Modern stream sands :Composition and sand generation indexGeology Department, Faculty of Science, Tehran university

General:
     Composition of modern sands of Poirud river in order to evaluate the contributions of different bedrock types in the Poirud watershed have been analyzed. Results of this work indicate that sand composition normally does not permit quantitative statements concerning sourceland composition. This is due to the fact that each rock type has a different potential to generate sand, depended on such properties as its mineralogy, average crystal size and microfabrics. This work introduce the concept of sand generation index (SGI), which is a relative measure of the capaity of one bedrock type to generate sand with respect to another in a compound source area. In case of a volcanic rock + carbonate sedimentary rock SGI of volcanic rocks is 2.1 and when thise rock type occures in association with terrigenous sedimentary rocks thrir SGI is 2.3. In a volcanic rocks + sedimentary rocks (carbonate and terrigeñous with together) association the SGI of volcanic rocks is 3.3 and at this case volcanics have the greatest potential for sand production. Finally, results of this work show that quantitative estimates of source land composition based on QFR diagrams are hazardous, if the concept of the SGI is disregarded.