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1.
Albian pelagic successions of the Nebeur area in northwestern Tunisia consist of radiolarian-bearing and organic-rich black shale beds, which represent the lower part of the Fahdene Formation. The carbonate content of the organic-rich beds ranges between 40 and 48%. Total organic carbon (TOC) analyses via Rock Eval pyrolysis yielded values ranging between 0.7 and 2.8% and a mixed marine/terrestrial origin. Tmax values vary between 424 and 450 °C, indicative of submature to mature organic matter. High resolution planktic foraminiferal and radiolarian biostratigraphy suggest that the black shales beds span the mid- to late Albian, confined to the middle part of the Ticinella primula zone, upper Biticinella breggiensis zone and lower appeninica + buxtorfi zone. Episodes of organic-rich deposition in the “Tunisian Trough” are interpreted as being the sedimentary record of the global oceanic anoxic events OAE1b, c, and d respectively. Age-diagnostic radiolarian assemblages recovered from late Albian organic-rich black shales lie within the UA13–UA14 boundary biochronozones. The abundance of radiolarian and calcispheres (i.e. pithonella) within the black shales suggests high productivity periods and eutrophic conditions probably triggered by upwelling currents.  相似文献   

2.
The late Cenomanian–early Turonian deposits in Tunisia recording the OAE2 event are commonly attributed to the Bahloul Formation. These deposits are composed of dark clayey limestone and organic-rich black shales with abundant planktic foraminifera. At the Bargou area, Oued Kharroub section, these organic-rich deposits include siliceous beds with abundant radiolarians recalling the “Livello Bonarelli”. A total of 25 radiolarian species are identified with a maximum of 18 species per sample. Moreover, the species skeleton preservation is usually poor to moderate in the most studied samples [estimated preservation index (PI) values ranging between 4 and 6]. Two successive assemblages are distinguished. The oldest one (RI) is dominated by nassellarians, which includes mainly Stichomitra, Pseudodictyomitra and Rhopalosyringium genera. It is included within the upper part of foraminiferal Rotalipora cushmani Zone and lower part of Whiteinella archaeocretacea zone. In contrast, the youngest assemblage (RII), which belongs to the upper part of the W. archaeocretacea zone and the earliest Turonian Watinoceras ammonite assemblage, is dominated by spumellarians and includes mainly Archaeocenosphaera, Cavaspongia, Pseudoeucyrtis and Pyramispongia genera. In addition, major and trace elements are analysed to test whether the geochemical record was synchronous to the biotic event or not. Thus, selected crossplots, Al2O3 vs. SiO2 and Al2O3 vs. TiO2, are used to evaluate the detrital input, and V/Cr vs. V/(V?+?Ni) and U/Th vs. V/(V?+?Ni) are used to evaluate the bottom redox conditions and the primary productivity within the Bahloul Formation. A strong Si/Al increase is marked in the lower part of the siliceous and organic-rich Bahloul Formation that could be explained by a local increase in upwelling-related biogenic SiO2 (silica-secreting organisms). Relative low abundance of terrigenous-related Ti/Al and K/Al ratios and enrichment of some productivity proxies such as Ba (organic matter related trace elements) suggests that the Bahloul was of relatively elevated primary productivity and minimal detrital input. Enrichments in redox-sensitive trace metals U and V in the Bahloul Formation deposits and redox indices, such as V/(V?+?Ni), U/Th and V/Cr, indicate that oxygen-restricted conditions prevailed during the late Cenomanian–earliest Turonian times and correlate well with relative abundances of some radiolarian and foraminiferal paleo-environmental indicators. In addition, indicators of detrital flux variations are used. Al2O3 shows a slight positive correlation with TiO2 and a less distinct correlation with SiO2.  相似文献   

