A red horizon underneath Damavand Volcano Quaternary pyroclastics in Polur area, Iran

Abstract

There is a red horizon immediately below the lavas of Damavand volcano, which has been considered as a palaeosol since 1970s, though highly debated in nature up to now the release of the geological map of the studied area in the . However, the nature of this red horizon is still debated. In this study, a thorough field work has been performed to recognize the stratigraphic position of this red horizon in the area and 65 samples were gathered from both lavas and underneath supposed sedimentary beds to prepare thin sections for petrography studies. In addition, XRD analysis was carried out on six fine grained subsamples to determine the mineralogical composition of silt and clay sized sediments located under the lavas. The results show that volcanic rocks in the Polur area consist of two types of intermediate and basic lavas, whereas the red horizon is composed of highly oxidized volcanic ashes combined with glass tuff and altered pyroclastics. The dominant clay minerals in the altered pyroclastics include illite and chlorite. The formation of red horizon is a consequence of simultaneous deposition of fluvial clastic sediments (such as shales, sandstones, and limestone from older formations) and pyroclastic ash fall deposits and later alteration and oxidation of Fe-bearing minerals in both fluvial sediments and ash fall deposits. The results show that the red horizon which is seen underneath the lava deposits of the Damavand volcano in the Polur area should thus no longer be considered as a palaeosol. This is in contrast to most of previous studies in this area and worldwide.