Estudio comparativo de la evolución sedimentaria y diagenética de los litosomas carbonatados arrecifales (pre-rifting) de la Cuenca de Cameros. Kimmeridgiense. La Rioja-Soria

Durante el Kimmeridgiense inferior se desarrolló en la Cuenca Ibérica noroccidental una plataforma carbonatica somera en la que se desarrollaron abundantes construcciones arrecifales de coral pertenecientes a la Fm. Torrecilla en Cameros, que constituye frecuentemente la base sobre la que se apoyan los depósitos de la Cuenca finijurásica-eocretácica de Cameros. Esta Tesis Doctoral ha consistido en el estudio, y posterior comparación, de l evolución sedimentaria y diagenética de los depósitos de esta Formación en tres sectores distintos: El Sector del Sur del Moncayo situado al Sur, fuera del ámbito de la Cuenca de Cameros, y el Sector de Torrecilla situado al norte de la cuenca. La sedimentación de la Fm. Torrecilla en Cameros en estos tres sectores tuvo lugar en una plataforma carbonática de tipo rampa en la que predominaron las construcciones arrecifales de coral pero que era afectada por frecuentes tormentas. La sedimentación estuvo controlada tanto por la tendencia eutástica positiva predominante durante el Kimmeridgiense inferior, como por la tectónica. El tipo de facies predominante así como el desarrollo los procesos diagenéticos más tempranos ocurridos en cada uno de los sectores condicionaron en gran medida el desarrollo de la evolucion diagenetica posterior de la unidad arrecifal. En el Kimmeridgiense superior la Fm.Torrecilla en Cameros fue expuesta. Durante este periodo de tiempo la linea de costa de Tethys se situaba al SE en zonas ya relativamente alejadas a los tres sectores. Sin embargo la linea de costa del Mar Cantabrico se situaba muy cerca del Sector de Torrecilla. Durante el periodo de rifting finijurásico-eocretácido que dio lugar a la Cuenca de Cameros, la evolución diagenética de los sectores de Soria y Torrecilla fue similar y estuvo controlada por el enterramiento de la unidad arrecifal. Sin embargo en el Sector del Sur del Moncayo la unidad arrecifal sufrió un enterramiento mucho menor y la diagénesis estuvo todavía controlada por las aguas meteóricas. En en Cretácido medio-superior tuvo lugar el metamorfismo hidrotermal de la Cuenca de Cameros, al cual estuvo asociado la precipitación de anquerita barroca en los tres sectores estudiados. Durante el Cretácico terminal tuvo lugar en los sectores de Soria y Torrecilla un proceso de fracturación. A partir del Paleógeno y durante la compresión alpina se produjo el levantamiento tectónico de la unidad arrecifal en los tres sectores al cual estuvo asociado la precipitaciónde las mineralizaciones de fluorita, celestina, esfalerita y dolomita y la llegada de hidrocarburos en el Sector de Torrecilla. Durante esta etapa también se produjeron fracturas que fueron cementadas por calcita a temperaturas relativamente elevadas en los tres sectores. Finalmente, y tambien en los tres sectores estudiados, durante la exhumación la unidad arrecifal tuvo lugar la alteración y remplazamiento de la anquerita y de la calcita ferrosa, inestables bajo las nuevas condiciones meteórica

Comparison of the Calcareous Shells of Belemnitida and Sepiida: Is the Cuttlebone Prong an Analogue of the Belemnite Rostrum Solidum?

The microstructure of the rostrum solidum of Jurassic belemnites is compared with that of Sepia cuttlebones, in order to examine possible convergences in their style of growth. For this study, transmitted and polarized light, cathodoluminescence, epifluorescence, scanning electron and backscattered electron microscopy have been employed. Despite differences in the primary mineralogy of the studied belemnites and sepiids, calcite and aragonite, respectively, many similarities have been observed between the microstructure of the belemnite rostra and the prong of Sepia cuttlebone: (1) In both, crystals start growing from successive spherulites, from which crystals emerge radially towards the apex and the external walls, displaying internally micro-fibrous texture. (2) Both display concentric growth layering, comprising an alternation of organic-rich and organic-poor layers, which, in turn, is traverse by the radially-arranged micro-fibrous crystals. (3) The highest organic matter content and porosity have been observed along the apical area of the Sepia prong, similarly to that interpreted for belemnite rostra. The strong convergences observed suggest that the growth of belemnites occurred similarly to that of the prong of sepiids and that the Sepia prong is the analog of the belemnite rostrum. Additionally, non-classical crystallization processes are proposed to be involved in the formation Sepia endoskeleton

