Interpretación sedimentaria de las calizas de crinoides del Carixiense Subbético

  1. Cristino José Dabrio González
  2. Dolores Polo
Aldizkaria:
Mediterránea: Serie de Estudios Geológicos

ISSN: 0212-4300

Argitalpen urtea: 1985

Zenbakia: 4

Orrialdeak: 55-77

Mota: Artikulua

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Beste argitalpen batzuk: Mediterránea: Serie de Estudios Geológicos

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The early Jurassic crinoidallimestones of central Subbetic Zone formed in shoals of a large shallow carbonate shelf placed in the inner part of the broad gulf that Thethys Ocean was at that time, next to the emergent 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 for 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 stages 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 highest wave action ocurred along the SE margin of the shoal, facing the prevailing waves generated by storms, hurricanes and, probably, some 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.

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