The application of petrography and geochemistry of ancient terrace sediments in determining the provenance and weathering rate, Birjand, South Khorasan

The studied area includes the Neogene alluvial deposits of Birjand city that is geologically considered as a part of the structural zone of eastern Iran. In petrographic studies, 200 gravel clasts and 10 sandstone samples were used, while for sediment chemistry analysis, 13 mudstone samples and 2 sandstone samples were used. The petrographic examination of the above samples shows the presence of particles with different mineralogy in the studied alluvial sediments, in which, in addition to quartz, the abundance of igneous and sedimentary rocks is significant. Drawing the results of the major oxide and minor elements analysis of the above samples in the presented graphs in this context, reflects the presence of mafic to intermediate igneous rocks and sedimentary rocks in the source area. The examination of the rock units around the studied area shows that the origin of the gravel fragments and the sand particles is probably from the Cretaceous ophiolitic units, Late Cretaceous to Eocene flysch units, Paleogene clastic-carbonate sedimentary rocks and younger volcanic and sedimentary rocks which are exposed around the target area. Study of mineralogy, textural and compositional maturity of sediments along with the evaluation of chemical weathering indicators show that the mentioned alluvial sediments were deposited near the source and suffered moderate to weak weathering.