Abstract Sandstone petrographic data have been used to investigate the provenance of the Matasiete, Pozo D-129, and Castillo formations (Chubut Group) in the Canadon Matasiete, San Bernardo Fold Belt at Golfo San Jorge Basin, Argentina. Available petrographical data provides evidence of several distinctive volcanic source areas, with minor contributions of metamorphic, plutonic, or sedimentary components. Paleo-volcanic contributions from basic-intermediate (lathwork and microlithic textures) and acidic compositions (felsitic, seriate, granular, and vitric textures) were mainly derived from the Middle-Upper Jurassic rocks of the Lonco Trapial Formation. Pyroclastic components (glass shards, pumice fragments, and eutaxitic textures) derived from coeval volcanic activity in the Andes Cordillera, generated either gradual or abrupt changes in the source of the lithic fragments along the study succession. The observed trends in detrital lithic provenance can be integrated with previous sedimentological and architectural studies of the Matasiete and Castillo formations, revealing that narrow-sheet geometries of the channel fill characterize stratigraphic intervals dominated by pyroclastic components, whereas stratigraphic intervals that contain a low content of pyroclastic materials in the channel fill show ribbon-shaped geometries. Sandstone petrography in volcaniclastic basins can be used to complement other indicators of the evolution of the fluvial systems, providing reliable information about the nature of the changes in the sediment supply within the drainage catchment.