3.
Late Aptian–early Albian limestones from the eastern Brazilian continental margin record the early evolution of the South Atlantic Ocean. In Tethyan and North Atlantic domains, a planktic foraminiferal turnover and organic-rich deposits related to Oceanic Anoxic Event 1b (OAE 1b) point to major ocean-climate changes through this interval. Coeval organic-rich deposits of the South Atlantic Ocean have been interpreted as the product of restricted circulation rather than attributed to a global event. However, previous investigations of the early marine phase of South Atlantic lack data from more distal facies, making correlations to global events difficult. Here, we present C, O, and Sr isotopes, elemental geochemistry, TOC and pyrolysis data, as well as a microfacies analysis of an upper Aptian–lower Albian distal section from the Campos Basin (southeastern Brazil). Our focus is on the paleoenvironmental characterization of and the possible association between organic-rich deposits and major perturbations related to Aptian–Albian transition. Five microfacies associations (MA) were identified in the informal units I and III, which were deposited in the neritic region on a carbonate ramp. Organic-rich deposits were described in unit III, composed of planktic-dominated wackestones interbedded with black shales, in a distal dysoxic to anoxic environment. The carbonates 87Sr/86Sr ratios showed a drastic increase (0.7072–0.7074), interpreted as enhanced chemical weathering, supported by the increase of continental input to the top of section. This trend was accompanied by a long-term δ13Ccarb negative excursion, which were assigned to the latest late Aptian–early Albian interval of the isotope reference curves, in accordance with the described occurrence of Colomiella recta. This scenario matches those proposed for the late Aptian–early Albian transition and OAE 1b set, as an enhanced greenhouse stage, pointing to the influence of the referred ocean-climate changes on the deposition of organic-rich deposits of the early South Atlantic Ocean. This investigation gives more evidences that these perturbations were a widespread event, as a product of broad-scale disturbances in the global carbon cycle which also controlled organic deposition and preservation on restricted settings.  相似文献   

4.
Variations in the sedimentary organic matter were documented throughout the Agua de la Mula Member (late Hauterivian) of the Agrio Formation, at a combined section in the type area of the Agrio Formation; the base at Agrio del Medio and the middle to top at Bajada del Agrio. A main organic-rich interval was identified in the basal Agua de la Mula Member, dominated by marine-derived Amorphous Organic Matter (AOM), coinciding with the highest Total Organic Carbon (TOC) content, between 1.4 and 3.8 wt.%, suggesting dysoxic conditions. The rest of the Member is predominantly characterized by terrestrially-derived organic matter, mainly phytoclasts, with low TOC values, around 1% or lower, indicating predominantly oxic depositional settings. By integrating stacking pattern and shell beds analysis, four depositional sequences SQ1–SQ4 were recognized. The organic-rich, finely laminated bituminous black shales of the Spitidiscus riccardii zone constitute a mayor and rapid inundation defining a Transgressive System tract (TST), related to a third order asymmetrical mesosequence dominated by a thick High System Tract (HST). It represents the most widespread and important flooding episode within the Neuquén Basin during the late Hauterivian. Inside sequences of higher frequency (SQ1, SQ2, SQ3, SQ4) of probably fourth order were recognized and analysed including several ammonids zones (Spitidiscus riccardii, Crioceratites schlagintweiti, C. diamantensis and Paraspiticeras groeberi). With the exception of the oxygen-controlled, basinal and outer ramp settings indicated for the TST1, which is equivalent to the TST of a lower order sequence, and the lower TST2, respectively, the prevalence of well oxygenated, inner to middle ramp depositional environments, is suggested for the rest of the sedimentary succession and emphasized in HST of SQ2, SQ3 and SQ4. Thus, a shallowing-upward trend with improved oxygenation is recorded through the Agua de la Mula Member, reflected by decreasing TOC, AOM content and preservation state of the organic matter. Based on the whole rock fluorescence analysis of the two selected organic-rich intervals from the Spitidiscus riccardii and the lowermost Crioceratites diamantensis zones, the presence of hydrocarbons suggests a very early in situ generation.  相似文献   

5.
Thick sequences of dark colored, organic carbon rich, finely laminated Santonian–Cenomanian claystones and homogeneous Albian siltstones were recovered from Ocean Drilling Program Sites 1257, 1258 and 1260 on the Demerara Rise in the western equatorial Atlantic Ocean. Total organic carbon (TOC) concentrations vary from 2 to over 20 wt% in the sequences of “black shales” that were deposited over a period of ～20 million years. Similarly long periods of elevated marine productivity implied by the high TOC concentrations are uncommon in the geological record and must have required unusual paleoceanographic conditions. The importance of nitrogen fixing bacteria to sustaining the amplified export production of organic matter is indicated by δ15N values that remain between ?4‰ and 0‰, a range that is notably less positive than the average of +5‰ for modern ocean sediments. Although containing mostly marine organic matter, the black shales have TOC/TN molar ratios between 20 and 40 that mimic those of land plant organic matter. The anomalously large TOC/TN ratios suggest selective organic matter degradation, probably associated with low oxygen conditions in the water column, that favored preservation of nitrogen poor forms of organic matter relative to nitrogen rich components. Deposition of black shales on the Demerara Rise was likely a consequence of the mid-Cretaceous warm and wet greenhouse climate that strengthened thermohaline stratification of this part of the Atlantic Ocean, which in turn encouraged bacterial nitrogen fixation, enhanced primary production, magnified organic matter export, and ultimately established anoxic conditions at the seafloor that improved preservation of organic matter for much of the 20 My period represented by these thick sequences.  相似文献   

