Resumo

PAREDES, José Matildo. Facies association and correlation of the sedimentites of Chenque Formation (Oligocene-Miocene) in the surrounding of Comodoro Rivadavia, Golfo de San Jorge Basin, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.1, pp.53-64. ISSN 1853-6360.

The Chenque Formation (upper Oligocene-lower Miocene) is widely represented in the Golfo San Jorge Basin. Around the Comodoro Rivadavia city it is composed of about 350 meters of siliciclastic and bioclastic sediments, with variables quantities of piroclastic material. The unit was deposited over the continental tuffs of the Sarmiento Formation (Eocene-Oligocene), in apparent conformity or locally by means of a subtle unconformity.Its base consists of few meters thick bioclastic sandstones and siltstones and represents a residual lag of a transgression. This facies has been recognized in all the profiles. This interval passes up to a 30 to 40-meter thick siltstone with hummocky cross-stratification and some evidence of tidal action, deposited in an inner offshore setting. Transitionally, it incorporates fine sandstones in lenticular lenses or continuous strata with a progradational pattern stacking, corresponding to a highstand system tract. Over an omission surface bioclastic conglomerates and medium sandstones that include bioclastic lenses were deposited under subtidal conditions, with a fining-upward pattern. This levels are interpreted as corresponding to a transgressive system tract. Glauconite, hardground and conspicuous bioturbation on the upper half of this level suggest a low sedimentation rate and a potential maximum flooding surface. A progradational set of very fine to medium sandstones with siltstone in the base were deposited in subtidal conditions. It shows unidirectional and bidirectional structures and has incorporated bioclastic beds of very different origin (in situ colonies, storm deposits and channelized biofacies). A 20-meter thick claystone and siltstone beds were deposited in an embayment, representing the final stage of the progradation associate to a highstand system tract. Over this claystones and siltstones and with an erosive contact a bioclastic conglomerate with bored hardground shows the base of a new depositional sequence. This residual lag is covered by fine to medium sandstones, with metric fining-upward cycles, showing the migration of 3-D subaqueous dunes. In the upper part of this cycles a hardground has been recognized. Also, bioturbation, wavebedding and flaser structures suggest a tide-dominated setting in very low sedimentation rate conditions, interpreted as a stillstand stage. A new depositional sequence, having the same characteristics of the previous cycles and a transgressive residual lag in the base is recognized. This progradational set were deposited under subtidal to intertidal conditions, with unidirectional and bidirectional structures. Some of the profiles are covered by polimictics conglomerates of Pliocene age known as Rodados Tehuelches.

Palavras-chave : Golfo San Jorge Basin; Marine Miocene; Chenque Formation; Sedimentology and sequence stratigraphy.

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