Deposition and reworking of primary pyroclastic detritus in Puesto La Paloma Member, Cretaceous Cerro Barcino Formation, Somuncurá-Cañadón Asfalto Basin, Patagonia, Argentina

UMAZANO, A.M.

Mar del Plata

Congreso; International Geological Congress of the Southern Hemisphere; 2010

The Puesto La Paloma Member (PLPM) constitutes the basal section of the Cretaceous Cerro Barcino Formation (Chubut Group), which was accumulated during sag conditions in the Somuncurá-Cañadón Asfalto Basin, Patagonia, Argentina. The PLPM overlies to Los Adobes Formation and is covered by Cerro Castaño Member. It is composed of continental, greenish, pyroclastic-rich strata without a detailed paleoenvironmental scheme to local, regional and temporal scales. Aim of this contribution is to analyze the sedimentary paleoenvironments of the unit with emphasis in the recognition of reworking processes of primary pyroclastic deposits. The PLPM was studied in four localities of the western sector of the basin within the Chubut province, from NW to SE they are: Cerro Los Chivos (43º13’10’’ S, 68º56’43’’ W; 42 m thick), Estancia La Payanca (43º29’43’’ S, 68º52’31’’ W; 20 m thick), Estancia La Madrugada (43º35’20’’S, 68º56’32’’W; 19 m thick) and Estancia La Juanita (43º48’2’’ S, 68º52’15’’W; 18 m thick). In all localities, the PLPM is mainly constituted of fine-grained tuffs and tuffaceous sandstones with minor amounts of tuffaceous mudstones, conglomerates and chert. Facies associations (FA) include sub-aerial primary pyroclastic deposits (FA1) and epiclastic deposits partially originated by reworking of the formers (FA2 to FA5). FA1 (ash-falls) comprises massive or plane parallel laminated, accretionary lapilli-rich, fine-grained tuffs disposed in bodies with mantle bedding and non erosive bases; intercalations of apedal paleosoils and burrowed levels are common. FA2 (unconfined fluvial flows) includes stacked and amalgamated sheet-like bodies composed of fine-grained tuffaceous sandstones; individual bodies have erosive bases and commonly show massive aspect in the base and lamination and roots in the top. FA3 (permanent fluvial channel belts) is mainly constituted by tuffaceous sandstone bodies with irregular and erosive bases and plane tops; internally, the bodies are composed of laterally stacked large-scale inclined strata-sets. FA4 (aeolian) is represented by a single body composed of medium-grained tuffaceous sandstones; the lower part (dunes) shows planar-tabular cross-beddings with frontal laminae dip 25° on average; the upper sector (interdunes) exhibits low-angle cross-bedding with intralamina inverse grading. FA5 (ponds) is a unique tabular tuffaceous mudstone body with massive aspect in the base which pass upward to levels with plane parallel lamination or asymmetrical ripples; coarse to fine-grained chert intercalations are relatively common. FA1 and FA2 occur in all localities, the first with reduced participation (<10%) and the later constituting always more than 50% of the sections. The remaining FA occur only in one locality; FA3 in Estancia La Payanca (≈ 40%) and FA4 and FA5 in Cerro Los Chivos (≈ 7% and ≈ 8%, respectively). Facies relationship suggests accumulation of sub-aerial ash-falls (FA1) and frequent removilization of the tephra by unconfined fluvial flows (FA2); locally, primary pyroclastic substrates were also reworked by channelled fluvial flows (FA3), aeolian processes (FA4) and shallow lacustrine sedimentation (FA5).