Paleoenvironmental implications of lacustrine Ostracodes (Crustacea) from the La Cantera Formation (Early Cretaceous), San Luis, Argentina

EVELYN L. BUSTOS ESCALONA; BENAVETE, CECILIA; MANCUSO, ADRIANA CECILIA

Reno, Nevada

Congreso; 6th International Limnogeology Congress; 2015

IAL

The La Cantera Formation is part of the infilling of the San Luis Basin (San Luis Province, Argentina). During the Cretaceous, as a result of the reactivation of a previous Triassic rift, the basins were developed. The La Cantera Formation is characterized by greenish‐gray finely horizontal laminated claystones and siltstones interbedded with gray ripple‐laminated sandstones that were interpreted as a lacustrine environment. The laminated mudrocks present freshwater fossils as crustaceans, fish and insects. The aim of this work was to analyze the ostracode (Crustacea) associations from the La Cantera Formation, from a paleolimnologic point of view, and to compare the results with those obtained from other groups of organisms from the same unit and other coeval formations. Three samples bearing ostracodes from different levels of the La Cantera Formation Type Locality were processed (M1, M2 and M3). The ostracode extraction process was mechanical and manual and cleaning of the sediment surface was done under binocular low‐magnification microscope. The specimens were photographed in the Scanning Electron Microscope (SEM) of the Laboratorio de Microscopía Electrónica y Microanálisis of the Universidad Nacional de San Luis (LabMEB, UNSL). Furthermore, Energy Dispersive Spectrometer (EDS) microanalysis was performed on specimens and sediment. The studied material is housed in the Fossil Collection (MIC) from the Facultad de Ciencias Físicas, Matemáticas y Naturales, UNSL. Three different taxa of the Class Ostracoda were recognized: Limnocytherinae sp. and Podocopida indet (Family Limnocytheridae) and Pattersoncypris angulata (Family Cyprideidae) (Bustos Escolana, 2015). Limnocytherinae sp. and Podocopida indet. were found in Level 10 and presents good preservation. Pattersoncypris angulata was found at levels 29 and 30, but they are more abundant in level 29. The preservation of the ostracodes from these levels is poor, with broken shells and sometimes only the molds are preserved. Microanalysis showed differences in elemental composition between the 3 levels; particularly between level 10 (sample M1) and the levels 29 and 30 (samples M2 and M3, respectively) (Table 1). In the M1, a high concentration of Si is observed, with a particular absence of Ca in the shells, but not in the bearing substrate. The samples M2 and M3 differ with M1 due to the presence of a high concentration of Ca in the shells and the absence of Si. The M2 substrate shows a higher concentration of Ca than Si, and in M3 the substrate has a similar concentration of Ca and Si. The ostracode microfauna from the La Cantera Formation appears to be a low diversity association characterized by two species of freshwater ostracodes distributed in two families. Representatives of Limnocytherinae are distributed in different continental Cretaceous basins of Argentina. Pattersoncypris angulata is recorded in Cretaceous successions in Brazil and Argentina. Singularly, the ostracode associations of the two lacustrine successions of the San Luis Basin have no similar elements. The association recorded in the La Cantera and the microanalysis suggests that the ostracodes were developed in freshwater environments with strong changes throughout its temporal evolution in their evaporation and salinity rates, mainly in the lacustrine shoreline.