Evolution of high latitude fluvial system in a humid climate conditions (Maastrichtian–Danian, Magallanes Basin, Chilean Patagonia)

Abstract

This study documents fluvial dynamics and paleoenvironmental evolution in a high-latitude foreland basin during the Maastrichtian–Danian, based on integrated sedimentological and paleontological analysis of a continental–coastal succession exposed in the Río de las Chinas Valley, Magallanes Basin, southern Chile. The fluvial record is characterized by sandy to gravelly channel bodies embedded within an extensive muddy floodplain, reflecting deposition on a low-gradient fluvial plain developed in the distal sector of the basin. Two main channel styles are recognized within the fluvial system: sinuous channels dominated by lateral-accretion point-bar deposits, and higher-energy braided channels composed of sandy to gravelly longitudinal bars. These channel styles occur laterally within the same fluvial corridor, indicating that channel morphology was primarily controlled by spatial variability in sediment supply and discharge rather than by downstream changes in slope or base level alone. Gravel-rich braided reaches are interpreted to reflect higher bedload input, whereas sandy sinuous channels developed in sectors dominated by suspended load. Dispersed paleocurrent patterns are interpreted as the result of channel migration and bar accretion processes within a low-gradient system, rather than evidence for independent drainage networks. Floodplain deposits record frequent overbank flooding, crevasse-splay activity, abandoned channels, and swampy environments, locally including deposits related to hyperconcentrated flows generated during high-magnitude floods under humid climatic conditions. Recurrent vertical and lateral intercalations of fluvial and shallow-marine facies indicate direct interaction between fluvial systems and a high-energy, linear coastline, emphasizing the role of accommodation changes linked to relative sea-level fluctuations at the end of the Cretaceous. Paleobotanical and taphonomic evidence supports humid conditions during the Maastrichtian, followed by a shift toward cooler and drier climates near the Cretaceous–Paleogene transition. Overall, the Río de las Chinas succession provides a well-exposed record of fluvial dynamics in a high-latitude foreland basin at the end of the Cretaceous.