermixed sand and calcareous clayey silt, not associated with a lithologic change, most likely represents a mass transport deposit (MTD) interval. Unit II continues in Hole U1413C to 366.45 mbsf with the same lithology, but heavy minerals, shell fragments, and occasional sandstone layers with sapropel and leaf fragments become more abundant with depth. The boundary between Units II and III (Section 344-U1413C-21R-3, 117 cm) is marked by the first appearance of an 18 m thick package of alternating sandstones and siltstones with common to abundant organic matter (sapropel) and shell and gastropod fragments. The matrix of the siltstone and sandstone is characterized by abundant magmatic and sedimentary lithic fragments, common feldspar, and volcanic glass fragments. Foraminifers are the most abundant components of the biogenic material. After a relatively thin siltstone interval (~42 m), the remainder of the hole (426.8–578.8 mbsf) consists of massive fine- to medium-grained sandstones that contain three tephra layers. The sandstones are normally graded and range from decimeter- to meter-thick layers with occasional internal laminations, particularly in the uppermost part of the section. Some cores in Unit III are particularly rich in gastropods and reworked, well-rounded carbonate clasts. These lithostratigraphic units can be correlated to some of the units found at Sites U1378 and U1379 during Expedition 334.

Biostratigraphy at Site U1413 was mainly constrained by calcareous nannofossils. The upper section of Hole U1413A (Samples 344-U1413A-1H-CC through 14H-CC) is assigned to Nannofossil Zone NN21. In Samples 344-U1413A-15H-CC through 19X-CC, the age is less well constrained and is assigned to Zones NN20–NN21 (1.89 Ma to present). The first appearance of Pseudoemiliania lacunosa, which defines the top of Zone NN19, is found in Sample 344-U1413A-20X-CC. The LO of Helicosphaera sellii is observed in Sample 344-U1413C-15R-CC and is assigned to 1.34 Ma. Radiolarians are present in the upper section of Hole U1413A, but few biostratigraphically useful species were found.

Benthic foraminifers were dominant and well preserved in Samples 344-U1413A-1H-1W to 19X-CC, but they range from common to few in the lower section of Hole U1413A and throughout Hole U1413C, with most of the foraminiferal samples showing signs of mechanical breakage. Benthic foraminiferal assemblages show remarkable changes downhole. The uppermost 150 m of sediment is characterized by an assemblage composed of Brizalina bicostata, Cassidulina tumida, and Cancris inflatus. These species are absent in the rest of the cored interval. From 160 mbsf (Sample 344-U1413A-20X-CC) to 478 mbsf (Sample 344-U1413C-32X-CC), there is a completely different assemblage composed of Brizalina spissa, Epistominella smithi, Uvigerina cf. excellens, and Hansenisca altiformis. The lower part of Hole U1413C (Samples 344-U1413C-36R-CC through 42R-CC) is characterized by the appearance of Brizalina cf. dilatata, which together with Uvigerina peregrina dominates the assemblages and constitutes as much as 80% of all foraminifers.

Faulting-related deformation starts to be present and abundant below 180 mbsf. Deformation is additionally localized along brecciated fault zones at 181, 230, 237–239, 365, 529–532, and 567 mbsf. Both normal and reverse faults were observed. Dip angles of these faults vary from subhorizontal to subvertical.

We collected 106 interstitial water samples from Site U1413. Hole U1413B was dedicated to high-resolution studies of the biogeochemical processes just below the SMTZ, which at this site occurs at 16 mbsf. Pore fluid profiles of sulfate, alkalinity, and ammonium concentrations in the uppermost ~150 m of Holes U1413A and U1413B reflect organic matter remineralization but are significantly impacted by sediment slumps at ~45 and ~150 mbsf. Calcium and magnesium concentrations decrease from seawater values at the seafloor to minima of 1.1 and 39.8 mM, respectively, at the lithostratigraphic Unit I/II boundary (44.60 mbsf), reflecting precipitation of authigenic carbonates. Cl concentrations decrease with depth to ~500 mbsf, but below this depth the Cl profile shows a trend of increasing concentrations reaching ~96% of seawater value. The reason for the lower than modern seawater Cl value throughout the cored section but in particular in the upper ~150 m is as yet unclear. Methane increases below the SMTZ, and the C₁/C₂ ratio in the upper 50 m indicates a biogenic origin for the gas. Below this depth, the C₁/C₂ ratio decreases steadily with depth and shows a marked decrease to values <100 below 480 mbsf, indicating a thermogenic component from a deeper source.

