Cyclostratigraphy of an upper Valanginian – lower Hauterivian mixed siliciclastic-carbonate ramp succession (Pilmatué Member of the Agrio Formation), Loma La Torre section, northern Neuquén Basin, Argentina
Résumé
Detailed sedimentological and stratigraphical studies were performed in the Pilmatué Member of the Agrio Formation at Loma La Torre area, to recognize orbital signals in the Valanginian–Hauterivian of the Neuquén Basin. Facies analysis allowed us to recognize 4 facies associations, interpreted as a mixed siliciclastic-carbonate ramp setting. A sequence stratigraphic analysis was performed allowing to recognize two composite sequences and five high-frequency sequences. This eurybathic/eustatic trend was subtracted from time series constructed by lithological rhythms (limestone/marlstone couplets) to stabilize its variance. Cyclostratigraphic analysis shows that the rhythmic late Valanginian – early Hauterivian succession presents a cyclical stratigraphic pattern composed by elementary cycles (limestone/marlstone couplets), bundles (4–5 elementary cycles) and superbundles (4–5 bundles) suggesting a climatic Milankovitch control. The found periodicities correspond to the precession cycle (∼20 ky), which is modulated by the eccentricity cycle (∼90 and ∼400 ky). Some statically low-significant periodicities of ∼42 ky and ∼33 ky can be assigned to the obliquity signal. That periodicities fit well with the so-called precession-eccentricity syndrome (PES) characteristic of mid and low latitudes, where obliquity is erratically identified. Our results indicate that the early Hauterivian has a minimum duration of ∼3 myr, considerably longer than the durations proposed in the GTS.