Sedimentological, petrographical and biostratigraphical characteristics of ODP Leg 190 volcanic ashes and siliceous claystones

Leg 190 was the first of a two-leg program across the Nankai accretionary prism and Trough, offshore Japan, aiming to evaluate existing models for prism evolution and to constrain syntectonic sedimentation, deformation styles, mechanical properties, and prism hydrology (Moore, Taira, Klaus, et al., 2001; Moore et al., 2001). More than 400 volcanic ash and siliceous claystone (altered ash) layers were penetrated and sampled during drilling of the six sites from two transects across the accretionary prism (Sites 1173-1178). In sites from the subducting Shikoku Basin (Sites 1173 and 1177) and in the trench axis (Site 1174), recognition of ash layers and diagenetically altered ashes was initially important in defining major lithostratigraphic units. However, it is clear that understanding the diagenesis of the volcanic ashes has considerable implications for prism evolution, mechanical properties, prism hydrology, geochemistry, and fluid flow in the accretionary prism and associated subducting sediments (cf. Masuda et al., 1996, doi 10.1346/CCMN.1996.0440402). Particle size, chemical composition, temperature, depth of burial, and time are all thought to be factors that may affect volcanic ash diagenesis and preservation (Kuramoto et al., 1992, doi:10.2973/odp.proc.sr.127128-2.235.1992; Underwood et al., 1993, doi:10.2973/odp.proc.sr.131.137.1993). The overall aim of this research is to evaluate factors influencing volcanic ash diagenesis in the Nankai Trough area. This data report presents just the results of the sedimentological and petrographic analysis of the volcanic ashes and siliceous claystones from Sites 1173, 1174, and 1177. It is anticipated that when the results of additional geochemical analysis of these lithologies is available a more meaningful evaluation of factors influencing volcanic ash alteration will be possible.

Organic sedimentation of ODP Sites 124-767 and 124-768

This study relates the organic sedimentation characteristics to the lithostratigraphic successionsthat were observed at Site 767 (Celebes Sea) and Site 768 (Sulu Sea) during ODP Leg 124. It is based on the total organic carbon content (TOC) of the sediments, on the petrographictype and maturity of the organic matter, and on the TOC accumulation rates calculated for the lithostratigraphic units. In the Celebes and Sulu Seas sediments, the organic matter is mainly of terrestrial origin with the highest concentrations and TOC accumulation rates occurring in the middle Miocene turbiditic sequences that correspond to a major compressive event between the Philippine Mobile Belt and the Palawan, Cagayan, and Sulu Ridges. Petrographic analysis of the Eocene and lower Miocene organic matter in the Celebes Sea shows that it consists only of highly degraded terrestrial particles. This observation and the very low TOC accumulation rates indicate poor conditions for organic carbon preservation during this open-ocean phase of the Celebes Basin formation. The organic matter, either of marine or terrestrial origin, is much better preserved in the younger sediments, suggesting physico-chemical changes in the depositional environment. Because of the dilution phenomena by turbidites, it is difficult to observe the progressive improvement of the organic matter preservation throughout the turbiditic series. The same change in preservation is broadly observed in the Sulu Sea from the early Miocene (rapid opening phase of the basin with massive pyroclastic deposits) to the present.

