Resumen

PAREDES, José M et al. Paleambiental analysis of the Matasiete Formation (Aptian) in its type area, Northwest of the Golfo San Jorge Basin, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.2, pp.81-101. ISSN 1853-6360.

The Aptian Matasiete Formation is exposed in the northwestern area of the Golfo San Jorge Basin. It comprises an exposed 650 m thick column of continental deposits, which are laterally related to the Pozo D-129 Formation, a widely distributed and mainly lacustrine unit, considered the main source rock of the basin. An unconformity on the top separates it from the Castillo Formation, an Albian continental unit. Previous works based on subsurface information in eastern locations have recognized an extensional and transtentional tectonic setting during the Aptian-Albian, with rapid thickness variations. The outcrops of the Matasiete Formation were studied in the past by oil geologists, but few works have been published. The Tertiary tectonic inversion in the western of the Golfo San Jorge Basin has exposed the Cretaceous deposits in the San Bernardo Fold Belt. The Matasiete Formation has been previously divided into a Lower Member (also called "Sección Conglomerádica Basal"), a Medium Member ("Sección Morada Inferior ") and an Upper Member ("Sección Morada Superior "). The base of the Lower Member is not exposed and the outcrops are composed of a 200 m thick package. Three cycles from 40-50 m containing multi-story channels separated by thick floodplain deposits with frequent paleosols are recognized. The lowest exposed cycle occurs in Puesto de Neme, where the typical facies association of the channels shows a finingupward arrangement, containing polimictics, well rounded, medium to fine gravel beds (mean size 2,5-3 cm) in their base. Amalgamation surfaces are frequent, and cut-and-fill structures are typical. Acid volcanic rocks, tuffs and ignimbrites are the dominant detrital phase. Some basic volcanic rocks and scarce quartzite clasts are also recognized. The presence of matrix-supported and trough-cross-bedded gravel beds inside each channel suggests changes in the water-sediment ratio and the flow conditions. Fines are represented by white or grey laminated mudstones and siltstones, typically rooted, and white tuffs. The mostly gravelly infilling of the channels, episodic discharge in high-energy conditions (presence of trunks of 1 m diameter incorporated in the flow) and the paleocurrent dispersion suggest a low-to-moderate sinuosity fluvial system. In the stratigraphicaly younger section of the Lower Member the infilling of the channels is multiepisodic, the most characteristic facies association grades from scarce fine, matrix-supported gravel beds, trough-cross-bedded fine gravel beds to coarse-to-medium grained cross-bedded sandstones. Few lateral and oblique accretion surfaces were recognized and avulsion was the main mechanism for migration of the channelized system. The size of the multi-story channels range from 300 m width and 14 m thick to 80 m width to 4 m thick. In the oldest section, exposed in Puesto de Neme, paleocurrent data were obtained from low-angle cross bedding and oriented trunks, and indicates a NE flow direction (mean: 64º, n = 89, S.D. = 21º), while in the medium section of the Lower Member the paleocurrent measurements indicate a ESE flow direction (mean: 110º, n = 63, S.D. = 31º) and in the upper section of the Lower Member paleocurrents are SSE (mean: 174º, n = 24, S.D. = 21º). The paleocurrent variations reflect paleogeographic changes, perhaps related to the interaction of longitudinal and transverse drainage systems in an extensional setting. The Middle Member of the Matasiete Formation has been referred as the lateral equivalent of the Pozo D-129 Formation. It is composed of a 215 m thick package of purple and grey mudstones and siltstones interbedded with channel fills of sandstones and fine gravels. For most of the member thick floodplain deposits separate isolated fluvial channels with lowconnectivity, indicating an increase in the accommodation or shifting of the channel belts to another location. In a general way, a less energetic flow is supported by the finer infilling. Frequently, mudstones and siltstones contain roots. Laterally extensive tuffs were deposited on the floodplain and some of them present reworking evidences. The scarcity of lateral accretion surfaces and the infilling of the channels mostly by gravels and coarse sandstone suggest high-energy conditions. In the upper section of the Middle Member extensive clastic and piroclastic deposits show evidences of bidirectional structures, suggesting the close presence of a marginal lake setting. The Upper Member of the Matasiete Formation comprises 220 m of reddish and white claystones and siltstones, white tuffs and isolated sandstones deposited in channels with multi-story fills. The most conspi-cuous facies association shows a multiple stacked fining-upward arrangement, from matrix and clast-supported medium-to-fine gravel and pebbly sandstones in the basal part up to coarse to fine-grained sandstones containing cross bedding from decimetre to meter scale. Lateral accretion surfaces are present inside some channels bodies. In floodplain facies the paleo-sols are frequent, and carbonate nodules or continuous carbonate layers of early diagenetic origin are pre-sents. The petrological composition of the sandstones shows intermediate volcanic rocks as the dominant fraction, followed by basic and acid volcanic rocks, ignimbrites and sedimentary clasts. The fluvial system had in some cases a meandering style, although some gravel and pebbly sandstone sheet beds indicate scarce high-energy episodes with braided characteristics. The paleocurrent measurements on trough cross bedding indicate a S (dominant) to SE direction. The Castillo Formation unconformably covers the unit. The main difference with the Matasiete Formation is the abundance of light green tuffs in the Castillo Formation.

Palabras clave : Golfo San Jorge Basin; Matasiete Formation; High-energy fluvial system; Aptian basin margin.

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