PROVENANCE OF MULTICYCLE QUARTZ ARENITES OF PLIOCENE AGE AT ARCOS, SOUTHWESTERN SPAIN

A typical deposit of multicycle quartz arenites of considerable econom ic significance located in Arcos (SW Spain) has been studied to establ ish the provenance of the grains and their stages of sedimentary evolu tion. The quartz grains were studied by SEM and the composition of the sand determined by modal analysis of the heavy and light minerals. Th e sources of the silica sands were the Numidic sandstones (known as 'A ljibe') in the Betic Cordilleras, and the late Miocene calcarenites of the Guadalquivir basin. Four phases in the evolution of the Arcos qua rtzose sands are: (a) the initial surface features, upturned plates; ( b) mechanical features (an association of conchoidal fractures and arc uate steps), formed in littoral environments, inherited from the Late Miocene calcarenites; (c) fluviatile features, such as mechanical V's and grooves; and (d) a final stage of chemical features (oriented etch pits, solution pits and hollows, and silica globules), developed in a hot, humid climate. The most important process producing the enrichme nt of the quartzose sands takes place during erosion-depositional cycl es.