Decoupling seasonal fluctuations in fluvial discharge from the tidal signature in ancient deltaic deposits: an example from the Neuquén Basin, Argentina

Fluvial discharge fluctuations are a fundamental characteristic of almost all modern rivers and can produce distinctive deposits that are rarely described from ancient fluvial or mixed-energy successions. Large-scale outcrops from the Middle Jurassic Lajas Formation (Argentina) expose a well-constrained stratigraphic succession of marginal-marine deposits with a strong fluvial influence and well-known tidal indicators. The studied deposits show decimetre-scale interbedding of coarser- and finer-grained facies with mixed fluvial and tidal affinities. The alternation of these two types of beds forms non-cyclic successions that are interpreted to be the result of seasonal variation in river discharge, rather than regular and predictable changes in current velocity caused by tides. Seasonal bedding is present in bar deposits that form within or at the mouth of minor and major channels. Seasonal bedding is not preserved in channel thalweg deposits, where river flood processes were too powerful, or in floodplain, muddy interdistributary-bay, prodelta and transgressive deposits, where the river signal was weak and sporadic. The identification of sedimentary facies characteristic of seasonal river discharge variations is important for accurate interpretation of ancient deltaic process regime

Magnetostratigraphy of the Jurassic through Lower Cretaceous in the Neuquén Basin

The first magnetostratigraphic scales for the Jurassic through Early Cretaceous from the Southern Hemisphere have been constructed over the last decades from marine sections in the Neuquén Basin. Paleomagnetic sites were tied to ammonite zones in order to achieve well-refined ages of studied sections. Diverse field tests for the paleomagnetic stability proved the primary origin of isolated magnetizations. In the case of Upper Jurassic–Lower Cretaceous studies, magnetostratigraphic and biostratigraphic data were combined with cyclostratigraphy. Finally, polarities were tied to Andean ammonite zones and from their correlation with the standard zones, calibrated to the GTS2016 (Geomagnetic Polarity Time Scale 2016). For the Early Jurassic, a composite magnetostratigraphic scale was derived out of five sections spanning the Hettangian–Toarcian. The magnetostratigraphic scale portrays 16 reverse (Jr1–Jr16) and 16 normal (Jn1–Jn16) polarity zones that encompass at least 19 ammonite zones. A major difference between both scales rises in the Hettangian involving the Jr1–Jr3 polarity zones. For the Middle Jurassic, the resultant magnetostratigraphy obtained in the Lajas Formation outlines a dominantly reverse polarity pattern. According to the correlation with the GTS2016, the studied section is assigned to the Lower-uppermost Middle Bathonian (Chrons M41 through M39). For the Late Jurassic–Early Cretaceous, the magnetostratigraphic scale obtained in the Vaca Muerta Formation comprises Subchrons M22r.2r through M15r, spanning the V. andesensis (Lower Tithonian)–S. damesi Zones (Upper Berriasian). The use of diverse chronostratigraphic tools such as biostratigraphy, magnetostratigraphy and cyclostratigraphy, enabled to determine with unprecedented precision the position of the Jurassic–Cretaceous boundary, as well as to assess durations of ammonite zonesFil: Iglesia Llanos, Maria Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias Geológicas; ArgentinaFil: Kietzmann, Diego Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias Geológicas; Argentin