Site U1413 porosity values decrease (and bulk densities increase) rapidly from 0 to 180 mbsf and more gradually below 180 mbsf. Magnetic susceptibility excursions appear to coincide with sand-rich layers that contain detrital magnetite. P-wave velocities average 1600 m/s above 20 mbsf. There are only a few scattered measurements between 20 and 200 mbsf because of core disturbance, and values average 1960 m/s below 200 mbsf. Thermal conductivity increases from 0.9 to 1.25 W/(m·K) from the seafloor to 200 mbsf and then gradually increases to 1.35 W/(m·K) at the bottom of Hole U1413C. Three temperature measurements yield a gradient of 49°C/km and a heat flow of 56 mW/m². Sediment strength generally increases from the seafloor to 140 mbsf. Below 140 mbsf, values are scattered but show an overall gradual increase. Formation factor increases rapidly within the upper 20 m and more gradually below 20 mbsf.

We performed split-core measurements of NRM and AF demagnetization of discrete samples to observe the magnetic properties of each lithostratigraphic unit recovered at Site U1413. In order to verify the section data, we demagnetized and measured 119 discrete samples using a progressive AF demagnetization technique. Based on biostratigraphic data, we were able to tentatively correlate certain parts of the magnetic polarity interval recorded in the sediment with the geomagnetic polarity timescale. In particular, the polarity shift from reversed to normal at 485 mbsf in Section 344-U1413C-42R-4 may represent the beginning of the Olduvai normal polarity subchron (1.778 Ma). The sediment accumulation rate for the uppermost 480 m of sediment in Hole U1413C is 269.97 m/m.y.

Downhole logging measurements were taken in Hole U1413C with three tool strings: a slick triple combo (without the radioactive source because of poor hole conditions), a UBI, and an FMS tool string. The tool strings could not be lowered below 187 mbsf because of a borehole obstruction; the last two tool strings were chosen to focus on the ultrasonic and resistivity imaging of borehole breakouts. Three logging units were distinguished. Logging Unit 1 (93–148 mbsf) contains clear borehole breakouts imaged by low reflection amplitudes and large traveltimes in the UBI ultrasonic log and by the orientation of the FMS pair of arms that measure the larger borehole radius. The orientation of the breakouts shows that the minimum principal horizontal stress is oriented approximately north–south. In contrast, the borehole is nearly circular and in gauge (~10 inch diameter) in logging Unit 2 (148–169 mbsf), which also has a higher natural radioactivity and higher resistivity than Unit 1, suggesting a more consolidated formation. The borehole is washed out in all directions in logging Unit 3 (169–184 mbsf), and low measured values of natural radioactivity and resistivity are likely artifacts caused by hole enlargement.

Site U1414 summary

Background and objectives

The primary objective of Expedition 344 was to sample and quantify the material comprising the seismogenic zone of an erosive subduction margin. Fundamental to this objective is an understanding of the nature of the sediment and oceanic crust entering the seismogenic zone, the hydrologic system, and the thermal state of the igneous oceanic crust. Site U1414 (proposed Site CRIS-19A) serves as a secondary reference site on the flank of the subducting aseismic Cocos Ridge (Fig. F6).

