Sedimentology and gamma-ray logs of the Pozo D-129 formation (Aptian) in the Sierra de San Bernardo, Chubut The study provides an outcrop analogue of productive hydrocarbon succesions of the uppermost part of the Pozo D-129 formation (Aptian) in the Golfo San Jorge basin. Although widely distributed in the subsurface of the basin, the Pozo D-129 formation is just exposed in three areas of the San Bernardo Fold Belt; this contribution presents the result of a sedimentological analysis carried out in the Sierra Silva (43 m thick) and Codo del Senguerr (140 m thick) anticlines. Facies analysis allowed the identification of thirteen lithofacies, which were grouped in three main lithofacies associations: (A) deep lacustrine, (B) sub-litoral lacustrine, and (C) shallow lacustrine. The later association consists of four sub-associations: C1 (carbonatic lake), C2 (distributary channels), C3 (distributary mouth bar) and C4 (interdistributary bay). Incomplete shallowing-upward cycles are common in both study areas, evidencing sedimentary progradation. Total and spectral gamma-ray (GR) measurements were obtained using a hand-held device. In Sierra Silva the shape of the U and total GR logs is similar, reflecting that minerals containing U are responsible of most of the emissions of this rocks; diagenetic enrichment of U occurred in a 4,5 m thick section, with measurements up to 500 ppm of U (378 g/tn de U3O8). At Codo del Senguerr, Th and K measurements in fine-grained rocks display better correlation with the total GR log than U, evidencing a different source of radioactive minerals. Integration of sedimentology and gammaray measurements bring data and lithological correspondences that could be useful for regional correlations and subsurface facies models.