Marine Deposits

bottom sediments of modern and ancient seas. They are more common than continental deposits, constituting more than 75 percent of the volume of the sedimentary shell of the earth’s continental crust. Marine deposits first formed when seas appeared in the Archean or even farther back in the geological past, about 3.5–4 billion years ago, and have continued to form throughout geological history. As a result of diagenesis, fossil marine deposits have been transformed into sedimentary rock. Marine deposits include most of the lime-stones, dolomites, marls, and siliceous rocks, a significant part of the clays and argillites, siltstones, sandstones, and conglomerates, and of the minerals, many iron and manganese ores, most of the phosphorites, and combustible shales. Many metamorphic rocks (gneisses, schists, and marbles) first accumulated as marine deposits.

The basic types of marine deposits—terrigenous, biogenic, chemogenic, and volcanogenic—as well as various combinations of these types, form from sedimentary material of various origins accumulating on the bottom of a body of water.

Different sediment formation conditions may exist simultaneously within particular sea basins depending on depth, distance from the shore, forms of bottom relief, currents, environmental conditions of sediment-forming organisms, and other factors; as a result different facies of marine deposits will develop. In the shallowest coastal zone, for example, terrigenous sands, pebbles, and shells accumulate through the action of waves; in calm sections and near the mouths of rivers, clays and silts are deposited. Shell and detrital biogenic limestone sediments and sands are common on submarine elevations and open shelves; clays and silts, sometimes rich in organic material, dominate in the basins of epicontinental seas; marly silt, lime mud, and siliceous ooze are encountered. Reefs of limestone or dolomite, often occurring in the midst of deep-sea deposits, are a special type of shallow-water marine deposit. Some sedimentary iron (oolitic) and manganese ores, bauxites, and phosphorites are also shallow-water marine deposits.

The deep-water deposits that accumulate primarily in the basins of deep seas are mostly fine-grained (clays, silts, lime mud, and siliceous ooze), but sands deposited by bottom currents are also encountered. Submarine slide deposits form on slopes. In the central parts of vast deep seas where the incoming supply of terrigenous material is small, marine deposits take on a pelagic appearance and resemble oceanic sediments (pelagic diatomites and limestones). Marine deposits rich in organic matter form on the bottom and at the base of the slopes of enclosed basins.

Fossil marine deposits are identified by the remnants or traces of the vital activity of marine organisms, but these signs may sometimes be absent, especially in deep-water deposits. Some authigenic minerals (glauconite) and the structural and textural characteristics of rocks serve as signs of marine deposits.

The masses of marine deposits and their composition and distribution on the earth’s surface depend primarily on the tectonic regime and climatic conditions. Tectonic movements determine the formation of marine basins, the configurations of marine basins, and the main features of the relief of the floor and adjoining shores; moreover, they cause transgressions and regressions of the sea and influence the intensity of sediment accumulation and the thickness of the layers of marine deposits that accumulate. Marine deposits in tectonically mobile geosynclinal regions and on relatively stable platforms differ substantially. The former are characterized by great thickness and the formation, in the initial and concluding stages of tectonic cycles, of deep-water marine deposits of siliceous and clay rocks, tuffs and tuffites, marls, and pelitomorphic pelagic limestones as well as polymictic and graywacke detrital deposits—that is, congloerates, sandstones, siltstones, rhythmically stratified flysch, submarine slide deposits, and reef limestones. Platform marine deposits include shallow-water organogenic limestones and dolomites and fine-grained terrigenous rocks (clays, marls, and fine-grained sandstones—for example, quartz and, frequently, glauconite sandstones); they accumulated primarily in the epicontinental seas and are not very thick.

The composition of marine deposits is systematically related to the climatic zonality of the earth. As study of modern seas indicates, marine sediment formation occurs in different ways in the main climatic zones. In the seas of the humid zones where rivers intensively transport the weathering products of rocks (both in the temperate and tropical belts), terrigenous deposits —sands, silts, and clays—dominate. Diatomaceous ooze accumulates at some places in the cold-water basins of the temperate zone. In the arid zone where less terrigenous sediments are transported, biogenic carbonate accumulation is more extensively developed and shell, bryozoan, foraminiferal, coccolith, and pteropod sediments form; in the warm waters of the tropical zone coral-algal reef complexes form. In some places chemogenic carbonate accumulation (oolitic calcareous sediments) occurs. In the ice zone glacial marine deposits become very important.

The conditions of formation of marine deposits have not remained unchanged throughout geological history. For example, during the Proterozoic and Paleozoic, chemogenic marine deposits accumulated on a much broader scale than in the Mesozoic and Cenozoic, when biogenic sedimentation was extensively developed. In the Precambrian and early Paleozoic marine dolomites were very common; in subsequent ages limestone was the principal rock. Unique marine deposits of ferruginous quartzites (jaspilites) are known only in the Proterozoic. These changes may be viewed as a reflection of the long-term evolution of the composition of the hydrosphere and the atmosphere and of the development of life on earth.