Recent progress in the bio- and chronostratigraphic understanding of the upper Frasnian (Upper Devonian) sedimentary sequences in Armenia

Upper Frasnian Bio‑ and Chronostratigraphy of Armenian Sedimentary Sequences

Abstract

Recently obtained biostratigraphic results in terrigenous sequences (shales and sandstones) intercalated between brachiopod-bearing limestones constitute an important breakthrough in our understanding of the chronostratigraphic framework of the Frasnian–Famenniantransitional interval. More precisely, miospores obtained from the Ertych section constrain the entire terrigenous sequence to the late Frasnian, while acritarches, prasinophytes and chitinozoans found in the same samples establish its accumulation in marine environments. Conodonts extracted from the Noravank section establish that the lower part of the terrigenous interval, dominated by shales, was accumulated during the linguiformis conodont zone. Taking into account the combined chronostratigraphic evidence in the two sections, it may be inferred that the terrigenous sequence accumulated in less than 240 thousand years. Our results give for the first time a relatively precise time framework to describe and reconstruct marine and terrestrial paleoecosystems that existed in the northern edge of the megacontinent Gondwana.