Abstract

The Ortasu area is located in the Central Iran zone, approximately 100 km north-west of Zanjan. Sandstone units of this area are belong to the Upper Red Formation. Based on field and petrographic studies, these sandstones are fine to coarse grain in size and show cross-bedding and traces of plant roots. Sorting of these sandstone is poor to moderate; having sub-angular to sub-rounded grains with a medium sphericity. Based on petrographic studies, these sandstones are litharenite (mainly chertarenite and phillarenite) containing monocrystalline and polycrystalline quartz, alkali-feldspar (orthoclase), and sedimentary and metamorphic rock fragments. The mineralogical and compositional maturity of these sandstones are immature to sub-mature. This may indicate close distances between the sedimentary environments of these sandstone with their origin. According to their constituents, origin of studied sandstone could be sedimentary and metamorphic rocks at the area.