Phosphatic Nodules Research Articles

Depositional and diagenetic controls of source-rock characteristics of the Lower Jurassic "Nordegg Member", Western Canada

Petrographic, mineralogical, and bulk-geochemical (XRD, XRF, and Rock-Eval) data indicate that depositional and early diagenetic processes were the principal controls on the organic and inorganic characteristics of the Lower Jurassic Nordegg Member source rock. This unit comprises organic-rich, calcareous mudstones to marlstones and marly limestones that were deposited in a predominantly anoxic, hypersaline, silled marine basin during two transgressive-regressive (T-R) events. Sediments deposited during marine transgression have a higher content of carbonate (calcite and dolomite) and organic matter. Fe (mainly as pyrite) content varies inversely with carbonate content. The basal Nordegg Member is phosphatic with stratiform apatite laminae and phosphatic nodules. Early diagenetic phosphatogenesis was probably mediated by bacteria under variably dysoxic conditions during the initial transgressive event. Regressive parts of T-R cycles are expressed as an increase in abundance of silicate (quartz, illite, kaolinite, and minor K-feldspar) and lower organic carbon contents. Deposition of abundant marine organic matter fueled vigorous bacterial reduction of sulfate and almost total bacterial reworking of the sedimented organic matter, resulting in significant production of reduced sulfur species and predominantly amorphous kerogen. Pyrite formation was ubiquitous but limited by the abundance of Fe. Where Fe contents fell below 3 wt %, excess sulfides were incorporated into the kerogen. This process resulted in the early diagenetic formation of the sulfur-rich kerogen (Type I/II-S) that typifies the Nordegg. Thus Fe availability in the depositional environment exerted an important control on the amount of S available for incorporation into the kerogen. Furthermore, content of organic S increases with increasing Hydrogen Index, indicating the importance of type and reactivity of organic matter on the extent of incorporation of sulfur into the kerogen during early diagenesis.

The geologic age of radiolarian assemblage from a phosphate nodule in the Chichibu Belt, Shikoku, Southwest Japan

The geologic age of radiolarian assemblage from a phosphate nodule in the Chichibu Belt, Shikoku, Southwest Japan

Rare-Earth Element Variation in Phosphate Nodules from Midcontinent Pennsylvanian Cyclothems

ABSTRACT The rare-earth element (REE) geochemistry of phosphate nodules from eastern Kansas and northeastern Oklahoma is dominated by patterns that are generally flat or are enriched in middle REE (MREE). Flat patterns are typical of phosphate nodules preserved in thick shales and in nodules from shales deposited nearest to detrital sources. The flat patterns are probably derived from terrigenous constituents in the host shale. MREE enrichment is evident in phosphate found in relatively thin shales and in distal shales. We suggest that the MREE-enriched pattern reflects the contribution of MREE-enriched fecal phosphate. The initial MREE enrichment mechanism may have been analogous to that in which some modern algae preferentially extract MREE from water of marine composition. The MREE-enric ed signature may be preserved only in phosphate nodules that formed where terrigenous input was so low that it did not mask the characteristic fecal pattern. Rare Ce depletion patterns reflect a primary seawater REE source that has not been obscured by fecal or detrital components.

Conditions of formation and diagenetic evolution of Upper Proterozoic phosphate nodules from southern Sweden: evidence from petrology, mineral chemistry and isotopes

