Post-depositional Oxidation Research Articles

Geochemistry of Mediterranean sediments. Bromine/organic carbon and uranium/organic carbon ratios as indicators for different sources of input and post-depositional oxidation, respectively

Organic carbon, U and Br (after removal of bromide) values were determined in several suites of Recent sediment samples from the Mediterranean. Br/C org ratios are found to be constant. Literature data revealed this ratio to be constant in various cases, though the numerical value of the constant varies. It is concluded that the numerical value is indicative for the presence of the relative amounts of terrestrial organic matter in the original environment of deposition. U/C org ratios are rather constant also. A small but significant systematic difference of the maximum values of U compared to those of C org with depth probably reflects post-depositional oxidation during a certain time span.

Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya

The Koobi Fora Formation, a Pliocene and Pleistocene sequence of sedimentary deposits northeast of Lake Turkana, has yielded numerous fossils and stone artifacts of early hominids. Stratigraphic correlation of the hominid‐bearing deposits throughout the Turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies. We have reinterpreted previously published magnetostratigraphy from the upper part of the Koobi Fora Formation because the original stratigraphy and dating of tuffs have been revised. In our reinterpretation we include previously unpublished data from the uppermost part of the formation. The upper magnetozones correlate with parts of the Brunhes Normal‐Polarity Chron and Matuyama Reversed‐Polarity Chron (about 0.6–0.85 Ma) and are separated from the magnetozones of the upper part of the Matuyama (2.0–1.25 Ma) by a disconformity. The Olduvai Normal‐Polarity Subchron is represented within the Matuyama, but the lower part of the Matuyama (2.4–2.0 Ma) is missing due to an erosional disconformity. We have also determined magnetozones in the lower part of the Koobi Fora Formation, which had not been sampled for paleomagnetism during the earlier studies. Our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of Koobi Fora tuffs with dated tuffs in the Shungura Formation of southern Ethiopia. The tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the Koobi Fora and Shungura formations. The lower part of the Koobi Fora spans the interval from about 4 Ma to 2.4 Ma, within the Gilbert Reversed‐Polarity and Gauss Normal‐Polarity chrons. Rock magnetic studies indicate that detrital magnetite carries most of the stable remanence, although hematite contributes to the remanence as indicated by thermal demagnetization. The hematite, which presumably formed by postdepositional oxidation of original iron oxide grains, carries chemical remanent magnetization (CRM) that diminishes the sharpness of polarity zone boundaries. The CRM accumulated continuously within the uppermost 10 m of sediment below the land surface, so that the CRM reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70,000 years in sections where the sedimentation rate was approximately 15 cm/1000 years.

Sulfur isotope systematics at the 21°N site, East Pacific Rise

Hydrogen sulfide in hydrothermal vent fluid at the 21°N site is enriched in 34S relative to Mid-Ocean Ridge basalts, probably by addition of H 2S reduced from seawater sulfate by FeO-bearing basalt. Metalliferous sulfides are depleted in 34S relative to the fluid from which they apparently precipitated, the degree of depletion reflecting the microenvironment in which each mineral crystallised and/or kinetic effects. Isotopic compositions of coexisting sulfides in a basal mound are consistent with equilibration at around 445°C, though heating to such a high temperature seems unlikely. Similar sulfides in a black smoker and in a dead chimney are out of isotopic equilibrium at any temperature, apparently reflecting a complex series of replacement mineralisations and post-depositional oxidation, respectively.

Maceral, Total Organic Carbon, and Palynological Analyses of Ross Ice Shelf Project Site J9 Cores

Analyses of macerals and total organic carbon indicate that the low organic content of core sediments from Ross Ice Shelf Project site J9 has been selectively reduced further, probably by postdepositional submarine oxidation. Palynological analysis revealed a reworked Paleogene dinocyst flora of low diversity (the transantarctic flora). This constitutes the most southerly dinocyst flora reported thus far. The antarctic distribution of the transantarctic flora supports the existence of a transantarctic strait during the Paleogene. The J9 sporomorph assemblage also is reworked and Paleogene in age.

