The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays

Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic ⁸⁷Sr from the clays into the surrounding pore waters.The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays.     

The extent of oxygen isotope exchange between clay minerals and sea water

The extent of oxygen isotopic exchange between detrital clay minerals and sea water was investigated by analyzing $\text{O}{}^{18}\text{O}{}^{16}$ ratios of separated fine-grained size fractions of deep-sea sediments from three North Pacific ocean cores. Isotopic results were interpreted according to models based on the assumption that the extent of isotopic exchange should increase with decreasing particle size and increasing time of exchange between the sediment and sea water. The data indicate that information concerning the provenance and mode of formation of detrital clay minerals can be obtained from the $\text{O}{}^{18}\text{O}{}^{16}$ ratios of the coarser-than-0.1 μm fraction of deep-sea sediments younger than several million years and the finer-than-0.1 μm fraction of deep-sea sediments younger than several tens of thousands of years. Furthermore, if the extent of chemical reaction between detrital clays and sea water is similar to the extent of oxygen isotopic exchange, such reaction may be important in regulating the chemistry of sea water.     

Mechanism of formation of Haddat Ash Sham ironstones (Oligo-Miocene), Makkah Al Mokaramah District, West Central Arabian Shield, Saudi Arabia

The present study aims to shed light on the mechanism of formation of the Oligo-Miocene oolitic ironstones of Haddat Ash Sham area, Saudi Arabia. These ironstones are enclosed within the middle part of the Oligo-Miocene siliciclastic succession of the western part of the Arabian Shield, western Saudi Arabia. The ironstone beds were formed during marine incursion and creation of short-lived starved time periods of high organic matter activities, ferrous iron, and low clastic input. The depositional and diagenetic processes involved in the formation of Haddat Ash Sham ironstones are summarized here as follows: (1) the deposition of detrital components (i.e., amorphous iron-bearing clays admixed with silt and sand-sized quartz grains) and their distribution by the waves and current actions in areas of different water depths (bars and inter-bar areas); (2) the deposition of the iron-bearing clays in slightly reducing transgressive conditions (dysaerobic zone) led to the authigenesis of green marine chamositic clays of variable mineralogical and chemical compositions according to the predominated depositional environments; and (3) in the upper parts of the depositional cycles, the iron-bearing clays become admixed with detrital quartz grains which resulted in the formation of silty and sandy ironstones of low iron content. The diagenetic processes led to the oxidation of the green chamositic clays and formation of amorphous Fe-oxyhydroxides, ferrihydrites, goethite, and hematite. These iron mineral phases are related to each other and show progressive steps of transformation during the diagenetic processes. The iron ooids represent in situ formed irregular domains formed during the diagenetic crystallization and dehydration of the amorphous iron oxyhdroxides resulted from the diagenetic oxidation of green chamositic clays. This is supported by the absence of detrital cores of the iron ooids, the gradational contact between the iron ooids and the enclosing matrix and also by the presence of many ooids of unclear and ill-defined internal structure.     

The geochemistry and mineralogy of the Permian red beds of Southwest England

Detailed mineralogical and chemical examination of some hundred rock samples representing the Permian red bed sequence of Southwest England has been carried out. Mineral—element correlations are presented and factor analysis of the chemical data shows that 80% of the variance can be explained by seven factors. Among these are recognised a clay factor; detrital minerals versus carbonate factor; halide and sulphate factors and a volcanic enrichment factor. The behaviour of elements in the strong oxidising environment associated with red bed deposition is discussed. In particular, zirconium is shown to be enriched in red bed clays, reflecting solution from the source area. Cerium, in its oxidised state as Ce⁴⁺ is shown to be strongly correlated with the carbonate phase, indicating its tendency to form relatively stable carbonate complexes. Strontium is positively correlated with the clays and negatively correlated with carbonate, while sulphur (as SO₄^2?) is associated with carbonate usually in the form of barytes. The origin of the red pigment is also discussed.     

