deltaic deposits[del′tā·ik di′päz·əts]
deposits of river drift in seas and lakes at the mouths of rivers. The cause of deposition is a sharp drop in water flow rate and partly in the coagulation of the fine particles brought by the river in suspension or as colloidal solutions upon encountering salt water. Favorable conditions for formation of thick layers of deltaic deposits include the absence at the mouths of the rivers of significant wave action and currents, which could carry away river drift, and a slow, steady sagging of the particular area of the earth’s crust. Deltaic deposits up to several kilometers thick can form under such conditions. The accumulative formation caused by deltaic deposits, the delta, is built up from the surface primarily by river and lacustrine-swamp sediment. The sediment of shallow parts of the sea, which have been desalinized by river waters, as well as lagoon and bar deposits, are part of deltaic deposits.
Deltaic deposits consist mainly of arenaceous-argillaceous rocks with occasional intercalations of limestones, less frequently of coal and other rock of organic origin; conglomerates are often found in piedmont regions. Deposits of coal and iron and copper ores are found in deltaic deposits. Fertile soils generally form on deltaic deposits characterized by a fine-grained nature and diversity of composition. Because of this, the deltas became centers of the farming culture and were densely populated even in the distant part (for example, the Nile delta). Deltaic deposits are also often encountered in the fossil state in sedimentary strata of past geological periods. They are usually characterized by considerable thickness, uniqueness of structure (such as the presence of definite types of cross bedding), and the combination of various marine, brackish-water, and continental deposits.