Publication: Laminación y bioturbación en carbonatos lagunares: interpretación genética (Cuenca del Guadiana, Badajoz)
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Publication Date
1986
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Consejo Superior de Investigaciones Científicas, Museo Nacional de Ciencias Naturales
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Abstract
El estudio sedimentológico al microscopio y en secciones pulidas de las muestras de la unidad de carbonatos laminados (1 a 2 m de espesor), situada a techo de la sucesión miocena, que rellena la cuenca neógena del Guadiana, revela que posee notables singularidades. La laminación se aprecia en muestra de mano y resulta de una alternancia de niveles masivos y laminados. En lámina delgada los niveles masivos constan de partículas intraclásticas, ooidales y peletoidales, mientras que los laminados presentan una alternancia de láminas claras y oscuras suavemente onduladas. Dentro de éstas y, en menor medida, en los niveles masivos pueden reconocerse: (a) numerosos vestigios de filamentos algales, (b) abundantes restos de caráceas, (e) pequeñas cavidades de unas 250 micras de diámetro, generalmente agrupadas, y otras mayores que contienen pellets, apareciendo ambos tipos en relación con bioturbación; (e) prismas dispersos de calcita semejantes a Microcodium, y (e) agrupaciones de cristales trapezoédricos de calcita de secciones rómbicas y lenticulares que son más abundantes en los niveles masivos. La interpretación sedimentológica de estos depósitos refiere una sedimentación en áreas encharcadas con oscilaciones periódicas del nivel del agua. Los niveles laminados, de naturaleza estromatolítica, corresponden a las etapas más secas y se forman bajo láminas de agua en ambientes tranquilos, mientras que los niveles masivos responden a un mayor influjo de agua a la cuenca que supone condiciones de energía más alta capaces de removilizar el material fragmentado por desecación en las etapas secas. La actividad vital de insectos Quironómidos produjo dos tipos de bioturbación: pequeñas cavidades arracimadas que corresponden a las puestas y eclosión de huevos y galerías mayores parcialmente rellenas por pellets fecales que registran la actividad minadora de las larvas.
The sedimentological study of both polished and thin sections of samples taken from the 1-2 m thick unit of laminated carbonates overlying the Miocene succession ínfilling the Neogene Guadiana Basin (western Spain) reveals the existence of prominent features tbat are discused in the papero. The lamination visible in hand specimens is due to the alternance of massive and laminated layers. In thin sections, the massive layers are made of intraclastic, ooidal pelletoidal particles, whereas tbe laminated ones show gently undulating, alternating light and dark-coloured layers. ]nside the laminated and, to a lesser extent, the massive leyers it is possible to recognize: (a) abundant vestiges of algal filaments, (b) abundant remains of ebaropbites, (e) small, usua1ly grouped-up, cavities (mean diameter about 250 mieroos) and bigger, pellet·filled avities, both types related lo burrowing; (d) sparse prisms of caleite resembling those of Microcodium, and (e) clusters of trapezohedral crystaIs of calcite with rhomboidal and lenticular sectioos, these being more abundant in the massive layers. The sedimentologica1 interpretation assumes that sedimentation took place in swamps with periodic osciUations of tbe water level. The laminated layers, of stromatolithic nature, correspond to quiet realms under constant water level during dry stages. The massive layers record higher environmental energy during flood stages, that is able to remobilize tbe cracked sediments of the former dry stages. The vital activity of Chironomid insects lead to the two described types of burrowing: the small grape-like cavities correspond to eclosing eggs whereas tbe bigger pellet-filled cavities resulted from mining by larvae.
The sedimentological study of both polished and thin sections of samples taken from the 1-2 m thick unit of laminated carbonates overlying the Miocene succession ínfilling the Neogene Guadiana Basin (western Spain) reveals the existence of prominent features tbat are discused in the papero. The lamination visible in hand specimens is due to the alternance of massive and laminated layers. In thin sections, the massive layers are made of intraclastic, ooidal pelletoidal particles, whereas tbe laminated ones show gently undulating, alternating light and dark-coloured layers. ]nside the laminated and, to a lesser extent, the massive leyers it is possible to recognize: (a) abundant vestiges of algal filaments, (b) abundant remains of ebaropbites, (e) small, usua1ly grouped-up, cavities (mean diameter about 250 mieroos) and bigger, pellet·filled avities, both types related lo burrowing; (d) sparse prisms of caleite resembling those of Microcodium, and (e) clusters of trapezohedral crystaIs of calcite with rhomboidal and lenticular sectioos, these being more abundant in the massive layers. The sedimentologica1 interpretation assumes that sedimentation took place in swamps with periodic osciUations of tbe water level. The laminated layers, of stromatolithic nature, correspond to quiet realms under constant water level during dry stages. The massive layers record higher environmental energy during flood stages, that is able to remobilize tbe cracked sediments of the former dry stages. The vital activity of Chironomid insects lead to the two described types of burrowing: the small grape-like cavities correspond to eclosing eggs whereas tbe bigger pellet-filled cavities resulted from mining by larvae.