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Evolution of Buntsandstein Fluvial Sedimentation in the Northwest Iberian Ranges (Central Spain)
Amparo Ramos, Alfonso Sopena, Marta Perez-Arlucea
The Buntsandstein sediments in central Spain (Permian and Triassic) are mainly continental redbeds that form the base of a sequence that passes up into siliciclastic and carbonate tidal sediments, the Muschelkalk facies. Buntsandstein sedimentation began with conglomeratic and sandy fluvial facies whose detailed sedimentological analysis is the main subject of this work.
Massive sheets are the most important conglomerate facies; they were formed by deposition as longitudinal bars. Channel-fill conglomerates are the second most important facies. Lateral accretion deposits also occur in the conglomerates; they evolved from longitudinal bars because of decreasing flow or because the bars had reached a certain height. Sandstones are composed of eleven different facies. Large linguoid or transverse bars were the main bedforms in this environment. These bedforms may be related downstream to large, oblique troughs developed when the tabular bedform grew to a certain size or when a tabular bedform reached another one that acted as an obstacle. Compound and composite compound bars consisting of large sets of trough cross-bedding with scoured, wavy bases are al o recognized in the sandstone facies.
Two major cycles occur in the complete conglomerate sequence which correspond to different physiographic models. The lower cycle was mainly formed by channels and bars that were smaller than those in the upper cycle. The upper cycle is more extensive than the lower one and has characteristics that indicate a higher stability. Paleocurrents for both cycles have a north-northeasterly trend. The vertical evolution of these sediments is clearly related to the different fault systems that controlled their distribution.
The paleogeography of the sandstone facies, together with paleocurrents (perpendicular to the conglomerate paleoccurents) indicate deposition from a separate dispersal system that extensively overlies the previous one. This unit, which can be followed extensively over most of the Iberian Ranges and even outside the Iberian Ranges, represents the growth of a fluvial braidplain over a large area of Iberia during the Lower Triassic.
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