Ferruginous Crust Research Articles

Hematite and Goethite from Duricrusts Developed by Lateritic Chemical Weathering of Precambrian Banded Iron Formations, Minas Gerais, Brazil

AbstractThe upper 15−20 m of a 200 m thick lateritic weathering profile on Precambrian itabirites of Capanema, Brazil, reveals a genetic pathway for the formation of hematitic and goethitic nodules in the ferruginous crust through a very fine grain Al-hematite and Al-goethite mixture, called here the brick-red-material (brm). This evolution develops between the soft saprolite and a 10 m thick indurated ferruginous crust. The soft saprolite retains the original structures of the itabirite and is characterized by almost complete dissolution of quartz, the development of goethite septa, and the partial dissolution of primary hematite. Near the contact with the overlying ferruginous crust, the brm is gradually filling voids as well as replacing primary hematite and goethite in the saprolite. In the upper indurated crust, the brm transforms into coarse structureless ferruginous nodules (aluminous hematites and goethites) and is the precursor of the hematito-goethitic nodules of the crusts. Crystallization of newly-formed Al-goethite and Al-hematite within the brm occurs without detectable amounts of amorphous iron oxides of ferrihydrite precursors.

La bioérosion des substrats magmatiquesen milieu littoral: L'exemple de la presqu'ile du Cap Vert (Sénégal Occidental)

Magmatic rocks (mainly dolerites and basanites) and thick ferruginous crusts (ironpans) outcroping on the shore of the Cap Vert peninsula (Western Senegal) are not uniformely abraded by borers and scrapers. Bioabrasion and bioattack are faint in basanites and thick ferrunginous crust. Conversely, dolerites are penetrated by microorganisms, essentially fungi, in particular related to the genus Ostracoblabe and actively bored by the sea-urchin Echinometra lucunter: Fungi are rather numerous throughout the intertidal zone, particularly in its upper half-part, and persist in the lower part of the supralittoral zone actively moistened by sprays. E. lucunter inhabits the lower part of the intertidal zone and the upper part of the subtidal zone. The study of thin sections showed that fungi first behave like chasmoendoliths before to act as euendoliths. Pyroxenes and peridots can be intensively permeated, whereas plagioclases are generally poorly affected by fungal attacks. Although very scarce, the cyanobacteria Hyella sp. can invade plagioclases situated at the sample surface, at least up to the half level of low tide. Frequencies of borers and scrapers were determined in the different biological horizons recognized by Sourie (1954). Boring activity of E. lucunter was surveyed veyed during 21 months. The deepening of their dwelling holes is about 1 mm/year in the lower half of the intertidal zone whereas it is only about 0,4 mm/year in the associated thick ferruginous crusts. The calculated rate of bioerosion can reach annually about 2 050 g/m 2 in the lower horizon of low tide where the population of E. lucunter is particularly important. The rate of biological erosion decreases rapidly in the overlying horizons. The bioerosion of the dolerites of the Cap Vert peninsula is an active process, contribution of which to the destruction of the rocky shore and to the coastal morphogenesis during the Quaternary is certainly significant.

Late cenozoic history of the eastern llanos of venezuela: Geomorphology and stratigraphy of the Mesa Formation

The study area corresponds to the Mapire River basin in the southern part of the State of Anzoátegui and constitutes a region of low relief covered by the Pleistocene Mesa Formation. This formation consists of horizontally bedded alluvial sediments, represented by light gray, coarse- to fine-grained quartz sand, with cross-bedding marked by ochre, red and pink alteration, and to a minor extent by marsh deposits, represented by laminated and spotted shale. The typical topography consists of mesas north of the left margin of the Orinoco River (the Musinacio and Mapire Mesas). To the north of the mesas there is a topography of low hills covered by blocks of ferruginous crusts. A typical tropical savanna climate exists in the region, with a strong dry season which lasts 4–6 months, followed by a rainy season with excess humidity. The dominant erosional process is scarp retreat, due to the action of the savanna climate on the lithology of the partly unconsolidated sediments; it has formed deep canyons in the headwaters, and is magnified by tectonic uplift and the lowering of the regional base level. However, some topographic forms have been preserved due to the hardening of the sands by iron oxides. Preliminary thermoluminescence dating of sand from two stratigraphic sections confirms the Pleistocene age of the Mesa Formation (0.5–1 Ma BP), previously suggested by its stratigraphic position and geomorphological interpretation.

