Geochemistry of beach sands along the western Gulf of Mexico, Mexico: Implication for provenance

This paper contributes to understanding the intractable problems in provenance study due to hydraulic sorting and geochemical heterogeneity in medium, fine, and very fine sands. For this purpose, detrital modes, major, trace, and rare earth element (REE) compositions of recent sands from the Playa Azul, Tecolutla, and Nautla beach areas of the western Gulf of Mexico have been investigated. Marked geochemical and petrographic differences occur among the three beach sands, even though they are separated just by 45km. The average quartz-feldspar-lithic fragment (QtFL) ratios for the Playa Azul, Tecolutla, and Nautla sands are respectively Qt₆₉–F₁₀–L₂₁, Qt₅₇–F₁₁–L₃₂, and Qt₃₇–F₅–L₅₈. The volcanic lithic fragment (Lv)−sedimentary lithic fragment (Ls)−[plutonic lithic (Lp)+metamorphic lithic fragments (Lm)] ternary diagram indicates that the Nautla sands are dominated by volcanic detritus, while the Tecolutla sands are dominated by sedimentary and volcanic detritus. The Playa Azul sands are dominated largely by sedimentary detritus. Geochemically, the three beach sands are quite distinctive from each other. The Playa Azul sands are higher in SiO₂ content (∼64–84wt.%) than in the Tecolutla sands (SiO₂=∼47–69wt.%). The Nautla sands are very low in SiO₂ content (<46wt.%). The contrasting geochemical compositions among the three beach areas are also confirmed by significant variations in Al₂O₃/TiO₂, Na₂O/K₂O, K₂O/Al₂O₃, Rb/Al₂O₃, and Cr/Ni ratios. The CIA values (∼39–69; chemical index of alteration) for the three beach areas suggest low to moderate weathering nature. In the three beach sands studied, the decrease in grain size is accompanied by a gradual decrease in SiO₂ content and an increase in TiO₂, Al₂O₃, Fe₂O₃, MgO, Zr, Hf, Cr, and V contents. Similarly, the ∑REE content increases with decreasing grain size. However, very fine sands in the Playa Azul and Nautla beaches are different in ∑REE content. This observation suggests that the provenance is more important in controlling the geochemical composition of beach sands than the grain size. The zirconium concentration in beach sands, however, is not related to the grain size. The comparison of REE patterns of beach sands with those of source rocks located relatively close to the study areas suggests that the Playa Azul sands were derived from felsic rocks, whereas a mixed provenance with contributions from felsic and intermediate rocks is more likely for the Tecolutla sands. In contrast, the REE distribution patterns of Nautla sands resemble those derived from basalt and basaltic andesite. However, selective concentration of magnetite grains in beach sands increases the LREE content and fractionates Eu resulting in a europium anomaly that is more negative than that displayed by Nautla sands. All of the above observations suggest that rivers delivering sands to the beaches are the important factors in controlling the composition of beach sands and that longshore currents play a less significant role.