Site 1175 Summary | Table of Contents


Site 1174 Summary
Site 1174 (ENT-03A) is located in the protothrust zone of the Nankai accretionary prism (Fig. 9) and is designed to sample a zone of incipient deformation. When combined with our reference Site 1173 (~11 km seaward) and Site 808 (~2 km landward at the frontal thrust), Site 1174 will provide a transect of structural, physical properties, and geochemical gradients across the deformation front of the accretionary prism.

We recognized five lithostratigraphic units and three subunits at Site 1174 (Figs. 8, 15). Unit I (slope-apron facies) is Quaternary in age and extends from the seafloor to a sub-bottom depth of 4.00 mbsf. This facies is composed mostly of mud that was deposited on the lowermost trench slope by hemipelagic settling. Unit II (trench-wedge facies) is Quaternary in age and includes three subunits. Subunit IIA (axial trench—wedge facies) extends from 4.00 to 314.55 mbsf and is characterized by thick sand turbidites, silt turbidites, and hemipelagic mud (Fig. 16). The lithologies of Subunit IIB (314.55—431.55 mbsf) are limited to silt turbidites and hemipelagic mud, whereas Subunit IIC (431.55—483.23 mbsf) is composed of hemipelagic mud, volcanic ash, and silt turbidites. The gradual transformation in facies character downsection is consistent with a change in depositional environment from the outer trench wedge to abyssal floor. Unit III (upper Shikoku Basin facies) is Quaternary to Pliocene in age and extends from 483.23 to 660.99 mbsf. Lithologies within this unit include hemipelagic mudstone and volcanic ash; the lower unit boundary coincides with the deepest identifiable bed of vitric tuff. In contrast, Unit IV (lower Shikoku Basin facies) contains mostly bioturbated mudstone with sporadic interbeds and nodules of carbonate-cemented claystone and siliceous claystone. Replacement of glass shards by smectite and zeolites (clinoptilolite or heulandite) increases gradually with depth and is more extreme in finer grained deposits. As a consequence, both ash to bentonite diagenesis and temporal changes in pyroclastic influx govern the lithologic distinction between the Upper and lower Shikoku Basin facies. The unit boundary shifts upsection as Shikoku Basin deposits migrate toward the Nankai deformation front and become increasingly affected by rapid burial and heating beneath the trench wedge. The lowermost stratigraphic unit at Site 1174, Unit V, begins at a depth of 1102.45 mbsf. We drilled only 8.86 m of variegated claystone in this middle Miocene volcaniclastic facies.

Deformation bands are well developed between 218 and 306 mbsf (Fig. 17) and are concentrated in two oppositely inclined sets striking at 033° with the acute bisectrix inclined 10°NW from vertical (Fig. 18). They occur immediately above a narrow but abruptly sheared interval which, with indications of reverse movement and a paleomagnetically restored southeast dip, seems to be a backthrust. Between 470 and 506 mbsf, fractured and markedly steepened bedding may represent a thrust; no significant deformation was seen in the cores equivalent to the thrust apparent on the seismic profile at 550 mbsf. Narrow, widely spaced zones of fractures and brecciation characterize the interval between 688 and 807 mbsf. Between 807.6 and 840.20 mbsf an irregular downward increase in intensity of inclined fractures and fineness of brecciation defines the basal décollement, thicker and more heterogeneous than at Site 808 but more thoroughly comminuted in its lower part (Figs. 19, 20). The underthrust sediments show little tectonic deformation apart from bed steepening between 950 and 1000 mbsf and, together with shearing, around 1020 mbsf.

Nannofossil assemblages are indicative of the Pleistocene (Subzone NN21b) to middle Miocene (Zone NN6) ages. Fifteen biostratigraphic events are recognized. Nannofossils are common and generally moderately preserved in the Pleistocene, whereas Pliocene and Miocene nannofossils are rare and mostly poorly preserved. Sedimentation rates based on biostratigraphy are 630—770 m/m.y. for the late Quaternary and are significantly lower (11—125 m/m.y.) for deposits >0.8 m.y.

Paleomagnetic results indicate that the Brunhes Chron (0—0.78 Ma) ranges from 0 to 543.15 mbsf and extends through the trench-wedge turbidites. The Matuyama Chron occurs from 543.15 to 685.95 mbsf, the Gauss Chron from 685.95 to 727.85 mbsf, and the Gilbert Chron from 727.85 to 802.07 mbsf. High magnetic intensities occur from 0 to ~550 mbsf, below which they drop to low values to the bottom of the hole.

