FLUVIAL REWORKING OF PYROCLASTIC DEPOSITS: THE CRETACEOUS LOWER MEMBER OF THE BAJO BARREAL FORMATION, SAN JORGE BASIN , PATAGONIA, ARGENTINA

ALDO MARTÍN UMAZANO; EDUARDO SERGIO BELLOSI; GRACIELA VISCONTI; RICARDO NESTOR MELCHOR

San Miguel de Tucumán

Simposio; 9th International Conference on Fluvial Sedimentology; 2009

The lower member of the Bajo Barreal Formation (Cretaceous of Patagonia) records fluvial sedimentation affected by explosive volcanism produced in the Patagonian Andes. Five sections were studied along a N-S transect in the San Bernardo Range. This member is composed of sandstone-conglomerate lenses interbedded with thicker and finer-grained tuffaceous sheets. Facies associations (Fa) defined include sub-aerial primary pyroclastic deposits (Fa1 and Fa2) and epiclastic deposits partially originated by reworking of the formers (Fa3 to Fa7). Fa1 (pyroclastic ash-falls) is composed of massive or laminated, fine-grained vitric tuffs showing mantle bedding and pedogenized tops. Fa2 (pyroclastic flows) consists of stratified lapilli tuffs with reverse grading of pumice that pass upward to cross-bedded tuffs, which are capped by massive tuffs. Fa3 (fluvial channel belts) includes fining-upward bodies with channel or ribbon geometry, increasing their thickness and width from N to S. Fa3 deposits begin with through cross-bedded conglomerates or horizontal stratified breccias with vertical transition to through cross-bedded or massive sandstones. Rarely, pause-planes were detected in the lower part of channel deposits suggesting intermittent sedimentation. Paleocurrents indicate paleoflow towards E-SE in all localities. Architectural analysis mostly suggests multi-channelled and subordinately single-channelled rivers with point bars. Fa4 (crevasse-splays) is characterized by massive and cross-bedded lobe-shaped sandstone bodies with subtle fining-upward trend, participation of mudstone drapes is rare. Fa5 (sheet-floods) comprises sheet-like strata of fine-grained tuffs with scoured bases; commonly they are massive in the basal part and laminated in the upper sector. Rarely, there are intercalations of tuffs and sandstones with different cross-beddings or pedo-features. Paleodrainage was near orthogonal to those recorded in Fa3 deposits. Fa6 (debris-flows) is characterized by sheets to plano-convex bodies dominated by massive, matrix-supported and intraclastic conglomerates. Intercalations of tuff levels with through cross-bedding or paleosols are scarce. Fa7 (ponds) includes sheet-like to channel-like tuffaceous mudstone bodies with plane parallel lamination, asymmetrical ripples or massive structure arranged in coarsening-upward cycles. Facies relationship suggests fluvial systems constituted by different types of fluvial channel belts (Fa3) and floodplains where accumulation of primary pyroclastic detritus occurred (Fa1 and Fa2), although the remobilization of these by different fluvial processes was common (Fa4 to Fa7).