6.
Four organic-rich shale units of the Proterozoic Vindhyan sedimentary succession have been scanned to reveal their origin and hydrocarbon potential. The wavy-crinkly nature of the carbonaceous laminae is suggestive of a microbial mat origin of the shales. These shales are thus different from Phanerozoic black shales which typically exhibit planar laminae. The hydrocarbon potential of the black shale units has been evaluated by Rock-Eval pyrolysis. Total organic carbon content of many of the shales exceeds 1%. The meanT max for the black shales translate to a vitrinite reflectance range of 2.05-2.40% Rm based on standard conversion techniques. These shales have reached the catagenetic stage near the beginning of anthracite formation.  相似文献   

7.
The Qreiya Beds that record the ‘mid-Paleocene event’ at Gabal Nezzazat occur within the Igorina albeari (P3b) Zone and constitute part of a 14-m thick shale succession that ranges in age from Early to Late Paleocene. They are composed of four alternating dark grey and brown shale beds, which are thinly laminated, phosphatic, organic-rich and extremely sulphidic. They are characterized by distinct enrichment and high peak anomalies in chalcophiles (Zn, Co, Ni, Cu and Pb) and organic association elements (V and Cr), especially within the brown organic-rich beds. It is concluded that these elements are incorporated into the phosphatic debris, sulphides and organic matter. In contrast, the grey beds are enriched in clay minerals and quartz. Clay mineral assemblages indicate alternating periods of warm/humid climate (high kaolinite) and dry climate (low kaolinite) during the formation of the grey and brown beds, respectively. The sediments of the Qreiya Beds yield lithological, biotic, geochemical and mineralogical data indicative of suboxic/anoxic marine environments as a result of high productivity and/or upwelling. The top metre of the succession below the Qreiya Beds is characterized by a progressive change from faunas dominated by praemurcurids to faunas dominated by Morozovilids, and by a progressive upward decrease in δ13Ccarb and δ18Ocarb values. The foraminiferal faunal change may reflect shallowing and warming preceding deposition of the Qreiya Beds. The change in isotopic values is inferred to be the result of surface weathering, fluvial input and diagenesis with no evidence of any primary change that could support presence of a hyperthermal event.  相似文献   

8.
Palaeotemperatures for the Cretaceous of India and Madagascar have been determined on the basis of oxygen isotopic analysis of well-preserved Albian belemnite rostra and Maastrichtian bivalve shells of from the Trichinopoly district, southern India, and Albian nautiloid and ammonoid cephalopods from the Mahajang Province, Madagascar. The Albian (possibly late Albian) palaeotemperatures for Trichinopoly district are inferred to range from 14.9 °C to 18.5 °C for the epipelagic zone, and from 14.3 °C to 15.9 °C for the mesopelagic zone, based on analyses of 65 samples; isotopic palaeotemperatures interpreted as summer and winter values for near-bottom shelf waters in this area fluctuate from 16.3 to 18.5 °C and from 14.9 to 16.1 °C, respectively. The mentioned palaeotemperatures are very similar to those calculated from isotopic composition of middle Albian belemnites of the middle latitude area of Pas-de-Calais in Northern hemisphere but significantly higher than those calculated from isotopic composition of Albian belemnites from southern Argentina and the Antarctic and middle Albian belemnites of Australia located within the warm-temperate climatic zone. Isotopic analysis of early Albian cephalopods from Madagascar shows somewhat higher palaeotemperatures for summer near-bottom shelf waters in this area (20.2-21.6 °C) in comparison with late Albian palaeotemperatures calculated from southern India fossils, but similar winter values (13.3-16.4 °C); however, the latter values are somewhat higher than those calculated from early Albian ammonoids of the tropical-subtropical climatic zone of the high latitude area of southern Alaska and the Koryak Upland. The new isotopic palaeotemperature data suggest that southern India and Madagascar were located apparently in middle latitudes (within the tropical-subtropical climatic zone) during Albian time. In contrast to the Albian fossils, isotope results of well-preserved early Maastrichtian bivalve shells from the Ariyalur Group, Trichinopoly district, are characterised by lower δ18O values (up to −5.8‰) but normal δ13C values, which might be a result local freshwater input into the marine environment. Our data suggest that the early Maastrichtian palaeotemperature of the southern Indian near-bottom shelf waters was probably about 21.2 °C, and that this middle latitude region continued to be a part of tropical-subtropical climatic zone, but with tendency of increasing of humidity at the end of Cretaceous time.  相似文献   

