Análisis del contacto entre las formaciones Vinchina y Toro Negro (Sierra de los Colorados, provincia de La Rioja, Argentina), sus implicancias tectónicas

the stratigraphic expression and geological significance of the contact between the vinchina (late miocene) and toro negro (late miocene-pliocene) formations (turner, 1964; ciccioli et al., 2010) is analyzed in this paper. these units with more than 8,000 m of sediments (ramos, 1970; tripaldi et al., 2001; ciccioli, 2008) represent the main units of the infill of the vinchina basin (ciccioli et al., 2010) (figs. 1-3) during the transition from a simple foreland basin stage to a broken foreland one (ciccioli, 2008; ciccioli, et al., in press). they mostly represent sedimentation by fluvial and aeolian processes in an inland basin during an overall warm and dry climate period (tripaldi et al., 2001; ciccioli, 2008). thus, major changes in sedimentation are though to correspond with tectonic events (ciccioli et al., in press). the lower member of the vinchina formation and the upper member of the toro negro formation present a more uniform and extended sedimentation pattern. in contrast, the upper member of the vinchina formation and the lower member of the overlying toro negro formation show important lateral (north-south) facies changes (fig. 4). the along-strike differences in the character of the discontinuity marking the boundary between the vinchina and toro negro formations, is the most meaningful of them all. six sections, from north to south (la aguada creek, los pozuelos creek, north la troya river, south la troya river, campo negro and del yeso creek) were measured across the boundary between the vinchina and toro negro formations along the sierra de los colorados (fig. 2). facies associations within this interval were defined and sedimentary paleoenvironments interpreted. the information gathered from these sections allowed interpreting the contact between the vinchina and toro negro formations as a high-relief incision surface that in some places suppresses up to 25% of the underlying vinchina formation (figs. 4 and 5). this surface forms a west-east orien