Resumen

ASTINI, Ricardo A  y  CANAS, Fernando L. La Formación Sassito, una nueva unidad calcárea en la Precordillera de San Juan: Sedimentología y significado estratigráfico y paleoambiental. Rev. Asoc. Argent. Sedimentol. [online]. 1995, vol.2, n.1-2, pp.19-36. ISSN 1853-6360.

A 24.5 m thick Late Ordovician (Caradocian-Lower Ashgillian) carbonate succession, the Sassito Formation (nom. nov.) is described and analyzed. The thinly stratified thickening-upward and strongly episodic unit is found in the Central Precordillera of San Juan. Argentina (Fig. 1), at the Sassito thrust. The formation represents the youngest carbonate succession described in the Lower Paleozoic of the Precordillera. Formerly, it had been considered as part of the San Juan Formation but recent surveys allow to determine the existence of an important erosive and nondepositonal gap between them, as shown by a thin horizon of chert and limestone conglomerates and coarse calclithites which cover the San Juan Formation. The conglomerates are succeeded by 5.5 m of condensed black shaly mudstones (Fig. 2). Several lithofacies are identified within the Sassito Formation: a) black calcilutites (shaly mudstones), b) coarse fossiliferous calcarenites (calclithites), seldom graded, c) thin-bedded calcarenite/calcisiltite rhythmites with variable degree of bioturbation, and d) laminated (with frequent low angle cross- laminations) fine-grained calcarenites. From a petrologic viewpoint the Sassito Formation includes both extraclats (detrital non-coeval carbonate grains) and autochthonous (coeval intrabasinal) carbonate components. For this reason, these rocks are seldom named calcarenites or calclithites, which according to Folk (1959, p. 36) contain over 50 percent carbonate particles derived from preexisting limestones and dolostones. Dunham (1962) classification cannot be applied properly. A certain amount of siliciclastic components, either rock fragments or isolated quartz and feldspars are also present. Coarse calclithites form part of the basal conglomerate and lenticular beds at half section. Calcisiltites and calcarenites largely predominate in the upper half of the section. Among the allochems over 90 % are rounded homogeneous micrite peloids. Grain types, in particular the remarkable absence of oolites, aggregate, and algal grains in the high-energy facies, the low depositional rates and the associated fauna, point to a temperate carbonate shelf environment for the development of the Sassito Fm. These evidences are in accordance with the regional sedimentological and paleontological data which show an intermediate to high latitude for the Precordillera during the Late Ordovician. The upper part of the Sassito Formation represents the episodic sedimentation of a storm-dominated carbonate shallowing-upward succession deposited on a shallow carbonate ramp, mostly between the storm wave base and the fair weather wave base (Fig. 4). A gradual stratofabric change from tabular to lenticular allow to interpret a progradational arrangement from mid-ramp to lower shoreface. In the mid-ramp a better preservation of trace fossils allows to differentiate pre and post event burrowing. In the shoreface some isolated gutter casts were found, whereas hummocky cross-stratification and swaley cross-stratification dominate the lenticular pattern. The aspect ratios as well as the sizes of hummocks and swales are lower than average, and in plan view out of phase (discordant) pyramidal oscillatory megaripples are present. The episodic nature of either the allochthonous non-coeval carbonates or the coeval intrabasinal ones allow the unusual preservation of trace fossils (Paleophycus, Planolites and Cruziana among the most diagnostic) although the scarcity of body fossils is remarkable, except for the lower half of the succession. In the Central Precordillera the Silurian strata (either the Tambolar Formation to the south or the La Chilca Formation to the north) unconformably overlie the Ordovician limestones (San Juan Formation) at different stratigraphic levels. The special paleogeographic situation of the Sassito section, where the pre-Silurian erosion has been less (Fig. 3), allows to study several stratigraphic features important to unravel the Mid to Late Ordovician history of the Precordillera basin, as well as the architecture and nature of the regional unconformity. Because of its erosive boundaries and shallowing-upward arrangement, the Sassito Fm. is interpreted to involve part of a transgressive systems tract and a highstand succession, in which the transgressive surface is coincident with the basal sequence boundary. An older (Early-Middle Ordovician) relative sea-level drop is evidenced by a chert and fossiliferous limestone conglomerate and red dolomitized breccia which caps the San Juan Fm. locally. The onlapping black calcilutites can be regarded as a highstand condensed deposit. The ongoing paleontological dating of the different units and key horizons will permit to construct a detail chronostratigraphic chart to allow a more accurate event-stratigraphic interpretation. Refinement on the mineralogy and geochemistry of the Sassito limestones may show new insights on the cooler conditions that affected the Precordillera during the Late Ordovician.

Palabras clave : Sassito Formation; Late Ordovician; Precordillera; Carbonate facies; Storm layers; Stratigraphy; Paleoclimate.

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