Análisis paleoambiental de la formación Matasiete (aptiano) en su área tipo, noroeste de la cuenca del Golfo San Jorge, Argentina

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 mudst