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sur le développement des zones arides et semi-arides

Titre : The role of tetrapod trace fossils in continental ichnofacies models in arid-semiarid climate environments

El rol de las trazas fósiles de tetrápodos en los modelos de icnofacies continentales en ambientes de climas áridos-semiáridos

Auteur : Krapovickas, Verónica

Université de soutenance : Universidad de Buenos Aires

Grade : Doctor 2010

Résumé
The ichnofacies model provides the main framework to understand the paleoenvironmental significance of trace fossils. Traditionally focused on invertebrate trace fossil, the ichnofacies model has only recently expanded to include tetrapod footprints. The purpose of this thesis is to study the role of tetrapod footprints in the development of continental ichnofacies models, as well as in the reconstruction of sedimentary paleoenvironments in arid and semiarid climate environments. An integrative perspective was employed to tackle this problem, taking into account depositional environments, as well as vertebrate and invertebrate trace fossil data. The various examples that constitute the empiric base of this study represent a new contribution to the ichnofaunistic knowledge of different lithostratigraphic units that outcrop in Argentinean localities -Patquía Formation, Paganzo, La Rioja province ; Candeleros Formation, Villa El Chocón area, Neuquén province ; Anacleto Formation, Sierra Barrosa, Neuquén province ; Vinchina Formation, Vinchina, La Rioja province -. The trace-fossil assemblage of the Lower Permian Patquía Formation consists of locomotion, resting and dwelling traces assigned to Cruziana problematica, Rusophycus carbonarius and Paleophycus tubularis, and occurs in floodplains deposits. In sand sheet deposits of the same formation, tetrapod fossil footprints, dwelling traces of invertebrates and locomotion arthropod traces are preserved and assigned to Chelichnus duncani in the case of tetrapods and Palaephycus tubularis and Skolithos isp. for invertebrates. Study of the Upper Cretaceous Candeleros Formation includes the classic localities of Villa El Chocón and Balneario Villa El Chocón, as well as a new locality (Cañadón de las Campanas). The trace-fossil association consists of invertebrates trace fossils (Arenicolites isp., Arenicolites carbonarius, Diplocraterion parallelum, Paleophycus tubularis, Scoyenia gracilis, Skolithos verticalis, Taenidium barretti), theropod dinosaur footprints (Irenesauripus isp.), and sauropods footprints (cf. Brontopodus birdi). It occurs in damp interdune and playa-lake deposits. The trace-fossil assemblage from the Upper Cretaceous Anacleto Formation includes avian and non-avian theropod footprints (Inotornis mcconnelli, Barrosopus slobodai, medium tridactyl footprints) and invertebrate trace fossils (Arenicolites type 1, type 2, Palaeophycus tubularis, cf. Digitichnus laminatus, large horizontal tubes). This assemblage occurs in deposits interpreted as formed in ephemeral water bodies developed in floodplain depressions. Associated structures attributed to megarizoliths occur in river-channels deposit. The trace-fossil assemblage from the Miocene Toro Negro Formation is very rich. Crevasse-splay deposits mainly contain meniscate feeding and dwelling traces (Taenidium barretti, Scoyenia gracilis and Paleophycus tubularis). Emerged-sandbar deposits include avian and mammalian footprints (Fuscinapeda sirin, Macrauchenichnus rector, cf. Venatoripes riojanus, small heteropod footprints, large incumbent footprint, slender anisodactyl footprint, kidney-like impressions, and oval monodactyl impressions), and scarce Palaeophycus tubularis and Helminthopsis hieroglyphica specimens. Finally, the pattern of distribution of vertebrate and invertebrate trace fossils in sedimentary environments developed under arid-semiarid climate conditions is evaluated. This analysis is based on the trace-fossil associations studied for this thesis, together with other classic ichnologic associations reported elsewhere, which have been compiled in a database. Previous tetrapod ichnofacies models established a relationship between fossil footprint associations and sedimentary paleoenvironments, but no insight on the controlling environmental parameters was provided. It is suggested that the distribution of tetrapod footprints and invertebrate trace fossils are not controlled by the same paleoenvironmental factors. The distribution of tetrapod trace fossils in terrestrial environments seems to be more

Mots clés : ICNOFACIES CONTINENTALES ; HUELLAS DE TETRAPODOS ; PERMICO ; CRETACICO ; MIOCENO ; AMBIENTES ARIDOS ; CONTINENTAL ICHNOFACIES ; TETRAPOD FOOTPRINTS ; PERMIAN ; CRETACEOUS ; MIOCENE ; ARID ENVIRONMENTS

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