MICROPALEONTOLOGIA (FORAMINIFEROS y OSTRACODOS) DEL CUATERNARIO DE PALMARES DO SUL (FORMACION CHUI), BRASIL
AbstractABSTRACT: MICROPALEONTOLOGY (FORAMINIFERA AND OSTRACODA) OF THE QUATERNARY FROM PALMARES DO SUL (CHUI FORMATION), BRAZIL. - First micropaleontological results of a research project, related with Quaternary subsurface strata of the Río Grande do Sul Coastal Plain, are presented. The studied G414-PDS-2 borehole is located in Palmares do Sul, and the core is 69,5 m long. The Coastal Plain, located in southern Brazil, is a long and narrow strip, almost parallel to the Atlantic shore, it is mostly made up of continental, marine, mixohaline and eolian deposits. The studied section, from the bottom to the top, comprises sediments of the Graxaim Formation (continental sands ), Chuí Formation (marine and mixohaline sands and clays) and the Itapoá Formation (eolian sands). These lithostratigraphic units overlie Cenozoic (Late Tertiary) marine deposits of the Pelotas basin. The Chuí Formation (50 meters), in the hole of Palmares do Sul yielded several Foraminifera and Ostracoda associations which revealed the existence, during the Quatemary, of relative sea level fluctuations; these changes gave origin, alternately, to shallow marine, coastal mixohaline (lagoons) and in part continental deposits. Typical shallow marine deposits are characterized by the foraminifera Buccella frigida (Cush.), Ouinqueloculina seminulum (L.), Pyrgo peruviana (d'Orb.), P. nasuta (Cush.), Bulimina marignata (d'Orb.), B. affinis (d'Orb.), B. patagonica (d'Orb.); elongata (d'Orb.), Discorbis williamsoni (Chapman & Parr ) , D. bertheloti (d'Orb.), Fursenkoina schreibersiana (Czízek ) , Bolivina striatula Cush., B. cuomoi Bolt., B. spathulata (Will.) and the ostracods Cytheretta punctata Sang., Kansarina sp. and Bensonia minipunctata Sanguinetti, whereas the coastal mixohaline environments, such as lagoons, are inferred ami based on the presence of a typical eurihaline assemblage of the foraminifera Ammonia beccarii (L.) parkinsoniana (d'Orb.), Elphidium galvestonense Korn., E. excavatum (T.), E. discoidale d'Orb. and the ostracods Cyprideis sp. and Minicythere heinii Orn. The last association is typical for littoral lagoons nowadays in the region. The age attributed to the Chuí Formation is Pleistocene-lower Holocene, based mainly on the known stratigraphic succession, the previous work of various authors in the area, and field observations. The above mentioned relative sea level fluctuations inferred for the analized stratigraphic suhsurface section, may represent hight and low sea-levels originating shallow marine and coastal mixohaline deposits, respectively. These facts may be probably correlative of major Quaternary events such as glaciations in the Northern Hemísphere, they are interpreted tentatively as glacieustatic responses to the Nebranskan, Kansan, Illinoian and the Wisconsin glacial stages and their interglacial units. Thirty foraminiferal species were found; the ostracode assemblage is composed of sixteen species, three of them new.
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