Resumen

BUSTOS, Ulises Daniel. Modelo sedimentario alternativo para el Devónico de la Precordillera central sanjuanina: Formación Punta Negra. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.1, pp.17-30. ISSN 1853-6360.

The Punta Negra Formation (Middle Devonian) is a thickenning and coarsening-upward succession of sandstones interbedded with mudstones of aproximately 1000 m thick that represents the culmination of the Devonian depositional cycle in the Central Precordillera of the San Juan Province (Fig. 1). The Formation was originally defined by Bracaccini (1949) and Padula et al. (1967). Gonzalez Bonorino (1975) and Gonzalez Bonorino & Middleton (1976) suggested for this deposits a submarine fan origin feeded by a single main channel and controlled by two north-south oriented submarine rises, located between the stable shield to the east and the oceanic area to the west. From a biostratigraphic viewpoint, subsequent works support the last interpretation (Peralta, 1985; Peralta & Ruzicky, 1990). The present work proposes an alternative depositional model for the Punta Negra Formation. Six detailed stratigraphic sections were measured and studied, which allowed the recognition of eight sedimentary facies that were grouped into three facies associations. The Facies Association "A" constitues the litologic intervals of about 5 to 10 m thick between the Talacasto and Punta Negra Formations (Fig. 4). Beds with high flow regime structures are gradually replaced by psamitic deposits related to turbidity currents of very low concentration. The first beds were interpreted as due to wave reworking, which might indicate a depositional environment above the storm wave base. The relatively high bioturbation observed, could support the last consideration. Hence, this association is interpreted as sedimentation in an external-middle shelf environment. Facies Association "B" is caracterized by fine-grained deposits of about 7 to 10 m thick and constitues topographic lows in outcrop (Fig. 5). In this association, fine-grained sandstones and shales were deposited by turbiditic currents of low concentration. The beds generally show decreasing flow regime conditions from base to top. The plant remains are abundant and bioturbation is a common feature. This association was considered as turbiditic-interlobes and interchannel deposits. Facies Association "C" is represented by intervals of mainly tabular medium to coarse sandstones of about 20 m thick, being the topographic highs in the field (Fig. 6). It is composed by beds with thickening-upward patterns and by subordinated thinning-upward tendencies. The first were deposited by unconfined turbiditic flows of very high concentration and the second by confined turbidity currents of moderate concentration and higher internal turbulence with associated tractional currents. The thinning-upwards designs would represent minor channel feeding systems, wherein medium scale cross-strata was identified. This association characterizes turbiditic-lobes and associated distributary channels prograding onto the shelf. The paleocurrent data shows unimodal and bimodal low-scattered designs with a mean direction of approximately 270o, resulting in patterns transverse to the basin axis (Fig.7). This areal uniformity of the data for more than 100 km, suggests that turbidity currents had multiple point sources aligned longitudinally that transport sediments from east to west. The lateral facies arrangement and architectural elements show multiple interdigitations between lobes and distributary channels. The shallow channels with lack of strong incisive features, allow to consider their origin as related with periodic avulsion. Scarce synsedimentary deformation together with the width and character of the channels, allow to infer a depositional substrate of very low gradient (less than 1o) (ramp-like). The abundance of plant debris and thorough bioturbation along with the presence of wave reworking, might indicate sedimentary processes operating in relative shallow waters, with depths of about 200 m at the most. Furthermore, being the formation a thickenning-upward sequence, being the sandstones the predominant lithology and showing the sedimentary structures and plant remains evidences of relatively shallow marine settings near to the shoreline, a deltaic model progradding over a ramp margin is suggested for the Punta Negra Formation (Astini, 1990) (Fig. 9). This model is in agreement with the recent finding by Cuerda et al. (1990) at the top of the unit of a 65-70 m thick of coarse-grained red sandstones with abundant trough cross-beds, interpreted as related to deltaic progradation.

Palabras clave : Punta Negra Formation; Paleoenvironment; Deltaic facies; Ramp margin; Devonian; Precordillera.

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