Ituzaingó Formation. A key for the interpretation of upper tertiary stratigraphy, Mesopotamia-Chaco Paraná Basin, Argentina

Abstract

Four years of sedimentological detailled study led me to reinterpretate the Upper Terriry Mesopotamia region stratigraphic squeme. Upon a detailed sedimentary environment analysis of the Mesopotamia – Chaco Paraná basin, I made a different interpretation about previous ideas of the several units present at this morphostructural domain. The Ituzaingo Formation was interpretated as continental (fluvial) in origin by several researchers based on an insecure fossil fauna of invertebrates, always present at the top of the layers. These fossil fauna his a broad and extend biochron over the Cenozoic time. The Paraná Formation infralies the Ituzaingó Formation. The idea of an erosion uncorformity between them was strongly proposed. The Paraná Formation carries marine fossil fauna of invertebrates dated as Middle Miocene. It generally is accepted that Toropí and Yupoí Formations rest over Ituzaingó Formation elsewhere. The structural contact was assumed as an erosional unconformity surface. Paraná Formation was dated by fossil invertebrate fauna as Middle Miocene. Topoí and Yupoí Formations were dated as of Midlle to Upper Pleistocene based on fossil vertebrate fauna. I assumed that the position of these fossils vertebrate fauna of is ofaloctonous for the Toropí and Yupoí muddy formations and I propose that the fossil vertebrate fauna is in Holocene sediments. I carried out detailled studies which include: architectural, granulometric and morphoscopic analysis, petrographic data, paleocurrent measurements, scanning electron microscopy, diffractometric x-ray measurements, detailled sedimentologic profiles, drill hole correlations and remote sensing digital processing image analysis. Architectural analysis me to distinguish the following internal sedimentary structures: hummocky cross stratification, tidal bundles, herringbone cross stratification, lenticular bedding, flaser bedding, bipolar cross stratification and several types of wavy stratifications. These studies led me to realize that the sedimentary environments and specially the relationship between these units are different from previously proposed studies. I propose a common shallow littoral marine origin (intertidal) for all these units in a Middle Miocene age, sincronous with the Paranense Sea ingression (12-14 MA) and an interdigitation between sand and mud lithofacies. A subsurface unit, near Paraná Formation, the Puelches Formatioin, is interpretated as belonging to this sandy-muddy typical shallow marine lithofacies, desoite of a previous work, that accepted a fluvial origin for this unit. An erosion unconformity was also proposed between Paraná and Puelches Formation. The Paraná and Ituzaingó Formations are composed by some 80% friable arenaceous facies and of about 20% of muddy facies. This is a typical heterolithic successioin. A neat but irregular ferricretization forehead is present, specially, in the sandy lithofacies. This physico-chemical phenomena led previous authors to a misinterpretation of the stratigraphic scheme. My results show that the contacts between forehead ferricretization in the sandy lithofacies was assumed as an erosion unconformity, which is not the case. In some place, the ferricretization formed ferrigenous sandstones, which I called ferrigenous duricrust. This is os a very recent age of about Upper Pleistocene-Holocene times. Calcretizatioin is also present in the outcrops. All these secondary processes hid the primary physical structures and chemical composition of the sandy-muddy lithofacies. Paraná, Ituzaingó, Toropí, Yupoí and Puelches Formations are suggested to have the same origin (intertidal) and the same age, Middle Miocene. They constitute a blanket body of about 1500 kilometers in extension at the northeastern portion of Argentina and have approximately 300 meters of thickness. They are composed only by medium, fine to very fine white sands and grey to beige mud lithofacies. When alternation is present (ferricretization), the sediments change its colours to reddish or yellowist, sometimes purple, brown or black. In a few cases the alternation (ferricretization) is very intense, then sands are converted into reddish to black sandstones. In this case, primary sedimentary tractive structures were always recognized. These units, Paraná and Ituzaingó Formations are sincronous and may be correlated with other similar outcrops at Uruguay, Brazil, Paraguay, Bolivia, Perú, Ecuador, Colombia, Venezuela and Guyanas. They were formed during Paranense-Amazon Seas.