Abstract

The base of the Triassic in the Iberian Range is represented by detrital sediments (Buntsandstein facies) deposited initially in a continental environment, finally evolving into a marine environment that is represented at the top of the sequence. The lithology of this facies is dominated by arkosic sandstones. The aim of this study is to reconstruct the nature and position of the source areas of these sandstones. Provenance research was carried out by quartz-grain typology. Eleven stratigraphic sections were sampled. The amount of interstitital matrix has been considered in selecting samples because mechanical compaction suffered by sandstones with little matrix may give rise to a significant increase in the undulosity of monocrystalline quartz. The Ollo de Sapo gneissic formation, located in the Hesperian Massif, has been mentioned as source rocks of feldspathic sandstones in previous works. In order to verify the provenance of feldspathic sandstones, in artificial sands derived by grinding gneisses, and sand samples collected at stream heads that drain the gneiss outcrops, we followed the methodology of Basu et al. (1975). Analytical results indicate that two different areas within the Triassic basin were notably influenced by different source areas: a) a western zone, the nearest to the gneissic source rocks, where monocrystalline, nonundulatory quartz grains predominate (Qm <= 5 degrees ); and b) an eastern zone farther from the gneissic source area, where polycrystalline quartz grains (Qp) and undulatory, monocrystalline quartz grains (Qm > 5 degrees ) increase. Sediment evolution during transport processes is markedly reflected by the increase in Qm <= 5 degrees /(Qp + Qm > 5 degrees ) ratios in the westernmost zone, away from the source area. Low values in the above-mentioned ratios in the eastern zone are interpreted as results of local influence by low-ranking metamorphic source areas. Finally, this method also allows for the monitoring of the evolution of sediment maturity throughout the basin.

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