Oncoid distribution in the shallow domains of a Kimmeridgian carbonate ramp (Late Jurassic, NE Spain)

Abstract

The distribution and abundance of oncoids in the shallow domains of a latest Kimmeridgian carbonate ramp was accomplished on the basis of a detailed sedimentological and petrographic analysis of the lower part of the Higueruelas Formation (Iberian Basin, northeastern Spain). Six types of oncoids are recognized based on the internal structure of the cortex, and show a preferential distribution on the carbonate ramp, which record from intertidal, lagoon, backshoal, shoal-sand blanket, foreshoal to offshore domains. Spherical type I oncoids, composed of continuous micritic laminae, occur in all the subenvironments in low proportions. Sub-spherical type II oncoids, constituted by micritic laminae and discontinuous organism-bearing laminae of mostly Bacinella irregularis, dominate in the higher-energy shoal-sand blanket domain. Type III oncoids, with alternating micritic and organism-bearing laminae of mostly Bacinella irregularis-Lithocodium aggregatum association, include two varieties: type IIIa oncoids, with micritic and organism-bearing laminae of similar thickness, preferentially formed in the foreshoal domain; and type IIIb oncoids, with thinner micritic laminae, formed in a low-energy sheltered lagoon. Irregular type IV oncoids characterize the low-energy settings, and include two varieties: type IVa oncoids, mainly consisting of a Bacinella-Lithocodium meshwork, which occur abundantly in the lagoon; and less abundant type IVb oncoids, composed of a continuous organism-bearing laminae of Lithocodium, which are found in the offshore domain. Water energy and platform morphology played an important role in formation and distribution of the oncoids in these shallow domains, where normal-marine water and oligotrophic conditions are highlighted by the high-diversity of light-dependant micro-encrusters in type III and IV oncoids. Comparable oncoid distribution patterns of diverse Late Jurassic shallow carbonate platforms enhance their potential use as reliable palaeoenvironmental proxies.