Dark-gray to black cryptocrystalline francolite occurs as nodules (∼ 2–8 cm in diameter) which are embedded in organic-matter-rich mudstones of the Visingsö Group (Upper Proterozoic, southern Sweden). The δ 13C PDB values (−18.4 to + 0.98‰) of structural-CO 2, complemented by studies of the petrographic character and strontium isotopic composition of francolite, indicate precipitation in the suboxic and sulfate-reduction zones. Francolite is intergrown with berthierine that has formed in the suboxic Fe-reduction zone. Calcite enriched in Mn and, to a lesser extent Fe, occurs as fracture- and void-fillings, and as replacement of francolite. Mn- and Mg-rich calcian siderites have precipitated as pseudomorphs after bacteria in the zone of microbial methanogenesis. The carbon ( δ 13 C PDB =−0.75 to +7.04‰ ) and oxygen ( δ 18 O PDB =−14.21 to −10.58‰ ) isotopes of calcite suggest derivation of carbonate ions from microbial methanogenesis and from thermal decarboxylation of organic matter at temperatures of 60° to 80°C. Some of the void- and fracture-filling calcites ( δ 13 C= −20.8 to −18.63‰ δ 18 O = −5.46 to −5.21‰ ) have, however, precipitated at lower temperatures (∼ 25–30°C) in the sulfate-reduction zone.

Technique for disaggregating mudstones to retrieve microfossils and non-clay residual particles

This technique is effective for completely breaking down moderately calcareous to noncalcareous mudstones and shales to produce a concentrated residue of microfossils and non-clay minerals. I have applied this treatment to Mississippian and Pennsylvanian age mudstones and have easily retrieved abundant conodonts, ostracodes, foraminifers, spores, wood fragments, fish teeth, vertebrate bone fragments, silt, mica, pyrite crystals and framboids, and phosphate nodules. All of the above items emerge from the treatment with fine structures such as spines and surface ornamentation undamaged and completely free of surficial clay. This technique is a modification of that of Lund (1970). The chemical used by Lund (Union Carbide Amine 220) is no longer manufactured and an effective alternative is reported here. Furthermore, the additional steps in processing reported here increase the efficacity of the technique for completely dissolving indurated mudstones.

Descripción y génesis de los bancos de oolitas ferruginosas en la base del Silúrico de la precordillera Argentina

Detailed outcrop and microscopic descriptions and current ideas about factors controlling the origin of oolitic ironstones coming from the Lower Silurian of the western Argentina Precordillera Basin are given. Chamosite and goethite comprise around 20 to 98 % of the oolitic ironstones. Phosphatic nodules are common components in these deposits. Lithofacies arrangement, outcrop architecture and bioturbation patterns allow to interpret them as shallow marine bars, which typically constitute parasequences boundaries. They are associated with widespread detrital facies. Their origin is related to sea-level fluctuations and a good correlation with global events can be established. The origin and distribution of the ironstones in the Precordillera basin is related with cool-water seas in high southern latitudes, during a high stand of sea-level, occurring after the Late Ordovician glacial event.

Sedimentology and early diagenesis of the Broadford Beds (Lower Jurassic), Skye, north-west Scotland

The Broadford Beds comprise a basal carbonate-dominated unit overlain by dark muddy sandstones and an upper cross-bedded sandstone unit. The limestones include coralliferous and oolitic lithologies and the sandstones include pisolitic berthierine ironstones and abundant phosphate nodules. The Broadford Beds have a maximum thickness of 140 m and can be subdivided into 17 parasequences, each initiated by an approximately 20 m rise in relative sea level. Marine flooding surfaces are most readily recognized in the lower parts of the succession, where diagenetic data can be utilized to locate emergence surfaces. There is no diagenetic evidence of early emergence in the upper parts of the sequence, but a variety of sedimentological data can be used to identify parasequence boundaries. Deposition of the Broadford Beds occurred largely around the shores of a number of islands in the northern Hebridean area. These islands partly isolated the Skye–Applecross area from a deeper marine basin to the west, and this may have enhanced the formation of authigenic phosphates and iron silicates. It is possible to use the regional variability of parasequence thicknesses to investigate the role of active local tectonism in governing differential subsidence and sediment supply.

