Bioestratigrafía del Silúrico (graptolitos) de la parte meridional de la Zona Centroibérica española

Abstract

Se analiza la bioestratigrafía de la sucesión silúrica en un vasto sector de la Zona Centroibérica meridional, a través del análisis de una treintena de secciones y localidades paleontológicas puntuales, seleccionadas entre más de 130 lugares con graptolitos silúricos. Se realizó un exhaustivo análisis bibliográfico a partir de los datos contenidos en 254 estudios que refirieron distintos aspectos del Silúrico en la región, lo que permitió revisar las unidades litoestratigráficas y precisar su correlación en las distintas estructuras variscas. El registro de graptolitos se circunscribe mayormente a las facies de pizarras negras vinculadas con fondos anóxicos entre el Telychiense y el Homeriense inferior. En casos puntuales se hallaron graptolitos del Aeroniense y el Ludfordiense en intercalaciones pizarrosas dentro de unidades dominadas por ambientes arenosos de alta energía. Se identificaron más de cien especies de graptolitos pertenecientes, al menos, a 32 géneros, de las que se ilustran algunas de las más importantes desde el punto de vista bioestratigráfico. Trece de las 15 biozonas y tres subzonas caracterizadas se distribuyen entre la base del Telychiense y el Homeriense basal. Las biozonas que registran una mayor diversidad de graptolitos, así como una amplia extensión en todo el ámbito del trabajo, son las de Oktavites spiralis y Torquigraptus tullbergi del Llandovery y la de Cyrtograptus perneri-Cyrtograptus ramosus del Wenlock. Las biozonas analizadas se relacionan bien con la sucesión bioestratigráfica del Silúrico perigondwánico tipificado por la sucesión del sinforme de Praga, en la República Checa.The biostratigraphy of the Silurian succession in a vast area of the southern Central Iberian Zone is analysed through the study of around thirty sections and isolated fossil localities, selected from over 130 sites containing Silurian graptolites. A thorough bibliographic analysis was carried out, based on data from 254 studies that analyzed various aspects of the Silurian in the region. This allowed for a review of lithostratigraphic units and the refinement of their correlation within the different Variscan structures. The graptolite record is largely restricted to black shale facies associated with anoxic bottoms, between the Telychian and the early Homerian. Occasionally, rare Aeronian and Ludfordian graptolite records may also occur within dark shale intercalations recorded in units dominated by high-energy sandstones. Palaeontologically, over one hundred species of graptolites were identified, belonging to at least 32 genera, with the most biostratigraphically significant species being illustrated. Thirteen of the 15 biozones and three subzones identified range from the base of the Telychian to the early Homerian. The biozones with the greatest diversity of graptolites, as well as the widest distribution across the study area, are those of Oktavites spiralis, Torquigraptus tullbergi and Cyrtograptus perneri−Cyrtograptus ramosus. All biozones studied herein allowed an accurate correlation with the Silurian biostratigraphic succession of peri-Gondwanan regions, as exemplified by the Prague Synform, Czech Republic, with the sole exception of the global Spirograptus guerichi Biozone at the base of the Telychian. In Bohemia, this biozone features unfavourable facies for graptolites in its lower part (Rastrites linnaei Biozone). From a biostratigraphic perspective, only one biozone is identified in the Aeronian (Lituigraptus convolutus Biozone), as pre-Telychian deposits are developed in almost exclusively sandy facies. The overlying black shales contain abundant Telychian graptolite assemblages, including the biozones of Spirograptus guerichi (with its three subzones: Paradiversograptus runcinatus−Monograptus gemmatus, Parapetalolithus palmeus, and Parapetalolithus hispanicus), Spirograptus turriculatus, Streptograptus crispus, Monoclimacis griestoniensis, Torquigraptus tullbergi, Oktavites spiralis, and Cyrtograptus lapworthi. In the Sheinwoodian black shales, the Cyrtograptus murchisoni, Monograptus riccartonensis, Monograptus belophorus−Cyrtograptus rigidus, and Cyrtograptus perneri−Cyrtograptus ramosus biozones are recognised. Finally, the Homerian and Ludfordian strata are represented by the Cyrtograptus lundgreni and Saetograptus leintwardinensis (?) biozones, which correspond respectively to the end of black sapropelitic deposition (diachronous in the study area), and to a local intercalation of this lithology within the sandy alternations in the upper part of the succession.