Integrated palynologic and paleontologic studies of a lower cretaceous fluvio-lacustrine sequence of Central Western Argentina

PRÁMPARO, M.; MILANA, J.P.; BALLENT, S.; GALLEGO, O.F.

Ontario, Canadá

Congreso; Proceedings of the 36° Annual Meeting of the American Association of Stratigraphic Palynologists. Joint Meeting with the Canadian Association of Palinologists and the North American Micropaleontology Section, SEPM; 2004

American Association of Stratigraphic Palynologists - Canadian Association of Palinologists and the North American Micropaleontology Section

The Lagarcito Formation lies over the sediments of the Gigante Group and is well exposed at the eastern side of Sierra de Guayaguas, south-eastern San Juan Province, central-western Argentina. It comprises siliciclastic fluvio-lacustrine sediments deposited in an extensional cretaceous basin, related to the breaking up of Gondwana. This Formation yields and important continental palynoflora and a fossil fauna of ostracoda and conchostraca. This integrated study is carried out in order to obtain more accurate palaeoecological data of the lacustrine sequence. Several types of lacustrine and fluvial environments are recorded throughout the deposition of the studied formation. Lacustrine environments range from a shallow meromictic low-saline lake, to a shallow stratified saline lake and to a saline sabhka. Fluvial systems are represented by ephemeral sandy systems (sheet-like channels, multistoried) and a terminal fluvial system that seems to represent a transition to the different lake systems. No delta-like systems were detected, supporting thus the presence of very shallow lakes. Although six main lake episodes have been detected reaching a stratificated phase, only the first and the second lake episodes have yielded palynological and paleontological remains. On both of them, fertile samples were located at the beginning of lake development, where lithology suggests a low-saline lake, which afterwards became saline. The ostracods fauna exhibites a very low diversity but a relatively well preservation. The assemblage is composed only by carapaces belonging to the superfamilies Cypridoidea (10 species) and Darwinuloidea (one specie). The conchostracan are assigned to the genera Dendrostracus, Pseudoestherites  and Estheriina, with probably at least, two new species. Four palynological assemblages were established based on the quantitative data of six sampled levels. The palynoflora is markedly dominated in all the assemblages by gymnospermous elements, mainly represented by plicate and rimulate grains. Classopollis is abundant (nearly 20%) in all the assemblages. Within the plicate grains, the different species of Cycadopites and a rich variety of Ephedra like pollen are the most frequent elements. The trilete spores are well represented by several species belonging to the genera: Verrucosisporites, Concavissimisporites, Cicatricosisporites, Cingutriletes, Crybelosporites, Dictyophyllidites. Bisaccate grains are scarse. Exesipollenites/ Spheripollenites are the main inaperturate group.  The aquatic forms are represented by the genera Leiosphaeridia, Scenedesmus, Botryococcus, Crucigeniella? and Ovoidites. A lacustrine environment, with a high nutrient content (eutrophic conditions) by the abundance of large Scenedesmus cenobia (32 cells), under warm and generally dry conditions, with high evaporation but locally moist conditions near the lacustrine bodies, as indicated by the high frequency and diversity of trilete spores, is suggested.