DOI: 10.6084/m9.figshare.26067376.v1
title: Large Fe-Mn oncoids from Early Jurassic Ammonitico Rosso Facies (eastern Sakarya Zone, NE Turkey): new insight into palaeogeographic conditions of Tethys Ocean

description: Oncoids, significant sedimentary structures within the Jurassic strata of the Mediterranean Region, serve as valuable indicators of paleoenvironmental conditions during their formation. One of the important examples of oncoids has been discovered in the Pliensbachian (Early Jurassic) Ammonitico Rosso Facies (ARF) of the eastern Sakarya Zone (Eastern Pontides, NE Turkey). This study presents the first comprehensive sedimentological and petrographic (textural and compositional characteristics) examination, as well as geochemical analyses (trace and rare earth elements) of these Fe-Mn oncoids. The oncoids consist of nuclei coated with irregular laminae and exhibit a range of sizes, from 10 to over 45 mm, displaying various colours such as brown, reddish, and metallic-looking. They mostly have a discoidal shape, although some are spherical. The nuclei consist of bioclastic wackestones, containing remnants of ammonoid shells. Some oncoids, especially the discoidal forms, have multiple nuclei. The cortex of the oncoid predominantly consists of wrinkled bands displaying micritic laminae, with an abundance of filamentous bodies and the local presence of encrusting microfossils. In the cortex of the oncoids, iron predominates over manganese, and there are compositional variations within the cortex, especially enriched in Si, Fe, and Ca. They predominantly fall within the region associated with hydrothermal Fe and Mn sediments, as indicated by discrimination diagrams involving Ni, Co, Zn, and plots of Ce/Ce* vs Y/HoN and Nd contents, suggesting a hydrothermal origin. Furthermore, their rare earth elements (REE) chemistry displays distinct Y/Ho, Sm/Yb, Ce/Ce, and Nd values, indicating the presence of seawater mixing with hydrothermal fluids. However, they show relatively low Sm/Yb, Y/Ho, Eu/Sm, Nd/YbN, and La/Yb values, suggesting a limited contribution from hydrothermal sources to the surrounding seawater. Nevertheless, the cortex of the oncoids exhibits a complex and variable mineralogy that changes over short distances, implying the dynamic nature of the depositional environment, characterized by fluctuating hydrological regimes, varying oxygen levels, elemental supply, and the saturation of specific elements in seawater. The increased presence of specific trace elements, including Fe, and REE during this period, is likely linked to hydrothermal fluid input into marine environments, coinciding with intensified syn-sedimentary tectonic activity, where ongoing extensional tectonic movements occasionally influenced paleoenvironmental conditions. In these conditions, the formation of oncoids is influenced by various sedimentary factors, including the availability of metal sources, specific paleoenvironmental conditions, and the presence of microbial organisms, thereby enriching our understanding of palaeogeography during the Early Jurassic period. Additionally, the studied oncoids, with their comparable stratigraphic position and petrographic characteristics to those in both the eastern and western parts of the Tethyan basin, underscore their significance in palaeogeography and stratigraphy.