Contribución al conocimiento sedimentológico y paleobotánico de la Formación El Freno (Jurásico Temprano) en el valle superior del río Atuel, Mendoza, República Argentina.

SPALLETTI, L.A.; MOREL, E.M.; FRANZESE, J.R.; ARTABE, A.E.; GANUZA, D.; ZÚÑIGA, A.

AMEGHINIANA

Asociación Paleontológica Argentina

Año: 2007 vol. 44 p. 367 - 386

0002-7014

Abstract. Contribution to the sedimentological and palaeobotanical knowledge of the El Freno Formation (Early Jurassic) in the upper Atuel River Valley, Mendoza, Argentina. Three detailed stratigraphic logs form the siliciclastic deposits belonging to the El Freno Formation (SInemurian) have been measured at Las Chilcas Hill and at El Cholo Anticline (upper reaches of the Atuel River Valley, Andean Cordillera of Mendoza, Argentina). The El Freno Formation is composed of clast-supported conglomerates, gravelly sandstones, sandstones, mudstones and heterolithic levels. The El Freno deposits were formed in fluvial systems which were characterised by systematic changes in architectural style due to vertical and regional variations in accommodation space, clastic supply, discharge and regional slope. Tractional bedload systems and mixed-load systems have been recognized. The former are characterised by vertical and lateral stacking of clast-supported coarse-grained deposits with common transitional flow-regime and low flow-regime tractional structures. The mixed-load systems consist of repeated cycles of channel-fill and aggradational flood plain deposits. Two main types of palaeochannel geometries (strings and belts) have been distinguished in the mixed-load systems. Six plantiferous levels have been identified in the measured profiles. One of them had been previously defined by Borrello and studied by Herbst. The floral assemblage of the El Freno Formation is composed of 13 taxa: Equisetites sp., Marattia münsteri (Goeppert) Zeiller, Cladophlebis mendozaensis (Geinitz) Frenguelli, Cladophlebis ugartei Herbst, Dictyophyllum (D.) atuelense Herbst, Dejerseya lobata (Jones and de Jersey) Herbst, Archangelskya proto-loxsoma (Kurtz) Herbst, Sagenopteris sp. cf. S. rhoifolia Presl, Kurtziana brandmayri Frenguelli, Otozamites bechei Brongniart, O. hislopi (Oldham) Feistmantel, Taeniopteris sp.b. and Elatocladus conferta (Oldham & Morris) Halle. This study, combined with the palaeobotanical analysis of five plantiferous levels in the cerro La Brea section, allowed us to define the overall palaeofloristic composition of the El Freno Formation. The El Freno palaeoflora was compared with other Jurassic palaeofloras of Argentina. A close relationship between the floristic assemblages of the El Freno Formation and the Lower Jurassic Nestares Formation (located in the northwestern sector of the North Patagonian Massif) has been established.