Oncoid growth and distribution controlled by sea-level fluctuations and climate (Late Oxfordian, Swiss Jura Mountains)

Abstract

Abundant lagoonal oncoids occur in the Late Oxfordian Hauptmumienbank Member of the Swiss Jura Mountains. Four oncoid types are observed in the studied sections and classiWed according to the oncoid surface morphology, the structure and composition of the cortex, and the texture and fauna of the encasing sediment. Micritedominated oncoids (types 1 and 2) have a smooth surface. Type 1 has a rather homogeneous cortex and occurs in moderate-energy environments. Type 2 presents continuous or discontinuous micritic laminae. It is associated with a low-diversity fauna and occurs in high-energy facies. Bacinella and Lithocodium oncoids (types 3 and 4) display a lobate surface. They are dominated by microencrusters (Bacinella irregularis and Lithocodium aggregatum) and are found in low-energy facies. The stratigraphic and spatial distribution of these oncoid types shows a correlation with the sequence-stratigraphic evolution of the studied interval, and thus with relative sea-level Xuctuations. It can be shown that these sea-level Xuctuations were controlled by orbital cycles with 100-and 20-kyr periodicities. At the scale of 100-and 20-kyr sequences, types 1 and 2 oncoids are preferentially found around sequence boundaries and in transgressive deposits, while types 3 and 4 oncoids are preferentially found around maximum Xoodings and in highstand deposits. This implies that changes of water energy and water depth were direct controlling factors.Discrepancies in oncoid distribution point to additional controlling factors. Platform morphology deWnes the distribution and type of the lagoon where the oncoids Xourished. A low accumulation rate is required for oncoid growth. Additionally, humidity changes in the hinterland act on the terrigenous inXux, which modiWes water transparency and trophic level and thus plays a role in the biotic composition and diversity in the oncoid cortex.

An Upper Jurassic-Lower Cretaceous carbonate platform from the Vâlcan Mountains (Southern Carpathians, Romania): paleoenvironmental interpretation

“…The large oncoids with lobate outlines, dominated by microencrusters (Bacinella and Lithocodium), correspond to oncoids of types 3 and 4 described by Védrine et al (2007) or to the organism-bearing oncoids of Dahanayake (1977). The above-mentioned authors ascribed these types of oncoids to the low-energy open-lagoonal environments, where the microbial meshwork had time to grow.…”

“…The above-mentioned authors ascribed these types of oncoids to the low-energy open-lagoonal environments, where the microbial meshwork had time to grow. In contrast, the smaller, spherical oncoids, with micritic, homogeneous cortex (type 1 oncoids of Védrine et al 2007) are often associated with packstone-grainstone and represent higher energy conditions. Encrustation, borings, breakage, burrowing and micritization are common, indicating low accumulation rates (Enos 1983).…”

“…Encrustation, borings, breakage, burrowing and micritization are common, indicating low accumulation rates (Enos 1983). Oncoid growth also requires low sedimentary rates (Peryt 1981;Flügel 2004;Védrine et al 2007). The presence of abundant oncoids in the Oxfordian of the Swiss Jura was correlated (Védrine et al 2007) with the beginning of a longterm sea-level rise.…”

“…Oncoid growth also requires low sedimentary rates (Peryt 1981;Flügel 2004;Védrine et al 2007). The presence of abundant oncoids in the Oxfordian of the Swiss Jura was correlated (Védrine et al 2007) with the beginning of a longterm sea-level rise. Furthermore, the authors correlated the different types of oncoids with small-scale sea-level fluctuations.…”

Late Jurassic facies architecture of the Złoczew Graben: implications for evolution of the tectonic-controlled northern peri-Tethyan shelf (Upper Oxfordian–Lower Kimmeridgian, Poland)

“…The presence of Bacinella irregularis Radoičić, 1959 suggests a shallow, subtidal, back-shoal environment. The Bacinella oncoids are genetically linked to low-energy, oligotrophic interior lagoons (e.g., Dupraz and Strasser 1999;Védrine et al 2007;Bádenas and Aurell 2010), although, in the Złoczew area, these occur in a higher-energy back shoal with ooid-dominated grainstone.…”