Calcareous nannofossils of ODP Leg 101 holes

Leg 101 of the Ocean Drilling Program drilled 19 holes at 11 sites to investigate the geology of the Straits of Florida and the northern Bahamas. Drilling at Site 626 indicated that the Gulf Stream has had significant flow through the Straits of Florida for at least the last 24 million years. Winnowed, foraminiferal grainstones and packstones with sparse nannofossil assemblages and the reworking of older nannofossils suggest strong bottom-current activity throughout this interval. Drilling north of Little Bahama Bank and in Exuma Sound documents the growth of platform slopes during the late Cenozoic. Nannofossil biostratigraphy of the upper Cenozoic sediments from the Little Bahama Bank and Exuma Sound slope transects indicates relatively continuous deposition, with only short breaks in the periplatform ooze and/or calciturbidite accumulation during the late Pliocene. These unconformities may be linked to sea-level lowstands. Nannofossil assemblages are generally poorly preserved owing to accelerated diagenesis caused by high aragonite and high magnesium calcite contents of bank-derived material. High rates of influx of bank-derived materials appear to coincide with highstands of sea level. Periplatform sediments are largely limited to the upper Cenozoic at Little Bahama Bank. Pelagic and/or hemipelagic conditions existed during the Late Cretaceous and Paleogene. A relatively complete, continuous section of Oligocene is present in the Little Bahama Bank area, although the rest of the Paleogene is thin. Paleogene material is also present in Northeast Providence Channel, although its thickness is uncertain. A thick upper Campanian chalk sequence with abundant, moderately to well-preserved nannofossils occurs in the Little Bahama Bank area. Hemipelagic nannofossil marls and marly chalks at Little Bahama Bank contain an excellent nannofossil record, which indicates a continuous lowermost to middle Cenomanian sequence overlying the upper Albian drowned platform. These hemipelagic sediments are significantly younger than the organic-rich, middle Albian limestones in Northeast Providence Channel. The latter indicate that a deep-water channel was already well established by the middle Albian.

Lithologic correlation of ODP Leg 115 holes

Whole-core (WC) measurements of low-field magnetic susceptibility (MS) provide an extremely simple, rapid, and nondestructive technique for high-resolution core logging and lithostratigraphic correlation between subsidiary holes at Ocean Drilling Program (ODP) sites. This is particularly useful for reconstructing composite, stratigraphically continuous sequences for individual ODP sites by splicing the uninterrupted records obtained from subsections of offset cores recovered from adjacent holes. Correlation between the WCMS profiles of holes drilled at different sites is also possible in some instances, especially when lithologic variations at each site are controlled by regional paleoceanographic or global (i.e., orbitally forced) paleoclimatic changes. In such circumstances, WCMS may also be used as a proxy paleoclimatic indicator, duly assisting climatostratigraphic zonation of the recovered sequence by more conventional microfossil and isotopic techniques. High-resolution WCMS profiles are also useful in detecting intervals of the recovered sequence affected by drilling disturbance, in the form of contamination by pipe rust or similar metallic artifacts as well as discontinuities related to repenetration of the corer or loss of material between successively cored intervals. Stratigraphic intervals that have been affected by early (suboxic) diagenesis resulting from a high initial organic matter content of the sediment are also readily identified by WCMS logging. The MS signal of horizons affected by suboxic diagensis is typically degraded in proportion to the duration and intensity (related to initial Corg concentration) of organic matter remineralization. The lowering of MS values during suboxic diagenesis results from "dissolution" (bacterially mediated ionic dissociation) of magnetic iron and manganese oxides and oxyhydroxides in the sediment. It is to be hoped that, on future ODP (or similar) cruises, WCMS logging will cease to be regarded as a mere adjunct to paleomagnetic measurements, but rather as a simple, yet powerful, lithostratigraphic tool, directly analogous to downhole geophysical logging tools, and complimentary to shipboard techniques for whole-core measurements of physical properties (e.g., P-wave logging, GRAPE, etc.).

