Consolidation properties and strength characteristics of ODP Site 204-1244 sediments

Eight whole-core samples from Ocean Drilling Program Site 1244, Hydrate Ridge, Cascadia continental margin, were provided to Massachusetts Institute of Technology (Cambridge, Massachusetts, USA) for geotechnical characterization. The samples were collected from depths ranging from 5 to 136 meters below seafloor (mbsf). Seven of the eight whole-core samples were located within the gas hydrate stability zone, whereas the eighth sample was located in the free gas zone. Atterberg limits testing showed that the average liquid limit of the soil is 81% and the average plastic limit is 38%, giving an average plasticity index of 43%. The liquid limit is sensitive to oven drying, shown by a drop in liquid limit to 64% when tests were performed on an oven-dried sample. Loss on ignition averages 5.45 wt%. Constant rate of strain consolidation (CRSC) tests were performed to obtain the compression characteristics of the soil, as well as to determine the stress history of the site. CRSC tests also provided hydraulic conductivity and coefficient of consolidation characteristics for these sediments. The compression ratio (Cc) ranges from 0.340 to 0.704 (average = 0.568). Cc is fairly constant to a depth of 79 mbsf, after which Cc decreases downhole. The recompression ratio (Cr) ranges from 0.035 to 0.064 (average = 0.052). Cr is constant throughout the depth range. In situ hydraulic conductivity varies between 1.5 x 10**-7 and 3 x 10**-8 cm/s and shows no trend with depth. Ko-consolidated undrained compression/extension (CKoUC/E) tests were also performed to determine the peak undrained shear strength, stress-strain curve, and friction angle. The normalized undrained strength ranges from 0.29 to 0.35. The friction angle ranges from 27 to 37. Because of the limited amount of soil, CRSC and CKoUC/E tests were also conducted on resedimented specimens.

Zeolite and silica diagenesis and sandstone petrography at DSDP Sites 57-438 and 57-439

We detected authigenic clinoptilolites in two core samples of tuffaceous, siliceous mudstone in the lower Miocene section of Hole 439. They occur as prismatic and tabular crystals as long as 0.03 mm in various voids of dissolved glass shards, radiolarian shells, calcareous foraminifers, and calcareous algae. They are high in alkalies, especially Na, and in silica varieties. There is a slight difference in composition among them. The Si : (Al+ Fe3+) ratio is highest (4.65) in radiolarian voids, intermediate (4.34) in dissolved glass voids, and lowest (4.26) in voids of calcareous organisms. This difference corresponds to the association of authigenic silica minerals revealed by the scanning electron microscope: There are abundant opal-CT lepispheres in radiolarian voids, low cristobalite and some lepispheres in dissolved glass voids, and a lack of silica minerals in the voids of calcareous organisms. Although it contains some silica from biogenic opal and alkalies from trapped sea water, clinoptilolite derives principally from dissolved glass. Although they are scattered in core samples of Quaternary through lower Miocene diatomaceous and siliceous deposits, acidic glass fragments react with interstitial water to form clinoptilolite only at a sub-bottom depth of 935 meters at approximately 25°C. Analcimes occur in sand-sized clasts of altered acidic vitric tuff in the uppermost Oligocene sandstones. The analcimic tuff clasts were probably reworked from the Upper Cretaceous terrain adjacent to Site 439. Low cristobalite and opal-CT are found in tuffaceous, siliceous mudstone of the middle and lower Miocene sections at Sites 438 and 439. Low cristobalite derives from acidic volcanic glass and opal-CT from biogenic silica. Both siliceous organic remains and acidic glass fragments occur in sediments from the Quaternary through lower Miocene sections. However, the shallowest occurrence is at 700 meters subbottom in Hole 438A, where temperature is estimated to be 21°C. The d(101) spacing of opal-CT varies from 4.09 to 4.11 Å and that of low cristobalite from 4.04 to 4.06 Å. Some opal-CT lepispheres are precipitated onto clinoptilolites in the voids of radiolarian shells at a sub-bottom depth of 950 meters in Hole 439. Sandstone interlaminated with Upper Cretaceous shale is chlorite- calcite cemented and feldspathic. Sandstones in the uppermost Oligocene section are lithic graywacke and consist of large amounts of lithic clasts grouped into older sedimentary and weakly metamorphosed rocks, younger sedimentary rocks, and acidic volcanic rocks. The acidic volcanic clasts probably originated from the volcanic high, which supplied the basal conglomerate with dacite gravels. The older sedimentary and weakly metamorphosed rocks and green rock correspond to the lithologies of the lower Mesozoic to upper Paleozoic Sorachi Group, including the chert, limestone, and slate in south-central Hokkaido. However, the angular shape and coarseness of the clasts and the abundance of carbonate rock fragments indicate a nearby provenance, which is probably the southern offshore extension of the Sorachi Group. The younger sedimentary rocks, including mudstone, carbonaceous shale, and analcime-bearing tuff, correspond to the lithologies of the Upper Cretaceous strata in south-central Hokkaido. Their clasts were reworked from the southern offshore extension of the strata. Because of the discontinuity of the zeolite zoning due to burial diagenesis, an overburden several kilometers thick must have been denuded before the deposition of sediments in the early Oligocene.

Shear experiments from IODP Expedition 322 and 333 samples

The mechanical behavior of the plate boundary fault zone is of paramount importance in subduction zones, because it controls megathrust earthquake nucleation and propagation as well as the structural style of the forearc. In the Nankai area along the NanTroSEIZE (Kumano) drilling transect offshore SW Japan, a heterogeneous sedimentary sequence overlying the oceanic crust enters the subduction zone. In order to predict how variations in lithology, and thus mechanical properties, affect the formation and evolution of the plate boundary fault, we conducted laboratory tests measuring the shear strengths of sediments approaching the trench covering each major lithological sedimentary unit. We observe that shear strength increases nonlinearly with depth, such that the (apparent) coefficient of friction decreases. In combination with a critical taper analysis, the results imply that the plate boundary position is located on the main frontal thrust. Further landward, the plate boundary is expected to step down into progressively lower stratigraphic units, assisted by moderately elevated pore pressures. As seismogenic depths are approached, the décollement may further step down to lower volcaniclastic or pelagic strata but this requires specific overpressure conditions. High-taper angle and elevated strengths in the toe region may be local features restricted to the Kumano transect.