Basin-scale exhumed Cretaceous fluvial systems (Chubut Group, Cañadón Asfalto basin), central Patagonia, Argentina

Abstract

This contribution describes basin-scale, Cretaceous (Late Aptian to Cenomanian-Early Turonian) fluvial systems outcropped over ∼30,000 km² that contain more than 7000 exhumed paleochannels in the Chubut Group, Cañadón Asfalto basin (central Patagonia), Argentina. Paleochannels are encased within a ∼350 m thick stratigraphic interval (∼20 My) including volcaniclastic and epiclastic fluvial units (Cerro Barcino and Puesto Manuel Arce formations). Geomorphologically, the exhumed paleochannels conform to isolated sandstones/conglomeratic ridges, a swarm of paleochannels, and elongated/equidimensional mesas (plateaus), characterizing much of the “mesetiform” landscape in central Chubut. The plan-view analysis of exhumed paleochannels mainly shows a mean WNW-ESE orientation (112°–292°) and predominance of narrow (83.1%) and very narrow (14.4%), low-sinuosity channels (96%). A plan-view multi-scale architectural analysis of sandbodies allows us to recognize different fluvial hierarchies: a) 3rd-order lithosomes (macroform growth increments), b) 4th-order lithosomes (point bars and crevasse-channels), b) 5th-order lithosomes (main channels), and c) 6th-order lithosomes (channel belts). Two main types of channel arrangements in the exhumed fluvial systems were recognized: fixed-channel and mobile channel belts. We observed a broad relationship between geomorphologic configurations and fluvial architectural units. Some macro spatiotemporal variation in the fluvial architecture was observed towards the youngest positions, including wider channels and a greater relative abundance of high-sinuosity channels with point bars. The basin-scale distribution of exhumed paleochannels would allow us to infer the predominance of shallow lacustrine or lagoon deposits in the western basin region, probably controlled by the inherited topography of the Jurassic- Early Cretaceous depocenters.