THE ANALYSIS OF QUARTZ GRAIN SURFACE-FEATURES AS A COMPLEMENTARY METHOD FOR STUDYING THEIR PROVENANCE - THE GUADALETE RIVER BASIN (CADIZ, SW SPAIN)

The analysis of quartz grain surface features from several sandy depos its of the Guadalete River basin-Guadalete terraces, Miocene calcareni tes and Aljibe sandstones-have yielded data of considerable value in t he study of the provenance of these materials. The majority of the qua rtz grains of the Miocene calcarenites present the same primitive surf ace features as those of the Aljibe sandstones (generalised polishing of their surface, mechanical marks enlarged by solution) and many conc hoidal fractures and grooves corresponding to the high-energy environm ent in which they were deposited. The quartz grains of the Guadalete t erraces are mainly derived from the Miocene calcarenites (they show nu merous fractures and polished grooves) and have undergone a later stag e of evolution, forming isolated Vs and silica deposits corresponding to a final fluvial means of transport. These data indicate a possible evolution as follows: (1) Aljibe sandstones; (2) Miocene calcarenites; (3) fluvial sands of the Quaternary terraces of the Guadalete River. This evolution has been confirmed by study of the heavy mineral associ ations and the petrology of these materials. Thus, quartz grain surfac e features may be an extremely useful instrument for studying provenan ce, determining the evolutionary path that various sediments of a regi on have taken, thereby complementing the data yielded by the mineralog y of heavy minerals and regional geology.