Provenance analysis of sediments and sedimentary rocks at coastal outcrops of Corralejo Natural Park (NE Fuerteventura Island)

Abstract

The textural, mineralogical and petrological analysis of sediments and sedimentary rocks that outcrop in littoral areas of the Corralejo Natural Park (NE of Fuerteventura Island), allow recognize better the provenance of clastic components, the sedimentary environment where they were formed and their evolution over geologic time. Therefore the aim of this research is to know the nature and relative abundance of the different components (bioclasts, lithoclasts and cements) forming sediments and sedimentary rocks. Thus, 18 samples of sands from the intertidal and supratidal zones of different beach-dune systems have been collected and also 38 samples of sedimentary rocks from different outcrops that appear in the coastal strip of this park. These rocks presents between them certain differences in the degree of cementation, in the sedimentary structures and the textural characteristics and they correspond to calcarenite strata of beachrocks, paleobars and eolianites, silty sandstone of paleosols and marine fossiliferous conglomerates of the Upper Pleistocene and Holocene in age. Several analytical techniques were carried out in coastal sediments, such as petrography, dry sieving and calcimetry, to determine the textural characteristics, carbonate content, and the nature and relative abundance of bioclastic and lithoclast components. Otherwise, it has been made geochemical analyses of sedimentary rocks cements using EMPA (Electron Microprobe Analyzer), and it has been defined their crystal morphology with SEM (Scanning Electron Microscope). This enables to determine the cement composition, the morphology of cysts and its cementation environments. The results of granulometric parameters obtained in the 18 sand samples, shown homogeneities in the average grain size and sorting between intertidal and supratidal zones. Even so, the grain size in the supratidal zone (aeolian sands) is classified like medium sands (0.25 and 0.5 mm) and show better sorting values (between 0.5 and 1). This is due to the wind action as transport and sedimentation agent, being more selective than wave’s action, tides and currents. The calcimetry analysis confirms a high carbonate content >80% (usually the obtained data are higher than 90%), evidencing the biogenic sand composition with scarce of lithic fragments provided by a terrigenous source area. The petrographic analysis of sediments, with the object of identifying microscopically the nature of the grains and its relative abundance counting points, shows that the sands contain bioclasts and lithoclasts. The bioclasts are mainly fragments of red seaweed meshes and mollusks and, in a minor proportion, foraminifers, equinoderms and bryozoes. The lithoclasts are essentially constituted by fragments of mafic volcanic rocks (basalts) and sedimentary rocks (intraclasts) and, in a minor proportion, fragments of mafic minerals (olivines, augites and Fe-Ti oxides) and glasses, and felsic minerals (feldspars). Also, the sediments have shown a great homogeneity of grain nature, in the two subenvironments, intertidal and supratidal. In relation to sedimentary rocks, petrographic results confirm similarity to marine sands but with a greater abundance of sedimentary intraclasts in the northernmost stretch coast of the park, which indicate the erosion of coastal calcarenite substrate. On the other hand, the low abundance of lithoclasts, both in sediments and sedimentary rocks, marks that erosion and transport agents have not acted significantly on the various volcanic rocks outcrops (middle Pleistocene basaltic lava flows, with porphyritic and aphyric textures, and containing olivine and clinopyrexene phenocrysts) and that appear discontinuously along the actual coastline. Also, it is rare the detrital grain inputs associated to incipient gullies that cross the Natural Park from west to east, eroding the underlying volcanic and sedimentary substrate. Considering the geochemical and mineralogical studies of sedimentary rock cements, these indicate a high-magnesium calcite cementation (HMC) with MgCO3 values of > 5% (data vary from 8.1 to 18.5%) and Sr contents between 1,186 and 1,563 ppm, demonstrating that these carbonate cements have been produced in a marine environment. Regarding to cements morphology, it has been observed isopach microsparite and sparite crystals (bladed) around the grains, filling intragranular fossils cavities of gastropods, foraminifera and bryozoans, and intergranular voids, pointing phreatic marine conditions. Moreover, the existence of mainly bioclastic calcarenite layers (grainstones and biosparites well sorted and with textural maturity) and, to a lesser extent, of calcirudites with rounded pebble of basalt and calcarenite groundmass, indicate shallow coastal sedimentation environments (beach and bars) with high energy. In addition, there are backshore sedimentation environments with calcarenites containing dunes planar cross lamination and browned paleosols levels characterized by silty sandstones with terrestrial molluscs and rizoliths. These sedimentary paleoenvironments are very similar to sedimentary coastal environments that appear today in the Natural Park of Corralejo.