Calcareous nannofossils and Milankovitch cycles: the example of the Albian Gault Clay Formation (southern England)

A quantitative study of calcareous nannofossil assemblages was carried out on the Albian Gault Clay Formation (England). This unit consists of grey marly claystones and marlstones with phosphatic nodules and glauconitic layers as minor lithologies. Two cores were analyzed in detail to focus on changes in nannofloral assemblages and consider their significance. Preservation of calcareous nannofossils is excellent and therefore diagenesis cannot be invoked to explain, not even partially, the changes registered in assemblages. Fast Fourier spectral analysis was performed on percentages of selected nannofossil species. Fluctuations in abundance of both Biscutum constans and Watznaueria barnesae, regarded as fertility and non-fertility indices, respectively, appear to be orbitally driven because spectral analysis showed periodicities of approximately 41 kyr (obliquity cycle), and 100 kyr (short eccentricity cycle). In all the spectra, the axial obliquityc cycle is a strong signal, as expected at these high latitude sites from the Boreal realm. The same pattern was observed in another fertility controlled taxon, Zygodiscus cf. erectus, but cross-correlation shows that its abundance plot is displaced with respect to that of B. constans. Orbital perturbations induce variations in climate and ocean circulation because they influence the Earth's insolation and seasonality. During times of high seasonality, strong gradients cause vigorous oceanic circulation, a better cycling of nutrients and higher fertility, whereas during times of low seasonality, low gradients induce more sluggish circulation and lower fertility. The carbonate content of the Gault Clay cores is not correlatable to the higher fertility indices, as the abundance curves of B. constans, W. barnesae, and Z. cf. erectus are displaced with respect to CaCO 3 content variations. This also means that cyclicity in nannofossil assemblages is not a result of diagenesis but an original signal. Abundance fluctuations of Repagulum parvidentatum and combined Parhabdolithus asper/P. splendens, colder and warmer water indicators, respectively, provide evidence for orbital periodicities suggesting that also cyclic variations of surface water temperature might have influenced the nannofossil assemblage patterns within the Gault Clay.

Paleozoic to early Mesozoic stratigraphy and sedimentary evolution of central Dolpo (Nepal Himalaya)

The stratigraphic succession of central Dolpo (Tarap-Atali area) begins with thick calcsilicate marbles of inferred Cambrol-Ordovician age, injected by pegmatitic dykes. The strongly deformed mid-Paleozoic sequence comprises fossiliferous marls, dolomitic quartzarenites and dolomites. The latter are unconformably followed by earliest Late Devonian biocalcarenites, capped by a maior ironstone layer. The overlying offshore black pelites contain quanzose sandstones and bioclastic limestones at several intervals, indicating deposition in shelfal environments at latest Devonian times. The Carboniferous is poorly represented in the surveyed area. A major disconformity, cutting deep into Early? Carboniferous fenestellid-rich biocalcarenites at Tarap, is overlain by white quartzarenites interbedded with dark pelites, sharply followed by a fossiliferous marker horizon yielding a large brachiopod fauna of mid-Permian age ( Costiferina arenites); the overlying fossiliferous shelfal pelites are unconformably followed by coarsening-upward quartzarenite sequences deposited in estuarine environments (Thini Chu Formation). In the Late Permian, another disconformity is overlain by conglomeratic to glauconitic arenites and shelfal pelites (Kuling Formation). The Triassic Tamba Kurkur Formation, with dolomitic subarkoses ar the base, consists of two condensed pelagic carbonate horizons of Dienerian and Smithian age, separated by dark pelites. Marly limestones and marls were deposited from the Spathian through the Carnian (Mukut Formation), and are overlain by the thick Tarap Shale. A thin basal condensed bed is followed by dark pelites yielding Early Norian ammonoids, by thick siltstones with phosphatic nodules and next by dark shales and calcareous siltstones with large Zoophycos -type burrows. The upper part of the unit contains up to fine-grained quartzo-feldspathic sandstones, nodular marly limestones and ironstone horizons (upper assemblage). The Late Triassic shallowing- upward succession is capped by dolomitic and oolitic quartzarenites with spectacular herringbone structures (Quartzite Series). The largely Lower Jurassic shallow-water Kioto Limestone, still containing metric intervals of up to medium-grained hybrid quartzarenites in the lower part, is followed by Middle Jurassic lumachelles (Laptal Formation). Younger terms of the succession are not exposed in Dolpo.

