Publication: Interpretación sedimentaria de las calizas de crinoides del Carixiense Subbético
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1985
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Universidad de Alicante. Departamento de Geología
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Abstract
Las calizas de crinoides del Carixiense del sector central de la Zona Subhética se formaron en bajíos dentro de una gran plataforma carbonatada adyacente al Macizo Hespérico, en el fondo del gran golfo que dibujaba el Tethys. A partir del análisis de las asociaciones y secuencias de texturas y estructuras sedimentarias primarias se deduce que la evolución paleogeográfica durante el Sinemuriense y Carixiense refleja una progresiva disminución de la profundidad relacionada con causas tectónicas que controlan también la morfología y distribución de las zonas palcohatimétricas. En las calizas de crinoides se diferencian dos subfacies. La inferior presenta rasgos que permiten asimilarla a apilamientos de sand waves mareales que se reactivan periódicamente. La reversión de las corrientes produce migraciones de las corrientes en sentido opuesto, aunque normalmente tiende a reflejarse sobre todo la corriente dominante; en ese caso, la subordinada produce superficies erosivas y, a veces, ripples y megaripples. Cuando vuelve a fluir la corriente dominante se reactivan los megaripples (sand waves) a partir de estas superficies. Otro rasgo mareal característico es la alternancia de láminas calcareníticas con otras pelmicríticas cuyo espesor varía regularmente durante la fase de migración activa que se supone coincidente con la marea viva. En la marea muerta los megaripples migran más lentamente (degeneración) y sufren retrabajado por el oleaje y los organismos. La estructura interna de la subfacies superior es estratificación cruzada en surco y laminación paralela y se atribuye a un aumento del efecto del oleaje sobre el fondo que se superpone a la acción de las corrientes de marea. Esta asunción viene apoyada por la mayor variedad de direcciones de migración de los megaripples. Se propone un modelo de bajío carbonatado con campos de sand waves y de megaripples que migran bajo la acción de la corriente de marea dominante en cada zona del bajío. Se asume que la mayor exposición al oleaje se produciría en el borde enfrentado al segundo cuadrante (SE) de donde vendrían los oleajes dominantes generados por tormentas, huracanes e incluso parte de los vientos diurnos. En esa parte se formarían barras que protegerían el bajío de los embates del oleaje, pasando sobre ellas sólo pequeñas olas que generarían ripples de oscilación de varias escalas y oriemaciones.
The early Jurassic crinoidal limestones of central Subbetic Zone formcd in shoals of a large shallow carbonate shelf placed in the inner part of ¡he broad gulf that Thethys Ocean was al that time, next to the emergem Hesperic Massif. From the study of associations and sequences of sedimentary textures and structures, a general shallowing-upward sequence can be deduced. A tectonic control is assumed for this sequence and also fm the regional paleobathimetric distribution. Two subfacies have been differentiated in the crinoidallimestones. The lower one corresponds to piles of sand waves that formed, migrated and degenerated periodically along with the tidal cycle. The reversion of flow is reflected in opposite-directed sets of cross bedding. However, most of the cross-bedded sets appear to be unidirectional due to the dominant tidal current; in this case, the subordinate current produces erosional surfaces and ripples or megaripples. As the dominant current sets again, reactivation of the sand wave takes place. A most typical tidal feature is the formation of bundles of calcarenitic laminae representing the avalanche under current action that alternate with a somewhat thicker pelmicrite laminae representing settling of fines from suspension during slack-water periods. The relative thickness of such an arrangement changes regularly during the active migration of the sand wave during springs tides. Low rates of migration at neap tides (degeneration stage:. of sand waves) result in wave and biogenic reworking. The internal structure of the upper subfacies consists of trough cross bedding and parallel lamination as a result of increasing wave effect upon the surface of the shoal that superimposes on the tidal currents. This assumption is supported by the higher variability of paleocurrent directions. A model of shallow carbonate shoal with tidal sand waves and megaripples of various types is proposed. It is assumed that the highesl wave action ocurred along the SE margin of the shoal, facing the prevailing waves generated by storms, hurrkanes and, probably, sorne of the diurnal winds. Calcarenite/oolite (?) bars are thought to have occurred along these areas shielding the shoal and giving a chance for the tidal features to form under limited wave reworking that resulted in wave ripples of various scales and orientation.
The early Jurassic crinoidal limestones of central Subbetic Zone formcd in shoals of a large shallow carbonate shelf placed in the inner part of ¡he broad gulf that Thethys Ocean was al that time, next to the emergem Hesperic Massif. From the study of associations and sequences of sedimentary textures and structures, a general shallowing-upward sequence can be deduced. A tectonic control is assumed for this sequence and also fm the regional paleobathimetric distribution. Two subfacies have been differentiated in the crinoidallimestones. The lower one corresponds to piles of sand waves that formed, migrated and degenerated periodically along with the tidal cycle. The reversion of flow is reflected in opposite-directed sets of cross bedding. However, most of the cross-bedded sets appear to be unidirectional due to the dominant tidal current; in this case, the subordinate current produces erosional surfaces and ripples or megaripples. As the dominant current sets again, reactivation of the sand wave takes place. A most typical tidal feature is the formation of bundles of calcarenitic laminae representing the avalanche under current action that alternate with a somewhat thicker pelmicrite laminae representing settling of fines from suspension during slack-water periods. The relative thickness of such an arrangement changes regularly during the active migration of the sand wave during springs tides. Low rates of migration at neap tides (degeneration stage:. of sand waves) result in wave and biogenic reworking. The internal structure of the upper subfacies consists of trough cross bedding and parallel lamination as a result of increasing wave effect upon the surface of the shoal that superimposes on the tidal currents. This assumption is supported by the higher variability of paleocurrent directions. A model of shallow carbonate shoal with tidal sand waves and megaripples of various types is proposed. It is assumed that the highesl wave action ocurred along the SE margin of the shoal, facing the prevailing waves generated by storms, hurrkanes and, probably, sorne of the diurnal winds. Calcarenite/oolite (?) bars are thought to have occurred along these areas shielding the shoal and giving a chance for the tidal features to form under limited wave reworking that resulted in wave ripples of various scales and orientation.