Origin of red beds in the Ringerike Group (Silurian) of Norway

The Ringerike Group is a meandering fluviatile succession which is about 60% red. Most of the red zones are formed of mudrocks and siltstones and correspond to the fine members of fining-upwards cyclothems. The majority of coarse members are drab coloured.Textural studies of thin and polished sections show that the red colour is caused by finely crystalline hematite as matrix and grain-coatings. This hematite apparently crystallized post-depositionally. Hematite also occurs in other textural sites: within altered phyllosilicates, as detrital grains and as totally pseudomorphed phyllosilicates. This, and the lack of consistency between colour and clay mineralogy, suggests that the red beds have had a long and complex diagenetic history.Iron analyses indicate that the red beds are enriched in Fe³⁺ and total iron (FeO) by about 1%. This is thought to have been derived from the pre-depositional weathering of iron minerals and introduced into the sediments as amorphous iron hydroxide or iron-bearing clays. Crystallization of iron hydroxide under oxidizing conditions and the post-depositional alteration of iron-silicates and oxides is thought to be responsible for the formation of the red beds.     

Detrital clay grain coats in estuarine clastic deposits: origin and spatial distribution within a modern sedimentary system,the Gironde Estuary (south‐west France)

Porosity and permeability may be preserved in deep sandstone reservoirs by clay coating (mainly chlorite) which limits quartz overgrowths. Chloritization around quartz grains results mainly from mineralogical transformations of pre‐existing clays. It is fundamental to study those clay precursors to better understand and predict the location and distribution of clay coatings in subsurface sandstones for petroleum or geothermal prospecting. This paper reports a high‐resolution analysis of the composition, distribution and fabric of clays along a modern estuary, the Gironde (south‐west France). The scale of the study ranges from thin sections, through sand bar bodies, up to the entire estuary. Results show that clays are detrital and deposited at the same time as sand grains despite strong hydrodynamic conditions. Clays bind to medium‐grained sands forming detrital clay grain coats. On average, 26% of detrital sand grains are coated along the entire length of the estuary. Coat thickness varies from 1 μm to more than 200 μm, and coat coverage exceeds 30% in some samples. The turbidity maximum zone position (surface water turbidity from 1 to 10 g l^?1) in the estuary, which is controlled by seasonal variations in hydrodynamic processes, significantly impacts the location of the maximum clay content and the abundance of coated grains in sandy facies along the estuary. Flocculation in the water column results in the accumulation of suspended matter and contributes to the high turbidity in the estuary. Exopolymeric substances produced by diatoms are observed both in the field and by cryo‐scanning electron microscopy, suggesting that they may play a major role in binding detrital clay around sand grains. Finally, tidal bars and heterolithic point bars in the estuary funnel and estuarine channels are prime sedimentological targets for finding clay coatings and potentially good reservoir quality in deeply buried sandstones.     

The Effects of Diagenesis on the Reservoir Characters in Ridge sandstone of Jurassic Jumara dome,Kachchh, Western India

Ridge sandstone of Jurassic Jumara dome of Kachchh was studied in an attempt to quantify the effects of diagenetic process such as compaction, cementation and dissolution on reservoir properties. The average framework composition of Ridge sandstone is Q₈₀F₁₇L₃, medium-to coarse grained and subarkose to arkose. Syndepositional silty to clayey matrix (3% average) is also observed that occurs as pore filling. The diagenetic processes include compaction, cementation and precipitation of authigenic cements, dissolution of unstable grains and grain replacement and development of secondary porosity. The major cause of intense reduction in primary porosity of Ridge sandstone is early cementation which include silica, carbonate, iron, kaolinite, illite, smectite, mixed layer illite-smectite and chlorite, which prevents mechanical compaction. The plots of COPL versus CEPL and IGV versus total cement suggest the loss of primary porosity in Ridge sandstone is due to cementation. Cements mainly iron and carbonate occurs in intergranular pores of detrital grains and destroys porosity. The clay mineral occurs as pore filling and pore lining and deteriorates the porosity and permeability of the Ridge sandstone. The reservoir quality of the studied sandstone is reduced by clay minerals (kaolinite, illite, smectite, mixed layer illitesmectite, chlorite), carbonate, iron and silica cementation but on the other hand, it is increased by alteration and dissolution of the unstable grain, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to serve as a reservoir is strongly related to sandstone diagenesis.     

