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Evolución de los filosilicatos y génesis de los yacimientos de pirita: su relación con las facies sedimentarias y el metamorfismo en la Cuenca de Cameros. Cretácico inferior. La Rioja-Soria
Abstract
Se ha caracterizado mineralógicamente el grupo enciso perteneciente a la cuenca de Cameros del cretácico inferior (Soria-La Rioja). Se han determinado las condiciones alcanzadas por este grupo en diferentes puntos, basándonos en las asociaciones minerales presentes y en el estudio de parámetros cristaloquímicos en filosilicatos (fundamentalmente ílita y clorita). Se han estimado condiciones de anquizona próximas a la epizona para las áreas depocentrales disminuyendo progresivamente el grado metamórfico al alejarnos de esta. También se han estudiado las mineralizaciones de pirita que aparecen en la cuenca. Estas mineralizaciones se generaron durante el metamorfismo hidrotermal que afecto a la cuenca por la interacción de fluidos ricos en azufre con niveles lutíticos con minerales de fe reactivos. El azufre procedía de la termorreducción de sulfatos y de la rotura térmica de piritas sedimentarias.
... The base of the sections corresponds to the contact between Jubera Fm and Leza Fm, which is gradational and, in this work, it is located at the first meter-scale carbonate bed found in the upper part of the Jubera Fm. The contact between the Leza Fm and the overlying Enciso Gr is also gradational and, in this work, the base of the Enciso Gr is considered to be marked by the first occurrence of decimetre-scale alternation of marls, sandstones and sandy ostracod-rich limestones, which are characteristic facies of the Enciso Gr Alonso-Azcárate, 1997;Doublet et al., 2003). 750 rock samples were collected from the stratigraphic sections, as well as from other outcrops. ...
... Geological maps (Figs. 3,4) show that the Leza Fm is always overlain by part of the Enciso Gr, which is a thick unit (up to 1100 m in its depocentre) formed by a wellbedded alternation of sandstones, siliciclastic mudstones, marls, limestones and dolomites Alonso-Azcárate, 1997;Alonso-Azcárate et al., 1999;Doublet et al., 2003), worldwide-known for the abundance of dinosaur footprints (Moratalla and Sanz, 1997;Pérez-Lorente, 2002;Moratalla and Hernán, 2010). Transition from the Leza to the Enciso deposits is also typically gradual, which causes interbedding of thin-bedded marls and sandy limestones of the Enciso Gr with thicker-bedded carbonates of the Leza Fm at the contact of both units (Fig. 8). ...
The Leza Formation is a carbonate unit of the northern Cameros Basin (N Spain) with controversial age, stratigraphic position, and sedimentological interpretation. It was deposited in a series of fault-bounded tectonic depressions along the northern margin of the basin. The Leza Fm overlies and changes laterally to the siliciclastic Jubera Fm, and the thickness of both units is also controlled by faults. Although the Leza Fm has been traditionally interpreted as lacustrine with sporadic marine incursions, detailed sedimentological analysis reveals new and very abundant evidences of marine influence: sedimentary structures of tidal origin, common marine fossils (dasycladales and foraminifers), and homogeneous populations of porocharacean charophytes, indicative of brackish conditions. Thus, this unit is interpreted as deposited in a system of coastal-wetlands with both fresh-water and sea-water influence, laterally related with the alluvial deposits of the Jubera Fm and the fluvio-lacustrine deposits of the Enciso Gr towards the centre of the basin. Using the chronostratigraphic ranges of the marine fossils and the lateral relationship with adjacent units, the Leza Fm is confirmed as part of a depositional sequence late Barremian - early Aptian in age. Furthermore, a transgressive trend is defined in the Leza Fm, which is compared with the eustatic evolution of the neighbouring Basque-Cantabrian, Iberian and Pyrenean marine basins, allowing correlation of the upper part of this unit with the widespread eustatic maximum of the middle-upper part of the early Aptian. This eustatic influence provided additional accommodation space to that created by tectonics.