9.
An integrated stratigraphic analysis has been made of the Tarcău Nappe (Moldavidian Domain, Eastern Romanian Carpathians), coupled with a geochemical study of organic-rich beds. Two Main Sequence Boundaries (Early Oligocene and near to the Oligocene–Aquitanian boundary, respectively) divide the sedimentary record into three depositional sequences. The sedimentation occurred in the central area of a basin supplied by different and opposite sources. The high amount of siliciclastics at the beginning of the Miocene marks the activation of the “foredeep stage”. The successions studied are younger than previously thought and they more accurately date the deformation of the different Miocene phases affecting the Moldavidian Basin. The intervals with black shales identified are related to two main separate anoxic episodes with an age not older than Late Rupelian and not before Late Chattian. The most important organic-rich beds correspond to the Lower Menilites, Bituminous Marls and Lower Dysodilic Shales Members (Interval 2). These constitute a good potential source rock for petroleum, with homogeneous Type II oil-prone organic matter, highly lipidic and thermally immature. The deposition of black shales has been interpreted as occurring within a deep, periodically isolated and tectonically controlled basin.  相似文献   

10.
Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian–Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, δ13Corg values lie mostly between −30‰ and −27‰, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C3 land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low δ13Corg values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian–Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.  相似文献   

11.
Using an integrated multidisciplinary approach the upper part of the Lower Cretaceous northeastern Tunisian Jebel Ammar sedimentary succession was examined in detail. The method applied included lithologic and microfacies analyses, micropalaeontology, sedimentology, variations in organic matter (OM) content and carbonate carbon stable isotope (δ13C) record. A major result was the identification in this sector of Tunisia of the Early Aptian (Bedoulian) OAE1a event in a biostratigraphically well-calibrated context, its location keyed to planktonic foraminiferal zones and isotopic stages.The most striking feature to the Jebel Ammar Aptian sequence is the presence of a 25 m interval of black limestones and marly limestones, of which the microfacies shows that these darker beds consist of wackestones with the presence of abundant radiolarians, a fair number of diversified planktonic and relatively rare benthic foraminifers, together an indication of a pelagic palaeoenvironment. The foraminiferal marker Schackoina (Leupoldina) cabri (still very rare at the beginning of its range) first occurs about six metres above the base of this interval, but becomes much more abundant in its uppermost part, together with the radially- elongate-chambered praehedbergellids. This “acme” of S. (L.) cabri is nearly contemporary with a radiolarian bloom. Rock Eval analyses show TOC values up to 4.59% and a Tmax ranging between 441 and 513 °C, which indicates an overmature OM. The δ13C isotope curve shows an evolution similar to those recognized worldwide. The lower part of the darker beds includes a marked shift in isotope values from −2.40 to +3.02 ‰/PDB. This increase is assumed to be equivalent to the isotopic C4 stage of Menegatti et al. (1998, Paleoceanography, 13, 530–545). The signature of the middle and upper part of the isotopic curve is tentatively interpreted as comprising the C5–C7 stages. The first occurrence of S. (L.) cabri is located in the upper part of C4 and its acme as well as the radiolarian bloom is situated within the span of the C7 stage. These results are fairly consistent, though with minor discrepancies, with what has been published from several parts of the North Tethyan margin and more particularly the Lower Aptian type area of southeastern France, where S. (L.) cabri first occurs at the C3/C4 stage boundary with an acme that begins in the lower part of C7.  相似文献   