Depositional depth of laminated carbonate deposits: Insights from the lower Cretaceous Valdeprado formation (Cameros Basin, Northern Spain)

The Lower Cretaceous (Berriasian) Valdeprado Formation (Cameros Basin, northern Spain) contains more than 900 m of laminated carbonates and pseudomorphs after sulfates. Traditionally, many sedimentary packages of different ages and lithologies have been interpreted as deep-water deposits based essentially on the abundance of laminations and the absence of subaerial exposure features. In contrast, the Valdeprado Formation provides an example of a shallow-water deposit dominated by laminations with scarce evidence of subaerial exposure, and gives criteria to solve the challenge of distinguishing shallow-water and deep-water, ancient laminated deposits. The two most abundant facies all along the Valdeprado Formation are: a) parallel-laminated limestone, formed by alternating carbonate mudstone and calcite and quartz pseudomorphs after displacive gypsum, and b) graded-laminated limestone, consisting of quartz, mica, ostracodes, and pseudomorphs after detrital gypsum grains at the base, which changes gradually upwards to carbonate mudstone. Parallel-laminated limestone and graded-laminated limestone could have been deposited in either deep or shallow environments as a result of salinity fluctuations driven by alternation of flooding and evaporation and by sediment resuspension processes, respectively. Subaerial exposure features, such as desiccation mudcracks, are scarce in most of the succession, except in a few meter-scale stratigraphic intervals where they are very abundant. Interestingly, in these intervals desiccation cracks are present at the tops of several successive laminae (up to 25 mudcracked laminae per meter of deposit), indicating that, at least during those periods of time, deposition occurred in shallow water bodies that were desiccated frequently. In the upper part of the stratigraphic section, parallel-laminated and graded-laminated limestones are associated with current-ripple and wave-ripple cross-laminated arenites, and ostracode mudstone to wackestone with centimeter-size pseudomorphs after lenticular gypsum, and abundant desiccation mudcracks and tepees, which also suggest sedimentation in shallow-water environments. Moreover, the laminated carbonates display continuous, parallel layering, and the same facies along the 40-km-long outcropping area. These deposits are directly interbedded with, and pass laterally to, siliciclastic sandy–muddy flat deposits in the western area of the basin, without clinoforms, slump structures, or slide masses in between. All of these features suggest deposition in shallow, perennial carbonate–sulfate water bodies and their peripheral mudflats, developed in a flat-bottomed basin with no marked gradients

Las dolomías del Cretácico Superior del borde sur del Sistema Central: estudio preliminar

This study is focused on the carbonate units of the Late Cretaceous of the South margin of the Central System (Caballar: Castrojimeno and Burgo de Osma Formations). These units are totally or partially dolomitized, and they have been studied in order to interpret the timing and the diage- netic environment in which dolomites precipitated and to infer the mechanism for dolomitization. Petrographic and geochemical data of dolomites of the three units suggest that they precipitated from the same fluids despite the different sedimentary environments in which these units were deposited. Moreover, data suggest that dolomites precipitated via reflux of hypersaline brines proceeding from the overlying latest Cretaceous to Paleogene evaporitic units

Diagénesis temprana meteórica de la Formación Torrecilla en Cameros (Kimmeridgiense inferior; Preriff) y de los carbonatos de la base del GrupoTera (Tithónico; Sinriff) en el sector de Soria. Cuenca de Cameros. N. España

The last marine Jurassic episode of sedimentation in the Soria area corresponds to the deposition of the Torrecilla en Cameros Fm. during Lower Kimmeridgian. This formation is mainly composed by sandstones, coral reefs and oolites, and is overlain by the Tithonian continental sediments (clays, conglomerates, sandstone and lacustrine and palustrine limestones) of the Âgreda Afm. (Tera Croup). The limit between both units corresponds to an edaphic surface overlain by a ferruginous crust. Associated to the subaerial exposure of the Torrecilla en Cameros Fm. during Upper Kimmeridgian, and prior to the deposition of Âgreda Afm., marine aragonite was dissolved. Both primary and secondary porosity were then cemented by meteoric non-ferroan calcite. Isotopic composition of this calcite ranges between -3.7 and -7.3 ‰ in δ13C and -4.7 and -5.9 ‰ in δ18. Carbonates situated at the base of the Âgreda Afm. correspond to some layers of palustrine limestones as well as conglomerates, basically composed by lacustrine and oolitic pebbles. The isotopic composition of both palustrine limestones and limestone pebbles is very similar to the isotopic composition of the meteoric cement precipitated in the Torrecilla en Cameros Fm. This suggests that the climatic conditions and the isotopic composition of the meteoric waters were very similar during precipitation of meteoric calcite in both Torrecilla en Cameros Fm. and the base of the Âgreda Afm

