Beach sand composition and provenance in a sector of the southwestern Mexican Pacific

Sandy sediment samples from eleven beaches in southwestern Mexico were texturally, petrologic and chemically analyzed. Our goals were to interpret the provenance of the sands in terms of grain size, petrogrology and geochemistry and to observe if the beach environment reflects accurately the source rock in three adjacent river basins dominated by sedimentary, volcanic and plutonic rocks respectively. Three littoral segments were divided in terms of lithological differences: the Cihuatlán (western), Armería (central) and Coahuayana (eastern) river basins and their respective beach segments. The Cihuatlán river basin is influenced by granitoids, whereas the Armería and Coahuayana river basins are dominated by the influence of intermediate to basic volcanic rocks and sedimentary rocks represented by limestones. This is supported by the enrichment of Fe2O3, CaO, MgO, TiO2, V, Cr, Co and Zn, and a depletion of SiO2, Al2O3, Na2O K2O, Ba, Sr and Rb from the western segment toward the eastern segment. The chemical index of alteration (C.I.A.) values were higher in beaches from the Armería and Coahuayana river segments. This is probably caused by weathering feldspars in the volcanic lithics due to more humid conditions in the source area. Sedimentary calcareous lithics in the Armería and Coahuayana river basins are depleted due to their low resistance to high energy of fluvial and river-marine conditions. Despite the fact limestone outcrops are more than 40 % in the central (Armería) and eastern (Coahuayana) basins, the beach sediments do not reflect limestone sources. Heavy minerals concentration is related to the influence of Armería and Coahuayana river basins and likelly produced by the erosion of intermediate to mafic Quaternary lavas.