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Submerged stabilized platform
OCEANOGRAPHIC
ABSTRACTS
This section contains a group of abstracts of current papers on general oceanography and related subjects. Together with the oceanographic bibliography it will provide a comprehensive service to scientists engaged in deep-sea research and oceanography in general. For ease of reference the names of authors are arranged in alphabetical order. ALLDREDGEL. R. and J. C. FITZ, 1964. Submerged stabilized platform. Deep-Sea Rex., 11 (6) : 935-942. During September 1963 an instrument platform was installed cu. 30m below the surface in the Catalina Basin 1280 m of water. Instruments were attached to the platform to measure its motion. The platform was held in place by three steel cables attached to anchors placed so as to form an equilateral triangle on the bottom. The cable catenary problem was avoided by attaching glass floats at 37-m intervals along the cables. The platform tilted with an amplitude of cu. & l/2” from a stable mean position with a period of ca. 1Osec which corresponded to the period of the surface swells. By comparing magnetic declination measurements made on the instrument platform with those ma& at nearby land stations, it is inferred that the rotation stability is similar to the tilt stability. There was, however, an apparent additional drift of 20 min of arc over a 5-hr period. An inverted echo sounder record clearly shows wave height, wave period and tidal range. It is bdieved that it is now practicable to use the three anchor stable platform system as a base for sensitive oceanographic instruments of many varieties in mid-an. ALLEN J. R. L., 1965. Sedimentation to the lee of small underwater sand waves: an experimental study. J. Geol., 73 (I): 95-116. Underwater sand waves growing by interaction between a movable sand bed and a fluid flow were simulated in a laboratory flume. Each sand wave advances in step with a flow system involving boundary-layer separation at the wave crest and an induced counter-current to the wave lee. Four sediment-structure elements result from this flow system, moving simultaneously downstream in the following or&r : (I) sand-wave foresets; (2) small-scale asymmetrical ripples formed beneath the counter-current, pointing toward the sand wave, and overridden by the sand-wave fore&s; (3) an erosion hollow where the counter-current first nnpinges on the flume bed; and (4) small-scale asymmetrical ripples pointing away from the sand wave and eroded by the downstream advance of the erosion hollow. Foresets, the most important of these four elements, accumulate from a combination of grain deposition from suspension creating lee-face instability and avalanching tending to create a stable face. Avalanching may be intermittent when periods of grain-stacking separate episodes of sliding; or it may be continuous when grains are in sliding motion at all times over all parts of the lee face, &pending on the flow speed over the sand wave and the rate of settling on the lee face in relation to the speed of avalanche descent. Each type of avalanching gives the sand wave a distinctive, internal sediment structure and vertical grading. This could prove a useful key to regime variability in cross-stratification studies. ALLEN J. R. L., 1965. Coastal geomorphology of eastern Nigeria: beach-ridge barrier islands and vegetated tidal fiats. Geol. en M&&ouw., 44 (1): l-21. The visible portion of the Niger delta consists, in eastern Nigeria, of three geomorphological units. Behind a chain of barrier islands formed of sand ridges of two types, occurs a broad tidal flat colonized by mangroves and marked by a reticulate drainage pattern. The tidal flat is succeeded inland by a forested river flood plain. The tidal flat is also partly in contact with a low terrace formed on Late Tertiary and possibly Pleistocene sediments. The nature of the barrier islands and the tidal flat is related to the forces at work in the region. The mangrove swamp of the eastern delta is growing at the expense of the barrier islands, the flood plain and the terrace because of erosion in tidal channels. It is suggested that the reticulate drainage pattern of the tidal flat is due to the stabilizing of deposited sediment by the mangroves and the manner in which these plants grow collectively. The Nigerian barrier complex, situated in the tropics, is compared with the temperate tidal flats of the Dutch coast. The main differences depend on the contrasted role of plants in the two areas, and thus linally on climate. ANTOINE J. W. and J. L. HARDING, 1965. Structure beneath continental shelf, northeastern Gulf of Mexico. Bull., Amer. Ass. Petrol. Geol., 49 (2): 157-171. Data from forty-seven reversed seismic refraction profiles in the northeastern Gulf of Mexico are presented in the form of four east-west and two north-south cross sections. Where possible.
