Resumo

DI PAOLA, Elda. Distribución y evolución de los depósitos cenozoicos de la provincia de San Luis entre los 32° 20' y 34° de latitud sur, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 1994, vol.1, n.1, pp.33-52. ISSN 1853-6360.

In the San Luis Province the southern outcrops of the Lower Paleozoic basement of Sierras Pampeanas and Mesozoic deposits in Sierra de Las Quijadas, El Gigante and Cerro Charlone are found. All of them, together with volcanic, andesitic and trachyandesitic bodies, which extend between La Carolina and Cerro El Morro, constituted the areas of provenance of the sediments forming the Cenozoic sequences. Their study was made in the band between 32° 20’ and 34° south latitude (Fig. 1). The different Cenozoic units have been the sedimentary answers to processes of mainly tectonic and volcanic character, which modified the base level, both locally and regionally. Cenozoic rocks are continental, red in colour, without diagnostic fossils and with discontinuous outcrops, which makes the correlation to distance unreliable. These red beds, which are presumably of Miocene and Pliocene age, can be found in the west and south of the Sierra de San Luis, and in the surroundings of the Sierra de Las Quijadas and El Gigante. To the east, the central and southeastern part of the Sierra de San Luis and the Cerro El Morro are covered by volcaniclastites which are contemporary with the red beds (Fig. 2). Finally, in the south of the Sierra de San Luis, deposits of gray beds Pleistocene in age and over 1,500 m thick predominate. The correlations are complicated and, in order to systematize it, the deposition of the different units in relation to regional and local unconformities was used as a criterium (Fig. 3 and Tabla 1). TERTIARY EPICLASTIC ROCKS. The first Cenozoic unconformity shows the beginning of the Miocene sedimentation on basement nuclei. Tectonism caused the formation of depressions and elevations, which formed intra- and intermountain basins. The filling is usually composed by fanglomeradic, fluvial and lacustrine deposits. The Miocene unit that has been best studied so far, is the San Roque Formation in the south of the Sierra de San Luis, which in the Potrero de Los Funes and in Las Chacras Depression has a calculated thickness of 1,500 m (Rivarola, 1990; Di Paola & Rivarola, 1992). Their equivalent in the West of the Sierra de San Luis are outcrops which stand in Nogolí (Unit I, Ojeda 1991; Sozzi, 1991) and in the surroundings of the Sierra de Las Quijadas and El Gigante (Figs. 4 and 5) where they also have fanglomeradic and fluvial architectural elements. On the second angular and erosive unconformity, of semiregional character, the Pliocene sequences constituted by the Cruz de Piedra Formation and the Río Quinto Formation are found. The first one is mainly the result of the erosion of the underlying San Roque Formation. At the base of the Tertiary sequences and perhaps previous to vulcanism, calcretes (Figs. 6, 7, 8 and 9) were found in different localities (Santa Rosa, Cerro El Morro, Paso de Las Carretas). They have developed from basement rocks in the Cerro El Morro and from fluvial accumulations in the rest of the localities. They are considered to be pre-Miocene in age, until further evidences are found. The petrographic studies made on the units of the south of the Sierra de San Luis show that the Miocene detrites are of local provenance. The red pigment is considered to be mainly of primary origin due to deferrization of opaque minerals, although in the surroundings of the Mesozoic sedimentary nuclei the pigment is inherited from underlying red beds. The Pliocene sequences show as main characteristics both the incipient formation of silcretes (Figs. 10 and 11) and the partially allochthonous origin of the detritic fraction. In the big intermountain depressions Tertiary Mio-Pliocene sensu lato units are found in the subsoil (Santa Cruz, 1978; Manoni, 1985). TERTIARY VOLCANICLASTIC ROCKS. In the east and south-east of the Sierra de San Luis and in the surrounding of the Cerro El Morro, volcaniclastites predominate. They are the response both to the pyroclastism and the physiographic changes due to emplacement of lava flows and andesitic and trachyandesitic dykes, the ages of which would range from 6,7 my to 1,9 my (Ramos et al., 1991). The pushing up of the domes caused small basins, like that where the Cantera de Santa Isabel stands. In that area a complete sequence remained exposed and is constituted by fluvial sedimentites with stromatolites and calcareous onyxes (Sedimentitas Calcareas Santa Isabel, Lacreu & Di Paola, 1992); on them the Volcaniclastitas Lomitas Unit lies in a unconformity constituted by conglomerates, surges deposits and lapillitic tuffs, which are the result of two tectonic-volcanic pulses. At the top of the sequence there are sandstones filling deep channels which eroded the underlying units, named Sedimentitas El Pantano. These channels would have formed part of a major drainage network related to the elevation of the volcanic hills, which could have constituted a climatic barrier to the humid winds blowing from northeast to southwest. PLEISTOCENE EPICLASTIC ROCKS. The third unconformity determines the Plio-Pleistocene boundary. On this unconformity gray fanglomeradic and fluvial deposits are found. They probably are of the Lower Pleistocene age, but diagnostic fossils have not been found yet. These units which are well exposed in the south of Potrero de Los Funes (Fanglomerado del Potrero Formation, Rivarola & Di Paola, 1991), is faulted and lying over the basement crystalline rocks (Fig. 13). The fluvial discontinous lithofacies outcrops from Donovan locality to Cerro Charlone (Fig. 14), lie in a unconformity both, on Miocene beds in the north (Donovan locality) and Mesozoic beds in the south; they are of pebble-sandy composition, with normal graded and crossbedding stratification. The calculated thickness is over 1,500 m, being proposed for them the name of Donovan Formation in the present paper. This unit is composed of clasts coming mainly from the crystalline basement, being calcite the principal cementing material. The fourth Cenozoic unconforrnity separates the Lower Pleistocene units from loose psephitic deposits. They are the Las Chacras Formation in the south of the Sierra de San Luis (Latrubesse et al., 1991), Psefitas Olguin, in the Cantera de Santa Isabel (Lacreu & Di Paola., 1992), Psefitas La Unida, in the Cerro El Morro (Grosso, 1993) and the Estancia La Petra Formation (Santa Cruz, 1978). These sequences could have been related to stages of great erosion due to changes of the sea level, with which the major fluvials systems (Rio Quinto and Rio Nogolí) draining the southern part of Sierra de San Luis, would be connected.

Palavras-chave : Sedimentary answers; Cenozoic; Continental deposits; Red beds; Gray beds; Volcaniclastites; Calcretes; Silcretes.

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