12.
Oceanic anoxic events are clues to ocean processes and are correlation datums. In North America only OAE 1a and 2 are well documented. Based on a low-resolution sampling program, a multi-proxy geochemical approach constrained by a biostratigraphic framework was utilized to identify OAE 1d in the upper part of the upper Albian Mesilla Valley Formation near El Paso, Texas. Chronostratigraphic and biostratigraphic evidence indicate that the OAE 1d event in the Mesilla Valley section is located in the lower part of the upper Albian–Cenomanian Ovoidinium verrucosum zone, which correlates with the uppermost Albian Parathalmanninella appenninica and Stoliczkaia dispar zones. The chronostratigraphic age of the geochemical event in the Mesilla Valley Formation is uppermost Albian (97.39–97.30 Ma).The classic geochemical signatures for OAEs are enriched total organic carbon (TOC) concentrations and coupled positive δ13C excursions. OAE 1d at this location records TOC values ranging from 0.25 to 0.69 wt.% throughout the Mesilla Valley Formation, where TOC increases during the OAE (21.0–40.0 m) to more than 0.40 wt.%. Interestingly, the organic matter in the Mesilla Valley is dominantly type III, which indicates a pervasive terrigenous source. Although marine organic matter is abundant from the base into the middle of the proposed OAE interval, it is progressively replaced by terrestrial material above the OAE section during progradation. The δ13Corganic values record a positive δ13C shift of +1.6‰ from −26.41 to −24.80‰ across the stratigraphic interval from 21.0 to 40.0 m, which correlates with OAE 1d.Mn and Fe geochemistry suggest the depositional conditions of the Mesilla Valley Formation were dominated by anoxic and possibly Fe-rich bottom waters, specifically during the time period associated with the OAE 1d event. This interpretation is supported by the presence of Fe enrichment recorded by FeTotal/Al and FeHighly Reactive/FeT with the lack of Fepyrite/FeHighly Reactive associated with Mn depletion.  相似文献   

13.
The Upper Cretaceous Phosphate Formation in the Western Desert of Egypt displays a characteristic facies association that includes marine phosphorites interbedded with black shales and glauconitic sandstones. The upper part of the formation is characterized by the presence of thin phosphatic beds, which are filled-extensively-with disordered and non stoichiometric (mean MgCO3 = 41.4 ± 0.34 mol%) authigenic dolomite cement. SEM and the back scattered images of these coarse crystalline dolomite cements reveal that they display planar euhedral crystal boundaries, polymodal crystal size distribution and variable inclusion pattern. The relatively low and wide ranged δ18O (− 0.87 to − 4.15‰ VPDB) values of the dolomite cements coupled with their depleted Sr (mean = 187 ± 26 ppm) and high iron and manganese values (mean = 6851 ± 554 ppm and 11599 ± 229 ppm respectively) invoke that they were formed from mixed hypo-saline fluids within a mixing marine-meteoric zone probably during a low stand period at the vicinity of the Maastrichtian/Early Tertiary unconformity. Meanwhile, their negative δ13C (− 1.31 to − 3.56‰ VPDB) values argue for a possible involvement of isotopically light carbon, derived from degradation of organic matter, during their precipitation.  相似文献   

14.
Recent studies have documented the expulsion of methane and oil to the Albian paleoseabed in the Basque-Cantabrian Basin. They interpret that hydrocarbon generation and expulsion were triggered by seismically recorded magmatic intrusions which metamorphosed organic-rich host sediments. An outcrop within the basin was selected to investigate organic matter evolution and sediment degassing due to an igneous body. This intrusion is a 5 m thick Late Albian basaltic sill that intruded mudstones of the Black Flysch Group, near Gorliz (north Iberia). Vitrinite and bitumen reflectance profiles and metamorphic mineral distribution in the overburden indicate that the sill produced a thermal effect that increases toward the intrusion, defining a 2 m thick (minimum) contact aureole.Geochemical profiles of TOC, S1, S2, HI and PC show a gradual decrease toward the sill indicating organic carbon loss and increase in the thermal maturity of the organic matter in the same direction. Concordantly, gas chromatograms show a loss of n-alkanes and a predominance of the shorter chain length homologues adjacent to the sill. Tmax and PI (S1/S1 + S2) values increase toward the sill which suggests an increase in the thermal stress and in the extent of kerogen pyrolysis, respectively.Organic carbon loss in the aureole was the result of carbon devolatilization and formation of CO2 and CH4 gases. The newly formed CO2 reacted with pore waters to precipitate 13C depleted carbonate minerals in the aureole and in sill fractures. CH4 escaped from the aureole via hydrofractures to the paleoseabed, where methane-derived authigenic carbonates were formed.  相似文献   