LATE DEVONIAN (FRASNIAN) CORALS FROM CENTRAL DOLPO, NEPAL

Rugosa and Tabulata from the Late Devonian of central Dolpo (Nepal) belong to the genera Kuangxiastraea , Scruttonia, Sinodisphyllum , Tabulophyllum (?), Fruehwirthia gen. n., Thamnopora , Cladopora , Alveolites and Alveolitella . The calcarenitic beds yielding the fossils are overlain by a prominent oolitic ironstone with thick ferruginous nodules and crusts. The age of the corals is probably Early Frasnian, thus younger than the Givetian fauna previously reported from eastern Dolpo. Biogeographic relations of the new fauna are with Western Europe and South China.

Geochemical exploration of the Maicuru alkaline-ultramafic-carbonatite complex, northern Brazil

The Serra do Maicuru represents an alkaline-ultramafic-carbonatite complex, emplaced in the Guiana Craton, which developed a thick lateritic profile, enriched in aluminum phosphates and especially titanium mineralization. The most important parent rocks are pyroxenites and syenites, plus some isolated apatitic bodies, glimmerites and carbonatite veins that cut all of these rocks. Above this lateritic cover and on its neighborhood, a geochemical survey was carried out by Docegeo, a subsidiary of Companhia Vale do Rio Doce ( cvrd). From the surficial laterites, divided into a ferruginous crust and lateritic soil, more than 1500 samples were collected, covering all of the Maicuru structure. They were analyzed for Fe 2O 3, P 2O 5, TiO 2, Mn, Ni, Co, Cr, Cu, Pb and Zn, including some analyses for REE and Nb. Strongly anomalous phosphorus in soils and in iron crust, as aluminum phosphates, furnished a good guide to primary apatite mineralizations, and showed the presence of high titanium as anatase, ilmenite and titanomagnetite and interesting anomalous amounts of Nb, Cu and REE. The geochemical data also helped identify and delineate the distribution of the main parent rock types.

Late Cretaceous chemical sediments related to a carbonate platform-foreland basin transition in the Oman Mountains

In the Oman Mountains, ferruginous, phosphatic and minor siliceous chemical deposits accumulated in the Late Cretaceous (? upper Turonian-Campanian) related to the transition from a carbonate platform to a foreland basin. This process was synchronous with large-scale emplacement of nappes derived from the Mesozoic Tethys ocean. The platform edge was uplifted and later subsided, controlled by crustal flexure. Following initial drowning of the Arabian platform (? Turonian), flexural uplift produced a regional unconformity, the “Wasia-Aruma” break and the outer shelf area was deeply eroded. Further inboard, ferruginous oolites formed on shoals in shallow protected seas, and were later reworked with neritic bioclastic sediment. Following inferred passage of a flexural bulge, subsidence began and ferruginous crusts and nodules precipitated on hardgrounds located along the erosion surface. Impure lime-muds then were deposited on the floor of a subsiding shelf (10s–100 m) that was progressively tilted, with accumulation of phosphatic deposits in deeper water (100s–1000 m), on unstable slopes. Then the thrust-sheets docked with the Arabian continent and the platform edge was downflexed to form a foredeep in which deep-water clastic sediments accumulated, and this was followed by overthrusting of nappes from the Tethys ocean.