Correlation of the Hythe Beds Formation (Lower Greensand Group: early-mid-Aptian), southern England

The outcrop and sub-surface stratigraphy of the Hythe Beds Formation shows a coarsening-up cyclicity in the arenaceous successions of the western Weald Basin (Hants. & Surrey), and in the glauconitic/argillaceous deposits of Sussex/Kent. Correlation can be achieved through the identification of hiatus surfaces, the most distinctive of which are dark, smectite-enriched clays with belemnites and phosphate nodule beds. These beds are interpreted to be the product of transgressive conditions. Correlation may be difficult in clastic shelf successions: the use of distinctive transgressive event beds provides a model for other successions displaying similar facies.

STRATIGRAPHY OF THE CHIKKIM AND FATU LA FORMATIONS IN THE ZANGLA AND ZUMLUNG UNITS (ZANSKAR RANGE, INDIA) WITH COMPARISONS TO THE THAKKHOLA REGION (CENTRAL NEPAL) : MID-CRETACEOUS EVOLUTION OF THE INDIAN PASSIVE MARGIN

Detailed biostratigraphic study of mid-Cretaceous Tethys Himalayan pelagic units shed new light on the evolution of the northern India passive continental margin. After a major episode of alkaline magmatism recorded from Zanskar to Nepal in the Early Cretaceous, condensed glauco-phosphorites were deposited during the Rotalipora subticinensis Subzone, and were overlain unconformably by pelagic mudstones during the Rotalipora ticinensis to Rotalipora appenninica Zones (Fatu La Formation in Zanskar and Muding Formation in Nepal). Throughout the Tethys Himalaya drowning of clastic shelves thus occurred with the same modalities and synchronously in Late Albian time. The base of the Chikkim Formation in Zanskar is younger, and overlies palimpsest arenites and reworked glauco-phosphorires deposited during the latest Cenomanian Whiteinella archaeocretacea Zone. Hiatuses several million years long, mainly coeval and similar in duration in the Chikkim and Fatu La Formations, characterize the Cenomanian and the Late Turonian to Campanian sections throughout the Zanskar Range. These hiatuses are ascribed to continuous resuspension of pelagic sediments on the upper slope, caused by the action of strong eastbound oceanic currents. Much higher accumulation rates were recorded in the early-middle Turonian, when deposition occurred at grearer depths below the mudline. Accumulation rates increased greatly in the Maastrichtian, when offshore marls with sparsc phosphate nodules were gradually rcplaced by inner shelf carbonate facies at the close of the Cretaceous. The studied succession shows a mirrorlike repetition of sedimentary facies, interpreted as part of a 65 to 70 my long Cretaceous supersequence. Sedimentary evolution reflected a complex interplay of global and regional phenomena, including the extensional and magmatic episodes leading to thc final fragmentarion of Gondwana-Land and opening of the Indian Ocean, a long-term tendency to eustatic rise, changing paleooceanographic circulation, and oceanic anoxic events.

Heterotrophic bacterial activity and growth rates in sediments of the continental margin of eastern Australia

