Architecture of Ituzaingó Formation (Pliocene), a comparison with deposits of Paraná River, Argentina

GEORGIEFF, SM., ANIS, K.B., ORFEO, O., RIZO, G.E.

Delft, Holanda

Conferencia; 8th International Conference on Fluvial Sedimentology; 2005

Universidad de Delft

The main goal of this work is to reconstruct ancient fluvial sandstone deposits and to compare them with modern bar deposits of some islands of Paraná River. Detailed descriptions of Ituzaingó Formation, Middle Pliocene, were performed in three ravines near Empedrado City, Corrientes Province, northeast of Argentina. The outcrops show a very good 2D lateral exposures, more than 40 m long and 2.5 to 3 m high. One of the outcrop is normal to the others and therefore a 3D partial perspective is obtained. The strata are almost horizontal and only a gentle deformation is observed like small faults with displacements from 2 to 5 cm. Grain size is medium sandstone (80-90% quartz, less than 5 to 10% feldspars and heavy minerals), even though the range is from fine to very coarse sandstone and some opal pebbles. Intraformational pebbles were observed at the base of some deposits. The source area of the sandstones is from Botucatú Formation (Upper Jurassic – Lower Cretaceous) in Brazil. Colour photomosaics were taken from vertical ravines, in order to refer the location vertical logs and trace of main and secondary contacts. Twelve detailed sedimentary profiles were measured along the outcrops to describe vertical and lateral facies and paleocurrent directions changes. Additional paleocurrent directions were recorded among profiles and they were located on the photomosaic. Through cross stratification is the main sedimentary structure, parallel lamination, ripples marks, bioturbation and convolute lamination are recorded at the top of the deposits and among through cross strata. Deposits have different kind of geometries (concave-, convex-upward and flat), and several accretion surfaces with systematic paleocurrent variations were recorded on the photomosaic and profiles. A map were performed by using a total station (infrared ray and miniprism), main contacts, location of profiles, outcrop orientations and a correlation of strata were recorded with this equipment. These deposits were reconstructed in a plan of view and the distribution and dispersion of paleocurrents were represented on a map, from the reconstruction, map and complementary data (contacts hierarchies, geometries and facies changes) we interpret these strata like lateral and central channel-bars, channels and chute-channels. Paleocurrent directions indicate altenative expansion and down stream migrations. We also compare the outcrops with the deposits of modern bars of Paraná River, specifically those belonging to Noguera and Choui[SG1]  islands (bars are 30 and 40 years old). We recorded similar grain size, sedimentary structure and geometries. Moreover, scale of sedimentary structures and deposits dimensions are very similar to those Pliocene outcrops described in the Empedrado. Thus, Ituzaingó Formation seems to be comparable ti the Paraná River deposits but the Pliocene deposits study here are smaller than those belong to the main river and then they seem to be more related to the channel-bars of Paraná’s islands.