Sediment provenance from the coastal aeolian sands of La Graciosa Island (Canary Islands)

Abstract

La Graciosa Island has been formed by volcanic and sedimentary processes since the Middle Pleistocene. It is an island characterized by low-lying plains, with a notable alignment of volcanic cones oriented NE-SW (Montaña Bermeja, Morros Negros, Las Agujas Grande and Chica, El Mojón, and Montaña Amarilla). Nearly half of the island is covered by aeolian sands (jables) and muddy sediments of endorheic basins. Field surveys have been carried out in the beach-dune systems of Las Conchas and Baja del Ganado in the NW of the island; Lambra beach and the jable of La Fragata in the N; Caletas beaches in the E; and Salado beach in the S. It has been observed that only the northern systems (Las Conchas and Lambra beaches) are currently active, while the others are semi-active (no marine sand enters and the existing sands are mobilized). To understand the composition and relative abundance of the sand grains on La Graciosa, 21 thin sections of aeolian samples and 5 from endorheic basins were petrographically studied. They are associated with six beach-dune systems: Las Conchas, Baja del Ganado and Lambra beaches, and the jable of La Fragata in the northern sector, and Salado and Las Caletas beaches in the central-eastern sector. The petrographic results indicate that the most abundant sands are bioclastic in nature, with an average relative abundance of 77.5% (σ = 8.6). Among these, mollusk fauna remains dominate with 55%, while flora remains represented by detrital fragments of red coralline algae meshes— account for 22.5%. These are followed in abundance by terrigenous grains (volcanic rock and mineral fragments, and sedimentary intraclasts), reaching 22.5%. Of these, sedimentary intraclasts show an average of 14.5% (σ = 5.9), and volcanic terrigenous grains account for 8% (σ = 1). The bioclasts mainly originate from marine areas, while the lithoclasts come from the erosion of volcanic materials and sedimentary rocks found in both the rocky substrates of the intertidal zone and the surrounding supratidal areas. The endorheic sediments consist of sands and silts forming sedimentary intraclasts, with an average abundance of 98.5%. The partial data on the abundance of the main components calculated in supratidal sands, foredune, nebkhas, and sandy sheet samples show slight variations within each beach-dune system studied and among the different beach-dune systems. These small differences are due to the location of the sample, the surrounding volcanic-sedimentary outcrops, and the presence of certain landforms. On the other hand, more notable differences have been observed in the composition and abundance data of the aeolian sands when comparing those studied on La Graciosa with those described in systems on other islands (Lanzarote, Fuerteventura, and Gran Canaria). Each system is influenced by its own local geological, geomorphological, and anthropogenic factors.