INCITAP   20787
INSTITUTO DE CIENCIAS DE LA TIERRA Y AMBIENTALES DE LA PAMPA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Early Cretaceous ichnofauna from eolian and associated deposits of Patagonia: the Cerro Barcino Formation, Chubut, Argentina
Autor/es:
PEREZ, M.; UMAZANO, A.M.; MELCHOR, R.N.
Lugar:
Santa Rosa
Reunión:
Simposio; II Latin-American Symposium on Ichnology; 2013
Resumen:
Trace fossil findings in Early Cretaceous eolian deposits are scarce. The Albian, alluvial eolianPuesto La Paloma Member is the basal section of the Cretaceous Cerro Barcino Formation(Chubut Group in the Somuncurá-Cañadón Asfalto basin). This unit offers a good opportunity tostudy organism-sediment interaction in Early Cretaceous eolian dunes and associated fluvial andlacustrine environments. For this contribution we measured four partial sections of the unit in threelocalities near Paso de Indios in the Chubut province. They are, from north to south, Cerro LosChivos (18 m thick, 43°13?9,78?? S - 68°50?36,18?? W), Huanimán (12 and 4 m thick, 43°7?54,44??S - 68°39?53,88??W) and Estancia La Payanca (18 m thick, 43°29?43,5?? S - 68°52?31?? W). Logged sedimentologic sections included trace-fossil bearing eolian deposits and associated fluvial, primary pyroclastic and lacustrine facies. Five types of deposits have been recognized including those of eolian dunes, pyroclastic ash falls, sheet-floods, shallow lakes and low-relief eolian bedforms. The last four deposits belong to interdune zone. Eolian dunes deposits are composed of fine-grained tuffaceous sandstones with tabular planar-cross bedding and subordinated trough, tabular tangential and low angle-cross bedding. The lack of grain-size contrast in eolian dune deposits makes recognition of trace fossils difficult. Skolithos isp. and root traces were found associated to second order surfaces (reflecting dune migration). Eolian dunes also contained, Taenidium barretti and Paleophycus tubularis developed parallel to foreset laminae, plus small mammal-like footprints, one tridactyl footprint and arthropod trackways. Mammal-like footprints are 27 mm long and 24 mm wide, have a triangular sole and three? short digit imprints with claw marks. Its morphology is roughly comparable with Brasilichnium. The tridactyl track is 29 mm long and 31 mm wide and the total divarication is 28º. Arthropod trackways are poorly preserved biserial rows of rounded imprints (about 3 mm wide) with series of 4 or 5 imprints. Low relief eolian bedform deposits are mainly composed of fine-grained tuffaceous sandstones with low angle-cross bedding and traslatent lamination, and subordinated massive tuffaceous sandstones. Trace fossil content is scarce and is represented by Skolithos isp., Taenidium barretti and rhizoliths. Sheet-flood deposits are occasionally associated with ash fall deposits showing accretionary lapilli. The former are composed of massive and laminated tuffaceous mudstones and fine-grained tuffaceous sandstones. Both deposits may overlie or underlie eolian dunes and display pedogenic features. One of the best developed paleosols (Cerro Los Chivos) contained a diffuse boxwork ichnofabric in the upper horizon, and a lower horizon with Edaphichnium lumbricatum, Taenidium barretti, Beaconites isp. and Skolithos isp. as discrete trace fossils. Another well developed paleosol (Estancia La Payanca) developed in tuffaceous mudstones and showing sphaerosiderite nodules and angular blocky peds, contained rhizoliths up to 10 cm long, Beaconites isp., Taenidium barretti and Skolithos isp. Finally, shallow lacustrine deposits covered by eolian dunes (Huanimán) are composed of massive and laminated tuffaceous mudstone levels with root traces, Taenidium barretti and Archaeonassa fossulata. The ichnofauna found in eolian dunes is similar to those reported from other roughly coetaneous eolian units from Brazil, namely the Guará and Botucatú formations (Upper Jurassic-Lower Cretaceous, Paraná basin). Elements shared by these units include mammal-like and tridactyl tracks, arthropod trackways and meniscate burrows.