Clays and other minerals in coal seams of the Moura-Baralaba area, Bowen Basin, Australia

The clays and other minerals in a succession of Late Permian coals of high-volatile bituminous to semi-anthracite rank have been identified, using low-temperature oxygen plasma ashing and X-ray diffraction, and evaluated to identify the relative roles in mineral matter formation of detrital input, early diagenesis in the peat swamp and late diagenesis associated with rank advance. Although well-ordered kaolinite of probable early diagenetic origin is abundant throughout the succession, the uppermost and lowermost seams of the sequence, regardless of rank, contain relatively abundant illite and/or interstratified illite/smectite, along with a small but significant proportion of chlorite. These clays are thought to be essentially of detrital origin, washed or blown into the peat deposit in relative abundance during the establishment and subsequent overwhelming of an extensive and long-lived swampy environment. Quartz is also abundant in the lower seams of the sequence, especially close to the regional sediment source area. Illite is unusually abundant in the topmost seam in both high- and low-rank parts of the succession, and thus appears to represent detrital input from a particular source material. Although significant changes are reported in the clays of the associated strata due to rank advance, the principal effect of rank advance on the minerals in the coal itself appears to be the development of an ammonium illite, and possibly some additional fine-grained chlorite, in the semi-anthracite material. Isolation within the organic matter of the coal is thought to have inhibited access for ions such as K⁺, which might otherwise have become involved in metamorphic reactions and given rise to mineralogical changes commonly found in non-coal sedimentary successions.     

西藏北部西雅尔岗地区白垩-第三系红层的成岩作用及成岩环境

西雅尔岗地区位于羌塘高原腹地，地理范围为东经88°00′至89°00′，北纬32°40′至33°40′.大地构造位置从属于羌塘地块的玛依岗日一阿木冈基底隆断带.在晚白坚一第三纪时期，受青藏高原整沐抬升的影响，沉积了一套巨厚的红色陆屑建造，其基本特征为沉积厚度大、粗粒沉积所占比例高、沉积类型复杂、横向及纵向上相变快、岩石的颜色多为红色及紫色.沉积相类型主要为冲积扇、扇三角洲及湖泊.化石类型卖要为植物〔柏型枝)，孢粉和鸟足印化石.反映了在干操气候条件下，快速堆积的沉积特征. 根据对该区岩石学的研究，发现岩石的矿物组合和结构类型皆因成岩作用的影响而发生了很大的变化.矿物组合的变化是由于沉积后附加的陆源机械渗人粘土，在地表条件下不稳定矿物及岩屑的溶解作用以及在成岩过程中自生矿物的形成作用所造成的.导致结构类型改变的原因是(1)机械渗人的碎屑粘土及自生的粘土矿物形成孔隙间的粘土质“成岩杂基”.而这些孔隙在最初沉积时则没有杂基存在;(2)松散沉积物在成岩过程中，结构颖粒的溶解作用;(3)由于压实及压溶作用而形成粉砂级及其它细小的颗粒;(4)交代作用及重结晶作用.这些成岩过程明显地反映了整个区域岩石的演化历史. 显微镜、扫描电镜的观察以及红外光谱的侧定结果表明该区红层的形成是由于在成岩过程中赤铁矿的沉淀而造成的.在扫描电镜下可清楚地识别出赤铁矿的自生晶形.由此可以断定赤铁矿在其形成过程中显然是起了染色剂的作用.它的形成主要受孔隙水的氧化还原条件所控制，似乎于气候的关系不大，尽管它是产于干旱条件的沉积物中. 基于对该区岩石的成岩作用及成岩历史演化序列的研究，建立了该区沉积后的成岩环境模式，亦即(l)早期的表生成岩浅埋藏环境，该环境明显受到沉积环境、沉积作用控制以及气候因素的影响;(2)晚期的成岩深埋藏环境，该环境则主要受地温梯度、压力、温度和孔隙水的地球化学条件的控制.     

