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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, marginal 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 away 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 deposi-tional 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 transgression 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.

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