Unravelling the microfacies signatures of parasequences using computer-optimized similarity matrices

The microfacies of a Lower Cretaceous carbonate drillcore from Oman are characterized using optimizing matrices of Jaccard's similarity coefficients of community. Other than systems tract boundaries, there is no obvious evidence of individual parasequences in the core. However, diagnostic patterns in microfossil distribution identify environmental gradients recording changes in water depth. These gradients are used to define individual parasequences, parasequence sets, stacking patterns and key surfaces. The patterns suggest that deposition was controlled by successive fourth- to fifth-order (high-frequency) relative sea-level cycles superimposed on an underlying third-order relative sea-level rise. Although the correlation of these depositional subunits to systematic changes in water depth and the rate of carbonate accumulation alone is not incontrovertible proof of such a sea-level control, concurrent multiorder relative sea-level cyclicity provides by far the most likely explanation. A microfacies deposited when the water depth was shallowing is characterized by a relay of microfossils with affinities that shallow upwards. Conversely, a microfacies that records a gradual increase in water depth has a relay of microfossils with affinities that deepen upwards. Microfacies characterized by an assemblage of microfossils with similar affinities record deposition when the benthic environmental conditions remained stable, either because of an equilibrium between shallow water carbonate deposition and rising sea level, or in deeper water where sediment composition was relatively insensitive to changes of water depth. Microfacies characterized by mixed affinity assemblages record syndepositional reworking. During periods of embedded multiorder sea-level changes, individual parasequences within systems tracts are shown to record more complex environmental gradients than simply the repetition of successive shallowing-up units as traditionally represented in carbonate sequence stratigraphic models. The microfacies of an individual parasequence may shallow up, or may record both deepening-up and shallowing-up depositional phases, as well as periods of sedimentation when benthic environments remained stable. Individual parasequence boundaries may be submarine or subaerial unconformities, or be conformable, as part of a predictable stratigraphic pattern related to the temporal position of an individual parasequence within the underlying third-order cycle of relative sea-level change. The hitherto ubiquitous use of assemblages to describe carbonate microfacies, coupled with the widespread use of the metre-scale shallowing-up template to identify parasequences, may have led to such complexities previously being overlooked

The three last Upper Aptian cycles of the Serdj Formation.

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Aplicación de análisis micropaleontológicos cuantitativos en estratigrafía secuencial : el Cretácico inferior de la cuenca de Organyà (Pirineos, España)

El análisis cuantitativo de los componentes fosilíferos de las microfacies en rocas carbonatadas se manifiesta como una herramienta útil para reconocer ciclos sedimentarios de diferente orden que pasarían desapercibidos en análisis únicamente cualitativos. El método cuantitativo utilizado se basa en el recuento de todos los organismos fósiles presentes en la misma superficie de cada lámina delgada o equivalente, los cuales son clasificados según su biotopo particular en función de su situación en la plataforma. Ello permite cuantificar las variaciones del medio por pequeñas que éstas sean e interpretar el aumento o disminución de la lámina de agua en una plataforma, así como otras características del medio, como el tipo de sustrato. El método se ha aplicado al estudio de los sedimentos urgonianos de la Cuenca de Organyà (Barremiense-Aptiense inferior), tanto de secuencias deposicionales mayores como de secuencias elementales

Stratigraphic distribution of Valanginian-Early Aptian shallow-water benthic foraminifera and algae, and depositional sequences of a carbonate platform in a tectonically-controlled basin: the Organyà Basin, Pyrenees, Spain.

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Aplicación de análisis micropaleontológicos cuantitativos en estratigrafía secuencial : el Cretácico inferior de la cuenca de Organyà (Pirineos, España)

El análisis cuantitativo de los componentes fosilíferos de las microfacies en rocas carbonatadas se manifiesta como una herramienta útil para reconocer ciclos sedimentarios de diferente orden que pasarían desapercibidos en análisis únicamente cualitativos. El método cuantitativo utilizado se basa en el recuento de todos los organismos fósiles presentes en la misma superficie de cada lámina delgada o equivalente, los cuales son clasificados según su biotopo particular en función de su situación en la plataforma. Ello permite cuantificar las variaciones del medio por pequeñas que éstas sean e interpretar el aumento o disminución de la lámina de agua en una plataforma, así como otras características del medio, como el tipo de sustrato. El método se ha aplicado al estudio de los sedimentos urgonianos de la Cuenca de Organyà (Barremiense-Aptiense inferior), tanto de secuencias deposicionales mayores como de secuencias elementales

Interpretation of gamma-rays logs: the distribution of Uranium in carbonate platform.

International audienceHigh gamma-radioactivity in carbonates is usually ascribed to uranium of detrital minerals and organic matter, and to thorium and potassium of clays. The present study based on Urgonian marls and marly limestones (France) shows that some of the most radioactive values correspond instead to some ‘pure' limestones. These peaks are generally associated with a sequence boundary or a maximum flooding surface. Low-level γ-spectrometry and ICP–MS analyses show that although high radioactivities are mostly associated with uranium, there is no obvious correlation between uranium enrichment and lithology. Also, correlation between high radioactivity and argillaceous beds might not be systematic