Study of the Mississippi Delta and several others shows that there are order and pattern in delta building. The characteristic stratigraphic sequences form during a delta cycle which consists of constructional and destructional phases.
Marine deltas are seaward-thickening embankments of sediments deposited during the constructional phase and modified by the destructional phase. Sediments of the embankments are mostly land-derived elastics deposited in orderly sequence on the sea floor about distributary mouths. They make up the top-set, fore-set, and bottom-set beds of classical delta literature. Because of the way deltas grow, sediments change vertically in completed deltas in the same way they change seaward during construction; definite similarities between different deltaic sequences can be seen when they are compared.
Changing sediment properties are produced by seaward-changing depositional environments. Sedimentary environments of modern deltas, and ancient ones as well, are complicated when studied in detail, but the general relationships now are well known. The environments are defined by (1) sediment sources, (2) processes and their intensities, and (3) rates of deposition. Source areas determine the raw materials, and these differ from delta to delta. Similar marine and fiuviatile processes, similar distributions of process intensity, and relatively high rates of deposition are the environmental properties that cause deltaic sequences to be similar. Relatively rapid deposition is the fundamental characteristic of deltas.
Deltas rarely build indefinitely in one direction; rather, the river shifts for a shorter route to the sea when it becomes over-elongated. The recently built delta is abandoned and modified by compaction coupled with marine wave and current action. This is the destructional phase, the time of winnowing of fine sediment and concentration of coarse material into thin veneers, beach ridges, and barrier islands.
Large alluvial plains at river mouths are built up in a step-by-step manner. Lqcal delta construction is followed by partial destruction, and later, by another constructional phase. A large alluvial plain consists of several imbricating deltas, each lying partly on the toes of earlier deltas and partly on the surface that existed prior to any delta building. The stratigraphic components of younger deltas become seaward extensions of their older counterparts. When the beds are buried beneath still younger ones, their full history can be understood only by recognizing their deltaic origin and by knowing how deltas are built.
Figures & Tables
Recent Sediments, Northwest Gulf of Mexico
All but one of the papers contained in this volume represent a symposium summarizing the results of work carried on in Project 51 of the American Petroleum Institute. This study of modern sediments along the northwest margin of the Gulf of Mexico contains 14 papers plus a consolidated bibliography. Paper titles are: Geologic framework of Gulf coastal province of United States; Sources and dispersion of Holocene sediments; Mississippi delta; Delta building and the deltaic sequence; Phytoplankton production in the Mississippi delta; Bays of central Texas coast; Sediments of Laguna Madre; Gulf Coast barriers; Sediments and history of Holocene transgression; Sedimentary patterns of microfaunas; Ecology and distributional patterns of marine macro-invertebrates; Rise of see level; Regional aspects of modern sedimentation; and Recent sedimentology.