15.
A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ13C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ13Ckerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ13C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO2-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO2” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.  相似文献   

16.
The Datangpo‐type manganese ore deposits, which formed during the Nanhuan (Cryogenian) period and are located in northeastern Guizhou and adjacent areas, are one of the most important manganese resources in China, showing good prospecting potential. Many middle‐to‐large deposits, and even super‐large mineral deposits, have been discovered. However, the genesis of manganese ore deposits is still controversial and remains a long‐standing source of debate; there are several viewpoints including biogenesis, hydrothermal sedimentation, gravity flows, cold‐spring carbonates, etc. Geochemical data from several manganese ore deposits show that there are positive correlations between Al2O3 and TiO2, SiO2, K2O, and Na2O, and strong negative correlations between Al2O3 and CaO, MgO, and MnO in black shales and manganese ores. U, Mo, and V show distinct enrichment in black shales and inconspicuous enrichment in Mn ores. Ba and Rb show strong positive correlations with K2O in manganese ores. Cu, Ni, and Zn show clear correlations with total iron in both manganese ores and black shales. ∑REE of manganese ores has a large range with evident positive Ce anomalies and positive Eu anomalies. The Post Archean Australian Shale (PAAS) normalized rare earth element (REE) distribution patterns of manganese ores present pronounced middle rare earth element (MREE) enrichment, producing “hat‐shaped” REE plots. ∑REE of black shales is more variable compared with PAAS, and the PAAS‐normalized REE distribution patterns appear as “flat‐shaped” REE plots, lacking evident anomaly characteristics. δ13C values of carbonate in both manganese ores and the black shales show observable negative excursions. The comprehensive analysis suggests that the black shales formed in a reducing and quiet water column, while the manganese ores formed in oxic muddy seawater, which resulted from periodic transgressions. There was an oxidation–reduction cycle of manganese between the top water body and the bottom water body caused by the transgressions during the early Datangpo, which resulted in the dissolution of manganese. Through the exchange of the euphotic zone water and the bottom water, and episodic inflow of oxygenated water, the manganese in the bottom water was oxidized to Mn‐oxyhydroxides and rapidly buried along with algae. In the early diagenetic stage, Mn‐oxyhydroxides were reduced and dissolved in the anoxic pore water and then transformed into Mn‐carbonates by reacting with HCO3? from the degradation of organic matter or from seawater. In the intervals between transgressions, continuous supplies of terrigenous clastics and the high productive rates of organic matter in the euphotic zone resulted in the deposition of the black shales enriched in organic matter.  相似文献   

17.
Marine black shales of the Lower Cambrian Niutitang Formation in southern China host Mo–Ni–platinum group elements (PGE) mineralization confined to a phosphate- and pyrite-rich stratiform body (max. 20-cm thick). The H/C atomic ratio, carbon isotopic composition, FTIR spectra of bulk organic matter, and spectra of extractable part of organic matter indicate similar sources and thermal evolution of organic matter in barren and mineralized black shales.The morphology and relative abundance of organic particles in barren and mineralized shales are different. In barren black shales, organic particles comprise only elongated bodies and laminae 2–10 μm across or elongated larger bodies (> 10 μm) with Rmax = 2.96–5.21% (Type I particles). Mineralized black shales contain Type I particles in rock matrix (90–95 vol%), small veinlets or irregular organic accumulations (Type II particles, 1–5 vol%) that display weak to well developed mosaic texture and a variable reflectance (Rmax = 3.55–8.65%), and small (< 1 to 5 μm) rounded or irregular Type III organic particles (1–4 vol%) distributed within phosphate nodules and sulphide rip-up clasts. Type III particles show similar reflectance as particles of Type I in rock matrix. Type I particles are interpreted as remnants of in situ bacterially reworked organic matter of cyanobacteria/algal type, Type II as solidified products or oil-derived material (migrabitumen), and Type III particles as remnants of original organic matter in phosphatized or sulphidized algal/microbial oncolite-like bodies. Equivalent vitrinite reflectances of Type I and III particles in barren and mineralized rocks are similar and correspond to semi-anthracite and anthracite. Micro-Raman spectra of organic particles in rocks display a wide belt in the area of 1600 cm− 1 (G belt) and approximately the same belt in the area of 1350 cm− 1 (D belt). The ratio of integrated areas of the two belts correlate with Rmax values.The Mo–Ni–PGE mineralized body is interpreted as to represent a remnant of phosphate- and sulphide-rich subaquatic hardground supplied with organic material derived from plankton and benthic communities as well as with algal/microbial oncolite-like bodies that originated in wave-agitated, shallow-water, nearshore environment.  相似文献   