The numbers, growth rates and productivity of heterotrophic bacteria were determined in sediment cores collected on the East Australian continental margin (28 and 32°S) at water depths ranging from 150 to 1000 m. The study was centered on a zone of modern phosphate nodule formation (350–460 m water depth), as bacteria had previously been implicated in the genesis of the modern phosphorites in this area. Numbers of bacteria in surface sediment ranged from 2 × 10 8 g −1 at 200 m water depth to 8 × 10 8 g −1 at about 400 m and 2 × 10 9 g −1 at 1000 m. Numbers decreased exponentially with depth in the sediments to 2 × 10 6 g −1 at 1 m depth. Bacterial growth rates, determined from rates of tritiated thymidine incorporation into DNA, were about 0.01–0.002 h −1 in the upper 10 mm of sediment from 150 to 460 m; 0.004 h −1 at 600 m, and 0.0005 h −1 at 1000 m. Bacterial productivity was greatest (80 mg C m −2 day −1 or 243 μmol C cm −2 y −1 integrated over the top 10 cm sediment depth) in the zone of modern phosphorite formation. About 80% of bacterial productivity occurred in the upper 2 cm of sediment, although bacteria found at depths up to 70 cm in the sediment were capable of metabolic activity. Rates of phospholipid and protein synthesis were also determined and, when converted to productivity, the values in the upper 2 cm of sediment were mostly within a factor of 2 or 3 of values determined from DNA synthesis. Similar values were obtained for the flux of the organic carbon into the sediments, required to support metabolic activity, whether calculated from values for bacterial productivity or from a geochemical model of oxygen diffusion and consumption, using profiles of oxygen concentration in the sediments. The organic carbon input into the sediments in the zone of modern phosphorite formation was much higher than anticipated, probably because the zone acts as a focus for the deposition of organic matter produced on the continental shelf. Rapid bacterial degradation of the organic matter in these sediments resulted in a shallow transition from oxic to anoxic conditions and to the release of phosphate to the pore-waters. These factors, in conjunction with sediment mixing and Fe-cycling processes, appear to be critical to phosphorite formation in this area.

Sedimentology and depositional setting of the Dartmouth Group, Bigbury Bay, south Devon

The Lower Devonian Dartmouth Group records some of the earliest stages in the evolution of the Variscan basins of southwest England. This thick, mudstone-dominated group has previously been interpreted as distal fluvial sediments, deposited on a coastal mudplain. The sequence at Bigbury Bay is made up of two large-scale sedimentary cycles. The base of each cycle consists of pebbly mudstones, graded siltstones and sandstones, massive mudstones, and heterolithic sediments with sporadic phosphatic nodules. These facies were deposited in substantial lakes; they were modified by phases of syn-sedimentary deformation which generated subaqueous debris flows and turbidites. The overlying parts of the cycle, making up the bulk of the succession, consist of mudstone and thin, sheet-like, sandstones. These are fluvial and were probably deposited in mud-dominated terminal fans. Each cycle represents a phase of rapid, probably fault-controlled, subsidence followed by relative tectonic quiescence. This behaviour may reflect pulses of rifting or extension in strike-slip basins, followed by thermal relaxation.

Sequence Boundary Modification by Submarine Erosion, Upper Cretaceous, San Juan Basin

Unconformities within shallow-marine depositional systems are commonly interpreted as exclusively a result of erosion during falling relative sea level. However, significant submarine erosion also occurs during transgressions and is a complicating factor in the interpretation of these unconformities. A major unconformity present in the latest Turonian and Coniacian regressive-transgressive cycle in the northwestern New Mexico, separates the Gallup Sandstone (HST) from the overlying Tocito Sandstone (TST and rarely LST). The basinward portion of the unconformity contains no evidence of subaerial exposure and is contained entirely within marine strata. Also, the associated lacuna decreases paleolandward. Above the unconformity, although there is addition of coarser sand grains, overlying strata contain concretions, glauconite and phosphate nodules, and are more heavily burrowed than underlying strata. These relationships suggest that the preserved unconformity reflects shoreline transgression and a decrease in the rate of allochthonous sedimentation. The stratigraphic relationships are interpreted in the following manner. HST of the Gallup Sandstone prograded from southwest to northeast forming a series of wave-dominated shorelines. A subsequent fall in relative sea level resulted in a forced regression and the formation of coarse-grained delta 'plumes' (some Tocito sandbodies) and inferred lowstand deltas. Sea-level rise, following lowstand, resulted in regional transgression. Erosionmore » of previously deposited strata was accomplished by both shoreface erosion (ravinement) and inner shelf erosion during the initial stages of this transgression.« less