Geochemistry of vein rocks of ODP Hole 118-735B

Rock samples from Hole 735B, Southwest Indian Ridge, were examined to determine the principal vein-related types of alteration that occurred, the nature of fluids that were present, and the temperatures and pressures of these fluids. Samples studied included veined metagabbro, veined mylonitic metagabbro, felsic trondhjemite, and late-stage leucocratic diopside-bearing veins. The methods used were standard petrographic analysis, mineral chemical analysis by electron microprobe, fluid inclusion petrography and analysis by heating/freezing techniques and laser Raman microspectroscopy, and oxygen isotopic analyses of mineral separates. Alteration in lithologic Units I and II (above the level of Core 118-735B-3OR; approximately 140 meters below the seafloor) is dominated by hydration by seawater-derived fluids at high temperature, up to about 700°C, and low water/rock ratio, during and immediately after pervasive ductile deformation. Below Core 118-735B-30R, pervasive deformation is less common, and brittle veining and brecciation are the major alteration styles. Leucocratic centimeter-scale veins, often containing diopside and plagioclase, were produced by interaction of hot (about 500°C) seawater-derived fluid and gabbro. The water/rock ratio was locally high at the veins and breccia zones, but the integrated water/rock ratio for the lower part of the hole is probably low. Accessory hydrous magmatic or deuteric phases formed from magmatic volatiles in some gabbro and in trondhjemite. Most subsequent alteration was affected by fluids that were seawater-derived, based on isotopic and chemical analyses of minerals and analyses of fluid inclusions. Many early-generation fluid inclusions, associated with high-temperature veining, contain appreciable methane as well as saline water. The source of methane is unclear, but it may have formed as seawater was reduced during low water/rock interaction with ultramafic upper mantle or ultramafic and mafic layer 3. Temperatures of alteration were calculated on the basis of coexisting mineral chemistry and isotopic values. Hydrothermal metamorphism commenced at about 720°C and continued to about 550°C. Leucocratic veining took place at about 500°C. Alteration within brecciated horizons was also at about 500° to less than 400°C, and the trondhjemite was altered at about 550° to below 490°C. Pressures calculated from a diopside-bearing vein, based on a combination of fluid inclusion and isotopic analysis, were 90 to 100 MPa. This pressure places the sample, from Core 118-735B-70R in Unit V, at about 2 km below the seafloor.

Composition of organic matter from black shales drilled in the Cape Verde Basin in DSDP Hole 41-367

Results of geochemical studies of organic matter in black shales from the Cape Verde Basin are reported. Based on these results, in combination with data of petrographic analysis, conclusions are made about sapropelic nature of their organic matter and low degree of its coalification. It corresponds to the proto-catagenetic substage of sedimentary rocks. Black shales of the Cape Verde Basin are classified as potential oil source strata.

(Table 1) Heavy mineral content in sediments of the southern Mediterranean Ridge, Nile Cone and Egyptian Shelf

Petrographic analysis of Quaternary terrigenous sand layers in eastern Mediterranean cores reveals distinct mineralogical differences between the Egyptian Shelf-Nile Cone region and the southern part of the Mediterranean Ridge. A compositionally and texturally immature suite in Ridge cores, mixed with a Nile-derived assemblage, identifies a fresh non-recycled mineral component derived from proximal igneous and metamorphic surface or near-surface exposures, probably in the south-central Ridge area rather than from distal African sources. The presence of such basement terrains would be consistent with a compressive thrust-belt origin for this part of the Mediterranean Ridge.

Quantitative component analysis of the coarse fraction of surface sediments from the Persian Gulf

