Visean-Serpukhovian (Mississippian) archaediscoid foraminifers of the northern Alborz, Iran

A relatively complete succession of the Visean to Serpukhovian rocks is exposed in northern Alborz, Iran, where two sections at Naserabad and Dozdehban have been selected for detailed biostratigraphic investigations. The archaediscoid foraminifers, which are probably the most precise biostratigraphic markers, have been investigated. During the early Visean, MFZ9 foraminiferal zone, the proposed ancestor of the archaediscoid Lapparentidiscus appears, followed by abundant occurrences of primitive archaediscoids, defining the MFZ10 zone. The MFZ11 can be well defined by the first occurrence of Uralodiscus rotundus. The base of MFZ12 in this area is characterized by the first occurrence of Nodosarchaediscus and its top by Paraarchaediscus at concavus stage, transitional to the angulatus stage. The lower upper Visean rocks might be absent from the northern Alborz, probably because a mid-Visean tectonic event occurred in this area. The middle late Visean is dominated by the acme of Archaediscus at angulatus stage and archaediscoids with occluded and semi-occluded lumen as well as by the first occurrence of Neoarchaediscus and Kasachstanodiscus. The Archaediscus/Paraarchaediscus at angulatus stage with transitional forms to the tenuis stage and Neoarchaediscus akchimensis mark the late Visean in the succession. The lower Serpukhovian (Tarusian-Steshevian) rocks contain some of the previously recorded species and new large Kasachstanodiscus and Archaediscus, but also a significant decrease in Paraarchaediscus, Nodosarchaediscus and Archaediscus at angulatus stage. The late Serpukhovian (Protvian) is marked by first occurrences of Brenckleina (especially Brenckleina rugosa) and Eosigmoilina.