Site U1414 is located within the 3-D seismic volume (Fig. F6) and is ~1 km seaward of the deformation front offshore the Osa Peninsula. Site U1414 is located near Line 2497 and crossing Line 2562 (Fig. F16). This site was chosen for multiple reasons. First, a clear seismic record of the plate stratigraphy is present at this site. The seismic section shows a 400 m thick sediment section resting on reflective basement interpreted as Cocos Ridge igneous crust. Secondly, the sedimentary section is thought to be composed of pelagic and hemipelagic sediments that may contain the Pleistocene to mid-Miocene sediment missing at Site U1381 as documented by biostratigraphy and a gap in the Costa Rica tephra record. Thirdly, a reflector of interest exists about two-thirds of the way through the sediment column. Finally, this site presented an opportunity to sample igneous basement.

The primary science goals at Site U1414 included (1) documenting the presence or absence of the hiatus observed at Site U1381, (2) documenting the presence or absence of tephras during the hiatus found at Site U1381, and (3) documenting the composition and alteration state of basement. Finally, because of oblique convergence of subduction at the Middle America Trench, sediment at Site U1414 likely reflects the sediment now under seismic Line BGR99-7. Documenting the physical state and water content of this sediment was an important objective.

Operations

After a 5.8 nmi transit from Site U1412, a positioning beacon was deployed at 0010 h and the vessel stabilized over Site U1414 at 0015 h on 3 December 2012. Hole U1414A (8°30.2304′N, 84°13.5298′W, 2459 m water depth; Table T1) was spudded at 0805 h. Cores 344-U1414A-1H through 22H were taken from 0 to 200.1 mbsf with the APC. APCT-3 formation temperature measurements were taken with Cores 344-U1414A-3H, 5H, 7H, and 9H. The coring system was switched to the XCB after having to drill over Core 344-U1414A-22H. Cores 344-U1414A-23X through 35X were taken from 200.1 to 311.9 mbsf, and XCB coring was terminated after penetration rates and recovery drastically decreased. The APC/XCB assembly was raised to just below the seafloor, a free-fall funnel was deployed, and the remainder of the drill string was brought to the surface. An RCB BHA was then assembled with a new bit and lowered to just above the seafloor. Hole U1414A was reentered, and Cores 344-U1414A-36R through 63R were recovered with the RCB by 1205 h on 9 December, including 96 m of igneous basement. Coring was terminated so that logging could begin.

In preparation for logging, the hole was swept clean with mud and the bit was released. The end of the drill string was set at 96 mbsf, and the logging tools were rigged up. The first logging run was with the triple combo–UBI tool string and reached a depth of 421 mbsf, where it encountered an obstruction. The second logging run was with the FMS-sonic tool string, and it reached the same depth. Logging activities were completed by 1200 h on December 10. The remainder of the drill string was pulled to the surface while the R/V JOIDES Resolution began a slow move toward land in dynamic positioning mode. The BHA cleared the rig floor at 1955 h on 10 December, ending Site U1414. The JOIDES Resolution picked up speed at 2015 h and continued the 95 nmi transit to Puntarenas, Costa Rica. First line ashore was at 0502 h on 11 December 2012, ending Expedition 344.

A total of 63 cores were recovered at this site: 22 APC cores, 13 XCB cores, and 28 RCB cores. The APC cored interval was 200.1 m, with 206.16 m recovered (103%). The XCB cored interval was 111.8 m, with 94.1 m recovered (84%). The RCB cored interval was 159.7 m, with 84.5 m recovered (53%). The overall recovery for Site U1414 was 81%. The total time spent on Site U1414 was 193 h or 8.0 days.

