Integrated Stratigraphy of the Jurassic/cretaeous Boundary Interval in the Neuquen Basin (argentina)

Over last decades, the issue of the Jurassic/Cretaceous (J/K) boundary interval witnessed a series of controversies, which has been extensively debated. In broader context, the Tithonian–Berriasian interval was marked by a major marine regression that promoted strong faunal provincialism and hindered supra-regional correlations, which resulted in a prolonged lack of consensus on the J/K stratigraphy. Recently, several studies have highlighted stratigraphic importance of the data collected via alternative (to biostratigraphy) approaches, including geochemistry (δ13C, δ18O, elemental geochemistry), astronomical calibration, magnetostratigraphy and/or radiometric dating. Nevertheless, none of paleoenvironmental (geochemical) events has yet been demonstrated to have a truly global extent, whereas some supra-regional inconsistencies remain when consider cyclo- and magneteostratigraphic frameworks. To address these issues, and to provide a consistent age model for the Jurassic/Cretaceous transition, this research was focused on magnetostratigraphic investigations of two biostratigraphically and radiometrically dated sections in the Neuquén Basin (Argentina): Las Alcantarillas and Las Loicas. Precise correlation between the sections was achieved using high-resolution gamma-ray spectroscopy (GRS). The resulting composite Tithonian–Berriasian record comprises the ~20 m thick uppermost continental sandstones of the Tordillo Formation and the overlying ~226 m succession of dark gray limestones, marls/claystones, and subordinate sandstones and marls of the Vaca Muerta Formation. Although thermal and alternating-field demagnetization revealed that many samples yielded ambiguous (primary or secondary) palaeomagnetic directions, the high-resolution sampling approach (223 horizons), combined with principal component analysis (PCA) and remagnetization-circle analytical methods enabled reliable magnetostratigraphic interpretation of collected data. An important interpretational difficulty arose from the fact, that South American platform occupied relatively stable paleogeographic position since the J/K transition; this, in turn, translates to near identic paleomagnetic directions of primary ‘normal’ magnetizations as well as potential mid-Cretaceous overprint. Accordingly, relatively low unblocking temperatures (usually below 400 °C) of the Vaca Muerta Fm samples prevents unanimous interpretation whether normal pre-folding magnetizations were acquired during sedimentation or later burial stages. Nevertheless, the studied succession is characterized by systematic occurrences of ‘reversed’ samples, including those in which traces of reversed polarity were discovered via remagnetization-circle approach. Consequently, as ‘reversed’ samples are thought to clearly document primary synsedimentary magnetizations, they were used for a construction of magnetostratigraphic framework of the Las Alcantarillas and Las Loicas sections, and their correlation with the geomagnetic polarity time scale. In result, the studied succession was found to provide a continuous, yet variable in terms of sedimentation rates record of the M22n–M15r magnetozones interval (lower Tithonian–upper part of the upper Berriasian). Ultimately, integration of magnetostratigraphic and cyclostratigraphic frameworks (investigated via spectral analysis of high-resolution organic carbon isotope dataset) with radiometric ages provided a coherent temporal framework for the Tithonian/Berriasian transition interval. The results of this study not only enable reliable and detailed correlations with other J/K successions, but also significantly improve the quality of the late Jurassic–early Cretaceous age model, which was so far lacking precise goechronologic callibration. This research was carried with a financial support from National Science Center, Poland MINIATURA grant no. 2024/08/X/ST10/00372 (leader: Damian Gerard Lodowski).