山西新太古界柏芝岩组条带状铁建造中的菱铁矿:成因机制与古环境意义

菱铁矿是前寒武纪条带状铁建造(BIF)中重要的矿物组分和古海洋信息载体,但它可能具有原生、早期成岩和晚期成岩多种成因,这在一定程度上限制了其在古海洋条件分析中的应用。虽然前人对菱铁矿开展了广泛的地球化学分析,但在岩相学研究方面相对薄弱。为进一步揭示BIF中菱铁矿的成因机制,以山西代县羊角沟矿区新太古界柏芝岩组的BIF为研究对象,开展了系统的岩相学工作。研究表明,该BIF主要由厘米级交互的富铁与富硅条带构成,其中普遍缺少水体扰动沉积构造,偶见交错层理和风暴碎屑,表明主要沉积于风暴浪基面之下。菱铁矿的主要产出形式有3种:(1)亚毫米级条带,其内"悬浮"有风暴成因碎屑颗粒,具有水柱或沉积物/水界面的原生成因特征;(2)在富铁条带中的晶体间隙充填,可能为早期成岩成因;(3)富硅条带中存在绿泥石层间脉状充填或截切石英和铁白云石的脉体,具有晚期成岩成因。原生菱铁矿的产出,表明新太古代在风暴浪基面之下的海水强烈缺氧、富铁并具有低硫酸盐浓度的特征。尽管原生菱铁矿条带的产出表明菱铁矿具有反映海洋化学条件的潜力,但多种成因菱铁矿的同时产出,也要求在应用菱铁矿分析古海洋条件时应当分组构进行。     

Continental and marine paleoenvironments reflected by west Pacific clay sedimentation

Various stratigraphic and paleoenvironmental results arise from mineralogical, geochemical and micromorpho logical investigations on Late Cretaceous and Cenozoic sediments recovered by the Deep Sea Drilling Project in the West Pacific:

Stratigraphic correlations. The deep pelagic red clays deposited on either side of the Shatsky Rise contain a variety of clay minerals, which point to the importance of eolian detrital supply and allow rough stratigraphic correlations.

Climate and paleocirculation. Neogene sediments of the Lord Howe Rise area reflect closely the relationship between the evolution of the Antarctic ice sheet and related aridification stages, and the atmospheric and oceanic circulation pattern since 14 my.

Regional and global tectonics. The post-Eocene deposits around New Zealand express a sequence of tectonic events and of relaxation stages, associated with changes in the circulation regime. Along the inner wall of the Japan Trench, clay mineral successions document the existence of an outer continental landmass until the late Oligocene, its collapse associated with volcanism close to the Paleogene-Neogene boundary, and an increased detrital influence from Japan since the early-middle Miocene.

Volcanism, diagenesis and detrital supply. The Aptian to Maastrichtian sediments in the Mariana Basin document a complex sequence of geochemical and geodynamical events: proximal volcanism and local supply from subaerial archipelagoes; early marine diagenesis during lithospheric subsidence and plate migration; and finally the occurrence and evolution of terrigenous input from distant sources distributed by marine currents.

     

山西新太古界柏芝岩组条带状铁建造中的菱铁矿:成因机制与古环境意义*

菱铁矿是前寒武纪条带状铁建造(BIF)中重要的矿物组分和古海洋信息载体,但它可能具有原生、早期成岩和晚期成岩多种成因,这在一定程度上限制了其在古海洋条件分析中的应用。虽然前人对菱铁矿开展了广泛的地球化学分析,但在岩相学研究方面相对薄弱。为进一步揭示BIF中菱铁矿的成因机制,以山西代县羊角沟矿区新太古界柏芝岩组的BIF为研究对象,开展了系统的岩相学工作。研究表明,该BIF主要由厘米级交互的富铁与富硅条带构成,其中普遍缺少水体扰动沉积构造,偶见交错层理和风暴碎屑,表明主要沉积于风暴浪基面之下。菱铁矿的主要产出形式有3种: (1)亚毫米级条带,其内“悬浮”有风暴成因碎屑颗粒,具有水柱或沉积物/水界面的原生成因特征;(2)在富铁条带中的晶体间隙充填,可能为早期成岩成因;(3)富硅条带中存在绿泥石层间脉状充填或截切石英和铁白云石的脉体,具有晚期成岩成因。原生菱铁矿的产出,表明新太古代在风暴浪基面之下的海水强烈缺氧、富铁并具有低硫酸盐浓度的特征。尽管原生菱铁矿条带的产出表明菱铁矿具有反映海洋化学条件的潜力,但多种成因菱铁矿的同时产出,也要求在应用菱铁矿分析古海洋条件时应当分组构进行。     

