Normal sedimentary process

In examining the particulars of the distribution, migration, and deposition of iron and silica in natural waters it is advisable to distinguish normal sedimentary processes and volcanic sedimentary processes. By normal sedimentary processes we mean the cycle weathered layer-transport-deposition in a sedimentary basin. The main distinguishing feature of volcanogenicsedimentary processes is the endogenetic source of the material and ex-ogenetic method of its deposition. 

In the works of Strakhov (1960) on the theory of lithogenesis, iron compounds are assigned to the third group, which is characterized by very 

Sihca is more soluble than ferric iron, and together with carbonates is assigned to the second group of compounds, which migrate to a substantial extent in dissolved form. Natural waters usually are sharply undersaturated in Si02, whieh apparently is explained by kinetic factors, i.e. by the low rate of solution of quartz and sihcates. Chemogenic deposition of sihca from present sea waters is not very likely biologic extraction predominates in them. 

From the geochemical data it follows that a substantial part of dissolved iron migrates not in ionic form, but as Fe(OH)3 sol shielded by organic matter (also colloidal) or by a sol of SiO, and also as colloidal (or truly dissolved) complex ferro-organic compounds, chiefly soluble iron humates (Strakhov, 1960). 

Perel man (1968) points out that in neutral waters true dissolved molecules of undissociated Fe(OH)3 can also occur in predominant amount (compared to Fe ion). Probably the molecules of Fe(OH)3 are stabilized by organic acids. It is important that transport in the form of true ionic solutions is possible only for divalent iron, while trivalent iron migrates mainly in the form of colloids and organic compounds. However, Castano and Garrels (1950) believe that aerated waters provide conditions adverse to the preservation of organic matter itself. The available data suggest that organic matter decomposes at the same rate at which it arises. 

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Sources. Iron ore deposits were formed by many different processes, eg, weathering, sedimentation, hydrothermal, and chemical. Iron ores occur in igneous, metamorphic, and sedimentary deposits. Normally, as-mined iron ore contains 25 to 68% iron. 

The Namorado Sandstone is composed primarily of massive fine- to very fine-grained arenites. Sedimentary structures are rare, but occasional normal grading is observed. The fluidization process is identified by unusual dish and pillar structures. Rarely, individual beds present ripple cross-laminated divisions at the top. Individual beds have an average thickness of Im. Several individual beds may be amalgamated, resulting in sandy intervals up to 10 m thick. Individual beds or cycles are capped by thin beds of shales and calcilutites. 

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