SUMMARY
Recent alluvial sediments are reviewed with respect to their geometrical, textural, structural and biological characteristics. These properties are related to the physiographic occurrence and hydraulic geometry of streams and to the dynamics of flowing water as controlling sediment transport-deposition and stream morphological activities. Based on this data, three-dimensional facies models are presented as an aid to the identification of ancient alluvial sediments, which are briefly reviewed also.

A review of the origin and characteristics of recent alluvial sediments

A study of global sand seas

Sand waves: A model of origin and internal structure

Transport of participate material across continental shelves is well demonstrated by the distributions on the seabed and in the water column of geological, chemical, or biological components, whose sources are found farther landward or farther seaward. This paper addresses passive (incapable of swimming) particles and their transport across (not necessarily off) continental shelves during high stands of sea level. Among the general factors that influence across-shelf transport are shelf geometry, latitudinal constraints, and the timescale of interest. Research studies have investigated the physical mechanisms of transport and have made quantitative estimates of mass flux across continental shelves. Important mechanisms include wind-driven flows, internal waves, wave-orbital flows, infragravity phenomena, buoyant plumes, and surf zone processes. Most particulate transport occurs in the portion of the water column closest to the seabed. Therefore physical processes are effective where and when they influence the bottom boundary layer, causing shear stresses sufficient to erode and transport particulate material. Biological and geological processes at the seabed play important roles within the boundary layer. The coupling of hydrodynamic forces from currents and surface gravity waves has a particularly strong influence on across-shelf transport; during storm events, the combined effect can transport particles tens of kilometers seaward. Several important mechanisms can cause bidirectional (seaward and landward) transport, and estimates of the net flux are difficult to obtain. Also, measurements of across-shelf transport are made difficult by the dominance of along-shelf transport. Geological parameters are often the best indicators of net across-shelf transport integrated over time scales longer than a month. For example, fluvially discharged particles with distinct composition commonly accumulate in the midshelf region. Across-shelf transport of particulate material has important implications for basic and applied oceanographic research (e.g., dispersal of planktonic larvae and particle-reactive pollutants). Continued research is needed to understand the salient mechanisms and to monitor them over a range of timescales.

/pdf/transport-of-particles-across-continental-shelves-4l0w7txf8x.pdf

Transport of particles across continental shelves

This study examines the morphology, sedimentology and genesis of the point bars and floodplain of the Beatton River. The formation of point bars occurs in distinct stages. An initial point bar platform composed mainly of coarse sediment is formed adjacent to the convex bank of a migrating meander bend, and is the base on which develops a single scroll bar of fine traction and suspended load. With continued sedimentation, the scroll bar grows, eventually supporting vegetation and becoming a floodplain ridge. Scroll bars form with greatest size and frequency in rapidly migrating bends, and the shape of the meander bend appears to determine both the location of the initial bar deposit, and its direction of growth up or downstream. Approximately one-half of the floodplain sediment is derived from suspended load, and the initiation of a scroll bar appears to be due to excessive deposition of suspended load in a zone of flow separation over a point bar platform. The critical flow condition for the initiation of a scroll bar does not occur with the same recurrence interval on different shaped meander bends, however, the average recurrence interval within the study reach is approximately every 30 years. Sedimentation rates on point bars and on the floodplain indicate two relatively distinct stages of floodplain alluviation. The most rapid is for surfaces less than 50 years old, although sediment accumulation still persists on surfaces up to 250 years in age. Although frequently flooded, surfaces older than this accumulate very little sediment. Despite 2–3 m of overbank deposition, the amplitude of floodplain ridges is maintained by secondary currents which sweep sediment from the swales towards the ridge crests.

Point bar and floodplain formation of the meandering Beatton River, northeastern British Columbia, Canada

Since 1952, geological mapping of the continental shelf around southern Britain has been carried out with echo-sounders, and more recently by the echo-ranging equipment on R.R.S. Discovery II, which produces acoustic maps of the sea floor. The character of the superficial sediments is here interpreted in terms of sedimentation processes and it is argued that these are controlled by the strong tidal currents. Regions of deposition and their supply paths are indicated and it is shown that only one sand stream is heading for the deep sea floor. The subsequent history of land deposits after a marine transgression and of younger material on present-day current-swept sea floors is stated and the facies that are being produced are mentioned.

Current-swept sea floors near the southern half of Great Britain

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Current-swept sea floors near the southern half of Great Britain