A model of loessic‐fluvial sedimentation and pedogenesis in a distal volcaniclastic setting (Late Cretaceous of Patagonia, Argentina)

ABSTRACT Pre‐Pliocene loess deposits are rare relative to the extensive Quaternary record. Furthermore, non‐classical tephric loess is poorly constrained, obscuring the understanding of fine‐grained volcaniclastic sedimentation in distal settings with frequent aeolian deposits. To address this, we examined a Cenomanian–Santonian succession of tephric loessites and palaeosols interbedded with fluvial deposits from the San Jorge Basin (Argentina). Facies analysis, stratigraphic architecture and pedotypes suggest the interaction of subaerial fine‐grained deposition, fluvial reworking and pedogenesis, expressed in three recurring sedimentary phases: aggradation, channelisation and stasis. These phases were likely driven by episodic aeolian volcaniclastic input and subsidence, under a subhumid, seasonal, temperate climate. Lateral variations in the sedimentary phases show that in the subsiding basin centre, aggradation was dominated by aeolian accumulation, frequently interrupted by stasis (recorded by strongly developed palaeosols) and channel incision. In contrast, basin margins reflect lower subsidence and continuous reworking inhibited soil development, resulting in fewer, weakly developed palaeosols and a single channelisation phase. This pattern suggests that accommodation may have exceeded sediment supply in the basin centre, leading to sediment‐starved conditions in which pedogenesis outpaced aggradation. This produced a reverse sedimentation/pedogenesis gradient, with more developed palaeosols concentrated in the depocentre. The Laguna Palacios Formation provides a key example of distal subaerial volcaniclastic sedimentation coupled with widespread pedogenesis, complementing existing models focused on proximal volcaniclastic systems.