Person:
Fernández López, Sixto Rafael

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First Name
Sixto Rafael
Last Name
Fernández López
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Department
Geodinámica, Estratigrafía y Paleontología
Area
Paleontología
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Now showing 1 - 10 of 20
  • Publication
    Fossilization of ammonites and sedimentary events in deep environments of carbonate platform (highest Middle to lowest Upper Oxfordian, Iberian Range, Spain).
    (Università degli studi di Milano. Dipartimento di Scienze della Terra "A. Desio", 2004-03) Fernández López, Sixto Rafael; Meléndez Hevia, Guillermo
    In the Ricla area (Zaragoza, Aragonese Branch of the Iberian Range), at the top of the Yátova Formation, grey-reddish wackestone limestones grade into yellow-green siliciclastic limestones of the Aldealpozo Formation. These changes of facies between the two successive formations are associated with syndepositional palaeoreliefs developed during the Late Oxfordian. The uppermost deposits of the Yátova Formation represent an Oxfordian condensed section, from the upper Bifurcatus Zone (Middle Oxfordian) and Hypselum Zone (Upper Oxfordian). These deposits are interpreted as developed in an open marine, moderately deep carbonate platform, showing uniform lowenergy conditions with extremely reduced carbonate and terrigenous background sedimentation, and very low sedimentation rates. The low diversity of the benthic fauna, scarce development of sponge bioherms and ammonite populations inhabiting the platform are palaeobiological criteria which corroborate these palaeoenvironmental conditions. Ammonite assemblages are composed of Sub-Mediterranean taxa. Over 900 ammonite specimens have been collected from the upper Bifurcatus and Hypselum zones. Oppeliidae (45,2%) and Perisphinctidae (37,9 %) are dominant. Aspidoceratidae (14,3%) are common. Haploceratidae (2,2%) are scarce. Two phylloceratids and a lytoceratid have been found. Ammonoids are commonly preserved as concretionary calcareous internal moulds of reelaborated elements. Resedimented shells are scarce. The degree of packing of ammonite remains and the stratigraphical persistence display high values. Taphonomic features indicative of sedimentary starving in deep carbonate platform environments are: 1) high concentrations of reelaborated ammonites, 2) taphonic population of type two, 3) phragmocones completely filled with sediment, and 4) homogeneous concretionary internal moulds, bearing no signs of abrasion, bioerosion or dense encrusting by organisms (such as serpulids, bryozoans or oysters). In conclusion, the occurrence of these ammonite associations confirms the development of an advanced deepening phase, within a 3rd order deepening/shallowing cycle, in the Aragonese platform, during the late Bifurcatus to Hypselum zones.
  • Publication
    Taphonomy of an ammonite recorded association (Murchisonae Zone, Aalenian, Iberian Range, Spain). Palaeoenvironmental implications.
    (International Subcommission on Jurassic Stratigraphy, 2002) Fernández López, Sixto Rafael; Martínez Gutiérrez, Gemma; Martire, Luca
  • Publication
    Aalenian Tmetoceras (Ammonoidea) from Iberia: taxonomy, habitats and evolution.
    (Kluwer, 1999) Fernández López, Sixto Rafael; Henriques, María Helena Paiva; Linares Rodríguez, Asunción; Sandoval Gabarrón, José; Ureta Gil, María Soledad; Olóriz Aguilera, Federico; Rodríguez Tovar, Francisco J.
    Several hundred Aalenian Tmetoceras from the Iberian Peninsula (N Lusitanian Basin, Asturias, Basque-Cantabrian Basin, NE Cameros, NW Iberian Basin, Aragonese Platform, Tortosa Platform, Castilian Platform, Majorca and Betic Basin) have been reviewed. Two species of Aalenian Tmetoceras have been identified on the basis of morphological, biochronological and palaeobiogeographical data: T. scissum and T. regleyi. T. scissum was dominant among the Tmetoceras populations developed in the Betic and Lusitanian basins during Opalinum, Murchisonae and Bradfordensis biochrons. Populations composed by evolute individuals of T. scissum inhabited shelfal or oceanic environments. A chronocline, from rectiradiate and primitive forms to flexicostate and derived forms, can be recognized in these populations of T. scissum. In contrast, shallow epicontinental platforms were inhabited by involute individuals of T. regleyi. This second species was phyletically derived from T. scissum, representing an adaptive radiation from populations of shelfal or oceanic basins to populations of epicontinental platforms.