STRATIGRAPHIC AND TECTONIC EVOLUTION OF LA PURISIMA AREA (BAJA CALIFORNIA, MEXICO) DURING LATE OLIGOCENE AND MIOCENE: IMPLICATIONS FOR THE PACIFIC MARGIN EVOLUTION

The study of the Neogene sequences outcropping in the La Purisima area, Baja California Sur, Mexico, has demonstrated the presence of regional unconformities separating the main sedimentary bodies. On the basis of stratigraphic and structural analyses, we propose a new statigraphic framework in terms of depositional sequences. The Lower Sedimentary Sequence (San Gregorio Formation Auctorum) is mainly composed of silicized shales with phosphatic nodules and subordinate tuffites. The sequence is heavily deformed at several sites, and shows N-S trending folds, thrusts and normal faults. The Middle Sedimentary Sequence (Isidro Formation and lowermost Comondu Formation Auctorum) consists of shallow marine fossiliferous sandstones with cineritic beds, capped by a thick fan of volcaniclastic sandstones. Small E-W trending thrusts and folds are associated to huge NNW-SSE and NW-SE open folds. The Upper Sedimentary Sequence is made of sub- horizontal sandstones and conglomerates probably representing the distal facies of the enormous alluvial fans of the Comondu Formation. Coquinoid limestones associated with volcaniclastic sandstones and conglomerates have been observed in between the lower and middle sequences, and are supposed to constitute a further allostratigraphic unit. The combined analysis of the stratigraphic and tectonic evidences is consistent with complex active wrenching along the Pacific margin.

Depositional history and palaeoecology of the Oxfordian/Kimmeridgian boundary beds at South Ferriby, South Humberside

SUMMARY The uppermost Ampthill Clay and lowermost Kimmeridge Clay are well exposed in a pit at South Ferriby. This provides an exceptional record of depositional conditions across the Oxfordian/Kimmeridgian boundary. These are investigated using a combined palaeoecological and taphonomic approach which reveals marked changes in the structural attributes of the benthic associations. The Ampthill Clay contains a low-diversity fauna dominated by deep infaunal bivalves and epifaunal serpulids. Sedimentation rates may have been high and benthic oxygen levels moderately low. The faunas of the Kimmeridge Clay are distinctly more diverse and they suggest a range of substrate types from soft to firm. The presence of phosphatic nodules and high fragmentation indices attests to low sedimentation rates. The phosphatic nodules are reworked only in the Oxytoma Cementstone, which coincides with the base of the baylei Zone and is here taken as the base of the formation. Benthic diversity declines to zero in the topmost beds at South Ferriby. This is attributed to the rapid onset of bottom water anoxia at an earlier time in the Kimmeridgian than anywhere else in Britain, possibly due to rapid local subsidence.

Geochemistry and occurrence of authigenetic phosphate nodules from the Desmoinesian cyclic Excello epeiric sea of the Midcontinent, USA