The chemistry and mineralogy of the clay fraction of sediments from the southern Barents Sea

The separated clay fraction (material <μ) of surface and sub-surface sediment samples from the southwestern Barents Sea is described. The partitioning of the major and minor elements within the different grain-size fractions of the sediment and between detrital and non-detrital phases demonstrates that the clay chemistry of these oxic shelf sediments is terrigenous in origin.The clays are a variable mixture of micaceous debris and illite with chloritic material, minor expandable clay and occasional kaolinite. Carbonate debris and amphibole occur locally. The semi-quantitative analysis suggests the existence of a considerable variation in the relative content of the principal mineralogical components and this is confirmed by the investigation of the major and minor elements. The terrigenous chemistry provides a more sensitive index of clay variability and allows the recognition of three distinct petrographic provinces.     

Mg,Ca and O18O16 exchange in the sediment-pore water system,hole 149, DSDP

The diagenesis and geochemical evolution of deep-sea sediments are controlled by the interaction between sediments and their associated pore waters. With increasing depth, the pore water of Hole 149 (DSDP) exhibits a strong depletion in Mg and a corresponding enrichment in Ca, while the alkalinity remains relatively constant. Dissolved SiO₂ is nearly constant in the upper 100 m of sediment, but is highly enriched in the deepest pore waters. The pore waters exhibit a depletion in K with increasing depth, and $\text{O}{}^{18}\text{O}{}^{16}$ pore water ratios also decrease.The sediment section has three zones of sedimentary regimes with increasing depth in the drill hole: an upper 100 m section of detrital clays, a middle section enriched in calc-akalic volcanics which have undergone submarine weathering to a smectite phase, and a lower section of siliceous ooze which still has a diagenetic smectite phase. The quartz-feldspar ratios and $\text{O}{}^{18}\text{O}{}^{16}$ composition of the silicate phases are in agreement with these interpretations.The submarine weathering of volcanics to a smectite can account for the observed pore water gradients. Volcanics release Ca and Mg to the pore waters causing the alkalinity values to increase. Smectite is formed, depletes the pore waters in Mg and O¹⁸ and causes the alkalinity to decrease. The net reaction allows for the observed relationship between pore water Ca and Mg gradients with little net change in alkalinity. Given the abundance of volcanics in many deep-sea sediments, especially in lower sections which often form near ridge crests, the submarine formation of smectite may be an additional oceanic Mg sink which has not yet been fully considered.     

Characteristics and distribution of clay minerals and their effects on reservoir quality: Huagang Formation in the Xihu Sag,East China Sea Basin

Abstract

The characteristics and distribution of clay minerals and their effects on reservoir quality in the Huagang sandstones in the Xihu Sag, East China Sea Basin were studied by using X-ray diffraction, casting thin-sections, scanning electron microscopy, electron microprobe analysis, fluid inclusion analysis, constant-rate mercury injection and nuclear magnetic resonance. Clay minerals consist of kaolinite, chlorite, illite and illite–smectite mixed layer (I/S); kaolinite forms from dissolved feldspars, chlorite occurs as clay coatings that are transformed from clay precursors owing to the flocculation of suspended detrital clays or the crystallisation of pore fluids, and illite forms from the illitisation of detrital smectite, authigenic kaolinite and K-feldspars. Clay distribution is controlled by sedimentary environments, burial history and lithologies. Typical reservoirs in the western sub-sag are thin and developed in braided river facies at relatively shallow burial depths with clays dominated by kaolinite. However, typical reservoirs in the central inversion tectonic zone are thicker and developed in a braided delta front facies at deeper burial depths with clays mainly consisting of chlorite, illite and I/S. High-quality reservoirs are characterised by coarse granularity, high quartz content and low clay content with widespread development of chlorite coatings that inhibit quartz cements at low temperatures. At higher temperatures, the high-quality reservoirs develop more pores providing growth space for quartz cements and result in the coexistence of chlorite coatings and quartz cements. The high-quality reservoirs are controlled by their lithological characteristics rather than chlorite coatings. Illite and I/S clays create severe damage to reservoirs by reducing the size and connectivity of pore-throats.     

