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

Abstract

The deposits of the ancient Ager deltaic system (Lerida, 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 Regola 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.