Estructuras de deformación (？sismitas?) en la Formación Río Negro, provincia de Río Negro, Argentina

soft-sediment deformation structures (ssd) are alterations produced almost simultaneously with sedimentation. they are directly related to internal characteristics of sedimentary materials as well as to external factors acting on them. results derived from such alterations are evidenced as injections, fractures, volcanoes and convolute laminations, among other forms, affecting stratification either totally or partially. soft-sediment deformation structures resulting from seisms are known as seismites. the present study aims at determining for the first time the presence of ssd structures in the río negro formation, located in the northern area of san matías gulf, near río negro lighthouse, argentina (fig. 1). to this end, structures were firstly identified and further described. samples were subsequently collected for the determination of grain-size, mineralogy and organic matter content. photographs of the different sectors evidencing deformations were taken in order to determine further comparative models. morphology in the study area is associated to cliffs with vertical, fractured fronts and with an average height of 70 m in whose base torn-down blocks are accumulated. the geological structure of the study area is related to the cuenca del colorado and the comarca nordpatagónica, whose basement is mainly composed of paleozoic and mesozoic crystalline rocks. the sedimentary tertiary cover from the miocene-pliocene is represented by light-blue sandstones of the río negro formation (andreis, 1965). this unit was formed in an aeolian environment with intercalations of clay-silt shallow lagoons and a marine episode located in the mid area of the río negro formation. at the top of the río negro formation there are pleistocene-holocene sedimentites having a thickness of up to 5 m. within the local structural framework of our study area there are fractures with a ne-sw and a nw-se direction, which are related with fractures n55o, n90o and n350o azimuth located in the ab