In the Persian Gulf and the Gulf of Oman marl forms the primary sediment cover, particularly on the Iranian side. A detailed quantitative description of the sediment components > 63 µ has been attempted in order to establish the regional distribution of the most important constituents as well as the criteria governing marl sedimentation in general. During the course of the analysis, the sand fraction from about 160 bottom-surface samples was split into 5 phi° fractions and 500 to 800 grains were counted in each individual fraction. The grains were cataloged in up to 40 grain type catagories. The gravel fraction was counted separately and the values calculated as weight percent. Basic for understanding the mode of formation of the marl sediment is the "rule" of independent availability of component groups. It states that the sedimentation of different component groups takes place independently, and that variation in the quantity of one component is independent of the presence or absence of other components. This means, for example, that different grain size spectrums are not necessarily developed through transport sorting. In the Persian Gulf they are more likely the result of differences in the amount of clay-rich fine sediment brought in to the restricted mouth areas of the Iranian rivers. These local increases in clayey sediment dilute the autochthonous, for the most part carbonate, coarse fraction. This also explains the frequent facies changes from carbonate to clayey marl. The main constituent groups of the coarse fraction are faecal pellets and lumps, the non carbonate mineral components, the Pleistocene relict sediment, the benthonic biogene components and the plankton. Faecal pellets and lumps are formed through grain size transformation of fine sediment. Higher percentages of these components can be correlated to large amounts of fine sediment and organic C. No discernable change takes place in carbonate minerals as a result of digestion and faecal pellet formation. The non-carbonate sand components originate from several unrelated sources and can be distinguished by their different grain size spectrum; as well as by other characteristics. The Iranian rivers supply the greatest amounts (well sorted fine sand). Their quantitative variations can be used to trace fine sediment transport directions. Similar mineral maxima in the sediment of the Gulf of Oman mark the path of the Persian Gulf outflow water. Far out from the coast, the basin bottoms in places contain abundant relict minerals (poorly sorted medium sand) and localized areas of reworked salt dome material (medium sand to gravel). Wind transport produces only a minimal "background value" of mineral components (very fine sand). Biogenic and non-biogenic relict sediments can be placed in separate component groups with the help of several petrographic criteria. Part of the relict sediment (well sorted fine sand) is allochthonous and was derived from the terrigenous sediment of river mouths. The main part (coarse, poorly sorted sediment), however, was derived from the late Pleistocene and forms a quasi-autochthonous cover over wide areas which receive little recent sedimentation. Bioturbation results in a mixing of the relict sediment with the overlying younger sediment. Resulting vertical sediment displacement of more than 2.5 m has been observed. This vertical mixing of relict sediment is also partially responsible for the present day grain size anomalies (coarse sediment in deep water) found in the Persian Gulf. The mainly aragonitic components forming the relict sediment show a finely subdivided facies pattern reflecting the paleogeography of carbonate tidal flats dating from the post Pleistocene transgression. Standstill periods are reflected at 110 -125m (shelf break), 64-61 m and 53-41 m (e.g. coare grained quartz and oolite concentrations), and at 25-30m. Comparing these depths to similar occurrences on other shelf regions (e. g. Timor Sea) leads to the conclusion that at this time minimal tectonic activity was taking place in the Persian Gulf. The Pleistocene climate, as evidenced by the absence of Iranian river sediment, was probably drier than the present day Persian Gulf climate. Foremost among the benthonic biogene components are the foraminifera and mollusks. When a ratio is set up between the two, it can be seen that each group is very sensitive to bottom type, i.e., the production of benthonic mollusca increases when a stable (hard) bottom is present whereas the foraminifera favour a soft bottom. In this way, regardless of the grain size, areas with high and low rates of recent sedimentation can be sharply defined. The almost complete absence of mollusks in water deeper than 200 to 300 m gives a rough sedimentologic water depth indicator. The sum of the benthonic foraminifera and mollusca was used as a relative constant reference value for the investigation of many other sediment components. The ratio between arenaceous foraminifera and those with carbonate shells shows a direct relationship to the amount of coarse grained material in the sediment as the frequence of arenaceous foraminifera depends heavily on the availability of sand grains. The nearness of "open" coasts (Iranian river mouths) is directly reflected in the high percentage of plant remains, and indirectly by the increased numbers of ostracods and vertebrates. Plant fragments do not reach their ultimate point of deposition in a free swimming state, but are transported along with the remainder of the terrigenous fine sediment. The echinoderms (mainly echinoids in the West Basin and ophiuroids in the Central Basin) attain their maximum development at the greatest depth reached by the action of the largest waves. This depth varies, depending on the exposure of the slope to the waves, between 12 to 14 and 30 to 35 m. Corals and bryozoans have proved to be good indicators of stable unchanging bottom conditions. Although bryozoans and alcyonarian spiculae are independent of water depth, scleractinians thrive only above 25 to 30 m. The beginning of recent reef growth (restricted by low winter temperatures) was seen only in one single area - on a shoal under 16 m of water. The coarse plankton fraction was studied primarily through the use of a plankton-benthos ratio. The increase in planktonic foraminifera with increasing water depth is here heavily masked by the "Adjacent sea effect" of the Persian Gulf: for the most part the foraminifera have drifted in from the Gulf of Oman. In contrast, the planktonic mollusks are able to colonize the entire Persian Gulf water body. Their amount in the plankton-benthos ratio always increases with water depth and thereby gives a reliable picture of local water depth variations. This holds true to a depth of around 400 m (corresponding to 80-90 % plankton). This water depth effect can be removed by graphical analysis, allowing the percentage of planktonic mollusks per total sample to be used as a reference base for relative sedimentation rate (sedimentation index). These values vary between 1 and > 1000 and thereby agree well with all the other lines of evidence. The "pteropod ooze" facies is then markedly dependent on the sedimentation rate and can theoretically develop at any depth greater than 65 m (proven at 80 m). It should certainly no longer be thought of as "deep sea" sediment. Based on the component distribution diagrams, grain size and carbonate content, the sediments of the Persian Gulf and the Gulf of Oman can be grouped into 5 provisional facies divisions (Chapt.19). Particularly noteworthy among these are first, the fine grained clayey marl facies occupying the 9 narrow outflow areas of rivers, and second, the coarse grained, high-carbonate marl facies rich in relict sediment which covers wide sediment-poor areas of the basin bottoms. Sediment transport is for the most part restricted to grain sizes < 150 µ and in shallow water is largely coast-parallel due to wave action at times supplemented by tidal currents. Below the wave base gravity transport prevails. The only current capable of moving sediment is the Persian Gulf outflow water in the Gulf of Oman.

