CONICET | Buscador de Institutos y Recursos Humanos

Two thick, fine-grained, siliceous pyroclastic beds are intercalated in a shallow marine to transitional Miocene siliciclastic succession of Patagonia. Both pyroclastic beds are mostly composed of fallout particles originated in explosive eruptions. Despite their common origin both deposits have different facies and geometry product of their reworking in different sedimentary environments with little or no mixing with the background sediments. The Patagonian succession was formed during a large transgression that covered most of Patagonia during the Early Miocene. In the study area, at the southern tip of Argentine Patagonia, it is known as the Estancia 25 de Mayo Formation and it is conformably overlaid by a terrestrial succession known as Santa Cruz Formation. The stratigraphically lower pyroclastic bed is interbedded with fine-grained fossiliferous marine sediments in the lower part of the Estancia 25 de Mayo Formation. The stratigraphically upper bed lies in the transition between this unit and the Santa Cruz Formation, within channelized sandstone bodies of fluvial origin. The Lower level is tabular in shape, with subtle thickness variations, laterally continuous by about 30 kilometers. Its maximum thickness is four meters, dying out gradually to both sides. Its base is sharp and plane. On the other hand, the Upper level is composed by a discontinuous net of lenticular bodies, reaching fifteen meters thick and one hundred meters width, with a concave up base and a plane top. Internally, the Lower level only exhibits parallel lamination at its base, with structureless fine grained tuffs in the upper part. Unlike the surrounding sediments, no bioturbation is visible. In contrast, the Upperlevel shows parallel lamination, ripple lamination, mud drapes and trough and tabular cross-stratification. Bioturbation is also not visible. Both levels are composed of siliceous, vitreous material (pumice and shards) with variable proportions of crystals (mainly quartz and plagioclase) and minor accidental fragments. The Lower level shows no mixing with epiclastic material and a fining upward trend. The Upper level shows a coarsening upward trend from fine tuffs to coarse tuffaceous sandstones, in concordance with an increasing degree of mixing of pyroclastic and epiclastic material. The similar composition and age of both levels suggest that the volcanic material was probably sourced by two different eruptions of the same volcano. The grain size distribution of the Lower level indicates that this source was located to the west or south west of the area, in the Andean range. The parallel lamination in the sandy tuffs of the Lower Bed is explained by sedimentation in upper flow regime. In a low energy marine environment, the most likely way of generating this type of currents is through hyperpicnal, sediment-laden flows. This is likely where pyroclastic material saturated the rivers entering the sea with an extremely large amount of sediments. The lack of bioturbation points to a very high sedimentation rate. The Upperlevel with many small-scale tractional sedimentary structures, suggest that they are not the result of direct ash fallout but are the product of deposition by currents within a fluvial channel. The upper flow regime parallel lamination, the small scale cross-bedding, plus the appearance of mud drapes, points to an origin by shallow streams with short slack water periods, that rapidly fulfilled the space available for sediment, preserving the channel shape. Both beds are thicker than the normal, distal, fine-grained fallout deposits. The big thicknesses of the two levels indicate remobilization of the pyroclastic material and deposition under very high sedimentation rates. In both cases, the minor or lack of mixing with epiclastic material could be explained by the rapid deposition and the inability of streams and marine currents to erode the substrate, as they were suddenly over-saturated with pyroclastic sediments.

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