  • Publication
    Criterios elementales de reelaboración tafonómica en ammonites de la Cordillera Ibérica
    (Instituto Nacional de Geología (España). Instituto Jaime Almera, 1984) Fernández López, Sixto Rafael
    Algunos moldes internos de ammonites, reelaborados, que están contenidos en calizas del Jurásico medio de la Cordillera Ibérica, muestran varias propiedades conservativas; los procesos de reelaboración tafonómica, por lo tanto, no deberían ser considerados como destructivos. Esta conclusión puede ser sacada de varios criterios observados y discutidos en este artículo. [ABSTRACT] Some ammonite steinkerns, reworked, that are contained in limestones from the Middle Jurassic Iberian Range, show several preservational features; processes of this kind therefore should not be called destructive. This conclusions can be draw from several criteria observed and discussed in this paper.
  • Publication
    Taphonomy of ammonite condensed associations. Jurassic examples from carbonate platforms of Iberia
    (Geologische Bundesanstalt., 1999) Fernández López, Sixto Rafael; Henriques, María Helena Paiva; Duarte, Luís Vitor
  • Publication
    Diagnóstico paleontológico de ciclos paleoambientales en plataformas epicontinentales carbonáticas. Ejemplos del Jurásico Medio de la Cordillera Ibérica
    (Alcalá de Henares (Madrid): Museo Arqueológico Regional, 2004) Fernández López, Sixto Rafael
    Aunque parece haber un acuerdo general entre los geólogos en cuanto a que el registro estratigráfico ha estado influenciado por ciclos paleoambientales, pocos estudios hacen referencia a la naturaleza del registro fósil y su carácter cíclico. Sin embargo, los ciclos estratigráficos representan fluctuaciones paleoambientales que también han influido en las entidades paleobiológicas y en la conservación de sus restos y señales. Por analogía con el modelo desarrollado en Estratigrafía Secuencial, la Paleontología puede proporcionar un modelo referente a procesos de distintas escalas representados por ciclos tafonómicos y ciclos paleobiológicos. Los conocimientos paleontológicos pueden ser aplicados en las investigaciones estratigráficas para identificar los ciclos y las discontinuidades del registro estratigráfico. De manera análoga, los conocimientos estratigráficos proporcionan un sistema conceptual en el que es posible contrastar los ciclos paleontológicos y las discontinuidades del registro fósil. La Estratigrafía Secuencial y la Paleontología Aplicada pueden desarrollar un sistema conceptual, integrado y predictivo, para interpretar y contrastar el carácter cíclico del registro geológico y los ciclos paleoambientales, analizando independientemente la ciclicidad del registro estratigráfico y la del registro fósil.
  • Publication
    Evolution tectono-sedimentaire et genese des associations d'ammonites dans le secteur central du Bassin Iberique (Espagne) pendant l'Aalenien.
    (Assoc. des Facultés Catholiques de Lyon, 1990) Fernández López, Sixto Rafael; Gómez Fernández, Juan José
    A la limite Lias-Dogger, le Bassin Ibérique correspondrait a une marge passive se disloquant en panneaux montrant des facies différents. Un des meilleurs exemples du comportement différentiel des blocs limités par des failles actives se trouve dans le secteur central du bassin, où la Faille de Noguera-Aguaton individualisait deux secteurs : celui d' Albarracin et celui de Menera. La discontinuité stratigraphique de plus grande amplitude se situe a la limite Toarcien-Aalénien dans le secteur d'Albarracin et a l'intérieur de l' Aalénien dans le secteur de Menera. Le changement paléogéographique le plus important pour la plate-forme se produit a la limite de l'Aalénien inférieur/Aalénien moyen . La sédimentation de ces secteurs correspond a un environnement de plate-forme externe tres peu profonde, soumise a des émersions locales répétées et les associationss uccessives d'ammonites sont surtout le résultat de nécrocinese régionale a partir d'aires épicontinentales plus septentrionales. [ABSTRACT] At the Lias-Dogger boundary the Iberian Basin corresponded to a passive margin where different facies conditioned by the presence of several faulted blocks were developped. One of the best examples showing the synsedimentary differential behaviour of the fault-bounded blocks was found in the central sector of the basin, where the Noguera-Aguaton Fault individualized two blocks : the Albarracin and the Menera ones. The most important stratigraphic discontinuity in the Albarracin Sector is located in the limit between the Toarcian and the Aalenian. However in the Menera Sector the most important stratigraphic discontinuity is located within the aalenian sediments. The biggest paleogeographical change in the platform occurred in the boundary between Early and Middle Aalenian. Sedimentation in this sectors corresponds to a very shallow external platform environment with repeated local emersions. The successive ammonite associations were mostly the result of regional nekrokynesis from septentrional epeiric seas
  • Publication
    Dispersión biogeográfica y tafonómica de ammonoideos filoceratinos hacia la Cuenca Ibérica durante el Jurásico Medio
