6533b861fe1ef96bd12c5934

RESEARCH PRODUCT

Ciclos pelíticos de bahía Interdistributaria del nivel intramareal inferior del grupo Roda-Oroel (sector occidental del paleodelta de Ager, Lérida)

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description

The deposits of the ancient Ager deltaic system (Lérida, Spain) are characterized in some way by the existence of cycles constituted, in a large scale, by alternances of pelites and sandstone bodies. According to geological literature these deposits represent bay marls and stream-mouth-bars, in the eastern-most counterpart. In this area sorne pelitic sections show a pronounced cyclical pattern devoid to repetitions of two terms: A marly bioturbated one, and the other one, constituted by undisturbed delicately laminated clays. Part of the deposits of the "Intramarea1 Inferior" level (''Tramo Superior" of the Roda-Oroel depositional system) in the area comprised between La Régola cementery and Ribellera gorge, have been studied taken specially into account the faunistic content, sedimentary structures sequences and lateral relationships of these pelitic cycles in order to characterize them. In that area, three cycles constituted by alternances of marls and clays have been distinguished. As a whole these three cycles are bounded at: the top and the bottom by sanstone bodies that have been interpreted as minor-mouth-bars. The mar1y terms of these cycles are light brown in colour and show a faunistic content that may be ascribed to a lagoon with normal salinity or slight1y hipersaline waters. On the other hand, fauna in the laminated clays is scarce and there is not evidence of bioturbation. The high content of plant remains ad evaporites in these clays suggest that they were developed in a reducing environment (probably anoxic). Boundaries between marls and laminated clays are always net and not erosive. They migth be originated by drastic changes in the physical-chemical characteristics of the sedimentary environment where the were deposited. Taking into account the sedimentary context of these clays, this cyclicity migth be interpreted as the response to the alternance of succesive periods of marine flood and periods of starved waters in a deltaic interdistributary bay when it was disconnected from the open sea.<br><br>Los depósitos deltaicos del Eoceno de Ager (provincia de Lérida) se caracterizan por presentar ciclos a gran escala, formados por alternancias de materiales. pelíticos y cuerpos de areniscas. Estos depósitos están formados, en su sector más occidental, por margas de bahía y lóbulos de frente de canal deltaico. En esta zona, algunos de los tramos pelíticos presentan una marcada ciclicidad, formada por la repetición de dos términos. Uno de ellos es margoso y está bioturbado; el otro es arcilloso y presenta una laminación fina, no distorsionada. Para la caracterización de estos ciclos pelíticos se ha estudiado detalladamente el contenido faunístico, secuencias de estructuras y relaciones laterales de parte de los materiales del "Nivel Intramareal Inferior" (''Tramo Superior" del grupo deposicional Roda-Oroel) en el sector comprendido entre el Cementerio de la Régola y el Barranc de Ribellera (Vall d'Ager, Lérida). Los términos margosos son de color ocre y poseen un contenido faunístico atribuible a un "lagoon" con aguas de salinidad normal o ligeramente hipersalinas. Por su parte. las arcillas laminadas apenas tienen fauna y no presentan signos de bioturbación. Su alto contenido en restos vegetales y evaporitas, sugiere que se depositaron en un ambiente reductor, posiblemente anóxico, Teniendo en cuenta el contexto sedimentario, dicha ciclicidad puede corresponder a períodos de inmersión y períodos de estancamiento, por pérdida de conexión con el mar abierto, de una bahía interdistributaria deltaica.