Abstract A newly developed logging-while-coring system was deployed during Ocean Drilling Program legs 204 and 209 off the coast of Oregon and near the Mid-Atlantic Ridge. The system consists of two existing devices modified to be used together — a Schlumberger Resistivity-at-the-Bit* tool, and a Texas A&M University wireline-retrieved core barrel and latching tool. The combination allows for precise core-log depth calibration and core orientation within a single borehole, and without a pipe trip. These tests, conducted in clay-bearing sediments (Leg 204) and in crustal peridotite and gabbroic rocks (Leg 209), mark the first simultaneous use of coring and logging-while-drilling technologies. Sediment cores were recovered with 33% recovery, on average, and as high as 68% to 75 m depth below the sea floor. Core recovery in crustal rocks was only 1–2%, however, penetrating to 21 m depth below sea floor, which is attributed to a problem with the core catcher. High-resolution logs were recorded in the downhole tool memory over the entire drilled intervals at both test sites. It is anticipated that logging-while-coring systems will be utilized more routinely where rig time constraints may otherwise preclude coring in difficult drilling environments.

Abstract This investigation of naturally occurring fractures in the mafic rocks of the Palisades dolerite sill characterizes the porosity of this crystalline rock sequence, and yields a method of determining the in situ porosity when complete down-hole information is not available. Two holes, 229 m and 305 m deep, were drilled 450 m apart through the sill and into the underlying Triassic sediments of the Newark Basin. Both holes were logged with geophysical tools, including the acoustic borehole televiewer (BHTV), to identify intervals of high porosity, fracturing, and potential zones of active fluid flow. Using the BHTV data, 96 and 203 fractures were digitally mapped within the sill in Well 2 and Well 3, respectively. Most fractures dip steeply (76–78°). There is a shift in fracture orientation between Well 2 and Well 3, although the lithology of the sill is continuous. The dolerite penetrated in both holes is fresh and unaltered, and intersects a 7-m thick olivine-rich layer about 15 m above the bottom of the sill. Several fractures identified in the sill have large apparent aperture (>6 cm) that correspond to high-porosity zones (6–14%), measured from both resistivity and neutron logs in Well 2. We use a relationship between porosity and apparent fracture aperture in Well 2 to infer the porosity in Well 3. This correlative method for estimating porosity may be applicable between holes in other crystalline rock environments where down-hole log data are incomplete. Changes in the temperature gradient log also indicate active fluid flow, although flow appears to be most active in fractured and high-porosity zones in the sediments.

Abstract The deployment of a down-hole dipole shear sonic tool in Hole 395A and Hole 735B marked the first two opportunities to measure high-resolution shear-wave velocity and V S anisotropy profiles in oceanic crustal rocks. In Hole 395A near the Kane Fracture Zone, dipole sonic logs were recorded from 100–600 mbsf, and allow azimuthal anisotropy to be determined as a function of depth in the crust. The magnitude of V S anisotropy varies with depth, from less than 3.2% in low-porosity flows at the bottom of the hole, to approximately 15.5% in highly fractured pillow basalts and breccias. The orientation of the fast V S direction also varies over depth, with a mean value between 75°N and 80°E, and aligns with the strike of steeply dipping structures observed by down-hole electrical and acoustic images. This fast V S angle orientation is locally oblique to the plate-spreading direction and to the Mid-Atlantic Ridge axis. In Hole 735B, drilled near the Atlantis Fracture Zone, dipole sonic logs from 23 to 596 mbsf indicate that V S anisotropy varies with depth, with averages of 5.3% in the foliated and deformed gabbros recovered at the bottom of the hole; 4.5% in undeformed olivine and oxide-rich gabbros around 300 mbsf; and 6.8% in highly deformed mylonitic zones at shallow depths. The fast V S angle also varies with depth, giving a mean orientation of approximately S45°E for well-resolved estimates in the upper interval of the hole. This direction aligns with the strike of steeply dipping fractures observed by down-hole imaging, and is locally oblique to the Southwest Indian ridge axis. Although the effects of regional stresses and local deformation of these holes may introduce anisotropy in the dipole sonic data, we conclude that crustal morphology in the vicinity of the holes contributes significantly to the magnitude and orientation of V S anisotropy.

Abstrack The structure and architecture of the oceanic crust, which underlies 70% of the earth’ surface, are still virtually unknown. Slices of assumed oceanic crust obducted above sea level (ophiolites) provide a three-layer model for the oceanic crust, comprising a volcanic sequence (seismic Layers 2A/2B) underlain by sheeted dikes (seismic Layer 2C) that are underlain by gabbros (seismic Layer 3). The last are considered to constitute the frozen magma chambers from which the overlying basaltic sequence was derived. Beneath Layer 3, the model shows peridotites of the upper mantle, which represent the host from which the entire overlying sequence was derived by partial melting. We can validate this ophiolite model and understand the variation in crustal architecture only by drilling and studying active oceanic crust. In the past 20 years, the Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) have drilled a few boreholes, with these objectives. Despite such efforts, no boreholes have penetrated the complete in-situ and assumed oceanic crustal sequence. Through studies of the few boreholes available, drilling has provided a wealth of information about the detailed lithologic architecture and its relationship to the gross permeability structure of active oceanic crust. With very poor core recoveries in basement holes (commonly less than 20%), much of the detailed information about the geologic sequence and physical properties can be derived only from wireline logs. This paper reviews the role of logging in the study of the oceanic basement, through discussion of these key boreholes where parts of the assumed oceanic model are represented. Holes 396A, 504B, and 896A provide a picture of volcanic layers 2A/2B at slow- and intermediate-spreading ridges. Hole 504B is the only significant section though Layer 2C and the seismic Layer 2/3 boundary, and Hole 735B provides an important section through Layer 3.