Upper Cretaceous Muti Formation: transition of a Mesozoic nate platform to a foreland basin in the Oman Mountains

ABSTRACTThe Upper Cretaceous (Turonian‐Campanian) Muti Formation (Sayja Member) documents the transition from a passive continental margin to a foreland basin, related to overthrusting of continental margin and ophiolitic nappes derived from the Tethys ocean. Upper Cretaceous northeastward subduction culminated in collision of a trench with the Arabian margin. As the trench docked with the margin the lithosphere was flexed, forming a peripheral bulge that migrated cratonward with time. The platform edge was initially uplifted (Turonian) and deeply eroded, creating the ‘Wasia‐Aruma break’. After passage of the peripheral bulge subsidence began, with accumulation first of ferruginous crusts on hardgrounds. Lime‐muds were then deposited on a deepening unstable sea‐floor, along with phosphatic nodules and crusts (Turonian‐Coniacian). Passage of the overthrust load over the Arabian continental‐margin edge downflexed the lithosphere (Santonian‐Campanian), resulting in drastic foundering of the old shelf edge to form a foredeep. Upper platform horizons collapsed as slump‐sheets and debris‐flows. Limestone blocks and lithoclastic debris‐flows were shed by mass‐wasting of the already deeply eroded old platform edge. Mud and silt were derived from the uplifted Arabian continent and deposited by mainly gravitational processes in a foredeep below the C.C.D. Subsidence of the Arabian platform edge allowed the Semail ophiolite nappe finally to override the Muti basin (late Campanian) with little internal deformation. Submarine emplacement is suggested by the absence of ophiolitic detritus in the Muti Formation. The stratigraphic evolution of the Muti Formation is in good general agreement with a model of the transition of an old, thermally mature, passive continental margin to a foreland basin, where the emplaced load is submerged.

Iron oxide properties versus strength of ferruginous crust and iron-glaebules in soils : Shadfan, H; Dixon, J B; Calhoun, F G Soil SciV140, N5, Nov 1985, P317–P325

Iron oxide properties versus strength of ferruginous crust and iron-glaebules in soils : Shadfan, H; Dixon, J B; Calhoun, F G Soil SciV140, N5, Nov 1985, P317–P325

EXPLORATION OF BURUNDI NICKELIFEROUS LATERITES BY ELECTRICAL METHODS*

AbstractThe laterites in Burundi, which are formed by weathering of ultrabasic rocks, show a complete profile with the following horizons: canga, the ferruginous crust capping, ferralite, consisting essentially of iron hydroxides, and saprolite, which contains a large quantity of hydrosilicate minerals. Nickel bearing minerals occur in the saprolite and the lower portion of ferralite.Resistivity well‐logging and resistivity sounding indicated that the electrical properties of rocks depend upon their composition: Canga and ferralite showed high resistivities of 6,500 Ωm and 800 Ωm, respectively. The resistivity of saprolite was found to be much lower, between 10 Ωm and 20 Ωm. The laterite is underlain by resistive peridotite. The chargeability of saprolite was found to be lower than that of the upper horizons and the bedrock.Electrolytic conductivity of laterite, which depends on the geometry of the deposit, was found to be low, because the laterite contains moisture and ground water, which are highly resistive. The relatively high conductivity of saprolite is caused by nickeliferous hydrosilicates, which exhibit the electrical properties of clay minerals, with an apparent maximum conductivity of 0.25 S/m. The conductivity of saprolite corresponds to a concentration between 30% and 50% of conductive silicate minerals distributed in the pore space of deposit. A nickel enrichment of up to 6% was estimated from the resistivity of the saprolite.Prospecting for laterites by electrical sounding showed that the development of laterite horizons in a nickel deposit correlates with the surface morphology of weathered ultrabasic massif. Thus the method can be used in preliminary exploration of such deposits.

Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England

ABSTRACTGeochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal‐inlet, on‐barrier and lagoonal sub‐environments of a carbonate barrier‐island complex. At Cowthick early composite (acicular‐bladed) radial‐fibrous cements, which pre‐date aragonite dissolution, completely fill intergranular pore‐space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO3) higher than those in allochems or later, ferroan cement (≅0.5 wt % MgCO3, 1.7 wt % FeCO3). At Castle Bytham early granular isopachous cements, which post‐date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore‐space in all three examples. Carbon and oxygen isotopic composition of whole‐rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late 18O depleted cements (δ18O – 8 to – 10). Early cements must have a marine isotopic composition; different δ18O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ18O, – 2.9 and δ13C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in 18O and 13C (δ18O = 0 δ13C ≅ 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high‐magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea‐water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds.

Sedimentation ralentie et diagenese sous-marine au se de la nouvelle-caledonie (dolomitisation, ferruginisation, phosphatisation)

This article is a petrological study of rocks dredged from between 500 and 600 m depth to the SE of New Caledonia. Two facies were distinguished: 1. (1) Dolomitic and ferruginous facies, comprising a shallow-depth biota (calcareous algae, benthonic foraminifera). Coralline algae are dolomitized and the perforations are filled with planktonic material showing that the rock has been transported to a greater depth. Lastly, they have been impregnated and encrusted by goethite. 2. (2) Calcareous, ferruginous and phosphatic facies consisting of a series of uncomformable sedimentary units: 1. Unit 1 : phosphatic limestone. Most bioclasts are pelagic and are not older than Pliocene in age; they are supported in a micritic, calcitic and phosphatic matrix. This unit is much bored. 2. Unit 2 : this is a thick ferruginous crust covering unit 1 and filling some of the bores, built up by many wrinkled laminations. It only contains encrusting organisms (bryozoa, serpula, foraminifera). 3. Unit 3 : more or less ferruginous white limestones, which are very rich in probably Pleistocene pelagic foraminifera. This fills the remaining cavities between 1 and 2. It consists of sub-units separated by surfaces of goethite impregnation. The phosphatization is attributed to the action of up-welling currents. The iron could be derived from the erosion of continental laterites. The ferruginous crust appears to have deposited it in stromatolitic form at great depth. This facies has the textural and mineralogical attributes of a hard-ground. It indicates sporadic sedimentation during the Plio-Pleistocene on this part of the continental slope.

Contribution a l'etude du Quaternaire des Idjibitene et de la bordure meridionale de l'Adrar (El Gletat, Mauritanie)

Abstract Two morphologically different regions are distinguished in the Quaternary of the Idjibitene mountains and along the southern border of the Adrar (El Gletat, Mauritania). Along the south and southwest border of the Adrar, deposits resulting from slope erosion are observed; to the south, dune structures and lake formations with white sands, brown sands, rare ferruginous crusts, and sand cemented by calcite (tarous) are seen. By observation of the terrain, collection of in place Acheulean industries, and study of the sands (granulometric and morphometric) it has been possible to reach an understanding of the stratigraphy. A chronology of Quaternary deposits of the Idjibitene is proposed. Close analogies with the Quaternary formations of Majabat are established.

Le Pliocene et les accidents periglaciaires de la plage de la Mine d'Or a Penestin (Morbihan)

Abstract Detailed study of the cliffs along the Mine d'Or coast provides results which indicate that two Pliocene stratigraphic sequences are separated by a period of emergence or semi-emergence locally evidenced by a slight unconformity and the presence of a ferruginous layer at certain points. Examination of the evidence of tectonic deformation confirms the existence of slight recent activity. Quaternary frost action, in areas where the arenaceous basement rock has been replaced by a white plastic clay, produced cryoturbation phenomena analogous to Recent pingos of the polar regions. Their morphology indicates that there were two successive glacial phases. The ferruginous crust observed at the base of and between the two Pliocene sequences is formed today in the present climate and under geologic conditions similar to those which existed during the Pliocene.