Principal results

The lithology of the upper part of Hole U1414A (Fig. F17) is characterized by a predominantly monotonous, terrigenous sequence of soft, light greenish gray hemipelagic silty clay to clay with some small sand layers in the uppermost part of Unit I (Subunit IA) and an increasing amount of calcareous nannofossils in the lower part (Subunit IB). Terrigenous material (lithic fragments, glass shards, and minerals) decreases with depth. The Unit I/II boundary at 145.34 mbsf separates greenish gray nannofossil-rich clay sediment from calcareous ooze. Unit II (145.3–309.4 mbsf) is a 164 m thick moderately consolidated interval that is characterized in the upper part by nannofossil calcareous ooze. In the lower part calcareous nannofossil ooze alternates with biosilica-rich calcareous ooze. Unit III (309.4–375.3 mbsf) is a 66 m thick interval that is characterized by a sequence of lithified, calcareous, and siliceous cemented silt- and sandstone that probably lost biogenic components as a result of diagenetic remobilization but still preserves the original sedimentary structures such as bedding and bioturbation. Tephra layers and pods are rare (1%), but despite their dark brown to black macroscopic appearance, the glass shards are mainly transparent under the microscope, implying an evolved rather than a mafic volcanic origin. Overall, 35 tephra layers were recovered in Units I–III.

Igneous basement, encountered at 375.2 mbsf, comprises a ~65 m thick sequence of massive basalt (Cores 344-U1414A-45R through 58R) overlying a 1.5 m thick interval of intercalated sediment. Below the sediment is a second (at least ~30 m thick) massive basaltic interval (Cores 344-U1414A-58R through 63R). We tentatively identify eight units. Based on changes in lava morphology, the presence of chilled margins, texture and phenocryst abundance we identify 8 units. Massive and thin sheet flows (Units 1–6) lie above the intercalated sediment (Unit 7). This is, in turn, above a massive flow (Unit 8).

Igneous Units 1–6 and 8 are massive and microcrystalline and exhibit groundmass textures including intersertal, intergranular, vesicular, porphyritic, and subophitic. Seriate and rare variolitic and glassy textures were observed in the uppermost 5 cm of Unit 2. Groundmass comprises plagioclase (44%–63%), clinopyroxene (14%–31%), magmatic opaques (4%–36%), and partially to completely replaced olivine (0%–8.6%). Mesostasis is typically hyalophitic and intersertal. Phenocryst abundance ranges from aphyric to highly plagioclase phyric, with phenocrysts typically more abundant in coarser grained intervals and the middle and uppermost portions of each unit. Unit 4 exhibits porphyritc texture throughout, comprising ~30% of the groundmass. Plagioclase, clinopyroxene, and olivine are all identified as phenocryst phases within Site U1414 basement. Olivine, however, is absent in Units 3, 4, and 6. Megacrysts of plagioclase up to 20 mm in size are observed in the lower portion of Unit 8.

Alteration of basement rocks at Site U1414 is the result of interaction with either seawater or modified “hydrothermal” fluids that partially replace groundmass and phenocryst mineralogy, fill fractures and vesicles, and form breccia. Overall alteration is slight to moderate; however, more intense alteration is concentrated around veins, vein-flanking alteration halos, vesicles, and chilled margins. Secondary mineralogy includes expanding clay minerals saponite and smectite, carbonate, silicates, secondary sulfides, and zeolite. No oxidative alteration assemblages, such as iron-oxyhydroxides or cleadonite, were observed. Cross-cutting relationships observed within veins and vesicles at Site U1414 suggest initial precipitation of expanding clays followed by sulfides and carbonate. The timing of quartz and zeolite remains uncertain. A number of veins at Site U1414 exhibit pressure-induced brecciation along the vein margins and multiple episodes of calcium carbonate precipitation. The most intense alteration observed at Site U1414 is a jigsaw-puzzle breccia in the uppermost 70 cm of Unit 3. The breccia comprises highly altered, subangular basaltic clasts and a smectite matrix. Rare chlorite (or chlorite/smectite) was tentatively identified within mesostasis between Sections 344-U1414A-48R-4 and 49R-1. Overall, our observations suggest that Site U1414 was subjected to an oxygen-starved alteration regime in which access to seawater was restricted.

Biostratigraphy in Hole U1414A is constrained by radiolarians and nannofossils. Nannofossils have moderate preservation overall, with evidence of recrystallization downhole. Radiolarians are well preserved overall, with intervals of poor preservation between Samples 344-U1414A-17H-CC and 21H-CC. The presence of Pseudoemiliania lacunosa and Collosphaera tuberosa indicates that the upper ~120 m was deposited in the Pleistocene. Radiolarian and nannofossil assemblages at the base of Hole U1414A constrain the age of the oldest sediment to the mid-Miocene.

