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Both igneous and sedimentary rocks affected by water–rock interaction commonly form rhythmic Fe‐oxide bands that provide information on palaeo‐groundwater. This study investigated the formation of Fe‐oxide banded patterns of concentrically banded concretions from the Jurassic Navajo Sandstone (United States) and banded claystone 'zebra rock' from the Neoproterozoic Johnny Cake Shale Member (Western Australia). Micro‐X‐ray fluorescence imaging of both samples revealed uniformly banded patterns of asymmetrical Fe content peaks. This study proposes that their formations were caused by diffusion, with a pH buffering reaction of Fe2+ and O2 due to dissolution of earlier materials by acidic fluids. Based on the model, it is suggested that the acid fluids for Navajo diffused towards the inner side of the concretion, and that for Kimberley they penetrated oblique or perpendicular to bedding. In addition, the Fe‐oxide precipitation has resulted from pH rises of acidic fluids because of diagenetic alteration of earlier calcite concretion in Navajo and that of earlier pyrite‐bearing claystone in Kimberley. The interpretation for Kimberley is based on the δ34S values (<−20‰) of zebra rock, suggesting that the alunite and Fe‐oxide resulted from the supply of SO42− and Fe2+ produced as a result of pyrite oxidation in earlier rocks. The formation rate of the Fe‐oxide band was estimated from the constant width of the Fe peaks and the expected diffusion coefficient for Fe2+ through the rock matrix, and it was found to be on the order of years for Navajo and on the order of tens of years for Kimberley. [ABSTRACT FROM AUTHOR]

A method of image processing is presented to extract a sequential profile from a map showing folded pattern of laminations with boudinage, overlapping noise and deficits in order to obtain reliable data for sequential analysis of sedimentary rocks on their cross-section. The raw map data is a digital photograph of any striped pattern of such rocks represented by 2D distribution of density of a certain quantity. The image processing consists of several steps. First, we derive the distribution of local slopes (strike) of laminations by means of statistical analysis of the first and the second spatial derivatives of the density map. Filtering of the local slopes leads to the elimination of noise amplified by differential operations. Secondly, a set of curved isochronous lines is determined by integrating the local slopes to trace smoothly the local strikes of laminae. Thirdly, the original folded pattern of laminations is converted to an “unfolded image” with straight laminations by using the isochronous lines. Finally, a set of a reliable 1D sequential profile of density and its uncertainty profile are derived by computing the mean and variance of density along each of the isochronous lines on the unfolded pattern of laminations. The computer code is tested successfully on the synthetic map data and applied to a set of abundance maps of major elements in Archean banded iron formation to characterize the striped pattern. High resolution is achieved objectively to recognize even thin and faint seams in the laminations. The computer code, lamination tracer, is expected to be useful for the analysis of other types of laminated pattern in various fields such as dendrochronology and sequence stratigraphy to decode the environmental variations in the geologic past. [Copyright &y& Elsevier]

Iron oxide concretions are found in sedimentary rocks on both Earth and Mars. On Earth, concretions are common in eolian formations, such as the Jurassic Navajo Sandstone in Utah, USA and those found in the Cretaceous Djadokhta Formation, Gobi Desert, Mongolia. Although it is known that the formation conditions of the iron oxide concretions were affected by the paleoclimate of these regions, quantitative modeling of such formations still requires development, especially concerning initial and diagenetic conditions. A 1-D diffusion–reaction simulation was conducted by assuming that a calcite concretion was initially located in a homogeneous layer of sandgrains. Favorable conditions for the formation of iron oxide concretions have been found to be 4.5 ⩽ pH ⩽ 6 , and 10 - 7 ⩽ [ Fe 2 + ] f O 2 ⩽ 10 - 5 , where [ Fe 2 + ] and f O 2 are the concentration of ferrous Fe 2 + ions and dissolved oxygen relative to the atmospheric value, respectively. An iron-rinded concretion from ferric Fe 3 + ions is not possible. For the case of Fe 2 + ions, the flow speed of the groundwater should be faster than 2 × 10 - 5 mm s - 1 . The formation timescale is determined by the diffusion flux of the hydrogen ion, and varies between 2.7 × 10 2 and 1.5 × 10 4 years for a calcite concretion with an initial radius of 15 mm. Formation conditions of iron-rinded concretions on Earth and Mars are discussed. [ABSTRACT FROM AUTHOR]