These data from the Cameros Basin are contrasted with a thorough compilation of paleogeographic data, showing that, during the early Aptian transgression, marine influence was likely to reach the northern Cameros Basin, not only coming from the SE Iberian Basin, as previously interpreted, but also from the NW Basque-Cantabrian Basin. This resulted in a probable link between Boreal and Tethyan marine realms
... La Supersecuencia o Megaciclo Jurásico terminal -Cretácico inferior se organi za en ocho secuencias deposicionales limitadas por discontinuidades estratigráficas, que se suelen manifestar como discordancias (Mas et al., 1993; Martín i Closas et al., 1998; Arribas et al., 2003) (Fig. 8). Este registro sedimentario es de carácter esencialmente continental (sistemas aluviales y lacustres) con sólo muy esporádicas incursiones marinas, que en las figuras 8 y 9 están señaladas con la letra M (Gómez Fernández, 1992; Mas et al., 1993; Mas et al 1997; Mas et al., 2003). Normalmente las secuencias deposicionales se organizan en ciclos que comienzan por depósitos clásticos aluviales que pasan hacia techo a cali zas de origen lacustre (Figs. 8 y 9: SD 1, SD 2, SD 3, SD 5 y SD 7), Aunque en la Cuenca de Cameros la velocidad de subsidencia y tasa de sedimentación son más elevadas que en el resto de las cuencas mesozoicas de la Cadena Ibérica, la pauta de relleno guarda un claro paralelismo secuencial con otras cuencas de la Cadena (las cuencas Ibérica meridional y del Maestrazgo) (Mas et al., 1993), En la figura 8 se indican las principales unidades litoestratigráficas que pueden diferenciarse en la cuenca, su posición dentro de las secuencias deposicionales, y sus principales ras gos sedimento lógicos , En general, el contenido paleontológico es escaso y solo de relativo valor cro noestratigráfico debido tanto al tipo de facies (esencialmente continentales) como a las transformaciones posteriores sufridas (fuerte diagénesis de enterramiento y metamorfismo local). ...
... de desbordamiento Los abundantes yacimientos de pirita del sector oriental de la Cuenca de Cameros son otro hecho distintivo genéticamente relacionado con este metamor fismo (Alonso-Azcárate et al., 1996). El estudio de los isótopos estables de azufre en los cristales de pirita (Alonso-Azcárate, 1997; ) ...
La Cuenca de Cameros, localizada en la parte NO de la Cordillera Ibérica, es una de las cuencas que constituyen el Sistema de Rift Mesozoico Ibérico o Cuenca Ibérica. Se formó en el contexto de la segunda fase de rifting intraplaca que, desde el Jurasico superior al Albiense inferior, tuvo lugar cuando Iberia se separó de Europa en relación con la apertura de la cuenca oceánica del Golfo de Vizcaya. Al mismo tiempo se formaron varias cuencas a lo largo del surco Ibérico de orientación NO-SE, siendo la de Cameros la más occidental en el Sistema de Rift Mesozoico Ibérico. El relleno de la Cuenca de Cameros (Titánico-Albiense inferior) corresponde a un gran ciclo o super-secuencia que está limitado por dos importantes discordancias en la base y en el techo. La Supersecuencia o Megaciclo Jurásico terminal - Cretácico inferior se organiza en ocho secuencias deposicionales limitadas por discontinuidades estratigráficas, este registro sedimentario es de carácter esencialmente continental (sistemas aluviales y lacustres) con sólo muy esporádicas incursiones marinas. Hay varios hechos distintivos que la diferencian de las otras cuencas del Sistema de Rift Ibérico: (1) influencia marina muy escasa; (2) retardo de los procesos de diastrofismo, pues el rifting empezó primero en la parte SE del surco ibérico (Kimmeridgiense en la Cuenca del Maestrazgo) y después se propagó hacia el NO(Titónico en la Cuenca de Cameros); (3) sin embargo, y a pesar de su posición interna, esta cuenca fue la más subsidente, registrando el mayor espesor de sedimentos, llegándose a acumular 5000 m de espesor vertical de sedimentos desde el Titónico hasta el Albiense inferior, que representan hasta 9000 m de registro estratigráfico en el sentido de desplazamiento de los depocentros de las sucesivas secuencias de depósito; (4) a pesar de su gran registro sedimentario, se trata de una cuenca sinclinal que, durante su formación, no estuvo limitada por grandes fallas; y (5) esta cuenca es la única entre las cuencas mesozoicas del Rift Ibérico, en la que sus depósitos se han visto afectados por metamorfismo. Se trata de un metamorfismo de bajo y muy bajo grado que, durante el Cretácico medio-superior, afectó a la parte oriental de la cuenca. Su génesis y evolución son explicadas mediante un modelo de cuenca de bloque de Lecho (bangingwal basin) formada sobre una rampa con buzamiento sur contenida en una falla extensional horizontal localizada en el basamento varisco. La Cuenca de Cameros sería, por tanto, una cuenca de rampa-extensional (extensional-ramp basin). Las reconstrucciones palinspásticas sugieren que durante la extensión el bloque de techo se desplazó unos 30 Km hacia el Sur. Desde el punto de vista estructural, la Cuenca de Cameros, que fue totalmente invertida durante la compresión Terciaria, forma parte de una lámina de cabalgamiento alpina que se encuentra desplazada hacia el Norte hasta un máximo de 28 Km sobre los materiales terciarios de las cuencas del Ebro y del Duero. Por ultimo, se establecen las relaciones entre los sistemas petrolíferos potenciales del área de Cameros y la evolución de esta cuenca desde su formación y relleno durante la extensión fíníjurásíca-eocretacíca hasta su inversión contractiva terciaría, enmarcando estos sistemas petrolíferos en la evolución fórmica de la cuenca.
