Modification of fluvial style in a volcaniclastic setting: an example from the Cretaceous Cerro Barcino Formation, Patagonia, Argentina

UMAZANO, A. M.; KRAUSE, J. M.; BELLOSI, E. S.; PÉREZ, M. ; VISCONTI, G.

Puerto Madryn

Congreso; 14°Reunión Argentina de Sedimentología; 2014

Asociación Argentina de Sedimentología

The Cretaceous Cerro Barcino Formation (Chubut Group, Somuncurá-Cañadón Asfalto basin) records different alluvial environments affected by influxes of volcanic ash-falls. In particular the Puesto La Paloma Member, lower section of the Formation, was mostly deposited by unconfined fluvial flows (Allard et al., 2010, Umazano, 2010) as consequence of a high pyroclastic sediment supply in regard to the underlying siliciclastic Los Adobes Formation (Villegas et al. this meeting). The Cerro Castaño Member, which was deposited over the Puesto La Paloma Member, represents fluvial channel-belts that crossed a vegetated floodplain (Manassero et al., 2000, Allard et al., 2010, Umazano et al., 2012). In this context, the goals of this contribution are: i) to provide a detailed palaeonvironmental interpretation of both members at a regional scale; and ii) to discuss the modification of the fluvial system from unconfined conditions to channelized flows. The study area is located in the north-central part of the Chubut province and includes the following localities (and section thickness): Huanimán (133 m), Tres Cerros (120 m), Puesto Mesa-Cerro León (267 m), La Payanca (124 m), La Madrugada (117 m) and La Juanita (68 m). Except for Tres Cerros, the remaining localities define an approximately N-S oriented transect, in which the thickness of both members reduces from Puesto Mesa-Cerro León to the south. As a generalization, in all localities the Puesto La Paloma Member is composed of sheet-like tuffaceuos strata dominated by sand-sized sediments, whereas the Cerro Castaño Member includes channeled sandstone bodies interbedded with tuffaceous and finer-grained sediments. Methodology integrated the measurement of sedimentary logs, and both facies and architectural analysis. Six facies associations were defined: sub-aerial ash falls (FA1), sheet-floods (FA2), shallow lakes (FA3), aeolian deposits (FA4), fluvial channel-belts (FA5) and debris flows (FA6). The Puesto La Paloma Member mainly records sheet-floods (FA2) in all localities except for La Payanca, where it is dominated by 2D aeolian dunes and dry interdune zones (FA4). Moreover, in this locality the sheet-floods are, in decreasing order of abundance, the second component of the member. Subordinately, this member locally has deposits originated from ash-falls (FA1), shallow lakes (FA3), Aeolian dunes and associated interdune zones (FA4). The overlying Cerro Castaño Member records perennial fluvial channel belts (FA5) and floodplains mainly constructed by sheet-floods (FA2) that drained into standing waters (FA3). In minor amount, and locally, debris flows (FA6) and primary pyroclastic deposits (FA1) were recognized in the floodplains. Architectural analysis of some FA5 bodies shows evidences of lateral accretion surfaces and longitudinal bars, suggesting a probable meandering-like fluvial system. The facies change suggests that, during deposition of Cerro Castaño Member, the ratio between sediment supply and accommodation space creation was lower in relation to depositional times of Puesto La Paloma Member. It is consistent with the generation of regional moderately-developed palaeosols near the contact between both units (Krause et al. this meeting). Coevally, the generation of a general low equilibrium profile during deposition of Cerro Castaño Member allowed that the rivers acquired a channelized style, as showed during sedimentation of the Los Adobes Formation