    (Universidad Complutense de Madrid, Servicio de Publicaciones, 1994) Fernández López, Sixto Rafael; Meléndez Hevia, Guillermo
    Los Phylloceratina (Filoceratáceos y Litoceratáceos) representan menos dell% de los ammonoideos en las asociaciones registradas del Jurásico Medio de la Cordillera Ibérica. Sin embargo, su presencia es relativamente constante en los materiales del Bajociense superior y del Calloviense inferior/medio. Estos dos episodios de dispersión de filoceratinos hacia la Cuenca Ibérica representan cambios en su comportamiento paleoecológico y tafonórnico debido a cambios relativos del nivel del mar durante el Jurásico Medio. El aumento relativo del nivel del mar durante el Bajociense superior favoreció la dispersión biogeográfica de algunas especies y la llegada ocasional de filoceratáceos inmigrantes. Algunos individuos juveniles dePhylloceras y Adabofoloceras llegaron a la Cuenca Ibérica, pero no hay evidencias de que encontraran condiciones favorables para su desarrollo, y representan individuos miodémicos. En cambio, los filoceratinos de las asociaciones registradas en los materiales del Calloviense inferior/ medio corresponden en su mayoría a individuos adultos. El descenso relativo del nivel del mar durante el Calloviense inferior/medio, asociado al incremento en la homogeneidad del relieve del fondo de la Cuenca Ibérica, aumentó la accesibilidad de las conchas de ammonoideos por deriva necroplanctónica y favoreció la concentración de restos de filoceratinos adémicos procedentes de otras áreas marinas más abiertas u oceánicas. [ABSTRACT] Phylloceratid ammonoids (Phyllocerataceae and Lytocerataceae) in the Middle Jurassic of the lberian Chain represent less than 1% of the whole spectra of ammonoidea. Their presence is, however, remarkably constant in two intervals, at the upper Bajocian and at the lower/middle Callovian boundary. These two separa te episodes of dispersa} for representatives of Phylloceratina towards the lberian Basin are regarded to represent meaningful changes in theirpalaeoecological and taphonomical behavior, as a consequence of regional sea-level changes during the Middle Jurassic. A relative sea-level rise during the upper Bajocian favored the biogeographic dispersa! of sorne species and the occasional arrival of immigrant filocerataceans. Sorne juvenile individuals ofPhylloceras and Adabofoloceras reached the lberian Basin, though no evidence that they could develop and evolve there can be found, so they are rather regarded as miodemic individuals. On the other hand, phylloceratid associations recorded at the lower/middle Callovian boundary, mainly formed by adult, polyspecific assemblages, had a very different origin, resulting from a relative sea-level fall and to a general homogenization of the shelf bottom, which led to an in crease of the accessibility of ammonite shells by nekroplanktonic drift, hence favoring the concentration of ademic individuals from more open to oceanic marine areas.
  • Publication
    Dimorphism and evolution of Albarracinites (Ammonoidea, Lower Bajocian) from the Iberian Range (Spain)
    (The Trustees of the Natural History Museum,, 2013-12) Fernández López, Sixto Rafael
    Several tens of specimens of Lower Bajocian Albarracinites (type species A. albarraciniensis Fernandez-Lopez, 1985), including microconchs and macroconchs from the Iberian Range, have been studied. This ammonite genus ranges in the Iberian Range from at least the Ovale Zone to the uppermost Laeviuscula Zone of the Lower Bajocian (Middle Jurassic). The macroconch counterpart is thought to be a group of stephanoceratids previously attributed to Mollistephanus, Riccardiceras and other new forms described in this paper. Two chronologically successive species of Albarracinites have been identified: A. albarraciniensis and A. submediterraneus sp. nov. The evolution of the Albarracinites lineage represents a hypermorphic peramorphocline starting from depressed, small and slender serpenticones of A. westermanni, to larger planorbicones with more cadiconic phragmocones and body chamber of subcircular cross section belonging to A. submediterraneus sp. nov., through A. albarraciniensis Fernandez-Lopez. In contrast, Mollistephanus planulatus (Buckman), M. cockroadensis Chandler & Dietze and M. mollis Buckman represent a peramorphocline by acceleration, producing adults of similar size but more compressed and with increasing ontogenic variation of shell ornament. Albarracinites and Mollistephanus subsequently developed two opposite peramorphoclines or gradational series of morphological changes undergoing greater development and ontogenic variation. These two genera show diverse palaeobiogeographical distributions too. Albarracinites is rarely recorded in the Mediterranean and Submediterranean from the Discites to the Laeviuscula Zone, whereas Mollistephanus is more common in north-western Europe and other biochoremas of the western Tethys from the Discites Zone to the Sauzei Zone. Albarracinites seems to be the earliest stephanoceratid lineage in western Tethys, branching off from the otoitid Riccardiceras by proterogenetic change and resulting in paedomorphosis at the Aalenian/Bajocian boundary.