Sediment Transport in Estuarine Areas
The Estuarine Sedimentation Research Group of King's College and Sir John Cass College has been studying the estuaries of the Thames, Medway and Taw-Torridge. Its members, who report some of their results here, have found that the sedimentary processes in the estuaries are varied and complex.
Seabed dynamics in a lage coastal embayment: 180 years of morphological change in the outer Thames estuary
This article analyses the morphological history of the outer Thames seabed, covering over 3,000 km2 from Aldeburgh (Suffolk), to Southend-on-Sea (Essex) and Margate (Kent). The region has been depicted on bathymetric charts since the sixteenth century, and has been formally charted since the eighteenth century. Charts published since the early 1800s incorporate sufficient grid reference or ground control detail for georectification onto a common coordinate system (British National Grid). The morphological history of the outer seabed was thus reconstructed through the digitisation and interpolation of soundings onto a regular grid (3D surface). The evolution of seabed morphology was examined using transects, bathymetric change maps and spatial statistics. The results show considerable spatial variability in seabed behaviour. Within the central Thames, banks have experienced significant depth changes can be associated with lateral shifts in individual banks. Some of the outer banks in this region exhibit progressive elongation. Shifts in bank position across the Suffolk shoreface appear to be more subtle, and there is evidence here of both onshore and offshore migration. There is no clear evidence of any regionally coherent response to large-scale historical forcing such as sea-level rise.
Regional Aspects of Modern Sedimentation in Northern Gulf of Mexico and Similar Basins, and Paleogeographic Significance
This paper summarizes some of the results of recent sediment studies in the northern Gulf of Mexico and Orinoco area in terms of their interest for stratigraphy and paleogeography. The discussion is limited to modern sediments formed in a broad, open basin with a vast hinterland, an abundant supply of terrigenous sediment, and a general predominance of sedimentation over tectonic effects. Studies of modern sediments are often based upon close observation grids, whereas in stratigraphy and paleogeography broad regional investigations using relatively open control are common. The importance of comparability in scale of comparative studies of ancient and modern sediments cannot be overemphasized. Many studies of recent sediments require generalization in terms of broad facies units and of descriptive features that are likely to be preserved, before they are suitable for use in the interpretation of ancient deposits. For the Gulf of Mexico a limited number of broad facies units can be established on the basis of simple and permanent lithologic, petrographic, and paleontologic criteria. Three of these are deltaic--the top set, fore-set, and bottom-set facies; three, margina non-deltaic--the littoral, subhumid, and semiarid bay facies; and four, marine--the shelf, slope, marginal reef, and nondepositional facies. The same facies units can be used in describing geologically similar modern areas, for example, the Orinoco shelf. In simplified form, the geographic distribution of these facies is a function of the relative rates of sediment supply from the land and redispersion by marine agents. The series delta-subhumid bay and littoral-semiarid bay and littoral reflects decreasing importance of land-derived sediment and increasing marine reworking and dispersion. Where the supply of land-derived sediment is small, marine forces produce complete separation of the material in a coarse fraction deposited in the littoral zone, and a fine fraction which is carried way in suspension. Where the current pattern supplies the suspended material and the water is deep and quiet enough to permit settling, the shelf facies is formed. Nondepositional areas occur where supply by marine currents is impossible (parts of the Gulf of Mexico), or long-period waves and shallow depth prevent deposition (Orinoco-Trinidad shelf). Under conditions of changing sea level, the rate of positive or negative sea-level change is a factor of importance in controlling facies development. The effects of this factor are still poorly known. In areas of high supply of sediment its importance appears small; changes in the relative positions of sea and land in deltaic areas are primarily due to erosional transgressions and depositional regressions. On the other hand, where sediment supply is reduced, as in the area of non-deltaic marginal and shelf facies, eustatic and tectonic sea-level changes exert important control. The littoral facies is best developed under conditions of stable or very slowly rising sea level; the resulting bay facies is completely separated from the open-marine environment. With an intermediate transgr ssion rate, open lagoons and low barriers are formed; with very rapidly rising sea level littoral sediments are deposited as a thin veneer without strong differentiation in bay and littoral facies. Little information is available concerning the influence of regressions. The stratigraphic sequence still appears the best means of distinguishing between transgressive and regressive deposits.