Petrographic description and geochemistry of basalts and altered volcanic rocks from ODP Leg 183 sites

The basalts recovered during Legs 183 and 120 from the southern, central, and northernmost parts of the Kerguelen Plateau (Holes 1136A, 1138A, 1140A, and 747C, respectively), as well as those recovered from the eastern part of the crest of Elan Bank (Hole 1137A), represent derivates from tholeiitic melts. In the northern part of the Kerguelen Plateau (Hole 1140A), basalts may have formed from two sources located at different depths. This is reflected in the presence of both low- and high-titanium basalts. The basalts are variably altered by low-temperature hydrothermal processes (at temperatures up to 120°C), and some are affected by subaerial weathering. The hydrothermal alteration led mainly to the formation of smectites, chlorite minerals, mixed-layer hydromica-smectite and smectite-chlorite minerals, hydromica, serpentine(?), clinoptilolite, heulandite, stilbite, analcime, mordenite, thomsonite, natrolite(?), calcite, quartz, and dickite(?). Alteration of extrusive basalts is mainly related to horizontal fluid flow within permeable contact zones between lava flows. Under a nonoxidizing environment of alteration, the tendency to lose most of elements, including rare earth elements, from basalts dominates. Under on oxidizing environment, basalts accumulate many elements.

Recalculated petrographic parameters and normalized microprobe analyses of volcanic glasses for DSDP Sites 63-467, 63-468 and 63-469

The southward passage of the Rivera triple junction and its effect on the North American plate are primary controls on the Miocene tectonic evolution of the outer borderland of California. Detrital modes of sand shed off the Patton Ridge and cored by the Deep Sea Drilling Project provide evidence of progressive tectonic erosion of the Patton accretionary prism and neartrench volcanism. Volcanic glass in the sediment is predominantly calcalkaline rhyolite and andesite, typical of subductionrelated volcanism, but also includes minor low-K2O tholeiitic basalt. We attribute these compositional features to interaction with a spreading ridge associated with a possible trench-ridge-trench triple junction along the Patton Escarpment from 18 to 16 Ma. This study suggests that evidence of ridge-trench interaction may be commonly preserved along submerged plate margins, in contrast to its more limited recognition and discussion in the literature based on exposed examples in Chile, Japan and Alaska.

Mineralogy and petrographic summary of ODP Leg 198 sites and DSDP Hole 62-463, Shatsky Rise

This petrological study of the lower Aptian Oceanic Anoxic Event (OAE1a) focused on the nature of the organic-rich interval as well as the tuffaceous units above and below it. The volcaniclastic debris deposited just prior to the OAE1a is consistent with reactivation of volcanic centers across the Shatsky Rise, concurrent with volcanism on the Ontong Java Plateau. This reactivation may have been responsible for the sub-OAE1a unconformity. Soon after this volcanic pulse, anomalous amounts of organic matter accumulated on the rise, forming a black shale horizon. The complex textures in the organic-rich intervals suggest a history of periodic anoxia, overprinted by bioturbation. Components include pellets, radiolarians, and fish debris. The presence of carbonate-cemented radiolarite under the OAE1a intervals suggests that there has been large-scale remobilization of carbonate in the system, which in turn may explain the absence of calcareous microfossils in the section. The volcanic debris in the overlying tuffaceous interval differs in that it is significantly epiclastic and glauconitic. It was likely derived from an emergent volcanic edifice.