Calcareous algae (dasycladales and charophytes), essential for the sedimentological interpretation of ancient coastal-lakes systems. The Barremian-Aptian Leza Fm., Cameros Basin, N Spain

Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu

‘Trapping and binding’: A review of the factors controlling the development of fossil agglutinated microbialites and their distribution in space and time

Trapping and binding of allochthonous grains by benthic microbial communities has been considered a fundamental process of microbialite accretion since its discovery in popular shallow-marine modern examples (Bahamas and Shark Bay). However, agglutinated textures are rare in fossil microbialites and, thus, the role of trapping and binding has been debated in the last four decades. Recently, renewed attention on this subject has produced new findings of fossil agglutinated microbialites (those mainly formed by ‘trapping and binding’ and analogous to modern examples), but they are still few and geologically recent (mainly post-Paleozoic) when compared to the 3.5 Gyr long record of microbialites. In order to better understand this discrepancy between modern and fossil examples, an extensive literature review is presented here, providing the first thorough database of agglutinated microbialites, which shows that all of them are formed in shallow-marine environments and most under tidal influence. In addition, a Lower Cretaceous example is described, including very diverse microbialites, each of them formed in a particular paleoenvironment. Some of these microbialites developed in grainy settings, but only those formed in marginal-marine tide-influenced environments accreted mainly by trapping and binding the surrounding grains, being analogous of modern agglutinated microbialites, and matching the environmental pattern observed in the literature database. The combination of the literature review with the case study allows to discuss the factors that control and enhance ‘trapping and binding’: a) occurrence of grains in the microbialite environment; b) frequent currents that mobilize the grains and supply them onto the microbialite surface; c) high concentration and diversity of electrolytes in the water to increase the adhesiveness of the extracellular polymeric substances (EPS) of the microbialite surface; and d) a CaCO3 saturation state not high enough to promote early and strong carbonate precipitation within EPS, which would eventually decrease its availability to adhere grains. Therefore, this review shows that the keys to solve the ‘trapping and binding’ debate may be environmental, because the conjuction of these hydrodynamic and hydrochemical parameters is preferentially achieved in shallow-marine settings and especially in those influenced by tides, at least since Mesozoic times. This explains the limited environmental and stratigraphic distribution of microbialites mainly formed by ‘trapping and binding’, and opens new ways to look, geologically and microbiologically, at this process, so often cited and yet so rare

Early Triassic-Anisian continental sediments from SE Iberian ranges: sedimentological and mineralogical features

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Strontium-isotope stratigraphy as a constraint on the age of condensed levels: examples from the Jurassic of the Subbetic Zone (southern Spain).

Condensed levels are often characterized by reworked fossils that may lead to incorrect age assessments. Strontium-isotope stratigraphy is an important chronostratigraphic tool that can be used to verify the biostratigraphic information from condensed beds. This paper describes a study of the 87Sr/86Sr isotope ratios of 56 belemnite samples collected from 28 stratigraphic sections of the boundary between the Upper Member of the Gavila´n Formation and the Zegrı´ Formation (Pliensbachian, Subbetic Zone). The petrographic and geochemical data (d18O, d13C, concentrations of Fe, Mn and Mg, and the Sr/Mn ratio) suggest that the belemnites have preserved their original marine geochemical composition. After plotting the samples in diagrams of 87Sr/86Sr values against time according to their biostratigraphic age, four different groups (A, B, C and D) were obtained with respect to the reference curve. In groups A and B, the age deduced from the Sr-isotope ratio is in total or partial agreement, respectively, with the biostratigraphic age; therefore the 87Sr/86Sr ratio is a good method for the dating, correlation and assessment of biostratigraphic results. In groups C and D, the SIS age and the biostratigraphic age do not coincide. A graphic procedure is presented as a suitable methodology to constrain the age of the samples showing an SIS age that differs from the relative age deduced (by biostratigraphy or stratigraphic correlation) for the bed they were collected in. These situations are interpreted as being the result of reworking of the belemnites (group C) or ammonites (group D) that are included in condensed levels. These condensed levels formed during the maximum flooding event that led to the drowning of the Gavila´n carbonate platform. The methodology supplied in this paper represents a valuable tool in identifying reworking processes, improving correlation and constraining biochronostratigraphic results. The values of 87Sr/86Sr represent a new contribution to the data set of 87Sr/86Sr ratios for the Pliensbachian