Pennsylvanian cyclothemic black shales of the Midcontinent, which were deposited in glacial-related epeiric seas containing high marine algal productivity due to the occurrence of swamps prior to maximum transgression, coastal upwelling and terrigenous runoff, are very rich in phosphate nodules. The high nutrient content of the epeiric sea supported phytoplankton populations that created favourable environments for the accumulation of phosphorites in anoxic bottom-waters. This close association of organic matter and phosphate-controlled nodule morphology (spherical, elongated, bladed, platy) appear to be related to progressive decreases in the nutrient supply of the sea water from the ancient Pacific Ocean (Panthalassa) in the south to the palaeoshorelines in the north. The major and trace element contents of ten phosphorite nodules from representative exposures along the 400-mile long outcrop belt from the northern Quachita to central Missouri were determined. Major elements of the phosphorites are P 2O 5 = 22.33–34.19, SiO 2 = 3.56–25.15, Fe 2O 3 = 0.73–2.94, CaO = 30.77–43.22, Na 2O = 0.30–4.51, K 2O = 0.35–2.27, Al 2O 3 = 4.31–8.02 and CO 2 = 1.2–2.0 wt%. The distribution of Mn, Zn, Cd and Cu shows a sharp increase at locations 40 and 50, where terrigenous runoff waters were dominant. The distribution of Ni, U, Co, Li, Cr, V and Mo does not show any significant change across the outcrop belt. Programmed pyrolysis indicates that the nodules are thermally immature with respect to hydrocarbon generation and kerogen types belong to Type II and Type III. Pyrolysis results are T max = 418–432° C, Ol = 17–168, Hl = 11–481, Pl = 0.04–0.4 and TOC = 0.65–7.07 wt%. Direct precipitation in micropores (1–4 μm in size) is shown by petrographic and SEM studies. However, the occurrence of nodules is explained by a calcite replacement model. The ubiquitous association of organic matter and phosphorus is vitally important for growing of phytoplankton, which extract phosphorus from sea water. When phytoplankton die, organic phosphorus oxidizes in the micropores to inorganic phosphorus species such as HPO 4 2−, which replaces calcite to form apatite as a function of alkalinity, pH, Eh and available ions. Concentric layers in nodules are due to progressive growth reaction with interstitial water. The model proposed for the occurrence of phosphate nodules in the Excello epeiric sea with bottom-water anoxia is similar to that for the highest productive regions of the world, such as Walvis Bay and off the coast of Peru and Chile.

Petrography and Origin of Bedded Barite and Phosphate in the Devonian Slaven Chert of Central Nevada

ABSTRACT Bedded barite deposits in the Slaven Chert in East Northumberland Canyon, Toquima Range, central Nevada, are part of the Roberts Mountains Allochthon which was thrust eastward onto the carbonate shelf by the Late Devonian to Early Mississippian Antler Orogeny. High concentrations of organic matter (up to 6.2%), well-preserved microfossil skeletal hash, and the presence of phosphate in interbedded barite and chert deposits indicate that the rocks formed in an anoxic environment. Rare earth element values from Early Paleozoic Cordilleran margin phosphates resemble patterns of phosphorites formed in Holocene zones of coastal upwelling at low latitudes versus phosphate formed in non-upwelling areas. These facts suggest that the barite, chert, and phosphate formed within an oxygen minimum one associated with coastal upwelling currents. The barite displays three main morphologies: massive, nodular, and composite. Massive barite is fine-grained, lacks matrix chert, and contains phosphate nodules. The massive beds formed as a rapid primary precipitate at or above the sediment/water interface. Nodular barite is composed of barite nodules associated with matrix chert. Chert laminations are bent by the barite nodules, indicating that the nodules grew prior to lithification of the chert and are therefore not a later diagenetic replacement of a precursor. Composite barite consists of barite nodules within a fine-grained barite matrix and lacks brown matrix chert, indicating that the nodules formed as a synsedimentary precipitate within a previously deposited fine-grained barite bed. Phosphate nodules are common to abundant in the chert beds and rare to common in the barite beds. Phosphate nodules within massive barite beds commonly contain abundant fine-grained barite rimmed by phosphate masses, occasionally the opposite relationship of phosphate rimmed by fine-grained barite was present. Similar relationships were observed between nodular barite and phosphate. Therefore, the phosphate and barite were contemporaneous to penecontemporaneous precipitates. Rare microbial mats exhibit a range in growth orientation within barite samples. The various orientations indicate that the mats were not light dependent, and geochemical analyses support a bacterial origin. Lab experiments on barite precipitates from sulfuric acid solutions demonstrate that the fine-grained barite precipitates rapidly out of solution, whereas the dipyramidal and bladed crystals precipitate more slowly, after the fine-grained barite. Although the medium in which the barite precipitated is different from any natural fluid, this sequence is very similar to the patterns observed in the rocks.

