Stratigraphy,paleogeography and depositional environments of the Upper Middle Carboniferous (Desmoinesian) cyclothemic Excello Black Shale of the Midcontinent Region,USA |
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| Institution: | 1. CONICET, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina;2. Facultad de Ciencias Naturales y Museo (FCNyM), Universidad Nacional de La Plata (UNLP), 1900 La Plata, Argentina;3. Instituto Patagónico de Geología y Paleontología (CONICET-CENPAT), Boulevard Brown 2915, Puerto Madryn, Chubut, Argentina;4. Divisão de Geologia e Georecursos Marinhos, Instituto Português do Mar e da Atmosfera (IPMA), Avenida Doutor Magalhães Ramalho 6, 1495-165 Alges, Portugal;5. Centre of Marine Sciences (CCMAR), Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal;6. Environmental Isotope Laboratory, Department of Geosciences, University of Arizona, Tucson, AZ, USA;7. Institute of Geosciences, University of Mainz, Germany;8. Instituto de Geocronología y Geología Aplicada, CONICET-UBA, Buenos Aires, Argentina;9. Instituto de Limnología \"Raúl A. Ringuelet\", CONICET-FCNYM, La Plata, Argentina;10. Centro de Investigaciones Geológicas, CONICET-UNLP, Diag. 113 esq. 64, 1900 La Plata, Argentina;11. Cátedra de Geoestadística, FCNyM (UNLP), Argentina;12. Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina;13. Departamento de Agrimensura, FI, Universidad de Buenos Aires, Buenos Aires, Argentina;1. Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18071 Granada, Spain;2. Departamento de Ciencias de la Tierra, Facultad de Ciencias Experimentales, Campus de El Carmen, Universidad de Huelva, Avda. 3 de Marzo, s/n, 21071 Huelva, Spain;3. CCTH - Centro de Investigación Científico Tecnológico, Universidad de Huelva, Avda. 3 de Marzo, s/n, 21071 Huelva, Spain;4. Department of Earth Sciences, Royal Holloway Univ. London, Egham, Surrey TW20 0EX, UK;5. Department of Environmental Sciences – Geology, University of Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland |
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| Abstract: | The Excello Shale is one of the best exposed examples of Pennsylvanian cyclothemic organic-rich shales in the midcontinent region. This study aimed to describe detailed stratigraphy of the study interval along the outcrop belt and to relate the environments of depositon, and paleogeography of the study area during the glacial-related paleoclimatic episodes. Certain present-day shallow silled basins were used as a possible analogue of the Pennsylvanian cyclic epeiric seas. Eustatic changes in sea-level were closely related to cyclic glaciation, global tectonics, basin subsidence and sedimentation pattern.Because of paleobathymetric relief in the cyclic epeiric seas, some units disappear and new ones appear, commonly with a change of facies. In many cases, cyclic coal beds underlie black shales. This suggests that swamp environments were present intermittently due to minor regression preceding episodes of cyclic maximum transgression. The occurrence of the cyclic seas and swamps between 5° and 8° N paleo-latitude all suggest a tropical-wet climate with possible seasonally controlled rainfall.Thin laminae, fine particle size and high TOC content (up to 17 wt%) indicate stagnant conditions with bottom-water anoxia in the Excello Sea. Anaerobic sediments were deposited in more than 100 m depth of water, poor circulation, shallow mixing with atmosphere, and high organic productivity and/or better preservation was present during maximum transgressive episode. The establishment of a density-stratified water column or pycnocline was the most important factor in development of the anaerobic sediments. Perhaps a halocline between bottom normal marine salinnity waters and a surface fresh-water layer probably caused the water stratification and anoxia leading to better preservation of organic matter in the Excello Sea. |
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