First record of miocene marine fossil diatoms from northeast patagonia, Argentina: Preliminary results

Abstract

Despite their paleontological relevance as paleoenvironmental and paleoecological indicators, studies of fossil diatom assemblages from Miocene deposits of Patagonia (Argentina) are still scarce. In this work, we present the first record of marine diatoms from the Gaiman Formation (lower Miocene). This marine unit is composed of whitish mudstones, fine sandstones and tuffaceous beds. The samples were collected from outcrops of the coastal area of northeast Chubut at Estancia Redonda Chica locality. Two bioturbated muddy sandstone samples were processed to concentrate diatoms, mounted on a slide and analysed under the microscope at 1000x in order to confirm the presence of diatoms, and to identify the taxonomic composition of the assemblages. The samples are stored at CNP-PIIc collection. Until now, the species Paralia sulcata and Cocconeis placentula and the genus Halamphora sp., Planothidium sp. and Lemnicola sp. were recognized. Due the presence of P. sulcata it is suggested that the sampled sediments were accumulated in a coastal marine environment, with influence of freshwater supply as it is indicated by C. placentula and Lemnicola sp. Moreover, the latter two taxa strongly dominate the samples, while the rest are less abundant. These results confirm the sedimentologic interpretation for the sampled sediments as shallow marine (inner shelf) deposits, although the strong influence of freshwater supply appears as new evidence from the diatom analysis. Finally, this study is part of a more extensive work, in progress, with the objective of providing relevant and complementary information on the paleoenvironmental and stratigraphic analysis of the Gaiman Formation.