18.
The Mesoproterozoic Srisailam Formation, exposed along the northern part of the Cuddapah basin, India, comprises mainly medium- to fine-grained siliciclastics, and is devoid of any carbonate sediment. Preliminary sedimentological studies helped in recognizing fifteen distinct facies (five facies associations) in Chitrial outlier of the Srisailam Formation deposited in continental half-graben basin(s). Black shales (sensu lato) are minor components of the Srisailam Formation, and inferred to have deposited in deep lacustrine and prodelta facies of the half-graben(s). The black shales show restricted thickness (up to 29.0 m), and are characterized by overall high ‘black shale' to ‘total shale' ratio (>0.51). Their geochemical characteristics were studied to constrain provenance, palaeoclimate, and tectonic setting of deposition of the Srisailam Formation. Further, an attempt has been made to use the Srisailam black shales as proxy for constraining the timing of breakup of the supercontinent Columbia.The Srisailam black shales are geochemically quite distinct. At similar SiO2 contents they are considerably different from PAAS. They are characterized by considerably lower ΣREE (Av. 136.0 ± 50.4 ppm) but a more conspicuous negative Eu-anomaly (Av. 0.34 ± 0.09) than PAAS. Al2O3/TiO2 and TiO2/Zr ratios coupled with Eu/Eu*, GdCN/YbCN, La/Sc, Th/Sc, and Th/Cr ratios suggest their derivation from granite and granodiorite. The CIA values (65–90, Av. 72 ± 9) as a whole indicate moderate chemical weathering under semiarid climate. Discriminating geochemical parameters indicate passive margin depositional setting. The combined sedimentological and geochemical characteristics reveal deposition of the Srisailam sediments in continental rift basin(s).Thick succession of black shales (with high CIA values) that deposited with shelf carbonates proxy for mantle superplume and supercontinent breakup events. The sedimentological characteristics and geochemical data of the Srisailam black shales plausibly exclude any large-scale breakup of Columbia during the interval (1400–1327 Ma) of deposition of the Srisailam Formation.  相似文献   

19.
《Gondwana Research》2011,19(4):632-637
In South China, the Datangpo black shales (663 Ma–654.5 Ma) were deposited during the Cryognian interglacial time between the Sturtian and Marinoan glaciations. Multi-geochemical proxies, including different iron speciation and relevant ratios (FeHR/FeT, FeP/FeHR and FeT/Al ratios) and molybdenum concentrations, were used to reconstruct the paleo-depositional environment of this black shale horizon. The ratios of different iron species (FeHR/FeT > 0.38 and FeP/FeHR < 0.80) suggest an overall anoxic conditions (ferruginous) over the deposition of the black shales, although intermittent euxinic (FeHR/FeT > 0.38 and FeP/FeHR ≈ 0.80) and oxic (FeHR/FeT < 0.38) intervals could have occurred. Furthermore, FeT/Al ratios (FeT/Al ≤ 0.51) confirm that water column may not be persistent euxinia during the deposition of the Datangpo black shales. Meanwhile, molybdenum concentrations show a decreasing trend towards the top of the black shales, reconciling the gradual oxygenating trend during this period as stated above. Compared to δ34SPy values in the Mesoproterozoic deep ocean, more positive δ34SPy values of this study may result from a small size of sulfate reservoir. The small-size sulfate reservoir and concurrent enrichment of molybdenum indicate that the ocean chemistry in the Cryogenian Period is similar to that in the Archean Eon.  相似文献   

20.
The Bonarelli Horizon is an important regional marker level in the Umbria-Marche Apennines. It is characterized by consisting of black bituminous shales that yield a distinctive radiolarian fauna. Black shales referable to the Bonarelli Horizon are also present in the Southern Alps. The horizon is of uniform lithology and contains radiolarian assemblages of late Cenomanian-early Turonian age throughout both the Umbria-Marche Apennines and Southern Alps. Several new radiolarian taxa have been recognised and described from the Bonarelli Horizon of both areas. Two new species are defined in this paper, namelySethocapsa bossoensisandXitus picenus.  相似文献   

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