HIGH-RESOLUTION CYCLOSTRATIGRAPHY AND EVALUATION OF POSSIBLE ORBITAL-SIGNAL TRANSFERENCE MECHANISMS IN THE UPPER JURASSIC-LOWER CRETACEOUS OF THE NEUQUÉN BASIN

KIETZMANN, D.A.; IGLESIA-LLANOS, M.P.; KOHAN MARTINEZ, M.; STURLESI, M.A.; TAHA, N.; WALDMAN, N.

Congreso; XVII Reunión Argentina de Sedimentología y VIII Congreso Internacional de Sedimentología; 2021

The Late Jurassic-Early Cretaceous in the Neuquén Basin (Mendoza Group) represents a time of tectonic quiescence that prompted the development of thick carbonate and mixed successions. These sequences display a marked rhythmicity of limestone-marlstone interpreted as controlled by variations of Earth´s orbit (Milankovitch cycles). Cyclostratigraphy has become an important tool in measuring geologic time and establishing floating astronomical time scales (ATS). In fact, almost the entire Jurassic and Cretaceous have been calibrated using astronomical cycles, yet high resolution tuning of the transition between those systems is still in development. In this study we present detailed cyclostratigraphical analyses carried out on eight Tithonian-Hauterivian sections of the Vaca Muerta and Agrio Formations exposed in southern Mendoza province (northern part of the Neuquén Basin). These units are characterized by decimeter-scale rhythmic alternations of marlstones and limestones, showing a well-ordered hierarchy of cycles, including elementary cycles, bundles and superbundles. According to biostratigraphy, elementary cycles have a periodicity of ~18-21 ky, which responds to the precession cycle of Earth. Spectral analysis based on time series of elementary cycle thicknesses allowed us to identify frequencies of ~400 ky and ~90-120 ky, which we interpret as the modulation of the precessional cycle by Earth?s eccentricity. A third frequency band of ~40 ky was also erratically identified that can be assigned to the obliquity cycle. In order to understand the transference mechanisms of the orbital signal to the sedimentary record, four variables were analyzed in almost 280 samples taken in c.a. 100 m- thick interval. These variables were compared with microfacial data to infer sedimentary processes: calcium carbonate content (%CaCO3), total organic carbon (%TOC), magnetic susceptibility (k) and anhysteretic remanent magnetization (ARM). Our results show that the first three variables respond to the cyclical forcing, while the ARM does not show any cyclical pattern. Carbonate content and TOC data suggest variations in carbonate productivity/fertility, while variations in the magnetic susceptibility cannot be related so far to any clear transference mechanism. Finally, ARM could be used as a proxy to evaluate the detrital contribution, although we observe no cyclic pattern. Whilst the coexistence of productivity and dilution as the dominant mechanisms is not ruled out, productivity as ?carbonate exportation? seems to be the dominant transference mechanism within the Vaca Muerta and lower Agrio Formations. Moreover, the current cyclostratigraphic analysis enables to finely tune in an almost continuous fashion, the time scale for the epochs covered by the sedimentary succession, hence providing an important contribution to global stratigraphy.