Hyperpycnal flow deposits and associated facies in upper Cretaceous reservoirs of the Magallanes Formation in the Campo Boleadoras-Estancia Agua Fresca-Puesto Peter area of southern Patagonia, Argentina, are documented based on subsurface data. The study represents one of the first ichnologic characterizations of a deltaic system dominated by hyperpycnal processes. The fluvial-deltaic system was sourced from uplifted areas located in Central Patagonia and the Rio Chico High and prograded toward the south and southeast. Integration of sedimentologic and ichnologic data allows establishing proximal-distal trends within a sediment transport system. The bulk of sandy hyperpycnal lobe deposits consists of coarse- to fine-grained sandstones that are either structureless or display a subtle parallel lamination that is commonly delineated by abundant plant remains. Vertical grain-size changes reflect flow fluctuations, with coarsening- and fining-upward intervals indicating waxing and waning flows, respectively. The concentration of plant remains indicates phytodetrital pulses in connection with direct fluvial discharges. These deposits are unburrowed to sparsely bioturbated, containing the Thalassinoides ichnofabric, which records opportunistic colonization during times of decreased sedimentation rate. However, the pervasive laminated fill of the burrows may reflect relatively high sedimentation rates, most likely because of suspension fallout of fine-grained material. The extreme compaction suggests burrow emplacement in a water-saturated soft substrate that underwent compaction subsequent to the bioturbation event. Associated heterolithic intervals contain the Planolites-Teichichnus ichnofabric, which characterizes marginal areas with respect to the hyperpycnal sand-rich lobes and times of quiescence between flows. During times of reduced sediment supply, material was reworked by wave processes, and hypopycnal conditions were dominant. The Thalassinoides-Teichichnus ichnofabric records colonization of these wave-reworked sandstone units, whereas the Terebellina-Phycosiphon ichnofabric reflects stable conditions that allowed intense bioturbation and the establishment of a moderately diverse benthic fauna. Fully marine offshore deposits are characterized by the Teichichnus-Phycosiphon ichnofabric, which display total biogenic reworking, high ichnodiversity, and a complex tiering structure.