Continental-Shelf and -Slope Studies
Published:January 01, 1991
Sedimentation in regions of high (100–1,000 Bubnoff units [B]) and superhigh accumulation rates (more than 1,000 B) differs drastically from that characterized by normal rates (less than 100 B), in terms of distribution, mass, composition and structure. One Bubnoff unit is 1 mm/1 ka. High-accumulation rates result in high-water content, high-sediment mobility, and high organic-carbon content. The ability of sediments to slide and flow into depressions increases, and the main transport processes are gravitational flows. Sedimentation associated with avalanche rates is the principal sedimentation process on Earth. The thickness of normal deposits in the pelagic realm does not exceed 1 km, but in the avalanche-sedimentation regions, thickness reaches as much as 10 to 15 km. Normal sedimentation is associated with dispersion of sedimentary material, whereas avalanche sedimentation is associated with the concentration and formation of thick, autonomously developing sedimentary bodies. Avalanche sedimentation occurs where suspension concentration exceeds 5 mg/l, where sediment accumulation rates of more than 100 B occur, and absolute masses of more than 5 mg/cm2/l ka accumulate.
According to the hypographic curve of the Earth, three global levels are revealed: 1) river mouths (river-sea border), 2) continental rises, and 3) deep-sea troughs. They are each separated by 3 to 6 km of topographic relief, but are related closely by sedimentary source material. Sediment translocation occurs with hiatuses and is a function of sea-level changes. With an increase in sea level, deposition occurs on the first level of concentration, and/or hiatuses form on the second and 3rd levels; with a decrease in sea level, hiatuses form on the first level and concentration occurs on the second and third levels. These three vertical levels of sediment deposition, related to each other in space and time, are unified by sediment composition (including organic carbon). These sedimentary systems (each system includes collection, transportation and accumulation of sediment) include subaerial and subaqueous aspects. Concentration of organic-rich material with isostatic crustal downwarping takes place in these zones, and in some cases results in oil and gas formation. This is the principal zone of oil and gas formation. Capture of the main supply, more than 90 percent of the river-suspension load on the first level, results in a deficiency of pelagic material. Precipitation of the dissolved load forms sediment by bioprocesses (CaCO3, SiO2 amorphous, C organic, etc.). However, river-suspension residues (7–8 percent), absorption, eolian, ice and endogenous-material input are most important. The concept of avalanche sedimentation requires new approaches to the questions of sedimentation, geochemistry and the formation of mineral deposits.
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From Shoreline to Abyss: Contributions in Marine Geology in Honor of Francis Parker Shepard
From Shoreline to Abyss: Contributions in Marine Geology in Honor of Francis Parker Shepard - Francis P. Shepard left a rich scientific legacy including more than 230 published papers and books primarily addressed to the study of submarine canyons and turbidity currents, continental shelves and associated sediments, coastal processes and sediments and marine physiography and tectonics. He is best remembered for his work on submarine canyons; however, his broad range of scientific interests and his remarkable ability to break new ground in each of these disciplines have served as a model for at least four generations of ?Shepard? students. This new work from these Shepard students addresses problems in marine geology from the global scale to the local outcrop scale. Relationships among tectonics, eustacy and both siliciclastic and carbonate sedimentation create a unifying theme. Special topics include coastal processes, shelf and slope evolution, and submarine canyon and fan systems.