Palynofacies analysis and sedimentary environment of Early Jurassic coastal sediments at the southern border of the Neuquén Basin, Argentina

This study shows how a multidisciplinary scope, integrating the structure of palynologic assemblages, organic facies, sedimentary facies associations, and the results of basin analysis, clarifies the complexities of environmental and stratigraphic relations. The main objectives include understanding paleoenvironmental dynamics through sedimentologic and palynofacies analysis to reveal how a pronounced geodiversity is conditioning high biodiversity within a near-coast ecotonal system. The Nestares Formation outcrops at its type locality Alicurá dam, northern Patagonia. Sedimentologic and palynofacies analysis reveals a coastal environment of the formation. Lithology and sedimentary structures indicate fluvial conditions in the lower part of the outcropping Nestares Formation with a high input of terrestrial-derived organic material, as shown by palynofacies. The middle and upper parts represent relatively quiet environments of an interdistributary paralic bay with tidal influence (flaser bedding) between deltaic channels, probably surrounded by marshes. Alternating palynofacies dominated by amorphous organic matter of algal origin suggest episodes of restricted deposition conditions in which energy levels change frequently. A marine Late Toarcian palynologic assemblage suggests that the Nestares sequence constitutes the basal stratigraphic unit of the Cuyano Group; thus, the marine transgression of the Neuquén Basin arrived at this southernmost part of the basin as late as the Late Toarcian. Together with the arrival of Callialasporites, the presence of the Parvocysta suite, with Phallocysta, Nannoceratopsis, and (?)Suessiaceae, supports a Late Toarcian age.