Fossiliferous sections in the Cerro Barcino Formation (Cretaceous, Patagonia): implications for mapping and sequence stratigraphy

KRAUSE, J.M.; UMAZANO, A.M.; POL, D.; CARBALLIDO, J.L.; STERLI, J.; CLADERA, G.; BELLOSI, E.S.; PUERTA, P.

Puerto Madryn

Otro; XIV Reunión Argentina de Sedimentología; 2014

Asociación Argentina de Sedimentología

The Cretaceous Cerro Barcino Formation (CBF), which is included in the Chubut Group, bears an important suite of vertebrate remains including titanosauriform sauropods,theropod dinosaurs, crocodyliforms, turtles and lepidosauromorphs. However, the lack of a well-defined and easily applicable lithostratigraphic scheme precluded establishing temporal relationships between the components of such fauna in the study area. In this contribution we analyze the stratigraphic relationship of the sections bearing fossil vertebrates and discuss the results regarding mapping, sequence stratigraphy and evolutionary implications.The studied outcrops are located in the north-central Chubut Province and include six localities: La Juanita, Turtle Town-La Madrugada, Puesto Mesa-Cerro León, Huanimán, Tres Cerros, and Chubutisaurus; all of them bearing vertebrate fossils. In the study area the substratum of the CBF is constituted by the Bardas Coloradas Member or by an irregular palaeo-relief mostly composed of Jurassic volcanic rocks.The Cretaceous succession is covered by Paleogene-Quaternary, continental and marine sequences. The Cerro Barcino Formation includes from base to top: the Puesto La Paloma Member (PLPM), mostly composed of green tuffaceous strata; the Cerro Castaño Member (CCM), largely composed of brownish to reddish tuffaceous sandstones and mudstones; and the Bayo Overo Member (BOM), which includes greenish grey tuffaceous sandstones and mudstones. Typically, the succession records fluvial sedimentation, which evolved from unconfined systems with local development of lakes and aeolian dunes (PLPM) to fluvial channels crossing a vegetated floodplain (CCM and BOM. The age suggested for the Chubut Group, estimated from macro and microfossils and field relationships, ranges from the Barremian to the late Cenomanian-early Turonian. Structural relationships and field evidences indicate that northern and north-western outcrops are gently older than eastern localities. In western outcrops (La Juanita, Turtle Town-La Madrugada, Puesto Mesa-Cerro León, Huanimán and Tres Cerros) the PLPM and CCM are present, while in Chubutisaurus section the BOM is recognized. The correlation among the studied localities allowed recognizing that most of fossil vertebrates derive from a unique discrete stratigraphic interval. This interval includes the top of the PLPM and the lower section of the CCM or equivalent beds. Three implications arise from these observations; first, some partial changes need to be implemented in the regional geologic charts, because it is evident that outcrops assigned to the BOM) could correspond to the PLPM/CCM. Thus, more detailed field works are necessary for a better delineation of the contact between the CCM and BOM. Second, the concentration of fossil vertebrates in a discrete stratigraphic interval, particularly in the lower part of the CCM, suggests the presence of a possible condensed section as the response to a relatively high base level of rivers and/or a reduced pyroclastic sediment supply. These interpretations are supported by the manifest change in the depositional systems and the presence of a stronger-developed hydromorphic paleosol complex of regional extension. Third, a well-constrained stratigraphic correlation among the sections will provide the framework to analyze the evolutionary history of the fossil fauna herein considered that constitutes one of the most complete and diverse faunas from the Cretaceous of Gondwana.