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

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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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

Evolution of the Urgonian shallow-water carbonate platform on the Helvetic shelf during the late Early Cretaceous

International audienceUrgonian platform carbonate (Cretaceous; Barremian, Aptian) forms an important lithostratigraphic unit in the Helvetic fold- and thrust unit of the northern Swiss Alps. Its widespread distribution and ubiquity allow for an integrated high-resolution study of macro- and microfacies, benthic foraminiferal biostratigraphy, sequence stratigraphy, and carbon-isotope and phosphorus records. The resulting data confirm the importance of environmental forcing and sea level change on the style of carbonate production and accumulation along the margin of the northern Tethys during the late Early Cretaceous. Stratal geometries, the succession of microfacies, the identification of major emersion surfaces, and biostratigraphy observed and analysed in twelve sections through the inner, middle, and outer platform, and in the panorama of the Churfirsten range, permit subdivision of the analysed succession into eight depositional sequences. The succession starts with a phase of sedimentary condensation (Altmann Member (Mb); late Hauterivian - late early Barremian; sequences H7, H8, and B1), followed by the deposition of hemipelagic sediments (Drusberg Mb; restricted to sequence B2 of the late early Barremian on the inner platform; and covering the late early Barremian to the middle late Barremian on the outer platform, and to the early Aptian on the outer shelf, thereby showing an important diachroneity of its upper boundary related to the inception and progradation of the Urgonian platform), and the development of predominantly lagoonal carbonate (Schrattenkalk Formation (Fm); early late Barremian - early Aptian; sequences B3, B4, B5, and A1). The Schrattenkalk Fm documents important progradation and aggradation of the carbonate platform, and the change from a ramp-like to flat-topped geometry. The oldest, allochthonous remains of the shallow-water carbonate platform were identified intercalated in and on top of the lower Barremian Drusberg Mb (sequence B2). Sequence boundary (SB) B3 resulted from an important regressive phase near the early-late Barremian boundary, which led to the emersion of the hemipelagic sediments of the Drusberg Mb in the inner part of the shelf and to the deposition of a lowstand systems tract at the base of the Lower Schrattenkalk Mb (late Barremian; sequences B3-5) in intermediate and distal domains. Deposition of in-situ platform carbonate started during the following transgressive phase in the middle late Barremian, which flooded the entire investigated area. The associated faunal assemblages and phosphorus contents indicate a concomitant increase in nutrient input, which led to a mixed photozoan-heterozoan platform association dominated by annelids and flat orbitolinids, and the formation of a condensed phosphate-rich bed on the outer shelf (Chopf Bed; middle late Barremian). The subsequent sea-level highstand allowed for the deposition of the first typical Urgonian carbonates rich in corals and rudists. This depositional sequence (B3) terminated by the important infilling of accommodation space combined with sea-level fall of at least 15 m. Later on, close to the Barremian-Aptian boundary, a further, major emersion phase (SB A1) was triggered by sea-level fall, estimated here as at least 16 m, which terminated this first phase in the deposition of rudist and coral-rich platform carbonates covering the middle late to latest Barremian (B3-B5). The overlying Rawil Mb (lowermost Aptian; transgressive systems tract A1) resulted from progressive deepening and document a phase of increasing eutrophication of the depositional environment, resulting in a mixed siliciclastic-carbonate platform build-up, characterized by sea-grass facies and the massive occurrence of Palorbitolina lenticularis. The overlying Upper Schrattenkalk Mb (lower Aptian; highstand systems tract A1) records recovery of the rudist-rich photozoan Urgonian platform. Its subsequent demise occurred well before the Selli oceanic anoxic episode (OAE 1a). It was initiated by emersion of the platform due to high-amplitude sea-level fall (SB A2), followed by eutrophication during the subsequent transgressive phase. The carbon-isotope records show an increase towards more positive values during the Lower Schrattenkalk Mb and the base of the Rawil Mb, interrupted in most sections by an excursion to lower values near the Barremian-Aptian boundary. A shift to lower values occurred also in the uppermost part of the Rawil Mb, followed by variable trends in the Upper Schrattenkalk Mb. These long-term trends are well correlated with the basinal record (Angles, La Bédoule). Deviations in the correlations are related to the influence of facies and microfacies, primary mineralogy, emersion phases, and post-depositional alteration. Figs. S13-S18 High-resolution compilations of stacked photos of each segment of the panorama across the Churfirsten, together with their interpretation with regards to facies and sequence stratigraphy. Legend in Fig. 21. Figs. S19-S30 Aerial photographs of the east side of the Churfirsten panorama provided by Hanspeter Funk (Baden), together with their sequence-stratigraphic interpretation. Fig. S31 δ18O - δ13C scatter plots of the whole-rock isotope analyses of all sections analysed. Fig. S32 Lithological logs, sequence-stratigraphic interpretations, environmental indicators, and isotope stratigraphy for the sections at Cluses (redrawn from Wermeille (1996) for the log and from Huck et al. (2013) for the isotope data), Le balcon des Ecouges (from Raddadi (2005) for the log and the data of environmental indicators; from Embry (2005) for the isotope data), Les gorges du Nant (from Raddadi (2005) for the log and the data of environmental indicators; from Arnaud-Vanneau (unpublished data) for the component counting; from Bastide (2014) for the isotope data)