Sedimentology and stratigraphic evolution of the Early Cretaceous regressive cycle at the northern edge of the Austral-Magallanes Basin, Santa cruz, Argentina

Abstract

The northern edge of the Austral-Magallanes Basin in southern Patagonia (Argentina) contains a 200–300 m thick regressive section of Barremian-Aptian age. The conformably stratigraphic sequence consists of outer-shelf deposits of the Río Mayer Formation, evolving to a shallowing-upward, mixed-energy deltaic sequences of the Río Belgrano Formation, covered by fluvial deposits of the Río Tarde Formation. We evaluated their sedimentary evolution by analyzing facies, paleocurrents, stratigraphic stacking patterns, and conducting a paleohydrological study of the fluvial succession in five areas along a north-south transect. Twenty sedimentary facies were identified and grouped into six facies associations: (I) prodelta, (II) delta front, (III) delta plain, (IV) fluvial channels, (V) proximal floodplain, and (VI) distal floodplain deposits. The progradation of the depositional systems occurred from north to south, exhibiting notable temporal and spatial changes in deltaic processes and the characteristics of its feeder rivers. A funnel-shaped marine physiography controls the relative influence of tidal and storm processes in the Río Mayer and Río Belgrano formations, whereas the river-dominated nature of the deltaic deposits is associated with fluvial systems evidencing significant discharge variations in a semiarid climate. Additionally, the Río Tarde Fm. Shows spatial changes in fluvial styles, a southward reduction in i) channel sizes, ii) associated water discharge, and iii) stacking density, allowing for interpretating a distributive fluvial system. Our results indicate that the evolution of the Lower Cretaceous units was controlled by the inherited topography of previous extensional settings, with no evidence of coeval tectonic activity during their deposition. Furthermore, the completeness of the deltaic components, lack of large erosional surfaces, and negligible seaward shift of facies suggest no significant base level drops. Currently, climatic influence is manifested only by a considerable variation in river discharge of the feeder fluvial systems to the coastal environment, but further studies should reinforce its importance in the organization of the depositional systems.