Abstract The mineral compositions of sediments of the Caribbean Sea have been ascertained on the basis of X-ray-diffraction analyses. As a result of the studies of the clay-size fraction (smaller than 2 µ) and the silt-size fraction (2–37 µ), it is possible to divide the Caribbean basin into at least two general petrographic provinces, a western province bordering the Central American coast and a southern province extending out from the coast of South America. There is probably another province adjacent to the Greater and Lesser Antilles, but the data from these regions were insufficient to outline the province clearly.

Chemical studies on the ultimate composition of plants, animals, and sediments of the marine environment have revealed the existence of high concentrations of such minor oceanic constituents as zirconium, titanium, and thallium. The fact that some of these elements have not yet been detected in sea water has made these observations all the more striking. It is inviting, therefore, to explore the paths of trace metallic elements through the various geospheres of the earth’s surface. The task of delimiting chemically the marine milieu is beset with complexities. First of all, although more than 90 per cent of marine waters exists at depths greater than 1000 meters, the vast majority of chemical investigations have been concerned with waters of shallower depths. For example, in a recent survey of published iron analyses of oceanic waters undertaken by the author, only 4 of the 39 investigations presented data for waters taken at depths greater than 1000 meters. The concentration of certain elements appears to vary by factors greater than two orders of magnitude. In the analyses cited above, the iron content in Japanese coastal waters shows a range of 0 to 800 mg/liter. Finally, many reported concentrations fail to differentiate elements between the solid phases of either organic or inorganic origin and the dissolved phase. Although the experimental separation of the solid and liquid phases is arbitrary, as it is somewhat dependent upon the porosity of the filter employed, such distinctions are of utmost importance. Table 1 presents the marine abundances compiled critically...

Sedimentary deposition in the Pacific Ocean is largely influenced by: (1) the great size of the ocean and the relatively small influx of river water; (2) high seismic and volcanic activity, especially in the marginal zone; (3) deep water characterized by relatively high alkalinity and high concentrations of silicate and phosphates; (4) well-developed equatorial current system resulting in high plankton production near the equator in the East Pacific; (5) trenches and other barriers around the margins partially protecting the central regions from influx of coarse continental materials. The area covered by red clay is both relatively and absolutely larger than in the other oceans, mainly because of the greath depths of the North Pacific Basin. Calcareous oozes are particularly extensive in the South Pacific but occur generally at lesser depths than in the Atlantic. Characteristic of the Pacific are large areas of siliceous oozes. Radiolarian ooze borders the northern fringe of the equatorial calcareous zone that underlies waters of high organic productivity. Diatom ooze forms bands in the northern Pacific and in the Antarctic Ocean. Chemical analyses indicate differences in the composition of some older and younger clays. The former are richer in Mn, Fe, and P relative to Ni, Ti, and Al respectively. The well-known high radioactivity of the recent pelagic clays is associated with the authigenic zeolite, phillipsite. A little quartz silt is ubiquitous. The iron-titanium and manganese-nickel ratios appear to be indicative of the proportion of hydrogenous matter. High barium and biogenous copper may indicate rapid accumulation and dissolution of organic remains. Deposits possibly laid down by slumping and turbidity currents occur under the flat bottom in large areas off the northwest coast of North America, and small “sediment lakes” are found at the bottoms of trenches. Direct evidence, such as graded bedding and mixtures of Quaternary and Tertiary fossils, has been found close to topographic highs in the basins of the Mid-Pacific Mountains, north of Alexa Bank, and between the Marquesas and the Tuamotus. In most topographic lows the slow deposition appears to be continuous and uniform. Much of the bottom of the Pacific is irregular; even on slight topographic highs, erosion, nondeposition, or greatly reduced deposition is evidenced. Outcrops of Tertiary calcareous oozes have been found on such highs. Unconformities are evident in some cores. Tertiary calcareous oozes occur at depths and in areas where clay is being deposited around them. Seismic-refraction and reflection profiles suggest sediment thickness of approximately 200 meters in the clay areas and 400 meters beneath the equatorial calcareous oozes. A radiocarbon measurement indicates for the last 14,000 years an average rate of deposition of 3 cm/1000 years in the area of maximum carbonate accumulation below the equatorial divergence. The corresponding rate of deposition of the nonbiogenous components would be about 0.26 cm/1000 years.