Foraminiferal work was conducted on 34 core catcher samples. Generally, the abundance of benthic foraminifers varied from common to few, with a few particular intervals where benthic foraminifers were only present. Preservation was good to moderate except for some samples contained in lithologic Subunit IIB, where foraminiferal preservation was poor with tests showing signs of recrystallization.

Overall, two main benthic foraminiferal assemblages can be distinguished in this hole. The upper assemblage (0–140 mbsf) includes Globobulimina group species Uvigerina auberiana and, as accessory species, Uvigerina cf. senticosa, Cassidulina carinata, and Melonis affinis. The lower assemblage (140–320 mbsf) is composed of species belonging to the “Cibicides” group, Globocassidulina subglobosa, Pullenia spp., and U. cf. senticosa. In Sample 344-U1414A-12H-CC, we found the LO of the Stilostomella group species, which indicates an age older than 0.6 Ma for that group. The base of the hole (Samples 344-U1414A-30H-CC through 34H-CC) contains specimens of Planulina renzi. This species died out during the mid-Miocene. This is in agreement with the biostratigraphic constraints for that interval.

Site U1414 has subhorizontal to gently dipping bedding. Few faults, generally normal, were observed in Units I and II. Unit III is characterized by strong foliation and veins. Unit III is slightly metamorphosed. Some shear zones are also recognized.

We completed chemical analyses on 61 interstitial water samples collected from Hole U1414A. The pore fluid composition in the uppermost 80 m shows trends characteristic of organic matter remineralization. In addition, coincident minima in ammonium and sulfate concentrations at 37 mbsf suggest that there may be favorable conditions for sulfate-reducing ammonium oxidation. The feasibility of this metabolic pathway in marine sediment was recently documented for the first time in sediment from the Bay of Bengal, India.

The sulfate concentration-depth profile at this site is also unusual in that it displays a second minimum at 330 mbsf, which corresponds to a sharp minimum in calcium and a maximum in barium concentrations. These data suggest lateral flow of a sulfate-depleted fluid, which originated from microbial oxidation of methane and/or other organic carbon sources landward of Site U1414 and migrated updip through the upper sediment of Unit III. The calcium minimum thus likely reflects a combination of in situ carbonate formation caused by the ongoing sulfate reduction and a contribution from the laterally migrating fluid having low calcium concentration. Similarly, the barium concentration, which mirrors the sulfate profile at depth, is controlled by the stability of the mineral barite (BaSO₄); thus, the increase in barium may reflect both in situ marine barite dissolution at the sulfate minimum plus barium contribution from the laterally migrating low-sulfate fluid. Postcruise isotopic analyses of the carbon species, solid-phase barite and carbonate analyses, and numerical modeling will be required to determine the origin and history of the fluid flow at depth in this input sediment section.

The incoming sediment also displays changes in composition driven by the nature of the lithology and associated diagenetic reactions. Lithologic changes at this site are apparent in the silica profile, which is characterized by typical silicate mineral diagenesis. The gradual increase in the amount of biogenic opal in the sediment from the silty clay/sand–dominated Unit I through the nannofossil-rich clay in Subunit IB is apparent as a monotonic increase in dissolved silica. The change from the nannofossil-rich clay of Subunit IB to the nannofossil-rich calcareous ooze of Subunit IIA coincides with a sharp decrease in Si concentrations. Another marked increase in Si concentrations across the lithologic Subunit IIA/IIB boundary reflects a change in fluid-rock reactions in Subunit IIB that are dominated by opal-A solubility at the in situ temperature of ~35°–45°C. Near the base of Subunit IIB is a distinct decrease in Si concentrations, which coincides with a clear decrease in dissolved K concentrations, consistent with clinoptilolite formation, a K-Si-rich zeolite, which is also apparent in the XRD data.