Concretions, isotopes, and the diagenetic history of the Oxford Clay (Jurassic) of central England

In interpreting the results of a petrographic and isotopic study of concretions, a range of subjects is discussed including the original texture of the Oxford Clay sediment, Jurassic palaeotemperatures, the diagenetic history of pore-waters and the palaeo-hydrology of central England. The concretions are all composed predominantly of calcite. They include precompactional, pyrite-rich concretions that later suffered an eposide of brecciation, and others that only commenced to form after compaction had crushed ammonite shells included in the bituminous clay sediment. Petrographic, chemical, and especially carbon isotope data demonstrate a dominantly organic source for the carbon in the early formed concretions. Oxygen isotopes indicate formation at the same temperatures (13-16°C) at which benthic molluscs were living. Concretion growth in pelleted, anaerobic mud proceeded concurrently with bacterial sulphate reduction and pyrite precipitation. Cracking of the concretions started at this stage: in a few concretions, the cracks were also partially filled with brown calcite. During post-compactional growth, δ¹³C increased and pyrite content decreased, showing waning organic influence; δ¹⁸O decreased. The brecciated concretions were intruded by clay in which baryte crystals grew; finally, most remaining voids were filled with strongly-ferroan calcite of δ¹⁸O about—7 PDB and δ¹³C about O PDB. This must indicate strong depletion of the pore waters in ¹⁸O. Mechanisms that might lead to this are reviewed. It is concluded that the sequence of mineralogical and chemical changes is most readily explained if originally marine porewaters, first modified by bacterial activity, were flushed from the compacting clays by water of ultimately meteoric origin. This had its source in palaeo-aquifers beneath the Oxford Clay. Speculative attempts are made to relate this history to the geology of the region.     

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

Clay minerals formed through chemical weathering have long been implicated in the burial of organic matter (OM), but because diagenesis and metamorphism commonly obscure the signature of weathering-derived clays in Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. Here we have analyzed the mineralogy, geochemistry and total organic carbon (TOC) of organic rich shales deposited in late Archean to early Cambrian sedimentary basins. Across all samples we have quantified the contribution of 1M and 1M_d illite polytypes, clay minerals formed by diagenetic transformation of smectite and/or kaolinite-rich weathering products. This mineralogical signal, together with corrected paleo-weathering indices, indicates that late Archean and Mesoproterozoic samples were moderately to intensely weathered. However, in late Neoproterozoic basins, 2M₁ illite/mica dominates clay mineralogy and paleo-weathering indices sharply decrease, consistent with an influx of chemically immature and relatively unweathered sediment. A late Neoproterozoic switch to micaceous clays is inconsistent with hypotheses for oxygen history that require an increased flux of weathering-derived clays (i.e., smectite or kaolinite) across the Precambrian-Cambrian boundary. Compared to previous studies, our XRD data display the same variation in Schultz Ratio across the late Neoproterozoic, but we show the cause to be micaceous clay and not pedogenic clay; paleo-weathering signals cannot be recovered from bulk mineralogy without this distinction. We find little evidence to support a link between these mineralogical variations and organic carbon in our samples and conclude that modal clay mineralogy cannot by itself explain an Ediacaran increase in atmospheric oxygen driven by enhanced OM burial.     

