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

KRAUSE, MARCELO; UMAZANO, ALDO M.; POL, DIEGO; CARBALLIDO JOSE L.; STERLI, JULIANA; CLADERA, GERARDO; PUERTA, PABLO

Puerto Madryn

Congreso; XIV Reunión Argentina de Sedimentologia; 2014

The Cretaceous Cerro Barcino Formation (CBF), which is included in the Chubut Group sensuCodignotto et al. (1978), bears an important suite of vertebrate remainsincluding titanosauriform sauropods,theropod dinosaurs, crocodyliforms, turtles and lepidosauromorphs. However, the lack of a well-defined andeasily applicable lithostratigraphic scheme precluded establishing temporal relationships between thecomponents of such fauna in the study area. In this contribution we analyze the stratigraphic relationship of thesections bearing fossil vertebrates and discuss the results regarding mapping, sequence stratigraphy andevolutionary implications.The studied outcrops are located in the north-central Chubut Province and include sixlocalities: La Juanita, Turtle Town-La Madrugada, Puesto Mesa-Cerro León, Huanimán, Tres Cerros, andChubutisaurus; all of them bearing vertebrate fossils (Figs. 1-2). In the study area the substratum of the CBF isconstituted by the Bardas Coloradas Member or by an irregular palaeo-relief mostly composed of Jurassicvolcanic rocks.The Cretaceous succession is covered by Paleogene-Quaternary, continental and marinesequences. 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 brownishto reddish tuffaceous sandstones and mudstones; and the Bayo Overo Member (BOM), which includes greenishgrey tuffaceous sandstones and mudstones (Codignotto et al. , 1978). Typically, the succession records fluvialsedimentation, which evolved from unconfined systems with local development of lakes and aeolian dunes(PLPM) to fluvial channels crossing a vegetated floodplain (CCM and BOM) (Umazano et al., this meeting).The age suggested for the Chubut Group, estimated from macro and microfossils and field relationships, rangesfrom the Barremian to the late Cenomanian-early Turonian (e.g., Proserpio, 1987; Marveggio and Llorens, 2013;Suárez et al ., 2014). Structural relationships and field evidences indicate that northern and north-westernoutcrops are gently older than eastern localities (Chebli et al ., 1976). In western outcrops (La Juanita, TurtleTown-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 allowedrecognizing that most of fossil vertebrates derive from a unique discrete stratigraphic interval. This intervalincludes the top of the PLPM and the lower section of the CCM or equivalent beds. Three implications arisefrom 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 (Proserpio, 1987; Anselmi et al., 2004) could correspondto the PLPM/CCM. Thus, more detailed field works are necessary for a better delineation of the contact betweenthe 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 responseto a relatively high base level of rivers and/or a reduced pyroclastic sediment supply. These interpretations aresupported by the manifest change in the depositional systems and the presence of a stronger-developedhydromorphic paleosol complex of regional extension. Third, a well-constrained stratigraphic correlation amongthe sections will provide the framework to analyze the evolutionary history of the fossil fauna herein consideredthat constitutes one of the most complete and diverse faunas from the Cretaceous of Gondwana.