SITE SUMMARIES

Site 1177 Summary
The science objective of Site 1177 is to study the stratigraphic, geochemical, and physical properties framework of a reference site along the Ashizuri Transect (Fig. 10). This transect includes Sites 297, 298, 582, and 583.

We recognized five lithostratigraphic units at Site 1177 (Figs. 8A, 26). Unit I (upper Shikoku Basin facies) is Pliocene in age (300.20 to 401.76 mbsf) and consists mainly of weakly indurated hemipelagic mud interbedded with fresh volcanic ash. Unit II (lower Shikoku hemipelagic facies) is late Miocene in age (401.76 to 449.30 mbsf) and is composed almost entirely of a more strongly indurated hemipelagic mudstone. Unit III (lower Shikoku turbidite facies) is early to late Miocene in age (449.30 to 748.35 mbsf) and consists of turbidite sand, silty sand, gravel, mudstone-clast conglomerate, and hemipelagic mudstone, plus a few thin layers of carbonate-cemented claystone and siliceous claystone. There are four sand-rich packets within this facies, and most of the siliciclastic sands contain abundant woody plant fragments (Fig. 27). Sediment dispersal evidently occurred through a broad system of coalescing submarine fans. Unit IV (volcaniclastic-rich facies) is early Miocene in age (748.35 to 831.08 mbsf). This unit consists of variegated mudstone to claystone, volcanic ash (Fig. 28), and silt turbidites with both volcaniclastic and siliciclastic compositions. Unit V is basaltic basement (831.08 to 832.13 mbsf) and is probably early Miocene in age. The basalt contains one pillow structure, and an intrusive contact with overlying sediment is highly altered(Fig. 29).

Deformation structures at this site, oceanward of the prism, are very sparse, more so than the reference sites at the Muroto Transect. This near absence at Site 1177 of structures and bedding dips >10° may result from slower rates of sedimentation, slight differences in lithology, or differences in topography of the substrate. Early soft-sedimentary structures are present between 748 and 831 mbsf; the main tectonic structure is a faulted and diagenetically altered interval between 579.45 and 581.10 mbsf. The basalt at the bottom of Hole 1177A, at 831 mbsf, exhibits glassy rinds at its contact with the overlying sediment and networks of veins bearing calcite and/or chlorite in complex interrelationships.

Biostratigraphic age control was provided by calcareous nannofossils, although their abundance and states of preservation were generally poor throughout the sequence; major intervals are barren of nannofossils. A total of 11 biostratigraphic events were identified. The continuous sedimentary section spans the time interval from the Pliocene (Zone NN18) through the early Miocene (Zone NN2). The biostratigraphic age estimates indicate an average sedimentation rate for the late Pliocene of 87 m/m.y. and a lower sedimentation rate of 28.7 m/m.y. for the lower Miocene to Pliocene sediments.

Magnetic inclination data of Hole 1177A after AF demagnetization at 30 mT were useful for interpretation of geomagnetic polarity changes from early the Miocene to Pleistocene. The Brunhes/Matuyama boundary is expected to occur above the initial coring depth of 300 mbsf. The Reunion Event (2.14 Ma) during the Matuyama Chron is interpreted to occur at 301.85 mbsf. The Matuyama/Gauss (2.581 Ma) and Gauss/Gilbert (3.58 Ma) boundaries are interpreted to occur at 328.55 and 384.25 mbsf, respectively. The beginning of Chron C3A (5.894 Ma) is identified to occur at 427.45 mbsf.

Sharp chemical discontinuities between and within lithostratigraphic units, particularly intense in the Cl, Na, K, sulfate, and alkalinity concentrations, and a high-sulfate turbidite unit in the middle half of the section are outstanding characteristics of Site 1177 pore fluid concentration-depth profiles. Discontinuities within lithostratigraphic units are unique for this site and were not observed at Sites 1173, 1174, or 808.The chemical discontinuities correspond with discontinuity boundaries in physical properties, suggesting that specific sediment intervals are and have remained for some time closed systems with respect to vertical diffusion. The chemistry within individual intervals reflects the concentrations at the time of "trapping" plus the in situ fluid-rock reactions since. The most conspicuous "sealed-off" interval was identified close to the bottom of the sediment section (775—805 mbsf) by its distinctly higher porosities, ~60%, relative to the sediments above and below having only ~40% porosity. The sharpest discontinuity occurs at ~410 mbsf, the depth of the incipient décollement, at the boundary between lithostratigraphic Units I and II. As yet it is unclear what sediment type is acting as a cap rock. Preliminary observations based on local Cl anomalies at the boundaries of the deep, high porosity suggest that thin clay-rich horizons, probably smectite rich and having high horizontal/vertical anisotropy and extremely low permeability, may be responsible for sealing off sediment packets. The concentration depth profiles are therefore only continuous in Units I and II but show unusual variance in Units III and IV.

