Upper Campanian-Paleogene from the Río Turbio coal measures in southern Argentina: micropaleontology and the Paleocene/Eocene boundary

Abstract

The Río Turbio coal measures area, Austral Basin, is one of the few areas in Argentina that records the Cretaceous/ Paleogene and Paleocene/Eocene boundaries within a thick marine sequence without major tectonic disturbances. More than 700 Upper Cretaceous and Paleogene samples, from outcrops and subsurface, were investigated for microfossils. The shallow environments recorded, unsuitable for planktonic elements, led to recognition of the following mostly low diversity benthic foraminiferal assemblages. In the lower part of the Cerro Cazador Formation, the Hoplitoplacenticeras fauna (Early Campanian) is associated with the Miliammina, the Haplophragmoides sp. cf. walteri and the Reticulophragmium sp. Assemblages, mostly composed of agglutinated foraminifera. In the middle and glauconitic part, containing the Maorites fauna, the Coryphostoma incrassata gigantea Assemblage, composed a high percentage of calcareous and infaunal species, suggests a marked environmental change and it represents a Maestrichtian transgressive episode that flooded all the Patagonian basins. In the overlying Cerro Dorotea Formation the Buliminella isabelleana procera Assemblage is the only one which contains planktonic foraminifera: Parasubbotina pseudobulloides, of Danian age. The unconformable overlying Rfo Turbio Formation (mostly Middle Eocene) bearing coal measures, contains the Karreria pseudoconvexa and Bythocypris sp. Assemblage in the lower Member, and Boltovskoyella argentinensis characterize the upper Member. The formational Cerro Dorotea/Río Turbio contact coincides with the Paleocene/Eocene boundary and with a kaolinite content peak related to the Paleogene climatic optimum. The cause or causes of this unconformable contact are not clear, because no major magmatic activity is known for this boundary, which is associated with a generalized regression in all Patagonia. The Río Turbio Formation is unconformable overlain by thick fluvial conglomerates of the Río Guillermo Formation, as a result of the Incaic diastrophic Phase. The Río Guillermo is overlain by the non-marine Río Leona Formation (Uppermost Eocene-Lower Oligocene) and unconformable overlaid by the marine Río Centinela Formation (Uppermost Oligocene-Lower Miocene) deposited during the major Cenozoic transgression in Patagonia.