The “Raña” unit, an intensely weathered Late Neogene product from SW Iberia: Petrographic and geochemical evidences

Abstract

The Raña is a morphosedimentary unit related to the last record of the Guadiana Basin and located in the southwestern Iberian Peninsula. The Guadiana Basin is an intracratonic basin filled with <200 m of Paleogene and Neogene clastic sediments. The Raña unit represents a relict unit that lies over the northern border of the Guadiana Basin and the Variscan basement of Neoproterozoic and Palaeozoic low-grade metasediments from the Iberian Massif. Raña deposits are alluvial fans abandoned by their feeding streams during the Late Neogene period. Outcrops correspond to the apex of the alluvial fans isolated from the fluvial network.

This study analysed Raña deposits using petrographic, mineralogical, and geochemical techniques, which suggested a source-to-sink analysis by contrasting data among the three involved elements: source areas, basin fill sediments, and Raña deposits. Sandstones from the basin infill show primarily quartzolithic petrofacies as a consequence of the low-grade metamorphic character of the sources. The Raña sandstones show similar petrofacies with a greater quartz content than basin infill deposits and an important population of argillaceous pisoids. Kaolinite is the dominant clay mineral associated with the Raña units. The aforementioned characteristics indicate a sedimentary provenance from intensely weathered products. The contrast of the geochemical composition of the parent rocks, basin-fill, and Raña deposits also confirms the forced maturation of sediments by chemical weathering. Chemical index of alteration values show evolution from infill sediments (76) to Raña deposits (90) as a consequence of mobile cation loss (Ca, Na, and K). This approach is related to different zones of alteration in soils, suggesting the formation of sapropelic deposits on weathering mantles at the source under hot and wet climates (tropical) during the formation of the Raña deposits. Finally, the provenance signals prevail despite intense weathering.