... • The Enciso Group (~400 m thick, upper Barremian -lower Aptian), composed of dark fluviolacustrine mudstones deposited in coastal wetlands and lagoons with restricted circulation (Alonso-Azcárate, 1997;Alonso-Azcárate et al., 1999;Mas et al., 1993;Suárez-Gonzalez et al., 2013). ...
The Mesozoic Cameros Basin, northern Spain, was inverted during the Cenozoic Alpine orogeny when the Tithonian – Upper Cretaceous sedimentary fill was uplifted and partially eroded. Tar sandstones outcropping in the southern part of the basin and pyrobitumen particles trapped in potential source rocks suggest that hydrocarbons have been generated in the basin and subsequently migrated. However, no economic accumulations of oil or gas have yet been found. This study reconstructs the evolution of possible petroleum systems in the basin from initial extension through to the inversion phase, and is based on structural, stratigraphic and sedimentological data integrated with petrographic and geochemical observations. Petroleum systems modelling was used to investigate the timing of source rock maturation and hydrocarbon generation, and to reconstruct possible hydrocarbon migration pathways and accumulations.
In the northern part of the basin, modelling results indicate that the generation of hydrocarbons began in the Early Berriasian and reached a peak in the Late Barremian – Early Albian. The absence of traps during peak generation prevented the formation of significant hydrocarbon accumulations. Some accumulations formed after the deposition of post‐extensional units (Late Cretaceous in age) which acted as seals. However, during subsequent inversion, these reservoir units were uplifted and eroded.
In the southern sector of the basin, hydrocarbon generation did not begin until the Late Cretaceous due to the lower rates of subsidence and burial, and migration and accumulation may have taken place until the initial phases of inversion. Sandstones impregnated with bitumen (tar sandstones) observed at the present day in the crests of surface anticlines in the south of the basin are interpreted to represent the relics of these palaeo‐accumulations.
Despite a number of uncertainties which are inherent to modelling the petroleum systems evolution of an inverted and overmature basin, this study demonstrates the importance of integrating multidisciplinary and multi‐scale data to the resource assessment of a complex fold‐and‐thrust belt.
... Gypsum interlayered with the dolomite is mostly structureless, but some of these beds show decimetric interlocking alabaster gypsum nodules. On the basis of illite and chlorite crystallinity and chlorite geothermometry in this sector of the basin, Azcárate et al. (1995Azcárate et al. ( , 1997Azcárate et al. ( , 1999) estimated peak metamorphic conditions as close to the anchizone limit. The layering is thus an original sedimentary feature but all gypsum is of secondary origin, i.e. original gypsum was transformed to anhydrite during diagenesis and very low grade metamorphism and rehydrated during the exhumation of the sediments. ...
Sulfate [delta]34S and [delta]18O and Sr isotope compositions are presented for Berriasian (Upper Jurassic) gypsum evaporites from the Cameros Basin, a continental basin in the Iberian ranges of northern Spain. Solute sources to the ephemeral lakes in which the evaporites formed are dominated by weathering of sediments containing older Keuper marine evaporites and granitic/metamorphic Variscan basement. Strontium isotopic ratios of the continental evaporites (87Sr/86Sr = 0.707882 to 0.707933) are elevated compared to the Keuper source (87Sr/86Sr = 0.707605 to 0.707799), most likely reflecting a radiogenic component from basement lithologies. Sulfate [delta]34S in the continental evaporites (at around 18.2 [per mille sign]V-CDT) is higher than any possible mixture of solutes to the basin. The degree of 34S enrichment is small ([approximate] 3.7[per mille sign]) and is most likely the result of partial bacterial reduction of lake-water sulfate. The same process would also enrich sulfate in 18O, but by only [approximate] 1[per mille sign]. Evaporite sulfate [delta]18O (at around 21.7[per mille sign]V-SMOW) is, in fact, enriched by [approximate] 10[per mille sign] relative to sulfate sources to the basin. This large effect is most likely the result of re-equilibration of oxygen isotopes during low grade metamorphism of the basinal sequence. Sulfur and strontium isotopes in the evaporites remained internally buffered and thus unchanged during metamorphism while oxygen isotopes were open to exchange with aqueous fluid and/or interbedded carbonates that constituted a large exchangeable oxygen reservoir. The potential of sulfate oxygen to undergo isotopic exchange at elevated temperatures must be taken into account when interpreting the significance of evaporite sulfate oxygen data from units that have undergone deep burial or low grade metamorphism.
