Oncoid growth and distribution controlled by sea-level fluctuations and climate (Late Oxfordian,Swiss Jura Mountains) |
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Authors: | Stéphanie Védrine André Strasser Wolfgang Hug |
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Institution: | (1) Department of Geosciences, University of Fribourg, Pérolles, 1700 Fribourg, Switzerland;(2) Office Cantonal de la Culture, Paléontologie A16, Rue de la Chaumont 13, 2900 Porrentruy 2, Switzerland |
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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 classified according to the oncoid surface morphology, the structure and composition
of the cortex, and the texture and fauna of the encasing sediment. Micrite-dominated 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 fluctuations. It can be
shown that these sea-level fluctuations 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 floodings 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 defines the distribution and type of the lagoon where the oncoids flourished.
A low accumulation rate is required for oncoid growth. Additionally, humidity changes in the hinterland act on the terrigenous
influx, which modifies water transparency and trophic level and thus plays a role in the biotic composition and diversity
in the oncoid cortex. |
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Keywords: | Oncoids Carbonate platform Sequence stratigraphy Milankovitch cycles Sea level Palaeoenvironments Palaeoecology |
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