The Loppio Oolitic Limestone (Early Jurassic,Southern Alps): A prograding oolitic body with high original porosity originated by a carbonate platform crisis and recovery |
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| Institution: | 1. Department of Sciences, University of Basilicata, Viale dell''Ateneo Lucano 10, 85100 Potenza, Italy;2. Department of Earth Sciences, Environment and Resources, University of Naples Federico II, Via Mezzocannone 8, 80134 Naples, Italy;1. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China;2. Department of Earth Sciences, Western University, London ON N6A 5B7, Canada;3. Institute of Paleontology and Paleoenvironment, School of Earth and Space Sciences, Peking University, China;1. Université de Lorraine, CNRS, Lab. GeoRessources UMR 7359, BP 70239, 54506 Vandoeuvre-lès-Nancy Cedex, France;2. 256 R Alexandre André, 45240 Ligny Le Ribault, France;3. Chrono-Environnement Université de Franche-Comté, 16 route de Gray, 25030 Besançon cedex, France;4. Laboratoire de Géologie de Lyon, Terre, Planètes et Environnement, Université Claude Bernard Lyon 1 et Ecole Normale Supérieure de Lyon, 2 rue Raphaël Dubois, 69622 Villeurbanne Cedex, France;5. UMR 7517 EOST, Université Louis Pasteur, 1 rue Blessig, F 67084 Strasbourg Cedex, France;6. RCJ—Office de la culture, Paléontologie A16 Hôtel des Halles, Case postale 64, CH-2900 Porrentruy, Switzerland;7. Circuit de la Pierre Commercy ADCPE—Villasatel, Hameau des Carrières, 55 200 Euville, France;8. ANDRA DS/Milieu Géologique 1/7, rue Jean Monnet F, 92290 Châtenay-Malabry Cedex, France;9. SNSB-Bayerische Staatssammlung für Paläontologie und Geologie and GeoBioCenter LMU, Richard-Wagner-Str. 10, D-80333 München, Germany |
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| Abstract: | The Loppio Oolitic Limestone is a lithostratigraphic unit of the Early Jurassic Trento Platform in the Southern Alps, Northern Italy, which deposited over an area of ca. 3500 km2. It appears as a roughly tabular or wedge-shaped sedimentary body with thickness gradually increasing from 0 to ca. 100 m toward the western platform margin. We investigated the sedimentology, petrography and bulk carbonate carbon isotope geochemistry of the Loppio Oolitic Limestone in order to shed light on its depositional setting and origin. The Loppio Oolitic Limestone is made almost exclusively of oolitic grainstone, and can be subdivided in two parts. In the lower part, ooids are poorly sorted and sedimentary structures are scarce or absent. In the upper part, sorting becomes good and sedimentary structures are common. The vertical succession of sedimentary structures and the upward increase in sorting suggest a shallowing upward trend within the oolite. A reddened surface, meteoric cements and dinosaur footprints occur at the top of the unit, testifying for a subaerial exposure which is also confirmed by carbon and oxygen stable isotopic data. In terms of sequence stratigraphy, the Loppio Oolitic Limestone represents a Highstand Systems Tract, bounded at the top by a subaerial exposure surface. Bulk carbonate stable carbon isotope curves across the Loppio Oolitic Limestone from 7 stratigraphic sections could be correlated over distances of tens of km on the whole Trento Platform. This correlation suggests that the deposition of ooids was nearly synchronous across the platform. A negative excursion of carbon isotopes with magnitude of ca. 1‰ VPDB was identified within a lime mudstone unit (“Nodular lithozone” of the Monte Zugna Formation) immediately below the Loppio Oolitic Limestone, which can be correlated to a global perturbation of the carbon cycle in the mid-Sinemurian. The flooding of a wide area of formerly peritidal carbonate platform below the wave base was interpreted as due to an ecological crisis that caused a drop of carbonate production. We suggest that the subsequent recovery of carbonate production is marked by the shallowing upward succession of the Loppio Oolitic Limestone, which quickly occupied the accommodation space formed in consequence of the crisis, thus preventing the platform drowning. The Loppio Oolitic Limestone deposited as an initially highly porous oolitic sand that was then topped by a clayey interval (base of the Rotzo Formation), giving origin to a structural and stratigraphic configuration that could be favourable for the accumulation of hydrocarbons in the subsurface. The recurrence of similar facies superpositions, formed in consequence of perturbations of the carbon cycle with documented climatic effects, is discussed with regard to the Tethysian record of Mesozoic carbonate platforms. |
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| Keywords: | Carbonate platform Jurassic Tethys Oolite Stable isotopes Sequence stratigraphy |
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