... The formation of pyrite deposits occurred in metapelite units with elevated contents of sedimentary chlorite (Alonso-Azcárate et al. 1995;Alonso-Azcárate 1997). The availability of Fe (aFe 2 + in k¡ and k 2 ) was thus controlled by the same factor-chlorite breakdown at the locus of pyrite formation-and thus similar at all deposits. ...
The low-grade metasediments of the Cameros Basin, northern Spain, host a number of deposits of spectacular quality pyrite mineralization. These formed during regional metamorphism and the pyrite crystals exhibit a wide range of morphologies. On the basis of pyrite crystal habit, the deposits can be classified into two groups: Group I comprises deposits with cubic, elongated or platy crystals; Group II comprises deposits characterized by pyritohedra and cubo-pyritohedra with striated faces, along with blocky crystals and fine-grained aggregates. Group I deposits are formed in sequences dominated by meandriform fluviatile sediments, while Group II is hosted by deltaic plain and lacustrine metasediments.
Temperature differences between deposits and As content are possible causes of the different pyrite morphologies in the deposits, but no significant variation exists between the two groups for either factor. Comparison with experimentally grown pyrite crystals suggests that Group I deposits have morphologies indicative of lower degrees of pyrite supersaturation than pyrite crystals in Group II deposits. The sedimentary facies hosting Group II deposits provides a greater availability of sedimentary sulphur (pyrite and sulphates). Moreover, reactions involving sulphate during metamorphism may have modified fluid chemistry, which would also act to produce higher degrees of pyrite saturation in fluids derived from the sulphate-rich deltaic plain and lacustrine metasediments hosting the Group II deposits. This hypothesis is confirmed by sulphur isotope data on the pyrites, which show a larger component of34S-enriched sulphate-derived sulphur in these deposits. Copyright
Variations in clay mineral assemblages, changes in Kubler index (KI), and the chemical composition of chlorites are used to identify source areas in the lacustrine materials in the Lower Cretaceous Leza Limestone Formation of the Cameros Basin, northern Spain. This formation has fairly homogeneous lithological characteristics and facies associations which do not allow for identification and characterization of local source areas. The Arnedillo lithosome of the Leza Limestone Formation contains a clay mineral association (Mg-chlorite, illite and smectite) indicative of its provenance. Chlorite composition and illite KI values indicate that these minerals were formed at temperatures higher than those reached by the Leza Formation which indicates its detrital origin. The similarity in the Mg-chlorite composition between the Arnedillo lithosome and the Keuper sediments of the area indicates that these materials acted as a local source area. This implies that Triassic sediments were exposed, at least locally, at the time of deposition of the Leza Formation. The presence of smectite in the Leza Formation is related to a retrograde diagenesis event that altered the Mg-chlorites in some samples.
New geochemical and sulphur isotopic data are presented for a number of pyrite deposits from the Late Jurassic–Early Cretaceous Cameros Basin, Spain. The deposits were formed at, or close to, the peak of metamorphism and are always related to sandstone units in the mainly metapelite sequence. Iron remained immobile and conservative, pyrite iron being derived by sulphidation of chlorite in the host metapelites. Reduced sulphur, however, was supplied from two external sources: thermochemical reduction of sulphate and release of S during metamorphism of sedimentary sulphides. These sources provided isotopically heavy and light S, respectively, with variation in pyrite isotopic composition between different deposits resulting from differences in their relative importance at each site. During metamorphism, the sandstone units acted as aquifers, carrying the sulphidic pore waters to locations where permeability provided by syn-depositional fractures on a scale of 0.5–5 m allowed its interaction with the metapelites. Transport distances for sulphide during metamorphism were of the order of hundreds of metres.
Vein deposits containing native sulfur, gypsum, quartz and rare sphalerite are described from Cervera del Rio Alhama, in the very low-grade metasediments of the Mesozoic Cameros Basin of NE Spain. The veins are hosted by lacustrine evaporites which comprise alternations of dolomite and gypsum (anhydrite during metamorphism) layers. Fluid inclusion homogenization temperatures and quartz–sulfate oxygen isotope geothermometry indicate formation of the veins at ≈225°C. Fluid inclusions contain S° along with a gas phase comprising H2S, N2, CO2 and minor CH4. These are all likely reactants and products of S° generation by thermochemical sulfate reduction (TSR) by organic matter, followed by partial re-oxidation of some H2S by SO42− to produce S°. TSR-type reactions during low grade metamorphism are thus concluded to be the origin of the S° veins. The TSR reactions we described differ from those observed in most petroleum-related sour gas settings. Firstly, there is no evidence for secondary carbonate precipitation. Secondly, significant S isotopic fractionations exist between sulfate (around +20‰) and reduced products (S° is around −11‰). This is attributed to relatively increased rates of isotopic equilibration compared to TSR that may be related to low availability of organic matter during the formation.
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