Title: Collected reprints / Atlantic Oceanographic and Meteorological Laboratories (and) Pacific Oceanographic Laboratories
Identifier: collectedreprin1976v1atla (find matches)
Year: 1968 (1960s)
Authors: Atlantic Oceanographic and Meteorological Laboratories; Pacific Oceanographic Laboratories; United States. Environmental Science Services Administration. Research Laboratories; Environmental Research Laboratories (U. S. )
Subjects: Oceanography Periodicals.
Publisher: Washington, D. C. : U. S. Dept. of Commerce, Environmental Science Services Administration, Research Laboratories : For sale by Supt. of Docs. U. S. G. P. O.
Contributing Library: Penn State University
Digitizing Sponsor: LYRASIS Members and Sloan Foundation

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312 C O N TI N K N T A I SHE! F SEDIMENTATION complex mosaic of sediment types, rather than a simple seaward-fining sheet. He suggested that these patches were deposited during Pleistocene low stands of the sea, rather than during Recent time. Emery (1952, 1968) raised this concept to the status of a new conceptual model. He classified shelf sediments on a genetic basis, as autlugenic (glauconite or phosphorite), organic (fcramini- fera, shells), residual (weathered from underlying rock), relict (remnant from a different earlier environment such as a now submerged beach or dune), and detrual, which includes material now being supplied by rivers, coastal erosion, and eolian or glacial activity. On most shelves, a thin nearshore band of modern detrital sedi- ment' is supposed to give way seaward to a relict sand sheet veneering the shelf surface. A third, more generalized model for shelf sedimenta- tion has been primarily concerned with the resulting stratigraphy. It incorporates elements of both the Johnson and Emery models. Like the Johnson model, it views the shelf surface as a dynamic system in a state of equilibrium with a set of process variables. The rate of sea-level change, however, is one of these variables; hence the effects of post-Pleistocene sea-level rise, as noted by Shepard and Emery, may be accounted for. The model may be referred to as the transgression- regression model, since it is generally expressed in these terms, or the coastal model, since it focuses on the behavior of this dynamic zone. It was first explicitly formulated by Grabau (1913), and more recently by Curray (1964) and Swift et al. (1972). In this model, the rate of sediment input to the continental shelf S, the character of the sediment G (grain size and mineralogy), the rate of energy input E, the sense and rate of relative sea-level change R, and slope L are seen as variables that govern the sense of shoreline movement (trans- gression or regression) and ultimately the character of shelf deposits. The relationship may be expressed in quasi-quantita- tive form as SG E R L oc T The processes controlling shelf sedimentation are much too complex to be adequately described by this equation and there is no way to evaluate its variables adequately. The expression is useful, however, in helping to sort out relationships. The first term, SG E, might be called the effective rate of coastal deposition. It increases with increasing 5\ the rate at which sedi- ment is delivered to the shore. It increases with increas- ing grain size G. since coarser sediments are less easily bypassed across the shelf. It decreases with increasing E, the rate of wind and tidal energy input, since a more rigorous hydraulic climate causes more sediment to be bypassed across the shelf. The second term, R/L, might be called the effective rate of sea-level movement. It increases with increasing /t, the absolute rate of sea-level movement (eustatic or tectonic), but decreases with increasing slope, L, of the coast. The steeper the slope, the greater the fall of sea level must be in order for the coast to advance a given distance. Also, with a greater slope, a greater volume of sediment must be delivered to the shoreline in order for the shoreline to prograde a given distance shoreward. The equation tells us that the rate and sense of shore- line movement, T, whether landward (negative) or seaward (positive), depends on the relationship between these two terms. Basic elements of the relationship are presented graphically in Fig. 1, according to a scheme of Curray (1964). In Fig. 2, the history of the Nayarit coast of Mexico has been plotted. The Coastal Boundary as a Filter: Shelf Sedimentary Regimes The fundamental determinants of shelf sedimentation are the areal extent of the adjacent continent undergoing denudation, and its relief, climate, and drainage pattern. These factors control the quantity of sediment delivered to the shoreline, and its textural and mineralogical composition. However, the rate and sense of shoreline movement, as determined by the parameters described above, have a modulating effect on the shelf sedi- mentary regime. It is helpful to think of the coastline as a '"littoral energy fence" (Allen, 1970b, p. 169) in which the net landward flow associated with bottom wave surge tends RELATIVE SEA LEVEL FALLING SEA LEVEL OR EMERGENCE RISING SEA LEVEL OR SUBSIDENCE RAPID SLOW STABLE SLOW RAPID
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FIGURE 1. Diagram of the effects of sea-level movement and the rate of coastal deposition on lateral migration of the shoreline. See text for explanation. From Curray (1964). 525

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