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ABSTRACTS
This section contains a group of abstracts of current papers on general oceanography and related subjects. Together with the oceanographic bibliography it will provide a comprehensive service to scientists engaged in deep-sea research and oceanography in general. For ease of reference the names of authors are arranged in alphabetical order. ALLDREDGEL. R. and J. C. FITZ, 1964. Submerged stabilized platform. Deep-Sea Rex., 11 (6) : 935-942. During September 1963 an instrument platform was installed cu. 30m below the surface in the Catalina Basin 1280 m of water. Instruments were attached to the platform to measure its motion. The platform was held in place by three steel cables attached to anchors placed so as to form an equilateral triangle on the bottom. The cable catenary problem was avoided by attaching glass floats at 37-m intervals along the cables. The platform tilted with an amplitude of cu. & l/2” from a stable mean position with a period of ca. 1Osec which corresponded to the period of the surface swells. By comparing magnetic declination measurements made on the instrument platform with those ma& at nearby land stations, it is inferred that the rotation stability is similar to the tilt stability. There was, however, an apparent additional drift of 20 min of arc over a 5-hr period. An inverted echo sounder record clearly shows wave height, wave period and tidal range. It is bdieved that it is now practicable to use the three anchor stable platform system as a base for sensitive oceanographic instruments of many varieties in mid-an. ALLEN J. R. L., 1965. Sedimentation to the lee of small underwater sand waves: an experimental study. J. Geol., 73 (I): 95-116. Underwater sand waves growing by interaction between a movable sand bed and a fluid flow were simulated in a laboratory flume. Each sand wave advances in step with a flow system involving boundary-layer separation at the wave crest and an induced counter-current to the wave lee. Four sediment-structure elements result from this flow system, moving simultaneously downstream in the following or&r : (I) sand-wave foresets; (2) small-scale asymmetrical ripples formed beneath the counter-current, pointing toward the sand wave, and overridden by the sand-wave fore&s; (3) an erosion hollow where the counter-current first nnpinges on the flume bed; and (4) small-scale asymmetrical ripples pointing away from the sand wave and eroded by the downstream advance of the erosion hollow. Foresets, the most important of these four elements, accumulate from a combination of grain deposition from suspension creating lee-face instability and avalanching tending to create a stable face. Avalanching may be intermittent when periods of grain-stacking separate episodes of sliding; or it may be continuous when grains are in sliding motion at all times over all parts of the lee face, &pending on the flow speed over the sand wave and the rate of settling on the lee face in relation to the speed of avalanche descent. Each type of avalanching gives the sand wave a distinctive, internal sediment structure and vertical grading. This could prove a useful key to regime variability in cross-stratification studies. ALLEN J. R. L., 1965. Coastal geomorphology of eastern Nigeria: beach-ridge barrier islands and vegetated tidal fiats. Geol. en M&&ouw., 44 (1): l-21. The visible portion of the Niger delta consists, in eastern Nigeria, of three geomorphological units. Behind a chain of barrier islands formed of sand ridges of two types, occurs a broad tidal flat colonized by mangroves and marked by a reticulate drainage pattern. The tidal flat is succeeded inland by a forested river flood plain. The tidal flat is also partly in contact with a low terrace formed on Late Tertiary and possibly Pleistocene sediments. The nature of the barrier islands and the tidal flat is related to the forces at work in the region. The mangrove swamp of the eastern delta is growing at the expense of the barrier islands, the flood plain and the terrace because of erosion in tidal channels. It is suggested that the reticulate drainage pattern of the tidal flat is due to the stabilizing of deposited sediment by the mangroves and the manner in which these plants grow collectively. The Nigerian barrier complex, situated in the tropics, is compared with the temperate tidal flats of the Dutch coast. The main differences depend on the contrasted role of plants in the two areas, and thus linally on climate. ANTOINE J. W. and J. L. HARDING, 1965. Structure beneath continental shelf, northeastern Gulf of Mexico. Bull., Amer. Ass. Petrol. Geol., 49 (2): 157-171. Data from forty-seven reversed seismic refraction profiles in the northeastern Gulf of Mexico are presented in the form of four east-west and two north-south cross sections. Where possible.
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