Woodford Shale (Upper Devonian-Lower Mississippian) and Associated Phosphate Nodules: ABSTRACT

A deep-water marine environment is inferred for the Woodford Shale formation (Upper Devonian-Lower Mississippian) in the Arbuckle and Ouachita mountains of southern Oklahoma. Black shales and associated phosphate nodules were deposited peripherally to the carbonate shelf adjacent to the North American craton. Analysis indicates that shales and interbedded cherts lack siliciclastic detritus and shallow-water fauna, and that they have an abundance of siliceous organisms. Depletion of Ce in the Woodford Shale relative to average shale compositions suggests the influence of deep oceanic water. Carbon isotope compositions of shales, cherts, and phosphates for {delta}{sup 13}C range from {minus}27.5 to {minus}30.2 {per thousand}. These values are comparable to Devonian-Mississippian values reported for distal marine sediments that get successively lighter farther from shore. In-situ phosphate nodules occurring in the top of the formation are early diagenetic features that formed in the upper few centimeters of organic-rich sediment in a poorly oxygenated subaqueous environment. A hierarchy of apatite morphologies in nodules indicates stages of diagenesis. Structureless collophane probably is representative of primary marine apatite. Higher order apatite phases successively include globular collophane, botryoidal apatite, and apatite crystallites. The latter commonly is associated with silicified nodules. Silicification of phosphate nodules represents the most advanced stagemore » of diagenesis prior to sediment lithification, and it implies that silica diagenesis within the Woodford occurred after nodules formed. Preserved radiolarians and sponge spicules observed in nodules also indicate that apatite precipitated before biogenic silica dissolved.« less

Pebbly phosphate and granular phosphorite (Late Cretaceous, southern Israel) and their bearing on phosphatization processes

Abstract Phosphate nodules, including common mineralized internal moulds of macrofauna, are found only at certain levels in the chalky, locally bituminous Ghareb Formation (uppermost Campanian-Maastrichtian) of southern Israel. The microstructure of these nodules suggests that they consist of marine sediment cemented by a microbially-mediated apatite, precipitated locally in protected microenvironments, e.g., within partly closed burrows and shells, or within the upper sediment layer. The globular microstructures commonly recognized in the phosphate cements of the pebbly phosphates are regarded as mineralized cells of endobenthic micro-organisms. The global occurrence of pebbly phosphate in diverse lithofacies throughout the geological column indicates the tolerance of these globular micro-organisms to a wide range of ecological conditions. The grains in the Upper Cretaceous phosphorites of the Negev (southern Israel) consist of bone fragments, phosphatized faecal pellets and ovular grains of different types. The commonest variety of ovular grain consists of microcrystalline phosphate (microphosphate) containing relics of globular microstructures, either forming the whole grain, coating a foraminifera or bone fragment, or cementing small clasts. The latter types resemble the pebbly phosphates and can be regarded as micronodules. The matrix of these granular phosphorites is mainly micritic, and in places argillaceous. In some Negev phosphorites the ovular grains and their matrix consist mainly of loosely packed tubules. These tubular microstructures resemble mineralized filaments of microphytes which form microbial mats on modern shallow seafloors lying within the photic zone. The origin of these granular phosphorites is discussed, and a comparison with the pebbly phosphates indicates that different ecological controls and microbial mediators resulted in the different kinds of phosphate bodies. Since most of the phosphate matter can be related to microbial activity, it is doubtful whether direct replacement of solid carbonate by phosphate contributes significantly to phosphogenesis in sediments. It is suggested, however, that in many deposits phosphate-precipitating micro-organisms have penetrated and mineralized the borings of algae and other endolithic groups, or have filled dissolution cavities within lithified carbonates.