Apports des phyllosilicates dans la différenciation entre altération hypogène et altération supergène dans le basalte triasique du Moyen Atlas (Maroc)Contribution of phyllosilicates to distinguish between hypogene alteration and weathering in Triassic basalt from Middle Atlas (Morocco)

Triassic basalt of the Middle Atlas has been subject to metamorphic transformation then weathering. Occurrence in both metabasalt and saprolite of ubiquitous clay minerals, such as smectite and mixed layers chlorite–smectite, makes it difficult to distinguish between the two alteration facies and explains the interest of complementary sources of information. In the Bhallil weathering profile, petrographical and mineralogical analyses of primary igneous minerals and their alteration products coupled with Fe oxidation state determination in clay fractions allow to identify three alteration facies: (i) metamorphic basalt, where iron occurs mainly as the ferrous form; (ii) the lower part of saprolite, where iron is partially oxidized to its ferric form; (iii) the upper part of saprolite, where iron is completely oxidized. To cite this article: A. Dekayir et al., C. R. Geoscience 334 (2002) 877–884.     

Geochemical and mineralogical investigations of the Lower Jurassic flint-clay bearing Mishhor and Ardon Formations,Makhtesh Ramon,Israel

The Lower Jurassic section at Makhtesh Ramon, in the northeastern Negev of Israel, comprises a sequence of fluviatile sediments of laterite composition. Post-depositional chemical weathering has resulted in marked facies changes whereby the clastic sequence passes laterally into high-alumina flint clays and bauxites (Goldbery, 1979). A geochemical and mineralogical investigation was initiated on 216 samples from several key sections of the facies change to obtain a more refined definition of the diagenetic overprint, to reconstruct the history of chemical weathering and finally to recognize the parent material of the unaltered laterite sediments.Results are presented herein on major and trace-element composition, normative mineralogical composition, kaolinite crystallinity and porosity and density measurements. Kaolinite crystallinity values proved to be a good indicator of intensity of leaching, showing a marked decline in ordering with increase of diagenetic reorganization. On the basis of mineralogical parameters it was possible to subdivide the section of flint clays and bauxites, generated under karstic conditions, into four units. Bivariate plots of Al₂O₃ vs. several elements within the section, plotted into four distinctive fields coinciding with the subdivision. Enrichment/depletion ratios, calculated from the element concentrations within these flint clays against element concentrations of the clastic parent material of the karstic infill, led to the reconstruction of a multi-cyclic leaching event, whose “signature” is left in each of the four units. At least 3 individual episodes of leaching, related to a rising water table and punctuated by “still-stands” could be recognized.Zr/TiO₂: Nb/Y plots of stable elements, using the diagram of Floyd and Winchester (1978) for determination of source rocks, indicated a rhyolite or alkaline rhyolite provenance for the clastic laterite detritus.     

Rock composition and origin of the Duwi Formation calcareous rocks, Upper Egypt

Upper Cretaceous phosphorite beds of the Duwi Formation, Upper Egypt, are intercalated with limestone, sandy limestone, marl, calcareous shales, and calcareous sandstone. Calcareous intercalations were subjected to field and detailed petrographic, mineralogical and geochemical investigations in order to constrain their rock composition and origin. Mineralogically, dolomite, calcite, quartz, francolite and feldspars are the non-clay minerals. Smectite, kaolinite and illite represent the clay minerals. Major and trace elements can be classified as the detrital and carbonate fractions based on their sources. The detrital fraction includes the elements that are derived from detrital sources, mainly clay minerals and quartz, such as Si, Al, Fe, Ti, K, Ba, V, Ni, Co, Cr, Zn, Cu, Zr, and Mo. The carbonate fraction includes the elements that are derived from carbonates, maily calcite and dolomite, such as Ca, Mg and Sr. Dolomite occurs as being dense, uniform, mosaic, very fine-to-fine, non-ferroan, and non-stoichiometrical, suggesting its early diagenetic formation in a near-shore oxidizing shallow marine environment. The close association and positive correlation between dolomite and smectite indicates the role of clay minerals in the formation of dolomite as a source of Mg^2＋ -rich solutions. Calcareous rocks were deposited in marine, oxidizing and weakly alkaline conditions, marking a semi-arid climatic period. The calcareous/argillaceous alternations are due to oscillations in clay/carbonate ratio.