Bulk density and porosity data are complicated by the combination of lithologic changes and variations in diagenesis. Compaction is slight in Unit I, with porosity values of 69% at 145 mbsf. Below 145 mbsf, porosity values decrease to 54% and then gradually increase to >75% near 225 mbsf before decreasing to the base of the hole. Background magnetic susceptibility values generally decrease with depth, reaching near-zero values below 350 mbsf. Excursions generally correspond to tephra layers. NGR values are generally stable in the upper 110 m, rise to a peak at 130 mbsf, and then decrease with depth to the Unit II/III boundary. NGR values are scattered in Unit III, with a peak near 350 mbsf. P-wave velocities from the split cores decrease in the upper 25 m and then gradually increase with depth. Overall, P-wave velocities in Unit I are low, averaging 1520 m/s. A local maximum at ~190 mbsf corresponds to a bulk density maximum and porosity minimum. P-wave velocities between 300 and 340 mbsf average 1850 m/s and then sharply increase to 3260 m/s below 340 mbsf. Thermal conductivity shows variations consistent with bulk density. Strength values rise steadily in Unit I and Subunit IIA. Four downhole temperature measurements yield a thermal gradient of 168°C/km.

We measured the NRM and performed AF demagnetization on all archive section halves and several discrete samples from Site U1414. In general, variations in NRM intensity follow changes in lithology, especially in the light green calcareous ooze of Hole U1414A. We demagnetized 55 discrete samples collected from the working halves with AF demagnetization up to 120 mT, with the main objective of recognizing the characteristic remanent magnetization. The upper part of Hole U1414A is characterized by a short interval with normal polarities and then a longer reversed interval followed by three normal intervals. The Brunhes Chron, if present, is very short or condensed in the uppermost ~10 m. Between 94 and 114 mbsf, four samples with homogeneous characteristics confirm the presence of a normal polarity interval. However, most of the samples from the middle part of Hole U1414A show reversed polarity. The lower part of the hole (below 282 mbsf) includes six samples with normal polarity, separated by only one sample with reversed shallow inclination.

The logging program in Hole U1414A included two runs with the triple combo–UBI and FMS-sonic tool strings. Logs were collected from 96 to ~410 mbsf and include spectral gamma radiation, bulk density, electrical resistivity, P- and S-wave velocity, and ultrasonic and resistivity images of the borehole wall. We distinguished four logging units in Hole U1414A. Logging Unit 1 (94–259 mbsf) is characterized by total gamma ray values that decrease from between ~40 gAPI at 120 mbsf to ~10 gAPI at the base of the unit. Bulk density, resistivity, and elastic wave velocities are generally low in this unit and show a decreasing trend toward the base of the unit, where the bulk density is 1.5 g/cm³, resistivity is 0.5 Ωm, and P- and S-wave velocities are 1.6 and 0.4 km/s, respectively. Logging Unit 2 (259–335 mbsf) displays an increase with depth of bulk density (1.6–1.8 g/cm³), resistivity (0.5–1 Ωm), P-wave velocity (1.7–2.1 km/s), and S-wave velocity (0.5–0.7 km/s). NGR values are generally 10–30 gAPI. Logging Unit 3 (335–375 mbsf) contains larger variations in physical properties, with gamma ray values ranging between 20 and 60 gAPI, bulk densities between 1.8 and 2.2 g/cm³, resistivities between 1 and 10 Ωm, P-wave velocities between 2 and 4 km/s, and S-wave velocities between 0.5 and 2.7 km/s. Finally, logging Unit 4 (375–410 mbsf) corresponds to the volcanic basement at Site U1414 and displays very low natural radioactivity (10 gAPI or less) and high values of bulk density (2.3–2.8 g/cm³), resistivity (2–100 Ωm), P-wave velocity (3.2–6.7 km/s), and S-wave velocity (1.7–3.8 km/s).

Top of page   |    Previous   |    Next