PALEOENVIRONMENTAL RECONSTRUCTION OF A NEOGENE DIATOMITE FROM NORTHERN PATAGONIA, ARGENTINA

ROCÍO FAYÓ; ESPINOSA MARCELA A.; VELEZ AGUDELO CAMILO A.

Congreso; 4th Palaeontological Virtual Congress; 2023

ONLINE PALAEONTOLOGY SCIENTIFIC MEETING PLATFORM

Diatomite deposits are siliceous biogenic sedimentary rocks composed mainly of diatoms. Paleoenvironmental reconstruction of them are scarce in Argentina. Micropaleontological and sedimentological characterization of a Plio-Pleistocene site of La Pampa (Puelén) is presented. The aim is identifying the diversity of fossil diatoms and the 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 with laminated white diatomite and three levels of volcanic ash. Seven samples 50 cm-equidistant were extracted and oxidized with H2O2 for diatomological analysis. 500 valves per slide were counted in order to calculate the relative frequency of each species. Diatom zones were defined using multivariate analysis of clustering (CONISS). Diverse assemblages with at least 73 morphospecies were determined, 34 of these have abundances > 4%. Dominant and ubiquity small fragilarioid diatoms (Punctastriata spp., Staurosiraspp., Staurosirella spp., Pseudostaurosira spp., among others), are accompanied by Rhopalodia spp., Aulacoseira spp., Cymbella spp., Epithemia adnata and Cocconeis placentula, indicating a lacustrine origin. Based on this new micropaleontological evidence, in the basal section it was inferred that a paleolake with a relatively alkaline pH, low concentrations of 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, brackish-freshwater species) and the decrease of all freshwater planktonic diatoms would indicate the shallowing of the water body and an increase in salinity, associated with periods of aridization.