Identifying organic-rich layers and reconstructing trophic levels in the Dalichai Formation in north Damghan

Abstract The Dalichai Formation in the Ahvanu section, north of Damghan city, with a thickness of 78 meters, comprises alternations of bluish-gray marls and limestones. Microfacies and palynofacies studies, along with fossil evidence, including foraminifera diversity and abundance, the ratio of epifaunal to infaunal benthic foraminifera, and the ratio of surface to deep infaunal, were utilized to identify organic-rich layers and reconstruct trophic levels. These factors confirm a mesotrophic environment with average oxygen and nutrient levels for the Dalichai Formation (Bajocian to Lower Callovian) in the studied section. The primary reasons for low organic matter in the Dalichai Formation are attributed to low production and poor preservation of organic matter. However, dark-colored deposits with high organic matter at the Bajocian–Bathonian boundary and the lower and upper Callovian boundaries suggest increased production rates in anaerobic and eutrophic conditions. Macrofossil fragments (ammonites and echinoids) within limestone deposits at the uppermost part of the Dalichai Formation, exhibiting low organic matter, and the presence of Zeophycus trace fossils, suggest an oligotrophic environment with dysoxic conditions in the Upper Callovian.