Large Fe-Mn oncoids from Early Jurassic Ammonitico Rosso Facies (eastern Sakarya Zone, NE Turkey): new insight into palaeogeographic conditions of Tethys Ocean

Abstract

Oncoids, significant sedimentary structures within the Jurassic strata of the Mediterranean Region,serve as valuable indicators of paleoenvironmental conditions during their formation. One of theimportant examples of oncoids has been discovered in the Pliensbachian (Early Jurassic)Ammonitico Rosso Facies (ARF) of the eastern Sakarya Zone (Eastern Pontides, NE Turkey). Thisstudy presents the first comprehensive sedimentological and petrographic (textural and composi-tional 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 exhibita range of sizes, from 10 to over 45 mm, displaying various colours such as brown, reddish, andmetallic-looking. They mostly have a discoidal shape, although some are spherical. The nucleiconsist of bioclastic wackestones, containing remnants of ammonoid shells. Some oncoids, espe-cially the discoidal forms, have multiple nuclei. The cortex of the oncoid predominantly consists ofwrinkled bands displaying micritic laminae, with an abundance of filamentous bodies and the localpresence of encrusting microfossils. In the cortex of the oncoids, iron predominates over manga-nese, 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, asindicated by discrimination diagrams involving Ni, Co, Zn, and plots of Ce/Ce* vs Y/HoN and Ndcontents, suggesting a hydrothermal origin. Furthermore, their rare earth elements (REE) chemistrydisplays distinct Y/Ho, Sm/Yb, Ce/Ce, and Nd values, indicating the presence of seawater mixingwith 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 sea-water. Nevertheless, the cortex of the oncoids exhibits a complex and variable mineralogy thatchanges over short distances, implying the dynamic nature of the depositional environment,characterized by fluctuating hydrological regimes, varying oxygen levels, elemental supply, andthe 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 marineenvironments, coinciding with intensified syn-sedimentary tectonic activity, where ongoing exten-sional tectonic movements occasionally influenced paleoenvironmental conditions. In these con-ditions, the formation of oncoids is influenced by various sedimentary factors, including theavailability of metal sources, specific paleoenvironmental conditions, and the presence of microbialorganisms, thereby enriching our understanding of palaeogeography during the Early Jurassicperiod. Additionally, the studied oncoids, with their comparable stratigraphic position and petro-graphic characteristics to those in both the eastern and western parts of the Tethyan basin,underscore their significance in palaeogeography and stratigraphy