A “cool-water”, non-tropical, mixed volcaniclastic–carbonate ramp from the Early Cretaceous of southern Chile (45°40’S)

Abstract The Aysén-Río Mayo Basin was a back-arc/marginal basin developed in southwestern South America (43°–47°S) between the Tithonian–Aptian. Its sedimentary fill corresponds to Coyhaique Group, which represents transgressive–regressive succession. Six lithofacies and five microfacies were defined for three outcrops exposed south of (45°40’S). have mixed calcareous–volcaniclastic composition assigned early transgressive Toqui Formation, i.e., lowermost part Group. These rocks comprise bioclastic–volcaniclastic conglomerate, gravelly allochemic sandstone, gravelly–sandy allochem limestone. Bedding is sharp amalgamated, sometimes rippled, depicting wave- storm-influenced, inner- mid-ramp. ramp over Valanginian, active volcanic terrain (Foitzick Volcanic Complex), source volcaniclastic sediments. Limestones are rich reworked bioclasts, controlled by calcitic organisms including gryphaeid oysters, non-geniculate red algae, echinoid fragments, defining heterozoan association (“maerl”-like sediments); less frequent ahermatypic corals, serpulids, carbonized wood. Based on their inferred paleolatitude (south 45°–50°S), fossil assemblage (heterozoan), kind carbonate platform (ramp-type), these calcareous Formation depict “cool-water” (sensu lato), non-tropical setting. includes oysters ( Aetostreon spp.), abundant algae attributed Archamphiroa jurassica Steinmann (1930), taxon previously known from upper Tithonian Cotidiano Argentina. A. here reported first time Lower Cretaceous Chile, suggesting broader Tithonian—Valanginian-Hauterivian? range species. facies model presented contrasts with depositional environments depicted correlative reefal Argentina (Tres Lagunas Formation), reflect “warm-water” In Basin, influence sea-water key physical variables sedimentation, as well position hydraulic regime platforms within basin, interaction volcanism still unclear.