Iron Oxides in Red Beds

Abstract

This study focuses on: (1) grains of black oxides, (2) pigment in matrix and cement, and (3) natural remanent magnetization in red beds. Black oxides in 13 samples of soils, 26 samples of alluvium in transit and Recent nonmarine deposits, 103 red and drab units in red bed sequences ranging in age from Precambrian to Pleistocene, and in 9 ancient nonmarine drab sequences, were identified by X-ray analysis, supplemented by polished-surface microscopy. These data and pertinent published information support the following conclusions: (1) Magnetite and ilmenite and their various intergrowths predominate in igneous and metamorphic source rocks, and they are delivered relatively unaltered to nonmarine and paralic environments. In oxidizing areas, inherited grains of oxide begin to alter to black hematite and leucoxene. Oxidation continues long after burial, so that successively older red beds contain relatively more hematite. In reducing environments, inherited oxide grains begin to dissolve. Loss of more easily destroyed magnetite increases the relative amounts of ilmenite and black hematite. The latter apparently developed en route or in a preceding oxidizing episode. (2) Muddy alluvium commonly contains brown amorphous and hydrated ferric oxide pigment which begins to age and also dehydrated to red hematite in oxidizing environments. Sandy alluvium with little matrix, after having accumulated in desert basins, acquires hematite pigment largely by postdepositional in situ alteration of iron-bearing silicates. (3) Most detrital red beds possess chemical remanent magnetization (CRM) which was imposed on some of the black grains of hematite after their postdepositional oxidation. Any detrital remanent magnetization (DRM) initially lodged in inherited black oxide grains is destroyed by small-scale organic and inorganic disturbance, as well as by the intrastratal hematitization. Red pigment in matrix and cement may also acquire CRM as the developing hematite grows through critical crystal sizes; in hematite-cemented quartz arenites this is the principal component of natural remanent magnetization (NRM).