Fernández López, Sixto Rafael

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First Name
Sixto Rafael
Last Name
Fernández López
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Geodinámica, Estratigrafía y Paleontología
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Now showing 1 - 6 of 6
  • 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
    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
    Abrasion surfaces on internal moulds of ammonites as palaeobathymetric indicators
    (Elsevier, 1994) Fernández López, Sixto Rafael; Meléndez Hevia, Guillermo
    Some abrasion surfaces displayed by internal moulds of ammonites, such as annular abrasion furrows formed on the venter surface and ellipsoidal abrasion facets preferentially developed on the last third part of the preserved whorl, are diagnostic criteria of taphonomic reworking. These taphonomic criteria indicate that the internal moulds have been exhumed and moved before their final burial. We explain the origin of these abrasion surfaces by the action of directional (non oscillatory) currents in very shallow water, most probably in intertidal environments. The distribution of ammonite moulds displaying such preservational features is correlated with the changes in the facies and the trend of the sedimentary sequences. Taphonomic analysis of Jurassic ammonites of the Iberian Range underlines the importance and usefulness of taphonomic gradients in sequence stratigraphy and palaeogeographic reconstructions.
  • Publication
    Trimarginia and Trimarginites (Ammonoidea) from the Iberian Basin.
    (Geologische Bundesanstalt., 1999) Fernández López, Sixto Rafael; Meléndez Hevia, Guillermo
  • Publication
    Trimarginia and trimarginites (Ammonoidea) from the Iberian basin.
    (Société géologique de France, 1997) Fernández López, Sixto Rafael; Meléndez Hevia, Guillermo; Enay, Raymond
  • Publication
    Taphonomic features of ammonite associations developed in deep carbonate platform environments (Oxfordian, Iberian Range, Spain).
    (International Association of Sedimentologists, 2004) Fernández López, Sixto Rafael; Meléndez Hevia, Guillermo; Pena dos Reis, Rui; Callapez, Pedro; Dinis, Pedro