PALEOENVIRONMENTAL RECONSTRUCTION OF A NEOGENE DIATOMITE FROM NORTHERN PATAGONIA, ARGENTINA

FAYÓ ROCÍO; ESPINOSA M. A.; VELEZ AGUDELO C.

Congreso; 4th Palaeontological Virtual Congress; 2023

© 2023 Palaeontological Virtual Congress

Diatomite deposits are siliceous biogenic sedimentary rocks composedmainly of diatoms. Paleoenvironmental reconstruction of them arescarce in Argentina. Micropaleontological and sedimentologicalcharacterization of a Plio-Pleistocene site of La Pampa (Puelén) ispresented. The aim is identifying the diversity of fossil diatoms andthe paleoenvironmental evolution based on diatom assemblages’composition. The “Dique” site (Fm. El Sauzal/Tehuelche, 38°08’40.5”S; 67°09’23.2” W) presents an outcropping section of 4.20m withlaminated white diatomite and three levels of volcanic ash. Sevensamples 50 cm-equidistant were extracted and oxidized with H2O2for diatomological analysis. 500 valves per slide were counted in orderto calculate the relative frequency of each species. Diatom zones weredefined using multivariate analysis of clustering (CONISS). Diverseassemblages with at least 73 morphospecies were determined,34 of these have abundances > 4%. Dominant and ubiquity smallfragilarioid diatoms (Punctastriata spp., Staurosira spp., Staurosirellaspp., Pseudostaurosira spp., among others), are accompanied byRhopalodia spp., Aulacoseira spp., Cymbella spp., Epithemia adnataand Cocconeis placentula, indicating a lacustrine origin. Based on thisnew micropaleontological evidence, in the basal section it was inferredthat a paleolake with a relatively alkaline pH, low concentrationsof nitrogen and high concentrations of phosphates (volcanic ash)favored the growth of a diverse diatom community (with epiphytes,planktonic, tychoplanktonic and benthic lifeforms). Towards the top,the dominance of small fragilarioids (tychoplanktonic, brackishfreshwater species) and the decrease of all freshwater planktonicdiatoms would indicate the shallowing of the water body and anincrease in salinity, associated with periods of aridization