Geochemistry at DSDP Holes 85-573B and 85-574C

Eocene-Oligocene metalliferous sediments and associated lithologies from the central equatorial Pacific are described in detail. Geochemical analyses of 54 sediment and 2 basalt samples are presented for 34 elements. Detailed stratigraphic and statistical analyses of these data, combined with mineralogic studies, indicate the presence of volcanic glass and seven main mineral phases: biogenic calcite and opal, Fe smectite, goethite, dMnO2, carbonate fluorapatite, and barite. Fe smectite formed by reactions between Fe oxyhydroxides and biogenic opal, causing the dissolution of calcite and the precipitation of barite. Diagenesis was oxic. Sediments have rare earth element distributions similar to those in seawater. The metal content of the sediments is related to competition between the supply rates of hydrothermal and biogenic particles, but has been enhanced by early diagenetic processes. Eocene-Oligocene metalliferous sediments compare closely to those currently being deposited in the Bauer Basin and on the flanks of the East Pacific Rise. There is, however, no evidence that they were deposited in close proximity to an active hydrothermal system.

Geochemsitry of minerals and glass shards at DSDP Leg 67 Holes

During Leg 67, the Middle-America Trench transect off Guatemala was drilled across the convergent margin of southern Mexico and Central America south of the Tehuantepec Ridge. The data of Leg 66, north of the Tehuantepec Ridge, and that of Leg 67 provided the opportunity to establish a continuous chronology of airborne volcanic ashes intercalated within the sediments (Aubouin et al., 1979; von Huene et al., 1980). Sites of both expeditions are favorably located for obtaining a good record of the explosive volcanicity of these areas, given the proximity of the volcanic sources and the position of the sites under the prevailing winds.

Petrology and relict mineralogy of serpentinites at DSDP Leg 84 Holes

Eleven serpentine samples from DSDP Leg 84 and four serpentinized ultramafic samples from Costa Rica and Guatemala were described and their relict mineral compositions measured by electron microprobe to try to determine the origin of the Leg 84 serpentinites and their relationship to the ultramafic rocks of the onshore ophiolites. The Leg 84 samples comprise more than 90% secondary minerals, principally serpentine, with hematitic and opaque oxides, and minor talc and smectites. Four distinct textural types can be identified according to the distribution of opaque phases and smectite. Remnants of spinel, olivine, orthopyroxene, and clinopyroxene occur variously in the samples; spinal occurs in all the samples. Textural evidence suggests that the serpentinites were originally clinopyroxene-bearing harzburgites. Relict mineral compositions are refractory and relatively uniform: olivine, Fo90.6-90.9; orthopyroxene, En90-91; clinopyroxene, Wo47 En50 Fs3; spinels, Cr/Cr + Al = 0.4-0.6. 567A-29-2, 30-35 cm has slightly more magnesian olivines (Fo92) and orthopyroxene, and more aluminous spinels (Cr/Cr + Al = 0.3). These compositions are similar to those inferred for refractory upper-mantle materials and also fall within the range of compositions for relict minerals in abyssal peridotites. They could be of oceanic origin. The onshore samples include serpentinites, a clinopyroxene-bearing harzburgite, and a clinopyroxenite. They too have magnesium-rich silicate assemblages, but relative to the drilled samples have more iron-rich olivines (Fogo) and more aluminous and sodic pyroxenes; spinels which are clearly relicts are very aluminum-rich (Cr/Cr + Al = 0.1-0.25). These samples are most likely mantle materials, but significantly less depleted. Their relationship to the drilled samples is unclear. Serpentinites were the most common basement materials recovered during Leg 84, and there appears to be a bimodal assemblage (basalt/diabase and serpentine) of igneous rocks sampled from the trench slope. Diapirism of serpentine throughout the trench slope and forearc is suggested as an explanation for this distribution of samples.

(Table 1) Sulfur isotopes, total sulfur, and degree of oxidation of basalt samples from DSDP Holes 69-504B and 70-504B

Whole-rock basalt samples from the upper half of Deep Sea Drilling Project Hole 504B have oxygen-isotope compositions typical of mid-ocean-ridge basalts which have experienced a moderate degree of low-temperature alteration by sea water. By contrast, d18O values in the lower half of the hole correspond to basalts which have experienced almost no detectable oxygen-isotope alteration. These observations suggest that the overall water/rock ratio was lower in the lower half of the drilled crust. A correlation between d18O values and 87Sr/86Sr ratios suggests that the water/rock ratio, rather than temperature variation, was the main factor determining basalt d18O values. Hydrogen-isotope data appear to be consistent with a low water/rock ratio in the lower part of the crust.