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a 'Diffusion-growth rate cross-plot' that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential. [ABSTRACT FROM AUTHOR]

In sedimentary rocks rich in Fe-oxide, such as red beds, white bleached spots that are free of Fe-oxide minerals are often observed. The spot formation has been explained by localized reduction reactions in relation to organic substances, such as fluids including hydrocarbon and organic debris as a precursor, and the recent major prevalent approach is the microbial activity in sediments. However, the evidence for microbial activities within the spots is rarely found, and it remains debatable whether the entire spots are of microorganism origin. We discovered bleached spots in zebra rock, which is the sedimentary rock from northern Australia with rhythmic Fe-oxide bands, and explained that the spots were formed by pH changes induced by the decomposition of primary pyrite. Our comprehensive petrological studies further demonstrate that the spots developed during the infiltration of the acidic fluid in relation to the Fe-oxide band formation, and the primary pyrite as a precursor is currently preserved as a pseudomorph composed of dickite and aggregation of hematite and goethite. This study demonstrates that the bleached spots in zebra rock were formed by inorganic chemical reactions, indicating that the proposal in other studies that spots of this kind can be used as a biomarker to find life on Mars is not always available. • Bleached spots are discovered in a sedimentary rock with rhythmic Fe-oxide bands. • Pseudomorphs consisted of dickite and Fe3+ minerals after pyrite are observed at the center of spots. • Bleached spots were formed during the infiltration of acidic fluid related to the Fe-oxide band formation. • This study shows that the bleached spots in zebra rock were formed by pH changes induced by the decomposition of pyrite. [ABSTRACT FROM AUTHOR]

Glendonite is generally accepted as a calcite pseudomorph after ikaite (CaCO 3 ·6H 2 O) and is commonly preserved in spherical carbonate concretions known as glendonite concretions in marine sedimentary rocks. Although they have widely attracted attention as environmental indicators, the detailed geochemical process underlying glendonite formation is not fully understood. Here to show the glendonite concretion formation process, we conducted detailed macro- and microscopic observations, carbon and oxygen isotopic analyses (δ13C and δ18O), chemical composition analyses, and elemental mapping of glendonite concretions from four Cenozoic marine sedimentary strata in Japan. The occurrence of glendonite and concretions around dead organisms such as bivalves, ghost shrimp, or soft-bodied organisms forming ichnofossils, and the negative δ13C values of glendonite concretions around −20‰ to −10‰ showed that organic matter in dead organisms served as a carbon source for both ikaite crystals and the surrounding concretions. Highly concentrated phosphorus and its distribution patterns in glendonite concretions showed that changes of phosphate concentration in porewater play a crucial role in the sequential formation process from ikaite crystallization to syngenetically surrounding calcite concretion growth. The sequential processes of glendonite concretion formation progressed rapidly owing to the supply of bicarbonate during the decomposition of organisms in marine sediments. Our work provides a phosphate concentration-controlled growth model of glendonite concretion involving organic decomposition that can be used to evaluate paleoporewater chemical conditions during early diagenesis. [ABSTRACT FROM AUTHOR]

Zebra rock, found in the eastern Kimberley region of northern Western Australia, is a Late Proterozoic sedimentary rock with a rhythmic Liesegang-type Fe-oxide banding. The striped rhythmic pattern in sedimentary rocks is an important key to infer chemical conditions of water-rock reactions. Although past studies have discussed the zebra rock formation for decades, the process remains unclear. Here, we introduce a new formation model, suggesting that zebra rock formed in an acidic hydrothermal system and that pH buffering of Fe2+-bearing acidic fluid, in a neutralization reaction with primary carbonate minerals, induced rhythmic Fe-precipitation. The Fe profile clearly shows a reaction front, indicating unidirectional diffusive fluid migration along bedding planes. The two types of zebra rock distinguished by color and by clay mineral assemblages correspond with typical acidic hydrothermal alteration zonation. Although a specific volcanic event related to the hydrothermal activity cannot be identified presently, this study provides a new model for the zebra rock, recording both hydrothermal alteration and post-Late Proterozoic fluid migration. [ABSTRACT FROM AUTHOR]