Within Unit III there is a general decrease in Cl concentration; the minimum value is ~7% fresher than modern seawater concentration. This freshening, probably not caused by fluid flow, is most plausibly produced by smectite dehydration enhanced by Cl uptake by an authigenic hydrous silicate. To a lesser extent Na and K profiles show similar trends. Sulfate reduction is complete in Units I and IV, driven by microbial activity. But in between, in Unit III, sulfate concentration is high, ~86% of the modern seawater value, indicating that since burial little microbial activity has occurred. This is probably the result of the very low content of nonwoody, labile organic matter available for microbial activity in the turbidites. Most of the labile organic matter was microbially oxidized when the turbidites were at or close to the seafloor and sulfate diffused into this low sedimentation rate section.

The important diagenetic reactions are ash alteration, particularly reflected in the Ca, Na, and K profiles; carbonate formation as reflected in the Ca and alkalinity profiles; and opal-A dissolution as reflected in the Si profile.

The total organic carbon contents ranged from 0.03 to 1.62 wt%, with an average value of 0.45 wt%. The highest carbon values were measured in the Shikoku turbidite facies sediments (Unit III), which contained a terrestrial component characterized by plant detritus and pieces of wood. The sulfur content ranged from 0 to 0.81 wt%, with the highest concentrations occurring between 400—520 mbsf and 650—770 mbsf. The C/N ratios indicated that a mixture of both marine and terrigenous sources were contributing to the overall sediment composition. Unlike Sites 1175 and 1176, the inorganic carbon (~0.78 wt%) and carbonate contents (~2.7 wt%) were low with the exception of some thin-bedded carbonate-cemented layers (up to 65 wt%) in the Shikoku turbidite facies (Unit III).

Methane concentrations in sediments below the sulfate reduction zone (~4.5 m down to 734 mbsf) are consistent with a bacterial origin. The C₁/(C₁+C₂) ratio for hydrocarbons in sediments below 750 mbsf plot within the mixing zone, suggesting that more than one source of hydrocarbons may be present.

Microorganisms were enumerated in 23 samples collected from 300 to 830 mbsf. Bacteria are present in all but two samples (687 and 830 mbsf) at abundances that are generally lower than expected based on results from previous ODP sites. A small, but statistically significant increase in bacterial populations occurs from 380 to ~740 mbsf that correlates with elevated sulfate concentrations in the interstitial water between these depths. The continued presence of sulfate is unexpected when bacteria are present at ~10⁶ cells/cm³, and this may be related to very low organic carbon concentrations in the sediment preventing significant amounts of bacterial sulfate reduction. A total of 21 whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.

Variations in physical properties at Site 1177 correlate well with the lithostratigraphic units. Units I and II are both characterized by low scatter in porosity. Unit I maintains a nearly constant porosity of 60%—65%. At the top of Unit II (402 mbsf), porosities begin to decrease rapidly with depth, decreasing to 45%—53% by 450 mbsf. Unit III is characterized by a gradual decrease in porosity with depth and by increased scatter that may be due to lithologic variations in this turbidite-rich sequence. Unit IV exhibits significant scatter and shows no clear trend with depth. An excursion to lower porosity (~40%) at 475—510 mbsf within Unit III occurs in a sandy section. Anomalously high porosity (~8%—15% higher than in surrounding sediments) within Unit IV occurs in a 30-m-thick zone between 765 and 795 mbsf. Low vertical P-wave velocities and formation factors also characterize this zone.

Most gas permeameter determinations at Site 1177 range around the values given by the background hemipelagites. Carbonate-cemented claystones at 540 and 591 mbsf give slightly higher values, as do the altered ashes of Unit IV, but the increase is small. The upper sands in Unit III account for all the high measurements at the site. Numerous wood-bearing silty sands were measured in the lower part of Unit III, but most give identical results to the background hemipelagites, suggesting blockage of the pore connections, perhaps by smectite.

In addition to serving as the reference site for the Ashizuri Transect, Site 1177 provides a comparison to the Muroto reference site (Site 1173). Comparison of the two sites will aid our understanding of the evolution of the Nankai Trough accretionary prism in two different geologic settings characterized by differing angles of prism taper.