The main characteristics of the interstitial water concentration-depth profiles at Site 1174 are similar to those at Site 808. There is an intense, and therefore a very shallow, sulfate reduction zone, alkalinity and ammonium concentrations peak in the uppermost 200 m of the section, and the solutes that are controlled by fluid-rock reactions, such as Cl, Na, and Si, have sharp changes in their gradients at a depth that corresponds to the boundary between the trench wedge and Shikoku Basin facies (lithostratigraphic Units II/III boundary). The chemical changes across the prime tectonic feature, the décollement, are subtler. At the depth that corresponds to the thrust intersection (~470 ± 5 mbsf), there are significant transient features, most distinctly exhibited in the Cl and Si concentrations, that may indicate active hydrologic activity. A high resolution record of pore fluid chemistry was recovered across and within the Nankai Trough décollement for the first time. A low-Cl zone in the 200-m interval below the décollement, with minimum concentrations that are ~17% diluted relative to seawater, occurs at an almost identical distance below the décollement at Site 808. The dilution, however, is ~21% at Site 808, ~17% at Site 1174, and considerably less (~9%) at reference Site 1173. In the lowermost ~100 m of the underthrust section, Cl concentrations increase, approaching seawater concentration at 1110 mbsf. Hydration reactions in the lower volcaniclastic or an underlying upper basement fluid flow system may be responsible for the increase in the Cl concentrations.

A local Cl maximum of 496 mM within the décollement has smoothly diffused ~50 m above the décollement, whereas there is a very sharp decrease (~10 mM) in the 10 m below the décollement. The cause of the Cl maximum in the décollement is as yet unclear.

Dissolved silica concentrations appear to be controlled by biogenic silica dissolution in the trench-wedge sediments, by volcanic ash diagenesis in the upper Shikoku unit, and by the low-Cl source plus in situ silicate reactions at >70° to ~130° in the Lower Shikoku unit. Dissolved sulfate increases below the sulfate reduction zone, 1—2 mM below the upper and lower Shikoku Basin boundary sediments, at ~660 mbsf, reaching 8—10 mM below the depth interval of the Cl minimum and remaining constant to the bottom of the section. At Site 1173 the first sulfate increase below the sulfate reduction zone is observed at a much shallower burial depth, ~400 m shallower than at Site 1174. The sulfate distributions at these sites may reflect a dynamic relationship among sedimentation rates, temperature, and microbial sulfate reduction rates.

Organic matter decreases with depth and low total organic carbon (TOC) values are low (0.90 to 0.11 w%; average = ~0.38 wt%) in the core. The C/N ratios indicate the presence of marine organic matter with only a slight increase in the upper trench—wedge facies (~200 mbsf) and in the lower Shikoku Basin facies below the décollement (~1000 mbsf). Discrete intervals of elevated methane concentrations are present between 225 and 700 mbsf. Minor amounts of ethane (C2; 200—800 mbsf) and propane (C3; 400—650 and 950—1110 mbsf) are likely because of some in situ thermal maturation of organic matter. There appears to be restricted flow of both C2 and C3 across the décollement, suggesting that the presence of higher hydrocarbons above the décollement may be due to migration.

Microorganisms were enumerated in 40 samples collected from the surface to 1100 mbsf at Site 1174. With the exception of two samples with low abundances (~1.8 ± 106 cells/cm3) in the sandy layers at 26 and 66 mbsf, abundances from the surface to 400 mbsf were close to values predicted based on data from previous ODP sites. Abundances were lower than predicted below 400 mbsf. The decrease may relate to the relatively high temperature gradient at Site 1174. Cell counts dropped below the detection limit at 528 mbsf and remained so until just above the décollement. Abundances at 778 and 789 mbsf were 4.8 and 4.2 ± 106 cells/cm3, respectively; no cells were detected below these depths. Nineteen whole-round samples were used to inoculate anaerobic growth media and were maintained at the estimated in situ temperature. Samples were chosen from the surface through the known hypothermophilic region (113°C), and subsamples at five depths were targeted for incubation at in situ pressure and temperature.

Porosities within the axial and outer trench—wedge facies (Subunits IIA and IIB) are characterized by high variability and decrease with depth. Porosity decreases at the top of the trench to basin transition facies (Subunit IIC). Within the transitional facies, porosities are less scattered and decrease slightly with depth. The upper Shikoku Basin facies (Unit III) is characterized by nearly constant porosities, which is a deviation from normal compaction trends. Surprisingly, a high velocity interval between 510 and 520 mbsf is associated with an interval of elevated porosity. At the top of the lower Shikoku basin (Unit IV; ~660 mbsf) another high-velocity interval occurs. Porosities within the lower Shikoku Basin facies resume a compaction trend of decreasing porosity with depth. Porosities increase sharply by 2%—4% at the top of the underthrust sequence. This porosity increase is accompanied by a decrease in velocity and increase of electrical conductivity. However, the anisotropy of electrical conductivity is higher in the underthrust sediments than above the décollement zone. Porosities and velocities increase with depth within the underthrust sediments, whereas electrical conductivities decrease.

Uncalibrated gas permeameter measurements were made throughout the section. Shallower than 600 mbsf, silt-rich and ash horizons showed higher values than the silty clays. The axial trench—wedge sands gave the highest values and the lowermost silty clays recovered gave the lowest.

In situ temperature measurements to a depth of 65.5 mbsf and laboratory thermal conductivity measurements indicate a heat flow of 180 mW/m2. If heat flow is purely conductive and steady state, a temperature of 140°C is projected for the bottom of the hole.

Site 1175 Summary | Table of Contents