The Golfo San Jorge basin has more than 100 years of history in the exploration and production of hydrocarbons in Argentina, with the main petroleum system associated to the Chubut Group. Nonetheless, the chronostratigraphic framework of this unit is immature, affecting the comprehension of the basin evolution. The Codo del río Senguerr anticline is the southernmost expression of the San Bernardo Fold Belt. There, the Chubut Group reaches a 1650 m thick succession, from where we obtained five samples and analyzed their U/Pb ages. The stratigraphic distributions of the samples involve all the outcropped formations: Pozo D-129 (n: 2), Castillo (n: 1), and Bajo Barreal (n: 2). Their sedimentological features and geochronological results evidence the dominance of reworked volcanic ash and subordinated distal primary fall-out. Taking this into account, we calibrated the Chubut Group chronostratigraphy using the youngest zircon ages. The upper deltaic levels of the Pozo D-129 Formation were reassigned to the Albian (103,1 Ma), while the ages of the overlying high-accommodation fluvial deposits were also modified: i) the Castillo Formation to the early Cenomanian (98,3 Ma), ii) the lower Bajo Barreal Formation to the middle Cenomanian (96,8 Ma), and the upper Bajo Barreal Formation to the early Coniacian (88,4 Ma). These results have an impact on all the components of the petroleum system, like the modeling of the subsidence rate and the timing of the fold belt construction which conditions the trap generation. Finally, the case study demonstrates the age variability in volcanic distal tuffs affected by different rework intensities.