Stratigraphy and depositional environment of the Kyeburn Formation (Cretaceous), a wedge of coarse terrestrial sediments in Central Otago

Abstract The Cretaceous Kyeburn Formation is a 4000-m thick terrestrial sequence dominated by coarse detritus, which was deposited in a fault-angle depression. The distribution of lithofacies, direction of fining and of increasing pebble roundness, nature of clasts, imbrication, and orientation of channels, indicate derivation of the sediment from the southwest and southeast, where the sequence is today bounded by the Dansey Pass and Waihemo Faults. Breccias close to the boundary faults represent talus deposits; alluvial fan, flolodplain, and lacustrine deposits occur in successively more distal (northward) parts of the basin. The more distal part of the sequence has scattered carbonaceous material and is dominantly grey in colour; the more proximal part is largely red because of a hematite stain, possibly formed by post-depositional oxidation of mafic minerals.

Deltaic and Associated Facies of Difunta Group (Late Cretaceous to Paleocene), Parras and La Popa Basins, Coahuila and Nuevo Leon, Mexico: ABSTRACT

ABSTRACT The Difunta Group is gray calcareous mudstone, siltstone, and sandstone that interfingers with wedge-shaped redbed units. These complexly interfingered units are divided into 13 formations and numerous members, tongues, and lentils; the group has a thickness of 10,000 feet. Low sinuosity, high-bedload streams transported volcanic and sedimentary rock debris from the Sierra Madre Oriental eastward to the ancestral Gulf of Mexico. Separate rivers fed the Parras and La Popa basins. Sediment accumulated in arcuate, wave-dominated deltas that prograded slowly into water several hundred feet deep. Some sediment was transported by marine processes to shallow delta-flank environments. Delta-plain lake and interfluve deposits are bioturbated mudstone containing sparse leaves, charophyte oogonia, dinosaur bones, and oyster debris. Many beds were reddened by postdepositional oxidation of iron-bearing minerals. Delta-front sheet sands are 20 to 60 feet thick, well sorted, flat bedded, and have sparse Ophiomorpha. Delta-platform deposits are characterized by fantastic sequences of ball-and-pillow structure and a sparse molluscan fauna. Prodelta deposits are interbedded graded sandstone and bioturbated mudstone; the sandstone beds were deposited by turbidity currents generated at the delta front by hyperpycnal inflow. Carbonate banks up to 1000 feet thick developed in distal prodelta (shelf) environments on submarine highs that were sea-floor expressions of rising evaporite diapirs. At least four major regressive-transgressive cycles developed in response to episodic basin subsidence. In contrast to the Rocky Mountain Cretaceous transgressions, the Difunta accumulated sheet sandstone units 20 to 60 feet thick. Parts of the transgressive units are delta-destructional deposits, but many are coalesced progradational sheet sands.

Diagenesis of Shaly Rocks: ABSTRACT

Argillaceous rocks show major chemical and mineralogic trends as a function of geologic age. If the trends are compared by eras, they are of global significance. The ratios of the various metal oxides to Al2O3 plotted against geologic age form 3 distinct behavioral groups and oxide-Al2O3 ratios, except those of K2O and FeO, diminish with increasing rock age. The first group, water (-H2O), CaO, and CO2, decreases with rock age from high to very low values. The covariance of H2O with CaO and CO2 is consistent with a gentle water leach of shale and loss of original calcium carbonate through time. MgO, Na2O, and SiO2 form a second group, but change with geologic age much less than the first group. The chemical trends of these second group oxides, along with K2O, reflect the differences in shale mineralogy as a function of age. The increase in illite percentage in older rocks results in a slight relative enrichment in K2O whereas the abundance of expanded clays in younger shales gives rise to Mesozoic-Cenozoic shales of higher Na2O, MgO, and Si 2 content. This interpretation requires that reactions of the following type obtain within shales in the first few hundred million years after burial: (low silica) kaolinite + (high silica) expanded clay + potassium = (intermediate silica) illite + MgO, Na2O, and SiO2 (lost from shale). The third group includes FeO and Fe2O3. There is a reciprocal relation between these oxides; young rocks are high in oxidized iron, old rocks low, but the total iron oxide concentration in shale is almost constant with geologic age. This trend partly reflects the postdepositional oxidation of organic matter and attendant reduction of iron in shales as they progressively age. These time-dependent chemical and mineralogic trends are in harmony with the general concepts of differential sedimentary cycling and chemical uniformitarianism; concepts that predict approximately the kinds of long-term, postdepositional, selective changes expected in argillaceous rocks as a function of geologic age, and that help to discriminate between primary and secondary chemical and mineralogic features. End_of_Article - Last_Page 858------------

Iron Oxides in Red Beds

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).