CONICET | Buscador de Institutos y Recursos Humanos

The discovery of carbonates in Pichaihue, northwestern Patagonia, improved our understanding of the first Atlantic transgression that reached the Andes foothills. This late Cretaceous-early Paleogene sea was termed ?Kawas sea? by Casamiquela (1978), a name taken from a mith of the Tehuelche indigenous people. The locality of Pichaihue is situated in the Neuquén Basin (36°40? SL). There are isolated outcrops where late Cretaceous pyroclastic flows are covered by ash-fall distal tuffs interbedded with carbonates of the Malargüe Group. A SHRIMP U-Pb dating of volcanic zircons of the tuffs yielded an age of 64.3 ± 0.9 Ma (Danian) (Aguirre-Urreta et al. 2011). The Pichaihue Limestones include massive bioclastic mudstones, stromatolites, and oncolites. The oncolites are isolated subspherical bodies up to 10 cm; their nuclei of silt probably caused by the disintegration and subsequent infilling of plant steams, and some oncoids have gastropods as nuclei as well. The stromatolites consist of domes of up to 1 meter high; and many of them preserve a nuclei composed by masses of thin serpulid tubes. Large columns indicate that the shoreline was exposed to wave action. Recent studies support the relationship between morphostructures versus water depth and energy. Microbial activity was suspected from outcrops and through thin sections, and SEM analysis showed undoubtedly the presence of cyanobacterial filaments, bacterias and coccoids microbes that clearly support the microbial origin of the carbonates. Physa wichmanni is the gastropod preserved as nuclei of the oncolites. This species is known from several campanian-maastrichtian localities in northern Patagonia. All fossil species of Physa have been registered in facies indicative of fluvial and shallow lacustrine environments. The microfossils comprise ostracods as Candona huantraicoensis and Ilyocypris triebeli preserved as internal molds in the tuffs layers. Charophyte gyrogonites are preliminary identified as Peckichara sp. and Pseudoharrisichara sp. and are abundant in a thin calcareous level. The plant fossil assemblage recovered from the pyroclasts is dominated by large ogs of pycnoxylic wood, palm and cycad trunks. This vegetation grew under a warm (tropical to subtropical) climate and in a relatively humid environment. Casamiquela (1978) characterized the paleoenvironment of the Kawas sea as a low lying coast bordered by lagoons with alternating marine and freshwater condition surrounded by an exhuberant forest of ferns and palms.