CONICET | Buscador de Institutos y Recursos Humanos

The Lower Cretaceous Castillo Formation (San Jorge Basin, Argentina) records fluvial and lacustrine sedimentary settings strongly influenced by explosive volcanism derived from the western part of Patagonia . The unit contains important hydrocarbon reservoirs in the subsurface of the basin and several outcrop studies focused on analog hydrocarbon-bearing strata (sandbodies) were made in the San Bernardo Range, where the Castillo Formation is exposed. However, the common volcaniclastic resedimentation of the unit has not been studied and its analysis represents the goal of this contribution. Two exposures of the Castillo Formation were studied, both within the San Bernardo Range, Chubut province: Cerro Colorado de Galvéniz (CG; 342 m thick) and Cañadón Puerta del Diablo (PD; 50 m thick). In both localities, the sedimentary succession is part of the western limb of the same anticline structure which has a N-S oriented axial trace. The unit is typically composed of sandstone lenses interbedded with thicker and finer-grained sheets of primary and reworked tuffs. Five facies associations (Fa) were recognized. Fa1 includes commonly fining-upward, sandstone-conglomerate bodies with erosive bases and channel-like geometry in cross-section. Internally, these bodies have through cross-bedding, massive structure or plane parallel lamination. Mean palaeocurrent indicate that the palaeoflow was towards N 82º (n = 26) at CG and N 93º (n = 24) at PD. It is interpreted as channel bars and fills and the architectural analysis suggest that the rivers were single-channels with channel-margin bars. Fa2 is composed of accretionary lapilli-rich, fine-grained vitric tuffs showing mantle bedding and internal diffuse parallel stratification or massive structure. These facies originated as sub-aerial pyroclastic ash-fall deposits. Fa3 consists of sheet-like strata of dominantly massive, occasionally plane parallel laminated or through cross-bedded, fine-grained tuffs showing intraclasts and scour and fill structures in their bases. In a few cases, deposits of Fa3 are capped by mottled tuffs bearing scarce thin root traces and vertical to horizontal burrows (Taenidium barretti, Skolithos and Palaeophycus) and possible pupal chambers of insects. It is interpreted as ash-fall deposits reworked by sheet-floods and later subjected to weak pedogenic modification. Stacked and moderately-developed palaeosols, observed in other exposures of the lower section, evidence lengthier periods of non-depositation and soil formation in the floodplain. Fa4 comprises sheet-like to channel-like tuffaceous siltstone bodies with mostly deformed plane parallel lamination, asymmetrical ripples or massive structure; some levels show desiccation cracks and horizontal trails. This facies association suggests deposition in shallow lakes with occasional periods of sub-aerial exposition. Fa5, only recognized at CG, is characterized by structureless, matrix-supported, conglomerate bodies with sheet or plane-convex geometry and locally erosive basal surfaces. These deposits are composed of andesitic and tuffaceous gravel-sized clasts floating in a matrix of fine-grained ash. Deposits of Fa5 are considered sub-aerial debris-flows partially originated from erosion of underlying sheet-flood (Fa3) and pyroclastic-fall (Fa2) deposits. In both localities, during deposition of the Castillo Formation the primary pyroclastic input arrived via sub-aerial ash-fall events (Fa2). Nevertheless, their deposits were commonly reworked by fluvial channels (Fa1) and sheet-floods (Fa3), the later represent the main aggradational process of the floodplain. Subordinately, the primary pyroclastic substrates also were remobilized in the overbank environment by sub-aerial debris-flow (Fa5) and unidirectional flows in shallow lakes (Fa4).

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