Resolving the role of longitudinal versus transverse sediment dispersal in ancient sedimentary basins is paramount for understanding filling history and the timing of source area exhumation. The southern Patagonian Andes provide a unique opportunity for constraining these relationships because Upper Cretaceous shallow- and deep-marine strata that record the longitudinal filling history of the Magallanes-Austral foreland basin are exposed along a 500+ km outcrop belt. New stratigraphic, sedimentologic, and facies analyses of the Cenomanian-aged Lago Viedma Formation indicate a protracted phase of a dominantly shoreface and foreshore depositional setting at the northern end of the basin (Austral basin sector), whereas 200 km to the south, age-equivalent strata of the Punta Barrosa Formation are characterized by southward-flowing deep-water fan systems. New sandstone compositional data from both formations are rich in intermediate volcanic grains and suggest an undissected to transitional volcanic arc source. However, detrital zircon populations from the shallow-marine Lago Viedma Formation are dominated by arc sources (126–75 Ma), whereas deep-water strata of the Punta Barrosa Formation contain much greater abundances of Jurassic (199–161 Ma) and pre-Jurassic (>200 Ma) ages. This indicates that deep-water fan systems were not linked to a shelfal sediment dispersal system by a simple northward point-source model, despite consistent southward-directed paleocurrents in deep-water strata. Time-transgressive provenance variations continue southward (along strike), where lithostratigraphic equivalents in the Ultima Esperanza and Fuegian sectors of the basin contain a mix of arc and pre-Jurassic metamorphic basement sources and a paucity of Jurassic ages. We interpret these along-strike provenance variations to be the result of significant local sediment contributions from transverse sources. The influence of transverse tributaries during the early history of the Magallanes-Austral foreland basin suggests that the diachronous onset of coarse clastic deposition in the basin was likely due to the progressive delivery (and initiation) of more locally derived coarse clastic sediment. We attribute this to southward-progressing thrust-belt development associated with progressive north to south collision (or suturing) of the parautochthonous Patagonian arc with attenuated continental crust of South America.