The Aburra Valley is a structurally controlled intramontane basin in the central mountain range of Colombia, where surficial slope deposits cover their hillsides. Samples obtained from geotechnical drilling into these sludge or debris-flow reservoirs, which are in the middle and lower parts of the slope on the west side of the Aburra Valley, exhibit a high degree of mechanical competence, which is atypical of soils. Through petrologic study and mineralogical analysis, this work makes the first characterization of the provenience and diagenetic processing of these deposits. By means of polarized-light optical microscopy and scanning-electron microscopy (SEM), we recognized evidence of pseudomorphic replacement, claystone and pore-filling autogenesis, dissolution of detrital minerals (iron-oxide coatings and moldic porosity), and the presence of cements (oxide, clay, and dolomite) indicating the progress of diagenetic processes. Also, the use of the Kübler index on diffractograms of illite, provided information on high-temperature conditions (>200°C) whose cause is still being assessed. From SEM and X-ray diffraction analysis, the presence of diagenetic minerals and changes of depositional environmental conditions were corroborated. The evidence indicates that the deposits are in an early stage of diagenetic processing without burial. A new lithostratigraphic unit called the Aburra Formation is proposed.

Keywords: Valley of Aburra, Slope deposits, Diagenesis, Weather conditions, Kübler index